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Light-emitting boron nitride nanoparticles encapsulated in zeolite ZSM-5
Microporous and Mesoporous Materials 40 (2000) 263±269
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Light-emitting boron nitride nanoparticles encapsulated in zeolite ZSM-5 Xiaotian Li a, Changlu Shao a, Shilun Qiu a,*, Feng-Shou Xiao a, Weitao Zheng b, Pinliang Ying c, Osamu Terasaki d a
Department of Chemistry and Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, Changchun 130023, People's Republic of China b Department of Material Science, Jilin University, Changchun 130023, People's Republic of China c State Key Laboratory of Catalysis, Dalian Institute of Chemistry Physics, Chinese Academy of Sciences, Dalian 116023, People's Republic of China d Department of Physics, Graduate School of Science, Tohoku University, Sendai 980-77, Japan Received 1 January 2000; received in revised form 3 March 2000; accepted 14 March 2000
Abstract We report strong visible photoluminescence (PL) at room temperature from BN nanoparticles encapsulated in ZSM5. The investigation of powder X-ray diraction, X-ray photoelectron spectroscopy, adsorption of N2 , UV±Vis absorption, and PL spectra shows that BN nanoparticles have been successfully encapsulated in ZSM-5. Intense blue PL can be obtained from the BN/ZSM-5 sample. Analysis of PL spectra leads us to propose that the luminescence may originate from the bound excitons at the defects or impurities in the BN nanoparticles, a quasi-direct-gap semiconductor material transferred from indirect-gap BN by encapsulation in ZSM-5. Ó 2000 Elsevier Science B.V. All rights reserved. PACS: 78.55.-m; 78.55.Cr; 78.66.Fd; 78.66.Jg Keywords: Photoluminescence; BN nanoparticles; Encapsulation; ZSM-5; Zeolite
1. Introduction Blue-emitting materials are in great demand for optoelectronic devices and display applications. As is well known, the discovery that porous silicon (PS) exhibits strong visible photoluminescence (PL) suggests promising applications for future Si-based optoelectronics [1]. There are several reports on blue-emitting PS [2±4], but the PL is weak, and the
*
Corresponding author. Fax: +86-431-5671974. E-mail address: [email protected] (S. Qiu).
stability and reproducibility are also low [2]. Many eorts have been made to produce other ecient blue-emitting materials. For example, a recent work on GaN, a wide-bandgap material, has shown excellent performance as light-emitting diodes operating in the blue region of the spectrum [5]. Generally, c-BN, which is the second hardest material after diamond and a similar wide-bandgap semiconductor, is used as hard-coating materials. In addition to hard-coating applications, c-BN has potentiality in high-power, high-temperature electronic devices [6]. However, because of the diculty in synthesizing nearly phase-pure
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and highly crystalline ®lms, there are few reports of the electronic properties of c-BN ®lms that are greatly inferior to typical GaN ®lms in crystalline quality [6]. In addition, c-BN has an indirect electronic bandgap [7], which is not ideal for lightemitting diodes. To produce ecient visible light from indirect-gap materials, such as c-BN, one might suppose that the only practical route would be to wander into the realms of low dimensionality. Notably, the encapsulation of semiconductor nanoclusters in the pores of molecular sieves has been actively explored as a means of producing quasi-direct-gap semiconductor light-emitting nanostructures from indirect-gap materials [8]. The well-de®ned and well-organized cavities provided by the zeolite hosts would serve as an ideal nanoreactor for the synthesis of single-sized semiconductor nanoclusters arrays. ``Ship-in-a-bottle'' approach has been used for the generation of numerous clusters and superclusters (cluster arrays) in zeolites [9,10]. The windows of the zeolite pores provide ready access for transporting reagents to the cavities, making a variety of synthesis schemes possible. The additional potential for producing novel clusters of electronic materials and stabilizing unusual geometries and connectivities by using the surface passivating and templating eects of the host lattice is also most attractive [11]. However, to our knowledge, luminescent BN has not yet been successfully achieved. Here, we try to realize the transference from indirect-gap BN to a quasi-direct-gap BN semiconductor material by encapsulation methods. In this paper, we report that light-emitting boron nitride nanoparticles can be successfully prepared by encapsulation methods, and intense blue PL can be obtained from the BN/ZSM-5 sample characterized by powder X-ray diraction (XRD), X-ray photoelectron spectroscopy (XPS), adsorption of N2 , UV±Vis absorption, and PL spectra. 2. Experimental section 2.1. Experimental process To prepare BN in ZSM-5, a superior synthetic approach is akin to that used for CVD or
