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Author's personal copy Journal of Non-Crystalline Solids 357 (2011) 105–109
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Journal of Non-Crystalline Solids j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j n o n c r y s o l
Rod-like nano-composite of inorganic nanowires encapsulated by mesoporous silica solid Wusheng Guo ⁎, Kun Wei ⁎, Cai Lin School of Materials Science and Engineering, South China University of Technology, Guangzhou, 510641, China
a r t i c l e
i n f o
Article history: Received 9 April 2010 Received in revised form 26 July 2010 Available online 4 November 2010 Keywords: HMS; Nanowire; Nano-composite; Praseodymium
a b s t r a c t A novel rod-like praseodymium-containing inorganic nanowire encapsulated by mesoporous silica, was synthesized via a self-assembly method. The specific surface area of the composite was 1198m2g− 1. The nanowires with 4–5 nm in diameter and 100 nm in length were encapsulated at the center of mesoporous solid showed by TEM images. The quantum size effects of the sample can be noted. The controversial absorption band at 960 cm− 1 in Fourier-transform infrared (FT-IR) spectra was proved to be not the evidence of the introduction of heteroatom. On the basis of the absorption spectra and select area electron diffraction (SEAD) pattern, the nano-composite was known as an amorphous phase containing praseodymium element, in which the praseodymium was trivalent. The absorption spectra indicated the 4f2 electronic configuration of Pr3+ ions. Four energy bands were attributed to the transitions from the ground state 3H4 to the excited levels 3P2, 3P1, 3P0 and 1D2, respectively. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved.
1. Introduction Since the pioneer work of Kresge et al in 1992 [1], mesoporous materials with highly ordered porous structure have attracted considerable attention all over the world, and there is an increasing interest in the discovery of new techniques to prepare mesoporous materials as well as the new technologies to process them into useful forms. Among all the mesoporous materials so far studied, mesoporous silica has been well recognized as an ideal substrate for adsorption, catalysis, chemical separations, drug delivery, and biomedical devices [2–4], due to its large surface area (700–1500 m2g− 1), uniform pore size (2–30 nm), high chemical and thermal stability. In recent years, various mesoporous silica materials with different morphology have been synthesized, such as sphere-like and fiber-like particles [5,6], films [7], monoliths [8], nanohelices [9] and nanotubes [10] etc. Praseodymium oxide, an important rare earth functional material, has been extensively investigated because of its promising applications for ceramic pigments, oxygen-storage components, high electrical conductivity materials, catalysts, promoters and stabilizers in combustion reactions [11]. Current research work on praseodymium oxide is mainly focused on the synthesis and characterization of onedimensional (1-D) nanostructured materials, including nanotubes, nanorods and nanowires [11–13], which exhibit unique physical and chemical properties resulted from their low dimensionality and the quantum confinement effects [14–16].
In this paper, we report a new core-shell nano-composite material with rod-like praseodymium oxide nanowire as core component and mesoporous silica as shell component. It is conceivable that the performance of the as-prepared 1-D nano-composite will be improved as a result of both shape-specific and quantum size effects, thus has great potential applications in the areas of fiber-reinforced high-performance, catalysts, oxygen-storage materials, sensors and lab-on-chips. 2. Experimental 2.1. Synthesis Pure conventional mesoporous silica solid (P-HMS) synthesized via the traditional process, was used as reference material [17]. Pr-HMS was prepared according to the following procedure: 1.464 g of Pr6O11 was dissolved in 5 g of nitric acid solution (32 wt%) at 80 °C, and then mixed with 44.6 ml tetraethyl orthosilicate (TEOS) and 40 ml distilled water at 70 °C. Subsequently, the mixture was dropped into a solution of pruryl amine (DDA), EtOH, cyclohexane and distilled water according to the mole ratio of DDA:TEOS:Pr6 O 11 :NO − 3 :EtOH:H 2 O:cyclohexane = 0.27:1:0.005:0.125:7.72:25:0.46 under constant stirring to form white precipitate. After further stirring for another 36 h, the precipitate was aged for 30 min, washed with distilled water, extracted with EtOH, dried at 80 °C for 3 h and finally calcinated in air at 640 °C for 4 h. 2.2. Characterization methods
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[email protected] (W. Guo),
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X-ray diffraction (XRD) was carried out using a Rigaku D/max-2200 (Japan, Cu Kα, λ = 1.54056 nm) with a voltage of 40 kV and a current of
