hierarchically structured anatase nanotubes and membranes

Hierarchically structured anatase nanotubes and membranes B. Ma a,b , G.K.L. Goh a, * , T.S. Zhang a ,J.Ma b a Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research), 3 Research Link, Singapore 117602, Singapore b School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798, Singapore article info Article history: Received 14 August 2008 Received in revised form 6 March 2009 Accepted 2 May 2009 Available online 9 May 2009 Keywords: TiO 2 Membranes AAO Mesoporous Photocatalysis abstract Hierarchically structured anatase membranes containing both macro- and mesopores were synthesized by liquid phase deposition of porous anodic aluminum oxide (AAO) templates at 50 °C. It was observed that titania initially forms nanotubes held together by residual AAO, then takes on completely the shape of the AAO template by displacing alumina and finally forms ‘capped’ membranes in which a layer of mesoporous titania covers the ends of the template. Besides the sub-250 nm diameter tube holes, nitro- gen physisorption also revealed that the tube walls were mesoporous with pore diameters centered around 4 nm with extremely high specific surface areas ranging from 179 to 552 m 2 /g. Active photodeg- radation of aqueous solutions of methylene blue indicated that the membranes were photocatalytically active and the highest degradation constant was observed for AAO templates that had been treated for 5 h by liquid phase deposition. Ó 2009 Elsevier Inc. All rights reserved. 1. Introduction Titania can be used as a photocatalyst for the decomposition of aldehyde gas [1,2], destruction of water borne pollutants [3–5] and water-splitting for hydrogen generation [6]. Titania nanostructures are incorporated in optical devices [7], gas sensors [8] and dye-sen- sitized solar cells [9]. In addition, titania structures are resistant to organic solvents, are mechanically stable and self-cleaning under ultraviolet radiation. As such, chemical solution methods of syn- thesizing and depositing titania films and nanostructures become important because of the relative simplicity and low costs of such methods. Crystalline titania structures including porous films, nanorods and nanoparticles can be formed by chemical methods such as pre- cipitation, sol–gel processing, liquid phase deposition and hydro- thermal deposition [10–14]. With increasing interest in using titania membranes not only for photocatalysis but also for filtra- tion in water purification, the use of anodic aluminum oxide (AAO) membranes for forming such bifunctional membranes be- comes an attractive method. Although titania nanotubes can be synthesized directly by anodic oxidation of titanium, the underly- ing impermeable substrate renders the assemblage unsuitable for filtration. Yamanaka and co-workers [15] have reported the use of porous AAO templates and liquid phase deposition (LPD) to fabricate TiO 2 and SnO 2 nanotubes. However, to avoid disruption of the template structure, the growth temperature was kept low (293 K), hence resulting in close packed oxide nanotubes that were amorphous and therefore not photocatalytic in the as-synthesized state. In the present work, self-supporting titania membranes were success- fully prepared by LPD of AAO templates at slightly elevated temper- atures (50 °C) that resulted in the formation of crystalline anatase membranes without the need for subsequent high temperature heat treatment. The present titania porous membrane contains the structure of the ordered hexagonal macropores from the AAO template, and also mesopores in the walls of the membrane. The membranes are shown to be photocatalytically active in the photo- degradation of methylene blue (MB), a common dye used in the tex- tile industry that is found as a pollutant in waste water. In the LPD process, the reaction equilibrium in Eq. (1) can be controlled by the addition of boric acid that consumes fluoride ions to form a more stable complex (Eq. (3)) [16]. ½TiF 6  2À þ nH 2 O ()½Tif 6Àn ðOHÞ n  2À þ nHF ð1Þ BO 3À 3 þ 4F À þ 6H þ ) BF À 4 þ 3H 2 O ð2Þ Yamanaka and co-workers [15] have proposed that alumina performs a similar role as boric acid via the formation of [AlF 6 ] 3À (Eq. (3)). Al 2 O 3 þ 3H 2 O þ 12F À () 2AlF 3À 6 þ 6OH À ð3Þ The fluoride ligand offers a slower and more controllable hydro- lysis because of the use of F À scavengers. This method allows TiO 2 films to be deposited over large areas, conformally on complex shapes and porous bodies and on temperature sensitive substrates such as organics due to the low deposition temperatures [17]. 