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Thin Solid Films 540 (2013) 36–40
Contents lists available at SciVerse ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Enhancing the visible-light-induced photocatalytic activity of the self-cleaning TiO2-coated cotton by loading Ag/AgCl nanoparticles Deyong Wu ⁎, Lianzhi Wang, Xinjian Song ⁎, Yuanbin Tan School of Chemical and Environmental Engineering, Hubei University for Nationalities, Enshi, Hubei, 445000, China
a r t i c l e
i n f o
Article history: Received 23 June 2012 Received in revised form 29 May 2013 Accepted 29 May 2013 Available online 11 June 2013 Keywords: TiO2 Cotton Self-cleaning Visible light Plasmonic photocatalysis
a b s t r a c t TiO2-coated cotton possesses excellent self-cleaning properties but requires ultraviolet irradiation for effective photocatalysis. It is highly desirable to develop self-cleaning cotton fabrics that can use visible light in high efficiency under sunlight irradiation. In this work, Ag/AgCl-TiO2-cotton was prepared by coating TiO2 films at low temperature and then loading Ag/AgCl nanoparticles via an impregnating precipitation photoreduction method. It was characterized by meanings of scanning electron microscope, X-ray diffraction, X-ray photoelectron spectroscopy and UV-visible diffuse reflectance spectrophotometer. In comparison with TiO2-cotton, Ag/AgCl-TiO2-cotton exhibits a highly visible-light-induced photocatalytic activity for degradation of methyl orange in water. The mechanism for the degradation of methyl orange over the Ag/AgCl-TiO2-cotton was discussed. Under visible light irradiation, Ag NPs are photoexcited due to surface plasmon resonance, and then the photoexcited electrons from Ag NPs inject into the TiO2 conduction band and the holes transfer to the surface of the AgCl particles. The produced radical groups, such as O2•−, H2O2, •OH and Cl0, can cause the photocatalytic degradation of organic pollutants. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Recently, many scientists have been working on TiO2-functionalized textiles which can be photoactivated under sun irradiation, inducing self-cleaning [1–5], UV-blocking [6,7], photo-oxidative [8] and bactericidal properties [1,2]. Several techniques have been applied for fixing nano-TiO2 on textile surface, and for increasing functionality and reactivity of TiO2-functionalized textiles, the structure of the textile surface was modified by pretreatment of RF-plasma, MW-plasma and vacuum-UV irradiation [4,9–11] and introducing a variable density of negative groups [6] and utilizing different approaches [8,12–14]. The mechanism of self-cleaning cotton is that TiO2 absorb energy equal to or more than its energy gap to generate electron–hole pairs and then form superoxide anion and hydroxyl radical [15,16]. However, due to its large band gap (3.2 eV for anatase), UV light (λ b 400 nm) is necessary to generate the electron–hole pairs, thus restricting its absorption of solar energy (about 4% of the total energy). For the sake of efficient utilization of the visible light (about 45% of the total energy), TiO2 modified with noble metals like Au and Ag has received more and more attention [17–21] because noble metal nanoparticles (Au, Ag) exhibit unique optical properties due to the surface plasmonic resonance (SPR). SPR can dramatically amplify the absorption of visible light and is therefore utilized to develop efficient visible-light-driven plasmonic photocatalysts [22–24]. Moreover, AgCl, AgBr and AgI, known as widely applied photosensitive materials, have ⁎ Corresponding author. Tel./fax: +86 718 8437531. E-mail addresses:
[email protected] (D. Wu),
[email protected] (X. Song). 0040-6090/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2013.05.113