MOCVD deposition of BN ®lm, for example, absorption of B2 H6 vapor followed by calcination in an atmosphere of NH3 . But B2 H6 is a heavily poisonous gas, and the synthetic process is dangerous. Here, we use an indirect method to prepare BN/ZSM-5. ZSM-5 is a kind of Si or Si±Al zeolite with two-dimensional open frame pores prepared by using work structure of 5±6 A hydrothermal synthesis in our laboratory [12]. We use Si±ZSM-5 as the host material. Firstly, the mechanical mixture of Si±ZSM-5 with a little B2 O3 (sample was over 99% pure with water as impurity and the XRD characteristic peak is around 28° (2h)) was heated at the dispersed temperature of 400°C for 36 h in a vacuum chamber (background pressure < 10ÿ3 Torr) to form B2 O3 /ZSM-5 composite materials, then, B2 O3 /ZSM-5 was treated in a nitriding instrument. The nitridation procedure is as follows: the sample was prepared by a temperature-programmed reaction of B2 O3 /ZSM-5 with ammonia. The temperature was increased from room temperature to 573 K in 0.5 h and from 573 to 973 K in 7 h, and was maintained at 973 K for an additional 2 h. The sample was cooled to room temperature in ¯owing ammonia, then passivated in a stream of 1% O2 =N2 so as to avoid violent oxidation. The resulting solid is stable in air. The bulk BN we used in UV±Vis absorption and XPS characterization was prepared by the reaction of B2 O3 with NH3 using the same procedure. 2.2. Instrumentation The BN nanostructure sample was characterized by powder XRD, XPS, adsorption of N2 , UV±Vis absorption, and PL spectra. The powder XRD data were collected by a Siemens D5005 diractometer using CuKa radiation, 40 kV, 35 mA with scanning rate of 0.2°/min (2h). The adsorption of N2 was measured with a nitrogen adsorption method (BET) by using an ASAP 2010M Micromeritics Instrument at 78 K. UV±Vis absorption spectra of various samples were obtained by a Perkin Elmer UV-Lambda-20 UV±Vis spectrophotometer with BaSO4 as the background, scanning range from 190 to 900 nm. PL
X. Li et al. / Microporous and Mesoporous Materials 40 (2000) 263±269
spectra were collected at room temperature by an MPF-4 ¯uorescence photospectrometer with correction for instrumental wavelength dependencies. The XPS for powder samples ®xed on double-sided tapes was measured on an ESCALAB MKII X-ray photoelectron spectrometer (VG Co.) using AlKa radiation. The base pressure was 10ÿ7 Pa. Corrections of energy shift, due to the steady state charging eect, were accomplished by assuming a 284.6 eV binding energy (BE) for the C1s peak, due to adventitious carbon contamination.
3. Results and discussion 3.1. X-ray diraction Fig. 1 shows the XRD patterns of B2 O3 /ZSM-5 samples with the weight ratio of 0.1 g/g before and after the heat treatment. The XRD peaks show the characteristics of B2 O3 and ZSM-5 structure. As observed in Fig. 1b, the XRD peaks at 28.5° (2h)
Fig. 1. XRD patterns of (a) B2 O3 /ZSM-5 samples with weight ratio of 0.1 g/g after the heat treatment, and (b) mechanical mixture of 0.1 g/g of B2 O3 /ZSM-5 before the heat treatment.
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represent the characteristics of B2 O3 , because the characteristic peak of bulk B2 O3 we used is around 28° (2h). It is of interest to note that the characteristic peaks assigned to B2 O3 disappear completely when the sample is heated at 400°C for 36 h, as given in Fig. 1a. The phenomenon might be explained by high dispersion of B2 O3 into the channel of the ZSM-5 zeolite, where B2 O3 no longer exists on the surface of ZSM-5 in crystalline state [13,14]. On increasing the B2 O3 loading in ZSM-5 zeolite to 0.2 and/or 0.3 g/g, the characteristic peaks assigned to crystalline B2 O3 around 28.5° (2h) appear again after the heat treatment. The phenomenon indicates that B2 O3 exists on the surface of ZSM-5 in crystalline state again. Comparison of B2 O3 with other metal oxides and salts, shows that the threshold (the critical dispersion capacity) is signi®cantly reduced [13,14]. Thus, the proper loading of B2 O3 is no more than 0.1 g/g (weight ratio). This value corresponds to ca. eight B2 O3 molecules per unit cell of ZSM-5. According to the spontaneous monolayer dispersion model [13], the dispersed B2 O3 molecules will form a monolayer or submonolayer. Because B2 O3 molecules are more dicult to be dispersed than other metal oxides or salts, it is impossible to disperse B2 O3 molecules into the channels of ZSM-5 on average. These channels, which are close to the surface of ZSM-5, are preferable. Thus, the BN nanoparticles, which were prepared by the reaction of B2 O3 with NH3 using the nitridation procedure, should exist at the same places. In order to increase the loading of B2 O3 , we should use tinysized ZSM-5 as host materials. Fig. 2 shows the XRD patterns of BN/ZSM-5 and ZSM-5 samples. Analyses of the BN/ZSM-5 material by powder XRD reveal that the crystallinity and integrity of the zeolite host ZSM-5 are maintained and there is no diraction peaks characteristic of BN whose characteristic peak is at 26.8° (2h). This indicates that BN, which is formed by a nitridation process, has been encapsulated in zeolite ZSM-5. The change of the intensities around 8° and 24° (2h) may be dealt with the encapsulation of BN into the channels of ZSM-5 at high temperature. The dispersion of active components onto the surface of supports, such as SiO2 , Al2 O3 , has
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did not change, which has been demonstrated from the analyses of XRD for the resultant nitridation samples. In other words, BN formed by nitridation reaction will be encapsulated in ZSM-5 to form nanoparticles. 3.3. Adsorption isotherms The BN nanoparticles encapsulated in ZSM-5 lead to the change in channel size of ZSM-5 signi®cantly. Fig. 3 shows the N2 isotherms of the ZSM-5 and BN/ZSM-5 samples. We observe that both the isotherms exhibit typical Langmuir adsorption on the samples with the adsorption amount of 75 and 18 cm3 /g. The dierent adsorption amounts may be related to the change in channel shape in ZSM-5, which strongly in¯uences the adsorption of N2 on the sample. Fig. 2. XRD patterns of BN/ZSM-5 and ZSM-5 samples.
