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Rahman, A., Ching, Y. C., Ching, K. Y., Awanis, N., Chakraborty, A. K., Chuah, C. H., and Liou, N. S. (2015). "Surface modification of natural fiber using Bi2O3/TiO2 composite for photocatalytic self-cleaning," BioRes. 10(4), 7405-7418

Abstract

A photocatalytic fiber was prepared by modifying the surface of jute fiber with a Bi2O3/TiO2 composite. Maleic acid was used as an organic linker, and the coating process was conducted with heat-treatment at 240 °C. At first, the Bi2O3/TiO2 composite was synthesized by incorporating TiO2 nanoparticles onto a Bi2O3 phase. Subsequently, the photocatalytic fiber was prepared by incorporating the Bi2O3/TiO2 composite onto the surface of the fiber. The Bi2O3/TiO2 composite-modified fiber was characterized by field-emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and UV-visible spectroscopy. The synthesized composite exhibited notably high photocatalytic activity under visible light irradiation of λ up to 420 nm, whereby it could decompose organic pollutants in the aqueous and gaseous phases. Because of increasing environmental concerns, this photocatalytic system could be an important candidate for decomposing organic pollutants.



Full Article

Surface Modification of Natural Fiber using Bi2O3/TiO2Composite for Photocatalytic Self-cleaning

Ashiqur Rahman,a Yern Chee Ching,a,* Kuan Yong Ching,b,c Nur Awanis,Ashok Kumar Chakraborty,e Cheng Hock Chuah,f and Nai-Shang Liou g

A photocatalytic fiber was prepared by modifying the surface of jute fiber with a Bi2O3/TiO2 composite. Maleic acid was used as an organic linker, and the coating process was conducted with heat-treatment at 240 °C. At first, the Bi2O3/TiO2 composite was synthesized by incorporating TiO2 nanoparticles onto a Bi2O3 phase. Subsequently, the photocatalytic fiber was prepared by incorporating the Bi2O3/TiO2 composite onto the surface of the fiber. The Bi2O3/TiO2 composite-modified fiber was characterized by field-emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and UV-visible spectroscopy. The synthesized composite exhibited notably high photocatalytic activity under visible light irradiation of λ up to 420 nm, whereby it could decompose organic pollutants in the aqueous and gaseous phases. Because of increasing environmental concerns, this photocatalytic system could be an important candidate for decomposing organic pollutants.

Keywords: Fiber; Bi2O3/TiO2; Photocatalysis; Self cleaning; Organic pollutants

Contact information: a: Department of Mechanical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia; b: National Centre for Advanced Tribology at Southampton, Faculty of Engineering and the Environment, University of Southampton, Southampton SO17 1BJ, United Kingdom; c: Department of Science, School of Foundation Studies, Southern University College, 81300 Skudai, Johor, Malaysia; d: Department of Chemical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia; e: Department of Applied Chemistry & Chemical Technology, Faculty of Applied Science & Technology, Islamic University, Kushtia-700, Bangladesh; f: Department of Chemistry, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia; g: Department of Mechanical Engineering, Southern Taiwan University of Science and Technology, Yungkang Dist., Tainan City 710, Taiwan R.O.C;

* Corresponding author: chingyc@um.edu.my

INTRODUCTION

Titanium dioxide (TiO2) could be a potential candidate for the degradation of organic materials because of its ability to initiate photocatalysis in the presence of UV light and its oxidant nature (e.g., O2/H2O). To initiate the photocatalysis reaction, TiO2 catalyst must undergo the photo excitation process. TiO2, which acts as a semiconductor, requires photons with energy greater than their energy band gap (Eg), located within the UV-light range of the electromagnetic spectrum, to induce the reaction. TiO2 is acknowledged as an excellent catalyst. However, when visible light is available as the energy input, its catalysis ability is limited because of its wide band gap of 3.2 eV (Sobana et al. 2006).

