TiO2 nanocomposite films with enhanced visible-light photocatalytic activity

Catalysis Communications 58 (2015) 30–33

Contents lists available at ScienceDirect

Catalysis Communications journal homepage: www.elsevier.com/locate/catcom

Short Communication

Preparation of transparent PVA/TiO2 nanocomposite films with enhanced visible-light photocatalytic activity Xiuyun Liu a, Qirong Chen b, Lizhen Lv a, Xiaoying Feng a, Xiangfu Meng a,⁎ a b

Beijing Key Laboratory for Optical Materials and Photonic Devices, Department of Chemistry, Capital Normal University, Beijing 100048, China Beijing Center for Physical and Chemical Analysis (BCPCA), Beijing 100089, China

a r t i c l e

i n f o

Article history: Received 10 June 2014 Received in revised form 21 August 2014 Accepted 22 August 2014 Available online 30 August 2014 Keywords: Photocatalyst Poly(vinyl alcohol) TiO2 Visible-light Nanocomposites

a b s t r a c t Polymer/semiconductor oxide nanocomposite films have been intensively investigated for various applications. In this work, we reported a simple hydrothermal method to fabricate highly transparent poly(vinyl alcohol)/titanium dioxide (PVA/TiO2) nanocomposite films with enhanced visible-light photocatalytic activity. The as-prepared PVA/TiO2 nanocomposite films showed high optical transparency in the visible region even at a high TiO2 content (up to 40 wt.%). The determination of photocatalytic activity by photodegradation of methyl orange (MO) and colorless phenol showed that PVA/TiO2 nanocomposite films exhibited enhanced visible-light photocatalytic activity and excellent recycle stability. This work provided new insights into fabrication of polymer/TiO2 nanocomposites as high performance photocatalysts in waste water treatment. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Visible-light-driven photocatalysis is currently a prime research area because of its potential application in clean and renewable energy as well as pollution abatement [1]. TiO2, as a promising photocatalyst in the purification of air and water, has attracted enormous attention in recent years. However, the poor utilization of solar energy and the difficulty of recovering TiO2 nanoparticles from suspension limit its practical applications. To achieve high photocatalytic performances and easy recover capability, several methods have been proposed to immobilize TiO2 nanoparticles on solid substrates, such as silica [2], zeolite [3], montmorillonite [4], and so on. However, these substrates are often opaque and lower light efficiency. Therefore, immobilization of TiO2 nanoparticles on transparent substrates will be an efficient approach to improve the photocatalytic activity. Poly(vinyl alcohol) (PVA) is a water-soluble polymer, which is widely used as functional membranes with high transparency. Recently, PVA has been extended to visible-light photocatalyst as a support. Wang et al. [5] reported a hydrothermal method to prepare TiO2/D-PVA microscale particles, which exhibited high visible-light photoactivity for methyl orange (MO) degradation. In the hydrothermal process, Ti(OH)4 precursor was degraded to produce TiO2 and PVA was degraded to produce conjugated unsaturated D-PVA that was doped onto the surface of TiO2. More recently, Lei et al. [6] used commercial TiO2 (P25) ⁎ Corresponding author at: No. 105, Xisanhuanbeilu, Haidian District, Beijing 100048. China. Tel.: +86 10 68902974. E-mail address: [email protected] (X. Meng).

http://dx.doi.org/10.1016/j.catcom.2014.08.032 1566-7367/© 2014 Elsevier B.V. All rights reserved.

to fabricate conjugation-grafted PVA/TiO2 nanohybrid for visible-light photodegradation of MO. During the thermal degradation process, the PVA molecular chains coacervated on the surface of TiO2 degraded to form conjugated structures and then attached onto the surface of TiO2 via the interfacial C–O–Ti bonds, which acted as the electron transfer pathway to accelerate the excited electrons transferring from conjugation structures to TiO2. However, the commercial P25 used in this case made PVA/TiO2 nanohybrid opaque and hence led to low light unitization efficiency. In our previous work [7], we developed a facile approach to prepare sunlight-driven anatase TiO2 nanoparticles based on water-soluble titania xerogel. The as-prepared anatase TiO2 showed higher photocatalytic activity than P25 under outdoor sunlight irradiation. In this work, we reported for the first time the highly transparent PVA/TiO2 nanocomposite films using water-soluble titania xerogel as a titanium precursor. In-situ growth of nano-sized anatase TiO 2 on the PVA chains was successfully achieved via a simple hydrothermal method. The asprepared transparent PVA/TiO2 nanocomposite films exhibited superior visible-light photocatalytic activity and recycle stability in photodegradation of MO. 2. Experimental 2.1. Preparation of transparent PVA/TiO2 nanocomposite film All chemical reagents were of analytical grade and used without further purification. The water-soluble titania xerogel was synthesized according to the method reported in our previous work [7]. In a typical

