Hydroxyproline Functionalization and Enhanced Photocatalytic Activity

Jul 2, 2008 - Materials, Xuchang UniVersity, Xuchang 461000, People's Republic of China, and State Key Laboratory of. Magnetic Resonance and Atomic ...
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J. Phys. Chem. C 2008, 112, 11379–11384

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In Situ L-Hydroxyproline Functionalization and Enhanced Photocatalytic Activity of TiO2 Nanorods Huimin Jia,†,‡ Wen-Jing Xiao,† Lizhi Zhang,†,* Zhi Zheng,‡,* Hailu Zhang,§ and Feng Deng§ Key Laboratory of Pesticide & Chemical Biology of Ministry of Education, College of Chemistry, Central China Normal UniVersity, Wuhan 430079, People’s Republic of China, Institute of Surface Micro and Nano Materials, Xuchang UniVersity, Xuchang 461000, People’s Republic of China, and State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, People’s Republic of China ReceiVed: April 7, 2008; ReVised Manuscript ReceiVed: May 15, 2008

In this study, L-hydroxyproline-functionalized anatase TiO2 nanorods were prepared by a nonaqueous sol-gel route at 80 °C. The resulting samples were characterized by XRD, TEM, HRTEM, XPS, nitrogen adsorption, and UV-vis diffuse reflectance spectroscopy. The presence of L-hydroxyproline on the TiO2 surface was confirmed by the analysis of FT-IR and 13C NMR. It was found that the functionalization of L-hydroxyproline could significantly enhance the photocatalytic activity of TiO2 on the degradation of rhodamine B in aqueous solution under simulated solar light. The reasons for photocatalytic activity enhancement were analyzed on the basis of characterizations. This study provides an alternative functionalization method to improve the photocatalytic activity of TiO2. Meanwhile, the modification of L-hydroxyproline on TiO2 is expected to find new applications in fields of organic catalysis, foodstuffs, cosmetics, and so on. 1. Introduction In the past three decades, nanosized titania has received much research attention because of its versatility in optical, electrical, and photochemical properties. It has been applied in highrefractive optics, oxide semiconductors, oxygen sensors, photovoltaics, and photocatalyst for environment cleanup.1–9 However, the photocatalytic activity of TiO2 is still not high enough for practical applications. Many methods have been developed to enhance the photocatalytic activity of TiO2. These methods include surface functionalization, metal or nonmetal ion doping, small band gap semiconductor coupling, and so on. For example, dye-sensitization is one of the widely used postfunctionalization methods to enhance TiO2 activity by extending TiO2 light absorption to the visible light region.10,11 Recently, different surface-modified TiO2 nanoparticles were used for the photodegradation of organic pollutants and heavy metals,12–15 and the preparation of organic-inorganic hybrid catalysts.16–20 Meanwhile, the surface modifications on TiO2 with noble metal nanoparticles or redox-inert anions were found to be able to significantly improve the photocatalytic activity of TiO2.21–28 For instance, Pelizzetti and co-workers reported that the surface fluorination of TiO2 can accelerate the photocatalytic oxidation of phenol.23 Korosi and co-workers revealed that phosphate could improve the thermal stability of the samples and increased the specific surface area and the band gap energy of TiO2.14 They also reported that the phosphate-modified TiO2 exhibited higher photocatalytic activity than Degussa P25 on ethanol oxidation in the gas phase.24 Recently, the modifications of TiO2 nanoparticles with organic chelating ligands have attracted more * To whom correspondence should be addressed. E-mail: zhanglz@ mail.ccnu.edu.cn, [email protected]. Phone/fax: +86-27-6786 7535. † Central China Normal University. ‡ Xuchang University. § Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences.

