Adsorption of Dyes on Hierarchical Mesoporous TiO2 Fibers and Its

ARTICLE pubs.acs.org/JPCC

Adsorption of Dyes on Hierarchical Mesoporous TiO2 Fibers and Its Enhanced Photocatalytic Properties Nan Bao,*,† Yuan Li,† Zhentao Wei,‡ Guangbin Yin,† and Junjian Niu† † ‡

School of Environmental Science and Engineering, Shandong University, Jinan 250100, China China North Industries Group Corporation, Institute 53, Jinan 250031, China ABSTRACT: In this Article, the surface characteristics of acidmodified continuous TiO2 fibers, the adsorption of dyes, and the photocatalytic degradation of dye pollutants under UV and visible light irradiation were investigated. After HF extraction of silicon, a hierarchical mesoporous structure is produced. The OH groups located on the surface were more resistant to exchange with F- because of the inductive effect on surface Ti4þ centers originating from adjoining electronegative F- ions. After protonation of the HF-modified fibers by HNO3 treatment, the photodegradation percentage of X-3B on the TiO2 fibers reached 98% in 45 min under visible light irradiation, and the first-order rate constant increased by a factor of 163.4. The high visible light activity was mainly attributed to a synergetic effect of the hierarchical mesoporous structure and the strong adsorption of X-3B upon the protonated surface. The adsorption isotherms showed that the adsorption of X-3B on acid-modified TiO2 followed the Langmuir model with a maximum adsorption capacity of 19.03 mg/g, 6-fold higher than the unmodified TiO2. The nature of the X-3B sorption was spontaneous and thermodynamically favorable. The sorption kinetics was best described by the pseudo-secondorder model.

1. INTRODUCTION Textile dye effluents are typically characterized by strong color and recalcitrance, even at very low concentration. These dyes can not only cause aesthetic problems, but also exhibit biotoxicity and possible mutagenic and carcinogenic effects. Given the complex and bioresistant character of textile effluents, conventional biological treatment methods are often ineffective for decolorization and degradation.1 An alternative for the degradation of texile effluents is to use heterogeneous semiconductor photocatalysis, which can effectively oxidize a wide range of organic compounds at moderate ambient conditions. In recent years, photocatalysis has become an appealing option for water and wastewater treatment.2,3 TiO2 stands out as a promising photocatalyst due to its biological and chemical inertness, strong photooxidation power, cost effectiveness, and long-term stability against photo- and chemical corrosion.4-6 However, its photodecomposition rate is intrinsically low due to the limited surface area. Nanosized TiO2 powders demonstrate superior photodegradation because of their large surface area and distinctly good dispersion capability.7 Unfortunately, nanoparticles tend to aggregate in suspension, leading to a rapid loss in active sites and photocatalytic efficiency. In addition, the postseparation in the slurry system poses a key obstacle to practical application. To maximize its functional activity, much work has been conducted on preparing various new morphologies such as flowers,8 tubes,9 fibers,10,11 and hollow spheres.12 Generally, one of the important advantages of one-dimensional fibers is their high surface area per unit volume. Large surface area TiO2 fibers would combine the r 2011 American Chemical Society

advantages of both adsorption and photocatalytic techniques: on one hand, TiO2 fibers work as adsorbents and concentrate the pollutants and intermediates around the TiO2; on the other hand, nanosized TiO2 photocatalysts can decompose the pollutants, thus regenerating the catalyst in situ. Electrospinning is an effective tool in the fabrication of microfibers, nanowires, nanofibers, and other nanostructures for various applications. Most studies on metal oxide nanofibers have focused on electrospinning from a metal precursor solution mixed with a polymer, such as poly(vinyl pyrrolidone) (PVP), poly(vinyl alcohol) (PVA), poly(vinyl acetate) (PVAc), and poly(ethylene oxide) (PEO) to achieve a certain viscosity.13-15 Although this approach has improved the mechanical strength of fibers, the functional activity of the metal oxide is restricted considerably because most are located inside the fibers, not at the surface. Our group has previously successfully prepared Si-doped TiO2 continuous fibers with a relatively high photocatalytic activity by a sol-gel method combined with centrifugal spinning without any template or binder polymer,16 but the prepared fibers showed a low photoresponse under visible light irradiation, which may be ascribed to the low transmittance-absorbance of Si-doped TiO2. It is necessary to extract an appropriate portion of the silicon to enhance light harvesting and generate a porous structure. HF etching has been demonstrated to be a simple and controllable Received: October 21, 2010 Revised: February 17, 2011 Published: March 11, 2011 5708

dx.doi.org/10.1021/jp1100939 | J. Phys. Chem. C 2011, 115, 5708–5719

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Figure 1. Dye structures: (a) Reactive Brilliant Red X-3B, and (b) Malachite Green.

Scheme 1. Schematic Diagram of TiO2 Fiber Preparation

method to synthesize hierarchical pore structures by silica extraction.17-19 The unique structure can provide size and shape selectivity for the guest molecules; besides, fluorinated TiO2 resulted in great improvement in the oxidation rate of phenol,20 benzene,21 and reactive brilliant red X-3B.22 However, fluorinated TiO2 led to a reduced degradation of Acid Orange 7 under visible light due to the hindered adsorption of substrates;23 thus a method to strengthen dye adsorption on TiO2 by modifying the surface fluorination is needed to improve both UV and visible light induced degradation capabilities. Adsorption of a compound onto a photocatalyst surface is one of the key reaction steps in photocatalysis; adsorption and enrichment not only affect the photodegradation rate, but also influence the mechanisms.24 Modeling of the adsorption data allows better understanding of the mechanism of the adsorption process. Lee et al.25 showed that surface protonation as well as uptake of NO3- anions on TiO2 surfaces after a simple HNO3 treatment enhanced dye adsorption and suppressed back electron transfer to TiO2 electrolytes. Their photoresponse can be extended to the visible region by dye sensitization. Photosensitization is an effective method to improve photodegradation rate and expand visible light response.26 The process involves excitation of the dyes to appropriate singlet or triplet states followed by electron injection into the conduction band of the semiconductor, whereas the dyes are converted to cationic

