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High adsorption capacity of V-doped TiO2 for decolorization of methylene blue
Applied Surface Science 258 (2012) 7299–7305
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
High adsorption capacity of V-doped TiO2 for decolorization of methylene blue Thanh-Binh Nguyen, Moon-Jin Hwang 1 , Kwang-Sun Ryu ∗ Department of Chemistry, University of Ulsan, Daehak-ro 93, Nam-gu, Ulsan 680-749, Republic of Korea
a r t i c l e
i n f o
Article history: Received 19 January 2012 Received in revised form 20 March 2012 Accepted 24 March 2012 Available online 13 April 2012 Keywords: Adsorption Metal ion-doped TiO2 Mesoporous Decolorization Methylene blue
a b s t r a c t In this study, pure TiO2 (V-TiO2 -0) and V-doped TiO2 (V-TiO2 -x, x = 1–10 mol%) were synthesized using a new sol–gel method. The adsorption capacity of the V-TiO2 -x samples was evaluated by measuring the removal of methylene blue (MB) from aqueous solution via decolorization. Since the adsorption capacity was affected by the specific surface area, the interaction between adsorbate (MB) and adsorbent (V-TiO2 x), and the structure of the adsorbent, the physicochemical properties of the samples were investigated. Among the V-doped TiO2 -x samples, the V-TiO2 -10 sample showed the highest adsorption capacity, which was 11.36 times greater than that of pure TiO2 , removing 85.2% of the MB after 2 h. Moreover, changing the molar ratio of the reactants in the V-TiO2 -10 sample improved the performance of the material so that 91.6% of the MB was removed after 2 h. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Industrial development and increasing populations have led to serious environmental problems; waste water disposal and air pollution have become global concerns. Adsorption technology plays a key role in industrial development because of its important purification functions, optimal environmental compatibility, and low operational cost [1–7]. Adsorption technology exploits the ability of some solid materials to remove color from dye-containing solutions. These adsorbent materials may be utilized for fluid purification and separation applications [1,2]. Recently, the synthesis of TiO2 or supported TiO2 on other substrates has been vigorously studied since TiO2 is considered to be an environmentally friendly and inexpensive catalyst with good dispersion properties under most conditions [5–14]. Nevertheless, TiO2 has a low adsorption capacity for most adsorbates. Therefore, several methods have been investigated to improve the performance of TiO2 , such as metal ion/non metal-doped TiO2 and deposited nanoparticle noble metals, due to their higher adsorption capacities [7]. Vanadium is a transition metal with multiple characteristics which improve TiO2 adsorptivity. For example, V-doped TiO2 with different valance state of the V and Ti (V0/1+/2+/3+/4+/5+ and Ti1+/2+/3+/4+ ) exhibits a difference in oxidation activity [7]. V doping was responsible for increases of superficial hydroxyl groups and electron transfer, resulting in faster interactions between the
∗ Corresponding author. Tel.: +82 52 259 2763; fax: +82 52 259 2348. E-mail address: [email protected] (K.-S. Ryu). 1 Energy Harvest-Storage Research Center, University of Ulsan, Daehak-ro 93, Nam-gu, Ulsan 680-749, Republic of Korea. 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.03.148
adsorbate and adsorbent [14–16]. V-doped TiO2 has been used as a catalyst for the selective reduction of NO to NH3 [11]. The ionic radius of V is almost the same as that of Ti, allowing it to be easily doped into TiO2 [8]. Previous doping methods have required long reaction times and high temperatures. For instance, Rodell [17] performed the doping reaction over 44 h, and Nguyen-Phan [5] used a 44 h reaction exclusive of preparation time of the precursor solution at high temperature (120 ◦ C for 12 h). In addition, these methods were carried out under acidic conditions, with HNO3 , HCl or CH3 COOH used as a catalyst [7–10]. In this study, different molar percentages (from 1 to 10 mol%) of vanadium doped into TiO2 were fabricated by a new sol–gel method at low temperature, with a short reaction time, and under alkaline conditions (ammonium hydroxide was a catalyst for the reaction), which were more energy efficient and increased the number of surface hydroxyl groups, an essential condition for improving material adsorptivity. Currently, there has been no demonstration of the adequate amount of V to be doped into TiO2 to provide an environmentally friendly adsorbent. We used 10 mol% of V as the critical value so that the V-TiO2 samples would be more compatible with a water environment. The adsorption capacity of the V-doped TiO2 samples was analyzed using decolorization of aqueous MB, and the effects of vanadium on the physicochemical properties of the samples were also studied.
2. Experimental method 2.1. Chemicals Titanium (IV) isopropoxide (Ti(OCH(CH3 )2 )4 , 97%, TTIP), isopropanol ((CH3 )2 CHOH, >99.8%, IPA) and vanadium (V) oxide (V2 O5 ,
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Fig. 1. Colors of V-TiO2 -x samples.
POROSITY-XQ, Mirae Scientific Instruments Co., Korea). An XPS study of fresh and spent catalysts was conducted via a Thermo Scientific K-Alpha instrument with an excitation source of Al K␣ radiation. Moreover, a binding energy scan range from 0 to 1350 eV for the identification of all detectable elements and detailed scans for chemical state identification and quantification were obtained. Ti 2p, O 1s, and V 2p spectra were deconvoluted using XPSPeak41 software. 2.4. Adsorption test
Scheme 1. Synthesis of V-TiO2 -x samples.
>99.6%) were obtained from Sigma-Aldrich Chemicals. Ammonium hydroxide (NH4 OH, 25–28%) was purchased from Dae Jung Chemicals & Metals, and distilled water was used to prepare all solutions.
