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Improved photodegradation properties and kinetic models of a solar-light-responsive photocatalyst when incorporated into electrospun hydrogel fibers
Journal of Colloid and Interface Science 346 (2010) 216–221
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Improved photodegradation properties and kinetic models of a solar-light-responsive photocatalyst when incorporated into electrospun hydrogel fibers Ji Sun Im a, Byong Chol Bai a, Se Jin In b, Young-Seak Lee a,* a b
Department of Fine Chemical Engineering and Applied Chemistry, BK21-E2M, Chungnam National University, Daejeon 305-764, Republic of Korea Department of Fire and Disaster Protection Engineering, Woosong University, Daejeon 300-718, Republic of Korea
a r t i c l e
i n f o
Article history: Received 23 December 2009 Accepted 18 February 2010 Available online 21 February 2010 Keywords: Photodegradation TiO2 Hydrogel Electrospinning Nano-fiber
a b s t r a c t The capacity of a photocatalyst system to degrade water pollutants was optimized using solar-light-sensitive TiO2 and the swelling behavior of a hydrogel. TiO2 synthesized via a sol–gel process was modified by multielement doping to change its solar-light-responsive properties. A hydrogel was used for the rapid absorption of both anionic and cationic water pollutants. TiO2 particles were immobilized in/on hydrogel fibers by an electrospinning method for the easy recovery of TiO2, and the ability of the hydrogel/TiO2 composite to degrade dye molecules was studied. The TiO2 particles were observed to have maintained their original anatase-type crystallinity in/on the electrospun hydrogel fibers. The dye degradation capacity of the hydrogel/TiO2 composite was investigated using both anionic and cationic dyes under sunlight. Two mechanisms were suggested by which the hydrogel/TiO2 composite can remove dye particles from the water: (1) the absorption of dyes by the hydrogel and (2) the degradation of the dye by the TiO2 in the hydrogel. Both of these mechanisms were investigated in this study. We found that the dye was effectively absorbed by the hydrogel fibers as demonstrated by the swelling behavior of the hydrogel and the nano-size effects. The dye was then introduced to the TiO2 particles for degradation. Ó 2010 Elsevier Inc. All rights reserved.
1. Introduction The photocatalytic degradation of various toxic organic compounds has been proposed as a viable method for the decontamination of wastewater for renewable use. Titanium dioxide has been used widely as a photocatalyst due its ability to oxidize toxic substances into nontoxic substances such as carbon dioxide and water in polluted aqueous systems [1–3]. The process is initiated by the photogeneration of hole/electron pairs in the valence and conduction bands, respectively, on absorption of UV light with an energy equal to or higher than that of the corresponding band gap (>3.2 eV, k < 387 nm). This band gap is one of the barriers for use in our environment because UV light accounts for only a small fraction (5%) of the Sun’s energy that reaches the Earth’s surface (visible light represents 45%). Research focused on shifting the optical response of titanium dioxide from the UV to the visible spectral range will have a profound positive effect on the photocatalytic efficiency of this material [4–8]. The other attempt to increase the efficiency of the photocatalyst was the preparation of a nano-scale photocatalyst, the small scale of which would maximize the active surface area. However, this * Corresponding author. Fax: +82 42 822 6637. E-mail addresses: [email protected], [email protected] (Y.-S. Lee). 0021-9797/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2010.02.043
attempt introduced a secondary problem with regard to the recovery of the nano-sized photocatalyst from the water. As a result, immobilization of the nano-sized photocatalyst on a larger support structure has been investigated for the efficient recovery of the photocatalyst while retaining the catalysts’ high pollutant degradation efficiency [9,10]. Recently, hydrogels have been investigated as composite materials to maximize the efficiency of decontamination of toxic organic compounds. Hydrogels swell significantly in water, resulting in the absorption of the toxic organic compounds. Thus, rapid decontamination of toxic organic compounds is possible, allowing quick treatment of polluted water [11,12]. In this paper, a solar-light-responsive hydrogel/TiO2 composite was prepared as a novel photocatalyst system. The solar-lightresponsive TiO2 was prepared by the modified method from our previous work [13]. The hydrogel selected was biocompatible so that it would be safe for use in environmental applications. Organic compounds can be effectively guided by the hydrogel to the TiO2 photocatalyst in the composite, and high sunlight transmission can induce the degradation of the organic compounds by TiO2. TiO2 was also immobilized in the hydrogel in order to obtain better photocatalyst recovery. Eventually, the hydrogel/TiO2 composite was prepared by an electrospinning method because the electrospun web had benefits related to improved photocatalytic effects
J.S. Im et al. / Journal of Colloid and Interface Science 346 (2010) 216–221
by virtue of the nano-sized effects presented in our previous work [14]. 2. Materials and methods 2.1. Materials
217
at 600 and 493 nm for procion blue and acridine orange, respectively. The volume of absorbed dye solution was measured by a scale in beaker after removal of hydrogel/TiO2 composite in a few seconds.
Titanium chloride (TiCl4, Knato Chemical) was used as a precursor of TiO2. Hydrochloric acid (HCl, Samchun chemicals) was used as a solvent for titanium chloride. Ammonia (NH3, 28–30%, Junsei) was used for the sol–gel process of preparing TiO2. Tetraethylammonium tetrafluoroborate (TEATFB, ðCH3 CH2 Þ4 Nþ BF 4 , Sigma–Aldrich) was used as a dopant for carbon, nitrogen, boron, and fluorine codoping. Polyvinyl alcohol (PVA, Mw: 120,000, Sigma–Aldrich) was used as the polymer source for the hydrogel. Glutaraldehyde (GA, 25 wt.% solution in water, KANTO) was used as a cross-linker. The dyes procion blue (anionic dye, CI number 61205) and acridine orange (cationic dye, CI number 46005) were purchased from Acros (USA).
