Ordered mesoporous crystalline titania with high thermal stability from comb-like liquid crystal block copolymers

Abstract

Ordered mesoporous crystalline titania with high thermal stability is directly synthesized in a simple one-pot procedure via an integrated approach of a solvent evaporation-induced self-assembly (EISA) technique and template-carbonization strategy by using a comb-like amphiphilic liquid crystal (LC) block copolymer (BCP) PEO-b-PMA(Az) with abundant sp²-carbons as a structure-directing agent. These BCP containing LC rigid segments with distinct hydrophilic/hydrophobic contrast can form robust micelles and co-assemble with titania sol into ordered mesostructures which are stable during the whole EISA process. Template carbonization in nitrogen after ageing is the key step to obtain the ordered crystalline mesostructures. The synthesized titania materials have a large pore size (∼4.87 nm) and pore volume (∼0.37 cm³ g⁻¹) and high surface area (∼239 m² g⁻¹), as well as high thermal stability (>700 °C). Additionally, mesoporous titania shows good photocatalytic activity for the degradation of rhodamine B in an aqueous suspension due to its highly crystalline framework and large surface area. This simple, yet facile method can therefore be expanded to other ordered crystalline mesoporous materials through high-temperature heat treatments.

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Introduction

Since the discovery of ordered mesoporous silica,^1–3 interest in the design and facilely controlled synthesis of mesoporous nanomaterials has been widely excited.^4–7 In particular, TiO₂, as an excellent semiconductor, has attracted enormous attention in recent years due to its promising applications in the photodegradation of organic dyes,^8–10 dye-sensitized solar cells (DSSCs),¹¹ lithium ion batteries (LIB),^12–14 photodissociation of water^15,16 and fixation of carbon dioxide,¹⁷ etc. Most of these applications strongly rely on the surface structure, specific surface area, tunable pore size as well as crystallization degree of TiO₂.¹⁸ Therefore, mesoporous TiO₂ with highly crystalline frameworks has been widely studied in this field. Compared to SiO₂ precursors, TiO₂ precursors, such as titanium chlorides and titanium alkoxides, exhibit a much faster hydrolysis-condensation rate, easily creating a phase separation between templates and titanium species. For instance, the hydrolysis rate of titanium alkoxides is about five orders of magnitude faster than that of silicate alkoxides.¹⁹ Different strategies, developed to synthesize mesoporous TiO₂ by careful selection of inorganic precursors, templates, solvents, etc, can be assigned into two classes: a hard templating approach and a soft one. The first case is the nanocasting process by using ordered mesoporous silicas or carbons as hard templates.^20–26 For example, Bruce and his co-workers synthesized mesoporous crystalline anatase TiO₂ by using mesoporous silica KIT-6 as the hard template and TiO(NO₃)₂ as the precursor through an incipient-wetness method.²³ The nanocasting method is an effective way to prepare mesoporous TiO₂ with crystalline frameworks. However, this procedure suffers from multiple, tedious steps because the templates are prerequisite and then, removal of the template is cumbersome and often includes the use of hydrofluoric acid. Moreover, it is difficult to completely fill the pores of the template, even after multiple-impregnation. The second method is a sol–gel process by using soft templates as the structure-directing agents. Commercial Pluronic triblock copolymers, such as EO₂₀PO₇₀EO₂₀ (P123) and EO₁₀₆PO₇₀EO₁₀₆ (F127), are the most widely used.^27–29 Stucky et al. synthesized ordered mesoporous TiO₂ by using P123 as a template, and titanium tetrachloride (TiCl₄) as the inorganic precursor.²⁷ Yan et al. synthesized ytterbium stabilized ordered mesoporous TiO₂ by using F127 as the structure-directing agent, titanyl acetylacetonate and Yb(NO₃)₃·5H₂O as the inorganic precursors.²⁹ However, due to the weak thermostability of polyethers, Pluronic copolymers can easily decompose at relatively low temperature (∼230 °C)³⁰ even in an inert atmosphere, which makes it difficult to synthesize ordered mesoporous TiO₂ with highly crystalline frameworks. Recently, because of the merits of designable compositions, block sequences, molecular weights and volume fractions, lab-made non-Pluronic block copolymers (BCPs) have been explored to synthesize mesoporous crystalline TiO₂. Poly(ethylene-co-butylene)-b-poly(ethylene oxide) (PHB-b-PEO referred to also as “KLE”) was firstly reported to synthesize highly crystalline cubic mesoporous TiO₂ films through dip-coating by Smarsly and co-workers, but the whole mesostructure collapsed at 700 °C.^31,32 Wiesner et al. reported an effective method, called combined assembly by soft and hard (CASH) chemistries, for highly crystalline mesoporous TiO₂ by using in situ generated sturdy, amorphous carbon derived from amphiphilic BCP polyisoprene-b-poly(ethylene oxide) (PI-b-PEO) as a rigid support to prevent the collapse of mesostructures during the crystallization.³³ However, the obtained mesostructured TiO₂ exhibits a wormhole-like structure. Moreover, the templates KLE and PI-b-PEO are generally prepared via anionic polymerization, which requires very harsh synthesis conditions and specialized operation. To avoid this, another home-made block copolymer, polystyrene-b-poly(ethylene oxide) (PS-b-PEO), prepared via atom transfer radical polymerization (ATRP) was utilized to fabricate mesoporous crystalline TiO₂, with the same procedure described by Wiesner et al.,³³ failed to form an ordered mesoporous structure of the resulting TiO₂.³⁴ Recently, Zhao et al. successfully improved the regularity of mesoporous TiO₂ by using PS-b-PEO as templates, and acetylacetone as a coordination agent, when the heat treated temperature was lower than 700 °C under nitrogen atomosphere.¹¹ Besides diblock copolymers, the lab-made triblock copolymer poly(styrene-b-2-vinyl pyridine-b-ethylene oxide) (PS-b-PVP-b-PEO) was used to synthesize mesoporous crystalline TiO₂ films with a partly disordered architecture, while thermal stability was just up to 600 °C.³⁵ Despite many efforts, up until today, it still remains a major challenge to directly synthesize highly ordered mesoporous TiO₂ with thermal stable crystalline frameworks, especially higher than 700 °C.

