Porous TiO2 Assembled from Monodispersed Nanoparticles

Porous TiO2 were assembled by evaporating or refluxing TiO2 colloid, which was obtained by dispersing the TiO2 nanoparticles with a crystallite size (dXRD) of 3.2 nm into water or ethanol without any additives. Porous transparent bulk TiO2 was obtained by evaporating the TiO2-C2H5OH colloid at room temperature for 2 weeks, while porous TiO2 nanospheres were assembled by refluxing the TiO2-H2O colloid at 80 °C for 36 h. Both of the porous TiO2 architectures were pore-size-adjustable depending on the further treating temperature. Porous TiO2 nanospheres exhibited enhanced photocatalysis activity compared to the nanoparticles. Electronic supplementary material The online version of this article (doi:10.1186/s11671-016-1372-2) contains supplementary material, which is available to authorized users.


Background
Size plays an outstanding role in determining the properties of materials. After several decades of intense research on size effects of single nanoparticle or small nanoparticle clusters, the trend goes in the direction of meso-and macroscale structures consisting of nanoscale building blocks [1]. Nanoparticles, which possess the characters of small size and large surface area, have received widely attention due to their unique properties of optics, magnetics, electrics and catalysis [2][3][4][5]. To expand the research on nanoparticles, there has been growing interest in constructing hierarchical structures consisting of aggregated nanoparticles [6][7][8]. The combination of size effects and collective properties gives rise to fascinating opportunities for the creation of unprecedented materials. It is important to control the organization and dispersion states to assemble nanomaterials [9][10][11]. For example, the homogeneous and disordered assembly of densely packed nanoparticles can result in bulk transparent film-like objects through gradually evaporating the high-quality homogeneous nanoparticle colloid [12,13]. The high transmittance benefits from the homogeneous particles, which are quite small and do not scatter visible light. The homogeneous assembly of densely packed nanoparticles results in a narrow range of pore sizes that are highly dependent on the size and uniformity of the nanoparticles [14,15].
As a most promising photocatalyst, anatase TiO 2 has been widely investigated to improve its photocatalytic performance. Porous TiO 2 has performed much higher photocatalytic activity than solids due to their stronger adsorption to substrates and/or higher light-harvesting ability [16][17][18][19]. The void space can modulate the refractive index, lower the density, increase the active area for catalysis and improve the catalysts' ability to withstand cyclic changes in volume. Furthermore, void space in hollow structures can be used to encapsulate and control release of sensitive materials such as drugs and cosmetics. TiO 2 nanomaterials with different morphologies have been reported, and it is still a challenge to assemble TiO 2 nanoparticles (TiO 2 -NPs) to form a porous structure [20][21][22].
In our study, monodispersed anatase TiO 2 -NPs were prepared by the hydrolysis of tetrabutyltitanate (TBT) in a cyclohexane/ethanol medium. The assembly of TiO 2 -NPs was investigated under different conditions and resulted in transparent bulk TiO 2 and porous TiO 2 nanospheres (TiO 2 -NSs).

Preparation of TiO 2 -NPs
All of the chemicals were of analytic grade and used without further purification. We prepared TiO 2 nanoparticles by the hydrolysis of tetrabutyltitanate (TBT) in a cyclohexane (C 6 H 12 )/ethanol (C 2 H 5 OH) medium under refluxing conditions as reported [23]. Briefly, 36 mL of TBT was dissolved in the mixed solvents (90 mL of ethanol and 90 mL of cyclohexane), and 9 mL of HCl (TBT/HCl (v/v) = 1:0.25) was added to it. The final homogeneous solution was refluxed at 67°C for 10 h and then cooled to room temperature. To precipitate the TiO 2 -NPs, 600 mL of ethanol was added dropwise to the solution under stirring. After standing overnight, a white precipitate was harvested by centrifugation. The powder was obtained by washing with ethanol three times and dried in air at room temperature. (Additional file 1 shows the X-ray diffraction (XRD) pattern, transmission electron microscopy (TEM) image and the size distribution histogram of TiO 2 -NPs).

Preparation of the Porous Transparent Bulk TiO 2
Ten milligrams of freshly prepared TiO 2 -NPs was added to 10 mL of ethanol in a 1000-mL beaker covered with plastic wrap. After standing for 2 days, the turbid suspension gradually changed to a transparent TiO 2 -C 2 H 5 OH colloid. Porous transparent bulk TiO 2 was obtained by evaporating the TiO 2 -C 2 H 5 OH colloid at room temperature for 2 weeks. To investigate the thermal stability, porous transparent bulk TiO 2 was heated at different temperatures for 2 h with a heating rate of 5°C/min in air.

Preparation of TiO 2 -NSs
The freshly prepared TiO 2 -NPs were dispersed into deionized water to form a TiO 2 -H 2 O colloid. Typically, 100 mL of a 10-g/L TiO 2 -H 2 O colloid was heated at 80°C for 36 h. Then, the precipitate was collected by centrifugation, washed with deionized water several times, and dried in air at room temperature. The thermal stability of the TiO 2 -NSs was studied by heating at different temperatures for 2 h with a rate of 5°C/min in air.

