Application of Pt . TiO 2 Nanofibers in Photosensitized Degradation of Rhodamine B

The surface properties of TiO2 nanofibers were utilized for the in-situ and ex-situ nucleation and growth of metal nanoparticles. Pt nanoparticles were generated on the titania surface without the use of external stabilizing agents. These nanoparticles were effectively anchored on the numerous pores present on the TiO2 surface and acted as sites for continued nanoparticle growth. The TiO2 surface exhibited extensive folding resulting in increased surface area that was then explored for the effective adsorption of reacting materials for heterogeneous catalysis. The resulting TiO2-based catalysts were found to facilitate UV/Visible light sensitized degradation of Rhodamine B (RH-B). The photodegradation followed first order kinetics with a rate constant of 0.0158 min. An investigation of the mechanism of RH-B degradation suggests a role of reactive oxygen species (ROS) as intermediates in the photodegradation process.


Introduction
Heterogeneous photocatalysis is an emerging area of research (Mills & Hunte, 1997;Bouras et al., 2007).The most commonly used photocatalyst has been titanium dioxide, which is a wide-band gap (~3.0-3.2 eV) semi-conductor metal oxide.The photocatalytic activity of a semi-conductor is due to the photo-induced electron-hole pairs created by illumination with light.These energetically excited species can be harvested to generate electricity in solar cells or in chemical processes for the degradation of organic pollutants (Carp et al., 2004;Guo et al., 2011).Titania-based materials have gained much interest in environmental remediation due to their transparency to visible light, high refractive index, low absorption coefficient, chemical stability, non-toxicity and cost effectiveness (Kwon et al., 2004;Wang et al., 2008).Some of the commercial applications of titania include antibacterial action and waste water treatment (Kwon et al., 2004;Im et al., 2009).Due to its wide band gap, UV light is required to generate electron-hole pairs.This significantly limits titania as an effective photocatalyst since UV light accounts for only 5% of total solar radiation.Additionally, the fast recombination rate of photogenerated electron-hole pairs lowers the photocatalytic efficiency (Zhu et al., 2008).To solve these problems, numerous research efforts are currently focused on the generation of novel materials that will shift the optical response of the photocatalyst to the visible range while preventing the recombination of the photogenerated electrons and hole (Friedmann et al., 2010;Fu et al., 2011;Sakthivel et al., 2006) A popular approach used to shift the optical absorption of titania from UV to visible range has been doping with transition metal elements (Wang et al., 2008;Zhu et al., 2008;Seery et al., 2007) non-metal ions (Im et al., 2009;Guo et al., 2010), dye-sensitization (Kozuka et al., 1996) and semi-conductor coupling (Liu et al., 2009;Wang et al., 2004).The role of the transition metal/non-metal doping is to introduce trapping sites that would scavenge the photogenerated electrons to separate the electron-hole pairs (Yalda et al., 2010;Ali et al., 2011), thus promoting redox processes on the catalyst surface (Bouras et al., 2007;Wang et al., 2008;Zhu et al., 2008).Further, dopants shift the absorption maximum of titania to the visible range by introducing energy states within the semi-conductor band gap that lower the energy required for excitation and hence the use of direct solar radiation (Bouras et al., 2007).The doped titania materials have been actively used in photosensitization reactions as useful means of detoxification of environmental pollutants (Wang et al., 2009;Kment et al., 2010).
Photosensitization reactions are also involved in effective therapies for the control of certain types of cancer, psoriasis, vitiligo and other skin diseases (Smith, 1989).Photosensitization technology has previously been employed for the purification of contaminated water, infectious blood and controlling neonatal bilirubemia (MacCormack, 2008;Pasale et al., 1976;Pillay & Salih, 2004).
The traditional methods used for the environmental remediation of organic pollutants have not been very effective, since they only transform the organic molecules from one form to another without complete degradation (Chumcheng et al., 2010).Furthermore, the mechanism for degradation of these dyes remains unclear.It is well established that reduction of particle size has a marked influence on increasing the system's catalytic activity.The shift to nanostructured materials for photocatalysis is a very active area of research in order to obtain materials with high surface areas and large pore volumes for enhanced dispersion and catalytic activity (Zhang et al., 2008;Libanori et al., 2009).The aspect ratios for nanostructured materials shortens the transportation length of electrons and holes from the crystal interface to the surface, thus accelerating their migration to the surface for redox reactions (Wang et al., 2008).This paper highlights the synthesis and characterization of a new type of nanostructured TiO 2 -based material with efficient electron-hole charge separation and high specific surface areas.We have explored the electronic and surface structures of titania nanofibers for the deposition of Pt nanoparticles via comparison of a modified in-situ with a newly developed ex-situ polyol synthesis procedure.Rhodamine B, a triphenylmethane dye (Yu et al., 2009;He et al., 2009aHe et al., , 2009b) ) was used to probe catalytic activity.Visible light was harvested by the doped TiO 2 for a more efficient and sustainable process while the nanoparticles acted as effective electron traps to prevent recombination of the TiO 2 electron-hole pair.An investigation of the involvement of ROS in Pt-TiO 2 sensitized degradation of the dye under UVA was undertaken to facilitate an understanding of as plausible mechanistic pathway of the reactive species on the TiO 2 surface.