1387-1811/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2009.05.007 * Corresponding author. E-mail address: g-goh@imre.a-star.edu.sg (G.K.L. Goh). Microporous and Mesoporous Materials 124 (2009) 162–168 Contents lists available at ScienceDirect Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso 2. Experimental methods The precursor solution was prepared by dissolving (NH 4 ) 2 TiF 6 (Aldrich) in distilled water to a concentration of 0.1 M. As a struc- ture director and a starting material, a commercial porous anodic aluminum oxide disc (anodisc 47, WHATMAN Inc.) with straight macroporous channels $250 nm in diameter was immersed in 25 ml of the precursor solution and maintained at 323 K for 1– 48 h. After reaction, the samples were removed from the solution and washed with distilled water. The samples were dried at 323 K for 24 h before characterization. The morphology of the membranes were examined by second- ary electron microscopy (JEOL JSM 6300F, Japan) while the crystal phases were identified by powder X-ray diffraction (XRD, Shima- dzu) using Cu K a radiation (50 kV, 40 mA) and a scan speed of 0.5°/min in h–2h mode. The divergence, anti-scatter and receiving slits were fixed at 1 o . The crystallite size of AlF n (OH) 3Àn (n = 1.5, 1.62 and 1.65) was calculated from the (1 1 1) peak based on Scherrer’s formula. Energy dispersive X-ray (EDX) spectroscopy was used to confirm the composition of the membranes at differ- ent locations. The anatase nanocrystallites in the nanotubes were studied by transmission electron microscopy (TEM) using a JEOL JEM 2010 electron microscope operating at 200 kV after dispersing the samples in ethanol using an ultrasonic bath for 30 min and col- lecting the fragments on holey carbon copper-supported grids. Nitrogen adsorption and desorption isotherms were recorded at 77 K using an Accelerated Surface Area and Porosimetry system after the membranes were vacuum dried at 373 K for 12 h to re- move surface moisture to obtain the mesopore size distribution using the Barrett-Joyner-Halenda (BJH) method. The photocatalytic activity of the treated membranes was eval- uated by the photodegradation of a methylene blue (MB) dye. An ultraviolet (UV) reaction chamber (RPR-100) was used to irradiate a glass vessel (the glass having 98% transmittance in the UV range) such that the intensity at 365 nm in the center of the container was 213 l W/cm 2 . Prior to catalytic testing, 6 ± 0.15 mg of the titania membranes were washed with distilled water and soaked in 50 ml distilled water for 24 h to equilibrate surficial hydroxyl groups. After drying at 323 K for 24 h, the membranes were im- mersed in 15 ml MB solution (3.7  10 À5 M) in the glass reaction vessel. The adsorption equilibrium of the dye by the membranes was established by keeping the container in the dark for 24 h until the concentration of MB stabilized. The container was then ex- posed to UV irradiation and the degradation of MB was monitored by a UV–Vis-NIR scanning spectrophotometer (Shimadzu UV- 3101PC) at a wavelength of 664 nm, which is nearly centered at a strong adsorption band of MB. 3. Results and discussion 3.1. Structure Recently, Shyue and co-workers [18] formed TiO 2 nanotubes by coating the walls of anodic aluminum oxide (AAO) templates by li- quid phase deposition, utilizing both the (NH 4 ) 2 TiF 6 precursor and boric acid. By lengthening the deposition period, nanowires were also obtained. The diameters of both nanotubes and nanowires ob- tained by them corresponded quite well with the diameter of the nanoholes of the AAO template ($250 nm), agreeing with their assertion that the TiO 2 first nucleated on the walls of the AAO tem- plate and then grew inwards, filling the nanoholes until nanowires were obtained. In contrast, this work and an earlier work [15] re- 10 20 30 40 50 60 70 80 2 θ (degree) Relative intensity (a) (b) (c) Fig. 1. XRD of (a) untreated AAO membranes, (b) AAO membranes treated by liquid phase deposition for 3 h with Ti/Al ratio of 1 (closed circles – AlF n (OH) 3Àn ; open circles – anatase TiO 2 ), and (c) AAO