Amylose–halloysite–TiO2 composites: Preparation, characterization and photodegradation

Applied Surface Science 329 (2015) 256–261

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Amylose–halloysite–TiO2 composites: Preparation, characterization and photodegradation Pengwu Zheng a , Yuanyuan Du a , Peter R. Chang b,d , Xiaofei Ma c,∗ a

School of Pharmacy, Jiangxi Science and Technology Normal University, 330013 Nanchang, Jiangxi, China Bioproducts and Bioprocesses National Science Program, Agriculture and Agri-Food Canada, 107 Science Place, Saskatoon, SK S7N 0X2 Canada Chemistry Department, School of Science, Tianjin University, Tianjin 300072, China d Department of Chemical and Biological Engineering, University of Saskatchewan, Saskatoon, SK S7N 5A9, Canada b c

a r t i c l e

i n f o

Article history: Received 30 October 2014 Received in revised form 23 December 2014 Accepted 23 December 2014 Available online 2 January 2015 Keywords: Amylose Halloysite TiO2 Photodegradation

a b s t r a c t A supramolecular structure was initially formed between amylose and halloysite (HNT) simply by using a mechanical force. Subsequently, amylose acted as a template for the growth of TiO2 nanoparticles. The thus-prepared amylose–HNT–TiO2 composite was characterized using Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), thermogravimetric analyses, and transmission electron microscopy (TEM). In comparison to its counterpart HNT-TiO2 composite, the amylose–HNT–TiO2 composite exhibited better dispersion, a larger specific surface area, and photocatalytic activity that was more effective for the photodegradation/removal of methylene blue (MB) and the persistent organic pollutant 4-nitrophenol (4-NP) under UV irradiation. After four successive UV irradiation treatments, amylose–HNT–TiO2 removed about 90% of 4-NP or MB. © 2014 Elsevier B.V. All rights reserved.

1. Introduction The unique properties of titanium dioxide (TiO2 ) photocatalysts, such as chemical stability, non-toxicity, low cost, and high photocatalytic activity without formation of secondary pollutants, have attracted much attention for the removal of industrial pollutants [1]. However, it is very difficult to recover the nanometer-scale photocatalyst particles from aqueous solution at the end-stage of the process [2]. Since many natural clay minerals exhibit the excellent properties including high specific surfaces areas, large pore volumes, chemical stability and good mechanical properties [3], these clays, e.g. rectorite [4], diatomite [2], montmorillonite [5], palygorskite [6] and halloysite (also known as halloysite nanotubes, HNT) [7] have been studied as supports for loading TiO2 nanoparticles to obtain excellent photocatalytic activity and easy separation of the photocatalyst. Aggregation of nanoparticles generally results in poor effectiveness of photocatalysis; therefore surfactants and polymers were introduced to improve dispersion. The cationic alkanediyl-␣,␻bis-N-dodecyl-N,N -dimethyl-ammonium bromide (Gemini) and anionic sodium dodecyl sulphate (SDS) surfactants were applied to control TiO2 nanoparticle aggregation [8]; and uniform nanoscale

∗ Corresponding author. Tel.: +86 22 27406144; fax: +86 22 27403475. E-mail address: [email protected] (X. Ma). http://dx.doi.org/10.1016/j.apsusc.2014.12.158 0169-4332/© 2014 Elsevier B.V. All rights reserved.

dispersion of TiO2 in the poly(p-phenylene vinylene) (PPV) matrix was obtained by the sol–gel process. The strong interaction between TiO2 nanoparticles and PPV in the media may prevent TiO2 nanoparticles from aggregating [9]. Biomass-based polysaccharides can act as templates to control nanoparticle growth of metal oxides and restrain aggregation [10]. Furthermore, those renewable polysaccharides and their derivatives may form complexes with metal ions due to their high number of coordinating functional groups (hydroxyl and glucoside groups). Not surprisingly, ZnO [10] and Sb2 O3 [11] nanoparticles were successfully encapsulated by carboxymethyl cellulose. In addition, starch was used as a template for growth of iron oxide nanoparticles, which were uniformly dispersed on the surface of carbon nanotube [12]. HNT has been used as a support for loading metal [13], metal oxides nanoparticles [14], and biomolecule immobilization [15,16] due to the versatile surface features of HNT with a negatively charged outer surface and a positively charged inner lumen within a range of pH values. Amylose has been wrapped on the surface of HNT to improve the dispersion of amylose–HNT in solution due to the supramolecular interaction between amylose and the outer surface of HNT [17]. We postulated that the amylose–HNT–TiO2 composite would exhibit higher photocatalytic activity because of better dispersion of the nanometer catalyst on the HNT support than the HNT–TiO2 composite. In this work, TiO2 nanoparticles were prepared on an amylose–HNT supramolecular structure. The thus prepared

