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Synthesis and characterization of mesoporous TiO2 assembled as microspheres
Journal of Alloys and Compounds 490 (2010) 311–317
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Synthesis and characterization of mesoporous TiO2 assembled as microspheres Ali Beitollahi a,∗ , Amir Hossein Haj Daie a , Leyla Samie a,b , Mohammad Mehdi Akbarnejad b a b
Nanomaterial Research Group, Dept. of Metallurgy and Materials Eng., Iran University of Science and Technology (IUST), Narmak, Farjam, Tehran, Iran Research Institute of Petroleum Industries, Tehran, Iran
a r t i c l e
i n f o
Article history: Received 18 July 2009 Received in revised form 30 September 2009 Accepted 1 October 2009 Available online 9 October 2009 Keywords: Mesoporous TiO2 Spray pyrolysis Self assembly
a b s t r a c t In the work presented here, we report the successful synthesis of highly porous (219 m2 /g) mesostructured titania powder assembled as microspheres of uniform wormlike channels of ∼6–7 nm widths by spray pyrolysis of a titania sol prepared by templating approach using P123 pluronic block copolymer (BC). This method involves the preparation of a Ti4+ -sol containing titanium tertraisopropoxide (TTIP), pluronic BC P123, 2-propanol, acetyl acetonate (ACAC) and hydrochloric acid (HCl) as well as H2 O followed by spray pyrolysis of the obtained sol into the hot zone of a pre-heated reactor at 800 ◦ C, 900 ◦ C and 1000 ◦ C. For all of these pyrolyzed samples, the formation of anatase as the major phase along with rutile as the minor one could be confirmed. The appearance of wormlike mesopores rather than highly ordered cubic or hexagonal mesostructures is related to the lack of appropriate drying conditions normally applied to induce self assembly as well as formation of rather large titania crystallite sizes. © 2009 Elsevier B.V. All rights reserved.
1. Introduction In the last decade, special attention is paid to the development of innovative material preparation routes with the aim to synthesize well-defined nano-objects and nanostructures such as nanofibers, nanotubes, quantum wells and mesoporous powders. The latter is of both scientific and technological interest due to its capability of interacting with atoms, ions as well as molecules both at their large surfaces and throughout their bulk leading to new applications [1,2]. Nanostructured particles exhibiting well-defined pore sizes and pore connectivities (1, 2 or 3D) have various potential applications such as: catalyst, controlled drug delivery, low dielectric constant fillers as well as custom engineered pigments and optical hosts [3,4]. Since the discovery of ordered mesoporous silica-based materials [5] numerous studies have been focused on the synthesis of these materials and other non-silica systems [6]. In this respect, a wide range of mesostructures of different pore size/shape, connectivity and symmetry has been synthesized using different templating approach based on the hydrolysis and cross linking of inorganic precursors at the surfaces of supramolecular surfactant assemblies [7]. High surface area mesoporous titania (MT) due to its enormous applications in the field of photocatalyst, separation, photoelectronics and electromagnetism [8,9] have gathered wide interest.
∗ Corresponding author. Tel.: +98 21 77459151; fax: +98 21 77459151. E-mail address: [email protected] (A. Beitollahi). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.10.002
MT powders/films have been successfully prepared by sol–gel method using templating approach which basically relies on supramolecular arrays and micellar organization formed by nonionic surfactant or block copolymer (BC) [10,11]. Polyethylene oxide–polypropylene oxide–polyethylene oxide (PEO–PPO–PEO) is one of the most widely used non-ionic surface active BC first introduced by BASF under the commercial name of Pluronics [12] as the template for the synthesis of ordered MTO. The structure of this copolymer consists of PEO oxide as the end blocks and PPO as the middle block. In case of low concentration solutions, surfactants will favor arrangement on the surface and unimers can be formed. The self assembly of BC chains in solutions can normally be initiated either by increase of concentration at a critical level known as CMC or by changing the temperature. In case of the latter, micelles do form at a critical micelle temperature (CMT) at fixed concentration. For a given system, the CMC and the CMT are interconnected and both affect remarkably the solution behavior of BC. In general, synthesis of MT in powder or thin film types by BC templating method consists of: (i) preparation of a stable sol, (ii) treatment of the deposited sol in a controlled humid atmosphere to induce self assembly referred as evaporation induced self assembly (EISA) process after which the surfactant micelles are arranged within the organic network [13] followed by (iii) drying and final heat-treatment processes to remove organics, stabilization of inorganic mesoporous channels and crystallization of titanium oxide. It is worth to mention that for some applications such as water treatment the utilization of ultra small nano-sized MT powder could impose both the problem of final separation from the reactor bed in use as well as environmental safety issues. However, uti-
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lizing a strategy to assemble MT particles as microspheres lends itself as a novel solution to solve the above mentioned drawbacks. Such a structure in case of having an interconnected pore channels could be of considerable interest as potential catalyst or sorbents considering the textural mesopores as well as the presence of interconnected pore systems of macrostructures. Various methods such as micro-molding using emulsion droplets, latex spheres and bacterial threads have been recently utilized to synthesize ordered inorganic structures with pore sizes in the range of micron to submicron [14–19]. Further, some groups [16–19] have also reported the synthesis of oxide materials with macro-mesoporous structures by combination of surfactant templating methods. Spray pyrolysis (SP) has been widely used to synthesize various kinds of oxide, non-oxide compounds and solid solutions of sphere-like morphology [20–25]. This is normally done by spraying jet of salt solution droplets into the hot zone of a pre-heated reactor forming solid/porous micro/nanospheres. This continuous synthesis method, compared to other powder preparation routes has several advantages such as: simplicity of manufacturing equipment, excellent control of chemical homogeneity/stoichiometry in multi component systems. The process consists of various steps: (i) generation of aerosol droplets from a liquid precursor by an appropriate aerosol generator, (ii) transfer of the produced droplets by a carrier gas during which solvent evaporation could take place followed by concomitant solute precipitation when the solubility limit is exceeded in the droplet (normally in case of a water based salt solution), (iii) decomposition and thermolysis of the precipitated particles in the hot zone of the reaction reactor to form micro/nano-sized particles, (iv) sintering of the formed particles to form hollow/dense particles and finally, collection of the produced particles. This preparation method has been widely utilized for the synthesis of single/multicomponent porous micro/nanospheres using ordinary metal salt solutions. However, most recently various researchers have reported the utilization of aerosol assisted self assembly for the synthesis of ordered/wormlike mesoporous silica particles of sphere morphology using BC [26–32]. Such a preparation strategy for the synthesis of mesoporous particles has several advantages compared to the conventional templating methods. These include (i) shorter production time, (ii) easier subsequent powder processing, (iii) possible formation of three-dimensional interconnected network of pores. Further, the mesoporous micro/nanoshperes formed by this process are also reported to incorporate rather high metal loading without much loss of surface area [26] which is desirable for catalyst applications. In this respect, Bore et al. [27] have shown that evaporation-induced self assembly of aerosols can be utilized for the synthesis of spherically ordered mesoporous microparticles from simple aqueous solution of tetraethoxysilane (TEOS) and cetyltrimethylammonium bromide (CTAB) as the surfactant. These authors confirmed the formation of hexagonally ordered particles with large surface areas (700–1300 m2 /g) and managed to grow Pt nanowires inside the hexagonally ordered pores. Kim et al. [28] also reported a saltassisted aerosol–gel approach to produce nano-sized mesoporous silica particles. They used sodium chloride both as an agent to accelerate the kinetics of silica gelation and simultaneously as a template. Most of the previous research work has focused on the synthesis and characterization of mesoporous silica of sphere-like morphology and less attention is paid to other compounds. In the work presented here, we report the successful synthesis of MT microspheres by spray pyrolysis method using a titania sol prepared by BC templating route. Further, the effects of fast drying and heat-treatment conditions induced by SP method on the degree of crystallization and mesostructure organization of the microspheres formed are also studied.