been widely investigated by Xie and Tang [13]. On comparing these supports with zeolite ZSM5, it is found that the speci®c surface area of ZSM-5 is higher. In some respects, the channel structure of ZSM-5 is expected to be more favorable to stabilize B2 O3 nanoparticles. We have successfully dispersed B2 O3 into the channels of ZSM-5 with the loading of 0.1 g/g (weight ratio). But we never succeeded in dispersing B2 O3 onto the surface of SiO2 or Al2 O3 with such loading. Thus, ZSM-5 has some superiority to be used as host materials.
3.4. X-ray photoelectron spectroscopy Because XPS is a surface-sensitive technique, BN, which is formed on the inner surface of the channels of ZSM-5 as nanoparticles, will give an XPS signal much stronger than that given by bulk BN. This prediction has been borne out well by XPS studies on our samples.
3.2. Nitridation procedure During nitridation procedure, NH3 was adsorbed into the pores of ZSM-5 where B2 O3 had been dispersed already. At the reaction temperature, NH3 reacted with B2 O3 to form BN. Because of the high reaction temperature and the ammonia erosion, the host material must have reliable thermal stability. ZSM-5, a kind of stable zeolite below 1000°C, is more suitable to be used as host materials than other zeolites, such as NaY. During nitridation process, the structure of ZSM-5 almost
Fig. 3. N2 isotherms of the ZSM-5 and BN/ZSM-5 samples.
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Fig. 4. XPS spectra for B 1s in (a) bulk BN and (b) BN/ZSM-5 sample.
Fig. 5. XPS spectra for N 1s in (a) bulk BN and (b) BN/ZSM-5 sample.
Figs. 4 and 5 show XPS spectra of BN/ZSM-5 and the bulk BN prepared by the reaction of B2 O3 with NH3 in the B 1s and N 1s energy regions. It can be seen that the main peak of B 1s in BN/ ZSM-5 is at 191.5±193.0 eV, and the main peak of N 1s in BN/ZSM-5 is at 398.7±399.1 eV, which suggests the existence of B and N atoms. The binding energy of B 1s in BN/ZSM-5 has been found to be about 1.2±2.7 eV higher than that for the bulk BN (Eb 190:3 eV). As to the binding energy of N 1s, a shift of 0.4 eV can be observed. It is reasonable to consider that the increase of the binding energy of B 1s and N 1s is due to the variation of BN forms. When the BN was prepared from bulk form to nanoparticles encapsulated in the channels of ZSM-5, the XPS signals became stronger. In other words, the increase of the binding energy is an evidence of the presence of low-dimensional BN. As shown in Figs. 4 and 5, the XPS spectra of B 1s and N 1s in BN/ZSM-5 are broader than that in bulk BN prepared by the reaction of B2 O3 with NH3 using the same nitridation procedure. The results may indicate that some B2 O3 cannot react
with NH3 completely to form BN, and some intermediate states may be formed during the complicated nitridation process. Thus, defects or impurities, which may act as the origin of luminescence, are irremovable in the BN particles. 3.5. UV±Vis spectra Fig. 6 shows the UV±Vis absorption spectra of ZSM-5, BN, and BN/ZSM-5. The spectrum of ZSM-5 has no obvious peaks. The spectrum of BN has one pronounced peak at 214 nm. As to the spectrum of BN/ZSM-5 material, it gives one intense peak at 206 nm, which may be assigned to the characteristic peak of BN. The blueshift from 214 to 206 nm can be considered as the contribution by quantum size eect. The phenomenon indicates that BN are present in the pores of ZSM-5. 3.6. Visible photoluminescence PL normalized spectra from the resultant samples are given in Fig. 7. When the excited wavelength is 450 nm, two broad PL peaks around 520
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Fig. 6. UV±Vis absorption spectra of (a) BN, (b) ZSM-5, and (c) BN/ZSM-5.
