TiO2 composite hollow fibers by electroless plating

TiO2 composite hollow fibers by electroless plating

Materials Science and Engineering A 466 (2007) 218–222 Preparation of Ni/TiO2 composite hollow fibers by electroless plating Haiqiang Lu a,b , Jianfe...

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Materials Science and Engineering A 466 (2007) 218–222

Preparation of Ni/TiO2 composite hollow fibers by electroless plating Haiqiang Lu a,b , Jianfeng Yao a , Lixiong Zhang a,∗ , Yuanyang Wang b , Jinping Tian b , Nanping Xu a a

College of Chemistry and Chemical Engineering, Key Laboratory of Materials-Oriented Chemical Engineering of Jiangsu Province, Nanjing University of Technology, No. 5 Xin Mofan Rd., Nanjing 210009, PR China b College of Materials Science and Engineering, Taiyuan University of Science & Technology, No. 66 Wa Liu Rd., Taiyuan 030024, PR China Received 16 September 2006; received in revised form 13 February 2007; accepted 27 February 2007

Abstract Ni/TiO2 composite hollow fibers with an outer diameter of ca. 560 ␮m and inner diameter of ca. 500 ␮m were successfully prepared by electroless plating using TiO2 hollow fibers as supports. The products were characterized by X-ray diffraction (XRD), scanning electron microscopy, energy dispersive X-ray spectroscopy (EDX), and tensile strength test. The results revealed that the Ni/TiO2 composite hollow fibers were made up of nickel outer layer and TiO2 inner layer and exhibited excellent tensile strength ascribed from the nickel layer. The thickness of the nickel layer could be readily controlled by varying the electroless time and the tensile strength of the composite hollow fibers increased with prolonging the electroless time. The tensile strength was up to 52.1 MPa when the nickel layer was ca. 22 ␮m. The composite hollow fibers showed stable selectivities of H2 over N2 from 6 to 10 and H2 permeation in the order of 10−9 mol m−2 s−1 Pa−1 at 453 K. © 2007 Elsevier B.V. All rights reserved. Keywords: Ni/TiO2 ; TiO2 hollow fiber; Electroless plating; Composites

1. Introduction Recent years have witnessed increasing interests in the preparation of inorganic hollow fibers because of their potential applications in inorganic membranes, microchannel reactors, catalysis, supports, etc. [1–3]. In the inorganic membrane area, inorganic membranes with hollow fiber geometry have drawn much attention because they provide a larger membrane area per unit volume. A series of inorganic hollow fiber membranes based on various inorganic materials were prepared [4–9]. On the other hand, composite hollow fiber membranes, such as TiO2 /Al2 O3 [10], polydimethylsiloxane (PDMS)/Al2 O3 [11], etc. were also fabricated. In the catalysis area, catalysts with a hollow fiber geometry could provide fast mass transfer and low pressure drop. For example, Pt-doped TiO2 fiber catalyst enhanced the photocatalytic activity for degradation of CHCl3 under UV irradiation [12]. On the other hand, hollow fibers can be used as a form of structured packing useful for distillation [13]. Thereby, they can be catalytically functional-



Corresponding author. Tel.: +86 25 83587186; fax: +86 25 83365813. E-mail address: [email protected] (L. Zhang).

0921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2007.02.114

ized as catalytic distillation packings. Recent advance in the microfluidic reactor also spurs preparation of various inorganic hollow fibers. Consequently, various methods have been developed to prepare inorganic hollow fibers, such as electronspun [14], template approach [15,16] and supermolecular assembly [17]. We also developed a versatile method to prepare TiO2 hollow fibers with an outer diameter of ca. 500 ␮m using poly(vinylidene chloride) hollow fiber microfiltration membranes as templates [18]. By preshaping the polymeric membranes into a spring structure, TiO2 hollow fiber spring could be prepared. Although the strength of the fibers was strong enough in normal handling, it could not meet the requirement for the usage as inorganic membranes, microreactors, catalysts, etc. In this paper, we introduced a simple method to improve the tensile strength of the TiO2 hollow fibers by depositing nickel layer on the surface of the TiO2 hollow fibers by electroless plating. The thickness of the nickel layer on the TiO2 hollow fibers was controllable by varying the electroless time. The permeation properties of the composite hollow fibers were examined with H2 and N2 . The resulting Ni/TiO2 composite hollow fibers have potential applications in membrane separation, microchannel reactor and catalysis.

