Microporous and Mesoporous Materials 139 (2011) 87–93

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Microporous and Mesoporous Materials

journal homepage: www.elsevier .com/locate /micromeso

A nanoparticle assembly method for the production of crystalline orderedmesoporous titanium oxide/carbon composites

Dan Liu a, Jia-heng Lei a,⇑, Li-ping Guo a, Ke-jian Deng b

a Department of Chemistry, School of Science, Wuhan University of Technology, Wuhan 430070, PR Chinab Key Laboratory of Catalysis and Materials Science of the State Ethnic Affairs Commission & Ministry of Education, South-Central University for Nationalities,Wuhan, 430070, PR China

a r t i c l e i n f o

Article history:Received 18 August 2010Received in revised form 14 October 2010Accepted 14 October 2010Available online 21 October 2010

Keywords:Nanoparticle assemblyOrdered mesoporous carbonsTitanium oxidesComposites

1387-1811/$ - see front matter � 2010 Elsevier Inc. Adoi:10.1016/j.micromeso.2010.10.023

⇑ Corresponding author. Tel./fax: +86 27 87756662E-mail addresses: daniellliu@whut.edu.cn, daniellli

a b s t r a c t

We demonstrate a simple and reproducible method to prepare thermally stable, crystalline mesoporousTi oxide/carbon composites. The composites were obtained using titania nanoparticles as inorganic pre-cursors, phenolic resols as carbon sources, and triblock copolymer F127 as a template based on the sol-vent evaporation-induced self-assembly process. The use of stable titania nanoparticles favored thereproducible preparation of mesoporous Ti–C composites over a wide range of Ti oxide/carbon ratioswithout the control of atmospheric humidity. Various analysis techniques have been used to investigatethe pore structure and crystallinity as a function of synthesis conditions. The resultant composites havean ordered 2D hexagonal mesostructure with high surface areas (200–800 m2 g�1). In addition to the cal-cination temperature, the carbon content also has significant effect on the crystalline transformation oftitanium species.

� 2010 Elsevier Inc. All rights reserved.

1. Introduction

In recent years, there has been increasing interest in titania/car-bon composites. The carbon-coated titania and titania-mountedactivated carbon has been prepared with the advantage of couplingthe photoactivity of anatase-type TiO2 with the adsorptivity ofactivated carbon [1,2]. Other studies have also demonstrated thatcomposition of titania and carbon could significantly improve theelectrochemical performances as Li-intercalation electrode materi-als [3–6]. The core–shell TiO2@carbon nanostructure supported Ptcatalysts showed improved catalytic activity and stability formethanol electrooxidation compared with those of conventionalcarbon supported Pt catalysts [7]. Three-dimensionally orderedmacroporous (3DOM) carbon/titania composites have been pre-pared by colloidal crystal templating method [8,9].

The synthesis of mesoporous titania/carbon composites hasbeen attempted to simultaneously achieve high crystallinity andthermal stability of titania frameworks with regular pore structure.The mesoporous titania/carbon composites with worm-like meso-structures have been prepared through self-assembly of blockterpolymers with titania sols, and the block terpolymers simulta-neously serves as templates and carbon sources [10]. Recently,Zhao’s group has successfully prepared ordered mesoporous tita-nia/carbon composites by triconstituent co-assembly followed by

ll rights reserved.

.u@163.com (J.-h. Lei).

the in situ crystallization technology [11]. The carbon componentin the titanium-based nanostructures can serve as a stabilizer,which can form a glass-like network to limit the growth of titaniananocrystals and retard the collapsing of ordered mesostructuresduring amorphous to crystalline transformation. However, the syn-thesis is in a humidity-controlled environment, which might neg-atively affect the reproducibility [12]. An underlying reason isthat the titania sols prepared by acid hydrolysis of TiCl4 were notso much stable in the solvent evaporation-induced self-assembly(EISA) process, especially when the phenolic resols as carbonsources were introduced into the reaction systems. The samegroup recently modified this approach to obtain mesoporous car-bon/anatase composites with high titania contents by the use ofacid–base pair (TiCl4 and Ti(OC4H7)4) as titania precursors insteadof single source [12]. It was shown that the humidity control wasunnecessary due to the presence of the relatively stable titania solsprepared using acid–base pair. However, the titania contents in thecomposites can only be tuned in a relatively narrow range.