Fig. 1. Demonstration of titania stock solution preparation procedure. 2. Experimental procedures 2.1. Raw materials preparation Block copolymer HO (CH2 CH2 O)20 (CH2 CH(CH3 )O)70 (CH2 CH2 O)20 H (Mav = 5800, designated as EO20 PO70 EO20 , pluronic P123) was received from Aldrich. Titanium tertraisopropoxide (Ti (OCH(CH3 )2 )4 , TTIP), 2-propanol (CH3 )2 CHOH, IPA), acetyl acetone (CH3 COCH2 COCH3 , ACAC) and hydrochloric acid (HCl) were purchased from Merck. All chemicals were of analytical grade. 2.2. Sol synthesis Fig. 1 summarizes the flow chart of the synthesized sol. The initial sol was prepared at room temperature in the following way: appropriate amounts of P123 was initially dissolved in IPA and mixed with a precursor solution separately obtained by mixing TTIP, ACAC and IPA while stirring for 2 h. The prepared solution was then carefully hydrolyzed by addition of an already prepared dilute HCl solution followed by aging at 50 ◦ C in a sealed Teflon bottle for 2 weeks. The molar ratios of the chemical components used were: 0.002P123:42IPA:0.7ACAC:1H2 O:1Ti. The level of concentration of P123 BC in the initial sol was selected below critical micelle concentration (CMC) value above which micellization process takes place. The magnitude of this parameter determined by solubilization method [33] was 1.72 wt% of P123 solution in IPA. 2.3. Spray pyrolysis Fig. 2 displays the schematic set up used for the spray pyrolysis apparatus used for the synthesis of MTO powder. The procedure involved atomizing already pre-
Fig. 2. Schematic demonstration of the spray pyrolysis system used for the synthesis of MTO microspheres.
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pared Ti4+ -sol at a rate of 1 l/h to form droplets by a homemade nebulizer. The droplets was entrained in a gas stream to form an aerosol that was transported to the heat zone of the SP reactor’s furnace pre-heated at 800 ◦ C, 900 ◦ C and 1000 ◦ C to drive the required reactions and to form the final product. Further, the obtained powders were finally deposited at the surface of a filter by using a backing pump that could also guide the evolved gases to the exhaust of the system (Fig. 2). 2.4. Characterization Simultaneous thermal analysis (STA) on the as-prepared sol was conducted in air with a NETZSCH STA 409 PC system with a heating rate of 10 K/min. Spectroscopic analyses of the pyrolyzed samples were also performed using Fourier transform infrared (FTIR) spectrometer, SHIMADZU IR Solution. Nitrogen adsorption–desorption isotherms at 77 K were also obtained using a Micromeritics Tristar 3000 apparatus. The Brunauer–Emmet–Teller (BET), equation was used to calculate the specific surface area. Pore size distributions were obtained using the Barret–Joyner–Halenda (BJH) model in the range of mesopores [34]. Wide angle X-ray powder diffraction (XRD) patterns were also obtained on a Philips X’PERT MPD diffractometer using Cu-K␣1 radiation. The crystallite sizes of the different phases were determined from the full-width at half maximum (FWMH) of their corresponding strongest diffraction peaks using the Williamson–Hall method [35] after applying the standard correction for instrumental broadening. Microstructural investigation was also carried out by field emission scanning electron microscope FESEM.
3. Results and discussion
1 (0.886AA /AR + 1)
Fig. 3. DTA/TGA graph of the sol stock at a heating rate of 10 K/min.
Table 1 calculated weight percents (wt%) of anatase and rutile phases and their corresponding crystallite sizes. PS Temperature (◦ C)
Fig. 3 demonstrates STA analysis of the as-prepared sol. As can be realized from this figure, the gradual weight loss occurring between room temperature and up to 250 ◦ C is related to the evolution of the IPA as well as surface adsorbed/molecular water of the prepared sol. The observed small rate of weight loss could be possibly related to rather high heating rate (10 K/min) applied. Further, the observed sharp weight loss detected between ∼250 ◦ C and ∼310 ◦ C is related to the loss of the P123 BC as well as other organic materials. Moreover, the exothermic peak appeared at ∼338 ◦ C is possibly attributed to the crystallization of TiO2 phase. However, one could not completely rule out the contribution from the burning out of the decomposed organic residue as well. It is already shown that the crystallization of anatase and rutile phases starts at about 350 ◦ C and 600 ◦ C, respectively [36]. Fig. 4 displays the XRD multiplot patterns of the synthesized sol pyrolyzed at 800 ◦ C, 900 ◦ C and 1000 ◦ C in the used SP reactor (Fig. 2). As can be understood from this multiplot, the higher the hot zone temperature of the reactor, the higher the degree of crystallization of the samples as evidenced by higher intensities of their corresponding diffraction peaks (Fig. 4). For all of these pyrolyzed samples, the formation of anatase phase as the major phase along with small amounts of rutile phase could be confirmed (Fig. 4). The weight fraction of rutile/anatase phases were calculated from the integrated intensities of anatase (1 0 1), rutile(1 1 0) diffraction lines using the Eq. (1) [37]: Wrutile =
Fig. 4. XRD multiplot of the samples pyrolyzed at 800 ◦ C, 900 ◦ C and 1000 ◦ C.