membranes treated by liquid phase deposition for 3 h with Ti/Al ratio of 2. Fig. 2. Cross-section SEM micrograph of AAO membrane treated by liquid phase deposition for 1 h near reaction front (white arrows highlight some AlF n (OH) 3Àn crystals while black arrows indicate partially reacted AAO membrane). Table 1 Grain size as determined with Scherrer formula. LPD time (h) TiO 2 (nm) AlF n (OH) 3Àn (nm) 1431 3429 5433 7425 12 4 35 B. Ma et al. /Microporous and Mesoporous Materials 124 (2009) 162–168 163 port that the AAO template actually dissolves according to Eq. (3), providing scavengers for the fluoride ion. This difference is because the fluoride ion scavengers in the work by Shyue and co-workers [18] are provided by the boric acid since it is already in solution and thus there is no significant dissolution of the AAO template. X-ray diffraction (XRD) supports the formation of [AlF 6 ] 3À in solution, as proposed in Eq. (3) that goes on to form a highly crys- talline hydrate, AlF n (OH) 3Àn , on precipitation as shown in Fig. 1. Scanning electron microscopy (SEM) of cross sections revealed that these AlF n (OH) 3Àn precipitates (larger particles) are found at the re- gion where the AAO template has not completely dissolved, a tran- sition zone between the unreacted AAO and TiO 2 nanotubes, as shown in Fig. 2. This transition zone is found to be further from the surface of the template, being $6 l m from the surface after 1 h, 7.5 l m after 3 h $ 10 l m after 5 h and so on. This latter obser- vation indicates that the AlF n (OH) 3Àn phase only precipitates in re- gions of low relative Ti 4+ concentration and its formation can be significantly suppressed by increasing the Ti/Al ratio, as confirmed by XRD in Fig. 1c. Application of the Scherrer formula to the XRD data concur with the SEM observations that parts of the AAO template has been con- verted to nanocrystalline anatase TiO 2 with average particle sizes (see Table 1) around 4 nm and that the AlF n (OH) 3Àn particles are Fig. 3. TEM image of wall of AAO membrane treated by liquid phase deposition for 1 h revealing presence of nanocrystalline anatase. Fig. 4. Plan-view SEM micrographs AAO membranes treated by liquid phase deposition for (a) 0 h, (b) 1 h, (c) 5 h, (d) 7 h, and (e) 12 h. 164 B. Ma et al. /Microporous and Mesoporous Materials 124 (2009) 162–168 larger, with sizes ranging from 25 to 35 nm, for growth periods of 1 to 12 h. The nanocrystalline nature of the nanotubes is further con- firmed by transmission electron microscopy (TEM), as shown in Fig. 3. Morphological observation of the ends of the AAO template after various periods of growth is shown in Fig. 4. After 1 h, the ar- ray of holes of the template are replaced by a close-packed array of tubes (Fig. 4b). This is similar to the observation by Yamanaka and co-workers [15] who carried out the growth at 293 K, except that in the present study (323 K), the TiO 2 tubes are crystalline, while in Yamanaka’s case, the tubes only become crystalline after a post growth heat treatment at 773 K. The replacement of the array of holes with an array of tubes is further evidence that the AAO tem- plate is dissolving and being replaced by TiO 2 . Further growth led to the apparent thickening of the tube walls with concurrent narrowing of the holes (Fig. 4 c). After 7 h growth, the holes begin to be completely filled (Fig. 4d). Fig. 5a shows that individual TiO 2 nanotubes can be obtained along the outer edges of the treated AAO templates. In fact, when a 0.1 M HCl solution is used to dissolve the AAO component, what is left are individual TiO 2 tubes when the growth period is less than 5 h, coalesced tubes for 5 h growth and capped tubes for 7 h growth and above, the lat- ter two observations being similar to the morphology before acid treatment, as in Fig. 5b and c. The capped tubes indicates that the tube holes are not filled with TiO 2 , as initially assumed from Fig. 4d, but instead a TiO 2 layer has formed to cap the ends of the coalesced tubes. This controllable decrease in the diameter of the ends of the tubes and eventual capping introduces a new parameter with which to tailor the design of TiO 2 membranes. For instance, the AAO/TiO 2 membrane could be used as a nano-selective reactor that would allow only reactants that have not been filtered out by the variable and