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Journal of Membrane Science 365 (2010) 225–231

Contents lists available at ScienceDirect

Journal of Membrane Science

journa l homepage: www.e lsev ier .com/ locate /memsci

o-sintering synthesis of bi-layer titania ultrafiltration membranes withntermediate layer of sol-coated nanofibers

inghui Qiu, Su Fan, Yuanyuan Cai, Yiqun Fan ∗, Nanping Xutate Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing University of Technology,in Mofan Road 5, Nanjing 210009, China

r t i c l e i n f o

rticle history:eceived 31 March 2010eceived in revised form 27 July 2010ccepted 6 September 2010vailable online 15 September 2010

eywords:eramic membrane

a b s t r a c t

The preparation of titania ultrafiltration membranes with intermediate layer of sol-coated nanofibers isbriefly described. In this process, titiania nanofibers cover on the porous substrate to produce uniformlayer with high porosity and flux. The use of titania nanoparticles from sol has been found to bringan improvement on the mechanical strength of the titania nanofiber membrane due to the formationof sintering neck between nanofibers with colloidal particles (sol) at lower sintering temperature. Inorder to reduce the pore size and achieve high separation efficiency, titania colloidal particulate solis used to coat on the top of the titania nanofiber layer and then co-sintered at the suitable sinteringtemperature of the titania gel (480 ◦C) to prepare ultrafiltration membranes. Furthermore, the coating

ltrafiltration

ol-coated nanofiberso-sintering

times of titania sol are optimized based on the measurements supplied by permeation and separationperformance and a delicate control of the coating times indeed plays a key role in the preparation of thedefect-free membrane with high performance. In this work it is found that the prepared membrane hashomogeneous surface without obvious defects in case of three coatings of titania sol co-sintered with thefiber layer at 480 ◦C. Property tests display that the pure water flux reaches 1100 L m−2 h−1 bar−1 while

-off is

the molecular weight cut

. Introduction

Nanofibers have attracted considerable interest due to theirnique properties and potential applications in many areas [1,2].ecently there have been some attempts to prepare microfiltrationr ultrafiltration membranes from metal oxide nanofibers [3–6].uch nanofiber membranes normally have an asymmetric struc-ure with a separation layer supported by a macroporous carrier.n these researches the separation layer is made of metal oxideanofibers with the diameter ranging from tens to hundreds ofanometers rather than common oxide particles. Ceramic mem-ranes with a separation layer of oxide nanofiber expectedly notnly inherit the advantage of conventional ceramic membranesuch as thermal and mechanical stabilities, chemical and microbio-ogical resistance, and long lifetime [7], but also adequately utilizehe aspects of nanofibers to attain high porosity and low flow resis-ance [8,9].

Compared with the conventional ceramic membrane preparedrom oxide particles, the nanofiber membranes have some advan-ages. Firstly, the porosity in the separation layer formed fromhe interconnected nanofibers can be over 70% to achieve higher

∗ Corresponding author. Tel.: +86 25 83172277; fax: +86 25 83172292.E-mail address: yiqunfan@njut.edu.cn (Y. Fan).

376-7388/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.memsci.2010.09.005

32,000.© 2010 Elsevier B.V. All rights reserved.

flux with similar separation performance. But conventional ceramicmembranes prepared from particles always have a porosity below36% and have some dead-end pores that make no contribution tothe flux [4]. Secondly, the dip-coating technique allows nanofibersto interconnect on the surface of supports, which could pre-clude infiltration of nanofibers. Thirdly, the randomly orientednanofibers have high elastic modulus and thermal stress resis-tance, decreasing the formation of pinholes and cracks duringthe drying and sintering process, and thus the sintered mem-branes with oxide nanofibers possess high thermal shock resistance[10]. Finally, it is possible to adjust the membrane pore sizesand surface roughness by controlling the stacking density ofnanofibers. Therefore, construction of asymmetrical membraneswith a separation layer of metal oxide nanofibers should bean effective way for preparing high-performance ceramic mem-branes.