2.2.5. Characterization of the hydrogel/TiO2 composite sample A JSM-6300 scanning electron microscope (SEM) (JEOL Ltd., Japan) was used to investigate the surface morphology of the hydrogel/TiO2 composite samples after Pt coating. The anatase structure of TiO2 was studied using X-ray diffraction (XRD). The XRD patterns of the samples were recorded on a Rigaku (D/MAX2200 Ultima/PC) diffractometer with Cu Ka radiation, and the samples were analyzed from 5° to 80° (2h), with a step size of 0.02° and a step time of 3 s. These results were used to assess the crystallinity of the catalyst. Ultraviolet-diffuse reflectance spectra (UV-DRS) measurements of undoped and doped samples were recorded with a Shimadzu (Model UV-2450, Japan) spectrophotometer. A Mecasys (Optizen 2120 UV) UV–Vis spectrophotometer was used to measure the concentration of the dyes in solution.
2.2. Methods
3. Results
2.2.1. Preparation of the TiO2 photocatalyst by a sol–gel process A solution of TiCl4 and HCl was prepared in a volumetric ratio (TiCl4:HCl = 1:12) with magnetic stirring at room temperature. The solution was titrated to pH 3.75 by stirring with ammonia to induce the formation of the gel. The gel-state suspension was washed with distilled water and filtered, and the filtered water was adjusted to approximately pH 7. The resulting powders were thermally treated at 350 °C for 12 h.
3.1. Surface morphology by SEM analysis
2.2.2. C, N, B, and F codoping on TiO2 TiO2 (3 g) was added to a TEATFB solution (20 ml, 0.1 M), and the mixture was stirred overnight. The mixture was exposed to 700 W microwave radiation for 30 min for multielement codoping. The resulting sample was washed using distilled water to remove the undoped salts and then dried at 110 °C. Further details are explained in our previous work [13,15]. 2.2.3. Preparation of an electrospun hydrogel web containing a photocatalyst PVA solution (10 wt.%, 30 ml) was prepared, and 3 g of multielement codoped TiO2 was added. The mixture was sonicated for 4 h to improve the dispersion of TiO2. GA (0.8 ml) was added to the prepared solution to cross-link the PVA. The prepared polymer solution was ejected from a syringe tip onto an aluminum foil-covered collector using an electrospinning apparatus. A schematic diagram of the electrospinning apparatus is depicted in our previous paper [16]. There are several factors that determine the electrospinning conditions [17–19]. Electrospinning was done under the following conditions: feeding rate of the polymer solution, 1 ml/ h; supplied voltage, 15 kV; tip-to-collector distance, 13 cm; and collector rpm, 100. The resulting electrospun fibers were treated at 110 °C for 6 h to obtain the cross-linked hydrogel fibers. 2.2.4. Dye degradation under solar light To evaluate the photocatalytic activity of the prepared catalysts, two aqueous dye solutions (procion blue and acridine orange, 500 ml, 10 ppm) were placed in a glass beaker with 100 mg of the photocatalyst. The suspension was irradiated with solar light. The solar intensity was measured with a digital illumination meter (INS, DX-200). Aliquots of a few milliliters of the aqueous suspension were collected at regular time periods during irradiation and were filtered through syringe filters to remove the catalyst particles. The dye concentration was estimated spectrophotometrically
The surface morphology of the hydrogel/TiO2 composite was investigated by SEM, as shown in Fig. 1. The nano-scale web consisted of the electrospun hydrogel fibers shown in Fig. 1a. The measured diameter of the fibers was 155 ± 25 nm. Clusters of TiO2 were observed among the fibers. In general, there were two types of TiO2 clusters observed: clusters were either located in/on or among the fibers, as shown in Fig. 1b and c, respectively. 3.2. Crystallinity study by XRD analysis Fig. 2 shows the XRD patterns of the doped catalysts embedded in the hydrogel fibers. The original XRD peaks of anatase-type TiO2 were observed as the predominant homogeneous crystalline phase by JCPDS number, indicating a tetragonal structure [20]. It is well known that the crystallinity and the crystal phase are crucial factors in the photocatalytic activity of TiO2 [20], with the crystalline anatase phase being the most active form of TiO2, while rutile and amorphous TiO2 are believed to be relatively inactive. There was no shift in the peak position caused by the C, N, B, and F codoping, suggesting that the dopant ions were uniformly distributed on the TiO2 surface, as explained in our previous work [13,15]. 3.3. UV diffuse reflectance spectra of undoped and doped TiO2 Fig. 3 shows the UV-DRS of the undoped and doped TiO2 samples. The data were measured three times and the average value was used with 2 ± % of error. The doped TiO2 show the shift in the band-gap transition. The shift of this type can be attributed to the charge-transfer transition between doped ions and TiO2 conduction or valence band [21]. The method of DRS was employed to investigate the band gap energies of the samples. First, to establish the type of band-to-band transition, the absorption data were fitted to Eq. (1) for direct band-gap transition. The minimum wavelength required to promote an electron depends on band-gap energy (Ebg) of the photocatalysts and is given by
Ebg ¼ hc=k ðeVÞ;
ð1Þ
where h is Planck’s constant (4.135667 1015 eV s), c is the velocity of light (3 108 m/s), and k is the wavelength (nm) of absorption onset [22].
