Received: April 11, 2008; Revised: May 27, 2008. *Corresponding author. Email: [email protected]; Tel: +86591-83445289. The project was supported by the Natural Science Foundation of Fujian Province, China (D0710009) and the Fujian Universities Foundation, China (2007F5032).
Silver Nanoparticles Filling in TiO2 Hollow Nanofibers by Coaxial Electrospinning Guoqing Chang, Xi Zheng, Riyao Chen, Xiao Chen, Liqin Chen, Zhen Chen*
College of Chemistry and Materials Science, Fujian Normal University, Fuzhou 350007, P. R. China
Abstract: This study investigated the coaxial electrospinning process of silver filling in TiO2 ultrafine hollow fibers using polyvinyl pyrrolidone (PVP) sol/titanium n-butyloxide (Ti(OC4H9)4) and PVP sol/silver nanoparticles as pore-directing agents. The bicomponent fibers were heat treated at 200 °C and calcined at 600 °C. Silver particles having diameters of 5 to 40 nm were deposited on the inner surface of the long hollow TiO2 nanofibers (outer diameter of 150−300 nm) with mesoporous walls (thickness of 10−20 nm). The morphological structure of the filled ultrafine hollow fibers has been studied by means of infrared (IR) spectrum, X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The diameters and wall thicknesses of the hollow fibers could be tuned by adjusting the electrospinning parameters. Compared with other nanostructured TiO2 materials, such as mesoporous Ag-TiO2 blending fibers, TiO2 hollow nanofibers, TiO2 nanofibers, and TiO2 powders, the silver filled TiO2 hollow fibers exhibited a higher photocatalytic activity toward the degradation of methylene blue.
There is considerable interest in the design and fabrication of nanostructures that exhibit tunable physicochemical proper-ties for advanced applications in material chemistry[1−3]. Re-cent research on nanocomposites of various noble metals and semiconductor oxides has encouraged the development of these functional materials, because of their specific metal- oxide interactions[1−7]. For example, noble metal nanoparticles have been recognized to promote electron-transfer processes of photocatalysis[8−10]. Silver-TiO2 is well known among these metal-semiconductor nanocomposites, because of the strong catalytic and antibacterial abilities. Recent studies have fo-cused on the interaction of Ag nanoparticles and TiO2
[9,11]. In the past decades, hollow inorganic fibers or tubules with
mesoporous walls have attracted much attention for their structural individuality. For example, the functional molecules or nanoparticles are significantly deposited on the inner or outer surfaces of carbon nanotubes[12]. The hierarchical tu-bules-within-a-tubule structure is important for its techno-logical applications, such as catalysis, selective separation, sensor arrays, waveguides, miniaturized electronic and mag-
netic devices, and photonic crystals with tunable band gaps[13]. In addition, it could act as a minitype reactor with potential applications.
Mou′s group[14] first reported the synthesis of MCM- 41(mobile crystalline material) tubules through a membrane- to-tubule transformation in highly alkaline solution. Since then, several fabrication methods for tubular mesoporous materials have been proposed, which can be classified into two types: hard-template synthesis and self-assembly[15]. The hard-tem-plate process has proved to be an effective approach to pro-duce hollow tubules, but the resultant material is limited by the morphology and structure of the templates. Using self-as-sembly techniques, mesoporous tubules with dimensions from nanometers to micrometers have been prepared, and their morphology is usually determined by the cooperative organi-zation of inorganic and organic species. However, the length of the obtained tubules is usually in the microscale or submi-croscale, and the formation of the tubular structures is always incidental[16]. To our knowledge, the fabrication of inorganic hollow nanofibers with independently functionalized inner
surfaces and mesoporous walls with length in centimeters has not been reported previously.
Since the co-electrospinning technique was first introduced by Loscertales′s group[17] and Greiner et al.[18], as a simple and convenient method, many achievements on the fabrication of core-shell fibers and hollow fibers including organic polymers and inorganic fibers have been reported[19,20]. In the present study, the silver/TiO2 composite core/shell hollow nanofibers having a length of centimeters with mesoporous walls were prepared by co-electrospinning of Ag nanoparticles, PVP and Ti(OC4H9)4, then following heat treatment and calcination. The Ag species were dispersed in the inner surface of this kind of TiO2 hollow nanofiber. The most interesting fact is that the Ag and Ag2O can interconvert in the TiO2 based hollow nano-fibers by alternating irradiation of UV-Vis light, and this composition conversion can influence the intrinsic properties of TiO2 itself, such as the photocatalytic ability.
