Pure titanate nanotubes and titanate nanotubes doped with Fe3+ and Cr3+ were fabricated by the hydrothermal treatment in methanol and NaOH mixture. The morphology, crysralline phase, composition were characterized by powdered X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Barrett-Joyner-Halenda methods (BET) and X-ray photoelectron spectroscopy (XPS). The results showed that nanotubes possess anatase phase and are composed up of 8-12nm in diameter and 360-400 nanometer in length. The band gap of the TiO2 nanotubes was determined using transformed diffuse reflectance spectroscopy according to the Kubelka-Munk theory, showed pronounced band gap decrease on doped TiO2 nanotubes. The photocatalytic activity of doped nanotubes were evaluated in terms of degradation of phenol and photoreduction of CO2 into methanol and ethanol under UV and IR radiation. It was found that Fe3+ and Cr3+ doped TiO2 nanotubes exhibited much higher photocatalytic activity than undoped titanate nanotubes.
Key Words: Cr/Fe doped TiO2, Photocatalyst, CO2 conversion, Phenol degredation, IR radiation, Sun radiation.
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Following the discovery of nanotube morphology in carbon, many studies have been devoted to the preparation and characterization of various inorganic nanotubular materials (Mohapatra et al., 2007). Among these nanostructures, the titania nanotubalar materials are of high interest due to chemical inertness, strong oxidizing power, large surface area, non toxicity, high photocatalytic activitry, high cation exchange capacity, strong oxidizing, low cost production and relatively good stability at elevated temperatures (Liang et al.,2009; Khan et al., 2006). It has been broadly explored as a material for water splitting. For the production of solar hydrogen (Chen et al., 2005). As a source of catalyst for the conversion of CO2 into valuable products like methane and methanol (Khan et al., 2006; Chen et al., 2005). Moreover these nanotubes are widely exploit in lithium ions batteries, electrochemical devices, gas sensors, photoluminescence ion exchange and in photovoltaic dye sensitized solar cells (Kuang et al., 2008).
Recent research on TiO2 nanotubes is aimed at understanding its strong photocatalytic activity which is useful in various environmental pollution remediations, such as air purification, hazardous waste remediation and water purification (Zaleska, 2008). The presence of pollutants such as CO2 , NO2 in air, organic dyes (methylene blue, reactive blue, methyl red etc), and many other organic hydrocarbons (phenol, formaldehyde and gasoline), produced as result of many industrial processes (manufacture of dyes, food processing, pesticides, polymers) have caused the sever environmental problems since couple of decades (Zaleska, 2008; Ismael et al., 2007). Titanium dioxide has sufficient band gap energies for promoting or catalyzing a wide range of chemical reactions of environmental interest Ismael et al., 2007). In particular it has been used to oxidize industrial pollutants and to convert them into harmless products of H2O, alcohols and other useful hydrocarbons. However, powdered photocatalyst have some limitations. It needs post treatment separation in a slurry system after photocatalyst reaction. Though this can be overcome by forming TiO2 into nanotubes which resulted in increase surface area and catalytic activity and catalyst can also be steadily separated after photocatalytic reaction (Fujishima et al., 2006). Interest in TiO2 nanotubes photocatalysis increased after the Fujishma and Honda in 1972 who discovered the photocatalytic splitting of water on TiO2 electrodes (Kochkar et al., 2009). Furthermore, Coating the nanotubes with transition metals results in boosting of their activity (Kochkar et al., 2009; Dmitry et al., 2005).
TiO2 based nanotubes (TNTs) of different geometrical shapes and microstructures have been developed through various approaches which compromises the electrochemical anodic oxidation of Ti, assisted tempelate method and alkaline hydrothermal treatment (Mohapatra et al., 2007; Liang et al.,2009; Khan et al., 2006). TiO2 based nanotubes were reported first time by Hoyer via template assisted method. Afterward TNTs were successfully fabricated by electrochemical anodic oxidation and hydrothermal treatment (Wang et al., 2004). There are respective advantages and limitations in each of above mentioned method and technical problems may arise from difficulties in attaining uniform inner diameter of titanium dioxide nanotubes (Liang et al., 2009; Wang et al., 2004).
Concerning the template-assisted method which utilizes porous alumina, carbon nanotubes or polymer membrane and amphiphilic surfactant as template to construct material with a regular morphology (Macak et al., 2007). However the disadvantage of this method is that it encounters difficulties of pre-fabrication and removal of template and usually results in impurities. The highly ordered and self assembled TiO2 nanotubes were discovered by Grimes in 2001 via anodization of Ti foil in fluoride based electrolyte. This method has limitations because it results in low yield and highly expense of fabrication apparatus (Wang et al., 2004; Macak et al., 2007).
