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Synergistic effects between TiO2 and carbon nanotubes (CNTs) in a TiO2/CNTs system under visible light irradiation a

b

a

Chung-Hsin Wu , Chao-Yin Kuo & Shih-Ting Chen a

Department of Chemical and Materials Engineering, National Kaohsiung University of Applied Sciences, Kaohsiung, Taiwan b

Department of Environmental and Safety Engineering, National Yunlin University of Science and Technology, Douliou, Yunlin, Taiwan Accepted author version posted online: 08 Feb 2013.Published online: 11 Mar 2013.

To cite this article: Chung-Hsin Wu, Chao-Yin Kuo & Shih-Ting Chen (2013) Synergistic effects between TiO2 and carbon nanotubes (CNTs) in a TiO2/CNTs system under visible light irradiation, Environmental Technology, 34:17, 2513-2519, DOI: 10.1080/09593330.2013.774058 To link to this article: http://dx.doi.org/10.1080/09593330.2013.774058

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Environmental Technology, 2013 Vol. 34, No. 17, 2513–2519, http://dx.doi.org/10.1080/09593330.2013.774058

Synergistic effects between TiO2 and carbon nanotubes (CNTs) in a TiO2 /CNTs system under visible light irradiation Chung-Hsin Wua∗ , Chao-Yin Kuob and Shih-Ting Chena a Department b Department

of Chemical and Materials Engineering, National Kaohsiung University of Applied Sciences, Kaohsiung, Taiwan; of Environmental and Safety Engineering, National Yunlin University of Science and Technology, Douliou, Yunlin, Taiwan

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(Received 9 October 2012; final version received 2 February 2013 ) This study synthesized a TiO2 /carbon nanotubes (CNTs) composite via the sol-gel method. The surface characteristics of the TiO2 /CNTs composite were determined by X-ray diffraction, transmission electron microscopy, specific surface area analyser, ultraviolent (UV)-vis spectroscopy, X-ray photoelectron spectroscopy and Raman spectrometer. The photocatalytic activity of the TiO2 /CNTs composite was evaluated by decolourizing C.I. Reactive Red 2 (RR2) under visible light irradiation. Furthermore, the effects of calcination temperature, pH, RR2 concentration, and the TiO2 /CNTs composite dosage on RR2 decolourization were determined simultaneously. The optimal calcination temperature to generate TiO2 and the TiO2 /CNTs composite was 673 K, as the percentage of anatase crystallization at this temperature was highest. The specific surface area of the TiO2 /CNTs composite and TiO2 were 45 and 42 m2 /g, respectively. The band gap of TiO2 and the TiO2 /CNTs composite was 2.97 and 2.71 eV by UV-vis measurements, respectively. Experimental data indicate that the Ti-O-C bond formed in the TiO2 /CNTs composite. The RR2 decolourization rates can be approximated by pseudo-first-order kinetics; moreover, only the TiO2 /CNTs composite had photocatalytic activity under visible light irradiation. At pH 7, the RR2 decolourization rate constant of 0.5, 1 and 2 g/L TiO2 /CNTs addition was 0.005, 0.0015, and 0.0047 min−1 , respectively. Decolourization rate increased as pH and the RR2 concentration decreased. The CNTs functioned as electron acceptors, promoting separation of photoinduced electron-hole pairs to retard their recombination; thus, photocatalytic activity of the TiO2 /CNTs composite exceeded that of TiO2 . Keywords: TiO2 ; carbon nanotubes; photodegradation; visible light; decolourization