(1)
900 1000
Anatase phase
Rutile phase
(wt%)
Crystallite size (nm)
(wt%)
Crystallite size (nm)
97 92
9 23
3 8
– –
where, AA and AR refer to the integrated intensities of anatase (1 0 1) and rutile(1 1 0) diffraction lines. Table 1 summarizes the calculated weight percent (wt%) of these two phases as well as their corresponding crystallite sizes of the samples pyrolyzed at 900 ◦ C and 1000 ◦ C. Titanium oxide can be mineralized in different crystallographic structures such as anatase, brookite and rutile. The stabilization of anatase phase at such high temperatures could be due to the short residence time of the particles in the hot zone area of the furnace used and the ultra small nano-sized crystallites formed for these samples. It is already shown that the surface energy associated to the different phases of TiO2 increases as: anatase < brookite < rutile [38]. In case of bulk titania although rutile is the most stable phase but for ultra small particles for which large amount of surface is present, anatase is expected to be stabilized [39]. Further, the higher level of rutile phase formed for the sample pyrolyzed at 1000 ◦ C (8 wt%) compared to that of the sample prepared at 900 ◦ C (3 wt%) could be mainly due to the higher temperature applied for this sample giving rise to the formation of larger crystallite sizes (Table 1). Moreover, it should be mentioned that XRD patterns of the samples pyrolyzed below 800 ◦ C did not confirm the formation of highly crystalline particles (not shown here) considering the overall short residence time (∼3–4 s) of the particles in SP reactor before accumulating on the surface of the filter employed at the end part of the reactor. Therefore, no attempt was made to prepare pyrolyzed samples below 800 ◦ C. Fig. 5 shows the typical SEM micrograph of the sample pyrolyzed at 1000 ◦ C highlighting the formation of nicely rounded microspheres. Fig. 6 also displays the micrograph of one single microsphere for improved clarity. As can be realized from this figure, the existence of large numbers of ultra small grains can be identified on the surface of this microsphere. During the pyrolysis process by passage of the as-prepared sol droplets through the first zone of the reactor furnace (drying zone) one can expect solvent and water evaporation from the surface of the sphere-like droplets. This could give rise to increased surfactant concentration gradient on the surface of the spheres formed. Further, the most outer surface of the droplets is assumed to have the
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Fig. 5. Typical FESEM micrograph of the sample pyrolyzed at 1000 ◦ C.
maximum BC concentration. This can also progressively increase in time [40]. Hence, by increasing the concentration of surfactant at the surface of the droplets the magnitude of the critical micelle concentration will initially exceed at the surface of the formed droplets. This eventually extends throughout the whole body of dried droplets by further progress of drying process giving rise to the formation of micelles and concurrent partitioning of the organic precursors into the micellar interiors and the inorganic precursors surrounding the micellar exteriors. On the other hand the rise of temperature would also maintain critical micelle temperature. The rise of surfactant concentration as well as temperature of the microspheres passing through the hot zone region of SP furnace is expected to induce further self assembly and organization of the micelles into liquid-crystalline mesophases as well. Moreover, one can not also ignore the existence of the liquid–vapor interface acting as nucleating surface giving rise to the formation of titaniasurfactant liquid-crystalline domains initially at the surface of the
Fig. 6. FESEM micrograph of one single microsphere obtained from the sample pyrolyzed at 1000 ◦ C.
droplets and their radial progressive growth towards the core of the formed particles [41]. The high curvature of the microspheres formed here by SP method can also possibly be expected to modify the general relationship between surfactant packing parameter and the formed mesostructure [42]. Fig. 7(a)–(c) also demonstrate the typical FESEM micrographs of the microsphere’s surface formed after pyrolyzing at 800 ◦ C at high magnifications. As can be noticed from these micrographs the formation of wormlike mesoporous channels on the surface of these microspheres could be clearly identified. The average widths of these channels were in the range of 7–8 nm (Fig. 7(c)). The formation of wormlike mesopores on the surface of the prepared titanium oxide microspheres rather than a highly organized cubic or hexagonal mesostructures is attributed to the lack of self-assembling processes.
Fig. 7. (a)–(c). Typical FESEM micrographs of the microsphere’s surface formed after pyrolyzing at 800 ◦ C at high magnifications. The formation of wormlike mesoporous channels on the surface of these microspheres can be clearly observed.