controllable diameter of the tube ends and/or decom- posed by the photocatalytically active TiO 2 , providing both size and chemical selectivity, as schematically illustrated in Fig. 6. In addi- tion, when the tube ends are capped from 7 h onwards, only chem- ical species would be allowed through the mesoporous end caps. 3.2. Porosity Nitrogen physisorption experiments show that treating the AAO template in the (NH 4 ) 2 TiF 6 solution for just 1 h caused the specific surface area to increase by 2 orders of magnitude, as detailed in Ta- ble 2. The adsorption–desorption isotherm in Fig. 7a reveals that this increase in specific surface area is due to the formation of mes- opores, as evidenced by the presence of a hysteresis loop. This means that the TiO 2 nanotube walls are mesoporous. The forma- tion of porous material is not surprising since the AAO material actually dissolves in the acidic fluoride solution according to Eq. (3) and then TiO 2 precipitates back onto the undissolved regions of the AAO membrane. Since the TiO 2 membrane is formed by con- tinuous precipitation (see last paragraph) and not crystal growth, Fig. 5. Cross-section SEM micrographs of AAO membranes treated by liquid phase deposition for (a) 1 h showing individual TiO 2 nanotubes, (b) 5 h showing coalesced TiO 2 nanotubes, and (c) 48 h showing a capped AAO/TiO 2 membrane. CHEMICAL SELECTIVITY Organic species are photocatalytically decomposed SIZE SELECTIVITY Varying diameter of entrance limits maximum size of reactant particle Fig. 6. Schematic of how AAO/TiO 2 membrane functions as a nanoreactor. B. Ma et al. /Microporous and Mesoporous Materials 124 (2009) 162–168 165 this leads to the presence of mesopores. At relative pressures more than 0.9, the sharp increase in the adsorption volume is due to the presence of macropores i.e. the holes of the nanotubes. The pore size distribution calculated from the adsorption branch by the BJH method is shown in Fig. 7b. It shows that most of the mesopores in the TiO 2 nanotube walls are $4 nm in diame- ter. The comparatively lower amount and more broadly distributed pores centered around 30–40 nm are believed to be the cylindrical pores located between the hexagonally packed nanotubes. In fact, geometrical considerations based on hexagonal packing of cylin- ders shows that the diameter, d, of a pore located between the nanotubes can be related to the outer diameter, D, of the surround- ing tubes according to [19], d ¼ Dð1 À cos30  Þ=cos30  ð4Þ Therefore, for a nanotube diameter, D, of 250 nm, the pore lo- cated between the surrounding tubes would have a diameter, d, of 38.7 nm, in excellent agreement with the experimental observation. The specific surface area values presented in Table 2 are for the treated AAO template i.e. a combination of AAO and TiO 2 . To get an idea of the specific surface areas of the mesoporous TiO 2 structures formed after the various soaking times, the volume ratio of TiO 2 to AAO is determined from the position of the transition zone. For example, the volume ratio of TiO 2 to AAO for a reaction time of 1 h would be 1:4 (the initial thickness of the untreated AAO mem- brane is 60 l m while the length of the transition zone after 1 h, 6 l m, is multiplied by 2 since the conversion of AAO to TiO 2 occurs from both ends of the template). Therefore, the volume, V, of TiO 2 in 1 g of treated AAO template after 1 h (i.e. 3  4V + 3.8  V =1)is 0.063 cm 3 and so the mass of TiO 2 is 0.239 g. Since we know that the specific surface area of AAO is 7.8 m 2 /g, it is easily determined that the specific surface area of the TiO 2 only is ((137.8 m 2 / g  1gÀ7.8m 2 /g  0.761 g)/0.239 g) 552 m 2 /g. Accordingly, the specific surface areas of mesoporous TiO 2 after 3 and 5 h reactions are 400 and 179 m 2 /g. These are extremely high specific surface areas and also show that the decreasing specific surface area trend from 1 h onwards is very significant. As more of the AAO template is transformed to mesoporous TiO 2 with increasing reaction time, one would expect the specific surface area to increase and not decrease as observed. Although the nanotube hole diameter decreases, the inner surface of the tube/hole does not contribute a significant portion of the specific surface area (maximum 7.8 m 2 /g if the AAO was made of dense TiO 2 instead). Generally, it is the mesoporosity