1. Introduction Titanium dioxide (TiO2 ) is the most extensively studied photocatalyst as it is non-toxic, and has powerful oxidizing activity and long-lasting chemical stability. However, TiO2 only can be activated under ultraviolent (UV) irradiation, which markedly hinders the its commercialization. Therefore, developing a new TiO2 -based composite that absorbs visible light irradiation is necessary for efficient utilization of solar energy. In addition, the challenge of preventing the recombination of photoinduced electronhole pairs in a TiO2 system must be overcome. Several investigations have synthesized sulphur-doped TiO2 [1,2] and nitrogen-doped TiO2 , [3,4] both of which extend the spectral response of TiO2 into the visible light region, thereby enhancing the photocatalytic activity of TiO2 . As carbon nanotubes (CNTs) are excellent electron conductors, they have garnered attention as an effective dopant and/or support for a TiO2 -based composite. The TiO2 /CNTs composite has enhanced photocatalytic activity because of electron transfer from TiO2 to CNTs, promoting charge separation and stabilization and retarding the recombination of electron-hole pairs. [5,6] ∗ Corresponding

author. Email: [email protected]

© 2013 Taylor & Francis

Several studies have synthesized the TiO2 /CNTs composite, and most Ti was sourced from Ti(OC4 H9 )4 [7–11] and Ti(OC3 H7 )4 [5,12–18]; only a few studies used TiCl4 [11] as the Ti source. Other studies mixed only TiO2 and CNTs to form a physical TiO2 /CNTs mixture [19,20]; however, the photocatalytic activity of the TiO2 /CNTs mixture was lower than that of the TiO2 /CNTs composite. [12,14,15] Wang et al. [12] demonstrated that the TiO2 /CNTs composite prepared by the sol-gel method has higher photocatalytic activity than the mechanical mixture with the same CNT content, as the CNTs were embedded in the TiO2 matrix in the TiO2 /CNTs composite. Tian et al. [11] used TiCl4 as the Ti source to generate a TiO2 /CNTs composite via the solvothermal synthesis method. No study has used TiCl4 as the Ti source for synthesizing the TiO2 /CNTs composite via the sol-gel method. The surface and photocatalytic properties of the TiO2 /CNTs composite synthesized from TiCl4 and CNTs remain largely unexplored. The photocatalytic activity of the TiO2 /CNTs composite for C.I. Reactive Red 2 (RR2) decolourization under visible light irradiation was studied, as RR2 is commonly used in the textile industry.

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The objectives of this study were to (i) generate the TiO2 /CNTs composite at different calcination temperatures; (ii) identify the surface characteristics of prepared photocatalysts; and (iii) evaluate the effects of calcination temperature, pH, RR2 concentration, and the TiO2 /CNTs composite dose on RR2 decolourization for the visible light/TiO2 /CNTs system.

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2.

Materials and methods

2.1. Materials Titanium chloride was the source of Ti (ACROS). The CNTs utilized were multiwall nanotubes (CBT, MWNTs2040) generated by pyrolysis of methane gas on Ni particles in chemical vapour deposition. The length and diameter of CNTs were 5–15 μm and 20–40 nm, respectively. The parent compound, RR2, was purchased from the SigmaAldrich chemical company. The formula, molecular weight, and maximum light absorption wavelength of RR2 were C19 H10 Cl2 N6 Na2 O7 S2 , 615 g/mol, and 538 nm, respectively. Solution pH was adjusted using HNO3 or NaOH. All compounds except CNTs were used as received without further purification. All solutions were prepared using deionized water (Milli-Q) and reagent-grade chemicals. Preparation of TiO2 and TiO2 /CNTs composite The purification of CNTs was performed by adding 0.5 g raw CNTs to 100 mL acid solution containing 2.5 M HNO3 (50 mL) and 0.5 M H2 SO4 (50 mL). The mixture was shaken in an ultrasonic bath (Delta, DC 400H) for 2 h. The CNTs were then filtered and washed with deionized water to reach pH 7. After purification, most impurities disappeared and the surface of CNTs became clean. Both TiO2 and the TiO2 /CNTs composite were prepared by the sol-gel method. To synthesize the TiO2 /CNTs composite, 0.6525 g purified CNTs were first ultrasonicated in 60 mL ethanol (95%) for 10 min for good dispersion and then mixed with 6 mL titanium chloride. The mole ratio of C/Ti in the TiO2 /CNTs composite was 1. The entire mixture was then stirred magnetically at 250 rpm for 24 h. After gel formation, 10 mL ethanol (99%) was used to rinse the gel, and the batch was dehydrated at 393 K. The dried mixtures were first ground and then calcinated (573, 673 or 773 K) for 3 h to generate the TiO2 /CNTs composite. The TiO2 particles were produced using the same procedure as that used to prepare the TiO2 /CNTs composite, but without adding CNTs. 2.2.