Herein, we introduce a reproducible method to fabricate ther-mally stable mesoporous Ti–C composites based on EISA process.Although EISA pathway is a powerful tool for the creation of or-dered mesoporous materials, its reproducibility in obtaininghigh-quality transition-metal-based materials, especially titania,is still a challenge due to the highly hydrolytic reactivity of metaloxide precursors (generally chlorides or alkoxides) [13–15]. Theformation is kinetically controlled and greatly influenced by theatmospheric conditions of the laboratory such as relative humidity,

88 D. Liu et al. / Microporous and Mesoporous Materials 139 (2011) 87–93

which can be varied from laboratory to laboratory [13]. This prob-lem might be partially resolved by using preformed nanoclustersas starting units due to their less reactivity than molecular precur-sors [15]. An alternative route is to employ the amorphous or crys-talline nanoparticles with the scale of a few nanometers asbuilding blocks [16,17]. In this work, the nanoparticulate titaniaprecursor has been utilized to avoid introducing the hydrolysis ofTiCl4 into the system, which makes the process simpler and morereproducible and can favor the stabilization of the synthesis sys-tem in triconstituent co-assembly process. Ti species/carbon ratiosin the composites can be tuned freely by varying the initial ratios ofreactants. The resultant composites exhibit an ordered 2D hexago-nal mesostructures with high surface areas (200–800 m2 g�1).

2. Experimental

2.1. Chemicals

Triblock poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) copolymers Pluronic F127 (EO106PO70EO106,Mav = 12,600) was purchased from Sigma–Aldrich Corp. Tetrabutyltitanate (Ti(OBu)4), phenol, formalin aqueous solution (37 wt.%),NaOH, conc. HCl (36 wt.%), acetic acid (HAc) and ethanol were pur-chased from Sinopharm Chemical Reagent Corp. All chemicalswere analytical grade and used as received without furtherpurification.

2.2. Synthesis

A 20 wt.% ethanolic solution of phenol/formaldehyde resols wasfirst synthesized according to the literature method [18,19], driedwith anhydrous MgSO4 and stocked in a refrigerator for the follow-ing usage. Titania nanoparticle sols were prepared in a HAc, HCland ethanol solution (AcHE) following a procedure by Fan et al.[20]. For a typical synthesis, 3.5 g of Ti(OBu)4 (10 mmol) was addedto a clear solution containing 2.0 mL of conc. HCl (24 mmol), 2.4 gof HAc (40 mmol), and 2.6 g of F127 in 23 g of ethanol. After themixture was stirred vigorously for 1 h at room temperature, a sta-bilized titania nanoparticle solution with particle size of about3.5 nm were obtained. Next, 5 g of 20 wt.% resols’ ethanolic solu-tion was added, and the solution rapidly became orange–red. Afterthe following stirring for 10 min, the homogeneous solution wastransferred into several open Petri dishes (diameter 200 mm).The ethanol was evaporated at 40 �C in an oven without additionalhumidity control. After 12 h, a membrane was produced on thedish, and then it was cured at 100 �C for another 24 h. The mem-brane was scraped from the dishes, and ground into powders.The as-made products were calcined at different temperature(350–1000 �C) under a nitrogen flow (100 mL min�1) for 3 h. Be-fore reaching the final temperature, furnace temperature was firstheld at 350 �C for 3 h to pyrolyze the templating agent (F127). Theramping rate was fixed at 1 �C min�1. The resultant products werelabeled as MCT-2-Y, where Y represents for the final calcinationtemperature. The as-made product was labeled as MCT-2-as.

Table 1Synthesis conditions and structural properties of the composites.

Sample Ti(OBu)4 (g) Resol (g) F127 (g) Carbon content (%)

MCT-1-600 3.50 0.50 2.10 30MCT-2-600 3.50 1.00 2.60 44MCT-3-600 1.75 1.00 1.80 60MCT-4-600 1.75 2.00 2.80 69MCT-4-1000 1.75 2.00 2.80 40

The other samples with different Ti–C compositions could alsobeen synthesized by adjusting the mass ratios of Ti(OBu)4 to resols.The initial Ti(OBu)4/resol/F127 compositions were listed in Table 1.In order to remain the constant titania particle size in the initialsols, the molar ratio of Ti(OBu)4/HCl/HAc/H2O/EtOH was fixed at10/24/40/82/500. The adding amount of F127 matched along withthat of Ti(OBu)4 and resols (i.e., 3.5 g of Ti(OBu)4 corresponding to1.6 g of F127; and 1 g of resols corresponding to 1 g of F127).