Instrumentation
The PMMA/TiP nanofibers were electrospun using a high KV Spellman SL 30 generator.The morphology and size of the fibers were determined using a Hitachi S-570 or Supra Zeiss 55VP Scanning Electron Microscope (SEM), equipped with an EDAX system for Energy Dispersive Spectroscopic (EDS) analysis.Transmission Electron Microscopy (TEM) images of the catalysts was obtained on a JEOL 2010 FETEM instrument.The samples were dispersed in ethanol by sonication.The resulting solution was placed onto a lacey carbon-coated Cu grid.UV-Visible determinations were done on an 8452A Hewlett Packard Diode Array spectrophotometer instrument in the 190 to 820 nm range.The ultraviolet irradiation system comprised a horizontal planar array of three, 4-foot long UVA-emitting (320-400 nm, TL-D 36 W), fluorescent tubes manufactured by Philips, Holland.The irradiance of the emitted light was measured by a VLX-3W UVR probe which was equipped with UVA (SX-365nm), UVB (290-320 nm, SX-312nm) and UVC (180-290 nm, SX-254nm) detectors, manufactured by Vilber Lourmat, Marne La Valle, France.The UVR dose is expressed in Joules (J = W/cm 2 x Second).The irradiance output of UVA was 5.8 mW/cm 2 at a distance of 10 cm from the source.The interference of UVB in the UVA source was below 0.2 mW/cm 2 .No UVC was detected in the UVA source.

Fabrication of TiO 2 Nanofibers
The detailed procedure has been described earlier (Obuya et al., 2011).Briefly, a solution of TiP was mixed with PMMA to achieve a viscous polymer blend.For the electrospinning process, a high voltage power supply (25 kV/cm) induced electrical charges on the polymer blend to stretch the solution into submicron length fibers.DMF was added to ensure a dielectric constant sufficient to withstand the high electric voltages.As the solvents evaporated, the stretched PMMA/TiP fibers were collected on an aluminum foil attached to the collector plate.TiO 2 nanofibers were then synthesized after heat treatment at 500 o C to remove the polymer matrix.

In-Situ Polyol Deposition
EG (10 ml) was heated at 110 o C in an oil bath (30 minutes) followed by addition of TiO 2 fibers.The EG/TiO 2 mixture was irradiated under UV light for 30 minutes.Subsequently, Pt 2+ and PVP (1:20 mole ratio) were added to the EG solution, which was left stirring for an additional 30 minutes to control the nucleation process of the nanoparticles.At the end of the reaction, the catalyst was washed several times with acetone, centrifuged and dried in an oven at 120 o C for 3 h to remove residual PVP and EG.

Ex-Situ Polyol Deposition
EG (10 mL) was heated in an oil bath at 110 o C (60 minutes), followed by addition of a 1:5 mole ratio of Pt 2+ : PVP to initiate the nucleation process.This solution was left stirring for 60 minutes, after which it was immediately cooled in ice to stop further growth of the Pt nanoparticles.The Pt nanoparticles (in 2 mL ethanol) were dispersed on the surface of the TiO 2 nanofibers with sonication (120 minutes).The catalysts were cleaned and dried via steps described in section 3.2.1.

Photodegradation of RH-B
Pt-TiO 2 catalyst (50 mg) was sonicated in 10 mL distilled water (15 minutes) to ensure complete dispersion of nanoparticles in solution.The catalyst was mixed with a 10 -5 M solution of RH-B in distilled water (50 mL) for 30 minutes in the dark to reach an adsorption/desorption equilibrium.The mixture was continuously stirred in air and irradiated under UV light.The distance between the UV lamp and the reaction beaker was maintained at 20 cm.The progress of reaction was monitored by recording the absorption spectra of irradiated samples after every 10 minutes.Aliquots (0.5 ml in 5 mL H 2 O) were centrifuged at 1550 RPM for 30 minutes to remove the dispersed catalyst before recording the absorption spectrum.Control experiments included: (i) dye with UV light in the absence of catalyst, (ii) both in-situ and ex-situ Pt-TiO 2 in sunlight, (iii) undoped TiO 2 nanofibers under UV light and (iv) undoped TiO 2 in sunlight.