at 20–80◦ . The morphology and structure of the prepared TiO2 /CNTs composite were characterized by transmission electron microscopy (TEM) (JEOL 3010, Japan). Specific surface area, pore size and pore volume of samples were determined by using nitrogen as the adsorbate at 77 K in a static volumetric apparatus (Micromeritics ASAP 2020, USA). The UV-vis spectroscopy (Jasco V-670, Japan) method profiled the absorbance spectrum of photocatalysts at wavelengths of 200–800 nm. The UV-vis diffuse reflectance spectra were used to calculate the band gap energy of photocatalysts. The X-ray photoelectron spectroscopy (XPS) measurements were recorded on a PHI Quantum 5000 XPS system (USA) with a monochromatic Al Kα source and a charge neutralizer. Raman measurements were performed by a Horiba Jobin Yvon HR 800 micro-Raman spectrometer (Japan). The pH of the point of zero charge (pHpzc ) of the particles was measured at 3–10 using a Zeta-Meter 3.0 (USA). 2.4.

Photodegradation of RR2

The RR2 concentration in all experiments was 20 mg/L, except in those used to determine the effects of the RR2 concentration. Reaction pH was 7 for all runs, except in those performed to delineate the effects of pH. The effects of TiO2 /CNTs composite dosages of 0.5, 1 and 2 g/L were evaluated. Decolourization experiments were conducted in a 3 L hollow cylindrical glass reactor. A 8 W lamp (410 nm, Philips) was placed inside the quartz tube as the light source. Reaction temperature in all experiments was 298 K. The reaction medium was stirred continually at 300 rpm to suspend the photocatalysts. In total, a 15 mL aliquot was withdrawn from the photoreactor at predetermined intervals. Suspended photocatalyst particles were separated by filtration through a 0.22 μm filter (Millipore). The RR2 concentration was measured by a spectrophotometer (Hitachi U-2001, Japan) at 538 nm. Some experiments were performed in triplicate and average values are reported. 3.

Results and discussion

3.1. Physical characterization of photocatalysts Figure 1 presents the XRD patterns of the TiO2 /CNTs composite generated under different calcination temperatures. Anatase content was determined from the integrated intensity of (1 0 1) anatase diffraction, IA , and that of (1 1 0) rutile diffraction, IR , using Equation (1). [21] Anatase (%) =

2.3. Characterization The crystalline structure of prepared TiO2 and the TiO2 /CNTs composite was analysed by X-ray diffraction (XRD) (PANalytical X’Pert Powder, Holland). Accelerating voltage and applied current were 40 kV and 30 mA, respectively. The XRD patterns were recorded as 2θ values

1 × 100 1 + 12.6 IIAR

(1)

The peaks, the diffractions of the various crystal planes of anatase, were as follows: 25.4◦ , (1 0 1); 37.9◦ , (0 0 4); 48.1◦ , (2 0 0); 55◦ , (2 1 1); and 62.5◦ (2 0 4) (JCPDS no. 211272). Other crystal phases corresponding to peaks at 27.4◦ , 36.1◦ , 41.2◦ , and 54.3◦ were assigned the diffraction peaks

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Figure 1. XRD patterns of TiO2 /CNTs composite (a) 573 K (b) 673 K (c) 773 K.