Dye solutions were bleached for quantitative measurement of the concentration of adsorbable components before and after treatment with the solid (V-doped TiO2 ) adsorbent. Methylene blue (C16 H18 ClN3 S·3H2 O, MB), a common chemical extensively used in chemical and biological processes, was obtained from Sigma–Aldrich and was chosen as a model pollutant. The adsorption capability of the V-TiO2 -x samples was evaluated by measuring the removal of methylene blue (C0 = 1 × 10−5 M) from aqueous solution. 10 mg of the V-TiO2 -x sample was dispersed in 100 ml of MB solution and stirred in the dark at a constant temperature (22 ◦ C). Aliquots of 3.5 ml were withdrawn at different times within 120 min and were put through a syringe filter. The MB concentration was characterized by UV–Vis spectrophotometry (OPTIZEN POP, Mecasys Co., Ltd., Korea) at max = 666 nm, and the percentage of MB remaining in solution was calculated using the following equation:
2.2. Synthesis of V-doped TiO2 MB = V2 O5 powder was dissolved in ammonium hydroxide solution (NH4 OH:H2 O = 75:25 volume) to make a homogeneous 0.1 M vanadium solution. All samples were designated as V-TiO2 -x, where x is the mol% of the V (x = 0, 1, 3, 5, 7, and 10). The V-TiO2 -x samples were synthesized as shown in Scheme 1. The A and B solutions were prepared at a molar ratio of TTIP, IPA, and H2 O of 1: 5: 100. This process was repeated with other molar ratios of TTIP, IPA, and H2 O for the samples containing various mol% of V to obtain optimal reactant ratios. 2.3. Characterization The morphology and composition of the samples were analyzed using scanning electron microscopy (SEM, Supra 40, Carl Zeiss Co., Ltd., Germany). The crystalline phase was determined from the X-ray diffraction patterns (XRD, Cu K␣ radiation, 40 kV, 100 mA, ˚ Rigaku ultra-X, Rigaku Co., Japan) at a step scan = 1.54059 A, rate of 0.02◦ /0.3 s in the 2 range of 10–80◦ . Inductively coupled plasma optical emission spectrometry (ICP-OES, iCAP 6500, Thermo Electron Corporation) was used to measure the weight ratio of Ti and V. The surface area was measured by BET analysis (nano
C × 100(%) C0
(1)
where C0 and C are the MB initial concentration and concentration after the adsorption time, respectively. 3. Results and discussion When the molar percentage of V was increased, the color of the solid samples changed from white to dark due to the color of V in the doped TiO2 , as shown in Fig. 1. The weight ratios of Ti and V in the samples are shown in Table 1. Comparison of actual data (ICP results) and calculations, the performance of the reaction is about 92–98%. SEM images of the V-TiO2 -x samples containing the different molar percentages of vanadium are shown in Fig. 2. The SEM images clearly indicate that the particle size was homogeneous with an average diameter of 10–15 nm. As the mol% of V was increased, agglomeration was reduced, and the particle size increased. For instance, the surface of the V-TiO2 -0 sample had an agglomerated shape and holes with 200 nm-diameter located between particles. The V-TiO2 -1 sample also contained holes, but the quantity and size of the holes were reduced. The surface of the V-TiO2 -1 sample
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Table 1 Physicochemical properties of V-TiO2 -x samples. Sample
2 (◦ )
˚ d-spacing (A)
Crystallite size (nm)
SBET (m2 /g)
Total pore volume (cm3 /g)
Average pore diameter (nm)
Weight ratios Ti/V
V-TiO2 -0 V-TiO2 -1 V-TiO2 -3 V-TiO2 -5 V-TiO2 -7 V-TiO2 -10
25.380 25.380 25.400 25.300 25.360 25.359
3.5065 3.5065 3.5039 3.5174 3.5092 3.5093
13.42 14.34 16.58 17.45 16.24 16.89
76.93 85.30 82.61 71.93 73.56 73.92
0.4088 0.2704 0.2081 0.2172 0.1873 0.1797
7.808 5.410 5.114 5.744 5.403 4.859
– 49.9/0.55 54.1/1.9 49.8/2.9 52.5/4.1 42.9/4.7
was smoother than that of the pure TiO2 , and the V-TiO2 -x samples (x = 5, 7, and 10) in particular had smooth surfaces. This phenomenon indicates that, as the mol% of V increased with increasing pH, the particle size increased. In addition, the vanadium content increased with a higher molar ratio of water in the reaction, leading to an increase in particle size and a reduction in agglomeration [12,13]. As shown in Fig. 3, the XRD patterns of the V-TiO2 -x samples were similar. The V-TiO2 -x (x = 0 and 1) samples included anatase and brookite phases, but the intensity of the brookite phase was low. The existence of anatase and brookite phases depended on the amount of water and the pH of the solution. In the present work, as the mol% of V was continuously increased (x = 3, 5, 7, and 10),
all phases of the samples exhibited anatase phase diffraction peaks because of the increased amount of water. However, no characteristic peaks of vanadium metal or oxides were observed, which implies that vanadium incorporation in the lattice of the anatase TiO2 or vanadium oxide was limited and highly dispersed [12]. On the other hand, the V-TiO2 -x samples had slightly shifted peaks, as shown in Table 1. The crystallite size of the samples was calculated following Scherrer’s equation: D=
k ˇ cos
(2)
where D is the crystallite size, k is a constant (shape factor, about 0.9), is the X-ray wavelength (0.154059 nm), ˇ is the full width at
Fig. 2. SEM images of the samples. (a) V-TiO2 -0, (b) V-TiO2 -1, (c) V-TiO2 -3, (d) V-TiO2 -5, (e) V-TiO2 -7, and (f) V-TiO2 -10.
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Fig. 5. BJH pore size distribution-adsorption of the V-TiO2 -x samples. Fig. 3. X-ray diffraction patterns of the V-TiO2 -x samples.
half maximum (FWHM) of the diffraction line, and is the diffraction angle. The crystallite size of the (1 0 1) peak increased with the increasing amount of V doped in the TiO2 , similar to the SEM result. Particle sizes ranged from 13.4 to 17.5 nm. The nitrogen adsorption–desorption isotherms of the V-TiO2 -x samples are shown in Fig. 4. All isotherms were identified as type IV according to the IUPAC classification and belong to the mesoporous family (radius from 2-50 nm) [18–20]. The V-TiO2 -x samples were classified as having three types of hysteresis loops. First, the V-TiO2 -x (x = 0 and 1) samples have hysteresis loops of type H3, which are usually produced by the aggregates of platy particles or adsorbent containing slit-shaped pores. According to the SEM results, the V-TiO2 -x (x = 0 and 1) samples had not only textural but also framework mesoporosity. These results also clearly show that the hysteresis loops include two areas. For instance, P/P0 values from 0.3 to 0.7 were found in the first area, with pores formed in the particles (framework mesoporosity). The second area contained P/P0 values greater than 0.7, in which pores were created between the particles (textural). However, in these samples, the textural pores were more significant than the framework mesoporosity. Second, the hysteresis loops of the V-TiO2 -x (x = 3 and 5) samples belong to the H2 type, denoting the existence of bottleneck pore channels. The average pore sizes of these samples (x = 3 and 5) were 5.1 and 5.7 nm, respectively, and most were textural pores.