2.3. Characterization

Powder X-ray diffraction (XRD) patterns were recorded on aRigaku D/MAX-RB diffractometer with a CuRa radiation operatingat 40 kV, 50 mA. Transmission electron microscopy (TEM) imageswere taken with a JEM 2100F electron microscope operating at200 kV. Nitrogen adsorption–desorption data were measured witha Quantachrome Autosorb-1 analyzer at �196 �C. Prior to the mea-surement, the samples were first degassed at 200 �C for at least 6 h.The surface areas were calculated by the Brunauer–Emmett–Teller(BET) method. The pore size distributions were derived from theadsorption branches of the isotherms using the Barrett–Joyner–Halenda (BJH) model. The micropore surface areas were calculatedfrom the V–t plot method. The t values were calculated as a func-tion of the relative pressure (P/P0) ranging from 0.05 to 0.35 usingthe de Bore equation, t (Å) = [13.99/(log (p0/p) + 0.0340)]1/2. Ther-mogravimetric (TG) analyses were carried out on a SDT Q600V5.0 Build 63 thermal analyzer with a heating speed of 5 �C min�1

and in a nitrogen or air flow of 100 mL min�1. X-ray photoelectronspectroscopy (XPS) data were collected using a Kratos XSAM800spectrometer.

3. Results and discussion

3.1. EISA syntheses of ordered mesoporous Ti–C composites

The EISA method has been employed to prepare a series of crys-talline ordered mesoporous Ti–C composites with various Ti/C con-tents. For the fabrication of multi-component mesostructuredhybrids, the obtaining of stable multi-constituent precursor solsis a key issue. In the work presented here, the stable titania nano-particles with particle size of several nanometers were chosen.Nanoparticle sols were prepared in a HAc, HCl and ethanol solution(AcHE), which have been fully featured in the previous work [20].The titania nanoparticles can form rapidly in AcHE solution andthen grow quite slowly to avoid macro-scale precipitates. Ourexperiments showed that when low-molecular-weight phenolicresins (resols) as carbon sources were introduced into the AcHEsolution of titania nanoparticles, the system could still remain sta-ble for a few hours. It allows the formation of homogeneous mes-ostructured composites by a relatively slow EISA process withoutthe control of humidity. Another advantage of the strategy is thatthe ratio of the titania nanoparticles and resols could be tunedfreely without the occurrence of macroscopic phase separation.Ordered mesoporous titanium oxide/carbon composites were

BET surface area(m2 g�1)

Total pore volume(cm3 g�1)

Pore size(nm)

Microporesurface area (m2 g�1)

237.6 0.17 3.16 48.1388.7 0.26 3.06 112.9307.2 0.24 3.86 88.2405.7 0.32 3.84 98.5787.9 0.73 4.88 207.9

2θθ (degree)0 1 2 3 4 5

hkl d /nm

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MCT-2-350

b

Fig. 2. (a) SAXRD patterns and (b) WAXRD patterns of the MCT-2-Y compositescalcined in N2 at different temperature (A, anatase; R, rutile).

D. Liu et al. / Microporous and Mesoporous Materials 139 (2011) 87–93 89

prepared from the co-assembly of performed titania nanoparticles,resols and copolymers F127, followed by thermal treatment in in-ert gas atmosphere. Here, we take the sample MCT-2-Y as anexample, which was synthesized with the initial mass compositionof Ti(OBu)4/resol = 3.5/1. The normal photographic image of uncal-cined mesostructured titania/polymer MCT-2-as (Fig. 1) showsthat it is an orange–red, transparent and flexible membrane, indi-cating that the homogeneous composite was formed withoutmacro-phase separation.

Evidence for the formation of ordered mesostructures is pro-vided by small-angle X-ray diffraction (SAXRD) patterns shownin Fig. 2a. The as-made sample shows only one weak diffractionpeak around 2h = 0.77�. After the removal of template F127 at350 �C in N2 flow, a nine times as strong peak and other two weakpeaks appear, due to the enhanced contrast in the electron densityfrom the matrixes [21]. These diffraction peaks, respectively corre-sponding to d-spacing of 10.38, 6.00 and 5.21 nm, can be indexedas (1 0 0), (1 1 0) and (2 0 0) reflections from 2D hexagonal p6mmsymmetry [22]. With further increasing temperature to 500 �C,the (1 0 0) peak shifts to larger 2h angle (1.02�) and the unit cellparameter (a0 = 2d100/