There are many studies on the modification of TiO2. For instance, TiO2 has been doped by metal and non-metal ions (Chao et al. 2003; Chatterjee and Dasgupta 2005; Chang and Doong 2006; Venkatachalam et al. 2007; Gao et al. 2010), dye photo-sensitized on its surface (Fujishima et al. 2000; Hilal et al. 2007), sensitized by a narrow band gap semiconductor (Bai et al. 2010; Zhao et al. 2010; Zyoud et al. 2010), deposited with noble metals (Hufschmidt et al.2002; Dobosz and Sobczyński 2003; Arana et al. 2004; Sobana et al.2006), and combined with semiconductors. All of these studies aimed at extending the absorption range of TiO2 into the visible range. Of the many approaches, noble metal-modified semiconductor nanoparticles have recently been recognized as one of the most viable solutions for maximizing the efficiency of photocatalytic reactions. Interestingly, noble metals doped or deposited on TiO2 may act either singly or simultaneously, depending on the photoreaction conditions. Noble metals may (i) act as electron traps, which promote electron-hole separation; (ii) prolong light absorption in the visible range and promote surface electron excitation by plasmon resonances excited by visible light (Leong et al. 2014); and (iii) modify the surface properties of photocatalysts (Sobana et al. 2006; Moafi et al. 2011;). For TiO2 to be photoactive under visible light, considerable efforts have been made to dope it with various metals (Ao et al. 2004; Abe et al. 2008; Mahdi et al. 2013) and non-metals (Arana et al. 2004; Brezesinski et al. 2010). A combination of semiconductors is considered an effective way to enhance the photostimulated electron-hole separation and effectively inhibit their recombination. The main feature of this technique is to assemble a heterojunction interface between wide and narrow band gap semiconductors with matching energy band potentials.

Consequently, an electric field assists in the transportation of charges from one particle to the adjacent one, which makes the catalysis process feasible at the interface of the electron-hole separations in the composite materials. Thus, the electron and hole can move to the surface of the semiconductors. The extensive research published on this composite system has mostly focused on TiO2-based photocatalysts such as WO3/TiO2, In2O3/TiO2, SiO2/TiO2, MgO/TiO2, Fe2O3/TiO2, Bi2O3/TiO2, and FeTiO3/TiO2 (Houas et al.2001; Dobosz and Sobczyński 2003; Kandavelu et al. 2004; Hameed et al. 2008; Kim et al. 2009; Chakraborty and Kebede 2012; Chakraborty et al. 2012). Bismuth (III) oxide Bi2O3, with a band gap of 2.8 eV, is known as p-type semiconductor and has proven to be a good photocatalyst for water treatment and the decomposition of pollutants under visible-light irradiation (Kun et al. 2006; Mozia et al. 2007; Salim et al. 2014).

In this study, Bismuth (III) oxide, Bi2O3 was used to increase the photoactivity of TiO2. Bismuth (III) oxide is known as the most industrially important compound of bismuth. About half of the production of bismuth is for bismuth compounds. Bismuth compounds are used in cosmetics, pigments, and a few pharmaceuticals. Notably Pepto-Bismol is used to treat diarrhea. Bismuth’s unusual propensity to expand upon freezing is responsible for uses in casting of printing type (typefounding). Bismuth can expand upon solidification, this makes it suitable to make castings for objects subjected to high temperatures. Scientific studies have confirmed that bismuth and most of its compounds are less toxic compared to other heavy metals such as lead, antimony, etc. Besides it is not bioaccumulative (Fujishima et al. 2000; Hilal et al. 2007; Chakraborty et al. 2012).

