Article pubs.acs.org/IECR
Visible−Light−Induced Photodegradation of Rhodamine B over Hierarchical TiO2: Effects of Storage Period and Water-Mediated Adsorption Switch Lun Pan,† Ji-Jun Zou,*,† Xin-Yu Liu,† Xiao-Jing Liu,† Songbo Wang,† Xiangwen Zhang,† and Li Wang† †
Key Laboratory for Green Chemical Technology of the Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China ABSTRACT: Hierarchical TiO2 was prepared via hydrolysis method and characterized by XRD, N2 adsorption−desorption, UV−vis diffusion, and NH3−TPD. With the increase of calcination temperature, the surface area and surface hydroxyls of prepared materials decrease rapidly. The photoactivity was evaluated using the self-sensitized photodegradation of rhodamine B under visible light. The fresh samples show higher activity than P-25 due to higher surface area and more effective light utilization. After stored in air for ca. half a year, the materials show significantly increased photoactivity, due to the prebonding of water on surface bridging hydroxyls, which induces water-mediated adsorption switch from covalent mode to electrostatic one. Water treatment further promotes the photoactivity of stored samples, because more water are bonded on TiO2 surface. It is found that the water-mediated effect closely depends on the surface area and amount of surface bridging hydroxyls. cationic−dye sensitization of TiO2.12 This so-called watermediated function opens a door toward facilely promoting the photocatalytic efficiency. Also, the primary work gives many important hints: (1) high surface area and abundant bridging hydroxyls would be favorable for the prebonding of water and (2) the storage period may show great influence on the performance of photocatalyst because water will be bonded gradually during this period. In this work, we prepared hierarchical macro-/mesoporous TiO2 (HMMT) with the aim to increase the surface area for dye adsorption and enhance light transmission depth. The photoactivity of fresh, stored, and water-treated HMMTs were evaluated via the self-sensitized photodegradation of RhB (a widely used representative cationic dye) to determine the effects of storage period and water-mediated adsorption switch. It is found that the combination of hierarchical structure and water-mediated adsorption switch gives significantly high photoactivity. Moreover, the storage period has considerable influence on the photoreaction.
1. INTRODUCTION Utilization of TiO2 under solar light has drawn great attention in the fields of photocatalysis, water splitting, solar cell, environmental remediation, and so on,1−11 and dye sensitization is a facile and effective method.12−21 The core of sensitization is the electron transfer (ET) from excited organic dye to conduction band (CB) of TiO2.2,4,5,19 Take the rhodamine B (RhB)/TiO2 system as an example (see Scheme 1a): RhB (+0.95 V vs NHE) is excited to RhB* (−1.42 V vs NHE)20 when irradiated by visible light, then RhB* injects electrons into the CB of TiO2 (−0.5 V vs NHE),21 subsequently the photocatalytic process occurs on TiO2 surface. For photodegradation, the electrons reduce the surfaceadsorbed oxygen into ·O2−, which induces the formation of oxidized species such as ·OOH and ·OH that are essential for photodegradation.12−19 The adsorption of dye on TiO2 surface is the prerequisite for the ET process.19 Many factors (e.g., hydratation and hydrophilicity) were reported to affect the adsorption mode and thus significantly influence the ET efficiency.19,22−24 There are several adsorption modes of dye on TiO2: covalent attachment, electrostatic interactions, and so on.19 Over clean TiO2, RhB is anchored on the surface in the form of monodentate linkage via the carboxyl group (Et2N=RnCOO···Ti, adsorption mode I, see Scheme 1b).14,15 Over fluorinated TiO2, the adsorption mode is switched to electrostatic interaction (Rn=N+Et2···F−, mode II).13,16 Preadsorption of anionic dodecylbenzenesulfonate (DBS) on the TiO 2 surface also induces such kind of adsorption (Rn=N+Et2···DBS−, mode III).17,18 The electrostatic interaction is believed to lead to more rapid photodegradation than the monodentate linkage under visible light.12−19 Recently, we found that prebonding of water on the surface bridging hydroxyls (OHbr) can modulate the surface electronic structure of TiO2, induce electrostatic adsorption mode (Rn=N+Et2···Obr−, mode IV), and significantly improve the © 2012 American Chemical Society
2. EXPERIMENTAL SECTION 2.1. Sample Preparation. HMMTs were synthesized via a direct hydrolysis using tetrabutyl titanate (TBT) as Ti source.25,26 10 mL of TBT was added dropwise in 150 mL of deionized water without stirring. After aging at room temperature for 3 days, the white participates were filtered, washed with deionized water, and dried at 80 °C. The obtained powders were named as T80, which was then calcined at 200, 300, 400, 500, 600, and 700 °C for 2 h at a heating rate of 2 °C min−1, namely T200, T300, T400, T500, T600, and T700, respectively. Received: Revised: Accepted: Published: 12782
July 18, 2012 August 31, 2012 September 6, 2012 September 6, 2012 dx.doi.org/10.1021/ie3019033 | Ind. Eng. Chem. Res. 2012, 51, 12782−12786
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surface hydroxyl groups were analyzed by the temperatureprogrammed desorption of ammonia (NH3−TPD), conducted on a Quantachrome ChemBET TPR/TPD apparatus. 2.3. Visible−Light−Induced Photodegradation of RhB. Self-sensitized photodegradation of RhB was conducted in a closed quartz chamber (150 mL) vertically irradiated by a 300 W high-pressure xenon lamp (PLS-SXE300UV, Beijing Trusttech. Co. Ltd.) located on the upper position. Visible light (>400 nm, 13.0 ± 0.5 mW cm−2 at 420 nm) were separated by UV−cut optical filter (Beijing Trusttech. Co. Ltd.). The irradiation area was about 20 cm2. Reaction conditions: temperature: 20 ± 2 °C; C0 (RhB) = 20 μmol L −1 ; photocatalyst: 0.3 g L−1; no acid or alkaline reagents added. After stirring for 20 min in black to achieve adsorption equilibrium, the reaction was conducted by magnetic stirring under atmosphere. Samples were withdrawn, centrifuged, and analyzed using a Hitachi U-3010 UV−vis spectrometer. The degradation rate was detected by monitoring the absorption intensity of reactant solution at 553 nm, and the data were fitted using a pseudo-first-order kinetic equation to calculate the reaction rate constant (k).