p3) is reduced from 11.98 to 9.99 nm. It rep-

resents a significant shrinkage (ca. 16.7%) of the framework, asso-ciated with the pyrolysis and carbonization of the bakeliteframework [18]. When the heating temperature is raised to 600and 700 �C, the strong (1 0 0) peaks can still be clearly observed,reflecting that the mesoscopic ordering is preserved. However,the (1 0 0) peak broadens and other two peaks become less-resolved, suggesting the gradual degradation of ordered meso-structures, mainly caused by the growth and crystallization ofthe titania nanocrystals in the pore walls (see below). Additionally,only slight lattice shrinkage (ca. 2.8%) is observed from 500 to700 �C. It indicates that Ti doping retards the contraction of carbonframeworks during the carbonization as compared with the puremesostructured bakelites [19]. After calcination at 900 �C, no SAX-RD diffraction peak can be detected, suggesting the complete dis-tortion of the pore regularity.

Wide-angle X-ray diffraction (WAXRD) was used to investigatethe changes of phase structure and crystallite sizes of the titaniumspecies with calcination temperature. The mesoporous compositeMCT-2-350 calcined at 350 �C shows amorphous phase (Fig. 2b).By 600 �C, five broad diffraction peaks are readily observed, whichcan be indexed as (1 0 1), (0 0 4), (2 0 0), (1 0 5), (2 0 4) reflectionsof anatase phase (JCPDS No. 21-1272). The broadening of the dif-fraction peaks is attributed to the small crystallite size or weakcrystallization of the sample [23]. Estimated from the (1 0 1) planeof anatase with Scherrer equation, the average crystallite size is4.6 nm. The single phase of anatase can be kept even after the heattreatment at 700 �C, with a slight growth of the crystallite size(4.8 nm). At 900 �C, the phase transformation from anatase to ru-tile (JCPDS No. 86-0148) occurs, but the anatase phase is still the

Fig. 1. Photographic image of the uncalcined MCT-2-as membrane scraped from aPetri dish.

predominant phase. Even by 1000 �C, the anatase phase still exists.Generally, pure anatase is transformed into rutile upon thermaltreatment at ca. 700 �C [1]. These differences are associated withthe interfaces between carbonized bakelite and titania in the com-posites, and the interfaces retard the crystallization and growth oftitania by limiting mass diffusion, similar to previously reportedtitania composites [24].

TEM image of MCT-2-600 with electron beam perpendicular topore channels ([1 1 0] direction) shows well-ordered hexagonalchannel arrangement in large domain (Fig. 3a), corresponding tothe SAXRD pattern. High-resolution TEM (HRTEM) investigationsreveal that randomly oriented anatase nanocrystals with well-defined lattice planes are embedded in amorphous carbon walls(Fig. 3b, c and d). The size of the nanocrystals is ca. 3–5 nm, whichis in good accordance with the average crystallite size calculatedfrom the WAXRD data.

3.2. Tuning the composition and crystalline phase in mesoporous Ti–Ccomposites

The Ti contents in the mesoporous composites can be adjustedby varying the initial mass ratios of Ti(OBu)4 to resols. Fig. 4 showsTG curves carried out under an air atmosphere for the calcinedcomposites with different carbon contents. All of the compositesshow a first weight loss of ca. 6 wt.% at the temperature lower than150 �C, mainly attributed to desorption of physically adsorbedwater. The significant weight loss in the temperature of 300–450 �C is due to the combustion of carbonized organic constituentswith residue to inorganic titanium species. Thus the carbon con-tents of the four samples calcined at 600 �C are ca. 30, 44, 60 and69 wt.% for MCT-1, MCT-2 MCT-3, and MCT-4, respectively.

The SAXRD patterns of the four samples calcined at 600 �C areshown in Fig. 5a. All of the samples display a diffraction peak atca. 2h = 1�, revealing stable 2D hexagonal mesostructures withsimilar unit cell parameters of ca. 10 nm. Furthermore, the samplesexhibit intensity-increasing and narrowing diffraction peak withincreasing carbon contents. WAXRD patterns show co-existenceof anatase and rutile phase for MCT-1-600 (Fig. 5b), but only ana-tase phase can be observed for MCT-2-600. When carbon contentsare further increased (MCT-3-600 and MCT-4-600), only two broaddiffraction peak at ca. 2h = 23 and 44� is observed, corresponding tothe (0 0 2) and (1 0 1) reflections of amorphous carbon frameworks,

Fig. 3. (a) TEM image and (b) HRTEM image of the MCT-2-Y composites calcined at600 �C in N2 viewed along the [1 1 0] direction. (c) and (d) is the enlarged image oftwo selected areas in (b).