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Undoped TiO2
Absorbance intensity
Doped TiO2
250
300
350
400
450
500
550
Wavelength (nm) Fig. 3. UV diffuse reflectance spectra of undoped and C, N, B, and F doped TiO2.
The band gap was 3.21 and 2.99 eV of undoped and doped TiO2, respectively. So it can be concluded that the band gap of TiO2 decreased about 7% by multielemental doping. 3.4. Kinetic model of dye removal by the hydrogel/TiO2 composite The dye absorption behavior of the hydrogel/TiO2 composite is presented in Fig. 4. The composite showed a similar absorbed volume ratio (absorbed volume/total volume) for both the anionic (procion blue) and the cationic (acridine orange) dye solutions, indicating that the swelling of the composite was not dependent on the charge of the dye. This result might be attributed to the non-ionic properties of the PVA hydrogel. In contrast, a pH-sensitive hydrogel, which had ionic properties, showed only limited absorption, depending on the type of dye [11,12]. In this case, the prepared composite showed an absorbed volume around 120 and 113 ml for the procion blue and acridine orange solutions, with absorption ratios of about 22.6 and 24.0 vol.% of the total dye solution volume (500 ml), respectively. This high absorption ratio might contribute to the effects of the nano-sized diameter fibers, which can provide high surface area for the efficient swelling of the hydrogel. This type of dye absorption behavior allows many kinds of dyes to be absorbed with high efficiency. To investigate the properties of dye removal by the hydrogel/ TiO2 composite, the dye concentration (MOC: measured outside concentration) was measured as a function of time. The efficiency of dye removal (g%) was calculated by the equation
g% ¼ ð1 MOC=C 0 Þ 100;
ð2Þ
where C0 is the initial concentration. Fig. 1. Surface morphology of the hydrogel/TiO2 composite.
4000
Absorbed volume (vol%)
(101)
25
15
10
Procion blue Acridine orange
5
(215) (301)
(116) (220)
(213) (204)
(105) (211)
1000
(200)
2000 (103) (004) (112)
Intensity
3000
20
0
0 10
20
30
40
50
60
2 Theta ( ) Fig. 2. Crystallinity measured by XRD peaks.
70
80
0
20
40
60
80
Time (min) Fig. 4. Absorbed volume ratio of dye solution by the hydrogel/TiO2 composite.
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(a)
70
60
Dye concentration (ppm)
The calculated efficiency of dye removal is shown in Fig. 5. Nearly 60% of the dye was removed within 10 min, showing a rapid dye-removal rate. After 20 min, around 80% of the dye had been removed. This quick dye absorption might contribute to the excellent swelling behavior of the hydrogel [11,12]. It can be concluded that the selective absorption of the dye solution was carried out between water ions and the dye ions by the hydrogel/TiO2 composite. It is suggested that the osmotic pressure played an important role because of the high concentration of the dye solution and the initial lack of dye in the absorbed water in the hydrogel. This result can be an advantage for dye-removal media because the dye can be removed by this prepared composite so quickly that the spread of pollutants is prevented.
50
PCH (Procion blue)
40
PCH (Acridine orange) MCH (Procion blue)
30
MCH (Acridine orange)
20
10 20
3.5. Kinetic model of dye degradation by TiO2 within the hydrogel/TiO2 composite
PCH ðppmÞ ¼ fC 0 TV ½ðTV AVÞ MOC=AVg;
ð3Þ
where PCH is the predicted dye concentration in the hydrogel/TiO2 composite, and TV and AV are the total and absorbed volumes, respectively. The measured dye concentration in the hydrogel/ TiO2 composite (MCH) was also measured. The PCH and MCH are plotted in Fig. 6a. The MCH was lower than the PCH. This large gap can be attributed to the excellent dye degradation ability of TiO2 in the composite. Based on this result, it can be suggested that the hydrogel does not block sunlight and thus does not prevent the photodegradation of dyes catalyzed by TiO2, due to the transparent properties of the hydrogel. This property of the hydrogel results in excellent dye removal in comparison to that obtained when using activated carbon and carbon nanotubes because the UV light and sunlight can be absorbed by activated carbon and carbon nanotubes, contributing to the decreased dye degradation efficiency of the photocatalyst. The kinetic model of TiO2 in the hydrogel is shown in Fig. 6b. For the kinetic model, the Langmuir–Hinshelwood model was applied as follows [23]:
Rate ¼ d½C 0 =dt ¼ kobs ½C 0 n ;
ð4Þ
where kobs is the apparent rate constant of dye removal (min1), and n is the order of the reaction. In this study, the following modified equation was used [24]:
60
80
Time (min)
(b)
1.2 1.0 0.8
ln(C0/C)
Even though the dye was removed by the hydrogel/TiO2 composite, this does not prove that the dye was degraded. To investigate dye degradation, the dye concentration in the hydrogel/TiO2 composite was measured by squeezing the hydrogel composite as shown in Fig. 6a. The dye concentration in the hydrogel/TiO2 composite was predicted by the equation
40
0.6 0.4
Procion blue (R2=0.98936) Acridine orange (R2=0.99086)
0.2
Regression line 0.0
0
20
40
60
80
100
Time (min) Fig. 6. Dye degradation (a) and relation between ln(C0/C) and time (b) by the TiO2 in the hydrogel composite as investigated by PCH and MCH (PCH, predicted concentration in hydrogel; MCH, measured concentration in the hydrogel).
lnðC 0 =CÞ ¼ kobs t n :
ð5Þ
The model was calculated by considering the volume absorbed by the hydrogel by Eqs. (5) and (6):
C ¼ f½MOC ðTV AVÞ þ ðAV MCHÞg=TV:
ð6Þ
The kinetic model followed the third-order model. One of the main reasons that different characteristics for the dye degradation were observed is that the dye volume in the hydrogel varied depending on the extent to which the hydrogel swelled. 4. Discussion
Dye removal efficiency (%)
100
4.1. Mechanism of dye decolorization by the prepared hydrogel/ photocatalyst composite
80
60
40
Procion blue Acridine orange 20
0
0
20
40
60
80
Time (min) Fig. 5. Dye-removal efficiency of the hydrogel/TiO2 composite.