Scheme 1. Sensitization Process between RhB and TiO2 (a) and Adsorption Mode of RhB on TiO2 Surface Modified by Fluorination, Anionic Surfactant Treatment, and Water Bonding (b)
3. RESULTS AND DISCUSSION 3.1. Textual Structure. XRD patterns of HMMTs are shown in Figure 1a, along with the derived structural parameters listed in Table 1. T80-T500 samples all show Table 1. Textual Structures of HMMTs Derived from XRD Patterns and N2 Adsorption−Desorption Isotherms crystal phasea samples
2.2. Characterization. The crystal structures of the prepared samples were assessed using a Rigaku D/max 2500v/pc X-ray diffractometer (XRD) equipped with a Cu Kα radiation source. The crystal phase content of TiO2 and crystal size of each phase were calculated according to peak fitting and Scherrer equation, respectively.27,28 The pore structures were evaluated through N2 adsorption−desorption isotherms at −196 °C on a Micromeritics TriStar 3000. The surface area and pore distribution were calculated using the BET and BJH methods, respectively. SEM images were obtained using a Hitachi S-4800 microscope. UV−vis diffuse reflectance spectra (UV−vis DRS) were recorded with a Hitachi U-3010 spectrometer equipped with a 60 mm diameter integrating sphere using BaSO4 as the reflectance sample. The
T80 T200 T300 T400 T500 T600 T700 a
content (%) (A)78.7/(B) 21.3 (A)76.9/(B) 23.1 (A)77.8/(B) 22.2 (A)77.2/(B) 22.8 (A)76.1/(B) 23.9 (A)63.2/(R) 36.8 (R)100.0
size (nm)
SBET (m2 g−1)
average pore size (nm)
(A)4.3/(B)3.3
351.0
3.32
(A)4.2/(B)2.8
207.1
3.58
(A)6.2/(B)6.1
162.7
4.21
(A)8.6/(B)8.0
109.7
5.53
(A)16.2/(B) 13.3 (A)43.9/(R) >100 (R)>100
55.0
7.40
11.1
11.15 NDb
0.078 b
Abbreviations: A, anatase; B, brookite; R, rutile. ND: not detected.
Figure 1. XRD patterns (a), N2 adsorption−desorption isotherms (b), and corresponding pore size distribution (c) of HMMTs. 12783
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Figure 2. SEM images of T80 and T600.
biphase of anatase and brookite with small grain size, while T600 possesses anatase and rutile, and only the rutile phase exists for T700 with large crystal size (>100 nm), indicating that the calcination causes crystal growth and phase transformation from anatase and brookite to rutile. The N2 adsorption−desorption isotherms and pore size distribution (Figure 1b,c) indicate the existence of mesopore in the prepared samples. The calculated BET surface area (SBET) and average pore size are shown in Table 1. With the increase of calcination temperature (from 80 to 700 °C), the SBET is significantly decreased and the pore size is greatly enlarged, informing the collapse of mesoporous channels. Macroporous structure is also present in all samples, see Figure 2. HMMTs possess a spongelike porous structure (ca. 500−800 nm) composed of randomly intercrossed nanosticks and pores, similar to the hierarchical structures of lotus leaf. Such a structure was reported to effectively increase the surface area for adsorption and enhance the light transmission depth.25,26 The UV−vis diffuse reflectance spectra of prepared HMMTs are exhibited in Figure 3. T80-T500 samples show identical
distinguish bridging (OHbr) and terminal hydroxyls (OHt), it can qualitatively represent the relative amount of OHbr over different samples. It should be noted that our previous in situ FT−IR characterization indicates that water removing or rebonding cannot affect the surface hydroxyl groups of TiO2.12 So the fresh, stored, and water-treated samples (see below) should possess the same amount of hydroxyls. Figure 4
Figure 4. NH3−TPD profiles of HMMTs.