In the present study, a Bi2O3/TiO2 system was developed by utilizing maleic acid as an organic binder. The role of maleic acid is to bind Bi2O3 and TiO2 using two end carboxylic functional groups (Chakraborty et al. 2014). The prepared Bi2O3/TiO2 was applied for the photocatalytic degradation of phenol in the aqueous phase and 2-propanol in the gaseous phase under visible-light irradiation. The capability of the non-toxic, inexpensive TiO2 nanoparticles to exhibit self-cleaning properties in different textile materials has been widely studied (Ding et al. 2000; Fujishima et al. 2000; Chao et al. 2003; Stylidi et al. 2004; Fruth et al. 2005; Gan et al. 2007; Uddin et al.2007; Venkatachalam et al. 2007; Uddin et al. 2008; Kim et al. 2009; Brezesinski et al. 2010; Gao et al. 2010). The compound of phenol and 2-propanol were selected as model pollutants in this study. This was due to the fact that both the phenol and 2-propanol are ubiquitous pollutants that cause pollutant to natural water resources. These pollutants come from the effluents of a variety of chemical industrial such as cool refineries, phenol manufacturing, pharmaceuticals and industries of resin paint, pulp mill, petrochemical, dying, textile wood, etc. (Gad and Saad 2008). These compounds can induce hematological, genotoxic, carcinogenic, immunotoxic, and physiological effects, and they have resulted in a high bioaccumulation rate along the food chain due to its lipophilicity. Thus, these pollutants represent a threat against the natural environment and also to human health (Hori et al. 2006). Bi2O3/TiO2composite can dissociate organic pollutants in the presence of UV-Visible light. For this reason, phenol and 2-propanol were selected as model pollutants in the photocatalytic and self-cleaning activity tests in this study.

In addition, self-cleaning materials have been explored for use in other applications, such as the windows of high-rise towers, automobile windshields, and even in industries that require this promising technique for sterilization, anti-fogging, room air cleaning, and deodorization (Houas et al. 2001). To enhance the catalyst’s performance in self-cleaning applications, fine TiOmust be immobilized onto suitable substrates. However, fabrication methods such as chemical vapor deposition (CVD), anodization, and thermal oxidation of Ti metal, ultrasonic nebulization, and pyrolysis operate at high temperatures which may result in unwanted cracking and peeling of the TiO2 film (Chao et al. 2003; Dobosz and Sobczyński 2003; Mozia et al. 2007). These defects are attributed to shrinkage during the crystallization of deposited amorphous films.

In this work, a facile and effective synthesis route to create self-cleaning coatings, based on titanium nanocomposites on fibers, was studied. The photocatalytic performance of the coated fibers was assessed. To the best of our knowledge, Bi2O3-doped TiO2 films for self-cleaning applications have not been studied previously.

EXPERIMENTAL

Materials

Titanium dioxide (TiO2), bismuth oxide (Bi2O3), maleic acid (C4H4O4), absolute ethanol (CH3CH2OH), phenol (C6H5OH), and acetone were purchased from Evonik Degussa GmbH (Germany) and Sigma Aldrich (Germany) and were used without further refinement. Distilled water was used throughout the experiments. The fibers were extracted from the bark of a jute tree. The fibers were treated with water and detergent at 80 °C for 45 min to remove all impurities, fats, greases, waxes, and other residue before use. Then, they were washed repeatedly by a large amount of distilled water until the pH was constant. Acetone was used to clean the fiber again before they were dried at ambient temperature for 36 h (Ching et al. 2015).

Preparation of Bi2O3/TiO2 Composite

In this study, a Bi2O3/TiO2 composite was prepared in a ratio of 5/95, meaning that the Bi2O3/TiO2 composite consisted of 5 mol% Bi2O3(mean particle size 20 to 30 nm) and 95 mol% TiO(mean particle size 4 to 13 nm). During preparation, 0.3070 g of Bi2O3 was first suspended in 40 mL of absolute ethanol. Then, 0.1987 g of maleic acid, dissolved in absolute ethanol, was added to the suspension. Subsequently, 1 g of TiO2 nanoparticles was added to the above suspension and stirred with a magnetic stirrer for 6 h at ambient conditions until the mixture was homogenous. After that, the suspension was centrifuged and the Bi2O3/TiO2 composite was observed as the residue. Then, the Bi2O3/TiO2 composite was washed several times with ethanol to remove unreacted maleic acid (Xu et al.2008; Liu et al. 2010; Chakraborty et al. 2014). Next, the composite was dried at 60 °C in an oven overnight. Then, the Bi2O3/TiO2composite was annealed at 100 °C for 3 h to increase its bonding strength. The Bi2O3/TiO2 composite obtained was then ready for coating on the surface of the jute fiber.