Lab-made amphiphilic BCPs mentioned above are composed of common linear molecules of both segments. Previously, we reported an amphiphilic liquid crystal (LC) BCP with a comb-like structure, PEO-b-PMA(Az), consisting of a hydrophilic poly(ethylene oxide) (PEO), and a hydrophobic polymethacrylate with azobenzene mesogen side chains (PMA(Az)), synthesized by the ATRP method.³⁶ Due to the rigid azobenzene-containing LC block of PMA(Az), microphase separation of PEO-b-PMA(Az) can easily occur, even in the case of a low molecular weight, and perpendicularly aligned PEO cylindrical domains within BCP films can be formed after thermal annealing.^37–40 Amphiphilic BCPs containing LC rigid segments with distinct hydrophilic/hydrophobic contrast can form robust micelles and combine TiO₂ sol into ordered mesostructures during the solvent evaporation-induced self-assembly (EISA) process.⁴¹ Inspired by these attributes, as well as sufficient sp²-hybridized carbons contained in the hydrophobic block, it is expected that this comb-like amphiphilic LC BCP as a structure-directing agent can contribute to form ordered mesoporous structures with excellent thermal stable crystalline frameworks. In this work, highly ordered mesoporous TiO₂ materials with crystalline frameworks were prepared via combining the EISA technique and template-carbonization strategy by using PEO-b-PMA(Az) as a structure-directing agent, titanium isopropoxide (TIPO) and TiCl₄ as inorganic precursors, and tetrahydrofuran (THF) as the solvent. Considering the acidic sensitivity of the ester bond and azobenzene group contained in this BCP, a non-hydrolytic sol–gel (NHSG) route to titanium oxide was used in the synthesis process, without extra acid. The first reaction of TIPO and TiCl₄ is a rapid exchange of the ligands, forming a mixture of halogenoalkoxides,⁴² similar to the ‘acid–base pair’,⁴³ which are the true precursors, making it easy to combine the templates into ordered mesostructures. Moreover, relying on the rigid support of in situ carbon derived from the comb-like PEO-b-PMA(Az) template during carbonization in nitrogen, highly ordered crystalline mesoporous TiO₂ with 2D hexagonal structure, high specific surface area, as well as excellent thermal stability can be obtained. The mesoporous titania also exhibits good photocatalytic activity in the degradation of rhodamine B in aqueous suspensions.