The mechanism of dye degradation using the hydrogel/TiO2 composite is suggested in Fig. 7. Initially, the hydrogel/TiO2 composite swelled in the dye solution. During this time, the dye ions were absorbed selectively compared with water ions due to the effect of osmotic pressure depicted in Fig. 7a and b. Then, selectively absorbed dye ions were decolorized by TiO2 due to the high light transparency of the hydrogel, as shown in Fig. 7c. Considering that the prepared TiO2 can work under both UV and visible radiation, as confirmed by UV diffuse reflectance spectra from our previous work [13], the dye degradation mechanism can be explained in two radiation areas. Under irradiated light energy greater than the original band gap of TiO2 (3.2 eV), the dye degradation reactions can be expressed as follows [24–27]:
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Fig. 7. Suggested mechanism of organic compound degradation by the electrospun hydrogel/TiO2 fibers.
þ
TiO2 þ hv ðUVÞ ! TiO2 ðeCB þ hVB Þ
effectively by this hydrogel/TiO2 composite system, as shown in Fig. 6d and e.
þ
TiO2 ðhVB Þ þ H2 O ! TiO2 þ Hþ þ OH þ
TiO2 ðhVB Þ þ OH ! TiO2 þ OH TiO2 ðeCB Þ þ O2 ! TiO2 þ O 2 þ O 2 þ H ! HO2
Dye þ OH ! degradation products þ
Dye þ hVB ! oxidation products Dye þ eCB ! reduction products: The resulting radical (OH), being a very strong oxidizing agent (standard redox potential +2.8 V) can oxidize most dyes to the mineral end products. The mechanism of photodegradation by visible radiation (k > 420 nm) is different than the mechanism by UV light. It can be expressed as follows [24,28–31]:
Dye þ hv ðVISÞ ! 1 Dye or 3 Dye 1
Dye or 3 Dye þ TiO2 ! Dyeþ þ TiO2 ðeCB Þ
TiO2 ðeCB Þ þ O2 ! O 2 þ TiO2
5. Conclusions A photocatalyst system was synthesized using solar-lightresponsive TiO2 and a hydrogel. TiO2 particles were immobilized in/on the hydrogel fibers successfully without any structural change to the TiO2, maintaining the anatase type. The rapid removal of anionic and cationic dyes was the result of the excellent swelling behavior of the hydrogel, showing a pseudo-first-order reaction. The absorbed dyes were guided to the TiO2 photocatalyst and then degraded. The dyes were decolorized by an apparent third-order reaction within the hydrogel composite. In conclusion, the hydrogel/TiO2 composite is expected to efficiently absorb both anionic and cationic water pollutants and to efficiently degrade the pollutants within this photocatalyst system. References [1] [2] [3] [4] [5] [6] [7] [8] [9]
Dyeþ ! degradation products:
[10]
The mechanism suggests that excitation of the adsorbed dye takes place by visible light to the appropriate singlet or triplet states, subsequently followed by electron ejection from the excited dye molecule onto the conduction band of the TiO2 particles, while the dye is converted into a cationic dye radical (Dyeþ ) that undergoes degradation to yield product [24,32]. The dyes were removed
[11] [12] [13] [14] [15] [16] [17] [18]
L. Zhang, H. Fu, Y. Zhu, Adv. Funct. Mater. 18 (2008) 2180. X. Song, J. Am. Ceram. Soc. 90 (2007) 4015. D. Mitoraj, H. Kisch, Angew. Chem., Int. Ed. 47 (2008) 9975. Y. Li, A. Lu, S. Jin, C. Wang, J. Hazard. Mater. 170 (2009) 479. W.K. Jo, J.T. Kim, J. Hazard. Mater. 164 (2009) 360. M.V.P. Sharma, V.D. Kumari, M. Subrahmanyam, Chemosphere 72 (2008) 644. G. Liu, Y. Zhao, C. Sun, F. Li, G.Q. Lu, H. Cheng, Angew. Chem., Int. Ed. 47 (2008) 4516. G. Liu, L. Wang, H.G. Yang, H. Cheng, G.Q. Lu, J. Mater. Chem. 20 (2010) 831. B. Barni, A. Cavicchioli, E. Riva, L. Zanoni, F. Bignoli, I.R. Bellobono, F. Gianturco, A.D. Giorgi, H. Muntau, L. Montanarella, S. Facchetti, L. Castellano, Chemosphere 30 (1995) 1847. F. Gianturco, L. Vianelli, L. Tatti, F. Rota, P. Bruzzi, L. Rivas, I.R. Bellobono, M. Bianchi, H. Muntau, Chemosphere 33 (1996) 1531. Y.S. Jeon, J. Lei, J.H. Kim, J. Ind. Eng. Chem. 14 (2008) 726. V. Bekiari, M. Sotiropoulou, G. Bokias, P. Lianos, Colloid Surf., A 312 (2008) 214. J.S. Im, S.M. Yun, Y.S. Lee, J. Colloid Interface Sci. 336 (2009) 183. J.S. Im, M.I. Kim, Y.S. Lee, Mater. Lett. 62 (2008) 3652. S.J. Kim, S.M. Yun, H. Kim, J.G. Kim, Y.S. Lee, Carbon Lett. 10 (2009) 123. J.S. Im, S.J. Park, Y.S. Lee, J. Colloid Interface Sci. 314 (2007) 32. G. Eda, J. Liu, S. Shivkumar, Eur. Polym. J. 43 (2007) 1154. J.Y. Park, I.H. Lee, G.N. Bea, J. Ind. Eng. Chem. 14 (2008) 707.