X. Liu et al. / Catalysis Communications 58 (2015) 30–33

procedure, 15 mL of tetrabutyl titanate (TBT) was added into 15 mL of trifluoroacetic acid solution (50 wt.%) at room temperature. After stirring for 3 h, a transparent solution was obtained. Then the solvent was removed through a rotary evaporator. Finally, yellowish titania xerogel was obtained. 0.125 g of titania xerogel and 0.5 g of PVA were dissolved into 16 mL of deionized water, respectively. Subsequently, the above prepared two solutions were mixed together under stirring and then transferred into a 40 mL Teflon-lined autoclave, which was heated to 150 °C for 3 h. After cooling down to room temperature, the solution was poured out to a plastic dish and formed the PVA/TiO2 nanocomposite films. The film thickness could be controlled by varying the amount of the casting solution. A series of nanocomposite films with TiO2 content of 5 wt.%, 10 wt.%, 20 wt.%, 30 wt.%, and 40 wt.% were prepared. For comparison, a commercial TiO2 (Degussa P25) was also used to prepare PVA/P25 films with 20 wt.% TiO2 content under the same procedure. Untreated PVA/TiO2 nanocomposite film with 20 wt.% TiO2 was directly prepared by solution casting the solution of titania xerogel and PVA, labeled as 20% untreated. 2.2. Characterization The distribution of TiO2 nanoparticles in PVA was characterized by transmission electron microscopy (TEM, FEI, Tecnai G20). TEM samples were prepared by dripping hydrothermally treated solution onto a Cu grid. Transmittance measurements were carried out using a UV–Vis spectrophotometer (UV-8000A, Shanghai Yuanxi). FT-IR measurements were performed on a Perkin-Elmer system 2000 infrared spectrophotometer in the wavenumber range of 370–4000 cm−1 with a spectra resolution of 4 cm−1. 2.3. Photocatalytic activity measurement The photocatalytic activity of PVA/TiO2 nanocomposite films was evaluated by photodegradation of MO under visible-light irradiation. PVA/TiO2 film with a size of 5 cm × 5 cm was dipped into 30 mL MO aqueous solution (15 mg/L). The irradiation source was a 300 W Xe lamp (Philips) with a 420 nm cutoff filter to ensure irradiation by visible light only. During the photocatalytic reaction, about 5 mL of MO was taken out at fixed intervals for concentration determination using UV–Vis spectrophotometer (UV-8000A, Shanghai Yuanxi). The photocatalytic activity was calculated by measuring the concentration variation of the MO solution. For photodegradation of phenol, the initial concentration of the phenol solution was 1 × 10−4 mol/L. The concentration of the phenol solution was determined by a 4-aminoantipyrene spectrophotometric method [8] through measuring the absorbance at 510 nm with the UV-8000A UV–Vis spectrophotometer.