Experimental section

Materials

Triethylamine (TEA), ether, methanol, HCl (37 wt%), sodium nitrite (NaNO₂), phenol, potassium hydroxide (KOH), potassium carbonate (K₂CO₃), potassium iodide (KI), acetone, chlorobenzene, and copper(I) chloride (CuCl) were purchased from Beijing chemical works. 4-Butylaniline, 11-bromoundecanol, methacryloyl chloride and 2-bromo-2-methypropionylbromide were purchased from Beijing Ouhe technology. Hexamethyltriethyenetetramine (HMTETA) was purchased from J&K Scientific Ltd. Monomethoxy poly(ethylene oxide) (PEO₁₁₄, molecular weight ≈ 5000), tetrahydrofuran (THF), and titanium isopropoxide (TIPO) were purchased from Aladdin Chemical co. Ltd. Titanium chloride (TiCl₄) was purchased from Beijing Changping Jingxiang Chemical Reagent (China). All other chemicals were used as received without further purification.

Synthesis of ordered mesoporous titanium dioxide

The templates of amphiphilic PEO-b-PMA(Az) diblock copolymers were prepared by the atom transfer radical polymerization (ATRP) method as reported previously.³⁶ The detailed process is described in ESI.† In a typical synthesis of mesoporous titania, equal molar amounts of TIPO and TiCl₄ (total weight is 0.20 g) were added to THF (4.00 g). PEO-b-PMA(Az) (0.10 g) was dissolved in 1.00 g of THF. Once dissolved, the two solutions were combined. The mixture was stirred for 5 h at room temperature and then poured into a petri dish to undergo a solvent evaporation process at 40 °C for 48 h. Finally, calcination was carried out at 350 °C in nitrogen for 3 h, or 600 °C for 3 h in nitrogen or 700 °C for 3 h in nitrogen, or 800 °C for 3 h in nitrogen (denoted as MT-350N, MT-600N, MT-700N, and MT-800N, respectively). After pyrolysis in nitrogen, the samples were calcined at 450 °C for 2 h in air (denoted as MT-600N-450A, MT-700N-450A, and MT-800N-450A, respectively).

Photocatalytic tests

The photocatalytic activity of the obtained ordered mesoporous titania was characterized by measuring the photodegradation of the xanthene dye rhodamine B, following the reported procedure.⁴⁴ Ten milligrams of catalyst were suspended in 50 mL of 10⁻⁵ M aqueous rhodamine B, in the dark for at least 30 min before illumination to reach an adsorption/desorption equilibrium. The stirred suspensions were illuminated with a 250 W mercury lamp ∼40 cm high over the solution, and the degradation rate of rhodamine B was monitored by measuring UV-vis absorption (553 nm) of the suspensions centrifuged at 8000 rpm to remove titania-containing microparticles at different interval periods.