and more attention because these modifications could offer TiO2 more functions including water or oil soluble properties, and so on.29 For example, Niederberger et al. developed a nonaqueous sol-gel method to synthesize water-soluble dopaminefunctionalized anatase TiO2 nanoparticles.30 Cozzoli and coworkers prepared the organic-capped anatase TiO2 nanorods at 80 °C. They found that the functionalized TiO2 could be dispersed in organic solvents to form optically clear concentrated solutions because of their oleic acid surface modification.31 Tahir and co-workers also prepared the dopmine-functionalized rutile TiO2 nanorods via a hydrothermal method.32 L-Hydroxyproline and its derivatives are water-soluble and commercially available. They have been widely used as small molecular catalysts for the asymmetric synthesis of organic compounds.33,34 In this study, we develop a simple route to prepare TiO2 nanorods with in situ surface functionalization of L-hydroxyproline by utilizing the nonaqueous sol-gel method reported by Niederberger.30 The L-hydroxyproline functionalized TiO2 nanocrystals were prepared by a simple injection of titanium tetrachloride into a solution of L-hydroxyproline and benzyl alcohol and thus with subsequent reaction under stirring at 80 °C for 4 days. We found that the resulting L-hydroxyproline-functionalized TiO2 nanorods possessed enhanced specific surface area. More interestingly, the L-hydroxyproline-functionalized anatase TiO2 nanorods exhibited the higher photocatalytic activity on degradation of the organic dye rhodamine B (RhB) than P25 and the pure TiO2 counterpart. 2. Experimental Section 2.1. Sample Preparation. All of the chemicals were of analytical grade and were used without further purification. In a typical synthesis, 320 mg of L-hydroxyproline was dissolved in 30 mL of benzyl alcohol at room temperature. Then, 1.5 mL of titanium tetrachloride was rapidly injected into the above L-hydroxyproline and benzyl alcohol solution in a sealed vial under vigorous stirring. The sealed vial was then heated to 80

10.1021/jp803002g CCC: $40.75  2008 American Chemical Society Published on Web 07/02/2008

11380 J. Phys. Chem. C, Vol. 112, No. 30, 2008 °C under continuous stirring for 4 days. The resulting fleshcolored suspension was centrifuged and thoroughly washed with chloroform. The collected precipitate was dried in an oven at 50 °C. For comparison, pure TiO2 was also prepared by the similar method without adding the L-hydroxyproline. 2.2. Characterization. The powder X-ray diffraction (XRD) patterns were recorded on a Rigaku D/MAX-RB diffractometer with monochromatized Cu KR radiation (λ ) 1.5418 Å). A transmission electron microscopy (TEM) study was carried out on a Tecnai 20 FEG electron microscope. The samples for TEM analysis were prepared by dispersing the final powders in ethanol followed by dropping on carbon-copper grids. FT-IR spectra of the as-prepared samples were collected with a ThermoNicolet Nexus 670 Spectrometer with a resolution of 4 cm-1. Solid state 13C NMR spectra were recorded on a Varian Infinityplus-300 spectrometer at room temperature. Specific surface areas were measured by the BET method at liquid nitrogen temperature, using N2 gas as an adsorbent (Micromeritics Gemini 2380). The samples were dried at 100 °C for 24 h and then degassed at 200 °C for 2 h prior to the analysis of the surface areas. XPS measurements were performed in a VG Scientific ESCALAB Mark II spectrometer equipped with two ultrahigh vacuum (UHV) chambers. All binding energies were referenced to the C1s peak at 284.6 eV of the surface adventitious carbon. A PerkinElmer Lambda 35 UV/vis spectrometer equipped with a labsphere diffuse reflectance accessory was used to obtain the reflectance spectra of the catalysts over a range of 200-700 nm. Labsphere RSA-PE-20 was employed as a reflectance standard. 2.3. Photocatalytic Activity Test. The photocatalytic activities of the samples were evaluated by the degradation of RhB in aqueous solutions. A 500-W tungsten halogen lamp was placed inside a cylindrical vessel and surrounded by a circulating water jacket for cooling. Typically, 0.1 g of photocatalyst was suspended in a 100 mL aqueous solution of 5 mg/L RhB. The solution was continuously stirred for about 1 h to ensure the establishment of an adsorption-desorption equilibrium between the photocatalyst and RhB before irradiation. Artificial solar light from a tungsten lamp was shown on solution. The distance between the light source and the bottom of the solution was about 15 cm, and the temperature of the RhB solution stirred by a dynamoelectric stirrer in an open reactor was kept at 25 °C. The concentration of RhB was monitored by colorimetry with a U-3310 UV-vis spectrometer (HITACHI). 3. Results and Discussion 3.1. Characterizations of the Samples. The X-ray diffraction was used to characterize the phase structure of the products. Figure 1 shows XRD patterns of the as-prepared pure TiO2 (T0) and L-hydroxyproline-functionalized TiO2 (T1) samples. These patterns can be well indexed to pure tetragonal anatase (JCPDS No. 71-1169, space group I41/amd (141)). No peaks of rutile or brookite phase are detected. Both the pure and functionalized TiO2 samples are well crystallized even at reaction temperatures as low as 80 °C. By applying the Debye-Scherrer formula on the anatase (101) diffraction peaks, the average crystallite sizes of the T0 and T1 were found to be 5.6 and 4.8 nm, respectively. This suggests the L-hydroxyproline functionalization could slightly inhibit the growth of TiO2 nanocystals. The size and morphology of the products were analyzed by TEM measurements. Figure 2a shows that most of the particles connected with each other to form plenty of nanorods in functionalized TiO2. The corresponding HRTEM image of the functionalized TiO2 nanorods was shown in Figure 2b. The