Fig. 7. PL spectra of BN/ZSM-5 under exciting wavelength of (a) 260 nm, (b) 365 nm, and (c) 450 nm.
and 590 nm can be obtained. As the excited wavelength changes to 365 nm, the spectrum exhibits a large blueshift from 520 to 434 nm. Under an excitation at 260 nm, we can observe an intense PL peak at 336 nm. The quantum eciency is considered to be high, because the observed emission intensity is intense compared with its excitation intensity. To evaluate the possible origins of the strong PL, we consider the optical properties of ZSM-5, B2 O3 /ZSM-5, and BN nanoparticles encapsulated in ZSM-5. Prior to measuring the PL of BN nanoparticles encapsulated in ZSM-5, the PL behavior of ZSM-5 and B2 O3 /ZSM-5 was evaluated, and no obvious emission was observed at room temperature, which indicates that the contribution from ZSM-5 and B2 O3 /ZSM-5 to PL can be neglected. The only possible origin of the strong PL is BN nanoparticles encapsulated in ZSM-5. In bulk crystal of indirect-gap BN material, electron±hole recombination is possible only through phonon emission or absorption because the wave-vector dierence between the conduction-band bottom and the valence-band top must be compensated [15]. According to the quantum con®nement theory on solid state physics, the lowest exciton energy in quantum dots increases with the decrease of their size. The energy of luminescence from indirect-type exciton also increases with the decrease of size in indirect-gap semiconductor particles [16,17]. Since the indirect-gap energy of bulk c-BN is ca. 6 eV (200 nm) [6], the energy peak of the luminescence from BN nanoparticles cannot be explained by the quantum con®nement theory simply. When indirect-gap BN is prepared into low-dimensional structures such as nanoclusters, superlattice, or quantum dots, the excitons will be bound by the quantum con®nement eect. We used NH3 gas to prepare BN nanoparticles. As bonds between B and N have strong covalency, dangling bonds are not preferred to being formed. The surface of the particles may be terminated by H atoms, which may play a role in an intense luminescence at visible region. Bulk h-BN with impurities of C or H atoms shows luminescence near UV region [18]. During nitridation procedure, the NH3 gas must be transferred through the channel of ZSM-5 to react with B2 O3 . The complicated
X. Li et al. / Microporous and Mesoporous Materials 40 (2000) 263±269
process makes it possible that some B2 O3 cannot react with NH3 completely, thus defects are irremovable in the BN particles. This prediction can be borne out by XPS studies on BN/ZSM-5 sample (see Figs. 4 and 5). The luminescence may originate from the bound excitons at the defects or impurities. Compared with free excitons, luminescence from bound excitons is usually intense. On exciting under dierent wavelength, excitons will be excited to dierent excited state, resulting in dierent visible PL from 590 to 336 nm. The size of BN nanoparticles in ZSM-5, the quantum yield of the luminescence, and the theoretical calculations of the excited states that corresponded to dierent PL will be reported later. 4. Concluding remarks In summary, we have successfully prepared nanometer-sized BN by an encapsulation method and observed strong PL. The investigation of powder XRD, XPS, adsorption of nitrogen, UV±Vis absorption, and PL spectra shows that BN nanoparticles can be encapsulated in the pores of ZSM5. The luminescence may originate from the bound excitons at the defects or impurities, which are irremovable in the BN nanoparticles during nitridation process. We can conclude that the encapsulation method which are used to prepare quasi-direct-gap semiconductor materials from indirect-gap materials, such as Si, BN, and SiC,
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etc., will have spectacular applications in optoelectronics. This work was supported by the National Natural Science Foundation of China. We are also grateful to the reviewer of this paper for the contribution to luminescence mechanism.