Mechanism of Photodegradation
RH-B degradation with the in-situ Pt-TiO 2 was carried out under three different conditions: (i) irradiation in 0.01M phosphate buffer, pH 7.0, (ii) irradiation in the presence of Pt-TiO 2 as a photosensitizer, pH 7 and (iii) irradiation in the presence of Pt-TiO 2 and selected scavengers of free radical/reactive O 2 species (ROS) scavenger.20 mL samples of a 6.25 × 10 -6 M solution of RH-B (prepared in pH 7 phosphate buffer) were irradiated in a Petri dish with or without a sensitizer (10.0 μg/mL) or varying amounts of a quencher under UVA at a dose ranging from 2 to 20 J.

Scanning Electron Microscopy and Energy Dispersive Spectroscopy
SEM analysis of the TiO 2 nanofibers indicated an enlarged surface area that was highly folded and wrinkled, providing favorable sites for heterogeneous nucleation of metal nanoparticles (Figure 1).In Figure 2, a highly magnified SEM image shows the presence of numerous pores that are indicative of a mesoporous structure of the TiO 2 that allows for the effective absorption of reacting molecules.This porosity, coupled with its ability to create photoinduced electron-hole pairs makes it a very effective support for the in-situ synthesis of nanoparticles without the need for external stabilizing agent.Figure 3 illustrates EDS image that shows the elemental composition and distribution of the in-situ Pt-TiO 2 catalysts, where the presence of a carbon peak on both spectra is indicative of incomplete combustion of PMMA.A summary of the elemental composition and relative percentages of the Pt-TiO 2 catalysts obtained from the XPS spectrum is given in Table 1.It provides the atomic percentages, binding energies, and peak areas for each assigned peak for Ti, O, C and Pt elements.These binding energies are used for peak identification against known values so as to establish the electronic states of the elements, while the atomic percentages and peak areas are used to calculate the percentage of O bonded to Ti, C and/or adsorbed on the TiO 2 surface.The Pt 4f5/2 peaks that were observed at binding energies of 70.6 and 74.0 eV were assigned to elemental Pt.

Catalytic Activity of Titania Nonofibers
The semiconductor nature of titania nanofibers was explored for the in-situ reduction of metal ion precursors on the TiO 2 surface using conduction band electrons.The presence of oxidizable additives, in our case EG, made use of the valence band holes thereby preventing electron-hole recombination.The ex-situ method was preferred for the Pt-TiO 2 catalyst since more of the nanoparticles were available on the TiO 2 fiber surface, which resulted in improved adsorption and degradation of organic pollutants.The objective of loading TiO 2 with nanoparticles was to maximize the extension of the existing wavelength response of the catalyst to longer wavelength (visible) region.The Pt nanoparticles also provided enhanced charge mobility on the TiO 2 surface thus preventing electron-hole recombination.The results were compared with well recognized 1 O 2 generating photosensitizers like RF and RB.While photosensitized RF and RB degraded dGuo by 99.1 ± 0.3% and 50.8 ± 1.6%, respectively, the Pt.TiO 2 photocatalyst was relatively ineffective (3.5 ± 0.1% degradation) under similar conditions.The photodegradation of dGuo by RF and RB was inhibited by over 95% with 10 mM NaN 3 (Joshi, 1985).The results of this study suggest that photosensitized degradation of dGuo by RF and RB is largely due to the involvement of 1 O 2 .However, Photosensitized Pt-TiO 2 did not show significant involvement in dGuo degradation providing additional evidence that its involvement in RH-B photodegradation was not due to 1 O 2 .In order to reconfirm this result, 1 O 2 determination test were carried out with Pt-TiO 2 , RF, RB and RH-B in an experiment described below.• in the Pt-TiO 2 sensitized (UVA, 20J) degradation of RH-B was determined by carrying out the photosensitized reaction in the presence of 25 U/μL SOD (Beauchamp & Fridovich, 1971).The photodegradation was not affected by SOD indicating no involvement of O 2 -• in the reaction.

Role of Hydroxyl Radicals ( • OH) in the Photodegradation of RH-B
The involvement of • OH in the Pt-TiO 2 sensitized degradation of RH-B was determined by carrying out the reaction in the presence of mannitol (10 mM; Goldstein & Czapski, 1984).Photodegradation of RH-B was inhibited by 74.4% in the presence of mannitol indicating that • OH was involved in the reaction.The involvement of H 2 O 2 in the photosensitized degradation of RH-B was determined by carrying out reaction in the presence of catalase (Seaver & Imlay, 2001).A stock solution of catalase (100 mg/mL) was prepared in phosphate buffer (0.01 M, pH 7) and 400μg/mL aliquot was added to the irradiation solution of RH-B.The Pt-TiO 2 sensitized degradation of RH-B was inhibited by 47.8% in the presence of catalase indicating partial involvement of H 2 O 2 in the reaction .