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Hydrothermal method was reported first time by Kasuga et al for preparation of TiO2 nanotubes in one step without use of template, by treating TiO2 with concentrated NaOH at elevated temperature in a tightly closed vessel. Titania nanotube produced by this method have good uniformity, controllable size, large surface area, high reactivity and in the amorphous state (Liang et al., 2009; Yury et al., 2000). These methods other than hydrothermal process are not appropriate for extensive production or not proficient to yield very low dimensional, well separated, and crystallized nanotubes. However, mechanism of formation and the morphology (diameter and length, wall thickness, size distribution and size of nanotube agglomerates) of nanotubes are still an issue of debate (Yury et al., 2000; Chen et al., 2002; Sun et al., 2003). Hydrothermal methods of TNTs formation using TiO2 as raw material (in the range of temperature between 110oC-180oC and time of 24-96 hrs) under high molar concentration of aqueous NaOH (10M), has been reported earlier. This limitation of long reaction time, high temperature, highly alkali concentration of alkali has been overcome in our method by using NaOH and methanol mixture rather than water. This resulted in nanotubes formation at low temperature (90 oC), reaction time (8 hrs) and at stirring rate of 300 rpm
In the present study we report a new synthesis procedure for the production of Fe/Cr doped and undoped TiO2 nanotubes via hydrothermal synthesis from TiO2 nanoparticles and its application for the degradation of organic waste, like phenol and carbon dioxide conversion to alcohol using UV radiation. The titania nanotubes fabricated through this method showed large surface are and high reactivity. Moreover, doping of titania nanotubes with transition metals like chromium and iron resulted in the enhanced photocatalytic properties. The effect of dopant on degradation of phenol, conversion of carbon dioxide to alcohols and on its optical properties were also studies for comparison.
Titanium dioxide, anatase (TiO2), sodium hydroxide (NaOH), Methanol, Hydrochloric acid (HCl), phenol, FeCl3, CrCl3 were obtained from Aldrich and were used without further purification. Ultra pure water was used without purification in the whole experiment.
2.2.1 Preparation of Titania nanotubes
Titania nanotubes were produced using hydrothermal method similar to that described in literature (Yury et al., 2000) with the modification of using methanol than water. In typical nanotube preparation, 3 g of the TiO2 powder was mixed with 300 ml of 2M NaOH in methanol, followed by hydrothermal treatment in a Teflon-lined autoclave high-pressure stainless steel at 90â-¦C for 8hrs and at stirring speed of 300rpm. After hydrothermal treatment the precipitates were separated through filteration and washed with 0.1M HCl and distilled water numerous times until the pH of the filterate turned to 7. The resulting sample was dried in oven at 100 â-¦C overnight to get TNTs and labelled as A-TNTs.
2.2.2 Prepration of co-doped Titania nanotubes;
Fe/Cr-codoped TiO2 nanotubes were prepared as follows: The synthesized titania nanotube (A-TNTs), 1 g, were stirred with 100mL aqueous solutions containing 0.1 g each of FeCl3, CrCl3 for 10min, The solution was transferred into a stainless steel autoclave with a Teflon lined autoclave, which was sealed and maintained at 90 OC for 2 hrs under H2-pressure of 25barr to dope titania nanotubes by ion exchange treatment. The H+ ion existing within nanotubes was ion exchanged with above referred metal ions. Subsequently, the resultant materials were centrifuged, washed and dried at 60 â-¦C for 4 h to get modified titania nanotubes. These samples were labelled as B-TNTs for Fe/Cr co-doped titania nanotubes.
2.2.3 Photocatalytic Activity
The degradation of phenol and conversion of CO2 were investigated in photocatalytic batch reactor setup as shown in Fig 1 (a&b). In two separate experiments 0.5g of A-TNTs and B-TNTs were added in 0.01M phenol for phenol degradation while for conversion of CO2, 0.2 gm of A-TNTs and B-TNTs were suspended into 0.2 N NaOH solution with pure CO2 bubbling at the rate of 100 mls/min. in both systems a mercury lamp (Ultra-Violet Products Inc., USA; LF-204.LS), UVC (254 nm) was placed on the top of reactor. The reaction was carried out for 1-2hrs with vigorous stirring at room temperature. After completion of reaction TNTs were separated through filteration using milipore filter paper. The filterate analyzed using GC/MS after the chloroform extraction method.