Coating of Bi2O3/TiO2 Composite on Fiber

To prepare a Bi2O3/TiO2 composite-coated fiber with ratio of 20 wt.% Bi2O3/TiO2 and 80 wt.% fiber, 0.2 g of Bi2O3/TiO2 composite was first suspended in 40 mL of absolute ethanol. Then, 0.1987 g of maleic acid and 0.8 g of fiber were added to the above suspension and stirred vigorously for 6 h at 25 °C. After that, the suspension was centrifuged and Bi2O3/TiO2 composite-coated fibers were observed as the residue. Then, the fibers were washed several times with ethanol to remove unreacted maleic acid. Afterward, the coated fiber was dried at 60 °C for 6 h in an oven. The same procedure was repeated to prepare Bi2O3/TiO2-coated fiber where fiber and Bi2O3/TiO2 composite were present at ratios of 90:10; 65:35; 50:50; 35:65; and 20:80.

Photocatalytic Test

Photocatalytic degradation of adsorbed phenol and 2-propanol on Bi2O3/TiOfilm coated fiber was investigated. The simulating solar light irradiation was carried out at 308 K by using a 300-W xenon lamp with a UV cutoff filter (λ≤420 nm, lamp spectrum: 220-2000 nm, Oriel Instrument, Singapore) was used for photocatalytic reactions (Fligge et al. 2001). The bulb and the Hfilter together yield a spectrum ranging from ultraviolet to infrared radiation (similar to natural sunlight).

In this study, aqueous solutions (0.05%, w/v) of reagent grade phenol and gaseous 2-propanol were prepared for impregnation of the unmodified and of Bi2O3/TiO2-coated fibers. The reaction medium was stirred by a magnetic stirrer for 30 min in darkness. The concentration of pollutants did not change after stirring for 30 min, which indicates that 30 min is enough to reach the adsorption equilibrium of organics (Chakraborty et al. 2014).

The phenol concentration was measured before the photocatalytic reaction. After that, the samples were remained overnight to complete the adsorption. The samples were then removed from phenol solution and gaseous 2-propanol and were dried at room temperature. From the decreased concentration value of phenol and 2-propanol, the concentration of pollutant compounds leave on the fiber was estimated to be 1.00×10-4 M. The samples containing adsorbed phenol and 2-propanol were then exposed to reproducible solar-like light (50 mW/cm2) for photoactivity test study. The phenol and 2-propanol photodecomposition reaction was monitored with a UV-Vis spectrometer in the reflectance mode by investigating the evolution of the absorbance upon light exposure (Uddin et al. 2007; Venkatachalam et al. 2007; Uddin et al. 2008). The remnant phenol and 2-propanol after the irradiation of visible light was analyzed from its characteristic absorption peak detected by UV-Vis spectroscopy (Chakraborty et al. 2014).

Characterization Methods

The morphologies of the pure and coated fibers were examined using a field-emission scanning electron microscopy (FESEM, FEI Quanta FEG 450, FEI Company, Redmond, USA). The elemental compositions of the Bi2O3/TiO2-coated fibers were identified with energy-dispersive X-ray spectroscopy (EDX) attached to FESEM. For X-ray diffraction measurements, a D8 Bruker Avance X-ray difractometer (Bruker, Germany) was used. FTIR analysis was performed using an FTIR Spectrum 400 spectrometer (Perkin Elmer, USA) to analyze the polymer chain quality of the fiber before and after treatment and after extended visible light exposure. UV-Vis reflectance spectra were recorded by a Shimadzu UV-1601 spectrophotometer (Shimadzu, Japan) to observe the photocatalytic dissociation of phenol and 2-propanol on the Bi2O3/TiO2-coated fiber.

RESULTS AND DISCUSSION

Morphological and Compositional Analysis

Morphological analysis of the pure and treated fibers was conducted by FESEM. Figures 1a and 1b display the surfaces of the unmodified fibers. Figures 1c, d show the fiber surface after it was coated with the Bi2O3/TiO2 (20 wt.%) composite in the presence of maleic acid. The samples were enclosed by discrete Bi2O3/TiO2 agglomerates. The Bi2O3/TiO2 agglomerates were irregular in shape, with dimensions of less than 100 nm. The Bi2O3/TiO2 agglomerates were unevenly distributed over the fiber surface.