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30 mA, respectively. The diffraction pattern was collected over the 2θ range 1–10° with a scanning speed of 1°/min and a step size of 0.02°. Thermogravimetry and differential scanning calorimetry (TG–DSC) were conducted on a NETZXSCH STA 449C thermal analyzer in the temperature range of 30–1000 °C with the heating speed of 10 °C min− 1 under nitrogen atmosphere. TEM and energy dipersion spectroscopy (EDS) were performed on FEI-Techai 1200 instrument with the accelerating voltage of 200 kV. Infrared spectra were obtained using a Fourier transform infrared spectroscopy (Nicolet 360). The textural porosity was measured with a Tristar Micromeritic Vacpre 061 in the liquid nitrogen atmosphere. Surface area was calculated by the Brunauer–Emmett–Teller (BET) method and the pore size distribution (PSD) was determined via non-local functional theory (NLFDT). 2.3. Uncertainty estimates The uncertainty estimate for each variable was determined from the instruments specifications or by repeatable tests. The maximum deviation of repeatable solid weight was ±0.002 g. Uncertainties of physicochemical properties of the samples were determined by calculating the deviation associated with the BET fit and repeatable tests for three times. 3. Results The TG and DSC curves of Pr-HMS are showed in Fig. 1a with a sharp endothermic peak at 70–100 °C corresponding to about 6% mass loss, which is assigned to the evaporation of the ethanol and deionized water remaining in the mesoporous structure. With the increase of temperature, another broad endothermic peak was observed at 150– 350 °C, indicative of the decomposition of residual DDA and also the formation of mesopores. The weak peak at 400–500 °C represents the release of CO2 formed during the heating process. In addition, the broad but weak endothermic peak at 500–800 °C showed little mass change, which can be explained as the evaporation of water produced in the polycondensation process of ≡Si–OH groups at high temperature. This point is further proven by FT-IR and N2 ad–desorption measurements shown below. In order to investigate the structure of the nano-composite materials calcinated under different temperatures, small-angle powder X-ray diffraction was carried out with the results shown in Fig. 1b. It is noted that all specimens displayed a single basal d100 reflection peak, indicating their typical HMS system mesoporous structure. Comparing with the Pr-HMS nano-composities, P-HMS showed the
strongest intensity of d100 diffraction peak, which demonstrated that the P-HMS has the highest degree of mesoporous orderness. In other words, the introduction of inorganic nanowires breaks the balance of Si–O–Si bonds and then changes the T–O–T bond angles, and thus reduces the orderly mesoporous structure [18]. Moreover, it can be observed that the intensity of d100 diffraction peak in Pr-HMS decreases with the increase of the sintering temperature. This behavior is mainly induced by the collapse of some mesopore structures at higher dealing temperature. The d100 reflect peak shifts to lower 2θ value when the calcination temperature increases from 640 to 800 °C, suggested a increase of basal spacing. Specifically, the basal spacing derived from the XRD patterns are 3.6, 3.9, and 4.5 nm for Pr-HMS calcination at 640, 730, and 800 °C, respectively, whilst the basal spacing of P-HMS is 3.4 nm. TEM images (Fig. 2a–c) show that Pr-HMS is a 1-D core-shell nanocomposite with rod-like praseodymium oxide nanowire as core component and mesoporous silica as shell component. The SAED pattern (Fig. 2d) indicates that the nano-composite is an amorphous phase. The nanowire with length of ~ 100 nm and diameter of 4–5 nm, are located at the center of rod-like nanosized mesoporous solid as shown in Fig. 2e. EDS (Fig. 2f) confirms the specimen is composed of Pr, Si and O elements and also implies the existence of carbon and Cu contaminations. The N2 ad–desorption isotherms (a) and PSD curves (b) (calculated from the adsorption isotherms) for all specimens are shown in Fig. 3. Both of them exhibit a typical IV isotherm curve with three distinct adsorption regions: a) a dramatic increase in the range of 0.2– 0.4 (relative pressure), reflecting the deposition of adsorbed layers on pore walls; b) a gentle increase in the range of 0.4–0.85, indicating the capillary condensation of nitrogen in pores; and c) a small hysteresis can be seen when relative pressure exceeds 0.85, representing multilayer adsorption on the external surface of the samples. Fig. 4a gives the FT-IR spectra of P-HMS and Pr-HMS calcinated at different temperatures. It's well known that the absorbance bands between 400–1500 cm− 1 can be attributed to the skeleton vibration of the nano-composite. For example, the band at 474 cm− 1 and 805 cm− 1 are assigned to the bending vibration and the symmetric stretching vibration of Si–O–Si, and the band at 1074 cm− 1 and a large shoulder at around 1224 cm− 1 are due to the antisymmetric stretching vibration of Si–O–Si in the tetrahedron skeleton [19,20]. Fig. 4b compares the diffuse reflectance UV–vis spectra of Pr-HMS640 °C and P-HMS. The weak peak of Pr-HMS-640 °C at 208 nm can be assigned to the ligand-to-metal charge transfer transition of O2− → Pr3+. Comparing with the peak of P-HMS-640 °C at 224 nm, the sharp peak
Fig. 1. (a) TG and DSC curves of the dried product which has not been calcinated; (b) Small angle XRD patterns of the specimens calcinated at different temperatures for 4 h in air.