been exploited as photocatalysts in recently [25–27]. Inspired by the photosensitive properties and the SPR effect of Ag NPs, high efficient plasmonic photocatalysts Ag/AgX have been developed and aroused broad interesting [28–37]. Huang et al. have prepared several kinds of plasmonic photocatalysts by a simple ion-exchange process and a light-induced chemical reduction reaction, such as Ag@AgBr [37], Ag@ AgCl [36], Ag@Ag(Br,I) [35], Ag@Ag(Cl,Br) and Ag@AgCl-AgI [31]. All the plasmonic photocatalysts show excellent visible light activation. Especially, the preparation and mechanism of Ag/AgCl have been researched by a lot of researchers [33,36,38–40]. In addition, the visible-light-driven plasmonic photocatalyst Ag/AgCl/TiO2 has been developed by a great diversity of methods to make the best of sunlight [29,41–43]. Based on the above discussions, to realize the immobilization of TiO2 on the substrates with low thermal resistance and the utilization of the visible light, Ag/AgCl-TiO2-cotton was prepared by coating TiO2 films on the surface of cotton at low temperature and then loading plasmonic photocatalyst Ag/AgCl nanoparticles on the TiO2 films. The prepared samples showed high visible light photocatalytic activity for the photocatalytic degradation of methyl orange (MO) and the mechanism has been discussed. 2. Experimental section 2.1. Materials Tetrabutyl titanate (Ti(OBu)4), silver nitrate (AgNO3), ammonium hydroxide (NH3.H2O), hydrogen chloride (HCl), nitric acid (HNO3) and isopropanol (C3H8O) were purchased from China National Medicines
D. Wu et al. / Thin Solid Films 540 (2013) 36–40
Corporation Ltd. All chemicals were analytical grade without further purification. Deionized water was used throughout this study. 2.2. Preparation of anatase TiO2 hydrosol A typical synthesis of the TiO2 nanosol can be described as follows. Tetrabutyl titanate (5 ml) was dissolved in isopropanol (5 ml), and then the solution was dropwise added into 0.04 M HNO3 solution (50 ml) under a vigorous stir at room temperature. Subsequently, the suspension was kept stirring for 12 h to ensure complete hydrolysis nucleation and growth of titania crystallites. Then the sol was refluxed at 100 °C for 6 h. 2.3. TiO2 coating process The cotton fabrics, which have been pretreated with the method described in our earlier study [5], were immersed in the TiO2 hydrosol for 1 min and then pressed at a pressure of 1.25 kg cm−2 for 5 min. The coated substrates were heated at 60 °C 5 min in preheated oven and then cured at 100 °C for 5 min to complete the formation of titanium dioxide. Finally, the fabrics were immersed in 70 °C hot water for 3 h to remove unattached TiO2 particles from the fabric surface. 2.4. Ag/AgCl loading process Firstly, TiO2-cotton fabrics were immersed 0.01 M [Ag(NH3)2]+ solution for 1 h in order to absorb large numbers of Ag+ as much as possible on the surface. Secondly, the cotton fabrics were immersed into 0.01 M HCl solution for 1 h so that the absorbed Ag+ on the cotton fabrics can react with Cl− to generate AgCl nanoparticles. Thirdly, the cotton fabrics were washed by a mass of water to remove unattached AgCl particles from the fabric surface. Finally, the prepared samples were irradiated with a 1000-W xenon lamp for 10 min to reduce partial Ag+ ions in the AgCl particles to Ag0 species by photochemical decomposition of AgCl or TiO2 photocatalytic reduction [29].
3. Results and discussion The microscope morphologies of the as-prepared TiO2-cotton and Ag/AgCl-TiO2-cotton were investigated by SEM observation. Fig. 1a shows a typical SEM image of the TiO2-cotton, which displays a homogeneous TiO2 coating on the surface of cotton with several cracks, resulting in a smooth surface covered by TiO2 nanoparticles. Fig. 1b is a low-magnification SEM image of the Ag/AgCl-TiO2-cotton, whose surface has been covered obviously with a large number of Ag/AgCl particles. However, the heterocomponents interface of Ag and AgCl cannot be verified in current analysis. XRD patterns of cotton, TiO2-cotton and Ag/AgCl-TiO2-cotton are shown in Fig. 2. Because TiO2 prepared at 100 °C has a relatively poor degree of crystallinity, the diffraction peaks of cotton are so strong that the diffraction peaks of TiO2 cannot be visibly displayed in the TiO2-cotton sample. As a matter of fact, the anatase is observed for the pure TiO2 nanoparticle powders [26]. In comparison with the diffraction profile of cotton and TiO2-cotton, new strong diffraction peaks in Ag/AgCl-TiO2-cotton appear at the position about 27.8°, 32.3° and 46.4°, corresponding to the peaks of (111), (200) and (220) diffraction peaks of AgCl, and at the position about 38.2°, 44.5° and 64.6° 77.6°, corresponding to the peaks of (111), (200), (220) and (311) diffraction peaks of Ag. The XRD pattern of the obtained products can be indexed to the cubic phase of Ag with lattice constant a = 4.074 Å (JCPDS file: 3-921) coexisting with the cubic phase of AgCl with lattice constant a = 5.5491 Å (JCPDS file: 31-1238) (Fig. 3). The surface components and chemical states of Ag/AgCl-TiO2-cotton have been investigated by XPS analysis. The position of Ti 2p peaks for the sample are at the value of 458.4 eV and 464.2 eV (Fig. 4a), which confirms that the Ti element exists in the form of Ti4+ in the sample. The Ag 3d spectrum consists of two peaks at about 367.4 and about 373.4 eV (Fig. 4b), which correspond to the binding energies of Ag