However, the mechanical strength of nanofiber membranes isnot sufficient for their application in a long run, especially for tubu-lar ceramic membrane. It is necessary to enhance the interactionbetween fibers [11]. Acid phosphate was firstly adopted as a binder

to improve the strength of the alumina fiber filters due to the bond-ing at the junctions of fibers [12]. Silica or alumina sol can also beused as binders to enhance strength of fiber membrane as fiberscan be connected by fine colloidal particles at relatively low sinter-ing temperature, with the minimal loss of flux. Furthermore, the

2 brane Science 365 (2010) 225–231

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26 M. Qiu et al. / Journal of Mem

sed colloidal particles and fibers materials of same type have theniformly thermal expansion [13,14].

In this work, we prepared tubular ceramic membranes with tita-ia nanofibers. They have unique features including hydrophilicity,nti-fouling [15], semi-conductivity, biocompatibility [16], andotential applications in photocatalytic membrane reactions [17].o improve the strength of titania nanofiber membranes, titaniaol was used to coat the surface of the nanofibers to increase theonding at the junctions of nanofibers during the sintering process.oreover, titania sol was coated repeatedly on the top of the fiber

ayer to reduce the pore size and achieve high separation efficiency.he performances of nanofiber membranes were characterized byeasuring pure water flux and retention rate.

. Experimental

.1. Fabrication of titania nanofiber membranes

Titania fibers, provided by our collaborator Prof. Lu’s Grouprom Nanjing University of Technology, were synthesized by ion-xchange reaction from potassium tetratitanate fiber (K2Ti4O9)ased on the hydrate conditions. In the hydration process, theydrate intermediate H2Ti4O9·1.2H2O were obtained when theolar ratio of Ti/K in solid phase were controlled at >20 by adjusting

he pH value to 2. With further treated of intermediate at 800 ◦C inuffle furnace for 2 h, the derivatives of anatase TiO2 nanofibersere synthesized [18,19]. The resultant raw anatase TiO2 fibersay contain coarse particles, removed by water scrubbing method

wing to gravity settling. Titania sol was synthesized by hydrolysisf tetrabutyl orthotitanate (TBOT) supplied commercially withouturther purification. TBOT, anhydrous ethanol, nitric acid, deion-zed water and acetylacetone were mixed to give volume ratios of5:250:0.55:5:600 at 85–90 ◦C. This mixed solution stirred for 4 ho ensure complete mixing and hydrolysis [20,21]. The pre-treatedanofibers without coarse particles were dispersed into titania sol0.75 wt%) to form suspension, in which the ratio of nanofibersnd nanoparticles from sol were controlled at 9:1. The deionizedater was added to the suspension to adjust the solid content of

itania. After stirring for 2 h, methylcellulose (MC) (Sigma–Aldrichorporation, molecular weight: 40,000) was introduced into theuspension as a polymer additive and stirred for another 30 min.o achieve well-dispersion, the suspension of sol-coated nanofiberas further treated in ultrasonic for 10 min and finally labeled “TF”.

The supports used were porous �-alumina tubes with theimensions of 12 mm in outer diameter, 2 mm in wall thickness,5 mm in length and 2–3 �m in mean pore diameter. The poros-

ty, surface roughness and pure water flux were 35%, ∼2 �m and0,000 L m−2 h−1 bar−1, respectively. Titania nanofiber membranesere fabricated by a dip-coating process on the inner surface of

he tubular supports. The treated support was dip-coated with TFuspension for 60 s, dried under an ambient environment for 12 hnd at 80 ◦C for 12 h, followed by sintering at 480 ◦C for 3 h in theuffle furnace (at a heating and cooling rate of 0.5 and 1 ◦C/min,

espectively).