of the (1 1 0), (1 0 1), (1 1 1) and (2 1 1) of rutile, respectively (JCPDS no. 21-1276). The peaks at 25.9◦ and 44.1◦ correspond to the (0 0 2) and (1 0 0) reflections of CNTs, respectively.[12,16,22] The characteristic peaks of CNTs cannot be identified from the patterns of the TiO2 /CNTs composite, as in the study by Wang et al. [12] This can be attributed to the (0 0 2) reflection of CNTs overlapping the anatase (1 0 1) reflection at 25.4◦ . The extent of crystallization of CNTs is much lower than that of TiO2 [5] and a trivial amount of CNTs in the TiO2 /CNTs composite meant that clearly recognizing the peaks of CNTs was impossible. [7] Table 1 lists the surface properties of TiO2 and the TiO2 /CNTs composite. The pore size of TiO2 was greater than those of the TiO2 /CNTs composite; conversely, the surface area of TiO2 was smaller than that of the TiO2 /CNTs composite. The CNTs introduced into TiO2 can prevent TiO2 particles from agglomerating, consequently increasing its surface area. For both TiO2 and the TiO2 /CNTs composite, the highest percentage of the anatase phase was under 673 K calcination. Heat treatment at a high temperature was responsible for aggregate formation; hence, surface area decreased as calcination temperature increased for TiO2 and the TiO2 /CNTs composite.

Table 1. posite.

Surface characteristics of TiO2 and TiO2 /CNTs comCrystallization (%)

Photocatalyst TiO2 (573 K) TiO2 (673 K) TiO2 (773 K) TiO2 /CNTs (573 K) TiO2 /CNTs (673 K) TiO2 /CNTs (773 K)

Anatase

Rutile

BET (m2 /g)

Pore size (nm)

62 87 49 67 70 65

38 13 51 33 30 35

47 42 38 64 45 24

9 14 23 7 12 22

Figure 2. TEM micrographs of TiO2 /CNTs composite (a) magnification: 40000 (b) magnification: 400000 (Calcination temp.: 673 K).

The TEM offered insight into the morphology and microstructure of the TiO2 /CNTs composite (Figure 2). The TiO2 particles attached to the surface of CNTs. The well-aligned lattice fringes of CNTs with adjacent fringe spacing were roughly 0.3–0.4 nm. The diameter of CNTs and TiO2 in the TiO2 /CNTs composite was approximately 40 and 35 nm, respectively. Figures 3(a)–(c) display the XPS spectra of C 1s, Ti 2p, and O 1s for the TiO2 /CNTs composite, respectively. The Ti 2p3/2 and Ti 2p1/2 spin-orbital splitting photoelectrons were located at binding energies of 459.5 and 465.6 eV, respectively. [14] Peaks at 532.3 and 530.7 eV were ascribed to O 1s of H2 O and TiO2 , respectively, [15] clearly indicating that all titanium cations in the TiO2 /CNTs composite were in oxidative state IV. According to the area of curves for Ti 2p, the amounts of Ti 2p3/2 and Ti 2p1/2 in TiO2 were 32% and 68% (figures not shown), respectively, and the amounts of Ti 2p3/2 and Ti 2p1/2 in the TiO2 /CNTs composite were 31% and 69%, respectively (Figure 3(b)). The C-C bond was assigned to the peak binding energy of 284.7 eV, while the peak at 285.3 eV was attributed to C-O bonds (Figure 3(a)). Kang et al. [23] demonstrated that binding energy at 282 eV is assigned to the Ti-C bond via the substitution of carbon for oxygen sites in TiO2 ; in addition, the binding energy at 288.6 eV was assigned to carbonate species via the substitution of carbon for the

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Figure 4. Raman spectra of CNTs and TiO2 /CNTs composite (Calcination temp.: 673 K).

Figure 3. Fitting XPS spectra for TiO2 /CNTs composite for (a) C 1s region (b) Ti 2p region (c) O 1s region (Calcination temp.: 673 K).

Ti site in TiO2 . The prepared TiO2 /CNTs composite had C-C and C-O bonds, without Ti-C bonds. Based on XPS analysis, the proportion of C-C and C-O bonds was 29% and 71% in the TiO2 /CNTs composite, respectively. In this study, the full-width at half-maximum (FWHM) value of the Ti 2p3/2 peak for TiO2 and the TiO2 /CNTs composite was 1.16 and 1.35 eV, respectively. Tian et al. [11] indicated that the increased FWHM value of the Ti 2p3/2 peak can be attributed to the bond between CNTs and TiO2 on the TiO2 /CNTs composite surface. Experimental results suggest that the Ti-O-C bond formed in the TiO2 /CNTs composite; moreover, the Ti-C bond was not observed.