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Fig. 1. SEM photos of a straight TiO2 hollow fiber (a) and a TiO2 hollow fiber spring (b). The scale bar is 500 ␮m for (a) and 1.0 mm for (b).

2. Experimental TiO2 hollow fibers were prepared according to the impregnation method we developed [18], using poly(vinylidene chloride) hollow fiber microfiltration membranes as templates. The magnetic Ni/TiO2 composite hollow fibers were prepared by a three-step procedure [19]. First, the TiO2 hollow fibers were sensitized in a solution prepared by mixing 1 g of SnCl2 , 2 g of HCl (35 wt.%) and 97 g of deionized H2 O for 1 min, followed by washing with sufficient deionized water. Both ends of the TiO2 hollow fibers were sealed with glue before they were dipped in the solution. Second, the sensitized TiO2 hollow fibers were activated in a solution prepared by mixing 0.025 g of PdCl2 , 0.25 g of HCl (35 wt.%) and 97.5 g of deionized water for 1 min and washed with sufficient deionized water afterwards. The process of sensitization and activation was repeated for 4–6 cycles until the TiO2 hollow fibers became brown in color. Finally, the brown TiO2 hollow fibers were submerged into the electroless plating solution [20] prepared by mixing 4 g of sulfate nickel, 5.9 g of sodium citrate, 3.2 g of ammonium chloride and 2.7 g of sodium hypophosphite with 84 g of deionized water at 80 ◦ C for 30, 60 and 90 min, respectively. The pH value of the solution was 8–9. Ammonia solution was added in the electroless plating solution every 30 min to control the pH. After electroless plating reaction, the fibers were washed with deionized water and dried at 80 ◦ C in vacuum oven overnight. The phase structure of the Ni/TiO2 composite hollow fibers was examined by X-ray diffraction (XRD, Bruker D8 Advance) with Cu K␣ radiation. Scanning electron microscope (SEM) images were taken with a Philips Quanta-200 microscope, and the elemental analyses of the samples were conducted by an energy dispersive X-ray spectroscopy (EDX, Oxford). The tensile strength of the fiber was tested with Almighty Test Machine (Shenzhen Sansi CMT4024) and the tensile rate was 10 mm/min. Single gas permeation measurements were carried out using H2 and N2 by a dead-end method. Three U-shaped Ni/TiO2 composite hollow fibers were sealed in one end of a stainless-steel tube by epoxy resin with the other end connected to a gas line. The sealed end of the stainless-steel tube was connected to a bubble flow meter to measure the flux of H2 and N2 through