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Obtaining a high level of self assembly for MTO powders/thin films prepared by templating approach demands careful control of drying process as well as appropriate heat-treatment conditions. The former can be maintained by a fine tuning of EISA parameters [13]. This method is shown to be one of the most promising synthesis strategies for the synthesis of ordered mesoporous powders/thin films. In this process introduced by Brinker et al. [43] for mesoporous silica films, the preferential evaporation of alcohol increases the concentration of the sol containing non-volatile surfactant and inorganic species before reaching to the equilibrium in the normal atmosphere. For the preparation of ordered MT powder/films such a process is normally carried out at appropriate relative humidity (RH) and near room temperature [13]. It is worth to mention that, the formation of a well-defined mesoporous hybrid requires synchronized formation of micelles and their organization in a liquid crystal template through the condensation of the inorganic framework. In fact, depending on the control of various variables involved in EISA process, mesoporous titanium oxides ranging from fully disordered wormlike to highly ordered one [13] can be obtained. In fact, obtaining completely ordered particles demands the maintenance of a liquid or liquid-crystalline state throughout the whole stages of EISA process. Since, premature solidification would prevent orderly self assembly of the emerging titania-surfactant mesophase. Crepaldi et al. [13], have identified five different stages during dip coating process of MT films at different levels of relative humidity, based on in situ 2D small angle X-ray scattering (SAXS) and interferometery measurements: the first, fast thickness reduction of the coated films due to the evaporation of the alcohol-rich vapors from the sol, the second stage related to a slower evaporation process attributed to the exit of H2 O/HCl-rich vapors without any sign of mesophase formation for RH values ≥45% suggesting the diffusion of water from the atmosphere within the coating, the third region for which no further thickness reduction was noticed but SAXS patterns revealed the formation of a randomly oriented mesostructure domains confirming the formation of micelles of defined sizes. Further, the fourth region was related to the formation of micellar aggregates and their corresponding organization/alignment along the film/substrate and film/air interfaces and finally the fifth region was attributed to the departure of residual water and HCl giving rise to the stabilization and contraction of the structure formed. Moreover, it is also emphasized that the degree of order obtained by EISA method is also highly dependent on the applied conditions [13] such as level of RH, water content of the initial sol, level of acidity, template type/Ti ratio and temperature: completely non-organized MTO films for RH ≤15%, a disordered mesostructure at RH = 20% as well as cubic mesostructure with increasing organization as RH further increased was seen to form. It should be mentioned that although using acidic medium is necessary to quench condensation but it is not sufficient to permit the disorder-to-order transition. Moreover, it is suggested [13] that a substantial quantity of water must be present to guarantee the necessary fluidity facilitating the organization of the template in a liquid crystal phase. Two sources of the available water in the initial sol and air moisture during the EISA process could initiate hydrolyzation of the inorganic species, making them hydrophilic and hence improving their interactions with the hydrophilic portions of the BC. Therefore, the appropriate increase of the water content of the sol is expected to act in favor of condensation which of course needs to be kept under control in acidic medium. It should be noted that the full accomplishment of the EISA stages normally demands long periods of time (few days to weeks). Such a process could not be simply adopted in SP method used here for the synthesis of MTO microspheres.
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On the other hand, applying appropriate heat-treatment conditions is also important for the maintenance of the state of the order achieved by EISA process. It is well known that heat-treatment is one of the most efficient methods for the removal of the organic phase of the MTO prepared by templating method. During this process, simultaneous dehydration, decomposition and crystallization [44] of the initial titania sol can take place. As mentioned before, during the passage of the titania sol droplets through the initial zone of SP furnace, one can expect stabilization of the hybrid mesophase composed of micellar domains embedded within the stable amorphous inorganic domain. By further transfer of the microspheres to the zones of higher temperatures in SP reactor, the decomposition and removal of the organic micelles also can occur, leaving porosity in the particles formed. Here, it is very important that the elimination of micelles take place when the inorganic network is rigid enough. Otherwise, the collapse of the whole porous mesostructure could happen. Moreover, by further rise of the temperature of microspheres, crystallization and growth of the inorganic network would happen. Crystallization process could take place through nucleation and growth of crystallized regions referred as seeds. The latter process can be initiated by diffusive sintering if sufficiently high temperatures are applied. It should be noted that, of course, homogeneous nucleation would act in favor of high level of pore ordering upon crystallization. In this respect, one can assume that fast nucleation giving rise to formation of large numbers of homogeneously dispersed seeds within the inorganic network would be of interest. The subsequent growth of these areas into nanocrystals would happen by diffusion of the neighboring atoms towards the surface of the formed seeds on the surface of microspheres. The process, however, depending on the applied temperature and residence time of the microspheres passing SP furnace is expected to stop when the neighboring amorphous inorganic species is completely consumed. In such a stage, once the surface of thus formed nanoparticles at the surface of microspheres spatially reach together, they can form inter-particle grain boundaries and the pore surface is created when the interface reaches the void forming the pore surface. However, if the temperature of the hot zone area of SP furnace exceeds the optimal conditions of the nucleation and growth processes one can expect extended diffusive sintering which involves further growth of the formed nanoparticles and possible surface area reduction due to the blockage of the mesopores. The formation of rather large size crystallites (Table 1) for the samples prepared here could have possibly also inhibited mesopore ordering as well. Since it is already shown [38] that the formation of titania crystallites larger than the formed mesopore size destroys the ordering. Fig. 8 displays the FTIR multiplot spectra of the pure pluronic P123 block copolymer as well as those pyrolyzed at 800 ◦ C and 1000 ◦ C. As can be seen from these spectra, the absorption peaks in the region of ∼2850–2930 cm−1 and 1100 cm−1 are attributed to C–H and C–O–C groups of P123 pluronic BC, respectively. The absorption peaks in the range of 3000–3500 cm−1 and 1640 cm−1 are related to the surface absorbed/molecular OH groups. Further the peaks appearing in the range of 500–700 cm−1 have originated from the stretched modes of Ti–O bonds [45,46]. Moreover, comparison of these spectra highlights the fact that most of the absorption peaks related to the inorganic compounds have mainly vanished for the samples pyrolyzed at 1000 ◦ C. The magnitude of the surface area of the sample pyrolyzed at 800 ◦ C was 219 m2 /g. This was comparatively higher than that of other samples. The rather high surface area of this sample (pyrolyzed 800 ◦ C) suggests the existence of highly open mesoporous channels in the synthesized microspheres. The existence of high surface area as well as rather uniform mesoporous channels (∼7–8 nm) reveals the success of using SP technique for the
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nicely uniform wormlike channels of about ∼6–7 nm widths by spray pyrolysis technique using BC templating approach. Such a synthesis strategy was capable of assembly of mesoporous TiO2 as microspheres which could have various interesting applications. BET and BJH measurement results obtained by N2 gas adsorption analysis confirmed the existence of average pore size of ∼6 nm which was in close agreement with our FESEM results. Acknowledgements We would like to acknowledge, the support of Iran Nanotechnology Initiative Council (INIC) and the Research Institute of Petroleum Industry (RIPI) for their kind collaborations. Further, kind help of Mr. Azad from department of materials of Darmstadt University, Germany, for obtaining FESEM micrographs is acknowledged.
References
Fig. 8. FTIR multiplot spectra of various samples: pure pluronic P123 block copolymer (A), titania sol pyrolyzed at 800 ◦ C (C) and 1000 ◦ C (E).
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Fig. 9. N2 adsorption–desorption isotherms of the sample pyrolyzed at 800 ◦ C as well as its corresponding BJH pore size distribution (shown as inset).
