Synthesis of Monolithic TiO2 Aerogels With Relatively Low Shrinkage and Improved Formability Assisted by CTAB

Monolithic TiO2 aerogels without severe shrink were obtained by the sol-gel method with the addition of the surfactant cetyltrimethylammonium bromide (CTAB) to control the hydrolysis and polycondensation process and acetonitrile solvent as the solvent to improve the crystallinity. After CO2 supercritical drying, the shrinkage ratio of monolithic TiO2 aerogels modified by CTAB decreased by up to ∼26.9%, compared with the pure TiO2 aerogel. Their apparent densities were all lower than 300 g/cm3. X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Fourier Transform infrared spectroscopy (FTIR) and BET Specific Surface Area Analysis were used to analyze the as-synthesized samples. The results revealed that all the samples were anatase-TiO2 phase with nanoporous network structures. The specific surface areas reached 250.2 m2/g confirmed by the BET (Brunaur–Emmett–Teller method) analysis. However, TiO2 aerogels without the addition of CTAB showed evident agglomeration and collapse of the network in comparison with CTAB-added samples. To further study the structure-property relationship, the photocatalysis performance of as-synthesized and 300°C-calcined aerogels was carried out contrastively. Interestingly, the influences of the CTAB adding amount of as-synthesized and calcined TiO2 aerogels are negative and positive, respectively, which is probably due to the synergistic effect of CTAB hindrance and grain refinement. Potentially, This kind of TiO2 aerogels assisted by CATB with low density, small shrinkage, improved formability, high specific surface area and fine crystalline grain may be applied in various applications, such as electrochemistry, photocatalysis, etc.


INTRODUCTION
Nano-TiO 2 materials have become a widely concerned research topic worldwide due to their excellent semiconductor properties (Chen, 2009;Muniz et al., 2011;Noman et al., 2019;Xie et al., 2019). Photocatalysis is one of the most popular topics (Znaidi et al., 2001;Topcu et al., 2016;Guo et al., 2018;Sanjay et al., 2019), and its efficiency is primarily affected by the specific surface area and grain size. To improve the photocatalytic effect, TiO 2 powder doped into different materials was discussed in previous studies (Anucha et al., 2021;Bathula et al., 2021). As aerogel is a kind of material with high specific surface area and large porosity (Wen et al., 2018;Yan et al., 2018;Bi et al., 2019;Wen et al., 2020), it is a better choice to study the photocatalysis of TiO 2 aerogel (Moussaoui et al., 2017;Qingge et al., 2018;Zhang et al., 2018). However, it is tough to prepare a kind of TiO 2 aerogel with low density and high porosity. The pure TiO 2 gel is very easy to shrink during the aging and drying process and easy to re-dissolve during the replacement process. Therefore, it is widely believed that molding the TiO 2 aerogel, fragile and easy to shrink, is really challenging.
Due to the fragile network, it is beneficial to combine TiO 2 aerogel with another nanomaterial to have more stable nanostructure and excellent porosity (Cheng et al., 2016;Xiang et al., 2018;Luna et al., 2020;Liu et al., 2021). Zu et al. have found that the composite aerogel obtained by combining SiO 2 and TiO 2 has good moldability and a superior photocatalytic effect (Zu et al., 2015). The TiO 2 /carbon composites have also been well studied as photocatalysts since carbon acted as a scaffold (Martins et al., 2017;Parale et al., 2019). An excellent supporting template can increase the formability and specific surface area, thereby further enhancing the photocatalytic performance. As a surfactant, cetyltrimethylammonium bromide (CTAB) has a perfect poreforming effect and could play the role of framework support. There have been many studies on the addition of CTAB to improve the performance of TiO 2 gel since it can effectively increase the specific surface area (Zhong et al., 2013;Wu et al., 2018;Dong et al., 2020). However, TiO 2 aerogel requires subsequent heat treatment to improve its crystallinity (Baia et al., 2006), which removes CTAB (Nadrah et al., 2017) and causes a severe collapse of the pore structure and limits the improvement of photocatalytic performance. It is necessary to find a way to avoid the calcination in order to obtain a kind of TiO 2 aerogel with larger specific surface area, as well as better formability. Studies have shown that the selected solvent can improve the crystallinity of TiO 2 aerogels (Hu et al., 1992). Therefore, if the traditional solvent ethanol is replaced by another solvent, it is possible to avoid the disadvantages of high-temperature treatment and make the aerogel obtained by supercritical drying have good crystallinity and improved formability.
In our work, we chose acetonitrile as the solvent, added different amounts of CTAB in the sol-gel process, and successfully obtained bulk TiO 2 aerogels with good formability and high porosity. This is a simple method by sol-gel process to prepare TiO 2 aerogel without the assistance of any other inorganic material. The combination of CTAB and acetonitrile realized to get a kind of monolithic TiO 2 aerogel with better formability, lower density and higher porosity. To further understand the role of CTAB, we also compared the differences in the aerogels' microstructure before and after heat treatment and the changes in photocatalytic performance.