References [1] L.T. Canham, Appl. Phys. Lett. 57 (1990) 1046. [2] X. Wang, G. Shi, F.L. Zhang, H.J. Chen, W. Wang, P.H. Hao, X.Y. Hou, Appl. Phys. Lett. 63 (1993) 2363. [3] M.K. Lee, K.R. Peng, Appl. Phys. Lett. 62 (1993) 3159. [4] D. R uter, T. Kunze, W. Bauhofer, Appl. Phys. Lett. 64 (1994) 3006. [5] H. Morkoc, S. Strite, G.B. Gao, M.E. Lin, B. Sverdlov, M. Burns, J. Appl. Phys. 76 (1994) 1363. [6] P.B. Mirkarimi, et al., Mater. Sci. Engng. R21 (1997) 47. [7] M.P. Surh, S.G. Louie, M.L. Cohen, Phys. Rev. B 43 (1991) 9126. [8] N.J. Herron, J. Incl. Phenom. Mol. Recognit. Chem. 21 (1995) 283. [9] N. Herron, Inorg. Chem. 25 (1986) 4714. [10] Y. Wang, N. Herron, J. Phys. Chem. 91 (1987) 257. [11] G.D. Stuchy, J.E. MacDougall, Science 247 (1990) 669. [12] S. Qiu, R. Ohnishi, H.J. Ichikawa, Phys. Chem. 98 (1994) 2719. [13] Y.C. Xie, Y.Q. Tang, Adv. Catal. 37 (1990) 1. [14] F.-S. Xiao, S. Zheng, J. Sun, R. Yu, S. Qiu, R. Xu, J. Catal. 176 (1998) 474. [15] T. Takagahara, K. Takeda, Phys. Rev. B 46 (1992) 15578. [16] W. Chen, G. McLendon, A. Marchetti, J.M. Rehm, M.I. Freedhoof, C. Myers, J. Am. Chem. Soc. 116 (1994) 1585. [17] Y. Masumoto, T. Kawamura, Phys. Rev. B 46 (1992) 1827. [18] K. Era, F. Minami, T. Kuzuba, J. Lumin. 24/25 (1981) 71.
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Light-emitting boron nitride nanoparticles encapsulated in zeolite ZSM-5 Xiaotian Li a, Changlu Shao a, Shilun Qiu a,*, Feng-Shou Xiao a, Weitao Zheng b, Pinliang Ying c, Osamu Terasaki d a
Department of Chemistry and Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, Changchun 130023, People's Republic of China b Department of Material Science, Jilin University, Changchun 130023, People's Republic of China c State Key Laboratory of Catalysis, Dalian Institute of Chemistry Physics, Chinese Academy of Sciences, Dalian 116023, People's Republic of China d Department of Physics, Graduate School of Science, Tohoku University, Sendai 980-77, Japan Received 1 January 2000; received in revised form 3 March 2000; accepted 14 March 2000
Abstract We report strong visible photoluminescence (PL) at room temperature from BN nanoparticles encapsulated in ZSM5. The investigation of powder X-ray diraction, X-ray photoelectron spectroscopy, adsorption of N2 , UV±Vis absorption, and PL spectra shows that BN nanoparticles have been successfully encapsulated in ZSM-5. Intense blue PL can be obtained from the BN/ZSM-5 sample. Analysis of PL spectra leads us to propose that the luminescence may originate from the bound excitons at the defects or impurities in the BN nanoparticles, a quasi-direct-gap semiconductor material transferred from indirect-gap BN by encapsulation in ZSM-5. Ó 2000 Elsevier Science B.V. All rights reserved. PACS: 78.55.-m; 78.55.Cr; 78.66.Fd; 78.66.Jg Keywords: Photoluminescence; BN nanoparticles; Encapsulation; ZSM-5; Zeolite
1. Introduction Blue-emitting materials are in great demand for optoelectronic devices and display applications. As is well known, the discovery that porous silicon (PS) exhibits strong visible photoluminescence (PL) suggests promising applications for future Si-based optoelectronics [1]. There are several reports on blue-emitting PS [2±4], but the PL is weak, and the
*
Corresponding author. Fax: +86-431-5671974. E-mail address: [email protected] (S. Qiu).
stability and reproducibility are also low [2]. Many eorts have been made to produce other ecient blue-emitting materials. For example, a recent work on GaN, a wide-bandgap material, has shown excellent performance as light-emitting diodes operating in the blue region of the spectrum [5]. Generally, c-BN, which is the second hardest material after diamond and a similar wide-bandgap semiconductor, is used as hard-coating materials. In addition to hard-coating applications, c-BN has potentiality in high-power, high-temperature electronic devices [6]. However, because of the diculty in synthesizing nearly phase-pure
1387-1811/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 7 - 1 8 1 1 ( 0 0 ) 0 0 2 6 3 - 8