Mechanism of RH-B Photodegradation
RH-B structure contains four N-ethyl groups on either side of the xanthene ring and a carboxylic acid group on the phenyl group connected to the xanthene ring (Figure 15).The presence of non-bonded lone pairs of electrons on the functional groups and the extended π conjugation makes this molecule susceptible to attack by ROS produced by oxidative and reductive pathways on the semiconductor surface (Wu et al., 1998).The positively charged amine groups assists in the adsorption of RH-B onto the negatively charged TiO 2 surface.Previous reports suggest that the photocatalytic cleavage of the RH-B molecule takes place via two competitive processes; N-de-ethylation and chromophore cleavage (He et al., 2009b).Subsequently, ring opening and mineralization occurs to give rise to various products.
For this study, UV-Vis absorption data were used to track the degradation process; chromophore cleavage of the dye led to the destruction of π conjugation across the RH-B molecule and thus the complete disappearance of the absorption maximum band at 554 nm (Figure 10).Due to its auxochromic properties, the stepwise removal of the N-ethyl group lead to a consistent hypsochromic shift of the absorption maximum band at 554 nm.A consistent decrease in the absorption at 554 nm with a simultaneous increase in absorption at ~248 nm (Figure 16) suggested the degradation of RH-B with the formation of lower wavelength-absorbing photoproducts.The identification of photoproducts is the subject of our ongoing research.Chromophore cleavage of RH-B under microwave irradiation using graphite-supported TiO 2 or Bi 2 WO 6 as catalysts proceeds through the formation of benzoic, hydroxybenzoic, phthalic and terephthalic acids, as identified by GC/MS analysis (He et al., 2009a(He et al., , 2009b;;Wu et al., 1998).Based on the experiments showing the participation of • OH and H 2 O 2 in RH-B degradation, a mechanism for RH-B degradation involving •OH has been proposed (Figure 17).The N-de-ethylation is initiated by the attack on an ethyl group of the RH-B molecule by • OH.The resulting radical undergoes β-cleavage to generate a stabilized radical and ethylene, which abstracts a hydrogen atom from water

Conclusion
An efficient Pt-TiO 2 catalyst was prepared by a modified ex-situ polyol method.The mesoporous nature of the TiO 2 nanofibers, coupled with a large surface to volume ratio provided a suitable anchorage for the Pt nanoparticles.Additionally, the TiO 2 surface was crucial for the adsorption of RH-B and the generation of ROS required for the degradation reaction.Maximum degradation using Pt-TiO 2 was observed in sunlight, which followed a first order reaction kinetics, with a rate constant of 0.0158 min -1 .Presence of Pt nanoparticles provided additional active sites for the adsorption of reactive intermediates as well as improved charge mobility on the TiO 2 surface, thereby preventing electron-hole recombination.Quenching studies suggested that • OH and H 2 O 2 were responsible for the Photodegradation of RH-B, while 1 O 2 and O 2 -• were not involved.These observations were in agreement with earlier studies on photosensitization reactions of Pt-TiO 2 complexes where damage to protein and DNA analogues was demonstrated via formation of • OH and H 2 O 2 (Macyk & Kisch, 2001;Hirakawa et al., 2004).This study provided useful information on the role of selective ROS as intermediates in the photocatalytic degradation of RH-B. Figure

Figure
Figure Figure 7.

FigureFigure 9 .
Figure 8. P Photodegr with time day but wi that in reac activity wa Photodegradat H-B was deter um azide (NaN erent concentra NaN 3 at any of or an indirect d /RNA (Joshi, 1 s studied to co osine (dGuo) w nder UVA for a

4. 3 . 2
Determination of 1 O 2 Generation by Pt-TiO 2 and Its Comparison With Riboflavin, Rose Bengal and RH-BThe production of 1 O 2 by photosensitized Pt-TiO 2 , RF, RB and RH-B under aerobic condition was measured in an aqueous solution by literature procedure(Kraljic & El Moshni, 1978). 1 O 2 forms transannular peroxide intermediate with histidine leading to the bleaching of RNO, which was quantitatively measured spectrophotometric ally at 440 nm.The results confirmed that Pt-TiO 2 and RH-B were not major producers of 1 O 2 as compared to RF and RB, Figure14.

Figure 14 .
Figure 14.Formation of 1 O 2 by various test compounds as determined by the bleaching of RNO (red) and quenching by sodium azide (blue) 4.3.2.3 Role of Hydrogen Peroxide (H 2 O 2 ) in the Photodegradation of RH-B

Figure 15 .
Figure 15.Chemical structure of Rhodamine B Figure

Table 1 .
A summary of the elemental composition and atomic percentage of the Pt-TiO 2 catalyst