31

3. Results and discussion 3.1. Characterizations of samples PVA is a hydrophilic polymer and its aqueous solution has a high stability in large temperature range. After hydrothermal treatment at 150 °C, a semitransparent solution was obtained without any solid deposition at the bottom of autoclave, suggesting that TiO2 nanoparticles were intimately contacted with PVA polymer chains via chemical bonds or hydrogen bonds [9,10]. Fig. 1 shows the TEM images of a PVA/TiO2 nanocomposite film with TiO2 content of 20 wt.%. It is clear that TiO2 nanoparticles are uniformly dispersed in PVA matrix and no large aggregation is observed (Fig. 1a). The average size of TiO2 nanoparticles is about 10 nm and the lattice fringes with an interplanar distance of 0.36 nm is well consistent with the (101) lattice plane of anatase TiO2 (Fig. 1b). The small TiO2 size may be due to the confined space of PVA macromolecular chains, which limit the growth of TiO2 nanoparticles during the hydrothermal process [11]. The small grain size with anatase phase of PVA/TiO2 nanocomposite films is facilitated for the improvement of photocatalytic activity. It can be seen that TEM characterization gives us a clear evidence of the uniform dispersion of TiO2 in PVA matrix. However, in order to follow the arrogation of nanoparticles, transmittance measurement is better to be performed on the PVA/TiO2 films because a small content of large domains existing could result in obvious transmittance loss. Fig. 2 shows the UV–visible transmittance spectra of neat PVA film and PVA/TiO2 nanocomposite films. It is clearly seen that all the films demonstrate high transparency in the whole visible region, even at a high TiO2 content up to 40 wt.%. This suggests that TiO2 particles with nanometer sizes are homogeneously dispersed in PVA matrix. Compared with pure PVA film, PVA/TiO2 nanocomposite films also show strong UV absorption due to the incorporation of UV-absorbing TiO2 component. High transparency and UV light adsorption are necessary prerequisites for increasing the light utilization efficiency in photodegradation of organic pollutant. 3.2. Photocatalytic activity of samples The photocatalytic performances of PVA/TiO2 nanocomposite films were evaluated using MO as model pollutant under visible-light irradiation. Fig. 3 shows the photocatalytic activity of all the as-prepared PVA/ TiO2 nanocomposite films. As expected, neat PVA film shows no photocatalytic activity for MO degradation. While PVA/TiO2 nanocomposite films prepared through our method exhibit much higher photocatalytic activity than that of PVA/P25 film. After 15 min irradiation, MO is almost photodegraded by PVA/ TiO2 nanocomposite films. This demonstrates that transparency plays a key role in the photodegradation of MO. For

Fig. 1. TEM of PVA/TiO2 (20 wt.%) nanocomposites film (a) low resolution and (b) high resolution.

32

X. Liu et al. / Catalysis Communications 58 (2015) 30–33

Fig. 2. UV–vis spectra of (a) neat PVA film and PVA/TiO2 nanocomposite films with different TiO2 contents: (b) 5 wt.%, (c) 10 wt.%, (d) 20 wt.%, (e) 30 wt.%, and (f) 40 wt.%. The thickness of all films is around 30 μm.

Fig. 4. FT-IR spectra of neat PVA and PVA/TiO2 nanocomposite film irradiated for different times under UV light.

highly transparent PVA/TiO2 nanocomposite film, the activity site on both sides of the film can be irradiated by light. While for opaque PVA/P25 film, only one side can be utilized. Moreover, the photocatalytic activity is slightly increased with increasing the TiO2 content from 10 wt.% to 40 wt.%, suggesting that increasing TiO2 content has limited effect on the improvement of photoactivity. Interestingly, untreated PVA/TiO2 nanocomposite film without hydrothermal treatment also shows very high photocatalytic activity. This may be due to the fact that titania xerogel has photocatalytic activity. In-situ modification of titania xerogel by TFA can reduce the recombination of photo-generated electrons and holes due to the strong electron-withdrawing effect of CF3 group and hence increase the photocatalytic activity [12,13]. As it is well known that MO may absorb visible light, the sensitization possibility for samples should be considered [14]. Therefore, to ensure the visible-light photocatalytic activity and exclude the dye sensitization under visible light, a colorless compound, phenol, is also chosen as a model pollutant since phenol shows no absorption in the visible region. Fig. 3b shows the photodegradation curves of phenol under visible light irradiation. It can be seen that phenol solution is almost photodegraded by PVA/TiO2 nanocomposite films after 5 h of visible-light irradiation although it has a slower photodegradation rate than that of MO

photodegradation. The visible-light photocatalytic activity of PVA/TiO2 nanocomposite films may be attributed to the surface fluorination of TiO2 nanoparticles. Yu et al. [15] reported that TFA could be partly decomposed under high-temperature hydrothermal condition and resulted in the production of F−. In the present work, TFA chemically bonded on titania xerogel can also be decomposed during hydrothermal growth and releases fluoride ions (F−), which further leads to the surface fluorination of TiO2 and improvement of the visible-light photocatalytic activity [16]. For practical application, photocatalyst is always expected to be useful for a relatively long time or recyclable for multi-cycle use. As shown insert of Fig. 3a, PVA/TiO2 nanocomposite films also exhibit stable photocatalytic activity during recycle degradation. It is particularly noteworthy that recovery of nanocomposite films from the reactor is very easy. Therefore, the present work gives us a new insight into practical applications of nanocomposite films for water treatment. 3.3. Stability of samples during photocatalysis In order to evaluate the stability of nanocomposite films during photodegradation, FT-IR is used to monitor the variation of carboxyl

Fig. 3. Photocatalytic activities of PVA/TiO2 nanocomposite films on the degradation of MO (a) and phenol (b) under visible light irradiation (N420 nm). Insert in (a): cycle runs of PVA/TiO2 (20 wt.%) nanocomposite film for degradation of MO under visible-light irradiation.