Fig. 4. N2 adsorption–desorption isotherms of the V-TiO2 -x samples.
Third, the hysteresis loops of the V-TiO2 -x (x = 7 and 10) samples are the H1 type, which are found in adsorbents with a narrow pore size distribution. In particular, the adsorption–desorption line was completely adsorbed, such that the vanadium may exist outside the TiO2 structure, and alternate structure between V and TiO2 is little. The specific surface area (SBET ) was calculated from the linear BET plot (P/P0 range of 0.05–0.2), and the total pore volume was measured at P/P0 = 0.99 [18–20]. The SBET , total pore volume, and average pore diameter are summarized in Table 1. As the mol% of V was increased, the crystallite size and SBET increased from 13.42 to 17.45 nm and from 71.93 to 85.3 m2 /g, respectively. The SBET values of the samples depended on the crystalline size, phase, and shape [18]. As the mol% of V was increased, the total pore volume and average pore diameter reduced. Adsorption selection kinetics is based on the size of the adsorbent pores and the dimensions of the diffusing adsorbate molecules. According to Fig. 5 about BJH pore size distribution-adsorption, pure TiO2 has a large pore size distribution, which is disadvantage adsorption capacity. When mol% of V was increased, pore size distribution of the samples was narrow size, which helps to increase adsorption capacity. Since the MB molecule can be regarded as an approximately rectangular volume ˚ the MB volume is greater than of dimensions 17.0 × 7.6 × 3.25 A, 1.5 nm, and the water molecule volume is about 0.26 nm [21]. Comparing the absorption selections for the two groups with ink bottle pore channels and narrow pores, the average pore sizes were similar. However, for the ink bottle pores, rb was always greater than rn , with a rn of about 1 nm. Therefore, for this pore size (about 2 nm), it would be difficult to trap 2–3 molecules of MB within the pores. The average pore diameter of the V-TiO2 -x (x = 7 and 10) samples was about 5 nm with narrow pores, which would allow easier trapping of MB compared with that in the ink bottle-shaped pores with the same average diameter. Hence, the MB removal adsorption kinetics of the V-TiO2 -x (7–10) samples was faster than those of the other samples [1,2]. The nature of the oxidation states of O 1s, Ti 2p, and V 2p were obtained by high-resolution XPS, as shown in Figs. 6–8 and Table 2. In Fig. 6, the Ti 2p XPS results of the V-TiO2 -x samples also indicated two main components related to 2p3/2 and 2p1/2 . The Ti 2p3/2 of the pure TiO2 sample has only oxidation states Ti4+ at 458.51 eV. As the mol% of V was increased, the other oxidation states Ti3+ , were more prevalent. For the Ti 2p3/2 of the V-TiO2 -5 sample, Ti3+ and Ti4+ were at 457.06 and 458.00 eV, respectively. For the V-TiO2 -10 sample, the %L–G of Ti3+ was increased to 37.39%. Thus, as the mol% of V was increased, the level of active ions increased, which is advantageous for adsorption capacity [7–9].
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Table 2 XPS fit parameters for the Ti 2p, O 1s, and V 2p signals of the samples. Level
Sample
Oxidation state
BE (eV)
% L–G
FWHM (eV)
Ti 2P1/2
1 2
Ti4+ Ti4+ Ti3+ Ti4+
464.17 462.49 459.80 463.81
31.93 42.13 10.63 30.43
2.07 4.35 1.30 2.56
Ti4+ Ti4+ Ti3+ Ti4+ Ti3+
458.51 458.00 457.06 459.02 457.81
68.07 16.91 30.33 32.18 37.39
1.25 1.62 1.17 2.14 1.19
– – – – – – – –
529.74 531.51 528.33 529.53 531.76 529.04 530.27 531.91
98.35 1.65 43.78 32.13 24.09 51.17 43.44 5.39
1.333 1.728 1.378 2.108 2.393 1.300 1.928 1.555
3 Ti 2P3/2
1 2 3
O 1s
1 2
3
V 2p1/2
2 3
V4+ V4+
523.28 523.74
25.46 41.67
3.904 4.441
V 2p3/2
2
V4+ V5+ V4+ V5+
515.41 518.13 516.09 517.31
24.47 50.07 28.76 29.57
2.026 6.401 2.230 4.410
3
(1), (2), and (3) are V-TiO2 -0, V-TiO2 -5, and V-TiO2 -10, respectively. BE: binding energy; %L–G: % Lorentzian–Gaussian.
Shown in Fig. 7 are the XPS spectra for the O 1s of V-TiO2 -x (x = 0, 5, and 10). Pure TiO2 consisted of two peaks, one at 529.74 eV related to OI and the other at 531.51 eV related to OII . As V was increasingly doped in the TiO2 , the oxidized component of the VTiO2 -x (x = 5, 10) increased. For example, these samples had three oxidation states that included M O (M-metal, V and Ti), hydroxyl groups, and physically adsorbed water. For the V-TiO2 -5 sample, the OH groups and physically adsorbed water increased to 32.13% and 24.09%, respectively. In particular, for the V-TiO2 -10 sample, the amount of OH groups increased to 43.44%, which corresponded to an increased adsorption capacity [7–9]. Fig. 8 shows the distinct distributions of V 2p1/2 and V 2p3/2 . With the V-TiO2 -5 sample, 2p3/2 included two peaks at 515.41 and 518.13 eV, corresponding to V4+ and V5+ , respectively, and 2p1/2 was at 523.28 eV [22–25]. The BE of the V-TiO2 -10 sample was shifted higher than that of the V-TiO2 -5 sample, which is easily seen by the shift of the dashed line. The %L-G of V5+ of the V-TiO2 -10 sample was reduced to 29.57%. Fig. 6. Ti 2p XPS of the samples: (a) V-TiO2 -0, (b) V-TiO2 -5, and (c) V-TiO2 -10.