The band gap of TiO2 (P25), TiO2 , and the TiO2 /CNTs composite was 3.06, 2.97 and 2.71 eV by UV-vis measurements, respectively. The band gap of the TiO2 /CNTs composite significantly decreased. A red shift means that the TiO2 /CNTs composite could be excited to produce more hole-electron pairs under visible light irradiation, which can increase photocatalytic activity. Min et al. [18] suggested that the decrease in the band gap can be attributed to the formation of Ti-O-C bonds. The pHpzc of CNTs, TiO2 and the TiO2 /CNTs composite was 3.8, 4.8 and 4.4, respectively. Under experimental conditions, the surface of TiO2 and the TiO2 /CNTs composite was charged negatively. Raman spectroscopy is used widely to characterize the electronic structure of carbon products. A change in the Raman band shift provides information about the carboncarbon bonds and defects. The D band is a common feature for sp3 defects in carbon, and the G band provides information on the in-plane vibration of sp2 bonded carbons. [24] Figure 4 shows the Raman spectra of CNTs and the TiO2 /CNTs composite. The Raman spectra of CNTs had two characteristic peaks; the peak at 1320 cm−1 is called the D band and the peak at 1571 cm−1 is called the G band. Compared with the Raman spectra of CNTs, three other spectra were observed at roughly 395, 515 and 637 cm−1 for the TiO2 /CNTs composite; these spectra represent the typical TiO2 frequencies. [13] The D and G bands of CNTs and three features of TiO2 in the TiO2 /CNTs composite were located. However, Zein and Boccaccini [16] did not identify the D and G bands of CNTs in the TiO2 /CNTs composite, suggesting that most CNTs are fully coated with TiO2 . The D band in the TiO2 /CNTs composite shifts upward (from 1320 to 1326 cm−1 herein) by stress induced by the TiO2 grown on the surface of CNTs. [18,19] According to Zhou et al. [10] a close interaction between TiO2 and CNTs causes the G band to shift upward (from 1571 to 1577 cm−1 herein). The Raman analysis results further confirm the formation of a chemical bond between TiO2 and CNTs, consistent with XPS results.

Environmental Technology

C/C0 (%)

80 60

TiO2 (573 K) TiO2 (673 K) TiO2 (773 K) TiO2/CNTs (573 K) TiO2/CNTs (673 K) TiO2/CNTs (773 K) P25

40 20 0

0

30

60

90 Time (min)

120

150

180

(b) 100 80 C/C0 (%)

At pH 7, no significant degradation occurred in the adsorption of TiO2 (1.5%) and the TiO2 /CNTs composite (15%), and in direct photolytic reactions ( P25 > TiO2 /CNTs mixture > pure TiO2 . Figure 5(b) presents the RR2 removal percentage at different pHs. After 180 min reaction, RR2 removal percentage for the TiO2 /CNTs composite at pH 7 and 9 was 37% and 19%, respectively. The pH of a solution influences adsorption and dissociation of the parent compound, catalyst surface charge, and oxidation potential of the valence band. When operating at pH < pHpzc of the TiO2 /CNTs composite (4.4), the surface charge of the TiO2 /CNTs composite becomes positively charged and gradually exerts an electrostatic attraction force toward the negatively charged

(a) 100

60 40

TiO2 (pH 7) TiO2 (pH 9) TiO2/CNTs (pH 7) TiO2/CNTs (pH 9)

20 0

0

30

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90 Time (min)

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(c) 100 80 C/C0 (%)

Photodegradation efficiency of TiO2 and TiO2 /CNTs composite under visible light irradiation

60 40 TiO2/CNTs (0.5 g/L) TiO2/CNTs (1 g/L) TiO2/CNTs (2 g/L)

20 0

0

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60

90 Time (min)

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(d) 100 80 C/C0 (%)

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60 40

RR2 (10 mg/L) RR2 (20 mg/L) RR2 (40 mg/L)

20 0

0

30

60

90 120 Time (min)

150

180

Figure 5. Effects of RR2 removal percentage at different experimental conditions (a) calcination temperature (b) pH (c) TiO2 /CNTs dosage (d) RR2 concentration.