of the tube/hole walls that contributes to the higher specific surface area. Also, a re-examination of Fig. 2 shows that pores within the tube walls can only be accessed via the holes from 5 h onwards. Before 5 h, the pores within the tube walls can be accessed from both the in- ner/larger holes and the smaller holes between the tubes. If the pores within the tube walls are interconnected all the way to the surface of the walls, then whether the pores in the walls can be ac- cessed from the larger and smaller holes, as compared with access from just the larger holes from 5 h onwards, should not make any difference to the measured specific surface area. But Ma and co- workers [11] have shown that film growth of TiO 2 by liquid phase deposition occurs by continuous nucleation and so new TiO 2 parti- cles may precipitate in existing pores and passages (Fig. 8), thereby blocking off pores below them from the wall surface. If pores could be accessed from both directions (inner and outer tube walls), then the blockage would not be a problem. Therefore, it is likely that a combination of pore blockage from newly precipitated TiO 2 within the tube walls and access from only the inner tube walls contribute to the drastic decrease in specific surface area. 3.3. Photocatalysis The photocatalytic activity of the membranes was evaluated by photocatalytic decolorisation of aqueous methylene blue solutions. The photodegradation observed pseudo-first-order reaction kinet- ics, as shown in Fig. 9, in which the degradation rate constant, k, can be determined from the equation ln(C o /C) =kt, where C o and C are the initial and final concentrations of the methylene blue dye after time t respectively. The apparent rate constant of the Table 2 Specific surface areas and calculated pore diameters for AAO membranes treated by liquid phase deposition (LPD) for various times. LPD time (h) Specific surface area (m 2 /g) Pore diameter (nm) 0 7.8 250 1 137.8 3 3 123.8 4 5 72.9 4 7 65.5 3 12 51.7 4 24 38.1 4 Fig. 7. (a) Adsorption–desorption isotherms (adsorption – closed circles; desorp- tion – open circles), and (b) BJH pore size distribution of AAO membranes treated by liquid phase deposition for 1 h. 166 B. Ma et al. /Microporous and Mesoporous Materials 124 (2009) 162–168 template is the same as that of the blank MB solution under UV irradiation (Table 3), confirming that the template does not con- tribute to photocatalytic activity, while all the anatase membranes accelerated the degradation of MB. AAO templates treated for 5 h in the (NH 4 ) 2 TiF 6 precursor solution displayed the highest degrada- tion rate constant of 0.0146 min À1 . The major factors contributing to the overall effectiveness of the photocatalytic activity are grain size, surface area and crystallinity. When titania is illuminated by UV radiation, an electron/hole pair is created. Both the electron and hole travel to the surface of the tita- nia particle where OH Å radicals are created that are mainly respon- sible for the degradation of the methylene blue (MB) dye. The grain size is important as smaller grain sizes translates to higher specific surface areas and so offer a greater number of surface sites at which adsorption and degradation of the MB dye can take place. Decreas- ing grain size also reduces the rate of recombination of the electron/ hole pairs in the bulk of the grain (the volume charge-carrier recombination rate). But there is a critical grain size at which the in- crease in photocatalytic activity with decreasing grain size changes to a decreasing trend as the rate of electron/hole recombination at the particle surface becomes more dominant (the surface charge- carrier recombination rate) with the ever escalating surface area to volume ratio [10]. In this study, grain size would not occupy a dominant role since the grain sizes do not vary significantly with treatment time, as shown in Table 1. Therefore, in order to explain the increase and then decrease of the degradation constant with treatment time, the surface areas and crystallinities of the membranes are now more closely exam- ined. Previously in Section 3.2, the specific surface areas of the TiO 2 portion of the treated AAO templates were shown to decrease drastically from 552 to 179 m 2 /g for treatment times between 1 and 5 h. But since the weight of the treated AAO