Fig. 7. O 1s XPS of the samples: (a) V-TiO2 -0, (b) V-TiO2 -5, and (c) V-TiO2 -10.
Fig. 8. V 2p XPS of the samples: (a) V-TiO2 -5 and (b) V-TiO2 -10.
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Fig. 9. The effect of vanadium mol% content on adsorption capacity.
The adsorption capacities of the V-TiO2 -x samples are shown in Fig. 9. After 120 min, pure TiO2 removed a small amount of MB (7.5%). As the mol% of V was increased (x = 3–10), the parameter (C/C0 × 100) was very quickly reduced. The MB decolorization process was categorized into three adsorption periods: (i) instantaneous adsorption of MB within 10 min, (ii) gradual attainment of an equilibrium from 10 to 60 min, and (iii) established equilibrium of MB molecules onto the V-TiO2 after 60 min. In stage (i), the MB concentration was reduced from 100% to 55% in the V-TiO2 10 sample. This result indicated the substantial effect of V on the adsorption ability of TiO2 . A higher V mol% resulted in increased adsorption ability of the V-TiO2 -10 sample, exhibiting the greatest adsorption capacity of the samples studied for decolorization of MB. However, the effect of the V-doped TiO2 (V-TiO2 -x, x = 1-10) samples was distributed among the three groups. The first group included the V-TiO2 -1, which had little effect on adsorption. The second group included the V-TiO2 -3, which had increased adsorption ability reflected in the C/C0 × 100 reduction from 92.5 to 31.4 for pure TiO2 . The third group included V-TiO2 -x (x = 5–10), which quickly removed MB from solution. This distribution is comparable to observations from the SEM images. A higher specific surface area provided more sensitive space for the penetration of MB. However, the third group had a lower specific area than the two other groups. Instead of a high specific area, the phase of the third group was very smooth, such that the MB could easily penetrate into the TiO2 . According to the BET results, the V-TiO2 -x (x = 7 and 10) samples were favorable for kinetic adsorption of MB into TiO2 . Various studies have shown that adsorption capacity is strongly dependent on the interaction between adsorbent (V-TiO2 -x samples) and adsorbate (MB) [13–15]. The adsorption of MB or other organic compounds is dominated by the interaction between the electrons of the MB ring and surface OH groups. The surface of TiO2 consists of multiple OH groups (Ti OH, Ti (OH)2 ). According to the XPS results, doping with V resulted in increased oxidation states of Ti, V, and O, and this increased oxidation activity is significant for adsorption capacity. A sol–gel method was applied to the synthesis of V-doped TiO2 , and the molar ratio of the starting materials was very important to the composition of the product [12]. In particular, the amount of water affected the particle size and agglomeration, material characteristics that have an effect on adsorption ability. The effect of reactant molar ratio on the doped TiO2 composition as measured by removal of MB is shown in Fig. 10. The molar ratio (TTIP:IPA:H2 O = 1:1:40) was optimal for the V-TiO2 10 sample, which removed 91.6% of the MB (C/C0 × 100 = 8.4) after
Fig. 10. The effect of molar ratio of the reaction on adsorption capacity.
120 min. V-doping of TiO2 significantly increased the effectiveness of MB removal, which increased from 7.5% to 85.2%, an 11.36-fold increase. In contrast, Liang [15] prepared V-doped Fe3 O4 which only increased MB removal from 41% to 93%, a 2.27-fold increase after 700 min, and Wenfang [9] prepared a V-doped TiO2 which increased the removal of MB from 12% to 32% (2.67-fold), after 8 h. 4. Conclusions A higher mole percent of V in V-TiO2 -x samples significantly promoted the adsorption capacity for decolorization of MB solution compared to the un-doped TiO2 sample. Incorporation of V changed the physical and chemical properties of TiO2 , improving its adsorption ability. The optimal sample V-TiO2 -10 (10 mol% V) removed about 85.2% of the MB, while pure TiO2 only removed 7.5% of the MB during 2 h. Optimization of the reactant molar ratio (TTIP:IPA:H2 O = 1:1:40) for the V-TiO2 -10 sample allowed 91.6% of the MB to be removed. The 11.36-fold (85.2/7.5) adsorptivity increase is significant in the context of wastewater treatment. Acknowledgments This work (grants no. 0039007-1) was supported by Business for Academic-industrial Cooperative establishments funded Korea Small and Medium Business Administration in 2009 and by the Priority Research Centers Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2009-0093818). References [1] F. Rouquérol, J. Rouquérol, K.S.W. Sing, Adsorption by Powders & Porous Solids: Principles, Methodology and Applications, Academic Press, 1999. [2] S. Lowell, J.E. Shields, Powder Surface Area and Porosity, Chapman and Hall, 1991. [3] M.A. Al-Ghouti, M.A.M. Khraisheh, M.N.M. Ahmad, S. Allen, The Journal of Hazardous Materials 165 (2009) 589–598. [4] M. Suzuki, Water Science and Technology 35 (1997) 1–11. [5] T.-D. Nguyen-Phan, M.B. Song, E.W. Shin, The Journal of Hazardous Materials 167 (2009) 75–81. [6] T.-D. Nguyen-Phan, C. Lee, J. Chung, E.W. Shin, Research on Chemical Intermediates 34 (2008) 743–753. [7] T.-D. Nguyen-Phan, M.B. Song, H. Yun, E.J. Kim, E.-S. Oh, E.W. Shin, Applied Surface Science 257 (2011) 2024–2031. [8] B. Liu, X. Wang, G. Cai, L. Wen, Y. Song, X. Zhao, The Journal of Hazardous Materials 169 (2009) 1112–1118. [9] Z. Wenfang, L. Qingju, Z. Zhongqi, Z. Ji, Journal of Physics D: Applied Physics 43 (2010) 035301. [10] J. Liu, R. Yang, S. Li, Journal of Rare Earths 25 (2007) 173–178.