P. Zheng et al. / Applied Surface Science 329 (2015) 256–261

2. Experimental 2.1. Materials

amylose-HNT-TiO2

amylose-HNT

Transmittance

amylose–HNT–TiO2 composite was characterized using Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), thermogravimetric analyses and transmission electron microscopy (TEM). The photodegradation of methylene blue (MB) dye and the persistent organic pollutant 4-nitrophenol (4-NP) was also studied.

257

amylose

2930

1160 HNT

HNT was provided by Chenxi Company (Hunan, China). HNT mineral was milled and sieved with a 50 ␮m sieve, and then purified according to the modified method of Liu et al. [18]. HNT was dispersed in water, and the solution was stirred for 10 min and left to stand for 5 min at room temperature. The upper solution was collected, and HNT was separated by centrifugation. HNT was washed with ethanol, and dried at 50 ◦ C. Amylose, isolated from potato starch, was obtained from Aladdin Industrial Inc. All other reagents were commercially available and of analytical grade. 2.2. Preparation 2.2.1. Preparation of amylose–HNT composite The amylose–HNT composite was prepared using the modified method of Chang et al. [17]. HNT (1 g) and amylose (0.5 g) were placed in a ball milling apparatus and the mixture was milled for 1 h at room temperature. The obtained powder was dispersed in a 3/2 (w/w) DMSO/water solution by stirring to dissolve amylose, and the suspension allowed to stand for 24 h to remove the precipitated HNT and collect the supernatant. Ethanol was added to the supernatant to precipitate the amylose–HNT composite; it was then filtered, washed with ethanol, and dried in a vacuum oven at 40 ◦ C for 4 h. 2.2.2. Preparation of amylose–HNT–TiO2 (or HNT–TiO2 ) composite Tetrabutyl titanate, Ti(OC4 H9 )4 , (1 or 2 mL) was added to 6 mL absolute ethanol. The mixture was stirred for 15 min at room temperature, followed by the addition of 0.8 g acetic acid and another 15 min of stirring. A quantity of 0.2 g of amylose–HNT (or HNT) was slowly added to the mixture, and the suspension was stirred for 1 h at 30 ◦ C. The solution composed of 0.5 g NH4 NO3 , 1.5 mL distilled water, and 1.4 mL absolute ethanol was added dropwise and the pH was kept in the range of 2–3. The mixture was stored at room temperature for 24 h. The amylose–HNT–TiO2 (or HNT–TiO2 ) composites were centrifuged, washed with water at least three times, and oven-dried for 12 h at 110 ◦ C. When 1 mL tetrabutyl titanate was used, the mass ratio of HNT to TiO2 was about 1:1, and the amylose–HNT–TiO2 and HNT–TiO2 composites were labelled as amylose–HNT–TiO2 and HNT–TiO2 . When 2 mL tetrabutyl titanate was added, the composites were labelled as amylose–HNT–TiO2 II and HNT–TiO2 II.

3692 3622 1080 4000

3600

3200

2800

2400

2000

1600

1010

1200

800

400

-1

wavenumber (cm ) Fig. 1. FT-IR spectra of HNT, amylose–HNT, HNT–TiO2 , and amylose–HNT–TiO2 .

Thermogravimetric analyses of amylose, HNT, and amylose– HNT were done on a STA 409 PC thermal analyzer (NETZSCH, Germany). The sample weights were about 10 mg and they were heated to 600 ◦ C at a heating rate of 15 ◦ C/min in a nitrogen atmosphere with a flow rate of 30 mL/min. HNT and amylose–HNT were respectively dispersed in DMSO using ultrasonication for 1 min. The UV–visible (UV–vis) spectra of DMSO solutions containing HNT and amylose–HNT, which were stored for 0 and 6 h, were recorded from 200 to 800 nm using a UV–vis spectrophotometer model U-1800, Hitachi Company. HNT, amylose–HNT, HNT–TiO2 , and amylose–HNT–TiO2 aqueous suspensions were dropped onto a copper grid, air dried, and analyzed using a JEM-1200EX transmission electron microscope (TEM). The atomic weight of Ti for amylose–HNT–TiO2 was recorded by energy dispersive X-ray spectroscopy (EDS). Nitrogen adsorption–desorption measurements were performed with an Autosorb-1 specific surface area analyzer (Quantachrome Instruments, USA). 2.4. Photodegradation experiments A quantity of 10 mg of HNT–TiO2 or amylose–HNT–TiO2 was dispersed in 10 mL 4-NP solution (10 mg/L). They were then exposed to UV irradiation or stored in the dark at 25 ◦ C. The UV light source was a 12 W UV lamp ( = 253 nm) and the irradiation intensity was approximately 350 ␮W/cm2 . Determination of the concentration of 4-NP was performed at 318 nm with a UV-vis spectrometer. In addition, a quantity of 10 mg of HNT–TiO2 or amylose–HNT–TiO2 was dispersed in 10 mL MB solution (32 mg/L) and exposed to UV irradiation or stored in the dark at 25 ◦ C. The UV light source was a 12 W UV lamp ( = 365 nm). The MB concentration was determined at 662 nm with a UV–vis spectrometer.