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and highly crystalline ®lms, there are few reports of the electronic properties of c-BN ®lms that are greatly inferior to typical GaN ®lms in crystalline quality [6]. In addition, c-BN has an indirect electronic bandgap [7], which is not ideal for lightemitting diodes. To produce ecient visible light from indirect-gap materials, such as c-BN, one might suppose that the only practical route would be to wander into the realms of low dimensionality. Notably, the encapsulation of semiconductor nanoclusters in the pores of molecular sieves has been actively explored as a means of producing quasi-direct-gap semiconductor light-emitting nanostructures from indirect-gap materials [8]. The well-de®ned and well-organized cavities provided by the zeolite hosts would serve as an ideal nanoreactor for the synthesis of single-sized semiconductor nanoclusters arrays. ``Ship-in-a-bottle'' approach has been used for the generation of numerous clusters and superclusters (cluster arrays) in zeolites [9,10]. The windows of the zeolite pores provide ready access for transporting reagents to the cavities, making a variety of synthesis schemes possible. The additional potential for producing novel clusters of electronic materials and stabilizing unusual geometries and connectivities by using the surface passivating and templating eects of the host lattice is also most attractive [11]. However, to our knowledge, luminescent BN has not yet been successfully achieved. Here, we try to realize the transference from indirect-gap BN to a quasi-direct-gap BN semiconductor material by encapsulation methods. In this paper, we report that light-emitting boron nitride nanoparticles can be successfully prepared by encapsulation methods, and intense blue PL can be obtained from the BN/ZSM-5 sample characterized by powder X-ray diraction (XRD), X-ray photoelectron spectroscopy (XPS), adsorption of N2 , UV±Vis absorption, and PL spectra. 2. Experimental section 2.1. Experimental process To prepare BN in ZSM-5, a superior synthetic approach is akin to that used for CVD or
MOCVD deposition of BN ®lm, for example, absorption of B2 H6 vapor followed by calcination in an atmosphere of NH3 . But B2 H6 is a heavily poisonous gas, and the synthetic process is dangerous. Here, we use an indirect method to prepare BN/ZSM-5. ZSM-5 is a kind of Si or Si±Al zeolite with two-dimensional open frame pores prepared by using work structure of 5±6 A hydrothermal synthesis in our laboratory [12]. We use Si±ZSM-5 as the host material. Firstly, the mechanical mixture of Si±ZSM-5 with a little B2 O3 (sample was over 99% pure with water as impurity and the XRD characteristic peak is around 28° (2h)) was heated at the dispersed temperature of 400°C for 36 h in a vacuum chamber (background pressure < 10ÿ3 Torr) to form B2 O3 /ZSM-5 composite materials, then, B2 O3 /ZSM-5 was treated in a nitriding instrument. The nitridation procedure is as follows: the sample was prepared by a temperature-programmed reaction of B2 O3 /ZSM-5 with ammonia. The temperature was increased from room temperature to 573 K in 0.5 h and from 573 to 973 K in 7 h, and was maintained at 973 K for an additional 2 h. The sample was cooled to room temperature in ¯owing ammonia, then passivated in a stream of 1% O2 =N2 so as to avoid violent oxidation. The resulting solid is stable in air. The bulk BN we used in UV±Vis absorption and XPS characterization was prepared by the reaction of B2 O3 with NH3 using the same procedure. 2.2. Instrumentation The BN nanostructure sample was characterized by powder XRD, XPS, adsorption of N2 , UV±Vis absorption, and PL spectra. The powder XRD data were collected by a Siemens D5005 diractometer using CuKa radiation, 40 kV, 35 mA with scanning rate of 0.2°/min (2h). The adsorption of N2 was measured with a nitrogen adsorption method (BET) by using an ASAP 2010M Micromeritics Instrument at 78 K. UV±Vis absorption spectra of various samples were obtained by a Perkin Elmer UV-Lambda-20 UV±Vis spectrophotometer with BaSO4 as the background, scanning range from 190 to 900 nm. PL
X. Li et al. / Microporous and Mesoporous Materials 40 (2000) 263±269
spectra were collected at room temperature by an MPF-4 ¯uorescence photospectrometer with correction for instrumental wavelength dependencies. The XPS for powder samples ®xed on double-sided tapes was measured on an ESCALAB MKII X-ray photoelectron spectrometer (VG Co.) using AlKa radiation. The base pressure was 10ÿ7 Pa. Corrections of energy shift, due to the steady state charging eect, were accomplished by assuming a 284.6 eV binding energy (BE) for the C1s peak, due to adventitious carbon contamination.
3. Results and discussion 3.1. X-ray diraction Fig. 1 shows the XRD patterns of B2 O3 /ZSM-5 samples with the weight ratio of 0.1 g/g before and after the heat treatment. The XRD peaks show the characteristics of B2 O3 and ZSM-5 structure. As observed in Fig. 1b, the XRD peaks at 28.5° (2h)
Fig. 1. XRD patterns of (a) B2 O3 /ZSM-5 samples with weight ratio of 0.1 g/g after the heat treatment, and (b) mechanical mixture of 0.1 g/g of B2 O3 /ZSM-5 before the heat treatment.