dye radicals. This injection produces a series of active oxygen species from reaction with molecular oxygen; the subsequent radical chain reaction can lead to the degradation of the pollutants. Interest in this photocatalytic route is enhanced by the fact that it can utilize an ideal energy source, solar light, which is free and inexhaustible. Most organic dyes in wastewater are photosensitizers, and the cooperation of photocatalysis and photosensitization can remove the residual dyes. On the basis of the principle of dye photosensitization, Choi et al.26 prepared visible light responsive TiO2. During the study, they found visible light induced dechlorination of carbon tetrachloride in aqueous solution. However, using optical active dyes as degradation targets to strengthen the photodegradation process has not yet been systematically studied.24,27 In the present study, a simple and low energy method for creating porous, surface-modified TiO2 is presented. Si-doped TiO2 fiber was immersed into 1.5% HF and further washed by 5% HNO3 solution. The effect of surface acid treatment of TiO2 on the photocatalytic degradation of X-3B and Malachite Green (MG) under both UV and visible light irradiation was investigated. The adsorption characteristics of X-3B and MG on the surface of Si-doped TiO2 and acid-modified TiO2 fibers were carefully examined and validated by direct evidence. Such studies can give insights into the mechanistic aspects of the adsorption and the degradation pathway.

2. EXPERIMENTAL SECTION 2.1. Chemicals and Materials. Titanium tetrabutoxide (TBOT), tetraethyl orthosilicate (TEOS), isopropyl alcohol (IPA), ethyl acetoacetate (EAcAc), and tetrahydrofuran (THF) were purchased from Sinopharm International Co. Ltd. (Shanghai, China). Reactive Brilliant Red X-3B and Malachite Green were purchased from Shanghai Dyestuff Chemical Plant, and their structures are shown as Figure 1. All of the chemical reagents were of analytical grade and used without further purification. Deionized water was used in all experiments. 2.2. Surface Modification Procedure. Si-doped TiO2 continuous fibers were prepared by a sol-gel method combined with centrifugal spinning without any template or binder polymer. The finished fibers were designated as STF. The detailed preparation procedure has been described elsewhere16 and is summarized in Scheme 1. The STF samples (0.1 g) were immersed in 25 mL of 1.5% HF for 90 min. The fluorinated TiO2 fibers were washed with one of the following four solutions: (1) 50 mL of deionized water; 5709

dx.doi.org/10.1021/jp1100939 |J. Phys. Chem. C 2011, 115, 5708–5719


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(2) 50 mL of 5% HNO3 solution; (3) 50 mL of 5% HNO3 solution and further washed by 50 mL of deionized water; and (4) 50 mL of 1 M NaOH solution. The resulting samples were kept in an oven at 423 K for 2.5 h. The modified fibers were designated as STF-F-H, STF-F-N, STF-F-N-H, and STF-F-B, respectively. For comparison with fluorinated TiO2 fibers, the STF samples (0.1 g) were also immersed in 25 mL of 5% HNO3 solution, which was named STF-N. The suffixes N, F, H, and B stand for the various modifiers, that is, HNO3, HF, deionized water, and NaOH, respectively. 2.3. Photoactivity Studies. All of the experiments were performed in an open fixed-bed photoreactor,28 fitted with a 250 W high-pressure mercury lamp (λmax = 365 nm). The irradiation intensity (15 W/m2) of the lamp at the surface of dye solution was measured with a digital illuminometer (TN2340, Taiwan). The visible light source was a 1000 W Xe lamp, and a glass optical filter was inserted to cut off the short wavelength components (λ < 420 nm). Before irradiation, an aqueous solution of X-3B and MG (100 mL) with an initial concentration of 30 mg/L in the presence of 0.4 g of prepared TiO2 fibers was stirred magnetically for 15 min in the dark to establish the adsorption/desorption equilibrium between the dyes and the photocatalyst surface. Next, the reaction mixture was illuminated under UV or visible light and cycled by a peristaltic pump (flow rate 20 mL/min) for the entire time span of experiment. At regular intervals, samples of about 5 mL in volume were taken and filtrated through a 0.45 μm syringe filter. The extent of X-3B and MG removal was determined on the basis of changes in color by measuring the absorbance value on a UVvis 1601 spectrophotometer (Shimadzu, Japan) at 536 and 664 nm, respectively. The determined absorbance was converted to concentration through the standard curve method. The degradation efficiency of the dyes was calculated by R ¼ ð1 - C=C0 Þ 100%

ð1Þ

where C0 and C were the concentration of dyes at reaction times 0 and t, respectively. In addition, the mineralization rate of X-3B was obtained by a total organic carbon analyzer (TOC-5000A, Shimadzu, Japan). 2.4. Adsorption Studies. Kinetic studies on the adsorption of dyes by STF and STF-F-N were carried out in the dark, and it was found that adsorption equilibrium was achieved within 2 h (data not shown). Thus, 2 h was chosen as the adsorption equilibrium time. Batch equilibrium adsorption experiments were conducted in the dark over a range of initial concentrations with a fixed weight (0.40 g/100 mL) of the TiO2 samples to obtain the adsorption isotherms of X-3B and MG on the catalysts. The suspension was placed in 250 mL stoppered conical flasks and mechanically shaken at 120 rpm in a water bath shaker (SHZ-82) at a constant temperature. After reaching equilibrium, the mixture was filtered through a 0.45 μm syringe filter. The absorbance of the X-3B and MG at 536 and 664 nm was measured to determine the equilibrium concentration. The amount of adsorbed dyes per gram TiO2 at equilibrium, qe (mg/g), was obtained by qe ¼

ðC0 - Ce ÞV W

ð2Þ

where C0 and Ce (mg/L) are the initial and equilibrium concentrations of X-3B and MG, respectively. V (L) is the volume of the solution, and W (g) is the weight of catalyst used.