Characterization and measurements

¹H-NMR spectra were measured by using a Bruker ARX 300 instrument spectrometer at 300 MHz with TMS internal standard as a reference for chemical shifts. Molecular weights of block copolymers were determined by using Waters equipped with UV and RI detectors in reference of a series of standard polystyrenes with THF as eluent. The small-angle X-ray diffraction (SAXRD) and wide-angle X-ray diffraction (WAXRD) measurements were performed using a Bruker D8 Advance X-ray diffractometer with Cu Kα radiation (40 kV, 40 mA) at room temperature. The nitrogen adsorption and desorption isotherms at 77 K were measured using an ASAP 2020 analyzer. Prior to the tests, samples were degassed in a vacuum at 180 °C for at least 6 h. The surface areas were calculated by the Brunauer–Emmett–Teller (BET) equation. Transmission electron microscopy (TEM) images were taken by the JEM-2100 under a working voltage of 200 kV. Fourier-transform infrared spectroscopy (FT-IR) was collected on a Nicolet Fourier spectrophotometer. The differential scanning calorimetry (DSC) curve was collected on a DSC Q2000. Thermogravimetric Analysis (TGA) measurement for the block copolymer PEO-b-PMA(Az) was carried out by using the NETZSCH STA 449F3 (Germany) from 25 °C to 800 °C under nitrogen with a heating rate of 10 °C min⁻¹.

Results and discussion

Diblock copolymer template

Herein, a lab-made amphiphilic LC diblock copolymer PEO-b-PMA(Az) with a comb-like structure was used as the structure-directing agent to synthesize ordered mesoporous TiO₂ materials. The PEO-b-PMA(Az) was synthesized by using the ATRP method according to a previous report.³⁶ Its chemical structure and composition were confirmed by ¹H-NMR spectroscopy (Fig. S1†), gel permeation chromatography (GPC, Fig. S2†), Fourier-transform infrared spectroscopy (FT-IR, Fig. S3†) and differential scanning calorimetry (DSC, Fig. S4†). The result of GPC shows a narrow molecular weight distribution and the polydispersity index (PDI) is 1.19. The molecular weight was measured to be ∼34 000 g mol⁻¹, and the composition can be formulated as PEO₁₁₄-b-PMA(Az)₆₀.

Ordered mesoporous titania after calcination in nitrogen

The as-made titanium–polymer composites before calcination in nitrogen are homogeneous saffron membranes, indicating a well-assembled mesostructure without macroscopic phase separation. The small-angle X-ray diffraction (SAXRD) patterns of ordered mesoporous TiO₂ materials calcined in nitrogen at different temperatures are shown in Fig. 1a. The sample calcined at 350 °C in nitrogen (denoted as MT-350N) is puce and shows a strong diffraction peak, which can be attributed to p6mm hexagonal symmetry in association with TEM observation (Fig. 2). The unit-cell parameter (a) is calculated to be about 17.0 nm. Heat treatment to 600 °C under nitrogen converted the as-made sample into black composites of carbon and highly crystalline TiO₂, indicating a further carbonization of the BCP templates. The formation of highly crystalline TiO₂ is evident from the wide-angle X-ray diffraction (WAXRD) analysis of the resulting material (see below). The diffraction intensity is higher than that of MT-350N, hinting that the mesoscopic order is well sustained. The unit-cell parameter (a) is calculated to be about 14.5 nm, smaller than that of MT-350N, which suggests a structural shrinkage of ∼14.7% because of the crystallization of the TiO₂ frameworks and the carbonization of the templates. The SAXRD pattern of the sample MT-700N shows a sharp diffraction peak, suggesting the mesoscopic order is still retained after calcination at even 700 °C under nitrogen, due to the support of the in situ carbon in the mesochannels. The cell parameter (a) is calculated to be ∼14.0 nm, a little smaller than that of MT-600N, which indicates a slight shrinkage. However, the diffraction intensity of MT-800N is very weak, indicating some collapse of ordered mesostructures due to the formation of large rutile particles (Fig. S5b†).

Fig. 1 (a) SAXRD and (b) WAXRD patterns of mesoporous titania calcined at different temperatures in nitrogen. A refers to anatase and R refers to rutile in (b).