the fibers. The gas permeance was calculated when the flow rate was almost constant for three measurements within 1 h. The ideal selectivity of H2 over N2 was expressed as the ratio of H2 permeance to N2 permeance. All the permeation measurements were conducted at 453 K with a transmembrane pressure of 0.3 MPa. 3. Results and discussion Fig. 1a shows an overview of a straight TiO2 hollow fiber. It could be seen that the fiber was longitudinally quite straight and cross-sectionally round in shape, with an inner diameter, outer diameter, and a thickness of about 330, 540 and 110 ␮m, respectively. Fig. 1b shows a picture of a TiO2 hollow fiber spring. Our previous research indicated that the spring-like TiO2 hollow fibers could be made with any length as needed. The strength of the fibers was high enough in normal handling [18]. Fig. 2 illustrates SEM images of cross-sections the Ni/TiO2 hollow fibers prepared for different electroless plating times. It could be seen that nickel layers were coated on the outer surfaces of the TiO2 hollow fibers. The thickness of the nickel layer increased with the increase of electroless plating time, indicating that nickel was constantly deposited on the outer surface of the support. The thickness of nickel layer was ca. 6, 15 and 22 ␮m when the electroless plating time was 30, 60 and 90 min, respectively. Fig. 3a shows SEM image of a full-scale view of the cross-section of a Ni/TiO2 composite hollow fiber prepared by electroless plating for 90 min. It was evident that the nickel layer was uniformly coated around the TiO2 hollow fiber. The outer diameter of the composite hollow fiber was ca. 560 ␮m. Fig. 3b shows the longitudinal view of the Ni/TiO2 composite hollow fiber. Compact structure and uniform distribution of nickel on the outer surface of the TiO2 hollow fiber could be clearly seen. The morphology of the nickel was the same as that of nickel coating prepared by electroless plating reported in literature [19]. No nickel layer could be apparently observed on the inner surface of the TiO2 hollow fiber. Fig. 3c shows the SEM image of a Ni/TiO2 composite hollow fiber spring. Compared with the TiO2 hollow fiber spring shown in Fig. 1b, the Ni/TiO2 hollow fiber spring kept the same shape, but with much stronger strength. Fig. 3d shows a close observation of the surface of the Ni/TiO2 hollow

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Fig. 2. SEM images of the cross-sections of the Ni/TiO2 composite hollow fibers prepared on TiO2 hollow fibers by electroless plating for 30 min (a), 60 min (b), and 90 min (c). The scale bar is 5.0, 20.0 and 20.0 ␮m for (a–c), respectively.

fiber spring. It could be seen that the nickel layer was composed of compact nickel particles with size of 2–4 ␮m. Fig. 4 illustrates the XRD patterns of the TiO2 hollow fibers and Ni/TiO2 composite hollow fibers prepared by electroless plating for 90 min. The TiO2 hollow fibers were prepared by impregnating tetrabutyl orthotitanate in poly(vinylidene chloride) hollow fiber microfiltration membranes, followed by calcination at 600 ◦ C for 3 h [18]. It was evident that the TiO2 hollow fibers were composed of anatase and rutile phases

(Fig. 4a) with a weight ratio of anatase of ca. 65% [18,21]. After the deposition of nickel, a characteristic peak at 44.6◦ attributed to nickel was found in the XRD patterns of the TiO2 hollow fiber (Fig. 4b). The diffraction peak intensities of TiO2 (anatase and rutile) became weak due to the presence of the nickel layer. These results confirmed that nickel was covered on the outer surface of the TiO2 hollow fibers. Fig. 5a exhibits the EDX spectrum of the nickel layer of the Ni/TiO2 composite hollow fibers to determine the elemental

Fig. 3. SEM images of the cross-section (a) and a longitudinal view (b) of a straight Ni/TiO2 composite hollow fiber and a general view (c) and close observation of the surface (d) of a Ni/TiO2 composite hollow fiber spring shape prepared by electroless plating for 90 min. The scale bar is 200 ␮m, 300 ␮m, 2.0 mm and 20.0 ␮m for (a–d), respectively.

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Fig. 4. XRD patterns of TiO2 hollow fibers (a) and Ni/TiO2 composite hollow fibers (b) prepared by electroless plating for 90 min.

composition. It clearly indicates that the nickel layer was composed of 93.6 wt.% of nickel and 6.4 wt.% of phosphorus. Fig. 5b shows EDX line scanning spectrum of the cross-section of the Ni/TiO2 composite hollow fibers to determine the distribution of each element along the cross-section. It could be seen that there was no elements Ti and O in the nickel layer. Meanwhile, only elements Ti and O were presented in the TiO2 layer. These results revealed that sealing of the two ends of the TiO2 hollow fiber by glue before the electroless plating reaction effectively prevented nickel from depositing on the inner surface of the TiO2 hollow fiber. Fig. 6 shows the tensile strength of the Ni/TiO2 composite hollow fibers prepared by electroless plating for 30, 60 and 90 min, respectively. Although the TiO2 hollow fibers were strong enough for normal handling, they were not strong enough to be tested by Almighty Test Machine. We could feel that the Ni/TiO2 composite hollow fiber was much stronger than the TiO2 hollow fiber. As could be seen from Fig. 6, the tensile strength of the Ni/TiO2 composite hollow fiber increased with the prolongation of the electroless plating time. It was 12.7 MPa when the electroless plating time was 30 min. When the electroless plating time was prolonged to 60 and 90 min, the tensile strength was increased to 45.9 and 52.1 MPa, respectively. These results strongly indicated that the nickel layer coated on the TiO2 hollow fiber improved the strength of the Ni/TiO2 composite hollow fiber.