2.5. Characterization of the Photocatalyst. The BrunauerEmmet-Teller (BET) specific surface area of fibers was analyzed by nitrogen adsorption in a Quadrasorb SI-MP system (Quantachrome, U.S.). The BET specific surface area was determined by a multipoint BET method using the adsorption data in the relative pressure (p/p0) range of 0.05-0.3. Pore size distribution was calculated from the N2 isotherm at 77 K based on the DFT (Density Functional Theory) method.29 The software used was QuadraWin from Quantachrome Instruments. DFT, based on a molecular-statistical approach, is applied over the complete range of the isotherm and is not restricted to a confined range of relative pressure or pore sizes. The pore size distribution was calculated by fitting the theoretical set of adsorption isotherms, evaluated for different pore sizes, to the experimental results. The pore volume and the average pore size were determined by nitrogen adsorption volume at a relative pressure of 0.991. All samples were degassed at 393 K for 8 h in a vacuum before BET measurements. The surface morphologies of the fibers were observed with a scanning electron microscope (S-520, Hitachi, Japan). The X-ray diffraction (XRD) patterns were obtained on a D/max-γA X-ray diffractometer (Rigaku, Japan) using Cu KR radiation (λ = 0.154178 nm) and at a scanning ratio of 10°/min in 2θ ranging from 20° to 70°. The accelerated voltage and applied current were 40 kV and 70 mA, respectively. The particle diameter (D) was calculated by applying the Scherer’s equation, D = 0.89λ/(B cos θ), where λ is the X-ray wavelength, θ is the Bragg angle, and B is the full width at half maxima. FT-IR spectra were recorded using an Avatar 370 spectrometer (Thermo Nicolet, U.S.). Solid-state UV-vis diffuse reflectance spectra were recorded at room temperature and in air by means of a UV-vis spectrophotometer (UV-3100, Shimadzu, Japan) equipped with an integrating sphere attachment using BaSO4 as background. X-ray photoelectron spectroscopy (XPS) measurements were carried out using an ESCALAB 250 spectrometer (Thermo Electron Corp., UK) with an Al KR source and a charge neutralizer, and all the spectra were calibrated to the C 1s peak at 284.6 eV. XPS experiments were used to detect the changes of the O 1s, N 1s, and F 1s binding energies in the samples.

3. RESULTS AND DISCUSSION 3.1. UV Light-Induced Photocatalytic Degradation of Dye Pollutants. The photocatalytic activity of the catalysts was

studied using X-3B and MG degradation experiments as explained in the Experimental Section. The decay of the original dyes is shown in Figure 2. In the absence of light, only an adsorption process took place. Adsorption of X-3B by unmodified TiO2 (STF) and three other samples (STF-F-H, STF-N, and STF-F-N-H) was negligible, but the adsorption became apparent with STF-F-N, suggesting a change of binding sites for X-3B to occupy on the surface of STF-F-N. After dark equilibration, the highest dye removal was 84 ( 2.0% for STF-F-N. The lower dye adsorption for STFF-N-H (33 ( 2.1%) is a result of decreased acid strength on the catalyst surface after washing with H2O, while just about 10% adsorption was observed for STF and the single acidtreated samples. The color of STF-F-N changed from white to brilliant red at the end of the dark experiment. The L-shaped adsorption profiles suggest that there is no significant competition between X-3B and water molecules during the adsorption process.30,31 5710



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Figure 2. Comparison of photocatalytic activity of samples for degradation of (a) X-3B and (b) MG under UV light.

Figure 3. UV-vis absorption spectra of X-3B solution.

When the light source was switched on, the photocatalytic reaction ensued. The photodegradation of X-3B and MG was negligible under light only without the photocatalysts. The kinetic data for the photocatalyzed degradation of dyes fit the apparent first-order rate equation, ln(C0/C) = kappt, where kapp is the rate constant; C0 and C are the concentrations of the dyes at irradiation time t = 0 and t, respectively. The X-3B dye in the STF-F-N dispersion disappeared completely within 20 min of irradiation, with a decay rate constant of kapp = 0.081 min-1. However, in the STF case, the same removal percentage of X-3B was observed after 120 min of irradiation (kapp = 0.017 min-1). The degradation rates of MG were 0.038 and 0.021 min-1 over STF-F-N and STF, respectively (Figure 2b). Considering that the photocatalysis reactions mainly took place at the surface of the catalysts rather than in the bulk, the adsorption capability of

Figure 4. Adsorption isotherms of X-3B onto STF-F-N at different temperatures: (a) Freundlich isotherm, (b) Langmuir isotherm, and (c) Redlich-Peterson isotherm (C0 = 20, 30, 40, 50, 60, 80, 100, 150, 200, and 300 mg/L, STF-F-N dosage = 0.4 g/100 mL, equilibration time = 120 min).