Fig. 2 TEM images (a) and (b) of MT-350N viewed along [001] and [110] orientations, (c) MT-600N, (d) MT-700N, and (e) the SAED pattern of MT-700N and (f) MT-800N, respectively.

In general, the crystal phase and crystallization of TiO₂ mainly depends on the calcination temperature. The WAXRD pattern (Fig. 1b) of the mesoporous TiO₂ sample MT-350N exhibits weak diffraction peaks on a flat baseline at 2θ of 25.3°, 37.8°, 48.0°, 53.9°, 55.0° and 62.7°, indexed to (101), (004), (200), (105), (211) and (204) reflections of anatase (JCPDS card 21-1272), respectively. According to the Debye–Scherrer equation, the crystallite size is calculated from the 101 reflection to be about 7.0 nm. The sample MT-600N displays higher diffraction peaks of anatase, suggesting that crystallinity increased with increasing temperature, and the crystallite size increases to about 9.0 nm. Further increasing the pyrolysis temperature to 700 °C in nitrogen, the sample MT-700N still retains the anatase phase, and diffraction peaks are sharper than those of MT-600N, indicating the further growth of crystalline grains in the frameworks. And the crystallite size increased to about 9.7 nm. The WAXRD pattern of the sample MT-800N obtained after pyrolysis at 800 °C for 3 h in nitrogen exhibits several diffraction peaks that correspond to the mixed anatase (JCPDS card 21-1272) and rutile (JCPDS card 21-1276) phases. The calculated crystallite sizes are ∼14.7 nm and ∼25.0 nm for the anatase phase and rutile phase, respectively.

Parts a and b of Fig. 2 show the TEM images of the sample MT-350N. The highly ordered 2D hexagonal arrangement of pores along the [001] direction (Fig. 2a) and the alignment of cylindrical pores along the [110] direction (Fig. 2b) are observed. Compared with PI-b-PEO³² and PS-b-PEO,³³ highly ordered mesostructures can be formed by using PEO-b-PMA(Az) as templates, mainly due to the more distinct hydrophilic/hydrophobic contrast. When the as-made sample was calcined at a high temperature (600 °C) in nitrogen, the ordered mesostructures were maintained (Fig. 2c), consistent with the results of SAXRD. While the as-made sample was directly calcined at 600 °C in air, the ordered mesostructures completely collapsed, and the large size of the anatase crystalline grains was obtained (Fig. S6†). The sharp contrast indicates that the in situ carbon from BCPs can support the ordered mesostructures during crystallization in nitrogen. After a further pyrolysis at 700 °C in nitrogen, the highly ordered mesostructures were still preserved (Fig. 2d). The corresponding selected area electron diffraction (SAED) pattern (Fig. 2e) shows a series of spotty diffraction pattern rings, which indicate a polycrystalline anatase phase, in compliance with WAXRD results. The TEM image (Fig. 2f) of the MT-800N shows that ordered mesostructures collapsed to some extent. The carbon nanotubes within TiO₂ nanostructures were observed in Fig. S5a,† proving that the in situ carbon can be formed from the macromolecular template. Moreover, large size grains of rutile were obtained (Fig. S5b†). As we all known, the thermal stability of rutile is better than that of anatase. Therefore, the crystalline transition from anatase to rutile occurs and forms relatively large particles at high temperature. The in situ carbon was insufficient to support the whole framework and part of the ordered mesostructure collapsed. It can be concluded that the obtained anatase phase with highly ordered mesostructures in this work can be maintained up to 700 °C, indicating excellent thermostability.