RR2. At pH > pHpzc of the TiO2 /CNTs composite, the TiO2 /CNTs composite will be negatively charged and repulse anionic RR2 in solution. Since the repulsive force between the TiO2 /CNTs composite and RR2 was greater at pH 9 than at pH 7, the decolourization efficiency of at pH 7 was higher than that at pH 9. Figure 5(c) lists the RR2 removal percentage at different TiO2 /CNTs dosages. After 180 min reaction, the RR2 removal percentage for the TiO2 /CNTs composite at 0.5, 1 and 2 g/L TiO2 /CNTs addition was 7%, 37%

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and 61%, respectively. The RR2 decolourization rate constant for the TiO2 /CNTs system under 0.5, 1 and 2 g/L TiO2 /CNTs addition was 0.0005, 0.0015 and 0.0047 min−1 , respectively. The RR2 decolourization efficiency of the TiO2 /CNTs system increased as the photocatalyst dosage increased, due to an increased number of available adsorption and catalytic sites on the surface of the TiO2 /CNTs composite. A further increase in photocatalyst dosage may cause light scattering and the screening effect, reducing the photocatalytic activity of the TiO2 /CNTs composite. [7,8] This study did not identify the light scattering and screening effect for 2 g/L TiO2 /CNTs addition. Figure 5(d) shows the effects of the RR2 concentration on RR2 decolourization. After 180 min reaction, the RR2 removal percentage for 10, 20 and 40 mg/L RR2 was 49%, 37% and 20%, respectively. The RR2 decolourization rate constant for the TiO2 /CNTs system under 10, 20 and 40 mg/L RR2 was 0.0025, 0.0015 and 0.0004 min−1 , respectively. Experimental results show that RR2 decolourization efficiency decreased as the initial concentration of RR2 increased. This can be due to RR2 blocking photocatalytically active sites on the surface of TiO2 /CNTs particles and reducing the interaction of photons with these sites. [7] In addition, increased amounts of RR2 and reaction intermediates competed with both hydroxyl radicals and active reaction sites on the TiO2 /CNTs surface when the initial RR2 concentration was increased. Since the amount of TiO2 /CNTs composite remained constant, the rate of formation of hydroxyl radicals on TiO2 /CNTs surface was constant. Hence, the fraction of hydroxyl radicals that attacked RR2 molecules and its reaction intermediates declined as the RR2 concentration increased. 4. Conclusions The TiO2 /CNTs composite was synthesized by the solgel method, and the surface properties and photocatalytic activity of the TiO2 /CNTs composite were determined. Introducing CNTs into TiO2 can prevent TiO2 particles from agglomerating, consequently increasing TiO2 /CNTs composite surface area. Furthermore, the Ti-O-C bond formed in the TiO2 /CNTs composite. The TiO2 /CNTs composite exhibited higher photocatalytic activity than pure TiO2 under visible light irradiation. The synergistic effects between TiO2 and CNTs can be attributed to electron transfer from TiO2 to CNTs, which promoted charge separation and stabilization and decreased the recombination of photoinduced electron-hole pairs in the TiO2 /CNTs composite. The RR2 decolourization rate in TiO2 /CNTs system increased as the TiO2 /CNTs composite dosage increased; conversely, as pH and the RR2 concentration decreased. Acknowledgements The authors would like to thank the National Science Council of the Republic of China, Taiwan, for financially supporting this research under Contract No. NSC 101-2221-E-151-038-MY3.

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CNTs system under visible light irradiation.

This study synthesized a TiO2/carbon nanotubes (CNTs) composite via the sol-gel method. The surface characteristics of the TiO2/CNTs composite were de...
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