the catalysts was a key factor for the decomposition of dyes. The observed difference between the two dyes was attributed to their different surface charge properties and molecular size. The UV-vis spectral change during the photodegradation of X-3B on STF-F-N is illustrated in Figure 3. The characteristic absorption band of the dye around 536 nm decreased drastically during dark adsorption, and X-3B was almost completely decolorized within 30 min. It can be seen that the strong absorption band around 230 nm results from the NO3- in the solution. 3.2. Adsorption Isotherms. By monitoring the concentration of X-3B in the bulk solution until the adsorption/desorption equilibrium was reached, adsorption isotherms for various initial concentrations of dye can be generated. The Langmuir, 5711



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Freundlich, and Redlich-Peterson isotherms were employed to determine the adsorption isotherm parameters, which were obtained by nonlinear curve-fitted (origin software) plots of qe versus Ce. The results are shown in Figure 4. The theoretical Langmuir isotherm32 assumes that adsorption occurs at specific homogeneous sites within the adsorbent and the capacity of the adsorbent is finite. The equation of Langmuir is represented as follows: qe ¼

KL q m C e 1 þ K L Ce

ð3Þ

where Ce is the equilibrium concentration (mg/L); qe is the amount of dye adsorbed at equilibrium (mg/g); qm is qe for a complete monolayer (mg/g), which gives the maximum adsorption capacity of adsorbent; and KL is the sorption equilibrium constant (L/mg). The Freundlich isotherm33 was shown to be consistent with exponential distribution of active centers and the characteristics of heterogeneous surfaces by adsorption. The Freundlich isotherm equation is expressed as: qe ¼ KF Ce 1=n

ð4Þ

where KF is the binding energy constant reflecting the affinity of the adsorption and n is the Freundlich exponent related to adsorption intensity. The Redlich-Peterson isotherm34 contains three parameters and incorporates the features of both the Langmuir and the Freundlich isotherms. It can be described as follows: ACe qe ¼ 1 þ BCe g

ð5Þ

A (L/g) and B (L/mg) are the Redlich-Peterson isotherm constants, and g (0 < g < 1) is the degree of heterogeneity. For g = 1, it can be assimilated to the Langmuir equation. A comparison of the coefficients of determination for the three isotherms was conducted and listed in Table 1. The coefficients of determination, R2, were between 0.8870 and 0.9345 for the Freundlich isotherm, while for the Langmuir isotherm they were between 0.9511 and 0.9659, indicating that the Langmuir isotherm better represented the experimental adsorption data at various temperatures. In addition, the values of χ2 of the Langmuir isotherms were smaller than those of the Redlich-Peterson and

Freundlich isotherms. Unlike linear analysis, different forms of the equation would affect R2 significantly and impact the final determination, whereas nonlinear, χ2 analysis would be a method of avoiding such errors. Consequently, the Langmuir isotherm was the most suitable model for this sorption system. The fact that Langmuir isotherm fits the experimental data very well suggests monolayer coverage of X-3B onto the STF-F-N surface as well as the homogeneous distribution of active sites on the STF-F-N surface, because the Langmuir isotherm assumes that the surface is homogeneous. The isotherm constants were determined and are shown in Table 2. The values of 1/n were found to be less than 1 for all temperatures, indicating that the adsorption was favorable. In most cases, the values of g tend to unity, which means that the isotherms are approaching the Langmuir form. The maximum adsorption capacity (qm) of STF-F-N for X-3B is 19.03 mg/g, 6-fold higher than the STF samples. Moreover, the values of qm and KL increased in temperature, indicating that the adsorption process is exothermic in nature. Figure 5 depicts the adsorption capacity of anionic dye X-3B and cationic dye MG on the photocatalysts at 298 K, and Table 3 lists the adsorption parameters. It was found that STF-F-N increases both the saturation amount (qm) and the adsorption strength (KL) for X-3B by factors of about 6.4 and 1.3, respectively. It is postulated that after acid treatment the surface of STF-F-N is positively charged: tTi - OH þ Hþ f tTi - OH2 þ

ð6Þ

while the X-3B dye molecule is negatively charged due to the sulfonic groups in its chemical structure; thus the electrostatic attraction facilitates the adsorption of the anionic dye of X-3B. This argument can be supported by the fact that STF-F-N exhibited less adsorption for the cationic dye MG relative to

Table 1. Comparison of χ2 and Coefficient of Determination for the Three Isotherm Models Freundlich

Langmuir

Redlich-Peterson

T (K)

R2

χ2

R2

χ2

R2

χ2

298

0.8870

2.67

0.9659

0.81

0.9673

0.88

308 318

0.9345 0.9321

0.88 0.34

0.9637 0.9511

0.49 0.25

0.9700 0.9555

0.46 0.26

Figure 5. Langmuir isotherms for the sorption of dyes on STF and STFF-N at 298 K (C0 = 20, 30, 40, 50, 60, 80, 100, 200, and 300 mg/L, STFF-N dosage = 0.4 g/100 mL, equilibration time = 120 min).