The physicochemical properties of the ordered mesoporous TiO₂ materials were characterized by nitrogen adsorption–desorption measurements. The result shows that MT-350N has a strange absorption and desorption isotherm (Fig. S7†), and its specific surface area is only 21 m² g⁻¹ (Table S1†). This is probably due to the fact that PEO, owing to the weak thermostability of polyethers, can easily decompose at 350 °C, while PMA(Az), because of the azobenzene mesogen, has higher thermostability (Fig. S8†) and still filled in the pores. The nitrogen-sorption isotherms (Fig. 3a) of MT-600N and MT-700N calcined in nitrogen show type IV curves with sharp capillary condensation steps in the relative pressure (p/p₀) of ∼0.7–0.9, indicating uniform mesopores with large pore size. For MT-600N, the specific surface area is 165 m² g⁻¹, and the total pore volume is 0.13 cm³ g⁻¹ (Table S1†). Meanwhile for MT-700N, both the surface area and total pore volume are increased to 193 m² g⁻¹ and 0.17 cm³ g⁻¹, respectively, mainly due to the further carbonization of BCPs. Fig. 3b shows pore size distribution curves obtained from the Barrett–Joyner–Halenda (BJH) adsorption model of the resulting samples. MT-600N has a bimodal distribution, probably resulting from the incomplete carbonization of the templates. Meanwhile for MT-700N, a unimodal distribution was obtained, and the average pore size of MT-700N is larger than that of MT-600N, hinting at a relatively complete carbonization. An atypical type IV curve was obtained for MT-800N. Even though part of the ordered mesostructures collapsed, the specific surface area was still as high as 239 m² g⁻¹.

Fig. 3 (a) Nitrogen adsorption–desorption isotherms and (b) pore-size distribution curves of the mesoporous titania materials: MT-600N, MT-700N and MT-800N, respectively. The value of dV/dD for MT-800N is divided by 2.

Ordered mesoporous titania after further calcination in air

Fig. 4 shows WAXRD patterns of the samples mentioned above sequentially calcined at 450 °C in air. It is clear that the pattern of MT-600N-450A is assigned to the anatase phase, and the calculated crystallite size from the 101 reflection is about 11.5 nm. The rutile phase appears on the curve of MT-700N-450A, whereas anatase is still the predominant one with a calculated crystallite size of ∼12.4 nm. For MT-800N-450A, the relative intensity of the 101 reflection for the anatase phase versus the 110 reflection for the rutile phase obviously increased compared with that of MT-800N (Fig. 1b), indicating that the formation or growth rate of anatase is faster than that of rutile when calcined at 450 °C in air. The crystallite sizes are calculated to be about 15.9 nm and 29.6 nm for anatase and rutile, respectively. The SAXRD patterns (Fig. S9†) of MT-600N-450A and MT-700N-450A illustrate that the ordered mesostructures are still retained,^29,45,46 which are confirmed by TEM observations (Fig. 5). Although the reflection peak for MT-800N-450A almost disappeared, some ordered mesostructures were observed, due to short-range regularity and gradual distortion of the frameworks by crystallization or grain growth.⁴⁶

Fig. 4 WAXRD patterns of the mesoporous titania after further calcination at 450 °C in air: MT-600N-450A, MT-700N-450A, T-700N-450A and MT-800N-450A, respectively. For T-700N-450A prepared by the same conditions of MT-700N-450A in the absence of PEO-b-PMA(Az), the intensity is divided by 3.64. A refers to anatase and R refers to rutile.

Fig. 5 TEM images of mesoporous titania after further calcination at 450 °C in air: (a) MT-600N-450A, (b) MT-700N-450A, (c) high magnification image of the part marked by a red circle in (b), and (d) MT-800N-450A, respectively.

Fig. 5 shows TEM images of the resulting samples calcined at 450 °C in air. It is obvious that highly ordered mesoporous TiO₂ with cylindrical pores along the [110] direction is observed from Fig. 5a (MT-600N-450A). Lattice fringes of nanocrystals can be clearly observed in the high-resolution transmission electron microscopy (HRTEM) image (Fig. S10†). After pyrolysis at 700 °C in nitrogen and calcination at 450 °C in air, the ordered mesostructures are retained (Fig. 5b). The wall thickness (∼9.3 nm) estimated from the TEM image is slightly smaller than the calculated crystallite size obtained from the (101) anatase peak of the WAXRD patterns (12.4 nm). This suggests the walls are highly crystallized rather than consisting of nanocrystals embedded in amorphous walls, further proven by the high magnification image (Fig. 5c) of the part marked by the red circle. For MT-800N-450A, short-range ordered structures are also observed in Fig. 5d, indicating excellent thermal stability.