1 Experimental
1.1 Chemicals
All solvents used were of analytical grade and were used as received. Silver nitrate (AgNO3, 99.95%), ethanol (AR), ethanal (37%), and titanium n-butyloxide (Ti(OC4H9)4, 99.9%) were obtained from the Beijing Chemical Company, China. Polyvinylpyrrolidone (PVP, Mw≈1300000) was purchased from the Aldrich Co., USA.
1.2 Preparation of nanofibers
First, 10 mL ethanal (37%) was added into 25.0 mL of de-ionized water, under stirring, and then 17.0 mL of AgNO3 (10%, 0.01 mol) was added dropwise to this solution. The mixture was stirred under the circumstance of supersonic waves for 15 min, and then filtrated and dried in air at 100 °C for 30 min. Thus, pale gray Ag nanoparticles were obtained. The products were characterized by scanning electron mi-croscopy (SEM) and X-ray diffraction (XRD) (supporting in-formation Fig.S1 is available freely from www.whxb.pku. edu.cn). The results confirmed that the products were Ag nanoparticles. The electrospinning setup was a home-built one, consisting of high voltage power supply, a syringe pump, a syringe, and an aluminum foil target. As shown in Fig.1, the spinneret consisted of two coaxial capillaries. The outer tube and inner capillary were made of stainless steel, and the two fluids were pressurized with N2 to drive the fluid to the tip of the spinneret, which was connected to a high-voltage supply (BGG-30 kV/20 mA), and a piece of aluminum foil was placed 15 cm below the tip of the needle, to collect the nano-fibers. The voltage was fixed at 12.5 kV. Compared with min-eral oil or different solution in the traditional method[20], PVP was employed as the inner and outer solution, to improve the heterojunction on both sides, and it demonstrated that the core/shell structure could be successfully fabricated at high
speed. In a typical procedure for the electrospinning process, a solution which consisted of PVP (4.0 g), titanium n-butyloxide (2 mL), acetic acid (2 mL), and ethanol (20 mL) was added to a syringe connected to the metallic needle, and another solution which consisted of PVP (4.0 g), and ethanol (20 mL) containing Ag nanoparticles was added to another sy-ringe connected to the silica capillary. These two liquids were fed using two syringe pumps (KDS-200, Stoelting, Wood Dale, 1 L). The typical feeding rates for the PVP solution 1 and PVP solution 2 were set at 0.4 and 0.5 mL·h−1, respec-tively. All experiments were conducted at room temperature in air, and the as-spun fibers were left in air for 1 h to allow the Ti(OC4H9)4 precursor to hydrolyze completely. The xerogel fibers were heated at 200 °C for 6 h and calcined at 600 °C for 6 h in air at a heating rate of 5 °C·min−1, from 200 to 600 °C, to completely eliminate the organics. After cooling to room temperature, mesoporous TiO2 hollow fibers with Ag2O nanoparticles on the inner face were obtained.
When the parameters were kept as described in Fig.1, Ag/TiO2 (core/shell) hollow fibers with tunable diameters and wall thicknesses were obtained by adjusting the inner and outer sol pressures. The spinnable sol was important for the preparation of the fibers, whose rheological properties deter-mined the morphology and diameter of the xerogel fibers. A lower viscosity always resulted in the formation of thinner fi-bers. In the present study, the rheological curve of the precur-sor sol exhibited the characteristics of a Newtonian fluid, in-dicating its spinnability. When the aging time was prolonged, its viscosity increased significantly. The experiments demon-strated that the viscosity was too small to electrospin when the aging time was less than 14 h. When the aging time was more than 17 h, the sol behaved as a non-Newtonian fluid and it was difficult to use it to prepare hollow fibers. The formation of mesostructured materials by the sol-gel process required more rigorous conditions, such as, the control of the pH value, sol composition, evaporation condition, and rheological properties of the sol and so on[21]. On the basis of the results of a series of experiments, the optimal viscosity of the precursor sol was 4−7 Pa·s corresponding to the aging time of 15 to 16 h, which was much smaller than the viscosity of sols employed in the mechanical spinning process[20]. The optimal mesostructure
was obtained when the solvent evaporated at a relative humid-ity (RH) of approximately 45% and a temperature of 25 °C.