2.3. Characterization 3. Results and discussion Fourier transform infrared spectroscopy (FTIR) spectra of HNT, amylose–HNT, HNT–TiO2 , and amylose–HNT–TiO2 were obtained on a Bio-Rad FTS3000 IR spectrum scanner. The sample powders were evenly dispersed in KBr and pressed into transparent sheets for testing. X-ray diffraction (XRD) patterns for HNT, amylose–HNT, HNT–TiO2 , and amylose–HNT–TiO2 were recorded in reflection mode in the angular range of 10◦ –80◦ (2), at ambient temperature, using a Bruker D8-S4 Pioneer operated at a CuK␣ wavelength ˚ of 1.542 A.

3.1. Characterization of amylose–HNT and amylose–HNT–TiO2 Fig. 1 shows the FTIR spectra of HNT, amylose–HNT, HNT–TiO2 , and amylose–HNT–TiO2 . The double peaks at 3692 and 3622 cm−1 were ascribed to the stretching vibrations of Al OH groups at the surface of HNT [19,20]. There are three characteristic peaks of C O stretching in the amylose spectrum. The peak at 1160 cm−1 was attributed to C O bond stretching of the C O H group, and the peaks at 1080 and 1010 cm−1 were ascribed to C O bond stretching

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100

H

H

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-10

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Fig. 2. X-ray diffraction patterns of HNT, amylose–HNT, HNT–TiO2 , and amylose–HNT–TiO2 (H: halloysite, A: anatase, Q: quartz).

400

500

600

o

Temperature ( C) Fig. 3. Thermogravimetric analyses of amylose, HNT, and amylose–HNT.

in amylose–HNT–TiO2 was calculated to be about 25 wt%. Assuming that there is no TiO2 mass loss over the range of 100–600 ◦ C, the amylose content (x) in amylose–HNT–TiO2 can be calculated as: x × 68.8% + (1 − 0.25 − x) × 15.2% = 1 × 19.8% [23]. The calculated amylose and HNT contents were 15.6% and 59.4%, respectively. The ratio of amylose to HNT was 0.26 for the amylose–HNT–TiO2 composite, which was lower than that (0.44) of the amylose–HNT composite. Amylose may be partly exfoliated during preparation of TiO2 . Fig. 4 exhibits the UV–vis spectra of HNT and amylose–HNT that were respectively dispersed with ultrasonication in DMSO and stored for 0 and 6 h. HNT precipitated after 6 h, so there was no obvious absorbance. The amylose component in amylose–HNT formed a good interaction with DMSO, so the amylose–HNT remained mostly dispersed in DMSO and had high absorbance intensity after 6 h. The position of the absorbance peak changed from 220 nm for HNT to 226 nm for amylose–HNT due to the interaction between amylose and HNT. After 6 h, the absorbance peak shifted to 222 nm for amylose–HNT because the interaction between amylose and DMSO weakened the interaction between amylose and HNT, and affected the HNT absorbance peak position. In Fig. 5(a), the TEM image of HNT shows nanotubes with an outer diameter of 50–200 nm, a length of about 1–2 ␮m, and a hollow cavity of about 10–20 nm in diameter. After HNT and amylose were milled, helical amylose and HNT were composited by mechanical force. The surface of HNT–amylose was rough after the