265
represent the characteristics of B2 O3 , because the characteristic peak of bulk B2 O3 we used is around 28° (2h). It is of interest to note that the characteristic peaks assigned to B2 O3 disappear completely when the sample is heated at 400°C for 36 h, as given in Fig. 1a. The phenomenon might be explained by high dispersion of B2 O3 into the channel of the ZSM-5 zeolite, where B2 O3 no longer exists on the surface of ZSM-5 in crystalline state [13,14]. On increasing the B2 O3 loading in ZSM-5 zeolite to 0.2 and/or 0.3 g/g, the characteristic peaks assigned to crystalline B2 O3 around 28.5° (2h) appear again after the heat treatment. The phenomenon indicates that B2 O3 exists on the surface of ZSM-5 in crystalline state again. Comparison of B2 O3 with other metal oxides and salts, shows that the threshold (the critical dispersion capacity) is signi®cantly reduced [13,14]. Thus, the proper loading of B2 O3 is no more than 0.1 g/g (weight ratio). This value corresponds to ca. eight B2 O3 molecules per unit cell of ZSM-5. According to the spontaneous monolayer dispersion model [13], the dispersed B2 O3 molecules will form a monolayer or submonolayer. Because B2 O3 molecules are more dicult to be dispersed than other metal oxides or salts, it is impossible to disperse B2 O3 molecules into the channels of ZSM-5 on average. These channels, which are close to the surface of ZSM-5, are preferable. Thus, the BN nanoparticles, which were prepared by the reaction of B2 O3 with NH3 using the nitridation procedure, should exist at the same places. In order to increase the loading of B2 O3 , we should use tinysized ZSM-5 as host materials. Fig. 2 shows the XRD patterns of BN/ZSM-5 and ZSM-5 samples. Analyses of the BN/ZSM-5 material by powder XRD reveal that the crystallinity and integrity of the zeolite host ZSM-5 are maintained and there is no diraction peaks characteristic of BN whose characteristic peak is at 26.8° (2h). This indicates that BN, which is formed by a nitridation process, has been encapsulated in zeolite ZSM-5. The change of the intensities around 8° and 24° (2h) may be dealt with the encapsulation of BN into the channels of ZSM-5 at high temperature. The dispersion of active components onto the surface of supports, such as SiO2 , Al2 O3 , has
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did not change, which has been demonstrated from the analyses of XRD for the resultant nitridation samples. In other words, BN formed by nitridation reaction will be encapsulated in ZSM-5 to form nanoparticles. 3.3. Adsorption isotherms The BN nanoparticles encapsulated in ZSM-5 lead to the change in channel size of ZSM-5 signi®cantly. Fig. 3 shows the N2 isotherms of the ZSM-5 and BN/ZSM-5 samples. We observe that both the isotherms exhibit typical Langmuir adsorption on the samples with the adsorption amount of 75 and 18 cm3 /g. The dierent adsorption amounts may be related to the change in channel shape in ZSM-5, which strongly in¯uences the adsorption of N2 on the sample. Fig. 2. XRD patterns of BN/ZSM-5 and ZSM-5 samples.
been widely investigated by Xie and Tang [13]. On comparing these supports with zeolite ZSM5, it is found that the speci®c surface area of ZSM-5 is higher. In some respects, the channel structure of ZSM-5 is expected to be more favorable to stabilize B2 O3 nanoparticles. We have successfully dispersed B2 O3 into the channels of ZSM-5 with the loading of 0.1 g/g (weight ratio). But we never succeeded in dispersing B2 O3 onto the surface of SiO2 or Al2 O3 with such loading. Thus, ZSM-5 has some superiority to be used as host materials.
3.4. X-ray photoelectron spectroscopy Because XPS is a surface-sensitive technique, BN, which is formed on the inner surface of the channels of ZSM-5 as nanoparticles, will give an XPS signal much stronger than that given by bulk BN. This prediction has been borne out well by XPS studies on our samples.
3.2. Nitridation procedure During nitridation procedure, NH3 was adsorbed into the pores of ZSM-5 where B2 O3 had been dispersed already. At the reaction temperature, NH3 reacted with B2 O3 to form BN. Because of the high reaction temperature and the ammonia erosion, the host material must have reliable thermal stability. ZSM-5, a kind of stable zeolite below 1000°C, is more suitable to be used as host materials than other zeolites, such as NaY. During nitridation process, the structure of ZSM-5 almost
Fig. 3. N2 isotherms of the ZSM-5 and BN/ZSM-5 samples.
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Fig. 4. XPS spectra for B 1s in (a) bulk BN and (b) BN/ZSM-5 sample.
Fig. 5. XPS spectra for N 1s in (a) bulk BN and (b) BN/ZSM-5 sample.