.2. Preparation of ultrafiltration membrane supported by theanofiber layer

Titania ultrafiltration membranes were prepared by coatingitania sol on the nanofiber layers. Fig. 1 shows the particle size dis-ribution of titania sol measured by Mastersizer 3000 particle size

nalyzer (Malvern, Britain) and exhibits a monomodal distributionentered at 25 nm. Then polyvinyl alcohol (PVA) (Sigma–Aldrichorporation, molecular weight: 13,000–23,000) was added into theol as a polymer additive and stirred for 30 min, followed by ultra-onic treat for 10 min. The resultant titania sol was labeled “TS”.

Fig. 1. Particle size distribution of titania sol (number analysis).

After coating TF on porous supports for 60 s, the wet membraneswere dried at room temperature for 12 h to serve as the substrate ofsol–gel membrane. Then the nanofiber layers were processed withthe repeating coating procedure with TS for 60 s, followed by dryingin a oven at 75 ◦C in a relative humidity of 75% for 10 h. In orderto make the defect-free top-layer, TS sol was used to coat severaltimes and dried with the same process. At last, membranes wereco-sintered in air at 480 ◦C for 3 h in muffle furnace (at a heatingand cooling rate of 0.5 and 1 ◦C/min, respectively).

2.3. Characterization of titania membranes

The viscosity of suspension TF and sol TS were measured byrotary viscosimeter (DV-II+, Brookfield Engineering Labs., Inc., USA)at 30 ◦C. Thermogravimetric analysis for the dried powders of TFand TS were accomplished using 10 ◦C/min heating rate (DTA/TG,Netzsch STA 409). The crystal structures of the sintered membraneswere studied by X-ray diffraction (XRD; Bruker D8 Advance), usingCu K� radiation with 2� from 20◦ to 80◦. Morphologies of the pre-pared membranes were examined by scanning electron microscopy(SEM) (Quanta 200, FEI, Netherlands) and field emission scanningelectron microscopy (FESEM) (HITACHI S-5000). The thickness (L)of the layer was determined by the increased weight of the tubularsupports after the formation of the membrane, and can be calcu-lated by the following equation:

L = W2 − W1

A�(1 − ε)(1)

where ε is the porosity of membrane layer, measured byArchimedes’ method used symmetric sample; A is the membranearea; � is the titania theoretical density; W1 is the weight of sup-port and W2 is the total weight of the support and the membranelayer.

The pore size distributions of the supports and membraneswere determined by the gas bubble pressure method (GBP) [22]and the liquid/liquid displacement porometry (LLDP) [23], respec-tively, according to the size of separation pore. The pure water flux(PWF) was determined in a tangential-flow filtration apparatusby collecting the permeation in a graduated cylinder and timingthe collection period. The separation performance was analyzed

by rejection molecular weight, following standard ultrafiltrationtests with aqueous dextran solutions [24]. The feed solution con-tained dextrans with molecular masses of 10,000, 40,000, 70,000and 500,000. The dextrans concentration were 2.5 g/L for 10,000,1 g/L for 40,000, 1 g/L for 70,000 and 2 g/L for 500,000. Analysis of

M. Qiu et al. / Journal of Membrane Science 365 (2010) 225–231 227

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ability between nanofiber layer and the support, and the sinteringprocess.

Firstly, the suspension of sol-coated nanofibers (TF) should bestable. The solid content and pH of the as-prepared suspensionsare controlled to be 5 wt% and 3 to ensure dispersion stability of

Fig. 2. The morphology of titania nanofibers sintered at differ

he feed and permeate solutions was conducted by gel permeationhromatography (GPC, Waters). The molecular mass of dextranorresponding to 90% retention was considered as the moleculareight cut-off.