Table 2. Isotherm Parameters for the Adsorption of X-3B on STF-F-N Freundlich T (K)

KF ((mg/g)(L/mg))

1/n

Langmuir

Redlich-Peterson

1/n

KL (L/mg)

qm (mg/g)

A (L/mg)

B (L/mg)

g

298 308

3.94 2.64

0.294 0.316

0.055 0.049

19.03 14.34

0.93 0.96

0.037 0.12

1.0 0.89

318

1.24

0.355

0.037

8.63

0.44

0.10

0.87

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Table 3. Adsorption Parameters Derived from the Best-Fit Langmuir Isotherm Model qm (mg/g)

KL (L/mg)

R2

X-3B

2.93

0.043

0.9677

MG X-3B

8.13 18.32

0.034 0.058

0.9492 0.9833

MG

5.23

0.045

0.9583

samples

dyes

STF STF-F-N

Table 4. Thermodynamic Parameters of STF-F-N for X-3B Adsorption T (K)

ΔG (kJ/mol)

ΔH (kJ/mol)

ΔS (kJ/mol K)

298

-26.19

-59.892

-0.114

308

-24.24

-59.892

-0.114

318

-23.91

-59.892

-0.114

STF. The increased adsorption of MG on STF as compared to X-3B adsorption is attributed to the smaller molecular of MG. In addition, the high specific surface area (127.7 ( 5.7 m2/g) and pore volume (0.25 ( 0.018 cm3/g) of STF enhanced the mass transport into and out of the pore structure.16 In this case, the Ti4þ ions could act as Lewis acids because of adjoining electronegative F- ions, and the X-3B dye could act as a Lewis base due to its π electron conjugation structure. Consequently, the formation of a Lewis acid-base complex between Ti4þ ions and X-3B dye in aqueous solution could chemically enhance the interface adsorption. 3.3. Adsorption Thermodynamics. The temperature affects the adsorption of dyes onto solid surfaces. In the present work, we evaluated the thermodynamic behavior of the adsorption of X-3B onto STF-F-N though the change in free energy (ΔG), enthalpy (ΔH), and entropy (ΔS). The thermodynamic parameters in Table 4 were calculated from the following equations:35,36 ΔG ¼ - RT ln K

ð7Þ

ΔG ¼ ΔH - TΔS

ð8Þ

where R is the gas constant, T is temperature in K, and K is the Langmuir constant at different temperatures. The values of ΔH and ΔS were determined from the slope and intercept of a plot of ΔG versus T. The negative ΔG values indicate the thermodynamically feasible and spontaneous nature of the adsorption. The absolute value of ΔG decreased as temperature increased, indicating that the spontaneous nature of adsorption is inversely proportional to the temperature. The negative values of ΔH for X-3B suggest the adsorption is exothermic. This is also supported by the decrease in the values of adsorption capacity of the STF-F-N with a rise in temperature. The negative entropy values (ΔS) resulted from the increased order during the adsorption of X-3B and reflect that no significant change occurs in the internal structure of STF-F-N during adsorption. 3.4. Adsorption Kinetics. The kinetics of adsorption that describes the solute uptake rate governing the contact time of the adsorption reaction is one of the important characteristics that define the efficiency of adsorption. Pseudo-first-order37 and pseudo-second-order38 equations were used to test the experimental data and thus elucidate the adsorption kinetic process.

Figure 6. Adsorption kinetics of X-3B onto STF-F-N: (a) pseudo-firstorder model, (b) pseudo-second-order model (C0 = 30 mg/L, STF-F-N dosage = 0.3 g/100 mL, 0.4 g/100 mL, and 0.5 g/100 mL, equilibration time = 120 min).

The pseudo-first-order model is expressed as: logðqe - qt Þ ¼ log qe -

k1 t 2:303

ð9Þ

where qt and qe are the amount adsorbed at time t and at equilibrium (mg/g), and k1 is the pseudo-first-order rate constant for the adsorption process (min-1). The pseudo-second-order model can be represented in the following form: t 1 1 ¼ þ t qt k2 q2e qe

ð10Þ

where k2 is the pseudo-second-order rate constant (g/mg min). The plots of log(qe - qt) versus t and the plots of t/qt versus t for different initial dye concentrations are shown in Figure 6. Kinetic constants calculated from the slope and intercept of plots for the two models are given in Table 5. It can be observed that the pseudo-second-order model fits the data very well and provided better correlation coefficients than did the pseudo-first-order model, suggesting the pseudo-secondorder model was more suitable to describe the adsorption kinetics of X-3B onto STF-F-N. The results supported the model’s assumption that the adsorption is mainly due to chemisorption, which is a monolayer adsorption.39 Moreover, the calculated qe (cal) values depending on the pseudo-second-order model were in good agreement with the experimental values qe (exp). The adsorption rate is one of the important kinetic parameters. As shown in Table 5, the values of k1 and k2 increased with increasing 5713



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Table 5. Kinetic Parameters for the Adsorption of X-3B onto STF-F-N at Different Dosages pseudo-first-order k1 (min-1)

qe, cal (mg/g)

pseudo-second-order R2

k2 (g/mg min)

qe, cal (mg/g)

R2

STF-F-N dose (g/L)

qe, exp (mg/g)

3

7.97 ( 0.36

0.0408

6.34

0.9916

0.0143

8.58

0.9960

4

6.44 ( 0.40

0.0416

4.77

0.9781

0.0279

6.74

0.9985

5

5.26 ( 0.38

0.0497

4.00

0.9812

0.0282

5.55

0.9978

Figure 7. Typical SEM images of TiO2 fiber: (a,b) STF and (c,d) STFF-N.

STF-F-N dosage. This might be attributed to the increased adsorption active sites on the surface with increasing STF-F-N dosage, so it is possible that the adsorption rate is higher when there are more active sites on the surface. A conclusion can be drawn from the kinetics study that the structures and the charge distributions on the surface of STF-F-N significantly affect the adsorption kinetics of X-3B on STF-F-N. 3.5. Morphology and Phase Structures. To investigate the morphology of the obtained samples, a comparison between the SEM images of the STF and STF-F-N is illustrated in Figure 7. As shown in our previous report, the surface of the STF photocatalytic fiber before etching with HF was relatively smooth with uniform diameters about 10-20 μm and average length about 1.1 m.16 In Figure 7c, parts of the TiO2 layer were removed from the core structure after immersion in the HF solution. The SEM images of the magnified surfaces STF and STF-F-N in Figure 7b and d show clearly that the STF samples treated by HF and HNO3 generate more abundant pores. The advantages of porous photocatalysts mainly lie in two aspects: (1) a high density of active centers for photocatalytic reactions, and (2) an enhanced light harvesting because of light reflection and scattering by the pores.40 An improvement in adsorptivity for reactants could be expected due to such a unique surface structure. Nitrogen adsorption-desorption isotherms were measured to determine the specific surface areas and pore size distributions of the STF and STF-F-N samples (see Figure 8). The adsorption isotherms (Figure 8a) corresponding to the STF and STF-F-N are both of type IV (IUPAC classification,

Figure 8. Adsorption isotherms (a) and distributions of pore size diameter (b) of STF and STF-F-N.