TiO2 powders, TiO2 fibers, TiO2 hollow fibers, and Ag(15.5%)-TiO2 blending fibers were prepared in accordance with the literature[22].
1.3 Characterization
The rheology and viscoelasticity of the sol were measured on a RS75 Rheometer (HAAKE Co.) at room temperature. X-ray diffraction (XRD) patterns of the samples were applied to identify the phase of fibers, and the voltage and electric cur-rent were maintained at 40 kV and 20 mA (2θ=10°−80°). The XRD patterns of the samples were recorded on a Bruker D8 Advance X-ray diffractometer with Cu Kα radiation (λ= 0.15418 nm) at a scanning rate of 1 (°)·min−1, at room tem-perature. The infrared (IR) spectra were recorded on a Nicolet Nexus 670 FTIR spectrometer using the KBr pellet method in the range of 400−4000 cm−1. Environmental scanning electron microscopic (ESEM) images were recorded on an FEI XL30 system to study the overall morphologies, nanofiber diameters, and the distribution of the freshly prepared nanofibers and hollow nanofibers. The nanofibers were dispersed in ethanol by ultrasonic treatment, and then dip-coated on the copper grids for transmission electron microscopy (TEM, EM-002B, TOPCON, Japan). Thermal gravimetric (TG) analysis was employed to evaluate the mass loss of the samples, under air flow of 20 mL·min−1, at a heating rate of 10 °C·min−1, using a thermal analyzer (TGA-7, Perkin Elmer Co.).
1.4 Photocatalytic activity measurements
The photocatalytic activities of the mesoporous TiO2 hollow fibers were investigated by photo-oxidizing methylene blue[23]. Deionized water was used as a blank. 1.0 mg sample was added to 100.0 mL methylene blue solution with a concentra-tion of 2.0 mg·L−1. The solution was irradiated with the help of a 125 W high-pressure mercury lamp (λ=320−400 nm, λmax=365 nm) with stirring. The change of absorption at 664 nm was applied to investigate the concentration of methylene blue as a function of irradiation time using a UV-Vis spectro-photometer (Lambda 35, Perkin-Elmer Co.).
2 Results and discussion
2.1 Microstructure of TiO2 hollow fibers
Fig.2 shows the SEM images of the freshly prepared Ag nanoparticles/Ti(OC4H9)4/PVP hybrid nanofibers before (Fig.2a) and after (Fig.2b) heated at 300 °C for 6 h. The di-ameters of the nanofibers before heated are approximately 300−500 nm, having a narrow diameter distribution. This suggests that the feeding solution can be electrospun in this condition to produce good hybrid nanofibers. After heated at 300 °C, the color of the hybrid nanofibers changes to pale gray, and the diameters of the nanofibers shrink slightly. This
phenomenon may be because the organic components have not been burned away, and the hybrid nanofiber cannot be de-stroyed at this temperature because of the high glass transition temperature of PVP. After calcination at 600 °C for 6 h, the color of the hybrid nanofibers changes to pale gray, as shown in Fig.2c and Fig.2d. It is obvious that the diameters of the nanofibers have shrunk markedly, to 150−300 nm, because of the burning away of the organic components.
2.2 TG analysis of Ag nanoparticles/TiO2/PVP hybrid nanofibers
The typical thermogravimetry (TG) curve (Fig.3) of the xe-rogel fibers shows four steps and a total mass loss of ap-proximately 88.56%. The first step of approximately 6.92% mass loss from 20 to 80 °C can be attributed to the desorption of the physical adsorbed water, and the second significant mass loss of approximately 8.05% between 80 and 270 °C can be assigned to the removal of water molecules from the hy-droxyls on titanium atoms and a small amount of residual or-ganic material. From 270 to 530 °C, there is a mass loss of approximately 72.50%, which is assigned to the decomposi-tion of the PVP (k-90) template in the nanochannels of the
Fig.2 SEM images of the produced hollow nanofibers
Ag nanoparticles/TiO2/PVP nanofibers before (a) and after (b) heated at 300 °C
for 6 h; (c) Ag filled TiO2 hollow nanofibers after calcination at 600 °C for
6 h; (d) smaller magnification of (c)
Fig.3 TG curve of Ag filled TiO2 xerogel hollow nanofibers
microtubule walls. These results show that the organic tem-plate is removable from the fibers upon calcination at 530 °C. There is no further mass loss above 530 °C, however, some weak exothermic peaks appear in the range of 530−900 °C in the differential scanning calorimetry (DSC) measurement. It indicates the slow transform action of the anatase phase of TiO2 to the rutile phase.