Preparation of TiO 2 Aerogels
In our work, we used tetrabutyl titanate as the precursor, acetonitrile as the solvent, acetic acid as the inhibitor, concentrated nitric acid as the pH value modifier, and a small amount of deionized water as well as CTAB to produce the TiO 2 aerogel. Different amounts (as shown in Table 1) of cetyltrimethylammonium bromide (CTAB) were dissolved in 30 ml of acetonitrile and 4.5 ml of distilled water at room temperature (RT) to achieve colorless transparent solution A, which was ready to adjust the microstructure of TiO 2 gel, especially the pore-size distribution. 6 ml of acetic acid, 450 μl of concentrated nitric acid, and 12 ml of tetrabutyl titanate were added successively into 30 ml of acetonitrile at 0°C in an ice bath to achieve pale yellow semitransparent solution B. The pH value of solution B was about 1. After stirring for 15 min, solution A was poured slowly into solution B. It is worth noting that the gelation time was shorter if more CTAB was added. Hence, the uniform mixture must be transferred to a proper container (such as plastic capsules) before it gelled. Its color turned opaque milk-white after the gelation process at room temperature. The wet gel was sealed and aged for 2 days, then removed and repeatedly substituted by acetonitrile every 8 h for 6 times to remove water and residual chemicals. At last, it was dried by CO 2 supercritical fluid to obtain TiO 2 aerogel.

Characterizations
A Model Rigaku D/Max-RB powder X-ray diffractometer (XRD, Rigaku, Tokyo, Japan) was operated to analyze the phase structure of our samples. Cu target Kα radiation (λ 0.15406 nm) was adopted in the test, with the 40 kV working voltage and 0.03 A current. The scanning step was adopted with the step length of 0.08°and the scanning range of the diffraction angle of 10°∼80°. The morphology was observed by a Philips XL30FEG scanning electron microscope (SEM, Royal Philips Electronics, Amsterdam, Netherlands) with an acceleration voltage of 10 kV; the sample was subject to gilding before observation. Transmission electron microscopy (TEM) was conducted with a Model JEM-2100 electron microscope (JEOL Corp, Tokyo, Japan) operating at 200 keV. Fourier Transform infrared spectra were recorded by a Bruker TENSOR27 spectrometer (FTIR, Bruker, Germany) from 4000 to 400 cm −1 . Nitrogen adsorption-desorption isotherms were measured at liquid nitrogen temperature (77 K) by a Quantachrome Autosorb-1MP analyzer (Quantachrome, Boynton Beach, FL, United States) after the samples were degassed in a vacuum at 100°C for at least 12 h. The specific surface area was calculated by Brunaur-Emmett-Teller (BET) method. The absorption spectra of all the samples were measured using a UV/vis/NIR spectrophotometer (UV-Vis, Jasco V-570, Japan).