International Union of Pure and Applied Chemistry),41 as indicated by a hysteresis loop at high relative pressures associated with capillary condensation of gases within the mesopores (2-50 nm). The hysteresis loop is of type H2 (IUPAC classification),41 which is consistent with pores with narrow necks and wider bodies (ink-bottle pores). As compared to STF, the adsorption and desorption branches of STF-F-N are steeper, indicating a more regular mesoporous structure. Table 6 also shows quantitative details on BET surface area, pore volume, porosity, and average pore size of STF and STF-F-N. It can be seen that STF exhibited a high BET surface area of 127.7 ( 5.7 m2/g, and treatment by HF hardly changed the surface area (132.3 ( 6.2 m2/g), but the pore volume increased from 0.25 ( 0.018 to 0.33 ( 0.020 cm3/g. The large surface area and pore volume could enhance the rate of photocatalytic reaction due to highly efficient adsorption of reactants and rapid diffusion of various products. The pore size distribution (Figure 8b) of STF-F-N shows one narrow peak in the range of 6-10 nm, and one broad distribution at 12-27 nm. These facts are attributed to leaching out of part of the Si and amorphous TiO2 by low concentration HF, causing widening of 5714



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Table 6. Physical Data for STF and STF-F-N samples

BET surface area (m2/g)

pore volume (cm3/g)

mean pore diameter (nm)

pore type

STF

127.7 ( 5.7

0.25 ( 0.018

8.3 ( 1.5

ink bottle shape

STF-F-N

132.3 ( 6.2

0.33 ( 0.020

7.0 ( 1.2 and 20.8 ( 2.1

ink bottle shape

Figure 9. XRD patterns of TiO2 fibers.

Figure 10. XPS spectra of all elements on the surface of TiO2 samples.

existing pores and creation of new pores on the surface of STF. The hierarchical structure is favorable for dye molecules and degradation intermediates to access the pores and to interact with TiO2. XRD was used to investigate the changes of phase structure of TiO2 samples modified by different methods (Figure 9). It can be seen that the diffraction peaks of all samples are ascribed to anatase diffraction peaks of (101), (004), (200), (105), and (204) at 25.4°, 37.9°, 47.8°, 55.1°, and 62.9° (JCPDS no. 21-1272). After surface modification, no new diffraction peaks appeared and the positions of the characteristic peaks of anatase TiO2 also remained unchanged, indicating that the etching of STF by 1.5% HF and washing with 5% HNO3 under the present conditions cannot change the crystalline phase and that F- is not incorporated into the bulk TiO2 particles. Structural analysis of STF after different surface modification processes suggests the crystallite size of all samples ranges from 9.9 to 10.3 nm. Although TiO2 can be transformed to TiOF2 by treatment with a high concentration of aqueous HF (50%),42 HF at low concentration (1.5%) in the present experiment seems to etch only the surface of anatase TiO2.

Figure 11. Binding energy of (a) oxygen, (b) fluorine, and (c) nitrogen in the samples.

3.6. Chemical Composition. The XPS technique was further employed to make clear the effect of acid treatment on the surface of STF. The XPS spectra (Figure 10) for STF, STF-F-N, and STF-F-B show peaks of binding energy (BE) for Ti 2p, O 1s, F 1s, N 1s, Na 1 s, and C 1s. All of the samples contain Ti, O, and C elements. The C element can be ascribed to the adventitious hydrocarbon from the XPS instrument itself. After etching by 1.5% HF, no peaks of Si were detected in both the STF-F-N and 5715