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Fig. 6. The tensile strength curve of the Ni/TiO2 composite hollow fiber prepared for different electroless plating time.

Ni membranes can be a cheaper alternative of Pd membranes for hydrogen separation [22]. Table 1 lists H2 and N2 permeation properties through the Ni/TiO2 composite hollow fibers at 453 K. It could be seen that the gas permeance decreased, while the H2 /N2 ideal selectivity increased with the increase of the electroless plating times. The decrease of the gas permeance was resulted from the increase of the thickness of the Ni layer after long electroless plating time. The increase of the H2 /N2 ideal selectivity would result from the decrease of defects of the Ni layer after long electroless plating time. The H2 permeance did not change after 1 month of measurement, indicating that the Ni layer was stable on the TiO2 support fiber. Therefore, the Ni/TiO2 composite hollow fibers have potential application in hydrogen purification as inorganic micro-membranes. On the other hand, the Ni/TiO2 composite hollow fibers were composed of Ni outer layer and TiO2 inner layer. If they are applied as a microchannel reactor, the TiO2 inner layer can be used as catalytic sites or catalyst support while the Ni outer layer as the channel. This provides a facile way to deposit catalysts in a microchannel reactor, which is more convenient than the method reported in literature [23] to immobilize the catalysts on the inner surface of a capillary microreactor. As we can prepare TiO2 hollow fibers with different structures by preshaping the polymeric membranes, Ni/TiO2 composite hollow fibers can also be fabricated.

Fig. 5. EDX spectra of the outer surface (a) and the cross-section (b) of the Ni/TiO2 composite hollow fibers prepared by electroless plating for 90 min.

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Table 1 Single gas permeances of Ni/TiO2 hollow fibers at 453 K with a transmembrane pressure of 0.3 MPa Ni/TiO2 hollow fibers prepared for different electroless plating times (min)

30 60 90

Permeance (10−10 mol m−2 s−1 Pa−1 )

Ideal selectivity

H2

N2

H2 /N2

31 14 6.2

5.3 1.7 0.6

5.9 8.1 10.2

4. Conclusions Ni/TiO2 composite hollow fibers with good tensile strength were successfully prepared by electroless plating using TiO2 hollow fibers as supports. The thickness of the nickel layer was 6–22 ␮m with the electroless plating time of from 30 to 90 min. Coating of the nickel on the outer surface of the TiO2 hollow fibers dramatically enhanced the tensile strength of the Ni/TiO2 composite hollow fibers. The nickel layer was composed of 93.6 wt.% of Ni and 6.4 wt.% of P. The composite hollow fibers exhibited H2 over N2 ideal selectivities from 6 to 10. Acknowledgement This work is supported by “Green-Blue” project of Jiangsu Province of China. References [1] A. Larbot, J.P. Fabre, C. Guizard, L. Cot, J. Gillot, J. Am. Ceram. Soc. 72 (1989) 257. [2] S.H. Hyun, B.S. Kang, J. Am. Ceram. Soc. 79 (1996) 279. [3] L.Q. Wu, N.P. Xu, J. Shi, AIChE J. 46 (2000) 1075. [4] S. Liu, G.R. Gavalas, Ind. Eng. Chem. Res. 44 (2005) 7633. [5] S. Liu, K. Li, R. Hughes, Ceram. Int. 29 (2003) 875. [6] X. Tan, S. Liu, K. Li, J. Membr. Sci. 188 (2001) 87.

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