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4. Conclusions
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Synthesis and characterization of mesoporous TiO2 assembled as microspheres Ali Beitollahi a,∗ , Amir Hossein Haj Daie a , Leyla Samie a,b , Mohammad Mehdi Akbarnejad b a b
Nanomaterial Research Group, Dept. of Metallurgy and Materials Eng., Iran University of Science and Technology (IUST), Narmak, Farjam, Tehran, Iran Research Institute of Petroleum Industries, Tehran, Iran
a r t i c l e
i n f o
Article history: Received 18 July 2009 Received in revised form 30 September 2009 Accepted 1 October 2009 Available online 9 October 2009 Keywords: Mesoporous TiO2 Spray pyrolysis Self assembly
a b s t r a c t In the work presented here, we report the successful synthesis of highly porous (219 m2 /g) mesostructured titania powder assembled as microspheres of uniform wormlike channels of ∼6–7 nm widths by spray pyrolysis of a titania sol prepared by templating approach using P123 pluronic block copolymer (BC). This method involves the preparation of a Ti4+ -sol containing titanium tertraisopropoxide (TTIP), pluronic BC P123, 2-propanol, acetyl acetonate (ACAC) and hydrochloric acid (HCl) as well as H2 O followed by spray pyrolysis of the obtained sol into the hot zone of a pre-heated reactor at 800 ◦ C, 900 ◦ C and 1000 ◦ C. For all of these pyrolyzed samples, the formation of anatase as the major phase along with rutile as the minor one could be confirmed. The appearance of wormlike mesopores rather than highly ordered cubic or hexagonal mesostructures is related to the lack of appropriate drying conditions normally applied to induce self assembly as well as formation of rather large titania crystallite sizes. © 2009 Elsevier B.V. All rights reserved.
1. Introduction In the last decade, special attention is paid to the development of innovative material preparation routes with the aim to synthesize well-defined nano-objects and nanostructures such as nanofibers, nanotubes, quantum wells and mesoporous powders. The latter is of both scientific and technological interest due to its capability of interacting with atoms, ions as well as molecules both at their large surfaces and throughout their bulk leading to new applications [1,2]. Nanostructured particles exhibiting well-defined pore sizes and pore connectivities (1, 2 or 3D) have various potential applications such as: catalyst, controlled drug delivery, low dielectric constant fillers as well as custom engineered pigments and optical hosts [3,4]. Since the discovery of ordered mesoporous silica-based materials [5] numerous studies have been focused on the synthesis of these materials and other non-silica systems [6]. In this respect, a wide range of mesostructures of different pore size/shape, connectivity and symmetry has been synthesized using different templating approach based on the hydrolysis and cross linking of inorganic precursors at the surfaces of supramolecular surfactant assemblies [7]. High surface area mesoporous titania (MT) due to its enormous applications in the field of photocatalyst, separation, photoelectronics and electromagnetism [8,9] have gathered wide interest.
∗ Corresponding author. Tel.: +98 21 77459151; fax: +98 21 77459151. E-mail address: [email protected] (A. Beitollahi). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.10.002
MT powders/films have been successfully prepared by sol–gel method using templating approach which basically relies on supramolecular arrays and micellar organization formed by nonionic surfactant or block copolymer (BC) [10,11]. Polyethylene oxide–polypropylene oxide–polyethylene oxide (PEO–PPO–PEO) is one of the most widely used non-ionic surface active BC first introduced by BASF under the commercial name of Pluronics [12] as the template for the synthesis of ordered MTO. The structure of this copolymer consists of PEO oxide as the end blocks and PPO as the middle block. In case of low concentration solutions, surfactants will favor arrangement on the surface and unimers can be formed. The self assembly of BC chains in solutions can normally be initiated either by increase of concentration at a critical level known as CMC or by changing the temperature. In case of the latter, micelles do form at a critical micelle temperature (CMT) at fixed concentration. For a given system, the CMC and the CMT are interconnected and both affect remarkably the solution behavior of BC. In general, synthesis of MT in powder or thin film types by BC templating method consists of: (i) preparation of a stable sol, (ii) treatment of the deposited sol in a controlled humid atmosphere to induce self assembly referred as evaporation induced self assembly (EISA) process after which the surfactant micelles are arranged within the organic network [13] followed by (iii) drying and final heat-treatment processes to remove organics, stabilization of inorganic mesoporous channels and crystallization of titanium oxide. It is worth to mention that for some applications such as water treatment the utilization of ultra small nano-sized MT powder could impose both the problem of final separation from the reactor bed in use as well as environmental safety issues. However, uti-
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lizing a strategy to assemble MT particles as microspheres lends itself as a novel solution to solve the above mentioned drawbacks. Such a structure in case of having an interconnected pore channels could be of considerable interest as potential catalyst or sorbents considering the textural mesopores as well as the presence of interconnected pore systems of macrostructures. Various methods such as micro-molding using emulsion droplets, latex spheres and bacterial threads have been recently utilized to synthesize ordered inorganic structures with pore sizes in the range of micron to submicron [14–19]. Further, some groups [16–19] have also reported the synthesis of oxide materials with macro-mesoporous structures by combination of surfactant templating methods. Spray pyrolysis (SP) has been widely used to synthesize various kinds of oxide, non-oxide compounds and solid solutions of sphere-like morphology [20–25]. This is normally done by spraying jet of salt solution droplets into the hot zone of a pre-heated reactor forming solid/porous micro/nanospheres. This continuous synthesis method, compared to other powder preparation routes has several advantages such as: simplicity of manufacturing equipment, excellent control of chemical homogeneity/stoichiometry in multi component systems. The process consists of various steps: (i) generation of aerosol droplets from a liquid precursor by an appropriate aerosol generator, (ii) transfer of the produced droplets by a carrier gas during which solvent evaporation could take place followed by concomitant solute precipitation when the solubility limit is exceeded in the droplet (normally in case of a water based salt solution), (iii) decomposition and thermolysis of the precipitated particles in the hot zone of the reaction reactor to form micro/nano-sized particles, (iv) sintering of the formed particles to form hollow/dense particles and finally, collection of the produced particles. This preparation method has been widely utilized for the synthesis of single/multicomponent porous micro/nanospheres using ordinary metal salt solutions. However, most recently various researchers have reported the utilization of aerosol assisted self assembly for the synthesis of ordered/wormlike mesoporous silica particles of sphere morphology using BC [26–32]. Such a preparation strategy for the synthesis of mesoporous particles has several advantages compared to the conventional templating methods. These include (i) shorter production time, (ii) easier subsequent powder processing, (iii) possible formation of three-dimensional interconnected network of pores. Further, the mesoporous micro/nanoshperes formed by this process are also reported to incorporate rather high metal loading without much loss of surface area [26] which is desirable for catalyst applications. In this respect, Bore et al. [27] have shown that evaporation-induced self assembly of aerosols can be utilized for the synthesis of spherically ordered mesoporous microparticles from simple aqueous solution of tetraethoxysilane (TEOS) and cetyltrimethylammonium bromide (CTAB) as the surfactant. These authors confirmed the formation of hexagonally ordered particles with large surface areas (700–1300 m2 /g) and managed to grow Pt nanowires inside the hexagonally ordered pores. Kim et al. [28] also reported a saltassisted aerosol–gel approach to produce nano-sized mesoporous silica particles. They used sodium chloride both as an agent to accelerate the kinetics of silica gelation and simultaneously as a template. Most of the previous research work has focused on the synthesis and characterization of mesoporous silica of sphere-like morphology and less attention is paid to other compounds. In the work presented here, we report the successful synthesis of MT microspheres by spray pyrolysis method using a titania sol prepared by BC templating route. Further, the effects of fast drying and heat-treatment conditions induced by SP method on the degree of crystallization and mesostructure organization of the microspheres formed are also studied.