0 100 200 300 400 500 600 700

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)

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MCT-1-600

Fig. 4. TG curves for calcined mesoporous titania/carbon composites under an airatmosphere.

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Fig. 5. SAXRD patterns (a, c) and WAXRD (b, d) patterns of the composites calcinedat 600 �C (a and b) and 1000 �C (c and d) in N2 (R, rutile).

90 D. Liu et al. / Microporous and Mesoporous Materials 139 (2011) 87–93

respectively [25,26], which implies the titanium species in both thecomposites are non-crystalline or only a small amount of TiO2 hascrystallized by calcination at 600 �C for 3 h. These results showsharp contrast to the thermal behavior of pure mesoporous TiO2,whose ordered mesostructure often collapses alongside the crys-tallization/grain growth of titania after calcination over 400 �C[13,27]. Therefore, it is concluded that the carbon componentcauses the suppression of phase transformation from amorphous

TiO2 to anatase and retards the collapsing of ordered mesostruc-tures. Like MCT-2-1000, WAXRD pattern of MCT-1-1000 showsthe co-existence of anatase and rutile phase (Fig. 5d). Interestingly,

0 1 2 3 4 5

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bTiO (JCPDS 77-2170)

TiN (JCPDS 38-1420)

Fig. 7. (a) SAXRD patterns and (b) WAXRD patterns of the MCT-4-Y compositescalcined in N2 at different temperature.

D. Liu et al. / Microporous and Mesoporous Materials 139 (2011) 87–93 91

when the carbon contents in composites further increase, anunusually phenomenon is observed. It can be seen that a newintergrowth phase appears except rutile phase for MCT-3-1000,which can also be observed in the WAXRD pattern of MCT-4-1000. The new phase is obviously not usual phase of titania (ana-tase or rutile), and the phase identification will be elucidated indetail (see below).

N2 adsorption–desorption isotherms and BJH pore size distribu-tion curves of the mesoporous composites with different carboncontents are shown in Fig. 6, and the corresponding pore structuralparameters including BET surface areas, micropore surface area,total pore volumes and pore sizes are summarized in Table 1.The three composites exhibit representative type IV curves withwell-defined capillary condensation steps, which are characteris-tics of ordered mesoporous materials (Fig. 6a) [28]. H2-type hyster-esis loops are observed for these composites, implying that thepore channels may be blocked. It may arise from incompletedecomposition of templates or growth of titania nanocrystals intopore channels [11]. However, compared with that of MCT-1-600and MCT-2-600 (both ca. 3.1 nm), MCT-3-600 exhibits a relativelarger mean pore size (ca. 3.9 nm), probably caused by less positionof titania nanocrystals into pore channels due to weak crystalliza-tion. It is in agreement with WAXRD measurements.

Similar to that of MCT-2-350 with low carbon contents, SAXRDpattern of MCT-4-350 exhibit (1 0 0), (1 1 0) and (2 0 0) reflectionsfrom 2D hexagonal p6mm symmetry (Fig. 7a). However, even whenthe heating temperature is raised to 1000 �C, the (1 0 0) peaks canstill be clearly observed, reflecting that the mesoscopic ordering ispreserved. The broaden (1 0 0) peak and less-resolved other twopeaks suggest the partial degradation of mesostructures. WAXRDpatterns present unique crystalline characteristics of MCT-4 sam-ple (Fig. 7b). At 500–900 �C, only two weak broad peaks could bedetected, corresponding to the (0 0 2) and (1 0 1) reflections ofamorphous carbon. By 1000 �C, five strong diffraction peaks arereadily observed. The experimental peak locations and their rela-tive intensities were carefully compared with powder diffractionfiles (PDF) contained in MDI JADE™ XRD Processing & Identifica-tion software (Materials Data, Inc., Version 5.0). All the strongpeaks can be indexed as (1 1 1), (2 0 0), (2 2 0), (3 1 1), (2 2 2)reflections of a face-centered-cubic structure [space group Fm3m(2 2 5)] with a unit cell parameter a0 = 0.4214 nm. The parameteris between the reported values of a0 = 0.4185 nm for TiO (JCPDSNo. 77-2170) and a0 = 0.4241 nm for TiN (JCPDS No. 38-1420),which implies that the crystallized phase is possibly titanium oxy-nitride TiOxN1�x [29]. Estimated from the (2 0 0) plane with

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b MCT-1-600 MCT-2-600 MCT-3-600

Fig. 6. (a) Nitrogen sorption isotherms and (b) BJH pore size distributions of thecomposites calcined at 600 �C in N2. The isotherms of MCT-2-600 and MCT-3-600are offset vertically by 50 and 100 cm3 g�1, respectively.