X. Liu et al. / Catalysis Communications 58 (2015) 30–33

adsorption band, which is generated during polymer photodegradation [17]. As shown in Fig. 4, there is no apparent degradation for neat PVA even under UV light irradiation for 100 h, indicating that PVA is sufficiently stable for long time irradiation. As for PVA/TiO2 nanocomposite film, the carboxyl adsorption at 1778 cm−1 is slightly increased after 100 h of UV irradiation, suggesting that PVA/TiO2 nanocomposite films are stable enough for long time recycle. Therefore, PVA/TiO2 nanocomposite films with cost-efficiency and long-time stability have a promising application in practical waste water treatment.

4. Conclusions In summary, we developed a simple hydrothermal route to prepare highly transparent PVA/TiO2 nanocomposite films based on watersoluble titania. PVA/TiO2 nanocomposite films showed high photocatalytic activity and multiple recycle stability. The low cost for fabrication and easy recovery may render the films a promising application in water treatment.

Acknowledgments This work is supported by the National Natural Science Foundation of China (No. 51203094), Joint program of Beijing Natural Science Foundation and Beijing Academy of Science and Technology (No. L140005), Beijing NOVA Programme (Z131101000413038) and Beijing Local College Innovation Team Improve Plan (IDHT20140512).

33

Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.catcom.2014.08.032. These data include MOL files and InChiKeys of the most important compounds described in this article. References [1] C. Chen, W. Ma, J. Zhao, Chem. Soc. Rev. 39 (2010) 4206–4219. [2] Z. Ding, X. Hu, G.Q. Lu, P.-L. Yue, P.F. Greenfield, Langmuir 16 (2000) 6216–6222. [3] S. Fukahori, H. Ichiura, T. Kitaoka, H. Tanaka, Environ. Sci. Technol. 37 (2003) 1048–1051. [4] C. Ooka, S. Akita, Y. Ohashi, T. Horiuchi, K. Suzuki, S.-i. Komai, H. Yoshida, T. Hattori, J. Mater. Chem. 9 (1999) 2943–2952. [5] Y.Z. Wang, M.Q. Zhong, F. Chen, J.T. Yang, Appl. Catal. B Environ. 90 (2009) 249–254. [6] P. Lei, F. Wang, S. Zhang, Y. Ding, J. Zhao, M. Yang, ACS Appl. Mater. Interfaces 6 (2014) 2370–2376. [7] X.F. Meng, L. Qi, Z.C. Xiao, S.Y. Gong, Q.L. Wei, Y. Liu, M.S. Yang, F. Wang, J. Nanopart. Res. 14 (2012) 1–7. [8] X.T. Hong, Z.P. Wang, W.M. Cai, F. Lu, J. Zhang, Y.Z. Yang, N. Ma, Y.J. Liu, Chem. Mater. 17 (2005) 1548–1552. [9] P.K. Khanna, N. Singh, S. Charan, Mater. Lett. 61 (2007) 4725–4730. [10] S. Liufu, H. Xiao, Y. Li, J. Colloid Interface Sci. 281 (2005) 155–163. [11] X. Wei, G. Xu, Z.H. Ren, C.X. Xu, G. Shen, G.R. Han, J. Am. Ceram. Soc. 91 (2008) 3795–3799. [12] J.G. Yu, W.G. Wang, B. Cheng, B.L. Su, J. Phys. Chem. C 113 (2009) 6743–6750. [13] J.C. Yu, W.K. Ho, J.G. Yu, S.K. Hark, K. Iu, Langmuir 19 (2003) 3889–3896. [14] X.L. Yan, T. Ohno, K. Nishijima, R. Abe, B. Ohtani, Chem. Phys. Lett. 429 (2006) 606–610. [15] J.G. Yu, L. Shi, J. Mol. Catal. A Chem. 326 (2010) 8–14. [16] H. Park, W. Choi, J. Phys. Chem. B 108 (2004) 4086–4093. [17] J.F. Rabek, Polymer Photodegradation: Mechanisms and Experimental Methods, Springer, 1995.