J.S. Im et al. / Journal of Colloid and Interface Science 346 (2010) 216–221 [19] J. Stanger, N. Tucker, A. Wallace, N. Larsen, M. Staiger, R. Reeves, J. Appl. Polym. Sci. 112 (2009) 1729. [20] Y. Park, W. Kim, H. Park, T. Tachikawa, T. Majima, W. Choi, Appl. Catal., B 91 (2009) 355. [21] V. Štengl, S. Bakardjieva, N. Murafa, Mater. Chem. Phys. 114 (2009) 217. [22] X. Yang, C. Cao, L. Erickson, K. Hohn, R. Maghirang, K. Klabunde, J. Catal., 260. [23] M.A. Behnajady, N. Modirshahla, R. Hamzavi, J. Hazard. Mater. 133 (2006) 226. [24] I.K. Konstantinou, T.A. Albanis, Appl. Catal., B 49 (2004) 1. [25] C. Galindo, P. Jacques, A. Kalt, J. Photochem. Photobiol., A 130 (2000) 35.
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Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Improved photodegradation properties and kinetic models of a solar-light-responsive photocatalyst when incorporated into electrospun hydrogel fibers Ji Sun Im a, Byong Chol Bai a, Se Jin In b, Young-Seak Lee a,* a b
Department of Fine Chemical Engineering and Applied Chemistry, BK21-E2M, Chungnam National University, Daejeon 305-764, Republic of Korea Department of Fire and Disaster Protection Engineering, Woosong University, Daejeon 300-718, Republic of Korea
a r t i c l e
i n f o
Article history: Received 23 December 2009 Accepted 18 February 2010 Available online 21 February 2010 Keywords: Photodegradation TiO2 Hydrogel Electrospinning Nano-fiber
a b s t r a c t The capacity of a photocatalyst system to degrade water pollutants was optimized using solar-light-sensitive TiO2 and the swelling behavior of a hydrogel. TiO2 synthesized via a sol–gel process was modified by multielement doping to change its solar-light-responsive properties. A hydrogel was used for the rapid absorption of both anionic and cationic water pollutants. TiO2 particles were immobilized in/on hydrogel fibers by an electrospinning method for the easy recovery of TiO2, and the ability of the hydrogel/TiO2 composite to degrade dye molecules was studied. The TiO2 particles were observed to have maintained their original anatase-type crystallinity in/on the electrospun hydrogel fibers. The dye degradation capacity of the hydrogel/TiO2 composite was investigated using both anionic and cationic dyes under sunlight. Two mechanisms were suggested by which the hydrogel/TiO2 composite can remove dye particles from the water: (1) the absorption of dyes by the hydrogel and (2) the degradation of the dye by the TiO2 in the hydrogel. Both of these mechanisms were investigated in this study. We found that the dye was effectively absorbed by the hydrogel fibers as demonstrated by the swelling behavior of the hydrogel and the nano-size effects. The dye was then introduced to the TiO2 particles for degradation. Ó 2010 Elsevier Inc. All rights reserved.
1. Introduction The photocatalytic degradation of various toxic organic compounds has been proposed as a viable method for the decontamination of wastewater for renewable use. Titanium dioxide has been used widely as a photocatalyst due its ability to oxidize toxic substances into nontoxic substances such as carbon dioxide and water in polluted aqueous systems [1–3]. The process is initiated by the photogeneration of hole/electron pairs in the valence and conduction bands, respectively, on absorption of UV light with an energy equal to or higher than that of the corresponding band gap (>3.2 eV, k < 387 nm). This band gap is one of the barriers for use in our environment because UV light accounts for only a small fraction (5%) of the Sun’s energy that reaches the Earth’s surface (visible light represents 45%). Research focused on shifting the optical response of titanium dioxide from the UV to the visible spectral range will have a profound positive effect on the photocatalytic efficiency of this material [4–8]. The other attempt to increase the efficiency of the photocatalyst was the preparation of a nano-scale photocatalyst, the small scale of which would maximize the active surface area. However, this * Corresponding author. Fax: +82 42 822 6637. E-mail addresses: [email protected], [email protected] (Y.-S. Lee). 0021-9797/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2010.02.043
attempt introduced a secondary problem with regard to the recovery of the nano-sized photocatalyst from the water. As a result, immobilization of the nano-sized photocatalyst on a larger support structure has been investigated for the efficient recovery of the photocatalyst while retaining the catalysts’ high pollutant degradation efficiency [9,10]. Recently, hydrogels have been investigated as composite materials to maximize the efficiency of decontamination of toxic organic compounds. Hydrogels swell significantly in water, resulting in the absorption of the toxic organic compounds. Thus, rapid decontamination of toxic organic compounds is possible, allowing quick treatment of polluted water [11,12]. In this paper, a solar-light-responsive hydrogel/TiO2 composite was prepared as a novel photocatalyst system. The solar-lightresponsive TiO2 was prepared by the modified method from our previous work [13]. The hydrogel selected was biocompatible so that it would be safe for use in environmental applications. Organic compounds can be effectively guided by the hydrogel to the TiO2 photocatalyst in the composite, and high sunlight transmission can induce the degradation of the organic compounds by TiO2. TiO2 was also immobilized in the hydrogel in order to obtain better photocatalyst recovery. Eventually, the hydrogel/TiO2 composite was prepared by an electrospinning method because the electrospun web had benefits related to improved photocatalytic effects
J.S. Im et al. / Journal of Colloid and Interface Science 346 (2010) 216–221
by virtue of the nano-sized effects presented in our previous work [14]. 2. Materials and methods 2.1. Materials
217
at 600 and 493 nm for procion blue and acridine orange, respectively. The volume of absorbed dye solution was measured by a scale in beaker after removal of hydrogel/TiO2 composite in a few seconds.