Characterization
Powder XRD patterns were recorded on a PANalytical B.V. Empyrean X-ray diffractometer with graphite-filtered Cu Kα radiation at 40 kV and 40 mA. A SU8020 electron microscope was applied to get scanning electron microscopy (SEM) images. High-resolution transmission electron microscopy (HRTEM) was performed on a Tecnai G2 S-TWIN F20 at an accelerating voltage of 200 kV. Before being studied by HRTEM, porous transparent bulk TiO 2 specimens were fabricated on an FEI Helios NanoLab 600i FIB/SEM dual-beam system. Infrared (IR) spectra of the samples were recorded on a Bruker IFS-66V/S FT-IR spectrometer with a resolution of 2 cm −1 . Thermogravimetric analysis (TG) was carried out on a NETZSCH STA 449 F3 with a heating rate of 20 K/min from room temperature to 800°C. Brunauer-Emmett-Teller (BET) measurements were performed via the nitrogen adsorption method on a Micromeritics ASAP 2420, and UV-Vis absorption was measured on a UV-2450 spectrophotometer.

Photodegradation
The photocatalytic activity of TiO 2 -NPs and TiO 2 -NSs was tested by measuring the degradation rate of methylene blue (MB) under UV light irradiation. Typically, 50 mg of the as-prepared TiO 2 catalyst was added to 50 mL of an aqueous solution containing 10 mg/L MB. The mixture was magnetically stirred in the dark at ambient temperature for 1 h to achieve adsorption-desorption equilibrium of TiO 2 with MB followed by exposure to UV light from a 125-W high-pressure mercury vapour lamp. Five milliliters of the suspension was extracted every 10 min, and the suspended solid was immediately separated by centrifugation. UV-Vis absorption spectra were measured to monitor the concentration of MB remaining in the aqueous system. For comparison, Degussa P25 was adopted to investigate the photocatalytic activity under the same condition.

Results and Discussion
The Assembly of Porous Transparent Bulk TiO 2 When freshly prepared TiO 2 -NPs with a crystallite size (d XRD ) of 3.2 nm (see Additional file 1) were dispersed in ethanol, the turbid suspension gradually became a transparent TiO 2 -C 2 H 5 OH colloid after standing for 2 days. The digital photographic image is shown in Fig. 1a. After evaporating at room temperature, the colloid slowly condensed and formed porous transparent objects with size of 1-3 cm as shown in Fig. 1b. The IR spectrum and TG analysis of the porous transparent bulk TiO 2 are present in Additional file 1: Figure S4 and Figure S5. IR spectrum (Additional file 1: Figure S4) shows the absorption peaks corresponding to the surface  Figure S5) shows the weight losses from room temperature to 200°C or higher, which indicates the loss of physically adsorbed water and the surface hydroxy groups. The hydrated layer formed around the nanoparticles and their hydrophilic nature leads to the formation of homogeneous and closely packed nanocrystals [12]. The nanoparticles dispersed in ethanol were homogeneously assembled into bulk materials after the evaporation of ethanol without the formation of inhomogeneous aggregates. During evaporation, TiO 2 -NPs in the colloid were drawn to the meniscus by convective transport; capillary forces pushed the organization of close-packed structures [24]. Figure 2a shows the SEM image of the smooth surface; no grain boundaries or cracks were observed on micrometre scale of the bulk objects. The micropores in porous transparent bulk TiO 2 are confirmed by TEM (Fig. 2d) and N 2 physisorption measurements and are formed in the interspaces of the densely packed TiO 2 -NPs. Figure 3b shows the N 2 adsorption-desorption isotherms and the corresponding pore size distribution curves of porous transparent bulk TiO 2 . The curve indicates a sharp uptake at low relative pressures and gradually increasing uptake at higher relative pressures. The adsorption isotherm is of type I according to the IUPAC classification, which indicates the presence of micropores [25]. The specific surface area of porous transparent bulk TiO 2 was 197 m 2 /g, and the main pore size was located around 1.3 nm. This result agrees well with the homogenous and disordered assembly of nanoparticles in the porous transparent bulk TiO 2 .
The as-prepared porous transparent bulk TiO 2 was thermally stable up to 500°C. No grain boundaries or cracks on micrometre scale were observed (Fig. 2b). After heating at 800°C for 2 h, boundaries were observed on the surface of the objects using SEM, as shown in Fig. 2c, and the phase was transformed from anatase to rutile during the thermal treatment (Fig. 3a). The BET specific surface decreased, and the pore size gradually increased with heating temperature. The shape of the isotherm of the sample heated above 400°C turns to type IV with type H2 hysteresis as shown in Fig. 3e, f. This result is consistent with the transformation of pore size from microporosity to mesoporosity as given in Table 1. After heating at 500°C, the BET specific surface area decreased to 39 m 2 /g, and the pore size was 6.9 nm, which indicates that the pore size of porous