2.5. Characterization of catalysts X-ray powder diffraction (XRD) patterns of samples were recorded on a powder X-ray diffractometer (D/max-2200/PC, Rigaku Corporation, Japan) with Cu Kα radiation, operating at 40 kV and 30 mA, where λ = 0.15418 nm for the Cu Kα line. XPS experiments were carried out on an RBD upgraded PHI-5000C ESCA system (Perkin Elmer, USA), and the shift of the binding energy due to relative surface charging was corrected using the C 1 s level at 284.6 eV as an internal standard. The structure and the morphology of the coatings were investigated by the field emission scanning electron microscopy (FESEM, FEI SIRION 200, FEI, USA). UV–vis diffuse reflectance spectra (DRS) of the samples were recorded on a UV–vis spectrophotometer (TU-1901) with an integrating sphere attachment. The analyzed range was 230–800 nm, and BaSO4 was used as a reflectance standard. 2.6. Photocatalytic activity measurements Methyl orange (MO) was selected as model chemicals to evaluate the activity of the samples in our homemade equipment. A 1000-W Xe lamp was used as the light source of the homemade photoreactor, surrounded with a water circulation facility at the outer wall through a quartz jacket. The short wavelength components (λ b 400 nm) of the light were cut off using a glass optical filter. The distance between the lamp and the center of the beaker was 100 cm. Before irradiation, 50 ml MO solution (5 mg/L) with 4 cm × 4 cm Ag/AgCl-TiO2-cotton fabrics was stirred in the dark for 30 min to achieve adsorption equilibrium. The MO solution was taken out at regular time intervals, and the MO concentration was monitored at 464 nm using a UV–vis spectrophotometer (UNICO 7200).
37
Fig. 1. SEM images of (a) TiO2-cotton and (b) Ag/AgCl-TiO2-cotton.
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D. Wu et al. / Thin Solid Films 540 (2013) 36–40
Fig. 2. The XRD patterns of cotton, TiO2-cotton and Ag/AgCl-TiO2-cotton.
3d5/2 and Ag 3d3/2, respectively, with a doublet separation of 6 eV [44]. For Cl 2p, two peaks are observed at binding energies of about 197.9 and 199.6 eV (Fig. 4c), corresponding to the binding energies of Cl 2p3/2 and Cl 2p1/2, respectively, with a doublet separation of 1.7 eV [33]. The XPS results of Ag/AgCl-TiO2-cotton show that the calculated surface content of Ti, Ag and Cl is 16.57 mol%, 3.28 mol% and 2.85 mol%, respectively. The Ag/Cl atomic ratio is approximately 1.15, which suggests that the as-obtained sample contains a small amount of Ag metal species. The UV–vis diffuse reflectance spectra of the samples are shown in Fig. 5. Pristine cotton has no ability to absorb UV and visible light. TiO2-cotton exhibits strong absorption in the UV region, which is attributed to the presence of TiO2 on the surface of cotton, which has a strong capability of UV light absorption [26,45]. In comparison to TiO2-cotton, Ag/AgCl-TiO2-cotton has strong absorption both in the ultraviolet and visible-light regions. The absorption in the ultraviolet region can be ascribed to the characteristic absorption of the AgCl and TiO2 semiconductors, and the strong absorption in the visible-light region can be attributed to the surface plasmon resonance of silver nanoparticles [29,36,39]. The strong absorption makes the sample use the sunlight more efficiently. The photocatalytic activity of Ag/AgCl-TiO2-cotton was measured by decomposing methyl orange (MO) under visible irradiation. MO is a typical recalcitrant organic dye with negligible photolysis and sensitization effect in the range of visible light. Fig. 6 shows the photodegradation of MO dye as a function of irradiation time over different samples. Under visible light irradiation, the degradation of MO over the pristine cotton is negligible and the photodegradation of MO over TiO2-cotton is very weak. However, the photocatalytic activity of Ag/AgCl-TiO2-cotton was greatly enhanced, and almost all of MO has been removed after 60 min
Fig. 3. XRD patterns of (a) Ag (JCPDS No. 3-921), (b) AgCl (JCPDS No. 31-1238) and (c) Ag/AgCl-TiO2-cotton.