2.3 IR spectra of Ag nanoparticles/TiO2/PVP hybrid nanofibers
The IR spectra (Fig.4) of the xerogel fibers and the fibers calcined at different temperatures confirm the results men-tioned earlier. The IR spectrum of the xerogel fibers shows a strong adsorption around 3434 cm−1, which is assigned to the hydroxyls from water, PVP (k-90), and Ti−OH, and the weak peak at 1720 cm−1 also indicates the presence of water. The adsorption peaks from 1520 to 1432 cm−1 are attributed to C−H vibrations of hydrocarbons in the xerogel fibers. The 1630 cm−1 band is considered to be the result of C=O mono-mer stretching vibration. The presence of the two bands at l650 and l300 cm−1 is attributed to the ν(COO) vibrations of the residual precursors. The shift from 3500 to 3400 cm−1 is assigned to Ti−OH vibration. After the xerogel fibers are cal-cined at 500 °C for 6 h in air, the strong absorptions around 3400 cm−1 weaken significantly, and absorptions around 530 cm−1 appear, which are attributed to the Ti−O and Ag−O vi-brations of anatase TiO2 and Ag2O. After calcination at 600 °C, the peaks from 1520 to 1432 cm−1 disappear indicating the complete removal of PVP.
2.4 XRD patterns of the Ag filled TiO2 hollow nanofibers
Fig.5 shows XRD patterns of the Ag filled TiO2 hollow nanofibers after calcination at 300, 400, 500, and 600 °C. All the three types of Ag filled TiO2 hollow nanofibers (Fig.5(b−d)) exhibit the usual anatase and rutile phases, simi-lar to pure TiO2 nanofibers, with similar peak intensities and shapes. It indicates that the states of the silver species do not influence the crystalline structures of the TiO2 itself. XRD
patterns indicate that the xerogel anatase phase TiO2 walls form after calcination at 400 °C. The intensity of the reflec-tions corresponding to the anatase structure increase as the calcination temperature increases from 300 to 600 °C. After the nanofibers are sintered at 600 °C for 6 h, the color of the nanofibers is nearly white. This indicates that Ag is further oxidized to Ag2O and disperses in the inner surface of the TiO2-based hollow nanofibers after calcination. When the xe-rogel fibers are calcined at 400 °C for 6 h, the mean size of the anatase TiO2 nanocrystals is approximately 10 nm, calculated according to the Scherrer equation. After the xerogel fibers are heat-treated at 600 °C, the Bragg peaks become higher and sharper. Fig.5c shows an expanded view of the XRD pattern (42° to 46°) of the nanofibers, after calcination at 500 °C, to show the peak for Ag at 44.2°. However, the diffraction peaks attributed to silver nanoparticles can be observed from Fig.5(b, d), because the X-ray beam itself can reduce Ag2O to Ag (Ag++e=Ag), just as in the case of UV irradiation. It has been observed that the white color of the silver filled TiO2 nanofi-bers after calcination turns gray after XRD measurement. Therefore, the XRD results are merely qualitatively useful and show evidence for the existence of silver. The diffraction peaks corresponding to Ag2O are not found in any of these samples.