Appearance and Structural Characterization
In our work, five different TiO 2 aerogels were prepared according to varying amounts of CTAB named CTAB-n (n 0, 1, 2, 3, 4, as shown in Table 1 was determined by m(CTAB)/m(Ti(OC 4 H 9 ) 4 ), where m(CTAB) and m(Ti(OC 4 H 9 ) 4 ) were the mass of added CTAB and the mass of tetrabutyl titanate during the sol-gel process. All of the samples obtained had monolithic appearances of regular cylinders. The bulk density of samples was determined by the weighting method. Table 1 lists their average values of densities, diameters and linear shrinkage ratios after repeated measurements. The linear shrinkage ratio was determined by 100 × (D C − D S )/D C , where D C and D S were the diameters of the container (40 mm) and the TiO 2 aerogels, respectively. It indicated that the shrinkage ratio of monolithic TiO 2 aerogels modified by CTAB decreased by up to ∼26.9%, compared with pure TiO 2 aerogels (CTAB-0). Figure 1 shows the appearances of five different TiO 2 aerogels. It was obviously observed that the more CTAB was added, the less shrinkage the TiO 2 aerogels had. Supplementary Figure S1 shows the diameter comparison of five TiO 2 aerogels with pure TiO 2 aerogel (CTAB-0) as the reference, which is much easier for us to understand the effect of CTAB. The XRD analysis results are shown in Figure 2. The diffraction peaks, located at 2θ 25.281°, 37.800°, 48.049°, 53.890°, 55.060°, 62.119°, and 75.029°, were detected and indexed as anatase-TiO 2 phase (PDF No. 21-1272). With acetonitrile as the solvent, the TiO 2 aerogels obtained have good crystallinity. Studies show that the calcination temperature obviously effects the phase formation (Qin et al., 2013;Zirakjou et al., 2015;Topcu et al., 2016). With ethanol as the solvent, it is amorphous TiO 2 aerogel if the calcination temperature is below 200°C (Qingge et al., 2018). Normally, the temperature should be above 450°C to obtain anatase TiO 2 . However, in our work, even though the TiO 2 aerogels didn't be calcined, the typical (101) , and (215) diffraction peaks were easily found, which indicated that using acetonitrile as the solvent was beneficial to improve the crystallinity. Meanwhile, the SAED (selected area electron diffraction) patterns ( Figure 3) show a series of concentric diffraction rings corresponding to the Miller indices of the tetragonal lattice of the anatase-TiO 2 phase, consistent with the XRD results. In order to study the functional groups of these samples, the FTIR technique was carried out and the spectra are shown in Supplementary Figure  S2. According to the FTIR spectra, the broadband at 3,424 cm −1 could be ascribed to the O-H stretching vibration of absorbed water attached to the surface, whereas the peaks at 1,630 cm −1 could be assigned to the bending vibrations of O-H (Chellappa et al., 2015;Yao et al., 2015). The presence of peak at 1,047 cm −1 was possibly due to the adsorbed water molecules. The characteristic absorption peak of CO 2− 3 appears at 1,385 cm −1 , which indicated that CO 2 adsorbed on the surface of TiO 2 reacted with H 2 O to form CO 2− 3 (Bezrodna et al., 2004;Wu et al., 2018). The band at 900-500 cm −1 was attributed to the vibrations of Ti−O−Ti bonds (Wen and Zhang, 2016;Feizpour et al., 2019;Jnido et al., 2019). Figure 4 shows the SEM diagrams (A-E) and the TEM images (F-J) of five different aerogels (CTAB-0, 1, 2, 3, 4), respectively. All the TiO 2 aerogels exhibited random nanoporous network structures, which consisted of regular spherical or near-spherical clusters and irregular pores. With the addition of CTAB, the aggregation of nanoparticles and the collapse of the network were reduced, which was in accordance with the appearance analysis.

Morphology and Microstructure Analysis
The nitrogen adsorption/desorption isotherms of five assynthesized TiO 2 aerogels are shown in Supplementary Figure S3. Based on the BJH (Barrett-Joyner-Halenda model) analysis on the desorption curves, their pore-size distributions are compared in Figure 5. Table 2 lists the specific surface areas, the average diameters, and the total volumes. According to the pore distribution and the data related to the pore structures, it is easily summarized that the addition of CTAB is beneficial to the increase of the pore size. But in our work, there is a suitable value of the CTAB adding amount. Here, the recipe of CTAB-3  was considered as a better choice because the sample has a lower density and a better nanoporous network structure.