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the STF-F-B, which further confirmed the Si exposed on the surface of TiO2 was dissolved by HF. For the high-resolution spectra of STF-F-N, The O (1s) spectrum (Figure 11a) is composed of two peaks whose binding energies are 530.3 eV (Ti-O) and 532.0 eV (Ti-OH).43 It can be seen that the surface hydroxyl content after HF and HNO3 treatment was higher than that before etching and washing, and the fraction of oxygen in OH- increased from 23.43% to 29.44%, which confirms that the TiO2 surface is significantly hydroxylated and protonated by the acid treatment.43 It is generally considered that there are three interaction modes between F- and TiO2: (1) F- dissociated at the surface of TiO2; (2) F- adsorbed on the surface of TiO2; or (3) F- incorporated into the TiO2 lattice. As shown in Figure 11b, the high-resolution F 1s XPS spectrum of STF-F-N was symmetric, and only one F 1s peak was observed at 684.4 eV, which is attributed to F- adsorbed on the surface of TiO2 (physically adsorbed F- or F- that replaces surface hydroxyl groups).23,44 However, the absence of F in the crystal lattice (BE = 688-689 eV) implies the oxygen in TiO2 lattice is not substituted by F. When the fluorinated TiO2 was further washed with 1 M NaOH solution, the peak of F 1s disappeared in STF-F-B. It has been reported that the surfaceexchanged F- on TiO2 can be easily replaced by OH- at alkaline pH’s.20,23,45 Considering that the XRD and XPS techniques reflect the characteristics of bulk materials and surfaces, respectively, it is reasonable to propose that, under the moderate conditions in our study, the effect of HF etching on the structure of TiO2 results in F- adsorption on the surface of TiO2. As shown in Figure 11c, the high-resolution N 1s XPS spectra of STF-F-N were asymmetric, which was derived from the presence of two different environments for the N element. The main contribution is attributed to nitrogen in surface nitrate ion groups with a XPS peak at 398.4 eV, and the other is lattice nitrogen (Ti-N-Ti) with a peak at around 394.8 eV, which is ascribed to lattice oxygen substituted by nitrogen via HNO3 treatment. The substitutional doping of N was the most effective because its p states contribute to the band gap narrowing by mixing with O 2p states.46 The presence of doped nitrogen could extend the optical absorption of TiO2 to the visible light spectrum and enhance the visible light driven photocatalysis. However, a semiquantitative XPS analysis shows that only 5.6% of the total nitrogen peak is bound to titanium (Ti-N-Ti), which means that the proportion of nitrogen at the surface of STF-F-N is largely in the form of NO3-. The FT-IR spectra of TiO2 samples with different compositions are shown in Figure 12. The band at 1080 cm-1 was attributed to the asymmetric stretching vibrations of the Si-O-Si bond.47 However, for the fluorinated TiO2 system, the stretching vibration band at 1080 cm-1 tended to disappear. This fact suggests again the Si was partly dissolved by HF: SiO2 þ HF f SiF4 v þ H2 O

ð11Þ

It is noteworthy that in all HF etching samples, a broad absorption spread over the 3550-3000 cm-1 range was observed. The set of signals was due to the undissociated water molecules (H-bond νOH) coordinatively adsorbed on surface Ti4þ ions.48 Such a large amount of adsorbed water molecules may be ascribed to an increase in the number of Ti4þ surface centers with a Lewis acid strength high enough to render the coordinative interaction with adsorbed water molecules resistant to the outgassing. The increase in the Lewis acid

Figure 12. FT-IR spectra of TiO2 samples.

strength should be ascribed to the inductive effect on surface Ti4þ centers originating from adjoining electronegative F- ions, as proposed in the case of fluorine-doped TiO2 powders,43 TiO2 fluorinated films,49 and AlF3.50 The increased hydrophobicity of the STF-F-N surface enhances the amount of adsorbed water, thus increasing the concentration of •OH radicals formed during illumination. A band at ca. 1621 cm-1 (Ti-O-H bending mode) with minor features was attributed to δOH experiencing a stronger coordinative interaction with Ti4þ centers.48 The increase of the peak in intensity (about 5.9%) at 1621 cm-1 supports that the surface of STF-F-N is hydroxylated and protonated during the acid treatment. This is in good agreement with XPS data in Figure 11a and previous research by Wang et al.51 The peak at 1356 cm-1 is associated with the presence of a NO3- group.52 The FT-IR spectrum shows that a large number of NO3- groups as well as protons are adsorbed on the TiO2 surface during HNO3 treatment. These facts further confirm that the combination of HF and HNO3 could optimize both the physical and the chemical properties of the surface of TiO2. After dark adsorption of the dyes for 2 h, the peaks of NO3and -OH decrease in intensity or disappear, indicating the functional groups on the surface of STF-F-N have undergone substantial changes. The characteristic color of X-3B is mainly attributed to the Ar-NdN-Ar conjugated chromophoric group; however, there was no absorption peak of X-3B in the IR spectra (top spectrum in Figure 12), possibly because the amount of dye is too low to detect. The enhancement in X-3B adsorption has been suggested to result from surface protonation.51,53 The acid treatment causes the surface of TiO2 fibers to become positively charged. The electrostatic attraction between the negatively charged end of the dye molecules and the positively charged TiO2 surface then strengthens their bond and increases the number of dye molecules adsorbed on the photocatalyst. We note that the protonation effect does not fully explain the adsorption performance in Figure 4. In this study, newly formed surface nitrate and hydroxyl groups also form strong bonds with the TiO2 surface, and the peak of NO3groups still remains after the HNO3 treated STF is soaked in the dye solution. The nitrate and hydroxyl ions on the surface of STF-F-N can exchange with sulfonic group and phenols of the dye matrix, which is another important factor to improve dye adsorption. However, some of the NO3- molecules can still stay on a significant portion of the surface of STF-F-N where the bulky dye molecules do not fit well. The specific 5716



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Scheme 2. Photocatalytic Mechanism of STF-F-N under Visible Irradiation

Figure 13. Photocatalytic degradation of dyes on TiO2 samples under visible light: (a) X-3B on STF, (b) X-3B on STF-F-N, (c) MG on STF, (d) MG on STF-F-N, and (e) the mineralization rate of X-3B on STF-F-N.