. Results and discussion

.1. Effect of sintering temperature on the structure of nanofibers

Sintering process has a significant effect on the morphologyf nanofiber and, consequently, on the microstructure of theanofiber membrane. Fig. 2 shows the morphology of titaniaanofiber before and after sintered at a series of temperatures.ccording to the SEM images present in Fig. 2(a), the raw materialxhibit good aspects with a diameter of 200–400 nm and a lengthf 5–10 �m. When the temperature was 400 ◦C, the morphologyf nanofibers maintained well (Fig. 2(b)). Once the temperaturencreased to 600 or 800 ◦C (Fig. 2(c) and (d) it was difficult inonstruction of nanofiber membrane owing to the fracture andgglomeration of titania nanofibers.

The influence of sintering temperature on the structure ofanofibers could be attributed to the phase transformation ofhe TiO2 membrane from anatase to rutile, which was confirmedy XRD analysis. Fig. 3 displays the XRD patterns of the titaniaanofibers after calcination at temperature range of 400–1000 ◦C.he sample sintered at the temperature lower than 600 ◦C is com-osed of pure anatase (JCPDS card No. 21-1272). As the temperature

ncreases from 600 to 800 ◦C, the anatase titania remains as theain phase, which is in good agreement with JCPDS card No. 65-

714. But new diffraction peaks centered at 38◦ corresponding tohe crystal face of (0 0 4) are observed. Higher temperature causeshase transformation from anatase to rutile for titania nanofibers.

t has been reported that the morphology of nanofiber is sensitiveo the phase transformation during the sintering process [25,26]. Toeep nanofiber in anatase phase, the nanofiber membrane shoulde sintered at the temperature less than 600 ◦C. When the nanofiberembrane sintered at a lower temperature, however, the less

mperature: (a) raw material, (b) 400 ◦C, (c) 600 ◦C, (d) 800 ◦C.

neck growth between nanofibers brings inevitable reduction ofstrength.

3.2. Preparation of nanofiber membrane sintered with the aid ofsol

Titania sol has a positive effect on improving the mechanicalstrength of nanofiber membrane because sintering neck betweennanofibers could be fabricated with colloidal particles. Fig. 4 illus-trates the nanofiber on the supports after co-sintering with sol. Thesurface of nanofibers is coated by titania particle sol to enhance thebonding at the junctions of nanofibers. Therefore, the key factorsfor controlling the structure of the nanofiber membrane are the dis-persion stability of sol-coated nanofiber suspension, the matching

Fig. 3. XRD patterns of sol-coated titania fiber sintered at 400–1000 ◦C for 3 h.

228 M. Qiu et al. / Journal of Membrane Science 365 (2010) 225–231

Fig. 4. Schematic illustration of the fiber sintered with the aid of sol.

Fig. 5. Thermogravimetry result of titania gel during sintering in air atmosphere.

Fig. 6. The surface morphology of TF membrane sin

Fig. 7. The pure water fluxes of nanofiber membrane before and after ultrasonictreatment.

aqueous suspension. The stability is judged by the change of thesuspension viscosity, which remains 4 cP in 10 h. The nanofiberlayer is formed on the porous support by dip-coating method. Aftercoating with TF suspension, the nanofibers lie randomly on theporous support with the thickness of 5 �m calculated by Eq. (1),indicating sufficient thickness to cover the entire rough surface ofthe alumina support.

The suitable sintering temperature of nanofiber membranestrongly depends on the sintering activity of sol because nanofibercould be connected by nanoparticles from sol. Fig. 5 displays thethermal evolution diagram of titania gel by the analysis of TG/DSC.It is shown that the sintering temperature for titania sol membranelower than 400 ◦C is not considered due to the risk of insufficientburnout of the organic additive. However, it is unreasonable forsol membrane to be sintered at higher than 500 ◦C because of thetransformation from anatase phase to rutile phase correspondingto an exothermic peak at 500–1000 ◦C in the DSC curve.