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High adsorption capacity of V-doped TiO2 for decolorization of methylene blue Thanh-Binh Nguyen, Moon-Jin Hwang 1 , Kwang-Sun Ryu ∗ Department of Chemistry, University of Ulsan, Daehak-ro 93, Nam-gu, Ulsan 680-749, Republic of Korea
a r t i c l e
i n f o
Article history: Received 19 January 2012 Received in revised form 20 March 2012 Accepted 24 March 2012 Available online 13 April 2012 Keywords: Adsorption Metal ion-doped TiO2 Mesoporous Decolorization Methylene blue
a b s t r a c t In this study, pure TiO2 (V-TiO2 -0) and V-doped TiO2 (V-TiO2 -x, x = 1–10 mol%) were synthesized using a new sol–gel method. The adsorption capacity of the V-TiO2 -x samples was evaluated by measuring the removal of methylene blue (MB) from aqueous solution via decolorization. Since the adsorption capacity was affected by the specific surface area, the interaction between adsorbate (MB) and adsorbent (V-TiO2 x), and the structure of the adsorbent, the physicochemical properties of the samples were investigated. Among the V-doped TiO2 -x samples, the V-TiO2 -10 sample showed the highest adsorption capacity, which was 11.36 times greater than that of pure TiO2 , removing 85.2% of the MB after 2 h. Moreover, changing the molar ratio of the reactants in the V-TiO2 -10 sample improved the performance of the material so that 91.6% of the MB was removed after 2 h. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Industrial development and increasing populations have led to serious environmental problems; waste water disposal and air pollution have become global concerns. Adsorption technology plays a key role in industrial development because of its important purification functions, optimal environmental compatibility, and low operational cost [1–7]. Adsorption technology exploits the ability of some solid materials to remove color from dye-containing solutions. These adsorbent materials may be utilized for fluid purification and separation applications [1,2]. Recently, the synthesis of TiO2 or supported TiO2 on other substrates has been vigorously studied since TiO2 is considered to be an environmentally friendly and inexpensive catalyst with good dispersion properties under most conditions [5–14]. Nevertheless, TiO2 has a low adsorption capacity for most adsorbates. Therefore, several methods have been investigated to improve the performance of TiO2 , such as metal ion/non metal-doped TiO2 and deposited nanoparticle noble metals, due to their higher adsorption capacities [7]. Vanadium is a transition metal with multiple characteristics which improve TiO2 adsorptivity. For example, V-doped TiO2 with different valance state of the V and Ti (V0/1+/2+/3+/4+/5+ and Ti1+/2+/3+/4+ ) exhibits a difference in oxidation activity [7]. V doping was responsible for increases of superficial hydroxyl groups and electron transfer, resulting in faster interactions between the
∗ Corresponding author. Tel.: +82 52 259 2763; fax: +82 52 259 2348. E-mail address: [email protected] (K.-S. Ryu). 1 Energy Harvest-Storage Research Center, University of Ulsan, Daehak-ro 93, Nam-gu, Ulsan 680-749, Republic of Korea. 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.03.148
adsorbate and adsorbent [14–16]. V-doped TiO2 has been used as a catalyst for the selective reduction of NO to NH3 [11]. The ionic radius of V is almost the same as that of Ti, allowing it to be easily doped into TiO2 [8]. Previous doping methods have required long reaction times and high temperatures. For instance, Rodell [17] performed the doping reaction over 44 h, and Nguyen-Phan [5] used a 44 h reaction exclusive of preparation time of the precursor solution at high temperature (120 ◦ C for 12 h). In addition, these methods were carried out under acidic conditions, with HNO3 , HCl or CH3 COOH used as a catalyst [7–10]. In this study, different molar percentages (from 1 to 10 mol%) of vanadium doped into TiO2 were fabricated by a new sol–gel method at low temperature, with a short reaction time, and under alkaline conditions (ammonium hydroxide was a catalyst for the reaction), which were more energy efficient and increased the number of surface hydroxyl groups, an essential condition for improving material adsorptivity. Currently, there has been no demonstration of the adequate amount of V to be doped into TiO2 to provide an environmentally friendly adsorbent. We used 10 mol% of V as the critical value so that the V-TiO2 samples would be more compatible with a water environment. The adsorption capacity of the V-doped TiO2 samples was analyzed using decolorization of aqueous MB, and the effects of vanadium on the physicochemical properties of the samples were also studied.
2. Experimental method 2.1. Chemicals Titanium (IV) isopropoxide (Ti(OCH(CH3 )2 )4 , 97%, TTIP), isopropanol ((CH3 )2 CHOH, >99.8%, IPA) and vanadium (V) oxide (V2 O5 ,
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Fig. 1. Colors of V-TiO2 -x samples.
POROSITY-XQ, Mirae Scientific Instruments Co., Korea). An XPS study of fresh and spent catalysts was conducted via a Thermo Scientific K-Alpha instrument with an excitation source of Al K␣ radiation. Moreover, a binding energy scan range from 0 to 1350 eV for the identification of all detectable elements and detailed scans for chemical state identification and quantification were obtained. Ti 2p, O 1s, and V 2p spectra were deconvoluted using XPSPeak41 software. 2.4. Adsorption test
Scheme 1. Synthesis of V-TiO2 -x samples.
>99.6%) were obtained from Sigma-Aldrich Chemicals. Ammonium hydroxide (NH4 OH, 25–28%) was purchased from Dae Jung Chemicals & Metals, and distilled water was used to prepare all solutions.