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Figure 1. XRD patterns of (a) pure TiO2 (T0) and (b) functionalized TiO2 powders (T1) prepared at 80 °C.

clearly resolved lattice fringes correspond to the (101) plane of the anatase TiO2. The diameters of the nanorods are in the range of 5-6 nm, which is in good agreement with the XRD calculation results. The HRTEM image also reveals the single crystal nature of these anatase nanorods, which grow along in the c-axis direction (Figure 2b). Figure 2b also indicates the growth of nanorods is through oriented attachment. Figure 2c shows the TEM image of the pure TiO2 sample. After comparing panels a and c of Figure 2, we notice that the functionalized TiO2 nanorods possess smaller crystal sizes than pure TiO2. This result agrees with the XRD observation. The pure and functionalized TiO2 nanorods were further characterized by means of FT-IR spectroscopy. Figure 3 shows the typical FT-IR spectra of the pure and functionalized TiO2 nanorods as well as L-hydroxyproline. Because the TiO2 samples were ultrasonically cleaned before characterization, the Lhydroxyproline physically attached on the TiO2 surface was thought to be completely removed. In the region above 2000 cm-1 in parts a and b of Figure 3, the IR spectra of both pure and functionalized TiO2 samples were dominated by the broad O-H stretching peak centered at 3300 cm-1. This peak is due to titanol groups or adsorbed H2O. On Figure 3b, the peaks at 1600 and 1450 cm-1 can be assigned to the COO- asymmetric and symmetric stretching vibrations, respectively. Their frequency difference (∆ν) was about 150 cm-1, smaller than that (200 cm-1) of the pure L-hydroxyproline (Figure 3c). Therefore, the mode of binding of carboxylate modified onto the TiO2 surface might be interpreted as chelating bidentate. The peak at 1375 cm-1 is attributed to C-H bonding vibrations of the pyrrolidinyl ring. The contributions of the CH2 twist and wagging vibrations of the pyrrolidinyl ring result in two peaks at 1320 and 1060 cm-1 (Figure 3b), respectively. Meanwhile, we can observe the peaks at 1020 cm-1 assigned to CCN stretching vibrations, and the characteristic vibrations of the inorganic Ti-O-Ti network in titanium dioxide below 950 cm-1 in Figure 3b. Therefore, we conclude that L-hydroxyproline might be modified onto the surface of TiO2 nanorods. We conducted the solid state 13C NMR analysis to further confirm the functionalization of L-hydroxyproline on TiO2 nanorods. Figure 4 shows the 13C NMR spectrum of the resulting L-hydroxyproline-functionalized TiO2. The five peaks at 174.9, 69.7, 60.2, 54.0, and 38.5 ppm could be assigned to the carbon atoms of C1-C5 in the L-hydroxyproline molecules (inset of

Enhanced Photocatalytic Activity of TiO2 Nanorods

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Figure 3. FT-IR spectra of (a) pure TiO2 and (b) L-hydroxyproline functionalized TiO2 nanorods and (c) pure L-hydroxyproline.

Figure 4. Solid state 13C NMR spectra of L-hydroxyproline functionalized anatase TiO2 nanorods.

Figure 2. TEM (a) and HRTEM (b) images of functionalized TiO2 nanorods and TEM image (c) of pure TiO2.

Figure 4), while the C6 signal at 126.8 ppm was attributed to the residues of the benzyl alcohol.