Figures 1e and 1f display the morphology of Bi2O3/TiO2-coated fiber after 25 washings. There was no observed change in the surface of the Bi2O3/TiO2-coated fiber. These figures demonstrate that the shapes of the particles were similar to each other and that these nanoparticles were irregular and likely to agglomerate. In general, the Bi2O3/TiO2 composite with the presence of maleic acid showed good interfacial adhesion with the fiber surface. Figure 1g displays the morphology of Bi2O3/TiO2-coated fiber without the presence of maleic acid. From the morphological result, no metal particle coating was observed on the jute fiber surface. This indicates that the maleic acid can play an important role as linker between the TiO2 and Bi2O3particles and between the Bi2O3/TiO2 composite to fiber surface.

The EDX analyses of unmodified fibers, Bi2O3/TiO2-coated fibers with a fiber-to-composite weight ratio of 80:20 before and after washing, and Bi2O3/TiO2 coated fibers without maleic acid are presented in Table 1. The EDX result of Bi2O3/TiO2-coated fiber without maleic acid was used as the control. Table 1 shows that the unmodified fiber contained high amounts of carbon and oxygen (Tan et al. 2015). After the fiber was coated with Bi2O3/ TiO2 composite, the elements Bi and Ti were observed. Thus, the Bi2O3/TiO3– coated fiber consisted of C, Bi, Ti, and O. However, the Bi2O3/TiO2-coated fiber without maleic acid did not show any element of Bi and Ti on the fiber surface. This indicates that maleic acid can play an important role to adhere the Bi2O3 and TiO2 particles on the fiber surface. After 25 washing cycles, significant amounts of Bi and Ti were still observable on the Bi2O3/TiO2-coated fiber surface. This shows that TiOparticles strongly adhered to the surface of the fibers. This might be due to the continuous and homogeneous configuration of the TiO2 films. Thus, the pollutant molecules impinging on the fiber-TiOcomposite could accumulate preferably with the TiO2phase covering the fibers.

Fig. 1. FE-SEM images of (a, b) unmodified jute fiber, (c, d) Bi2O3/TiO2-coated fiber in the presence of maleic acid, (e, f) Bi2O3/TiO2-coated fiber (with maleic acid) after 25 washings (weight ratio of fiber: Bi2O3/TiO2 = 80:20) and (g) Bi2O3/TiO2-coated fiber without maleic acid.

Table 1. EDX Data of Unmodified Fiber, Bi2O3/TiO2-coated Fiber (without maleic acid), Bi2O3/TiO2-coated Fiber (with maleic acid) before Washing, and Bi2O3/TiO2-coated fiber (with maleic acid) after 25 Washing Cycles

(Weight ratio of fiber: Bi2O3/TiO2 = 80:20)

XRD Analysis

The XRD patterns of unmodified fiber, Bi2O3, TiO2, and the Bi2O3/TiO2 composite are shown in Fig. 2a. As shown from the typical XRD pattern of fibers, one intense peak at 23.1° was observed because of the crystalline phase and two broad peaks at 15.10° and 16.80°, respectively, were attributed to the amorphous phase. The Bi2Ocurve had diffraction peaks at 22.10°, 27.30°, 33.10°, 46.69°, and 54.10°. The TiO2 curve exhibited peaks at 25.2°, 37.9°, 55°, and 62.70°, clearly designating the presence of an anatase phase, and at 44.10°, 57.80°, and 64.10°, establishing the existence of a rutile phase. The XRD patterns of Bi2O3/TiO2-coated fiber matched the diffraction peaks of Bi2O3 and TiO2 phases without any other impurity phases. This indicates that there was no significant chemical reaction between Bi2O3 and TiO2 during the preparation of the Bi2O3/TiOcomposite and subsequent heat treatment at 100 °C.