Structure-Property Relationship
To further study the role of CTAB, these five TiO 2 aerogels were placed in a muffle furnace at 300°C for 1 h with a heating rate of 3°C/min. The calcined TiO 2 aerogels were also characterized by X-ray diffraction (XRD), a scanning electron microscope (SEM), a transmission electron microscope (TEM), and specific surface area analysis (BET). The results are shown in detail in the Supplementary Material. Supplementary Figures S4, S5 indicated that the 300°C-calcined TiO 2 aerogels were anatase-TiO 2 phase (PDF No. 21-1272) and had higher crystallinities. However, there were apparent particle agglomeration and collapse of the network according to the results of SEM (Supplementary Figure S6), TEM (Supplementary Figure S6), and BET (Supplementary Figures S7, S8; Supplementary Table S1).
The particle sizes of as-synthesized and 300°C-calcined aerogels were evaluated by measuring the diameters of particles (50 measurements for each sample) in TEM images. Supplementary  Figures S9, S10 were two examples of the measurements. Besides, the crystallite sizes of these two types of aerogels were calculated by the Scherrer formula (Gnaser et al., 2011;Suryanarayana et al., 2011;Sharma et al., 2019;Dong et al., 2020). Here, we chose the typical (101), (004), (200), and (213) diffraction peaks to compare the size in different crystal plane orientations. According to the comparisons in Figures 6, 7, for aerogels that have not been heat-treated, the particle size and crystal grain size showed the opposite changing trends. In contrast, the calcined samples showed a similar changing trend. It was likely that CTAB was coated on the outside of the TiO 2 particles.
The more CTAB was added, the thicker the coating layer was. This may explain the result in Figure 6A. In this case, the coating of CTAB could also strengthen the nano-skeleton structure and improve the moldability of the TiO 2 aerogels. Figure 7 confirmed that the more CTAB was added, the smaller the TiO 2 crystal particles grew. It is possible that the coating of CTAB could limit the growth of TiO 2 particles to a certain extent. This effect of grain refinement is beneficial to the photocatalysis (Luna et al., 2020).
To further study the structure-property relationship, the photocatalysis performance of as-synthesized and 300°Ccalcined aerogels was carried out contrastively. The degradations of gentian violet (GV) and methylene blue (MB) were both tested. 10 mg of TiO 2 aerogel was dispersed in 40 ml of dye solution (GV or MB, 20 mg/L) and placed in the dark for 1 h before UV-light irradiation. Then the dye solution in presence of TiO 2 aerogel was exposed under a UV lamp for 120 min. The absorption spectra of the dye solution were measured by a UV/ vis/NIR spectrophotometer. Figure 8 shows the dye solution concentration percentages, which present the photocatalysis performance of different aerogels. Here, C 0 and C were the initial dye solution concentration (20 mg/L) and the final dye solution concentration after UV-light irradiation. Meanwhile, the adsorptions of dye solution (GV or MB, 20 mg/L) for all the assynthesized and 300°C-calcined aerogels were measure as well and the results are shown in Supplementary Figure S11. The degradation rate curves with time of CTAB-0 and CTAB-4-300 are compared in Supplementary Figure S12, which are consistent with the results in Figure 8. Interestingly, the influences of the CTAB amount of as-synthesized and calcined TiO 2 aerogels were negative and positive, respectively, which is probably due to the synergistic effect of CTAB hindrance and grain refinement. In other words, with the increase of CTAB added, the photocatalysis of uncalcined samples became worse. However, as for the calcined samples, due to the addition of CTAB, the grain size could become smaller, thereby contributing to photocatalysis. Therefore, the performance of photocatalytic performance may be influenced by the combination of the two above effects. As a result, the CTAB-3 TiO 2 aerogel was considered as a suitable choice because it had good performance of photocatalysis with finer crystalline grain, lower density, higher specific surface area, and better formability. What's more, this kind of monolithic TiO 2 aerogel with low density is a perfect template to prepare the selfsupporting nanoporous titanium aerogel .

CONCLUSION
In our work, we successfully prepared monolithic CTAB-modified TiO 2 aerogels without severe shrink using acetonitrile as the solvent to improve the crystallinity. Five TiO 2 aerogels were compared to study the effect of the CTAB adding amounts. To further understand the role of CTAB, we discussed the structure-property relationships of the as-synthesized and 300°C-calcined aerogels. It indicated that the performance of photocatalysis was probably influenced by the synergistic effect of CTAB hindrance and grain refinement. The CTAB-3 TiO 2 aerogel was considered as a good choice with a relatively excellent performance of photocatalysis since it had a lower density (175.8 mg/cm 3 ) and a higher specific surface area (250.2 m 2 / g). Potentially, this kind of TiO 2 aerogels assisted by CTAB with low density, small shrinkage, improved formability, high specific surface area and fine crystalline grain could play an important part in various applications, such as electrochemistry, photocatalysis, etc.

DATA AVAILABILITY STATEMENT
The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding authors.

AUTHOR CONTRIBUTIONS
BZ, AD, and TN designed the study. TN carried out the experiments and wrote the original draft manuscript. TN and FIGURE 7 | Crystallite size analysis of (A) TiO 2 aerogels without the calcination, (B) 300°C-calcined TiO 2 aerogels. ZeZ carried out the characterization tests with the support of ZhZ and JS. JY and XJ made the data interpretation. BZ and AD revised and approved the submitted version.