The XPS characterization of the samples was also performed to identify the chemical bonding nature of the L-hydroxyproline on the TiO2 nanorods. Figure 5A shows a typical XPS survey spectrum of the functionalized TiO2, revealing that the sample surface is composed of three elements of Ti, O, and C. The C 1s core level spectrum (Figure 5B) was fitted with three peaks, which can be assigned to -C5-CH2- and adventitious elemental carbon at 284.6 eV, -C2-OH, -HN-C3-CO2H, and -HN-C4-CH2OH at 286.3 eV, and OdC1-OH at 288.7 eV, respectively. Figure 5C shows the XPS spectrum of the O 1s core level. This O 1s core level spectrum could be fitted with two peaks that are assigned to O-Ti bonds (529.8 eV) in TiO2 and O-H, O-C bonds (531.5 eV) in L-hydroxyproline, respectively. The XPS Ti 2p core level (Figure 5D) was observed at binding energies of around 458.5 eV (Ti 2p3/2) and 464.3 eV (Ti 2p1/2), in good agreement with that in pure TiO2. All these XPS results confirm that the L-hydroxyproline has been chemically attached to the surface of TiO2 nanorods by covalent or ionic interactions and form a Ti-O-C structure, not physical adsorption. The nitrogen adsorption-desorption isotherms of pure and functionalized TiO2 samples were also recorded and shown in Figure 6. Both the pure and the functionalized TiO2 samples exhibit the type IV isotherm with an H3 type hysteresis loop,35 suggesting the existence of mesoporous structures in the resulting materials. These mesopores should be formed by the agglomeration of TiO2 nanocrystals. The Brunauer-EmmettTeller (BET) specific surface areas of pure and functionalized

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Figure 5. XPS spectra of (A) survey spectrum, (B) C 1s, (C) O1s, and (D) Ti 2p for L-hydroxyproline functionalized TiO2 nanorods.

Figure 6. Nitrogen adsorption-desorption isotherms of (a) pure TiO2 and (b) functionalized TiO2 nanorods.

Figure 7. Plots of (Rhν)1/2 vs the energy of absorbed light and UV-vis diffuse reflectance spectra (inset) of the sample P25, pure TiO2 (T0), and functionalized TiO2 nanorods (T1).

TiO2 samples were measured to be 170 and 215 m2 g-1, respectively. Therefore, the L-hydroxyproline functionalization could enhance the surface area of TiO2 nanorods to some degree, indicating a better adsorption of functionalized TiO2 nanorods to organic molecules. The UV-vis diffuse reflectance spectra of pure and functionalized TiO2 samples as well as Degussa P25 (30% rutile and 70% anatase, surface area 54 m2 g-1) were compared (Figure 7). As TiO2 is an indirect transition semiconductor, plots of (ahV)1/2 vs the energy of absorbed light afford the band gaps

of the pure and functionalized TiO2 samples and P25. The band gaps optically obtained in such a way were approximately 3.17, 3.10, and 3.05 eV for the samples T1, T0, and P25, respectively. This reveals that the band gaps of samples T1 and T0 are slightly larger than that of P25. This is because P25 is a mixture of 30% rutile and 70% anatase and the band gap of anatase is larger than that of rutile. The larger band gap of sample T1 than that of sample T0 is attributed to the smaller crystal size of functionalized TiO2 than that of pure TiO2, as revealed by XRD and TEM observations.

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J. Phys. Chem. C, Vol. 112, No. 30, 2008 11383 TABLE 1: Summary of Physiochemical Properties of Functionalized TiO2 Nanorods, Pure TiO2, and P25

Figure 8. Photocatalytic decomposition curves of RhB by P25, pure TiO2 (T0), functionalized TiO2 nanorods (T1), and self-degradation (SD) under simulated solar light irradiation.