Figs. 4 and 5 show XPS spectra of BN/ZSM-5 and the bulk BN prepared by the reaction of B2 O3 with NH3 in the B 1s and N 1s energy regions. It can be seen that the main peak of B 1s in BN/ ZSM-5 is at 191.5±193.0 eV, and the main peak of N 1s in BN/ZSM-5 is at 398.7±399.1 eV, which suggests the existence of B and N atoms. The binding energy of B 1s in BN/ZSM-5 has been found to be about 1.2±2.7 eV higher than that for the bulk BN (Eb 190:3 eV). As to the binding energy of N 1s, a shift of 0.4 eV can be observed. It is reasonable to consider that the increase of the binding energy of B 1s and N 1s is due to the variation of BN forms. When the BN was prepared from bulk form to nanoparticles encapsulated in the channels of ZSM-5, the XPS signals became stronger. In other words, the increase of the binding energy is an evidence of the presence of low-dimensional BN. As shown in Figs. 4 and 5, the XPS spectra of B 1s and N 1s in BN/ZSM-5 are broader than that in bulk BN prepared by the reaction of B2 O3 with NH3 using the same nitridation procedure. The results may indicate that some B2 O3 cannot react
with NH3 completely to form BN, and some intermediate states may be formed during the complicated nitridation process. Thus, defects or impurities, which may act as the origin of luminescence, are irremovable in the BN particles. 3.5. UV±Vis spectra Fig. 6 shows the UV±Vis absorption spectra of ZSM-5, BN, and BN/ZSM-5. The spectrum of ZSM-5 has no obvious peaks. The spectrum of BN has one pronounced peak at 214 nm. As to the spectrum of BN/ZSM-5 material, it gives one intense peak at 206 nm, which may be assigned to the characteristic peak of BN. The blueshift from 214 to 206 nm can be considered as the contribution by quantum size eect. The phenomenon indicates that BN are present in the pores of ZSM-5. 3.6. Visible photoluminescence PL normalized spectra from the resultant samples are given in Fig. 7. When the excited wavelength is 450 nm, two broad PL peaks around 520
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Fig. 6. UV±Vis absorption spectra of (a) BN, (b) ZSM-5, and (c) BN/ZSM-5.
Fig. 7. PL spectra of BN/ZSM-5 under exciting wavelength of (a) 260 nm, (b) 365 nm, and (c) 450 nm.
and 590 nm can be obtained. As the excited wavelength changes to 365 nm, the spectrum exhibits a large blueshift from 520 to 434 nm. Under an excitation at 260 nm, we can observe an intense PL peak at 336 nm. The quantum eciency is considered to be high, because the observed emission intensity is intense compared with its excitation intensity. To evaluate the possible origins of the strong PL, we consider the optical properties of ZSM-5, B2 O3 /ZSM-5, and BN nanoparticles encapsulated in ZSM-5. Prior to measuring the PL of BN nanoparticles encapsulated in ZSM-5, the PL behavior of ZSM-5 and B2 O3 /ZSM-5 was evaluated, and no obvious emission was observed at room temperature, which indicates that the contribution from ZSM-5 and B2 O3 /ZSM-5 to PL can be neglected. The only possible origin of the strong PL is BN nanoparticles encapsulated in ZSM-5. In bulk crystal of indirect-gap BN material, electron±hole recombination is possible only through phonon emission or absorption because the wave-vector dierence between the conduction-band bottom and the valence-band top must be compensated [15]. According to the quantum con®nement theory on solid state physics, the lowest exciton energy in quantum dots increases with the decrease of their size. The energy of luminescence from indirect-type exciton also increases with the decrease of size in indirect-gap semiconductor particles [16,17]. Since the indirect-gap energy of bulk c-BN is ca. 6 eV (200 nm) [6], the energy peak of the luminescence from BN nanoparticles cannot be explained by the quantum con®nement theory simply. When indirect-gap BN is prepared into low-dimensional structures such as nanoclusters, superlattice, or quantum dots, the excitons will be bound by the quantum con®nement eect. We used NH3 gas to prepare BN nanoparticles. As bonds between B and N have strong covalency, dangling bonds are not preferred to being formed. The surface of the particles may be terminated by H atoms, which may play a role in an intense luminescence at visible region. Bulk h-BN with impurities of C or H atoms shows luminescence near UV region [18]. During nitridation procedure, the NH3 gas must be transferred through the channel of ZSM-5 to react with B2 O3 . The complicated
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process makes it possible that some B2 O3 cannot react with NH3 completely, thus defects are irremovable in the BN particles. This prediction can be borne out by XPS studies on BN/ZSM-5 sample (see Figs. 4 and 5). The luminescence may originate from the bound excitons at the defects or impurities. Compared with free excitons, luminescence from bound excitons is usually intense. On exciting under dierent wavelength, excitons will be excited to dierent excited state, resulting in dierent visible PL from 590 to 336 nm. The size of BN nanoparticles in ZSM-5, the quantum yield of the luminescence, and the theoretical calculations of the excited states that corresponded to dierent PL will be reported later. 4. Concluding remarks In summary, we have successfully prepared nanometer-sized BN by an encapsulation method and observed strong PL. The investigation of powder XRD, XPS, adsorption of nitrogen, UV±Vis absorption, and PL spectra shows that BN nanoparticles can be encapsulated in the pores of ZSM5. The luminescence may originate from the bound excitons at the defects or impurities, which are irremovable in the BN nanoparticles during nitridation process. We can conclude that the encapsulation method which are used to prepare quasi-direct-gap semiconductor materials from indirect-gap materials, such as Si, BN, and SiC,
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etc., will have spectacular applications in optoelectronics. This work was supported by the National Natural Science Foundation of China. We are also grateful to the reviewer of this paper for the contribution to luminescence mechanism.
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