Fig. 8. TEM images (a, b, and d) and HRTEM images (c and e) of the MCT-4-Ycomposite calcined at 600 �C (a–c) and at 1000 �C (d and e) in N2, viewed along the[0 0 1] (a and c) and [1 1 0] (b, d and e) directions. (f) and (g) are the enlargedimages of two selected areas in (e).

Scherrer equation, the average crystallite size is 7.0 nm. Fig. 8shows the TEM and HRTEM images of MCT-4-600 and MCT-4-1000. After calcination at 600 �C, the TEM images show well-de-fined 2D-hexagonal mesostructure (Fig. 8a and b), and no crystal-line phase can be observed in the corresponding HRTEM image(Fig. 8c), consistent with the results from XRD measurement. By1000 �C, the TEM image shows that the ordered mesostructure still

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Fig. 9. The XPS spectra of the composite MCT-4-1000. (a) Survey spectrum; (b) Ti2pspectra along with their deconvolution; (c) N1s spectrum. Note: for the analysis ofthe Ti spectrum, a Shirley-type background was used. The deconvolution and fittingof the peaks was performed by using Gaussian/Lorentzian sum functions ofXPSPEAK 4.1 software.

92 D. Liu et al. / Microporous and Mesoporous Materials 139 (2011) 87–93

exists (Fig. 8d). HRTEM analyses reveal that lots of nanocrystalswith the size of ca. 5–10 nm are embedded in the pore wall ofMCT-4-1000 (Fig. 8e). As shown in Fig. 8f and g, the mean interpla-nar distances can be measured using Gatan DigitalMicrograph™software (version 3.5.2), and their values are in good agreementwith calculated d-spacing from WAXRD.

As mentioned above, the XRD patterns of TiO, TiOxN1-x, and TiNis virtually difficult to distinguish, and thus the crystalline phaseshould be considered with care. The chemical bonding of the sam-ple MCT-4-1000 was further investigated by XPS measurements,and the results are given in Fig. 9. The XPS Ti2p could be deconvo-luted into four peaks with binding energies (B. E.) of 464.0, 461.6,458.4 and 456.2 eV (Fig. 9b). The two strong peaks at 464.0 and458.4 eV are corresponding to Ti2p1/2 and Ti2P3/2, respectively,which can be assigned to that of TiO2 [30]. The another pair ofpeaks at 461.6 and 456.2 eV are close to Ti2p1/2 and Ti2P3/2 peaksfor TiO [2,29–32], however, the reported Ti2p3/2 B. E. for TiN is at455.3 eV [33]. The N1s peak (Fig. 9c) is located at a characteristicB. E. of nitride (396.3 eV) [31], and its weak intensity implies thatthe sample MCT-4-1000 has a negligible N content. The above data,combined with WAXRD analyses, reveal that titanium species inthe MCT-4-1000 are not fully crystallized even after calcinationat 1000 �C in N2 for 3 h, and TiO is dominant crystalline phase.The formation of TiO is attributed to occurrence of carbothermalreduction reaction (TiO2 + C = TiO + CO) [34,35]. It should be notedthat the carbothermal reduction reaction is usually highly endo-thermic and proceeds above 1200 �C [34,36]. Here, the betterhomogeneity of TiO2 nanoparticles with carbon and the high spe-cific energy would result in the decrease of the formation temper-ature of TiO to 1000 �C.