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Fig. 2. (a–c) TEM images of Pr-HMS-640 °C under different magnifications; (d) SAED pattern of a single particle of the rod-like nano-composite; (e) a single nanowire encapsulated by mesoporous solid; (f) EDS spectra of Pr-HMS-640 °C.
of Pr-HMS-640 °C at 226 nm induced by Si–O–Si structure shows a little blue shift to higher energy, proving the quantum size effect. 4. Discussion As the calcination temperature increased, the PSD curves shifted toward lower value and their symmetry decreased accordingly. The
possible reasons for this phenomenon could be explained in the following three aspects: Firstly, additional mesopores can be formed in the interface of nanowires and mesopores at higher temperature. Similar results were also observed by Vasiliev's study [8]. And the gap between different nanosized rod-like particles also have great contributions to the PSD; Secondly, with the growth of temperature, condensation reaction could occur between the residual hydroxyl
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Fig. 3. (a) N2 ad–desorption isotherms and (b) PSD curves for all the samples.
groups which contained in mesoporous structure to form additional Si–O–Si bonds, thickening the pore wall and hence narrowing the pore size and reducing the total pore volume; Thirdly, some of the mesopores may likely collapse at high treatment temperature. However, it should be pointed out that the specific surface area is still greater than 980 m2 g− 1 even if some mesopores are destroyed at high temperature, which is mainly due to the contribution of the nanosize particles. Table 1 summarized the main textural and physiochemical parameters of the specimens. As can be found, the introduction of Pr species results in a decrease of pore volume, but not always in pore size and specific surface area. Generally speaking, the total pore volume of mesoporous materials is mainly determined by the mesoporous orderliness, while the specific surface area is closely related to the mesoporous orderliness as well as the particle size. Therefore, although the total pore volume is reduced owing to the decrease in mesoporous orderliness, the specific surface area of PrHMS calcinated at 640 and 730 °C is still higher in comparison with the P-HMS, which is mostly aroused by the smaller particle size. An intensive band at around 960 cm− 1 is detected in the spectra of P-HMS and Pr-HMS, but the relevant explanation for it is not consistent in the literature. Chaudhari et al. [21] assigned it to the
polarized Si–Oδ−···Zrδ+ bonds in Zr-MCM-41, and the defect sites of silica hydroxyl in pure silicon MCM-41. Araújo et al. [22] thought it is an evidence of the incorporation of metallic heteroatoms to the purely siliceous MCM-41. In our case, the peak at 960 cm− 1 should be assigned to in-plane deformation bending vibration of O–H bonds in water produced in the reaction process. When the sintering temperature increased, this peak changed from a wide peak to a broad shoulder, and even disappeared after the temperature reached 800 °C, indicative of the complete evaporation of the as-produced water. Meanwhile, with the increase in calcination temperature, the shoulder at about 1224 cm− 1 is broadened, and the intensity of peaks at about 474, 805 and 1074 cm− 1 increased to a certain extent. Except for those peaks mentioned above, the peak at 1638 cm− 1 can be ascribed to the O–H deformation vibration , verifying the existence of trace water adsorbed in the nano-composite. Whilst the broad peak at 3447 cm− 1 resulted from the stretching vibration of hydroxyl groups. In contrast to P-HMS, the absorbance peak of Pr-HMS at 1638 cm− 1 became very weak, which can be attributed to the decreasing adsorption capacity for water induced by the presence of nanowires. Similarly, the peak of Pr-HMS at 3447 cm− 1 declined remarkably compared with P-HMS, indicating the fewer free hydroxyl groups in
Fig. 4. (a) FT-IR spectra of all the samples; (b) the absorption spectra of Pr-HMS-640 °C and P-HMS-640 °C; (c) determination of UV edge energy from UV–visible spectra of Pr-HMS640 °C and P-HMS-640 °C.