Titanium chloride (TiCl4, Knato Chemical) was used as a precursor of TiO2. Hydrochloric acid (HCl, Samchun chemicals) was used as a solvent for titanium chloride. Ammonia (NH3, 28–30%, Junsei) was used for the sol–gel process of preparing TiO2. Tetraethylammonium tetrafluoroborate (TEATFB, ðCH3 CH2 Þ4 Nþ BF 4 , Sigma–Aldrich) was used as a dopant for carbon, nitrogen, boron, and fluorine codoping. Polyvinyl alcohol (PVA, Mw: 120,000, Sigma–Aldrich) was used as the polymer source for the hydrogel. Glutaraldehyde (GA, 25 wt.% solution in water, KANTO) was used as a cross-linker. The dyes procion blue (anionic dye, CI number 61205) and acridine orange (cationic dye, CI number 46005) were purchased from Acros (USA).
2.2.5. Characterization of the hydrogel/TiO2 composite sample A JSM-6300 scanning electron microscope (SEM) (JEOL Ltd., Japan) was used to investigate the surface morphology of the hydrogel/TiO2 composite samples after Pt coating. The anatase structure of TiO2 was studied using X-ray diffraction (XRD). The XRD patterns of the samples were recorded on a Rigaku (D/MAX2200 Ultima/PC) diffractometer with Cu Ka radiation, and the samples were analyzed from 5° to 80° (2h), with a step size of 0.02° and a step time of 3 s. These results were used to assess the crystallinity of the catalyst. Ultraviolet-diffuse reflectance spectra (UV-DRS) measurements of undoped and doped samples were recorded with a Shimadzu (Model UV-2450, Japan) spectrophotometer. A Mecasys (Optizen 2120 UV) UV–Vis spectrophotometer was used to measure the concentration of the dyes in solution.
2.2. Methods
3. Results
2.2.1. Preparation of the TiO2 photocatalyst by a sol–gel process A solution of TiCl4 and HCl was prepared in a volumetric ratio (TiCl4:HCl = 1:12) with magnetic stirring at room temperature. The solution was titrated to pH 3.75 by stirring with ammonia to induce the formation of the gel. The gel-state suspension was washed with distilled water and filtered, and the filtered water was adjusted to approximately pH 7. The resulting powders were thermally treated at 350 °C for 12 h.
3.1. Surface morphology by SEM analysis
2.2.2. C, N, B, and F codoping on TiO2 TiO2 (3 g) was added to a TEATFB solution (20 ml, 0.1 M), and the mixture was stirred overnight. The mixture was exposed to 700 W microwave radiation for 30 min for multielement codoping. The resulting sample was washed using distilled water to remove the undoped salts and then dried at 110 °C. Further details are explained in our previous work [13,15]. 2.2.3. Preparation of an electrospun hydrogel web containing a photocatalyst PVA solution (10 wt.%, 30 ml) was prepared, and 3 g of multielement codoped TiO2 was added. The mixture was sonicated for 4 h to improve the dispersion of TiO2. GA (0.8 ml) was added to the prepared solution to cross-link the PVA. The prepared polymer solution was ejected from a syringe tip onto an aluminum foil-covered collector using an electrospinning apparatus. A schematic diagram of the electrospinning apparatus is depicted in our previous paper [16]. There are several factors that determine the electrospinning conditions [17–19]. Electrospinning was done under the following conditions: feeding rate of the polymer solution, 1 ml/ h; supplied voltage, 15 kV; tip-to-collector distance, 13 cm; and collector rpm, 100. The resulting electrospun fibers were treated at 110 °C for 6 h to obtain the cross-linked hydrogel fibers. 2.2.4. Dye degradation under solar light To evaluate the photocatalytic activity of the prepared catalysts, two aqueous dye solutions (procion blue and acridine orange, 500 ml, 10 ppm) were placed in a glass beaker with 100 mg of the photocatalyst. The suspension was irradiated with solar light. The solar intensity was measured with a digital illumination meter (INS, DX-200). Aliquots of a few milliliters of the aqueous suspension were collected at regular time periods during irradiation and were filtered through syringe filters to remove the catalyst particles. The dye concentration was estimated spectrophotometrically