Fig. 6 presents morphologies of TF membrane sintered at 480 ◦C.It reveals that the surface has no visible pinholes and cracks, andthe sol particle between fibers can be observed. To achieve a betterunderstanding of the effect of the sol in nanofiber membrane, thenanofiber membranes sintered without sol were prepared at thesintering temperature of 480 ◦C and their strength was comparedto that of the nanofibers membranes sintered with the aid of sol.The pure water fluxes of both membranes before and after ultra-sonic treatment have been measured and displayed in the Fig. 7.The results can verify that the nanofiber membrane sintered withthe aid of sol can withstand ultrasonic vibration (160 W, 40 kHz)for 30 min without damage. But the nanofiber membrane sinteredwithout sol show unsatisfactory interfacial adherence.

3.3. Co-sintering synthesis of bilayer titania membranes

The ceramic membrane with a separation layer of titaniananofibers can be prepared with the aid of titania sol. To reducethe pore size and achieve high separation efficiency, titania col-loidal particulate sol (TS) has been coated on the top of the titania

tered at 480 ◦C: (a) 10,000× and (b) 50,000×.

M. Qiu et al. / Journal of Membrane Science 365 (2010) 225–231 229

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Fig. 8. Schematic diagram of bi-layer mem

anofiber layer to prepare ultrafiltration membranes via the sol–gelrocess. In addition, the co-sintering technique should be applied toabricate the present ultrafiltration membrane because the bondingtrength of the nanofiber layer was controlled by the neck connec-ion between colloidal particles rather than nanofibers. Fig. 8 showshe schematic diagram of bi-layer membrane from the sol-coatedber as transition layer, and titania colloidal sol as separation layer.he coating and drying steps of sol layer were usually repeated sohat the membrane can be sufficiently covered and crack-free [27].

Fig. 9 gives the pore size distribution of the support andanofiber membrane characterized by gas bubble pressure method.

t is shown that the mean pore size of the alumina support is up to–3 �m (curve a in Fig. 9), and the titania nanofiber membrane

as pores of 1–2 �m (curve b in Fig. 9), which is in agreementith the results from morphologies of TF membrane (Fig. 6).onsidering a diameter of 200–400 nm of the titania fiber, it isifficult to further reduce the pore size of the nanofiber mem-rane. The pore size distribution of the bi-layer titania membrane

ig. 9. Pore size distribution of porous alumina support, nanofiber membrane andi-layer titania membrane with three coatings of titania sol.

e with medium layer of sol-coated fibers.

with three coatings of titania sol was also displayed in Fig. 9(curve c). By coating the titania sol layer, the sizes of the filtra-tion pores could be reduced to 12 nm, to enhance selectivity of themembrane.

Fig. 10 displays the morphologies of membrane with three coat-ings of titania sol co-sintered with the fiber layer at 480 ◦C. Fig. 10(a)and (b) reveals that the surface of top-layer membrane is homoge-neous without evident defects. It is composed of titania sphericalparticles which can cover the surface of fiber layer completely. FromFig. 10(c) it can be found that the sub layer is composed of fibers andhas an approximate thickness of 5 �m, coinciding with the thick-ness calculated by weighing. And the top-layer membrane with thethickness of 800 nm exhibits uniform and excellent interface withgood adherence to fiber layer.

3.4. Performance of titania membranes

The permeability and separation efficiency of the membraneswere measured by pure water flux (as shown in Fig. 11) and dex-tran retention test (as shown in Fig. 12), respectively. After coatingwith sol-coated fibers, the flux of the nanofiber layer was close to4000 L m−2 h−1 bar−1, which was approximately 30% of the flux ofthe porous alumina substrate, while rejection efficiency of TF layerfor dextran was enhanced obviously. Further coating of titania solon the nanofiber layer, the top layer membranes have a higherrejection but lower flux. For sol–gel method, the integrity of themembrane is strongly related to the thickness, which can be con-trolled by the concentration of the sol. Moreover, the concentrationof the sol solution has effect on the colloidal diameters [28]. The solwith the concentration of 0.75 wt% and with the colloidal diameterof 25 nm is too dilution to form adequate coating on the substrate,and thus the repeated coating process of the sol was needed toincrease the membrane thickness. The results of flux and retention

in Figs. 11 and 12 indicate that three coating times may be suf-ficient for complete coverage to form a defect-free membrane inour work. By contrast with the tubular titania/�-alumina compos-ite membrane of high (>90%) filtration efficiency on BSA (66 kDa)[29,30], the present membrane in this work possess better separa-

230 M. Qiu et al. / Journal of Membrane Science 365 (2010) 225–231

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Fig. 11. Pure water flux through TiO2 membranes, as a function of coating times ofTiO2 sol, co-sintered at 480 ◦C for 3 h.