2.5 TEM and HR-TEM images of silver filled TiO2 hollow nanofibers
TEM and high resolution-transmission electron microscopy (HR-TEM) images of the silver filled TiO2 hollow nanofibers after calcination at 600 °C for 6 h have also been examined, shown in Fig.6. Fig.6a shows that the Ag particles adhere to the inner surface of the TiO2 hollow nanofibers. The diameters of the hollow fibers in the cross section are 150−300 nm, and the wall thickness is approximately 10−20 nm, marked by black arrow(1). The TEM image of the nanofibers is shown in Fig.6b. It can be seen that the Ag nanoparticles are distributed in the inner surface of the hollow nanofibers in the range of 5 to approximately 40 nm. HR-TEM image of the silver filled
Fig.4 IR spectra of xerogel fibers (a) and the fibers calcined at
different temperatures (b−e) for 6 h in air atmosphere T/°C: (b) 300, (c) 400, (d) 500, (e) 600
Fig.5 XRD patterns of xerogel Ag filled hollow fibers after calcined at different temperatures for 6 h in air atmosphere
TiO2 hollow nanofibers provides further information about the crystalline structures as shown in Fig.6c. The crystalline planes show the interplanar spacings, of 0.352 and 0.236 nm, attributed to the anatase (101) plane and Ag (111) plane, re-spectively. The electron dispersion X-ray (EDX) spectrum of the nanofibers (Fig.6d) indicates the composition of the nano-fibers presenting Ag (15.54%) and Ti (50.12%), after UV-Vis light irradiation.
2.6 Photocatalytic mechanism and evaluation
The photocatalytic activity of TiO2 is determined primarily by its physicochemical properties, such as morphology, parti-cle size, surface area, and porosity[24,25]. A possible method for improving the efficiency of the electron-transfer dynamics is through the modification of semiconductor nanoparticles with a noble metal deposit. Semiconductor-metal composites have been widely used in photocatalysis[26]. Au-TiO2 and Ag-TiO2 nanocomposites exhibit enhanced charge-transfer efficiency by shifting the Fermi level to more negative potentials[27].
Using this strategy, the authors were able to transfer elec-trons from the excited TiO2 shell into the Ag core[8,28]. The flow of electrons from the conduction band of TiO2 (ECB=−0.5 V (vs NHE)) into the silver core with low lying Fermi level (−0.45 V) is energetically favored. Electrons stored in the sil-ver core can be readily discharged on-demand to an electron acceptor. Basic understanding of the charge transfer process of metal core/semiconductor shell systems will probably favor the development of the next generation catalysts (Fig.7).
The reactions of the silver filled TiO2 hollow nanofibers under alternating irradiation can be explained in terms of the following:
Out of the hollow fibers: TiO2+hv(UV)=h++e (1) dye*+h+=dye+ (2)
In the hollow fibers: TiO2(e)+Ag=TiO2+Ag(e) (3) dye++Ag(e)+H2O=Ag+degradation (4)
The photocatalytic properties of the mesoporous Ag filled TiO2 hollow fibers were evaluated by photo-oxidation of me-thylene blue. Methylene blue was hardly degraded under UV light irradiation in the absence of photocatalyst. The adsorp-tion of methylene blue at 664 nm almost disappeared, and the
Fig.6 (a) TEM image of Ag nanoparticles filled TiO2 nanofibers obtained via the electrospinning method after UV-Vis light irradiation for
1 h in air; (b) and (c) HR-TEM images of Ag nanoparticles filled in the inner surface of TiO2 hollow nanofibers; (d) EDX spectrum of the nanofibers after UV-Vis light irradiation for 1 h
Fig.7 Photoinduced charge injection and charge separation in Ag filled TiO2 hollow nanofiber
Fig.8 Time courses for UV photodegradation of methylene blue
c0=2.0 mg·L−1; (a) without photocatalyst; (b) mesoporous TiO2 powders, the
pore size was the same as that of the hollow fibers; (c) mesoporous TiO2 fibers;
blue color of the solution vanished completely after irradiating for 40 min in the presence of Ag filled TiO2 hollow fibers, as shown in Fig.8(f). As a comparison, the photodegradation in the absence of photocatalyst, mesoporous TiO2 powders, TiO2 fibers, TiO2 hollow fibers, and Ag-TiO2 blending fibers pre-pared under the same conditions were also measured (Fig.8). Ag filled TiO2 hollow fibers showed good photocatalytic ac-tivity for the decomposition of methylene blue. The photode-gradation rate of methylene blue in the case of Ag filled TiO2 hollow fibers was faster than that of mesoporous Ag-TiO2 blending fibers, TiO2 hollow fibers, TiO2 fibers, and TiO2 powders.
