Synthesis of co-existing phases Sn-TiO2 aerogel by ultrasonic-assisted sol-gel method without calcination

Materials Letters 228 (2018) 379–383

Contents lists available at ScienceDirect

Materials Letters journal homepage: www.elsevier.com/locate/mlblue

Synthesis of co-existing phases Sn-TiO2 aerogel by ultrasonic-assisted sol-gel method without calcination Xu Wei a,b, Huidong Cai a, Qingge Feng a,b,⇑, Zheng Liu a, Dachao Ma a, Kao Chen a, Yan Huang a a b

School of Resources, Environment and Materials, Guangxi University, Nanning 530004, People’s Republic of China School of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004, People’s Republic of China

a r t i c l e

i n f o

Article history: Received 12 March 2018 Received in revised form 5 June 2018 Accepted 14 June 2018

Keywords: Sn-TiO2 aerogel Ultrasonic-assisted Sol-gel preparation Co-existing phases Crystal structure Tetracycline hydrochloride degradation

a b s t r a c t Generally, TiO2 aerogel prepared by the sol-gel technique is amorphous and it exhibits special catalytic property only in the crystalline state after calcination at high temperature, but the calcination will significantly decrease aerogel’s specific surface area. Thus the aim of this study is to prepare co-existing crystalline phase Sn-TiO2 aerogel without annealing. It was synthesized by ultrasonic-assisted sol-gel method using stannous chloride as dopant. Results indicated that the uncalcined Sn-TiO2 aerogel composed by amorphous and anatase phase (denoted as co-existing phases) has a specific surface area of 172.4 m2/g. Optimal degradation efficiency of 96% for tetracycline hydrochloride was achieved within 30 min of UV irradiation using the co-existing phases Sn-TiO2 aerogel, due to the synergistic effect of adsorption and photocatalytic activity. Ó 2018 Elsevier B.V. All rights reserved.

1. Introduction The anatase TiO2 aerogel has been considered as a promising photocatalytic material due to its extraordinary physicochemical properties such as high catalytic activity, high porosity, high specific surface area, low toxicity, and extremely low density [1–4]. Generally, TiO2 aerogel prepared by the sol-gel technique is amorphous and it exhibits special catalytic property only in the crystalline state. Thus, to crystallize, the amorphous TiO2 aerogel should be calcined at high temperature; but the surface area and porosity of the aerogel would be significantly decreased after calcination. Recently, many research efforts have been focused on direct synthesis of anatase TiO2 aerogel by using the high temperature supercritical drying technique [5–9]. However, high temperature supercritical drying is dangerous in operation due to the high temperature and high pressure. Furthermore, the specific surface area of TiO2 aerogel prepared by high temperature supercritical drying is not high because of the collapse of aerogels’ frameworks. Therefore, to promote photocatalytic activity and industrial application of TiO2 aerogel, direct synthesis of anatase crystalline phase TiO2 aerogel with high specific surface area is of great significance.

⇑ Corresponding author at: School of Resources, Environment and Materials, Guangxi University, Nanning 530004, People’s Republic of China. E-mail address: [email protected] (Q. Feng). https://doi.org/10.1016/j.matlet.2018.06.050 0167-577X/Ó 2018 Elsevier B.V. All rights reserved.

It has been reported that Sn doping had an important effect on transformation of TiO2 gel to anatase, and the lowest transformation temperature had decreased to 329 °C [10]. However, to the best of our knowledge, few attempts have been made to directly synthesize crystalline phase TiO2 aerogel by Sn doping. In our earlier study, pure TiO2 aerogel with a high surface area of 563.6 m2/g was synthesized using an ultrasonic-assisted sol-gel method [11], but to obtain crystal TiO2 aerogel, calcination at high temperature was still needed. Considering the advantages of Sn doping and the ultrasonic-assisted sol-gel method for TiO2 aerogel synthesis, we expected that desired TiO2 aerogel could be obtained by combining these two methods. Thus, the aim of this study was attempted to directly synthesize crystalline phase Sn-TiO2 aerogel without annealing by doping stannous chloride in ultrasonic-assisted solgel process. The photocatalytic activity of the as-prepared aerogels towards pollutant degradation was evaluated by degradation of tetracycline hydrochloride, which is one of the widely used antibiotics and a potential contaminant because its residues can induce higher antibiotic resistance and serious environment problems.

2. Materials and methods The co-existing phases Sn-TiO2 aerogels was synthesized according to our previous report with a little modification through the ultrasonic-assisted sol-gel method and vacuum drying [11] (see ESI, Fig. S1y). Briefly, ethanol, hydrochloric acid (HCl) and

380

X. Wei et al. / Materials Letters 228 (2018) 379–383

stannous chloride (SnCl22H2O) with different molar ratios of Sn:Ti from 1:6 to 1:20 were mixed in a beaker under the ultrasonic cleaner, and then tetrabutyl titanate (TBOT) was added. Then distilled water was added in shower watering way to attain yellow Sn-TiO2 alcogel. Afterwards, the alcogel was exchanged by n-hexane at 45 °C for 12 h, and then dried in vacuum at 60 °C for 24 h. For comparison, pure TiO2 aerogel was also synthesized using a similar procedure mentioned above but without adding SnCl22H2O. The aerogel samples were calcined at 500 °C for 2 h. Various techniques, including X-ray diffraction (XRD) patterns, Brunauer-EmmettTeller theory (BET), Raman spectra, high resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS), were used to characterize the as-prepared materials (more details of test condition see ESI).