Figure 14. UV-vis diffuse reflectance spectra of TiO2 samples.

surface area obtained from BET experiments for STF-F-N is 132.3 ( 6.2 m2/g; from the value of qm, a surface concentration of 2.34 10-7 mol/m2 is estimated. Assuming full coverage of the TiO2 surface, this corresponds to an area of 7.1 nm2 per X-3B and indicates that a wide surface of the TiO2 was occupied by X-3B, while that of a single NO3- ion is only a few square angstroms. 3.7. Photocatalytic Activity under Visible Light and Possible Mechanism. The photocatalytic performance of the STF and STF-F-N is shown in Figure 13. In the STF system, only ca. 32 ( 2.1% and 7 ( 1.7% of MG and X-3B are degraded after the 120 min illumination period (kapp = 0.0025 and 0.00041 min-1, respectively). These results strongly point to the need for prior adsorption of dyes on the TiO2 fiber surface for efficient degradation. As discussed above, surface acid treatment can markedly change the surface charge and functional groups of TiO2, and consequently vary the adsorption strength and sites. The increase in both the adsorption amount and the adsorption strength for anionic dye X-3B on STF-F-N (Table 3) can promote the electron transfer from the excited dyes to STF-F-N so that the decomposition of X-3B was almost complete in 45 min. The rate constant increased to 0.067 min-1, which is almost equivalent to that under UV illumination (0.081 min-1). It increased by a factor of 163.4 as compared to the STF system. Meanwhile, the TOC (total organic carbon) decreased from 30.04 to 10.74 mg/L, corresponding to a mineralization rate as high as 64 ( 2.0%, which indicates the

occurrence of radical reactions that degrade X-3B. The retarded degradation of cationic dye MG on STF-F-N can be attributed to the lower adsorption rate (degradation efficiency is 31 ( 2.0% in 120 min, and rate constant is 0.0026 min-1). Park and Choi23 have proposed that the surface tTi-F group acts as an electron-trapping site due to the strong electronegativity of fluorine. Accordingly, the driving force for the electron recombination from the conduction band to the formed dye cationic radicals would also decrease. Both changes tend to enhance the electron separation. Wang et al.51 suggested that surface protonation, induced by acid treatment, leads to a positive shift of a TiO2 flat band, which can increase the driving force for electron injection from the excited state of the dye to the conduction band. These are probably reasons underlying the promotion of the degradation of dyes on STF-F-N. After dark adsorption equilibrium for X-3B (30 mg/L) onto STF-F-N (0.4 g/100 mL) for 15 min, the samples were filtered and dried at 378 K for 2 h. The UV-vis diffuse reflectance spectra (DRS) of the obtained samples were measured as shown in Figure 14. The absorption onset wavelength of STF-F-N is in the range 280-330 nm, which shows poor absorption in the visible region. It is worth noting that the STF-F-NþX-3B sample exhibits an additional broad absorption for the region of 420650 nm as compared to the STF-F-N. This has been suggested to result from surface protonation and ion exchange to enhance X-3B adsorption on the surface of STF-F-N. Because TiO2assisted photocatalytic degradation of dyes under visible irradiation requires the direct interaction between the dyes and the surface of TiO2 to achieve efficient charge injection, the increase in both the adsorption amount and the adsorption strength for anionic dye X-3B on STF-F-N (Table 3) can promote the electron transfer from the excited dyes (denoted as dye*) to TiO2. This is pictorially illustrated in Scheme 2.54 Electron injection at the TiO2 surface is on a subnanosecond time scale, while electron back-transfer from the conduction band to the dye radical occurs at a rate several orders of magnitude slower than the forward charge injection. Such a rapid electron injection offers more chance for conduction band transport of the injected electrons to surface reaction sites and for the oxidized dyes to react. In turn, the injected electron on the STF-F-N is captured by oxygen molecules dissolved in the suspension to yield the superoxide radical anion •O2- and hydrogen peroxide H2O2. The newly formed intermediates can interact to produce hydroxyl radical •OH, which is a powerful oxidizing agent to degrade X-3B. 5717



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The process may be described in detail by the following equations: dye þ hv f dye

ð12Þ

dye þ TiO2 f •dyeþ þ e-

ð13Þ

O2 þ e- f •O2 -

ð14Þ

•O2 - þ Hþ f •OOH

ð15Þ

•OOH þ •O2 - þ Hþ f O2 þ H2 O2

ð16Þ

H2 O2 þ •O2 - f •OH þ OH- þ O2

ð17Þ

•dyeþ þ ð•OH, •O2 - , and=or O2 Þ f degraded products ð18Þ

4. CONCLUSIONS Si-doped TiO2 fiber modified with HF and HNO3 hardly changed the crystalline phase but significantly influenced the surface properties of TiO2. The surface of TiO2 fibers was hydroxylated and protonated during the acid treatment. Hierarchically structured anatase fibers with high specific surface areas (132.3 m2/g) facilitate mass transfer into and out of TiO2. Both F- and NO3- were adsorbed on the surface of TiO2. Positively charged STF-F-N influenced the adsorption capacity of the composite toward X-3B, thus affecting the degradation rate of dye molecules on the surface of the catalyst. The rate of adsorption was much higher for X-3B as compared to MG. The Langmuir isotherm model provided the best fits to predict the adsorption equilibrium for the dyes onto the catalysts. Thermodynamic study showed that the adsorption process was spontaneous and exothermic because ΔG and ΔH were both negative. The kinetic study confirmed that the adsorption followed the pseudo-second-order model. It is proposed that the X-3B can be rapidly degraded on the TiO2 fiber surface under visible light illumination after acid treatment because of the enhanced adsorption and it involves electron injection by photosensitized TiO2. ’ AUTHOR INFORMATION Corresponding Author

*Tel.: þ86 13645418916. Fax: þ86 53188364513. E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (50908133). We thank Dr. Pamela Holt for proofreading the manuscript. ’ REFERENCES (1) Robinson, T.; McMullan, G.; Marchant, R.; Nigam, P. Bioresour. Technol. 2001, 77, 247–255. (2) Legrini, O.; Oliveros, E.; Braun, A. M. Chem. Rev. 1993, 93, 671–698. (3) Fox, M. A.; Dulay, M. T. Chem. Rev. 1993, 93, 341–357. (4) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem. Rev. 1995, 95, 69–96.

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Adsorption of Dyes on Hierarchical Mesoporous TiO2 Fibers and Its

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