Fig. 1. Demonstration of titania stock solution preparation procedure. 2. Experimental procedures 2.1. Raw materials preparation Block copolymer HO (CH2 CH2 O)20 (CH2 CH(CH3 )O)70 (CH2 CH2 O)20 H (Mav = 5800, designated as EO20 PO70 EO20 , pluronic P123) was received from Aldrich. Titanium tertraisopropoxide (Ti (OCH(CH3 )2 )4 , TTIP), 2-propanol (CH3 )2 CHOH, IPA), acetyl acetone (CH3 COCH2 COCH3 , ACAC) and hydrochloric acid (HCl) were purchased from Merck. All chemicals were of analytical grade. 2.2. Sol synthesis Fig. 1 summarizes the flow chart of the synthesized sol. The initial sol was prepared at room temperature in the following way: appropriate amounts of P123 was initially dissolved in IPA and mixed with a precursor solution separately obtained by mixing TTIP, ACAC and IPA while stirring for 2 h. The prepared solution was then carefully hydrolyzed by addition of an already prepared dilute HCl solution followed by aging at 50 ◦ C in a sealed Teflon bottle for 2 weeks. The molar ratios of the chemical components used were: 0.002P123:42IPA:0.7ACAC:1H2 O:1Ti. The level of concentration of P123 BC in the initial sol was selected below critical micelle concentration (CMC) value above which micellization process takes place. The magnitude of this parameter determined by solubilization method [33] was 1.72 wt% of P123 solution in IPA. 2.3. Spray pyrolysis Fig. 2 displays the schematic set up used for the spray pyrolysis apparatus used for the synthesis of MTO powder. The procedure involved atomizing already pre-
Fig. 2. Schematic demonstration of the spray pyrolysis system used for the synthesis of MTO microspheres.
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pared Ti4+ -sol at a rate of 1 l/h to form droplets by a homemade nebulizer. The droplets was entrained in a gas stream to form an aerosol that was transported to the heat zone of the SP reactor’s furnace pre-heated at 800 ◦ C, 900 ◦ C and 1000 ◦ C to drive the required reactions and to form the final product. Further, the obtained powders were finally deposited at the surface of a filter by using a backing pump that could also guide the evolved gases to the exhaust of the system (Fig. 2). 2.4. Characterization Simultaneous thermal analysis (STA) on the as-prepared sol was conducted in air with a NETZSCH STA 409 PC system with a heating rate of 10 K/min. Spectroscopic analyses of the pyrolyzed samples were also performed using Fourier transform infrared (FTIR) spectrometer, SHIMADZU IR Solution. Nitrogen adsorption–desorption isotherms at 77 K were also obtained using a Micromeritics Tristar 3000 apparatus. The Brunauer–Emmet–Teller (BET), equation was used to calculate the specific surface area. Pore size distributions were obtained using the Barret–Joyner–Halenda (BJH) model in the range of mesopores [34]. Wide angle X-ray powder diffraction (XRD) patterns were also obtained on a Philips X’PERT MPD diffractometer using Cu-K␣1 radiation. The crystallite sizes of the different phases were determined from the full-width at half maximum (FWMH) of their corresponding strongest diffraction peaks using the Williamson–Hall method [35] after applying the standard correction for instrumental broadening. Microstructural investigation was also carried out by field emission scanning electron microscope FESEM.
3. Results and discussion
1 (0.886AA /AR + 1)
Fig. 3. DTA/TGA graph of the sol stock at a heating rate of 10 K/min.
Table 1 calculated weight percents (wt%) of anatase and rutile phases and their corresponding crystallite sizes. PS Temperature (◦ C)
Fig. 3 demonstrates STA analysis of the as-prepared sol. As can be realized from this figure, the gradual weight loss occurring between room temperature and up to 250 ◦ C is related to the evolution of the IPA as well as surface adsorbed/molecular water of the prepared sol. The observed small rate of weight loss could be possibly related to rather high heating rate (10 K/min) applied. Further, the observed sharp weight loss detected between ∼250 ◦ C and ∼310 ◦ C is related to the loss of the P123 BC as well as other organic materials. Moreover, the exothermic peak appeared at ∼338 ◦ C is possibly attributed to the crystallization of TiO2 phase. However, one could not completely rule out the contribution from the burning out of the decomposed organic residue as well. It is already shown that the crystallization of anatase and rutile phases starts at about 350 ◦ C and 600 ◦ C, respectively [36]. Fig. 4 displays the XRD multiplot patterns of the synthesized sol pyrolyzed at 800 ◦ C, 900 ◦ C and 1000 ◦ C in the used SP reactor (Fig. 2). As can be understood from this multiplot, the higher the hot zone temperature of the reactor, the higher the degree of crystallization of the samples as evidenced by higher intensities of their corresponding diffraction peaks (Fig. 4). For all of these pyrolyzed samples, the formation of anatase phase as the major phase along with small amounts of rutile phase could be confirmed (Fig. 4). The weight fraction of rutile/anatase phases were calculated from the integrated intensities of anatase (1 0 1), rutile(1 1 0) diffraction lines using the Eq. (1) [37]: Wrutile =
Fig. 4. XRD multiplot of the samples pyrolyzed at 800 ◦ C, 900 ◦ C and 1000 ◦ C.