Dye solutions were bleached for quantitative measurement of the concentration of adsorbable components before and after treatment with the solid (V-doped TiO2 ) adsorbent. Methylene blue (C16 H18 ClN3 S·3H2 O, MB), a common chemical extensively used in chemical and biological processes, was obtained from Sigma–Aldrich and was chosen as a model pollutant. The adsorption capability of the V-TiO2 -x samples was evaluated by measuring the removal of methylene blue (C0 = 1 × 10−5 M) from aqueous solution. 10 mg of the V-TiO2 -x sample was dispersed in 100 ml of MB solution and stirred in the dark at a constant temperature (22 ◦ C). Aliquots of 3.5 ml were withdrawn at different times within 120 min and were put through a syringe filter. The MB concentration was characterized by UV–Vis spectrophotometry (OPTIZEN POP, Mecasys Co., Ltd., Korea) at max = 666 nm, and the percentage of MB remaining in solution was calculated using the following equation:
2.2. Synthesis of V-doped TiO2 MB = V2 O5 powder was dissolved in ammonium hydroxide solution (NH4 OH:H2 O = 75:25 volume) to make a homogeneous 0.1 M vanadium solution. All samples were designated as V-TiO2 -x, where x is the mol% of the V (x = 0, 1, 3, 5, 7, and 10). The V-TiO2 -x samples were synthesized as shown in Scheme 1. The A and B solutions were prepared at a molar ratio of TTIP, IPA, and H2 O of 1: 5: 100. This process was repeated with other molar ratios of TTIP, IPA, and H2 O for the samples containing various mol% of V to obtain optimal reactant ratios. 2.3. Characterization The morphology and composition of the samples were analyzed using scanning electron microscopy (SEM, Supra 40, Carl Zeiss Co., Ltd., Germany). The crystalline phase was determined from the X-ray diffraction patterns (XRD, Cu K␣ radiation, 40 kV, 100 mA, ˚ Rigaku ultra-X, Rigaku Co., Japan) at a step scan = 1.54059 A, rate of 0.02◦ /0.3 s in the 2 range of 10–80◦ . Inductively coupled plasma optical emission spectrometry (ICP-OES, iCAP 6500, Thermo Electron Corporation) was used to measure the weight ratio of Ti and V. The surface area was measured by BET analysis (nano
C × 100(%) C0
(1)
where C0 and C are the MB initial concentration and concentration after the adsorption time, respectively. 3. Results and discussion When the molar percentage of V was increased, the color of the solid samples changed from white to dark due to the color of V in the doped TiO2 , as shown in Fig. 1. The weight ratios of Ti and V in the samples are shown in Table 1. Comparison of actual data (ICP results) and calculations, the performance of the reaction is about 92–98%. SEM images of the V-TiO2 -x samples containing the different molar percentages of vanadium are shown in Fig. 2. The SEM images clearly indicate that the particle size was homogeneous with an average diameter of 10–15 nm. As the mol% of V was increased, agglomeration was reduced, and the particle size increased. For instance, the surface of the V-TiO2 -0 sample had an agglomerated shape and holes with 200 nm-diameter located between particles. The V-TiO2 -1 sample also contained holes, but the quantity and size of the holes were reduced. The surface of the V-TiO2 -1 sample
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Table 1 Physicochemical properties of V-TiO2 -x samples. Sample
2 (◦ )
˚ d-spacing (A)
Crystallite size (nm)
SBET (m2 /g)
Total pore volume (cm3 /g)
Average pore diameter (nm)
Weight ratios Ti/V
V-TiO2 -0 V-TiO2 -1 V-TiO2 -3 V-TiO2 -5 V-TiO2 -7 V-TiO2 -10
25.380 25.380 25.400 25.300 25.360 25.359
3.5065 3.5065 3.5039 3.5174 3.5092 3.5093
13.42 14.34 16.58 17.45 16.24 16.89
76.93 85.30 82.61 71.93 73.56 73.92
0.4088 0.2704 0.2081 0.2172 0.1873 0.1797
7.808 5.410 5.114 5.744 5.403 4.859
– 49.9/0.55 54.1/1.9 49.8/2.9 52.5/4.1 42.9/4.7
was smoother than that of the pure TiO2 , and the V-TiO2 -x samples (x = 5, 7, and 10) in particular had smooth surfaces. This phenomenon indicates that, as the mol% of V increased with increasing pH, the particle size increased. In addition, the vanadium content increased with a higher molar ratio of water in the reaction, leading to an increase in particle size and a reduction in agglomeration [12,13]. As shown in Fig. 3, the XRD patterns of the V-TiO2 -x samples were similar. The V-TiO2 -x (x = 0 and 1) samples included anatase and brookite phases, but the intensity of the brookite phase was low. The existence of anatase and brookite phases depended on the amount of water and the pH of the solution. In the present work, as the mol% of V was continuously increased (x = 3, 5, 7, and 10),
all phases of the samples exhibited anatase phase diffraction peaks because of the increased amount of water. However, no characteristic peaks of vanadium metal or oxides were observed, which implies that vanadium incorporation in the lattice of the anatase TiO2 or vanadium oxide was limited and highly dispersed [12]. On the other hand, the V-TiO2 -x samples had slightly shifted peaks, as shown in Table 1. The crystallite size of the samples was calculated following Scherrer’s equation: D=
k ˇ cos
(2)
where D is the crystallite size, k is a constant (shape factor, about 0.9), is the X-ray wavelength (0.154059 nm), ˇ is the full width at
Fig. 2. SEM images of the samples. (a) V-TiO2 -0, (b) V-TiO2 -1, (c) V-TiO2 -3, (d) V-TiO2 -5, (e) V-TiO2 -7, and (f) V-TiO2 -10.
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Fig. 5. BJH pore size distribution-adsorption of the V-TiO2 -x samples. Fig. 3. X-ray diffraction patterns of the V-TiO2 -x samples.
half maximum (FWHM) of the diffraction line, and is the diffraction angle. The crystallite size of the (1 0 1) peak increased with the increasing amount of V doped in the TiO2 , similar to the SEM result. Particle sizes ranged from 13.4 to 17.5 nm. The nitrogen adsorption–desorption isotherms of the V-TiO2 -x samples are shown in Fig. 4. All isotherms were identified as type IV according to the IUPAC classification and belong to the mesoporous family (radius from 2-50 nm) [18–20]. The V-TiO2 -x samples were classified as having three types of hysteresis loops. First, the V-TiO2 -x (x = 0 and 1) samples have hysteresis loops of type H3, which are usually produced by the aggregates of platy particles or adsorbent containing slit-shaped pores. According to the SEM results, the V-TiO2 -x (x = 0 and 1) samples had not only textural but also framework mesoporosity. These results also clearly show that the hysteresis loops include two areas. For instance, P/P0 values from 0.3 to 0.7 were found in the first area, with pores formed in the particles (framework mesoporosity). The second area contained P/P0 values greater than 0.7, in which pores were created between the particles (textural). However, in these samples, the textural pores were more significant than the framework mesoporosity. Second, the hysteresis loops of the V-TiO2 -x (x = 3 and 5) samples belong to the H2 type, denoting the existence of bottleneck pore channels. The average pore sizes of these samples (x = 3 and 5) were 5.1 and 5.7 nm, respectively, and most were textural pores.