The nitrogen adsorption–desorption isotherms for the samples after further calcination in air are shown in Fig. 6a. All the isotherms are type IV curves with H1 shaped hysteresis loops characteristic for mesoporous materials.⁴⁷ It is found that after the removal of carbon, the specific surface areas decreased to 126, 137 and 108 m² g⁻¹ for MT-600N-450A, MT-700N-450A and MT-800N-450A, respectively. This behavior is related to the in situ formed carbon. Amorphous carbon typically has a higher surface area than metal oxide owing to the presence of micropores,⁴⁸ and hence, the removal of carbon results in a decrease in the surface area. The condensation steeps were slightly shifted to greater relative pressures from MT-600N-450A to MT-800N-450A, indicating larger mesopores. Fig. 6b shows that narrow pore size distributions (3–5 nm) are maintained though the diameter increases as the calcination temperature in nitrogen increases. The large surface areas and narrow pore size distributions combined with excellent thermal stability enhance the potential applications of the mesoporous titania, such as catalysis and energy conversion.

Fig. 6 (a) Nitrogen adsorption–desorption isotherms and (b) pore size distribution curves of mesoporous titania: MT-600N-450A, MT-700N-450A and MT-800N-450A, respectively.

Formation process of ordered mesoporous titania

In this work, a lab-synthesized amphiphilic block copolymer PEO-b-PMA(Az) with a comb-like structure containing azobenzene LC groups with abundant sp²-hybridized carbons was used as a structure-directing agent via an integrated approach of the solvent EISA technique and template-carbonization strategy to fabricate highly ordered mesoporous titania with a 2D hexagonal structure and crystalline framework. Scheme 1 illustrates the proposed synthesis process. In the synthesis system, the NHSG method makes an appropriate pH value for the mixed solution which is insufficient to impact ester bonds and azobenzene groups, and avoids a rapid condensation rate for Ti species. With the evaporation of THF, PEO-b-PMA(Az) copolymers are induced to microphase separate to form cylindrical micelles with hydrophobic PMA(Az) segments as the core surrounded by hydrophilic PEO shells, which are selectively swelled by TiO₂ sol through hydrogen bonding. Simultaneously, the azobenzene LC groups in the hydrophobic domains self-organize to form alignments vertical to hydrophilic domains,^39,49 which makes the cylindrical micelles more stable. As THF continuously evaporates, highly ordered mesostructured hybrids can be formed by a solvent EISA process. To obtain highly crystalline TiO₂, the as-made inorganic–polymer composites are subsequently calcined under nitrogen at different temperatures and under air. During the calcination process in nitrogen, PEO blocks easily decompose, whereas the more thermally stable PMA(Az), containing azobenzene LC groups with abundant sp²-hybridized carbons, is converted to a sturdy carbon material within the walls of the resulting cylindrical pores. The in situ carbon is sufficient to act as a rigid support for the mesostructured frameworks, preventing collapse when heat treated to temperatures required for forming highly crystalline TiO₂ materials. Finally, the products are calcined in air to remove the residual carbon species, leaving a well-organized, highly crystalline mesoporous TiO₂.

Scheme 1 Proposed synthesis process of ordered and highly crystalline mesoporous titania with a 2D hexagonal structure from the template-carbonization method.