(1)
900 1000
Anatase phase
Rutile phase
(wt%)
Crystallite size (nm)
(wt%)
Crystallite size (nm)
97 92
9 23
3 8
– –
where, AA and AR refer to the integrated intensities of anatase (1 0 1) and rutile(1 1 0) diffraction lines. Table 1 summarizes the calculated weight percent (wt%) of these two phases as well as their corresponding crystallite sizes of the samples pyrolyzed at 900 ◦ C and 1000 ◦ C. Titanium oxide can be mineralized in different crystallographic structures such as anatase, brookite and rutile. The stabilization of anatase phase at such high temperatures could be due to the short residence time of the particles in the hot zone area of the furnace used and the ultra small nano-sized crystallites formed for these samples. It is already shown that the surface energy associated to the different phases of TiO2 increases as: anatase < brookite < rutile [38]. In case of bulk titania although rutile is the most stable phase but for ultra small particles for which large amount of surface is present, anatase is expected to be stabilized [39]. Further, the higher level of rutile phase formed for the sample pyrolyzed at 1000 ◦ C (8 wt%) compared to that of the sample prepared at 900 ◦ C (3 wt%) could be mainly due to the higher temperature applied for this sample giving rise to the formation of larger crystallite sizes (Table 1). Moreover, it should be mentioned that XRD patterns of the samples pyrolyzed below 800 ◦ C did not confirm the formation of highly crystalline particles (not shown here) considering the overall short residence time (∼3–4 s) of the particles in SP reactor before accumulating on the surface of the filter employed at the end part of the reactor. Therefore, no attempt was made to prepare pyrolyzed samples below 800 ◦ C. Fig. 5 shows the typical SEM micrograph of the sample pyrolyzed at 1000 ◦ C highlighting the formation of nicely rounded microspheres. Fig. 6 also displays the micrograph of one single microsphere for improved clarity. As can be realized from this figure, the existence of large numbers of ultra small grains can be identified on the surface of this microsphere. During the pyrolysis process by passage of the as-prepared sol droplets through the first zone of the reactor furnace (drying zone) one can expect solvent and water evaporation from the surface of the sphere-like droplets. This could give rise to increased surfactant concentration gradient on the surface of the spheres formed. Further, the most outer surface of the droplets is assumed to have the
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Fig. 5. Typical FESEM micrograph of the sample pyrolyzed at 1000 ◦ C.
maximum BC concentration. This can also progressively increase in time [40]. Hence, by increasing the concentration of surfactant at the surface of the droplets the magnitude of the critical micelle concentration will initially exceed at the surface of the formed droplets. This eventually extends throughout the whole body of dried droplets by further progress of drying process giving rise to the formation of micelles and concurrent partitioning of the organic precursors into the micellar interiors and the inorganic precursors surrounding the micellar exteriors. On the other hand the rise of temperature would also maintain critical micelle temperature. The rise of surfactant concentration as well as temperature of the microspheres passing through the hot zone region of SP furnace is expected to induce further self assembly and organization of the micelles into liquid-crystalline mesophases as well. Moreover, one can not also ignore the existence of the liquid–vapor interface acting as nucleating surface giving rise to the formation of titaniasurfactant liquid-crystalline domains initially at the surface of the
Fig. 6. FESEM micrograph of one single microsphere obtained from the sample pyrolyzed at 1000 ◦ C.
droplets and their radial progressive growth towards the core of the formed particles [41]. The high curvature of the microspheres formed here by SP method can also possibly be expected to modify the general relationship between surfactant packing parameter and the formed mesostructure [42]. Fig. 7(a)–(c) also demonstrate the typical FESEM micrographs of the microsphere’s surface formed after pyrolyzing at 800 ◦ C at high magnifications. As can be noticed from these micrographs the formation of wormlike mesoporous channels on the surface of these microspheres could be clearly identified. The average widths of these channels were in the range of 7–8 nm (Fig. 7(c)). The formation of wormlike mesopores on the surface of the prepared titanium oxide microspheres rather than a highly organized cubic or hexagonal mesostructures is attributed to the lack of self-assembling processes.
Fig. 7. (a)–(c). Typical FESEM micrographs of the microsphere’s surface formed after pyrolyzing at 800 ◦ C at high magnifications. The formation of wormlike mesoporous channels on the surface of these microspheres can be clearly observed.
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Obtaining a high level of self assembly for MTO powders/thin films prepared by templating approach demands careful control of drying process as well as appropriate heat-treatment conditions. The former can be maintained by a fine tuning of EISA parameters [13]. This method is shown to be one of the most promising synthesis strategies for the synthesis of ordered mesoporous powders/thin films. In this process introduced by Brinker et al. [43] for mesoporous silica films, the preferential evaporation of alcohol increases the concentration of the sol containing non-volatile surfactant and inorganic species before reaching to the equilibrium in the normal atmosphere. For the preparation of ordered MT powder/films such a process is normally carried out at appropriate relative humidity (RH) and near room temperature [13]. It is worth to mention that, the formation of a well-defined mesoporous hybrid requires synchronized formation of micelles and their organization in a liquid crystal template through the condensation of the inorganic framework. In fact, depending on the control of various variables involved in EISA process, mesoporous titanium oxides ranging from fully disordered wormlike to highly ordered one [13] can be obtained. In fact, obtaining completely ordered particles demands the maintenance of a liquid or liquid-crystalline state throughout the whole stages of EISA process. Since, premature solidification would prevent orderly self assembly of the emerging titania-surfactant mesophase. Crepaldi et al. [13], have identified five different stages during dip coating process of MT films at different levels of relative humidity, based on in situ 2D small angle X-ray scattering (SAXS) and interferometery measurements: the first, fast thickness reduction of the coated films due to the evaporation of the alcohol-rich vapors from the sol, the second stage related to a slower evaporation process attributed to the exit of H2 O/HCl-rich vapors without any sign of mesophase formation for RH values ≥45% suggesting the diffusion of water from the atmosphere within the coating, the third region for which no further thickness reduction was noticed but SAXS patterns revealed the formation of a randomly oriented mesostructure domains confirming the formation of micelles of defined sizes. Further, the fourth region was related to the formation of micellar aggregates and their corresponding organization/alignment along the film/substrate and film/air interfaces and finally the fifth region was attributed to the departure of residual water and HCl giving rise to the stabilization and contraction of the structure formed. Moreover, it is also emphasized that the degree of order obtained by EISA method is also highly dependent on the applied conditions [13] such as level of RH, water content of the initial sol, level of acidity, template type/Ti ratio and temperature: completely non-organized MTO films for RH ≤15%, a disordered mesostructure at RH = 20% as well as cubic mesostructure with increasing organization as RH further increased was seen to form. It should be mentioned that although using acidic medium is necessary to quench condensation but it is not sufficient to permit the disorder-to-order transition. Moreover, it is suggested [13] that a substantial quantity of water must be present to guarantee the necessary fluidity facilitating the organization of the template in a liquid crystal phase. Two sources of the available water in the initial sol and air moisture during the EISA process could initiate hydrolyzation of the inorganic species, making them hydrophilic and hence improving their interactions with the hydrophilic portions of the BC. Therefore, the appropriate increase of the water content of the sol is expected to act in favor of condensation which of course needs to be kept under control in acidic medium. It should be noted that the full accomplishment of the EISA stages normally demands long periods of time (few days to weeks). Such a process could not be simply adopted in SP method used here for the synthesis of MTO microspheres.