The surface morphology of the hydrogel/TiO2 composite was investigated by SEM, as shown in Fig. 1. The nano-scale web consisted of the electrospun hydrogel fibers shown in Fig. 1a. The measured diameter of the fibers was 155 ± 25 nm. Clusters of TiO2 were observed among the fibers. In general, there were two types of TiO2 clusters observed: clusters were either located in/on or among the fibers, as shown in Fig. 1b and c, respectively. 3.2. Crystallinity study by XRD analysis Fig. 2 shows the XRD patterns of the doped catalysts embedded in the hydrogel fibers. The original XRD peaks of anatase-type TiO2 were observed as the predominant homogeneous crystalline phase by JCPDS number, indicating a tetragonal structure [20]. It is well known that the crystallinity and the crystal phase are crucial factors in the photocatalytic activity of TiO2 [20], with the crystalline anatase phase being the most active form of TiO2, while rutile and amorphous TiO2 are believed to be relatively inactive. There was no shift in the peak position caused by the C, N, B, and F codoping, suggesting that the dopant ions were uniformly distributed on the TiO2 surface, as explained in our previous work [13,15]. 3.3. UV diffuse reflectance spectra of undoped and doped TiO2 Fig. 3 shows the UV-DRS of the undoped and doped TiO2 samples. The data were measured three times and the average value was used with 2 ± % of error. The doped TiO2 show the shift in the band-gap transition. The shift of this type can be attributed to the charge-transfer transition between doped ions and TiO2 conduction or valence band [21]. The method of DRS was employed to investigate the band gap energies of the samples. First, to establish the type of band-to-band transition, the absorption data were fitted to Eq. (1) for direct band-gap transition. The minimum wavelength required to promote an electron depends on band-gap energy (Ebg) of the photocatalysts and is given by
Ebg ¼ hc=k ðeVÞ;
ð1Þ
where h is Planck’s constant (4.135667 1015 eV s), c is the velocity of light (3 108 m/s), and k is the wavelength (nm) of absorption onset [22].
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Undoped TiO2
Absorbance intensity
Doped TiO2
250
300
350
400
450
500
550
Wavelength (nm) Fig. 3. UV diffuse reflectance spectra of undoped and C, N, B, and F doped TiO2.
The band gap was 3.21 and 2.99 eV of undoped and doped TiO2, respectively. So it can be concluded that the band gap of TiO2 decreased about 7% by multielemental doping. 3.4. Kinetic model of dye removal by the hydrogel/TiO2 composite The dye absorption behavior of the hydrogel/TiO2 composite is presented in Fig. 4. The composite showed a similar absorbed volume ratio (absorbed volume/total volume) for both the anionic (procion blue) and the cationic (acridine orange) dye solutions, indicating that the swelling of the composite was not dependent on the charge of the dye. This result might be attributed to the non-ionic properties of the PVA hydrogel. In contrast, a pH-sensitive hydrogel, which had ionic properties, showed only limited absorption, depending on the type of dye [11,12]. In this case, the prepared composite showed an absorbed volume around 120 and 113 ml for the procion blue and acridine orange solutions, with absorption ratios of about 22.6 and 24.0 vol.% of the total dye solution volume (500 ml), respectively. This high absorption ratio might contribute to the effects of the nano-sized diameter fibers, which can provide high surface area for the efficient swelling of the hydrogel. This type of dye absorption behavior allows many kinds of dyes to be absorbed with high efficiency. To investigate the properties of dye removal by the hydrogel/ TiO2 composite, the dye concentration (MOC: measured outside concentration) was measured as a function of time. The efficiency of dye removal (g%) was calculated by the equation
g% ¼ ð1 MOC=C 0 Þ 100;
ð2Þ
where C0 is the initial concentration. Fig. 1. Surface morphology of the hydrogel/TiO2 composite.
4000
Absorbed volume (vol%)
(101)
25
15
10
Procion blue Acridine orange
5
(215) (301)
(116) (220)
(213) (204)
(105) (211)
1000
(200)
2000 (103) (004) (112)
Intensity
3000
20
0
0 10
20
30
40
50
60
2 Theta ( ) Fig. 2. Crystallinity measured by XRD peaks.
70
80
0
20
40
60
80
Time (min) Fig. 4. Absorbed volume ratio of dye solution by the hydrogel/TiO2 composite.
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(a)
70
60
Dye concentration (ppm)
The calculated efficiency of dye removal is shown in Fig. 5. Nearly 60% of the dye was removed within 10 min, showing a rapid dye-removal rate. After 20 min, around 80% of the dye had been removed. This quick dye absorption might contribute to the excellent swelling behavior of the hydrogel [11,12]. It can be concluded that the selective absorption of the dye solution was carried out between water ions and the dye ions by the hydrogel/TiO2 composite. It is suggested that the osmotic pressure played an important role because of the high concentration of the dye solution and the initial lack of dye in the absorbed water in the hydrogel. This result can be an advantage for dye-removal media because the dye can be removed by this prepared composite so quickly that the spread of pollutants is prevented.