2.5 2.0 1.5

Absorbance

of the C O C group in the anhydroglucose ring [17]. The broad band at 2930 cm−1 resulted from the C H stretching vibration of methylene groups [21]. The above mentioned peaks all appeared in the FTIR spectra of amylose–HNT except for the characteristic double peaks of HNT, this indicated that the HNT structure was either destroyed or the double peaks of HNT overlapped with the OH groups of amylose at 3400 cm−1 . In FTIR of amylose–HNT–TiO2 , the C O and C H stretching vibration peaks of amylose were not obvious. This illustrated that amylose could fall off the HNT surface during preparation of TiO2 , or that these amylose peaks overlapped with Si O groups in HNT at 1000 cm−1 [3]. The lower intensity double peaks of the HNT Al OH groups suggest that HNT structure was partly destroyed in amylose–HNT–TiO2 . The XRD patterns of HNT, amylose–HNT, HNT–TiO2 , and amylose–HNT–TiO2 are exhibited in Fig. 2. Both Halloysite 7 A˚ and Halloysite 10 A˚ were indentified with JCPDS Card No. 291487 and 291489, and minor amount of quartz (JCPDS Card No. 520784) also coexisted with HNT. Unlike HNT, amylose–HNT had no obvious peaks at 2 values of about 10◦ and 20◦ indicating that preparation of amylose–HNT partly destroyed the HNT structure [22]. HNT–TiO2 displayed the characteristic reflections of anatase (TiO2 ) at 2 values of 37.7, 47.6, 54.5 and 62.5 [14]. The amylose–HNT–TiO2 exhibited a pattern similar to HNT–TiO2 except for the disappearance of the HNT peaks at 2 values of about 10◦ and 20◦ . In amylose–HNT–TiO2 , the anatase formed on the damaged HNT structure, and quartz component divorced from HNT. Fig. 3 shows the thermogravimetric (TG) and derivative thermogravimetric (DTG) curves for amylose, HNT, amylose–HNT and amylose–HNT–TiO2 . The thermal decomposition temperature is the temperature at the maximum rate of mass loss, which for amylose appeared at 278 ◦ C on the DTG curve with a 68.8% mass loss from the TG curve. Amylose–HNT showed a mass loss of about 31.7 wt% with a higher thermal decomposition temperature of 288 ◦ C, showing that the interaction between amylose and HNT may improve the thermal decomposition temperature of amylose. HNT had a mass loss of about 15.2 wt% at the thermal decomposition temperature of 485 ◦ C. Using the equation y × 68.8% + (1 − y) × 15.2% = 1 × 31.7% [23], the amylose content (y) of amylose–HNT was calculated to be 30.7%. The ratio of amylose to HNT was about 0.44 for the amylose–HNT composite. The amylose–HNT–TiO2 composite showed a mass loss of about 19.8 wt% with a thermal decomposition temperature of 298 ◦ C, which could be related to the interaction between amylose and TiO2 particles. The atomic weight for Ti (14.88 wt%) in amylose–HNT–TiO2 was recorded by EDS, and the TiO2 content

226nm 222nm

amylose-HNT

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2.5 2.0

220nm

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HNT

1.0 0.5

0h

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wavelength(nm) Fig. 4. UV–vis spectra of HNT and amylose–HNT in DMSO, stored for 0 and 6 h.

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Fig. 5. TEM micrographs of HNT (a), amylose–HNT (b), HNT–TiO2 (c), amylose–HNT–TiO2 (d), HNT–TiO2 II (e) and amylose–HNT–TiO2 II (f).

smooth HNT was covered with amylose, as revealed in Fig. 5(b), and the structure of HNT was destroyed by the mechanical force. Comparing the TEM image of HNT–TiO2 (Fig. 5(c)) with that of amylose–HNT–TiO2 (Fig. 5(d)) showed that TiO2 particles were uniformly dispersed without apparent aggregation on the surface of HNT in amylose–HNT–TiO2 , while agglomerated TiO2 particles were clearly observed on HNT–TiO2 . Using the N2 -BET method, the specific surface areas of HNT–TiO2 and amylose–HNT–TiO2 were determined to be 175.3 and 408.8 m2 /g, respectively. The larger specific surface area of amylose–HNT–TiO2 was ascribed to the

higher dispersion of TiO2 particles in the presence of amylose templates on the HNT. Nucleation of TiO2 could preferentially occur on the HNT surface because HNT is a good adsorbent of metal ions due to cation exchange between Na+ on HNT and Ti2+ [24]. Many coordinating amylose functional groups (hydroxyl and glucoside groups) could form complexes with metal ions as well [11]. Therefore, the crystal growth of TiO2 was controlled by amylose on HNT. In this process, polysaccharides presented dynamic supramolecular associations by inter- and intra-molecular hydrogen bonding, and could act

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0.0

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HNT-TiO2(dark)

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degradation rate(%)