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Table 1 Main textural and physiochemical parameters of the specimens. Specimens
d [Å]a
SBET [m2g− 1]b
Total pore volume [cm3g− 1]
a0 [Å]c
Pore size [Å]d
Wall thickness [Å]e
HMS WPr-HMS-640 °C WPr-HMS-730 °C WPr-HMS-800 °C
34.442 ± 0.002 36.232 ± 0.002 39.453 ± 0.001 45.119 ± 0.001
1161.0 ± 1.2 1197.5 ± 1.4 1193.7 ± 0.5 980.4 ± 0.6
0.93 ± 0.01 0.73 ± 0.03 0.69 ± 0.01 0.39 ± 0.01
39.771 ± 0.001 41.837 ± 0.002 45.556 ± 0.001 52.098 ± 0.001
25.85 ± 0.05 30.12 ± 0.11 27.03 ± 0.07 23.12 ± 0.10
13.92 ± 0.10 11.72 ± 0.05 18.52 ± 0.07 29.97 ± 0.10
a b c d e
d(100) spacing of the extracted materials from XRD. SBET calculated over the relative pressure range P/P0 = 0.05–0.25. 2dð100Þ a0= pffiffiffi . 3 Pore size was determined by the NLDFT. Estimated from a0 — pore size.
the Pr-HMS nano-composite materials. This is mainly caused by the consumption of hydroxyl groups to bond the mesoporous silica and praseodymium oxide nanowire. More importantly, the intensity of this peak decreased as the sintering temperature increased from 640 to 800 °C. Therefore, a conclusion can be drawn that the condensation reaction happened among the residual hydroxyl groups to form additional Si–O–Si bonds in the mesoporous structure is much easier to proceed at higher sintering temperature. These results of FT-IR characterization are in good agreement with the DSC and N2 ad– desorption analysis. The broad absorption shoulder located at 245–318 nm in UV– absorption spectra of P-HMS may be arose from the coordination of water with ≡ Si–OH groups. The disappearance of this peak in PrHMS-640 °C clearly proves the loss of ≡ Si–OH groups, which is in good accordance with the analysis of FT-IR spectra. The 4f2 electronic configuration of Pr3+ is also noted from the absorption spectra, and four energy bands located at about 444, 472, 484, and 590 nm are corresponding to the transitions from the ground state 3H4 to the excited levels 3P2, 3P1, 3P0 and 1D2, respectively. Furthermore, the edge energy (Eg) of transitions are determined by finding the intercept of the slope in the low energy rise of a plot of [F(R∞) × hυ]2 against hυ (Fig. 4c), where F(R∞) is the Kubelka–Munk function for infinitely thick sample and hυ is the incident photon energy [23,24]. The correlation coefficient of the fit are as follows: 2
FðR∞ Þ = ð1−R∞ Þ = 2R∞ ; h pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi R∞ = K + S− ðK + 2SÞ; hυ = hC = λ: Among them, K and S represent the absorption and scattering coefficient whilst h and λ are plank index and wavelength, respectively. And C stands for the speed of light. The calculated Eg of Pr-HMS-640 °C and P-HMS-640 °C are 4.7 eV and 4.5 eV, respectively, which manifests as an apparent quantum size effect. The level 3P2 is the most intense and the sharpest band, and the 1D2 band is broadband type with the largest bandwidth. These observations are similar to Jana's research [25]. 5. Conclusions A novel core-shell nano-composite material with rod-like praseodymium oxide nanowire as core component and mesoporous silica as shell component were synthesized with a self-assembling technology. The XRD patterns of all samples exhibited a single basal d100 reflection of the typical HMS system mesoporous materials. The specific surface area of the composite was 1198 m2 g− 1. The diameter and length of nanowires encapsulated at the center of the mesoporous solid are about 4–5 nm and 100 nm, respectively. The 4f2 electronic configu-