Photocatalytic properties

The photocatalytic activity of mesoporous titania calcined at different temperatures was examined by monitoring the photodegradation of rhodamine B in an aqueous suspension of TiO₂ with a concentration of 0.2 g L⁻¹ following a reported procedure.⁴⁴ Fig. 7a shows that, in the blank experiment (without catalyst), there is almost no degradation of rhodamine B molecules after irradiation for 300 min, while the degradation rates were significantly enhanced in the presence of the catalysts. Under irradiation for 300 min, the degradation percentages of rhodamine B for MT-600N-450A, MT-700N-450A and MT-800N-450A are 74.7, 82.1 and 99.3%, respectively. The photodegradation of rhodamine B follows roughly a pseudo-first-order reaction (Fig. 7b), which is consist with previously reported results.^44,50–52 The rate constants are calculated to be about 4.45 × 10⁻³, 5.74 × 10⁻³ and 1.84 × 10⁻² min⁻¹ for MT-600N-450A, MT-700N-450A and MT-800N-450A, respectively. These results indicate that MT-600N-450A exhibits the lowest photocatalytic activity, whereas MT-800N-450A shows the highest photocatalytic activity. Generally, the photocatalytic activity is determined by a compromise between the specific surface area and crystallinity.⁵⁰ The samples calcined at higher temperature exhibited enhanced activity because of their improved crystallinity. For comparison, the commercial Degussa P25 TiO₂ was tested for the photodegradation of rhodamine B under the same conditions. The rate constant for P25 is measured to be 5.69 × 10⁻² min⁻¹, which is a little higher than that of MT-800N-450A. This phenomenon is similar to previously reported works,^44,50 probably due to the smaller size of the crystalline anatase TiO₂ and poorer dispersion in solutions than those of P25. Moreover, the sample prepared by the same conditions of MT-700N-450A in the absence of PEO-b-PMA(Az) was synthesized, denoted as T-700N-450A. The degradation rate is only ∼5%, and the rate constant is 1.85 × 10⁻⁴ min⁻¹, far lower than that of MT-700N-450A. From the WAXRD patterns in Fig. 4, it is clear that the crystalline structure of T-700N-450A is similar with that of MT-700N-450A, with a relatively larger crystallite size (20.5 nm, Table S1†). Meanwhile the surface area of the sample prepared in the presence of BCP is much larger than that of the sample without BCP, indicating that the surface area affects the catalytic properties substantially.

Fig. 7 (a) Photocatalytic degradation of rhodamine B monitored as normalized concentration change versus irradiation time in the presence of mesoporous titania calcined in air, (b) respective apparent first-order rate constant determined from the linear graph of ln(c/c₀) versus irradiation time.

Conclusions

In summary, well-organized mesoporous crystalline titania with a high thermal stable framework was directly synthesized in a simple one-pot procedure by using a comb-like amphiphilic LC PEO-b-PMA(Az) containing abundant sp²-carbon in the hydrophobic block as the structure-directing agent. Two calcination steps were adopted, and the calcination in nitrogen was important to form ordered crystalline mesostructures. The obtained mesoporous titania has uniform pore size (∼4.87 nm), large pore volume (∼0.37 cm³ g⁻¹) and high specific surface area (∼239 m² g⁻¹), as well as excellent thermal stability (>700 °C). The mesoporous titania shows good photocatalytic activity toward the degradation of rhodamine B in an aqueous suspension, which may be attributed to its highly crystalline framework and high specific surface area. This approach is simple and reproducible, and can be expanded to other ordered mesoporous crystalline metal oxides with multifunctional properties.

Acknowledgements

This work was financially supported by National Natural Science Foundation of China (No. 51472018, 51272010), Beijing Nova Program (No. XX2013009), The Research Fund for the Doctoral Program of Higher Education (No. 20121102120001), Program for New Century Excellent Talents in University (NCET-12-0035), and the fund of State Key Laboratory of Chemical Resource Engineering (CRE-2014-C-106).

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Footnote

† Electronic supplementary information (ESI) available: These might include comments relevant to but not central to the matter under discussion, limited experimental and spectral data, and crystallographic data. See DOI: 10.1039/c6ra10352a

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