3. Results and discussion Fig. 1 shows XRD patterns, N2 adsorption-desorption isotherms and the pore size distribution curve of the as-synthesized TiO2 aerogels. According to the standard diffraction spectrum (JCPDS No: 21-1272), the TiO2 aerogels and Sn-TiO2 aerogels calcined at 500 °C for 2 h were pure-phase anatase TiO2. In addition, uncalcined TiO2 aerogel was amorphous [11], while uncalcined coexisting phases Sn-TiO2 aerogel has anatase crystal face (1 0 1), (0 0 4), (2 0 0), (1 0 5), and (2 0 4) without the peak for SnO2 (Fig. 1(a)). Fig. 1(b) reveals that the peaks of anatase arose gradually with the molar ratios of Sn:Ti decreased from 1:6 to 1:20, implying that doping a small amount of Sn (n (Sn:Ti)  1:8) could promote the phase transformation from amorphous to anatase phase upon this ultrasonic-assisted sol-gel method. It might be

Fig. 1. (a), (b) XRD spectrogram, (c) the N2 adsorption-desorption isotherm and (d) pore size distribution of aerogels.

Table 1 Comparison of SBET of aerogels synthesized via different ways. Aerogel

Synthetic method

Drying condition

Highest SBET (m2 g1)

References

TiO2 aerogel Eu-TiO2 aerogel TiO2 aerogel TiO2 aerogel TiO2 aerogel co-existing phases Sn-TiO2 aerogel

Supercritical drying in isopropanol Supercritical drying High temperature supercritical drying Supercritical ethanol drying High temperature supercritical drying Ultrasonic assisted sol–gel and vacuum drying

300 °C, 100 bar Highest about 82 bar 60  110 bar – 300 °C, 100 bar 60 °C, 0.08 bar

92 96 82 132.3 89 172.4

[5] [6] [7] [8] [9] This study

X. Wei et al. / Materials Letters 228 (2018) 379–383

381

Fig. 2. Raman spectra and HRTEM images (a-b) uncalcined co-existing phases Sn-TiO2 aerogel (1:10), (c-d) Sn-TiO2 aerogel-500 °C/2h, (e-f) uncalcined TiO2 aerogel, and (g-h) TiO2 aerogel-500 °C/2 h.

382

X. Wei et al. / Materials Letters 228 (2018) 379–383

due to the closed ionic radii values of both Ti (0.068 nm) and Sn (0.071 nm), which can lead to easily form a uniformly dispersed solid solution [12,13]. According to the IUPAC classification [14], the isotherm of co-existing phases Sn-TiO2 aerogel exhibited similar behavior to type I isotherms in the H4 hysteresis loop, while the sample calcined at 500 °C/2h was agreed well with type IV isotherms in the H2 hysteresis loop (Fig. 1(c) and (d)), implying that the co-existing phases Sn-TiO2 aerogel possessed micropore structures and they could be converted into mesoporous structures after calcination at 500 °C for 2 h. As can be seen from Table 1, the coexisting phases Sn-TiO2 aerogel had a specific surface area of 172.4 m2/g, which was higher than other anatase TiO2 aerogels calcined at high temperature or prepared by high temperature supercritical drying method. The results of Raman spectra and HRTEM were given in Fig. 2. As shown in the Fig. 2(a), (c) and (g), the characteristic anatase TiO2 Raman peaks appeared at 144, 400, 515, and 640 cm1 could be assigned to the stretching mode of Eg(1), B1g(1), B1g(2), and Eg(3) [15,16], respectively, indicating that the co-existing phases Sn-TiO2 aerogel had formed anatase phase similar to TiO2 aerogel and SnTiO2 aerogel calcined at 500 °C for 2 h. The average crystalline sizes of the co-existing phases Sn-TiO2 aerogel nanoparticles were 2–10 nm (Fig. 2(b)), while the grain sizes increased to 10–20 nm when calcined at 500 °C for 2 h (Fig. 2(d)). Analogously, the sizes of the uncalcined amorphous TiO2 aerogel were 2–5 nm (Fig. 2(f)), and the values increased to 10–30 nm after calcination (Fig. 2(h))