Fig. 12. Molecular retention curves of support, titania nanofiber membrane and topultrafiltration membrane.

ig. 10. The SEM morphology of the bi-layer TiO2 membrane co-sintered at 480 ◦C.a) Surface image 10,000×; (b) surface image 180,000×; (c) cross-section image.

ion efficiency with MWCO of 32 kDa while maintaining a high fluxf 1000 L m−2 h−1 bar−1.

Based on the correlation between the molecular radius (a) andolar mass (M) (a = 0.33M0.46), the diameter of dextran corre-

ponding to the cut-off of molecular weight (32 kDa) is calculated tonm, which is a litter less than the pore size of the membrane deter-ined by the liquid/liquid displacement porometry. It is reasonable

hat the dextran molecule with smaller size than filtration poreould be rejected due to the deposition and adsorption of dextrann the membrane surface and within the pores.

Further increasing thickness by more number of coatings tendso crack easily when being dried or sintered and, hence, rejection

ecrease. When the coating times of sol are more than three, theux increases while the separation efficiency for dextran lowerown. The reason is that the present ultrafiltration membranesere prepared by repeated coating and drying procedure, fol-

Fig. 13. The surface morphology of bi-layer TiO2 membrane after fivefold cycle ofsol.

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M. Qiu et al. / Journal of Mem

owed by the co-sintering process, not via conventional methodf multiple sintering cycles. Although the thickness of each layer isufficiently thin to decrease the risk of crack-formation during dry-ng period, many cracks and defects occurred after fivefold cycleollowed by one step sintering process, which can be clearly seenn Fig. 13.

. Conclusions

Titania nanofibers are used to construct separation layer onorous supports. The sintering temperature is investigated by thenalysis of TG/DSC, XRD and SEM characterization. It is shownhat nanofiber should be sintered at the temperature lower than00 ◦C in order to prevent phase transformation and keep nanofiberorphology. Titania sol has a positive effect on improving theechanical strength of nanofiber membrane because sintering

eck between nanofibers could be fabricated with colloidal parti-les. Therefore, the nanofiber membrane can be sintered at 480 ◦Cith the aid of sol. In order to reduce the pore size and achieveigh separation efficiency, titania colloidal particulate sol has beenoated on the top of the titania nanofiber layer to prepare ultra-ltration membranes via the sol–gel process. Both layers can beo-sintered at 480 ◦C which is the suitable sintering temperaturef the titania gel. Furthermore, the coating times of titian sol areptimized based on the measurements supplied by permeationnd separation performance. The results show that three coatingimes should be sufficient for complete coverage to form a defect-ree membrane. When the membrane with three coatings of titaniaol co-sintered with the fiber layer at 480 ◦C, the prepared mem-rane has homogeneous surface without obvious defects. And theure water flux reached 1100 L m−2 h−1 bar−1 while the moleculareight cut-off is 32,000.

cknowledgements

This work was supported by the National Basic Research Pro-ram of China (no. 2009CB623400), the National High Technologyesearch and Development Program of China (no. 2007AA030303nd no. 2009AA033005) and the National Nature Science Founda-ion of China (no. 20636020). We sincerely thank Prof. Xiaohuau from Nanjing University of Technology for providing high-erformance titania nanofibers. And we also thank Dr. Xuebin Kerom Queensland University of Technology for his helpful discus-ion.

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