3 Conclusions
Long Ag filled TiO2 hollow fibers (outer diameter of 150− 300 nm) with mesoporous walls (wall thickness of 10−20 nm) and high surface areas were prepared by the two-capillary spinneret electrospinning technique. The diameters and wall thicknesses of the hollow fibers could be tuned by adjusting the electrospinning parameters. The silver particles with di-ameters in the range from several to 40 nanometers grew on the inner surface of the TiO2 nanotubes. The Ag filled TiO2 hollow fibers with mesoporous walls showed higher photo-catalytic activities toward decomposition of methylene blue than other nanostructured TiO2 materials, such as mesoporous Ag-TiO2 blending fibers, TiO2 hollow nanofibers, TiO2 nano-fibers, and TiO2 powders. Supporting information available freely online at http:// www.whxb.pku.edu.cn
References
1 (a) Kamat, P. V. Prog. React. Kinet., 1994, 19: 277 (b) Bell, A. T. Science, 2003, 299: 1688 (c) Schmid, G. Clusters and colloids: from theory to application. Weinheim: VCH, 1994 (d) Rolison, D. R. Science, 2003, 299: 1698 (e) Zhang, J. Z. Acc. Chem. Res., 1997, 30: 423
2 (a) Rao, C. N. R.; Vijayakrishnan, V.; Santra, A. K.; Prins, M. W. J. Angew. Chem. Int. Ed. Engl., 1992, 31: 1062 (b) Valden, M.; Lai, X.; Goodman, D. W. Science, 1998, 281: 1647 (c) Xu, C.; Lai, X.; Zajac, G. W.; Goodman, D. W. Phys. Rev. B, 1997, 56: 13464
3 (a) Roucoux, A.; Schlz, J.; Patin, H. Chem. Rev., 2002, 102: 3757 (b) Lewis, L. N. Chem. Rev., 1993, 93: 2693 (c) Kiwi, J.; Gratzel, M. J. Am. Chem. Soc., 1979, 101: 7214 (d) Brugger, P. A.; Cuendet, P.; Gratzel, M. J. Am. Chem. Soc., 1981, 103: 2923 (e) Reets, M. T.; Helbig, W. J. Am. Chem. Soc., 1994, 116: 7401 (f) Grunert, W.; Bruckner, A.; Hofmeister, H.; Claus, P. J. Phys. Chem. B, 2004, 108: 5709
4 (a) Ma, Q.; Klier, K.; Cheng, H.; Mitchell, J. W.; Hayes, K. S. J. Phys. Chem. B, 2001, 105: 9230 (b) Xu, B. Q.; Wei, J. M.; Yu, Y. T.; Li, Y.; Li, J. L.; Zhu, Q. M. J. Phys. Chem. B, 2003, 107: 5203 (c) Dong, W.; Feng, S.; Shi, Z.; Li, L.; Xu, Y. Chem. Mater., 2003, 15: 1941 (d) Soejima, T.; Tada, H.; Kawahara, T.; Ito, S. Langmuir, 2002, 18: 4191 (e) Tada, H.; Soejima, T.; Ito, S.; Kobayashi, H. J. Am. Chem. Soc., 2004, 126: 15952 (f) Kiyonaga, T.; Mitsui, T.; Torikoshi, M.; Takekawa, M.; Soejima, T.; Tada, H. J. Phys. Chem. B, 2006, 110: 10771
5 (a) Gerischer, H. J. Phys. Chem., 1984, 88: 6096 (b) Sun, B.; Vorontsov, A. V.; Smirniotis, P. G. Langmuir, 2003, 19: 3151 (c) Einaga, H.; Harada, M.; Futamura, S.; Ibusuki, T. J. Phys. Chem. B, 2003, 107: 9290 (d) Einaga, H.; Harada, M. Langmuir, 2005, 21: 2578 (e) Cozzoli, P. D.; Fanizza, E.; Comparelli, R.; Curri, M. L.; Agostiano, A.; Laub, D. J. Phys. Chem. B, 2004, 108: 9623 (f) Cozzoli, P. D.; Comparelli, R.; Fanizza, E.; Curri, M. L.; Agostiano, A.; Laub, D. J. Am. Chem. Soc., 2004, 126: 3868 (g) Pena, D. A.; Uphade, B. S.; Reddy, E. P.; Smirniotis, P. G. J. Phys. Chem. B, 2004, 108: 9927 (h) Sun, B.; Smirniotis, P. G.; Boolchand, P. Langmuir, 2005, 21: 11397