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0 1

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cycle time

cycle time Fig. 6. (a) Photocatalytic degradation of 4-NP by HNT–TiO2 and amylose–HNT–TiO2 . (b) Stability of HNT–TiO2 and amylose–HNT–TiO2 after 4 cycles of photocatalytic decomposition of 4-NP.

as templates for nanoparticle growth [12] preventing the particles from aggregating, and maintaining nanoparticles of smaller sizes with high dispersibility. Compared with amylose–HNT, amylose–HNT–TiO2 exhibited a lower amylose component which may be related to the partial exfoliation of amylose during preparation of amylose–HNT–TiO2 . This was also evident in thermogravimetric analysis and FTIR. As shown in Fig. 5(e) and (f), for HNT–TiO2 II and amylose–HNT–TiO2 II many TiO2 particles were not attached to HNT because the superfluous TiO2 particles could be not loaded on the HNT surface, or because the aggregated of TiO2 particles easily fell off of HNT. The superfluous TiO2 introduction also weakened the effect of amylose on the high dispersion of TiO2 nanoparticles.

3.2. Photodegradation of amylose–HNT and amylose–HNT–TiO2 Fig. 6(a) shows the photodegradation of 4-NP by HNT–TiO2 and amylose–HNT–TiO2 under UV irradiation or in the dark. Results showed poor ability to absorb 4-NP by HNT–TiO2 and amylose–HNT–TiO2 in the dark; however under UV irradiation, HNT–TiO2 and amylose–HNT–TiO2 exhibited good photodegradation of 4-NP. When 4-NP was exposed to UV irradiation for 4 h, about 70% and 90% of the 4-NP was removed by HNT–TiO2 and amylose–HNT–TiO2 , respectively. The better photodegradation of 4-NP by amylose–HNT–TiO2 could be ascribed to the better dispersion of TiO2 than HNT–TiO2 , and to the direct exposure of more TiO2 particles to UV irradiation.

Fig. 7. (a) Photocatalytic degradation of MB by HNT–TiO2 and amylose–HNT–TiO2 . (b) Stability of HNT–TiO2 and amylose–HNT–TiO2 after 4 cycles of photocatalytic decomposition of MB.

To determine its usefulness, HNT–TiO2 and amylose–HNT–TiO2 were reused four times for evaluation of photodegradation of 4-NP under UV light. As shown in Fig. 6(b), after four cycles the degradation effectiveness decreased from 90% to 80% for the amylose–HNT–TiO2 composite and much more significantly for HNT–TiO2 , from 80% to 53%. This result indicated that photodegradation by amylose–HNT–TiO2 remained steady and more effective than HNT–TiO2 under successive UV light treatments. Fig. 7(a) reveals the effects of UV irradiation and exposure time on the photodegradation of MB in the presence of HNT–TiO2 or amylose–HNT–TiO2 . When stored in the dark for 10 h, about 35% of the MB was removed by HNT–TiO2 and by amylose–HNT–TiO2 due to the adsorption of MB by HNT [25]. When exposed to UV-light irradiation for 10 h, HNT–TiO2 and amylose–HNT–TiO2 exhibited higher removal of MB dye because of photodegradation. And the higher dispersion of TiO2 in amylose–HNT–TiO2 contributed to the removal of more MB under UV-light irradiation. About 91% MB was removed by amylose–HNT–TiO2 , as compared with about 81% MB removal for HNT–TiO2 . Similarly, photodegradation with amylose–HNT–TiO2 remained more effective and steady for MB than HNT–TiO2 after four cycles, as illustrated in Fig. 7(b). 4. Conclusions Amylose assisted in the uniform distribution and growth of TiO2 nanoparticles on the surface of HNT due to the supramolecular interaction between the metal ions and hydroxyl/glucoside groups in amylose. Consequently, the amylose–HNT–TiO2 composite exhibited steadier and more effective photodegradation

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ascribed to better dispersion of TiO2 nanoparticles than the typical TiO2 aggregation in the counterpart HNT–TiO2 composite. As the inert ingredients, minor quartz could be regarded as the part of HNT, which would not affect the photodegradation. The amylose–HNT supramolecular structure can serve as a potential template and facilitate the growth of metal or metal oxide nanoparticles without apparent aggregation, which thereby improves its catalytic effectiveness. Acknowledgements This research was supported by the Science and Technology Project of Jiangxi Provincial Office of Education (KJLD12082 and Innovation Platform “project 311”) and Nature Science Foundation of Jiangxi Province (20132BAB 206006) and the National Nature Science Foundation of China (51162011 and 51462009).

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