shows the NH3−TPD profiles of the prepared HMMTs. All samples show maximum signal at ca. 350 and 400 °C, corresponding to the removal of NH3 adsorbed on surface hydroxyls.29 Also, it is noted that the amount of OHsurf decreases quickly with the calcination temperature, due to the decrease of surface area and oxygen vacancy. 3.3. Photocatalytic Performance. For each photocatalyst, three kinds of sample were evaluated in this work, see Figure 5. One is the as-prepared fresh sample with clean surface. The second is the stored sample being stored in a desiccator for ca. half a year. During this period, some water molecules in the atmosphere are bonded on OHbr. The third is the water-treated one with the stored sample being stirred in deionized water for 12 h to make more water molecules bonded. The mineralization process, including stepwise N-de-ethylation and cycloreversion, can be examined by UV−vis absorption spectra.17,18,30,31 As shown in Figure 6b, the final solution shows nearly no absorption in the whole UV−vis range, indicating that RhB can be mineralized completely. As discussed above, different adsorption modes of RhB exist over different surfaces. For the fresh HMMTs, the photo-
Figure 3. UV−vis diffuse reflectance spectra of HMMTs.
optical absorption (band gap of 3.2 eV), but the red−shift happens for T600 and T700 due to the formation of rutile phase (band gap of 3.0 eV),2 consistent with the XRD results. 3.2. Surface Hydroxyl Groups. The amount of surface hydroxyls (OH surf ) on the as-prepared HMMTs was determined by NH3−TPD according to the method reported by Carneiro et al.29 Although this technique does not 12784
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After storage for ca. half a year, the photodegradation rate is increased by 120%, 58%, 54%, 17%, 15%, and 12% for T80T600, respectively. This indicates that the bonding of water on OHbr occurs even when the materials are exposed to relatively dry atmosphere. So some electrostatic adsorption modes (mode IV in Scheme 1b) are formed over the stored HMMTs, which can promote the photodegradation considerably. The effect of water-mediated function decreases from T80 to T600, closely related to the decrease of SBET and surface bridging hydroxyls. The bonding of water is very slow during the storage. To make more water bonded on the TiO2 surface, the stored samples were further stirred in water for 12 h before the photocatalytic test. As expected, significantly high efficiency is obtained. The activity of water-treated T80-T600 is increased by 260%, 146%, 144%, 84%, 68%, and 46% in comparison with the fresh counterparts, respectively. Especially, the reaction rates over water-treated T80-T400 are much higher than that of water-treated P-25 (9.4 × 10−3 min−1).12 The order of watermediated function also agrees with that of SBET and OHbr amount. Moreover, this may not be the end point for some samples. Prolonging the treatment time can further increase the photodegradation rate, see Figure 6. After water treatment of 36 h, the reaction rate over T200 is increased for ca. 2.6 times. However, care should be taken when extending the treatment time, because the particles tend to agglomerate with each other and decrease the surface area, which counteracts the watermediated function.12 This explains why T700 shows decreased activity after water treatment.
Figure 5. Photodegradation rate of RhB under visible light over HMMTs.
4. CONCLUSIONS Hierarchical macro-/mesoporous TiO2 synthesized by direct hydrolysis possesses a high surface area and a large amount of surface hydroxyls, which are gradually reduced with the increase of calcination temperature. Storage and water treatment induce the prebonding of water on surface bridging hydroxyls and switch the adsorption mode of RhB from covalent mode to electrostatic one. As a result, in the self-sensitized degradation of RhB under visible light, the stored sample shows considerably higher photoactivity than the fresh counterpart, and the water-treatment can further increase the degradation rate significantly. The effect of the water-mediated function is closely related to the SBET and amount of surface bridging hydroxyls, which are affected by the calcination temperature. This work shows that the combination of hierarchical structure and water-mediated adsorption switch can obtain very high photoactivity. In addition, the storage period and environment have considerable influence on the photocatalytic performance of TiO2, so care must be taken when evaluating the inherent performance of a photocatalyst.
Figure 6. Visible−light−induced photodegradation of RhB over stored T200 with different water-treated time: (a) pseudo-first-order kinetic fits based on absorption intensity at 553 nm and (b) absorption spectra (200−650 nm) of RhB solution in the presence of stored T200 with water treatment for 0 and 36 h.
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degradation happens through self-sensitization via the covalent adsorption mode (mode I in Scheme 1b). The textual structure is the dominant factor determining the photodegradation efficiency. With the increase of calcination temperature, the photodegradation rate decreases quickly, consistent with the decline of SBET. It is also noted that the HMMTs (T80-T400) show higher photoactivity than P-25 (SBET: 47.1 m2 g−1; photodegradation rate: 6.2 × 10−3 min−1)12 under identical conditions. This shows the advantages of hierarchical macro-/ mesoporous structure: high surface area and more efficient light utilization.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors appreciate the support from the National Natural Science Foundation of China (21222607, 20906069), the Foundation for the Author of National Excellent Doctoral 12785
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Dissertation of China (200955), and the Program for New Century Excellent Talents in Universities (NCET-09-0594).
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