50
PCH (Procion blue)
40
PCH (Acridine orange) MCH (Procion blue)
30
MCH (Acridine orange)
20
10 20
3.5. Kinetic model of dye degradation by TiO2 within the hydrogel/TiO2 composite
PCH ðppmÞ ¼ fC 0 TV ½ðTV AVÞ MOC=AVg;
ð3Þ
where PCH is the predicted dye concentration in the hydrogel/TiO2 composite, and TV and AV are the total and absorbed volumes, respectively. The measured dye concentration in the hydrogel/ TiO2 composite (MCH) was also measured. The PCH and MCH are plotted in Fig. 6a. The MCH was lower than the PCH. This large gap can be attributed to the excellent dye degradation ability of TiO2 in the composite. Based on this result, it can be suggested that the hydrogel does not block sunlight and thus does not prevent the photodegradation of dyes catalyzed by TiO2, due to the transparent properties of the hydrogel. This property of the hydrogel results in excellent dye removal in comparison to that obtained when using activated carbon and carbon nanotubes because the UV light and sunlight can be absorbed by activated carbon and carbon nanotubes, contributing to the decreased dye degradation efficiency of the photocatalyst. The kinetic model of TiO2 in the hydrogel is shown in Fig. 6b. For the kinetic model, the Langmuir–Hinshelwood model was applied as follows [23]:
Rate ¼ d½C 0 =dt ¼ kobs ½C 0 n ;
ð4Þ
where kobs is the apparent rate constant of dye removal (min1), and n is the order of the reaction. In this study, the following modified equation was used [24]:
60
80
Time (min)
(b)
1.2 1.0 0.8
ln(C0/C)
Even though the dye was removed by the hydrogel/TiO2 composite, this does not prove that the dye was degraded. To investigate dye degradation, the dye concentration in the hydrogel/TiO2 composite was measured by squeezing the hydrogel composite as shown in Fig. 6a. The dye concentration in the hydrogel/TiO2 composite was predicted by the equation
40
0.6 0.4
Procion blue (R2=0.98936) Acridine orange (R2=0.99086)
0.2
Regression line 0.0
0
20
40
60
80
100
Time (min) Fig. 6. Dye degradation (a) and relation between ln(C0/C) and time (b) by the TiO2 in the hydrogel composite as investigated by PCH and MCH (PCH, predicted concentration in hydrogel; MCH, measured concentration in the hydrogel).
lnðC 0 =CÞ ¼ kobs t n :
ð5Þ
The model was calculated by considering the volume absorbed by the hydrogel by Eqs. (5) and (6):
C ¼ f½MOC ðTV AVÞ þ ðAV MCHÞg=TV:
ð6Þ
The kinetic model followed the third-order model. One of the main reasons that different characteristics for the dye degradation were observed is that the dye volume in the hydrogel varied depending on the extent to which the hydrogel swelled. 4. Discussion
Dye removal efficiency (%)
100
4.1. Mechanism of dye decolorization by the prepared hydrogel/ photocatalyst composite
80
60
40
Procion blue Acridine orange 20
0
0
20
40
60
80
Time (min) Fig. 5. Dye-removal efficiency of the hydrogel/TiO2 composite.
The mechanism of dye degradation using the hydrogel/TiO2 composite is suggested in Fig. 7. Initially, the hydrogel/TiO2 composite swelled in the dye solution. During this time, the dye ions were absorbed selectively compared with water ions due to the effect of osmotic pressure depicted in Fig. 7a and b. Then, selectively absorbed dye ions were decolorized by TiO2 due to the high light transparency of the hydrogel, as shown in Fig. 7c. Considering that the prepared TiO2 can work under both UV and visible radiation, as confirmed by UV diffuse reflectance spectra from our previous work [13], the dye degradation mechanism can be explained in two radiation areas. Under irradiated light energy greater than the original band gap of TiO2 (3.2 eV), the dye degradation reactions can be expressed as follows [24–27]:
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Fig. 7. Suggested mechanism of organic compound degradation by the electrospun hydrogel/TiO2 fibers.
þ
TiO2 þ hv ðUVÞ ! TiO2 ðeCB þ hVB Þ
effectively by this hydrogel/TiO2 composite system, as shown in Fig. 6d and e.
þ
TiO2 ðhVB Þ þ H2 O ! TiO2 þ Hþ þ OH þ
TiO2 ðhVB Þ þ OH ! TiO2 þ OH TiO2 ðeCB Þ þ O2 ! TiO2 þ O 2 þ O 2 þ H ! HO2
Dye þ OH ! degradation products þ
Dye þ hVB ! oxidation products Dye þ eCB ! reduction products: The resulting radical (OH), being a very strong oxidizing agent (standard redox potential +2.8 V) can oxidize most dyes to the mineral end products. The mechanism of photodegradation by visible radiation (k > 420 nm) is different than the mechanism by UV light. It can be expressed as follows [24,28–31]:
Dye þ hv ðVISÞ ! 1 Dye or 3 Dye 1
Dye or 3 Dye þ TiO2 ! Dyeþ þ TiO2 ðeCB Þ
TiO2 ðeCB Þ þ O2 ! O 2 þ TiO2
5. Conclusions A photocatalyst system was synthesized using solar-lightresponsive TiO2 and a hydrogel. TiO2 particles were immobilized in/on the hydrogel fibers successfully without any structural change to the TiO2, maintaining the anatase type. The rapid removal of anionic and cationic dyes was the result of the excellent swelling behavior of the hydrogel, showing a pseudo-first-order reaction. The absorbed dyes were guided to the TiO2 photocatalyst and then degraded. The dyes were decolorized by an apparent third-order reaction within the hydrogel composite. In conclusion, the hydrogel/TiO2 composite is expected to efficiently absorb both anionic and cationic water pollutants and to efficiently degrade the pollutants within this photocatalyst system. References [1] [2] [3] [4] [5] [6] [7] [8] [9]
Dyeþ ! degradation products:
[10]
The mechanism suggests that excitation of the adsorbed dye takes place by visible light to the appropriate singlet or triplet states, subsequently followed by electron ejection from the excited dye molecule onto the conduction band of the TiO2 particles, while the dye is converted into a cationic dye radical (Dyeþ ) that undergoes degradation to yield product [24,32]. The dyes were removed
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