6 (a) Dawson, A.; Kamat, P. V. J. Phys. Chem. B, 2001, 105: 960 (b) Subramanian, V.; Wolf, E.; Kamat, P. V. Langmuir, 2003, 19: 469 (c) Kamat, P. V.; Flumiani, M.; Dawson, A. Colloids Surf. A, 2002, 202: 269 (d) Subramanian, V.; Wolf, E. E.; Kamat, P. V. J. Phys. Chem. B, 2003, 107: 7479 (e) Sudeep, P. K.; Kamat, P. V. Chem. Mater., 2005, 17: 5404 (f) Drew, K.; Girishkumar, G.; Vinodgopal, K.; Kamat, P. V. J. Phys. Chem. B, 2005, 109: 11851
7 (a) Jakob, M.; Levanon, H.; Kamat, P. V. Nano Lett., 2003, 3: 353 (b) Chandrasekharan, N.; Kamat, P. V. J. Phys. Chem. B, 2000, 104: 10851 (c) Stathatos, E.; Lianos, P.; Falaras, P.; Siokou, A. Langmuir, 2000, 16: 2398 (d) Subramanian, V.; Wolf, E.; Kamat, P. V. J. Phys. Chem. B, 2001, 105: 11439 (e) Haick, H.; Paz, Y. J. Phys. Chem. B, 2003, 107: 2319 (f) Chandrasekharan, N.; Kamat, P. V.; Hu, J.; Jones II, G. J. Phys. Chem. B, 2000, 104: 11103 (g) Hirakawa, T.; Kamat, P. V. J. Am. Chem. Soc., 2005, 127: 3928
8 (a) Link, S.; El-Sayed, M. A. J. Phys. Chem. B, 1999, 103: 8410 (b) El-Sayed, M. A. Acc. Chem. Res., 2001, 34: 257 (c) Kamat, P. V. J. Phys. Chem. B, 2002, 106: 7729 (d) Toshima, N. Pure Appl. Chem., 2000, 72: 317
(e) He, J.; Ichinose, I.; Kunitake, T.; Nakao, A.; Shiraishi, Y.; Toshima, N. J. Am. Chem. Soc., 2003, 125: 11034 (f) Hirakawa, T.; Kamat, P. V. Langmuir, 2004, 20: 5645 (g) Subramanian, V.; Wolf, E. E.; Kamat, P. V. J. Am. Chem. Soc., 2004, 126: 4943
9 (a) Pradhan, N.; Pal, A.; Pal, T. Langmuir, 2001, 17: 1800 (b) Ghosh, S. K.; Kundu, S.; Mandal, M.; Pal, T. Langmuir, 2002, 18: 8756 (c) Claus, P.; Hofmeister, H. J. Phys. Chem. B, 1999, 103: 2766 (d) Sudrik, S. G.; Maddanimath, T.; Chaki, N. K.; Chavan, S. P.; Chavan, S. P.; Sonawane, H. R.; Vijayamohanan, K. Org. Lett., 2003, 5: 2355
10 (a) Jana, N. R.; Pal, T. Langmuir, 1999, 15: 3458 (b) Sau, T. K.; Pal, A.; Pal, T. J. Phys. Chem. B, 2001, 105: 9266 (c) Li, Y.; Boone, E.; El-Sayed, M. A. Langmuir, 2002, 18: 4921 (d) Chan, S. C.; Barteau, M. A. Langmuir, 2005, 21: 5588
11 (a) Fujishima, A.; Hashimoto, K.; Watanabe, T. TiO2 photocata-lysis: fundamentals and applications. Tokyo: BKC, 1999 (b) Fujishima, A.; Rao, T. N.; Tryk, D. A. J. Photochem. Photobiol. C, 2000, 1: 1 (c) Fujishima, A.; Zhang, X. T. C. R. Chim., 2006, 9: 750
12 (a) Chen, J.; Hamon, M. A.; Hu, H.; Chen, Y.; Rao, A. M.; Eklund, P. C.; Haddon, R. C. Science, 1998, 282: 95 (b) Choi, H. C.; Shim, M.; Bangsaruntip, S.; Dai, H. J. Am. Chem. Soc., 2002, 124: 9058 (c) Lee, J.; Kim, H.; Kahng, S. J.; Kim, G.; Son, Y. W.; Ihm, J.; Kato, H.; Wang, Z. W.; Okazaki, T.; Shinohara, H.; Kuk, Y. Nature, 2002, 415: 1005