Fig. 4. N2 adsorption–desorption isotherms of the V-TiO2 -x samples.
Third, the hysteresis loops of the V-TiO2 -x (x = 7 and 10) samples are the H1 type, which are found in adsorbents with a narrow pore size distribution. In particular, the adsorption–desorption line was completely adsorbed, such that the vanadium may exist outside the TiO2 structure, and alternate structure between V and TiO2 is little. The specific surface area (SBET ) was calculated from the linear BET plot (P/P0 range of 0.05–0.2), and the total pore volume was measured at P/P0 = 0.99 [18–20]. The SBET , total pore volume, and average pore diameter are summarized in Table 1. As the mol% of V was increased, the crystallite size and SBET increased from 13.42 to 17.45 nm and from 71.93 to 85.3 m2 /g, respectively. The SBET values of the samples depended on the crystalline size, phase, and shape [18]. As the mol% of V was increased, the total pore volume and average pore diameter reduced. Adsorption selection kinetics is based on the size of the adsorbent pores and the dimensions of the diffusing adsorbate molecules. According to Fig. 5 about BJH pore size distribution-adsorption, pure TiO2 has a large pore size distribution, which is disadvantage adsorption capacity. When mol% of V was increased, pore size distribution of the samples was narrow size, which helps to increase adsorption capacity. Since the MB molecule can be regarded as an approximately rectangular volume ˚ the MB volume is greater than of dimensions 17.0 × 7.6 × 3.25 A, 1.5 nm, and the water molecule volume is about 0.26 nm [21]. Comparing the absorption selections for the two groups with ink bottle pore channels and narrow pores, the average pore sizes were similar. However, for the ink bottle pores, rb was always greater than rn , with a rn of about 1 nm. Therefore, for this pore size (about 2 nm), it would be difficult to trap 2–3 molecules of MB within the pores. The average pore diameter of the V-TiO2 -x (x = 7 and 10) samples was about 5 nm with narrow pores, which would allow easier trapping of MB compared with that in the ink bottle-shaped pores with the same average diameter. Hence, the MB removal adsorption kinetics of the V-TiO2 -x (7–10) samples was faster than those of the other samples [1,2]. The nature of the oxidation states of O 1s, Ti 2p, and V 2p were obtained by high-resolution XPS, as shown in Figs. 6–8 and Table 2. In Fig. 6, the Ti 2p XPS results of the V-TiO2 -x samples also indicated two main components related to 2p3/2 and 2p1/2 . The Ti 2p3/2 of the pure TiO2 sample has only oxidation states Ti4+ at 458.51 eV. As the mol% of V was increased, the other oxidation states Ti3+ , were more prevalent. For the Ti 2p3/2 of the V-TiO2 -5 sample, Ti3+ and Ti4+ were at 457.06 and 458.00 eV, respectively. For the V-TiO2 -10 sample, the %L–G of Ti3+ was increased to 37.39%. Thus, as the mol% of V was increased, the level of active ions increased, which is advantageous for adsorption capacity [7–9].
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Table 2 XPS fit parameters for the Ti 2p, O 1s, and V 2p signals of the samples. Level
Sample
Oxidation state
BE (eV)
% L–G
FWHM (eV)
Ti 2P1/2
1 2
Ti4+ Ti4+ Ti3+ Ti4+
464.17 462.49 459.80 463.81
31.93 42.13 10.63 30.43
2.07 4.35 1.30 2.56
Ti4+ Ti4+ Ti3+ Ti4+ Ti3+
458.51 458.00 457.06 459.02 457.81
68.07 16.91 30.33 32.18 37.39
1.25 1.62 1.17 2.14 1.19
– – – – – – – –
529.74 531.51 528.33 529.53 531.76 529.04 530.27 531.91
98.35 1.65 43.78 32.13 24.09 51.17 43.44 5.39
1.333 1.728 1.378 2.108 2.393 1.300 1.928 1.555
3 Ti 2P3/2
1 2 3
O 1s
1 2
3
V 2p1/2
2 3
V4+ V4+
523.28 523.74
25.46 41.67
3.904 4.441
V 2p3/2
2
V4+ V5+ V4+ V5+
515.41 518.13 516.09 517.31
24.47 50.07 28.76 29.57
2.026 6.401 2.230 4.410
3
(1), (2), and (3) are V-TiO2 -0, V-TiO2 -5, and V-TiO2 -10, respectively. BE: binding energy; %L–G: % Lorentzian–Gaussian.
Shown in Fig. 7 are the XPS spectra for the O 1s of V-TiO2 -x (x = 0, 5, and 10). Pure TiO2 consisted of two peaks, one at 529.74 eV related to OI and the other at 531.51 eV related to OII . As V was increasingly doped in the TiO2 , the oxidized component of the VTiO2 -x (x = 5, 10) increased. For example, these samples had three oxidation states that included M O (M-metal, V and Ti), hydroxyl groups, and physically adsorbed water. For the V-TiO2 -5 sample, the OH groups and physically adsorbed water increased to 32.13% and 24.09%, respectively. In particular, for the V-TiO2 -10 sample, the amount of OH groups increased to 43.44%, which corresponded to an increased adsorption capacity [7–9]. Fig. 8 shows the distinct distributions of V 2p1/2 and V 2p3/2 . With the V-TiO2 -5 sample, 2p3/2 included two peaks at 515.41 and 518.13 eV, corresponding to V4+ and V5+ , respectively, and 2p1/2 was at 523.28 eV [22–25]. The BE of the V-TiO2 -10 sample was shifted higher than that of the V-TiO2 -5 sample, which is easily seen by the shift of the dashed line. The %L-G of V5+ of the V-TiO2 -10 sample was reduced to 29.57%. Fig. 6. Ti 2p XPS of the samples: (a) V-TiO2 -0, (b) V-TiO2 -5, and (c) V-TiO2 -10.