template (a com- bination of AAO and TiO 2 ) was kept constant for the photocatalytic tests, the amount of TiO 2 present in the MB test solutions would be different for the different treatment times since more AAO would have been converted to TiO 2 with longer treatment times. The ac- tual surface areas of TiO 2 present are easily calculated to be 132, 118 and 68 m 2 for treatment times of 1, 3 and 5 h, respectively. This is still a significant decrease in surface area over which the degradation of the MB dye can take place and thus the variation in surface areas cannot explain the increase in photocatalytic activ- ity with increasing treatment time from 1 to 5 h. Ma and co-workers [13] pointed out that in liquid phase depo- sition of titania taking place in an oven set at 50 o C, the growth solution takes approximately 90 min to reach the temperature of the oven and the maximum crystallinity attained even after 12 h is only 75%. In addition, the crystallinity of the precipitated mate- rial is still increasing from 70% even after 4 h. This is because the measured crystallinity is an average of the maximum crystallinity attained after 90 min with the amorphous or very low crystallinity material precipitated before 90 min. The presence of amorphous titania is detrimental to the photocatalytic efficiency as it contains a variety of trap sites and recombination centers that decreases the concentration of electron/hole pairs generated from UV irradiation. Therefore, it is most likely that the photocatalytic efficiency of membranes treated from 1 to 5 h increases despite the decrease in surface area available because of the increase in titania crystal- linity. After 5 h, the photocatalytic efficiency decreases as the surface area decreases, as noted in Table 2. 4. Conclusions Anatase membranes containing both macro- and mesopores were synthesized by liquid phase deposition of porous anodic alu- minum oxide (AAO) templates at 50 o C. Instead of boric acid, the alumina from the AAO template served as the F À scavenger re- quired in the conversion of the (NH 4 ) 2 TiF 6 precursor to TiO 2 .As such, the alumina dissolves and is replaced by precipitated TiO 2 . Fig. 8. Illustration of how continuous nucleation of new material can block off pores and reduce total internal surface area. Fig. 9. Photodegradation kinetics of AAO membranes treated by liquid phase deposition for various times (open circle – 1 h; filled triangle – 3 h; open triangle – 5 h; filled square – 12 h; closed circle – blank reference). Table 3 Degradation rate constants for AAO membranes treated by liquid phase deposition (LPD) for various times. LPD time (h) Blank 0 1 3 5 12 Apparent rate constant k (min À1 ) 0.0006 0.0006 0.0046 0.007 0.0146 0.0045 B. Ma et al. /Microporous and Mesoporous Materials 124 (2009) 162–168 167 It was observed that TiO 2 initially forms a close-packed array of nanotubes held together by residual AAO. As the AAO template is treated for longer periods, a TiO 2 membrane is formed having sub-250 nm holes. This membrane is finally covered on both ends by a layer of mesoporous TiO 2 . XRD confirms that crystalline TiO 2 is present in the membranes for all treatment times while nitrogen physisorption also revealed that the tube/membrane walls were mesoporous with pore diameters centered around 4 nm. BET calcu- lations showed that these mesoporous structures had extremely high specific surface areas starting from 552 m 2 /g after 1 h treat- ment in the LPD solution and then decreasing to 179 m 2 /g after 5 h treatment. The photodegradation of aqueous solutions of methylene blue observed pseudo-first-order reaction kinetics and the photocata- lytic activity first increasing and then decreasing. The highest deg- radation constant, k, of 0.0146/min was observed for AAO templates that had been treated for 5 h. It is believed that despite the lower surface area of the 5 h treated membrane compared to the 1 and 3 h membranes, the higher crystallinity of the 5 h membrane was the reason for its maximum photocatalytic activity. Acknowledgment The authors thank T.J. 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Zhang a ,J.Ma b a Institute of Materials Research and. 2009 Keywords: TiO 2 Membranes AAO Mesoporous Photocatalysis abstract Hierarchically structured anatase membranes containing both macro- and mesopores were