because of the crystalline phase transformation and crystalline growth. Note that some Raman peaks of organic groups could be observed in Fig. 2(a) and (e), which might be due to the remaining n-hexane and ethanol. As shown in Fig. 2(a) and (e), the peaks at 1132–1460, 2872, 2916–2936, and 2952–2969 cm1 could be assigned to –CH3 bending vibration mode, –CH3 stretching mode, –CH2- asymmetric stretching mode, and –CH3 asymmetric stretching mode, respectively [17]. The biggest Raman spectra difference between uncalcined co-existing phases Sn-TiO2 and uncalcined TiO2 aerogel was that the peak of Eg(1) assigned to O-Ti-O bending vibration of anatase phase could be observed for uncalcined coexisting phases Sn-TiO2, while this peak was not observed for uncalcined TiO2 aerogel. It suggested that the uncalcined coexisting phases Sn-TiO2 aerogel possessed anatase crystallization mixed with a small amount of organic groups. Also, the anatase phase was further confirmed by the selected area electron diffraction (SAED) pattern, as shown in the inset of Fig. 2(b). The SEM and EDS analyses were provided in Figs. S2 and S3y(ESI). As shown in the Fig. 3(a), the co-existing phases Sn-TiO2 aerogel showed the highest total tetracycline hydrochloride degradation compared to calcined Sn-TiO2 aerogel and pure calcined TiO2 aerogel due to its preferable adsorption activity. The degradations of tetracycline hydrochloride using the aerogels with Sn:Ti ratios among 1:10–1:20 were greater than those with Sn:Ti ratios of 1:6–1:10, possibly owing to more complete anatase phases of the former aerogels (Fig. 3(b)). In short, the co-existing phases

Fig. 3. The adsorption and photocatalytic activities towards tetracycline hydrochloride degradation (a) different calcined and uncalcined samples, (b) different molar ratios of Sn:Ti samples.

X. Wei et al. / Materials Letters 228 (2018) 379–383

Sn-TiO2 aerogel exhibited a good catalytic activity for degradation of tetracycline hydrochloride due to the synergistic effect of adsorption and photocatalytic activity.

383

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.matlet.2018.06.050.

4. Conclusion References In summary, the co-existing phases Sn-TiO2 aerogel was successfully synthesized by an ultrasonic assisted sol-gel and vacuum drying method using stannous chloride as doping agent. The most remarkable conclusion is that uncalcined Sn-TiO2 aerogel had coexisting phases, which was confirmed by XRD, Raman, and HRTEM. The results revealed that the co-existing phases Sn-TiO2 aerogel had large specific surface area with micropores, which could be converted into mesopores by calcination. The co-existing phases Sn-TiO2 aerogel exhibited a good catalytic activity for degradation of tetracycline hydrochloride due to the synergistic effect of adsorption and photocatalytic activity. Hence, it is a promising prospect for the preparation of TiO2 aerogel to avoid high temperature calcination or high temperature supercritical drying in industrial application. More importantly, it also can be expanded to directly prepare other crystalline aerogels by seeking a suitable dopant. Acknowledgment The project was supported by the National Natural Science Foundation of China (51362003, 51762004).

[1] X. Chen, S.S. Mao, Chem. Rev. 107 (2007) 2891. [2] M.D. Hernándezalonso, F. Fresno, S. Suárez, J.M. Coronado, Energy Environ. Sci. 2 (2009) 1231. [3] Y. Lan, Y. Lu, Z. Ren, Nano Energy 2 (2013) 1031. [4] H. Maleki, N. Hüsing, Appl. Catal. B 221 (2017) 530. [5] Y.V. Kolen Ko, A.V. Garshev, B.R. Churagulov, S. Boujday, P. Portes, C. ColbeauJustin, J. Photochem. Photobiol. A 172 (2005) 19. [6] H.S. Kibombo, A.S. Weber, C.M. Wu, K.R. Raghupathi, R.T. Koodali, J. Photochem. Photobiol. A 269 (2013) 49. [7] S.K. Parayil, R.J. Psota, R.T. Koodali, Int. J. Hydrogen Energy 38 (2013) 10215. [8] Y. Kong, X.D. Shen, S. Cui, Mater. Lett. 117 (2014) 192. [9] R. Moussaoui, K. Elghniji, M.B. Mosbah, E. Elaloui, Y. Moussaoui, J. Saudi. Chem. Soc. 21 (2017) 751. [10] Z.M. Shi, L. Yan, L.N. Jin, X.M. Lu, G. Zhao, J. Non-Cryst. Solids 353 (2007) 2171. [11] Q. Feng, H. Cai, H. Lin, S. Qin, Z. Liu, D. Ma, Y. Ye, Nanotechnology 29 (2017) 75702. [12] F. Sayilkan, M. Asiltürk, P. Tatar, N. Kiraz, E. Arpaç, H. Sayilkan, J. Hazard. Mater. 144 (2007) 140. [13] E.M. El-Maghraby, Phys. B 405 (2010) 2385. [14] K.S.W. Sing, Pure Appl. Chem. 57 (1985) 603. [15] T. Ohsaka, F. Izumi, Y. Fujiki, J. Raman Spectrosc. 7 (1978) 321. [16] S. Kelly, F.H. Pollak, M. Tomkiewicz, J. Phys. Chem. B 101 (1997) 2730. [17] P. Patnaik, J.A. Dean, Dean’s Analytical Chemistry Handbook, McGraw-Hill, 2004.