3.2. Photocatalytic Activity. Photocatalytic activities of the pure and functionalized TiO2 nanorod samples as well as Degussa P25 were investigated on the degradation of RhB in aqueous solution. The red color of the RhB solution gradually diminished upon the simulated solar light irradiation in the presence of photocatalysts, illustrating the degradation of RhB. Total concentrations of RhB were simply determined by the maximum absorption measurement. Under the illumination of artificial solar light from the high-pressure mercury lamp and in the presence of the titania, the photodecomposition of the RhB will proceed in both photocatalytic pathway and photosensitization pathway. Both of these processes are dependent on the photocatalytic activity of the titania.36,37 Therefore, the photodegradation of RhB in water under simulated solar light can reflect the photocatalytic activity of the titania powders. RhB shows a maximum absorption at about 555 nm. In the photodegradation process, the major absorption of the degraded solution exhibited hypsochromic shifts with irradiation time, implying the cleavage of the conjugated structure of RhB and the decomposition of a series of N-deethylated intermediates.36,38,39 Figure 8 shows the variation in adsorption of RhB at 555 nm with the irradiation time and self-degradation of RhB under simulated solar light irradiation. It was found that the RhB was slightly self-degraded under simulated solar light irradiation. This slight self-degradation should be attributed to the presence of UV light in the simulated solar light. The degradation of RhB became obvious in the presence of photocatalysts. After 4 h of light irradiation, 39.1% RhB was photodegraded by P25, while 88% and 97% of RhB were degraded on pure TiO2 (T0) and functionalized TiO2 nanorods (T1), respectively. As is shown in Figure 8, the degradation of RhB follows pseudofirst-order kinetics. The functionalized TiO2 nanorods exhibited the highest photocatalytic activity with a constant k ) 0.9983 h-1. Table 1 summarizes the physiochemical properties of functionalized TiO2 nanorods, pure TiO2, and P25. Several reasons may account for the high activity of the L-hydroxyproline-functionalized TiO2 nanorods. First, its large specific surface area could enhance the photocatalytic activity,40,41 because a larger specific surface can absorb more hydroxyl radicals and other reactive species. Second, the attached hydroxyproline groups may scavenge the photogenerated electrons in the conduction band of TiO2 to reduce the electron-hole

samples

crystal size (nm)

ABET (m2/g)

estimated band gap (eV)

degradation kinetic constant (h-1)

P25 T0 T1

20.0 5.6 4.8

54 170 215

3.05 3.10 3.17

0.1519 0.5276 0.9983

recombination. Third, the as-prepared functionalized TiO2 nanorod sample possesses a larger band gap than the pure TiO2 sample and P25. It is commonly accepted that a larger band gap corresponds to a more powerful redox ability. Because the photocatalytic process system can be considered similar to an electrochemical cell, the increase in band gap results in an enhanced oxidation-reduction potential.25 The larger band gap of L-hydroxyproline-functionalized TiO2 nanorods suggests a higher energy level of the conduction band, which could be considered helpful for the production of active oxygen radicals (e.g., O2•-, •OOH, •OH),42 and thus enhance the degradation of dye RhB. 3.3. Possible Formation Pathway of Anatase TiO2 Nanorods in Situ Functionalized with L-Hydroxyproline. We proposed a possible pathway for the formation of in situ L-hydroxyproline-functionalized TiO2 nanorods as follows. Rod formation requires anisotropic crystal growth, which is usually realized when the surface free energies of the various crystallographic planes differ significantly. Organic molecules such as surfactants can act as surface ligands to control the shape and size of the growing particles, as revealed by Banfield and co-workers.43 In our reaction system, as soon as titanium tetrachloride was injected into the L-hydroxyproline benzyl alcohol solution, it would form 6-fold coordinated [TiClm(OC6H5CH2)6-m]2- complex species and HCl. Then the [TiClm(OC6H5CH2)6-m]2- species reacted with L-hydroxyproline to form [Ti(RCO2)nClm(OC6H5CH2)6-n-m]2- or [Ti(OR′)nClm(C6H5CH2)6-n-m]2- complex species and generate Ti-O-Ti bonds by nonhydrolytic condensation and/or elimination of an ester. Here L-hydroxyproline may serve as a chelating ligand to inhibit the growth rate along some crystallographic directions. These [Ti(RCO2)nClm(OC6H5CH2)6-n-m]2- or [Ti(OR′)nClm(C6H5CH2)6-n-m]2- species typically consist of a compact Ti-O-Ti framework of hexacoordinated Ti atoms, surrounded by a hydrocarbon periphery. They can be thought of as nanotitania cores protected by carboxylate ligands. Therefore, the L-hydroxyprolines not only have the ability to control the formation of TiO2 nanorods and tune the size of TiO2 nanorods, but also act as “monomers” for the development of an extended Ti-O-Ti network. At the initial stage, the quick injection of a large amount TiCl4 precursor into benzyl alcohol could promote nonhydrolytic condensation to generate numerous primary particles. When the reaction time was extended, the primary particles would grow into nanorods through oriented attachment as revealed in Figure 2b. There is certainly a strong thermodynamic driving force for the oriented attachment, because the surface energy is reduced substantially when interface is eliminated.44 This driving force should partially originate from the L-hydroxyproline functionalization on the TiO2 surface. 4. Conclusions In this paper, TiO2 nanorods in situ functionalized with were successfully synthesized at a temperature as low as 80 °C by a simple injection of TiCl4 into the L-hydroxyproline benzyl alcohol solution. The functionalized TiO2 nanorods showed the enhanced photocatalytic activity in L-hydroxyproline