N2 sorption measurements show that both of MCT-4-600 andMCT-4-1000 display type IV isotherms (Fig. 10a). For MCT-4-600,the hysteresis hoop is of a common H2-type. However, an H3-typehysteresis hoop is observed for MCT-4-1000 [28]. The H3-type loop,which does not clearly exhibits an adsorption plateau at relativepressures close to the saturation vapor pressure, is usually relatedto the existence of slit-like pores or a mixture of micropores andmesopores in materials [37]. The BET surface area increases from405.7 m2 g�1 for MCT-4-600 to 787.9 m2 g�1 for MCT-4-1000, themicropore surface area increases from 95.8 to 207.9 m2 g�1 andthe total pore volume also increases from 0.32 to 0.73 cm3 g�1

(Table 1). These results indicate that micropores generate withincreasing pyrolysis temperature. By contrast, the carbonizationof pure mesoporous bakelite from 600 to 1000 �C presents gentleincreasing BET surface areas and total pore volumes [19]. Thus,the consumptions of partial carbon during the carbothermal pro-cess may result in a large quantity of micropores in the pore walls.The loss of carbon contents from 69 wt.% for MCT-4-600 to 40 wt.%for MCT-4-1000 can also support above conclusions (Table 1).

TG analyses were carried out under a N2 atmosphere to investi-gate the thermal process of as-made composite from room temper-ature to 1400 �C (Fig. 11). TG curves show that an almost sameweight loss of ca. 50 wt.% occurs from 200 to 350 �C for MCT-2-as and MCT-4-as, mainly attributed to the decomposition of F127copolymer into volatile small molecules. The reaction results inlow carbon residues of F127 template, in favor of generating theopen mesopores after the thermal treatment in N2. However, TGcurves at higher temperature show an obvious difference betweenMCT-2-as and MCT-4-as. MCT-2-as shows ca. 23 wt.% weight lossesin the range of 350–970 �C, associated with the carbonization ofbakelite and condensation of titanium species. However, the signif-icant weight loss of MCT-4-as persists until ca. 1200 �C. Consider-ing that the carbonization of bakelite only presents the weightloss lower than 2% above 900 �C [19], it is believed that MCT-4-as start to encounter a carbonization reduction reaction at ca.1000 �C, but not for MCT-2-as. Combined with the above XRD,

TEM and XPS analyses, it can be concluded that the carbon contentalso has significant effect on the phase transformation of titaniumspecies except for the calcination temperature. Very recently,Huang et al. investigated the effect of experimental conditions onthe crystalline phases for mesoporous Ti–C composites preparedusing TiCl4 sols as precursor, and found that Magneli phases and

100

200

300

400

500

a MCT-4-1000

Volu

me

Ad

sorb

ed (

cm3

g-1

ST

P)

Relative Pressure (P/P0)

MCT-4-600

0.0 0.2 0.4 0.6 0.8 1.0 0 2 4 6 8 10 12 14 160.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

dV

/dD

(cm

3 g

-1 n

m-1

)

Pore size (nm)

b MCT-4-600 MCT-4-1000

Fig. 10. (a) Nitrogen sorption isotherms and (b) BJH pore size distributions of thecomposites calcined at 600 �C and 1000 �C in N2.

0 200 400 600 800 1000 1200 14000

20

40

60

80

100

Wei

gh

t (%

)

Temperature (°C)

MCT-2-as MCT-4-as

Fig. 11. TG curves for the MCT-2-as and MCT-4-as samples under a N2 atmosphere.

D. Liu et al. / Microporous and Mesoporous Materials 139 (2011) 87–93 93

TiC could be obtained at 1000 �C [38], which has some differencewith the results presented here. This difference indicates that theTi precursors also have important effect on the crystalline formof Ti species in the mesoporous Ti–C composites.

4. Conclusions

In brief, we have described a robust supramolecular templatingapproach to prepare thermally stable, crystalline mesoporous Tioxide/carbon composites by using performed titania nanoparticlesand resols as inorganic and organic precursors and triblock copoly-mer F127 as a template. All the composites possess ordered 2Dhexagonal mesostructures with high surface areas and uniformpore sizes. Their composition can be controlled easily in a widerange by varying the initial ratios of reactants. In addition to thecalcination temperature, the carbon content also has significant ef-fect on the crystalline transformation of titanium species. After cal-cinations in N2 atmosphere, titanium species in the composites

with low carbon contents exhibit nanosized TiO2 (anatase or rutile)dispersed within the amorphous carbon frameworks. When thecarbon contents in the composites increase to a relatively high le-vel, TiO is dominant crystalline phase from a carbothermal reduc-tion process. The method appears quite general and should beadaptable to the fabrication of other metal oxide/carbon system.

Acknowledgement

This work was supported by the National Natural Science Foun-dation of China (50272048).

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