The Assembly of TiO 2 -NSs
Freshly prepared TiO 2 -NPs can be dispersed into deionized water to form a colloid without any additives [23], and porous TiO 2 -NSs can be prepared by heating 10 g/L TiO 2 -H 2 O colloid at 80°C for 36 h. The SEM image of the TiO 2 -NSs is shown in Fig. 4a, and the results indicate that the nanoparticles assembled into spheres due to the heat treatment. The assembled spheres had a spherical diameter of 100-270 nm. Detailed information of a single TiO 2 -NS was provided by TEM (Fig. 4b). The nanospheres had a rough surface and were assembled by nanoparticles (Fig. 4c). In addition, the porous interior is shown in Fig. 4b, c. As revealed in the TEM images, the spheres were composed of TiO 2 -NPs.
The HRTEM micrograph shown in Fig. 4d indicates that each nanoparticle was fully crystallized with a grain size (d TEM ) of ca. 5 nm. The result is consistent with the analysis of XRD data of untreated TiO 2 -NSs shown in Fig. 5a, which is assigned to anatase TiO 2 (JCPDS card no. 21-1276) with crystallite size (d XRD ) of 4.6 nm. Heating treatment in the water encouraged the grain growth  due to the Ostwald ripening mechanism and resulted in a larger crystallite size of 4.6 nm compared to the original size of 3.2 nm. The pore size of the TiO 2 -NSs was confirmed by N 2 physisorption measurements. Figure 5b-f shows the N 2 adsorption-desorption isotherms (type IV) [25] and the corresponding pore size distribution curves of the samples heated under different temperatures. The BET specific surface area of the untreated sample was 169 m 2 /g, and the average pore size was 1.9 nm (Fig. 5b), which was larger than the transparent bulk TiO 2 due to the bigger crystallite size of the units. After heating at different temperatures, the BET specific surface area and pore size of the assembled nanospheres changed gradually, which can also be ascribed to the increase of the crystallite size, as shown in Table 2. The TiO 2 -NSs heated at 600°C exhibited the largest pore size (6.4 nm), and the untreated TiO 2 -NSs exhibited the largest BET specific surface area (169 m 2 /g).

Photocatalytic Activities of TiO 2 -NPs and TiO 2 -NSs
The photocatalytic activities of TiO 2 -NPs and TiO 2 -NSs were evaluated by the degradation of MB under UV light irradiation. As shown in Fig. 6, the photocatalytic result indicates that the photocatalytic performance was substantially improved after the assembly of TiO 2 -NPs into TiO 2 -NSs. MB was almost completely removed in 40 min by the TiO 2 -NSs heated at 400°C, which exhibit superior performance among the photocatalysts we studied, including P25.
The number of effective active sites on the surface is an important character of the activity of a photocatalyst, which would be affected by many factors. Photocatalysts with smaller particle size usually exhibit higher activity. The reason may be that when the particle size is smaller, the specific surface area is larger, and more photocatalysis active sites are available. Whereas, when the particle size is too small, most of the electrons and holes are generated close to the surface and surface recombination is faster than interfacial charge carrier transfer processes. It is reported that when the particle size of TiO 2 is smaller than 10 nm, the photocatalytic activity will decrease [26,27]. But the crystallite size (d XRD ) of as-prepared TiO 2 -NSs in this work was 4.6 nm which is far less than 10 nm. In addition, the spheres were assembled from nanoparticles by the heat treatment. Surface states play an important role in the recombination process of electron and hole [28]. We believe that the effective contact among nanoparticles affected the surface states which could reduce the recombination chance of electron and hole. Therefore, the photocatalytic activity of TiO 2 -NSs improved compared to TiO 2 -NPs. Moreover, the defect in nano semiconductor photocatalysts plays an important role in photocatalysis [29][30][31]. The photocatalytic enhancement upon the calcination is attributed to the fact that increasing calcination temperature leads to a decrease in the concentration of bulk defects in TiO 2 due to the increased crystallinity evidenced by XRD (Fig. 5a), thus reducing the recombination of photogenerated electrons and holes. As can be seen from Fig. 6 and Table 2, the specific surface area of the TiO 2 -NSs samples decreases with increasing calcination temperature, but actually it should be understood as the increase of the specific photocatalytic activity (per unit surface area), that leads the photocatalytic activity of TiO 2 -NSs heated at 400°C higher than that of uncalcined TiO 2 -NSs and other calcinated TiO 2 -NSs.

Conclusions
Porous transparent bulk TiO 2 was formed upon slow evaporation of TiO 2 -C 2 H 5 OH colloid. The crystalline building blocks are clearly distinguishable in the freestanding macroscopic bodies. Porous TiO 2 -NSs were assembled by refluxing the TiO 2 -H 2 O colloid and