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On the other hand, applying appropriate heat-treatment conditions is also important for the maintenance of the state of the order achieved by EISA process. It is well known that heat-treatment is one of the most efficient methods for the removal of the organic phase of the MTO prepared by templating method. During this process, simultaneous dehydration, decomposition and crystallization [44] of the initial titania sol can take place. As mentioned before, during the passage of the titania sol droplets through the initial zone of SP furnace, one can expect stabilization of the hybrid mesophase composed of micellar domains embedded within the stable amorphous inorganic domain. By further transfer of the microspheres to the zones of higher temperatures in SP reactor, the decomposition and removal of the organic micelles also can occur, leaving porosity in the particles formed. Here, it is very important that the elimination of micelles take place when the inorganic network is rigid enough. Otherwise, the collapse of the whole porous mesostructure could happen. Moreover, by further rise of the temperature of microspheres, crystallization and growth of the inorganic network would happen. Crystallization process could take place through nucleation and growth of crystallized regions referred as seeds. The latter process can be initiated by diffusive sintering if sufficiently high temperatures are applied. It should be noted that, of course, homogeneous nucleation would act in favor of high level of pore ordering upon crystallization. In this respect, one can assume that fast nucleation giving rise to formation of large numbers of homogeneously dispersed seeds within the inorganic network would be of interest. The subsequent growth of these areas into nanocrystals would happen by diffusion of the neighboring atoms towards the surface of the formed seeds on the surface of microspheres. The process, however, depending on the applied temperature and residence time of the microspheres passing SP furnace is expected to stop when the neighboring amorphous inorganic species is completely consumed. In such a stage, once the surface of thus formed nanoparticles at the surface of microspheres spatially reach together, they can form inter-particle grain boundaries and the pore surface is created when the interface reaches the void forming the pore surface. However, if the temperature of the hot zone area of SP furnace exceeds the optimal conditions of the nucleation and growth processes one can expect extended diffusive sintering which involves further growth of the formed nanoparticles and possible surface area reduction due to the blockage of the mesopores. The formation of rather large size crystallites (Table 1) for the samples prepared here could have possibly also inhibited mesopore ordering as well. Since it is already shown [38] that the formation of titania crystallites larger than the formed mesopore size destroys the ordering. Fig. 8 displays the FTIR multiplot spectra of the pure pluronic P123 block copolymer as well as those pyrolyzed at 800 ◦ C and 1000 ◦ C. As can be seen from these spectra, the absorption peaks in the region of ∼2850–2930 cm−1 and 1100 cm−1 are attributed to C–H and C–O–C groups of P123 pluronic BC, respectively. The absorption peaks in the range of 3000–3500 cm−1 and 1640 cm−1 are related to the surface absorbed/molecular OH groups. Further the peaks appearing in the range of 500–700 cm−1 have originated from the stretched modes of Ti–O bonds [45,46]. Moreover, comparison of these spectra highlights the fact that most of the absorption peaks related to the inorganic compounds have mainly vanished for the samples pyrolyzed at 1000 ◦ C. The magnitude of the surface area of the sample pyrolyzed at 800 ◦ C was 219 m2 /g. This was comparatively higher than that of other samples. The rather high surface area of this sample (pyrolyzed 800 ◦ C) suggests the existence of highly open mesoporous channels in the synthesized microspheres. The existence of high surface area as well as rather uniform mesoporous channels (∼7–8 nm) reveals the success of using SP technique for the
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nicely uniform wormlike channels of about ∼6–7 nm widths by spray pyrolysis technique using BC templating approach. Such a synthesis strategy was capable of assembly of mesoporous TiO2 as microspheres which could have various interesting applications. BET and BJH measurement results obtained by N2 gas adsorption analysis confirmed the existence of average pore size of ∼6 nm which was in close agreement with our FESEM results. Acknowledgements We would like to acknowledge, the support of Iran Nanotechnology Initiative Council (INIC) and the Research Institute of Petroleum Industry (RIPI) for their kind collaborations. Further, kind help of Mr. Azad from department of materials of Darmstadt University, Germany, for obtaining FESEM micrographs is acknowledged.
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Fig. 8. FTIR multiplot spectra of various samples: pure pluronic P123 block copolymer (A), titania sol pyrolyzed at 800 ◦ C (C) and 1000 ◦ C (E).
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
Fig. 9. N2 adsorption–desorption isotherms of the sample pyrolyzed at 800 ◦ C as well as its corresponding BJH pore size distribution (shown as inset).
production of MT microspheres. Further, Fig. 9 shows the N2 adsorption–desorption isotherms of the sample pyrolyzed at 800 ◦ C and its corresponding BJH pore size distribution (shown as inset in this figure). As can be realized from these figures the existence of a type IV gas adsorption isotherm normally appearing for materials containing large mesoporous channels [47] could be confirmed. The hysteresis loops of this sample represent more H1 type characteristics than H2 type suggesting the existence of rather uniform mesopores. The average pore diameter calculated by BJH model for this sample was ∼6 nm which closely matches with the results obtained by FESEM results.
[22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36]
4. Conclusions
[37] [38] [39]
In summary, we report successful synthesis of mesoporous titania microspheres of rather high surface area (219 m2 /g) with
[40]
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