ration of Pr3+ was noted from the absorption spectra, and four energy bands were attributed to the transitions from the ground state 3H4 to the excited levels 3P2, 3P1, 3P0 and 1D2, respectively. The calculated edge energy of the Pr-HMS sample is 4.7 eV, corresponding to 4.5 eV of P-HMS, which manifested as an apparent quantum size effect. The FT-IR absorption band at 960 cm− 1 of all the HMS materials may be ascribed to in-plane deformation bending vibration of O–H bonds in water, which was produced in the process of condensation reaction to form additional Si–O–Si bonds among the residual ≡ Si–OH groups contained in the mesoporous structure at high sintering temperature. This approach may be generally applicable to the synthesis of many other rare earth silica composites. Acknowledgments This work was financially supported by the National Nature Science Foundation of China (Grant no. 50572029) and high technology study plan of China (Grant no. 2007AA021908). References [1] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 359 (1992) 710–712. [2] B.G. Trewyn, I.I. Slowing, S. Giri, H.T. Chen, V.S.-Y. Lin, Acc. Chem. Res. 40 (2007) 846–853. [3] F. Torney, B.G. Trewyn, V.S.-Y. Lin, K. Wang, Nat. Nanotechnol. 2 (2007) 295–300. [4] J.M. Rosenholm, A. Duchanoy, M. Lindén, Chem. Mater. 20 (2008) 1126–1133. [5] M. Chen, L.M. Wu, S.X. Zhou, B. You, Adv. Mater. 18 (2006) 801–806. [6] S. Kubo, K. Kosuge, Langmuir 23 (2007) 11761–11768. [7] V.R. Tirumala, R.A. Pai, S. Agarwal, J.J. Testa, G. Bhatnagar, A.H. Romang, C. Chandler, B.P. Gorman, R.L. Jones, E.K. Lin, J.J. Watkins, Chem. Mater. 19 (2007) 5868–5874. [8] P.O. Vasiliev, Z. Shen, R.P. Hodgkins, L. Bergström, Chem. Mater. 18 (2006) 4933–4938. [9] T. Delclos, C. Aimé, E. Pouget, A. Brizard, I. Huc, M.-H. Delville, R. Oda, Nano Lett. 8 (2008) 1929–1935. [10] X. Wu, J. Ruan, T. Ohsuna, O. Terasaki, S. Che, Chem. Mater. 19 (2007) 1577–1583. [11] L. Yan, R. Yu, G. Liu, X. Xing, Scr. Mater. 58 (2008) 707–710. [12] P.X. Huang, F. Wu, B.L. Zhu, G.R. Li, Y.L. Wang, X.P. Gao, H.Y. Zhu, T.Y. Yan, W.P. Huang, M. Zhang, D.Y. Song, J. Phys. Chem. B 110 (2006) 1614–1620. [13] L. Ma, W. Chen, J. Zhao, Y. Zheng, X. Li, Z. Xu, Mater. Lett. 61 (2007) 1711–1714. [14] K. Hiruma, M. Yazawa, T. Katsuyama, K. Haraguchi, K. Ogawa, M. Kouguchi, J. Appl. Phys. 77 (1995) 447–452. [15] J. Hu, T.W. Odom, C.M. Lieber, Acc. Chem. Res. 32 (1999) 435–445. [16] J.D. Justin, K.P. Johnston, R.C. Doty, B.A. Korgel, Science 287 (2000) 1471–1473. [17] P.T. Tanev, M. Chibwe, T.J. Pinnavaia, Nature 368 (1994) 321–323. [18] K. Wei, W. Guo, C. Lai, N. Zhao, X. Li, J. Alloy. Comp. 484 (2009) 841–845. [19] H. Rahiala, I. Beurroies, T. Eklund, K. Hakala, R. Gougem, P. Trens, T.B. Rosenholm, J. Catal. 188 (1999) 14–19. [20] B.H. Lee, Y.H. Kim, H.J. Lee, J.H.Y., Microporous Mesoporous Mater. 50 (2001) 77–90. [21] D. Chaudhari, R. Ball, T.K. Das, A. Chandwadkar, D. Srinivas, S. Sivasanker, J. Phys. Chem. B 104 (2000) 11066–11074. [22] R.S. Araújo, D.C.S. Azevedo, E. Rodríguez-Castellón, A. Jiménez-López, C.L. Cavalcante Jr, J. Mol. Catal. A Chem. 281 (2008) 154–163. [23] D. Wei, H. Wang, X. Feng, W.–.T. Chueh, P. Ravikovitch, M. Lyubovsky, C. Li, T. Takeguchi, G.L. Haller, J. Phys. Chem. B 103 (1999) 2113–2121. [24] A.A. Christy, O.M. Kvalheim, R.A. Velapoldi, Quantitative analysis in diffuse reflectance spectrometry: a modified Kubelka–Munk equation, Vibrat. Spect. 9 (1995) 19–27. [25] S. Jana, S. Mitra, J. Alloy. Comp. 457 (2008) 477–479.