11384 J. Phys. Chem. C, Vol. 112, No. 30, 2008 comparison with the pure TiO2 counterpart. This photocatalytic activity enhancement of functionalized TiO2 nanorods could be attributed to its larger surface area, the inhabitation of electron-hole recombination, and larger band gap arising from L-hydroxyproline functionalization. Furthermore, we proposed a possible pathway for the formation of functionalized TiO2 nanorods. We believe that these L-hydroxyproline-functionalized TiO2 nanorods are promising for organic catalysis, cleaning up environmental pollutants, and so on. Acknowledgment. This work was supported by the National Basic Research Program of China (973 Program) (Grant 2007CB613301), the National Science Foundation of China (Grants 20503009, 20574058, 20673041, and 20777026), the Program for New Century Excellent Talents in University (Grant NCET-07-0352), the Key Project of Ministry of Education of China (Grant 108097), and the Youth Item Foundation of Xuchang University, China (Grant 2008107). References and Notes (1) Vogel, R.; Meredith, P.; Kartini, I.; Harvey, M.; Riches, J. D.; Bishop, A.; Heckenberg, N.; Trau, M.; Rubinsztein-Dunlop, H. ChemPhysChem 2003, 4, 595. (2) Frach, P.; Gloss, D.; Goedicke, K.; Fahland, M.; Gnehr, W. M. Thin Solid Films 2003, 445, 251. (3) Du, X. Y.; Wang, Y.; Mu, Y. Y.; Gui, L. L.; Wang, P.; Tang, Y. Q. Chem. Mater. 2002, 14, 3953. (4) Hansel, H.; Zettl, H.; Krausch, G.; Kisselev, R.; Thelakkat, M.; Schmidt, H. W. AdV. Mater. 2003, 15, 2056. (5) Kron, G.; Rau, U.; Werner, J. H. J. Phys. Chem. B 2003, 107, 13258. (6) Bosc, F.; Ayral, A.; Albouy, P. A.; Guizard, C. Chem. Mater. 2003, 15, 2463. (7) Nakamura, R.; Imanishi, A.; Murakoshi, K.; Nakato, Y. J. Am. Chem. Soc. 2003, 125, 7443. (8) Sung, Y. M.; Lee, J. K. Cryst. Growth Des. 2004, 4, 737. (9) Shin, Y. K.; Chae, W. S.; Song, Y. M.; Sung, Y. M. Electrochem. Commun. 2006, 8, 465. (10) Bae, E.; Choi, W. EnViron. Sci. Technol. 2003, 37, 147. (11) Shang, J.; Chai, M.; Zhu, Y. EnViron. Sci. Technol. 2003, 37, 4494. (12) Calza, P.; Pelizzetti, E.; Mogyorosi, K.; Kun, R.; Dekany, I. Appl. Catal., B 2007, 72, 314. (13) Kun, R.; Szekeres, M.; Dekany, I. Appl. Catal., B 2006, 68, 49. (14) Korosi, L.; Dekany, I. Colloids Surf. A 2006, 280, 146. (15) Mogyorosi, K.; Dekany, I.; Fendler, J. H. Langmuir 2003, 19, 2938. (16) Makarova, O. V.; Rajh, T.; Thurnauer, M. C.; Martin, A.; Kemme, P. A.; Cropek, D. EnViron. Sci. Technol. 2000, 34, 4797.

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