Fig. 4. XPS spectra of Ti 2p, Ag 3d and Cl 2p of Ag/AgCl-TiO2-cotton.
irradiation. It is well known that photocatalytic oxidation of organic pollutants follows first-order kinetics. The apparent rate constant has been chosen as the basic kinetic parameter for different photocatalysts,
Fig. 5. UV–vis diffuse reflectance spectra of the samples.
D. Wu et al. / Thin Solid Films 540 (2013) 36–40
39
which was fitted by the equation ln(C/C0) = − kt, where k is the apparent rate constant, C is the solution-phase concentration of organic pollutant (in this case, MO) and C0 is the initial concentration of MO. The rate constant for the photodegradation of the dye over Ag/AgCl-TiO2-cotton (0.0391 min− 1) is about 23 times higher than that over TiO2-cotton (0.0017 min−1). The excellent visiblelight-induced photocatalytic activity of Ag/AgCl-TiO2-cotton is likely related to the surface plasmon resonance effect from the Ag nanoparticles. The photocatalytic activity of the Ag/AgCl-TiO2-cotton can be understood by the following suggested mechanism (as shown in Fig. 7). Under visible light irradiation, photogenerated electron–hole pairs are formed in Ag NPs due to surface plasmon resonance. The photoexcited electrons at the silver NPs are injected into the TiO2 conduction band, and the active species (O2•−, •OOH, H2O2, •OH) will be formed [46]. Meanwhile, the holes transfer to the surface of the AgCl particles and cause the oxidation of Cl ions to Cl0 atoms, which are reactive radical species and can oxidize organic pollutant [47,48]. The major reaction steps in this plasmonic photocatalytic mechanism under visible light irradiation are described by Eqs. (1)–(8) as follows [29,41]: ⁎
ð1Þ
Ag NPs þ hν→Ag NPs
⁎
þ
Ag NPs þ TiO2 →Ag NPs • þ TiO2 ðeÞ
ð2Þ
Fig. 7. Schematic diagram for the charge separation in the visible light irradiated Ag/AgCl-TiO2-cotton system.
4. Conclusions −
TiO2 ðeÞ þ O2 →TiO2 þ O2 •
−
ð3Þ
þ
O2 • þ H →•OOH
ð4Þ
þ
•OOH þ TiO2 ðeÞ þ H →H2 O2 þ TiO2
−
H2 O2 þ TiO2 ðeÞ→•OH þ OH þ TiO2
þ
−
Ag NPs • þ Cl →Ag NPs þ Cl
0
Organic pollutant 0 − þ Cl ðor • OH or O2 • or H2 O2 Þ→Degraded or mineralized products
ð5Þ
ð6Þ
ð7Þ
ð8Þ
Ag/AgCl-TiO2-cotton with visible-light-driven self-cleaning properties has been prepared by depositing AgCl NPs on the surface of TiO2-cotton and then reducing partial Ag+ ions in the surface region of the AgCl particles to Ag0 species under xenon lamp irradiation. The prepared Ag/AgCl-TiO2-cotton displays good visible-light absorption and high visible-light-induced photocatalytic activity, which should be attributed to the surface plasmon resonance absorption of silver NPs under visible light irradiation and the charge separation at the silver NPs, which includes electrons transferring from the plasmon-excited silver to TiO2 to form the active species (O2•−, •OOH, H2O2, •OH) and the holes transfer to the surface of the AgCl particles to form Cl0 atoms. Thus, Ag/AgCl-TiO2-cotton can be used as efficient visible-light-induced self-cleaning materials and is promising to be employed for wastewater treatment and air purification. Acknowledgments This project was supported by the State Ethnic Affairs Commission (12HBZ001), the Foundation of Hubei Educational Committee (Q20122908), the National Natural Science Foundation of China (21267009) and the Natural Science Foundation of Hubei Province of China (2012FFB01104). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
Fig. 6. Photocatalytic activities of cotton, TiO2-cotton and Ag/AgCl-TiO2-cotton.
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