Fig. 7. O 1s XPS of the samples: (a) V-TiO2 -0, (b) V-TiO2 -5, and (c) V-TiO2 -10.
Fig. 8. V 2p XPS of the samples: (a) V-TiO2 -5 and (b) V-TiO2 -10.
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Fig. 9. The effect of vanadium mol% content on adsorption capacity.
The adsorption capacities of the V-TiO2 -x samples are shown in Fig. 9. After 120 min, pure TiO2 removed a small amount of MB (7.5%). As the mol% of V was increased (x = 3–10), the parameter (C/C0 × 100) was very quickly reduced. The MB decolorization process was categorized into three adsorption periods: (i) instantaneous adsorption of MB within 10 min, (ii) gradual attainment of an equilibrium from 10 to 60 min, and (iii) established equilibrium of MB molecules onto the V-TiO2 after 60 min. In stage (i), the MB concentration was reduced from 100% to 55% in the V-TiO2 10 sample. This result indicated the substantial effect of V on the adsorption ability of TiO2 . A higher V mol% resulted in increased adsorption ability of the V-TiO2 -10 sample, exhibiting the greatest adsorption capacity of the samples studied for decolorization of MB. However, the effect of the V-doped TiO2 (V-TiO2 -x, x = 1-10) samples was distributed among the three groups. The first group included the V-TiO2 -1, which had little effect on adsorption. The second group included the V-TiO2 -3, which had increased adsorption ability reflected in the C/C0 × 100 reduction from 92.5 to 31.4 for pure TiO2 . The third group included V-TiO2 -x (x = 5–10), which quickly removed MB from solution. This distribution is comparable to observations from the SEM images. A higher specific surface area provided more sensitive space for the penetration of MB. However, the third group had a lower specific area than the two other groups. Instead of a high specific area, the phase of the third group was very smooth, such that the MB could easily penetrate into the TiO2 . According to the BET results, the V-TiO2 -x (x = 7 and 10) samples were favorable for kinetic adsorption of MB into TiO2 . Various studies have shown that adsorption capacity is strongly dependent on the interaction between adsorbent (V-TiO2 -x samples) and adsorbate (MB) [13–15]. The adsorption of MB or other organic compounds is dominated by the interaction between the electrons of the MB ring and surface OH groups. The surface of TiO2 consists of multiple OH groups (Ti OH, Ti (OH)2 ). According to the XPS results, doping with V resulted in increased oxidation states of Ti, V, and O, and this increased oxidation activity is significant for adsorption capacity. A sol–gel method was applied to the synthesis of V-doped TiO2 , and the molar ratio of the starting materials was very important to the composition of the product [12]. In particular, the amount of water affected the particle size and agglomeration, material characteristics that have an effect on adsorption ability. The effect of reactant molar ratio on the doped TiO2 composition as measured by removal of MB is shown in Fig. 10. The molar ratio (TTIP:IPA:H2 O = 1:1:40) was optimal for the V-TiO2 10 sample, which removed 91.6% of the MB (C/C0 × 100 = 8.4) after
Fig. 10. The effect of molar ratio of the reaction on adsorption capacity.
120 min. V-doping of TiO2 significantly increased the effectiveness of MB removal, which increased from 7.5% to 85.2%, an 11.36-fold increase. In contrast, Liang [15] prepared V-doped Fe3 O4 which only increased MB removal from 41% to 93%, a 2.27-fold increase after 700 min, and Wenfang [9] prepared a V-doped TiO2 which increased the removal of MB from 12% to 32% (2.67-fold), after 8 h. 4. Conclusions A higher mole percent of V in V-TiO2 -x samples significantly promoted the adsorption capacity for decolorization of MB solution compared to the un-doped TiO2 sample. Incorporation of V changed the physical and chemical properties of TiO2 , improving its adsorption ability. The optimal sample V-TiO2 -10 (10 mol% V) removed about 85.2% of the MB, while pure TiO2 only removed 7.5% of the MB during 2 h. Optimization of the reactant molar ratio (TTIP:IPA:H2 O = 1:1:40) for the V-TiO2 -10 sample allowed 91.6% of the MB to be removed. The 11.36-fold (85.2/7.5) adsorptivity increase is significant in the context of wastewater treatment. Acknowledgments This work (grants no. 0039007-1) was supported by Business for Academic-industrial Cooperative establishments funded Korea Small and Medium Business Administration in 2009 and by the Priority Research Centers Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2009-0093818). References [1] F. Rouquérol, J. Rouquérol, K.S.W. Sing, Adsorption by Powders & Porous Solids: Principles, Methodology and Applications, Academic Press, 1999. [2] S. Lowell, J.E. Shields, Powder Surface Area and Porosity, Chapman and Hall, 1991. [3] M.A. Al-Ghouti, M.A.M. Khraisheh, M.N.M. Ahmad, S. Allen, The Journal of Hazardous Materials 165 (2009) 589–598. [4] M. Suzuki, Water Science and Technology 35 (1997) 1–11. [5] T.-D. Nguyen-Phan, M.B. Song, E.W. Shin, The Journal of Hazardous Materials 167 (2009) 75–81. [6] T.-D. Nguyen-Phan, C. Lee, J. Chung, E.W. Shin, Research on Chemical Intermediates 34 (2008) 743–753. [7] T.-D. Nguyen-Phan, M.B. Song, H. Yun, E.J. Kim, E.-S. Oh, E.W. Shin, Applied Surface Science 257 (2011) 2024–2031. [8] B. Liu, X. Wang, G. Cai, L. Wen, Y. Song, X. Zhao, The Journal of Hazardous Materials 169 (2009) 1112–1118. [9] Z. Wenfang, L. Qingju, Z. Zhongqi, Z. Ji, Journal of Physics D: Applied Physics 43 (2010) 035301. [10] J. Liu, R. Yang, S. Li, Journal of Rare Earths 25 (2007) 173–178.
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