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Fig. 2. XRD curves of (a) unmodified fiber substrate, TiO2, Bi2O3, and Bi2O3/TiO2 composite; and (b) Bi2O3/TiOcomposite-coated fiber prepared with Bi2O3/TiO2 composite ranging from 10 to 80 wt.%

The Bi2O3/TiO2 composite-coated fiber with various weight percentages of Bi2O3/TiO2 composite exhibited the diffraction peaks of Bi2Oand TiOwithout any other impurity phases, as shown in Fig. 2b. This indicates that there was no chemical reaction place during the introduction of various weight percentages of Bi2O3/TiO2composite onto the fiber. The intensity of the peaks of Bi2O3/TiO2 at 26.10° steadily diminished with reduction in the weight of Bi2O3/TiO2 composite applied to the fiber. This suggests that the annealing temperature of 100 °C was sufficient to provide good interfacial adhesion between Bi2O3, TiO2 nanoparticles, and the fiber surface. This good interfacial bonding resulted in good interparticle electron transfer between Bi2O3 and TiO2.

FTIR Analysis

FTIR spectra of unmodified fiber, Bi2O3/TiO2-coated fiber, and Bi2O3/TiO2-coated fiber after 48 h of solar light exposure are shown in Fig. 3. The spectrum of fiber shows inter and intra chains of OH-O groups at 3600 to 3100 cm-1. The attendance of interstitial or absorbed water, (H2O) is indicated by the peak at 1600 cm-1. The absorption at 300 to 2800 cm-1 and 1450 to 1350 cm-1 indicated the presence of ν (CH) and δ (CH), respectively. The most intense peak, located at 1200 to 900 cm-1, specifies the existence of C-O-C groups on the fiber (Ali et al. 2015)

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Fig. 3. FTIR spectra of unmodified fiber, Bi2O3/TiO2-coated fiber, and Bi2O3/TiO2-coated fiber after 48 h of solar light exposure (weight ratio of fiber: Bi2O3/TiO2 = 80:20)

After introduction of a Bi2O3/TiOcomposite coating on the surface of the jute fiber, there was no major change on the observed curve. This indicates that the Bi2O3/TiO2 coating layer did not structurally change the fiber surface. Upon exposure to solar light, the Bi2O3/TiO2-coated fiber did not experience any chemical changes (Uddin et al. 2007). This might be because the contribution from the external alcoholic groups, which are expected to actively participate in the anchoring process and hence to be consumed by the coating procedure, was too small or negligible due to the small diametr of the fibers (∼10μm) (Uddin et al. 2007). This is demonstrated by the spectrum of the Bi2O3/TiO2-coated fiber (curve b), which is substantially unaltered. This means that due to the low external surface area of the supporting fibers, the use of FTIR spectroscopy is not informative and it is quite remarkable that the FTIR spectrum of the coated sample is only dominated by the spectrum of the fiber. The contribution of the Bi2O3/TiO2 phase is also negligible.

The photostability of the Bi2O3/TiO2-coated fiber after 48 h of solar light exposure is illustrated in curve c. From the curve, it can be seen that the absorption band at ∼3400 cm−1 and the complex absorption in the range 1650 to 1050 cm−1 characteristic of the Bi2O3/TiO2-coated fiber are mostly unchanged after 48 h of solar light exposure. This indicates that the chemical structure of Bi2O3/TiO2-uncoated and -coated fiber was not substantially altered upon exposure to solar-like light. The homogeneous nature of the Bi2O3/TiO2 film can protect the fibers from O2- and OH• attack when exposed to sunlight (Uddin et al. 2007; Uddin et al. 2008).

UV-Vis Diffuse Reflectance Spectra

Figure 4 illustrates the UV-Vis diffuse reflectance spectra of the as-prepared Bi2O3/TiOcomposite-coated fibers. The band gaps of Bi2O3 and TiO2 were reported to be 2.8 and 3.2 eV, respectively (Hameed et al. 2008; Xu and Schoonen 2000). The optical absorptions of Bi2O3/TiO2 composite-coated fibers start at about 400 nm due to the absorption edge of Bi2O3 composite.

The second absorption edge appearing at ~ 387 nm is attributed to TiO2 particles. Figure 4 illustrates that the absorption in visible-light of the coated fiber increased with the increasing of Bi2O3/TiO2component in the Bi2O3/TiO2-coated fibers system. This result indicates that the Bi2O3/TiOcomposite is efficient in absorbing the photon in the visible region of the solar spectrum.