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The microstructures and morphology of the A-TNTs and B-TNTs were observed using Transmission Electron Microscope S-3500 N with Absorbed Electron Detector S-6542 (Hitachi Science System Ltd) and SEM scanning electron microscopy with energy dispersive X-ray spectrometer (SEM-EDX, Philips XL30, EDAX DX4). XRD patterns were recorded using a Scintag XDS 2000 D8 diffractometer equipped with CuKÎ± radiation of wavelength of 1.5406 A0 in the 2Î¸ range 200-800 with a step size of 0.020 to evaluate the crystallinity, phase and structure. Brunauer-Emmett-Teller (BET) measurements were done to determine specific surface area nitrogen adsorption-desorption and average pore volume was also evaluated from the data using the Barret-Joyner-Halender (BJH) method. EDX (Inca Energy, Oxford Instrument Microanalysis, Ltd) were used to determine the surface comparison of samples under specified conditions of 20 keV, 25mm working distance and magnification varying from x 300-1000. Diffuse reflectance spectroscopy (DRS) was used for the optical characterization of TiO2 nanotubes. All spectra were taken at room temperature in the range of 200-1000nm on Lambda-950 Perkin-Elmer. The chemical status and shift of the catalyst surface due to doping were analyzed by X-ray photoelectron spectroscopy (XPS) using a VG Microtech MT500 spectrometer, operated with a constant pass energy of 50 eV and with Mg KÎ± radiation as the excitation source (hÎ½ = 1253.6 eV). The catalyst was pressed into a pellet, and then adhered on sample holder by carbon tape.
4. Results and discussions
The phase and crystal structure of TNTs before and after calcinations in air at 450oC for 2hrs,was investigated by XRD pattern and shown in Fig 2(a&b) for udoped and doped TNTs. The intensities and positions of the observed peaks are in agreement with literature values (Zhuang et al., 2007). The XRD peaks were sharp which suggests that synthesized nanotubes have relatively high crystallinity and attributable to the anatase phase (indicated by peak 24).The peaks became sharper after annealing which demonstrates that cryatallanity of nanotubes enhances with increasing the heat treatment.
BET specific surface area of doped and undoped TNTs is much higher to the TiO2 nanoparticles. BET surface area has no effect on the phase of the TNTs but has an effect on the photcatalytic degradation reaction (Hussain et al., 2009). Doping on titania nanotubes with metals resulted in increase in BET surface area and pore volume. Larger surface area has increased adsorption/desorption capacity, consequently good photocatalytic activity (Hussain et al., 2009; Diebold et al., 2002). Table 1 presents the measured surface area of the prepared titania nanotubes, the surface area of the commercial TiO2 is also included for comparison.
Table 1: BET Surface area and pore volume of doped and undoped samples and commercial Degussa P25.
Surface area (m2g-1)
Pore Volume / (cm3Â·g-1)
Fig 3(a &b) and Fig 4(a&b) shows the SEM and TEM image of doped and undoped TNTs. As seen in the SEM image, TiO2 nanotubes have had obvious tubular and uniform morpholgy with an average diameter of 8-12nm and with narrow size distribution.. The dispersion of Fe/Cr on the nanotubes is uniform as depicted in Fig. 3b .
TEM analysis verified that tubes were hollow with an open end having needle shape structures. Furthermore TEM pattern shows that these tubes are uniform, single walled and are upto several hundred nm in length. No TiO2 nanoparticles exist around the nanotubes, proving high conversion of nanoparticles into nanotubes under employed experimental conditions. Moreover, the chemical composition and respective percentage of each atom present in the samples, was analyzed by EDX. Table 2. Presents the percentages of O, Ti, Na, Fe, Cr elements confirming the formation of doped and undoped TNTs.
Table 2: EDX analysis of doped and undoped TNTs.
4.1 The DRS Studies
The band gap of samples were measured by Kubelka-Munk function F(R) which is related to the the diffuse reflectance, R, of the sample according to the following relation (Hussain et al., 2009; Diebold et al., 2002)
F(R) = (1-R)2/2R
Where Ì€RÎ„Ì€ is the absolute value of reflectance. The energy bandgap of the doped and Fe/Cr doped TNTs were calculated from their diffuse-reflectance spectra by plotting the square of the Kubelka-Munk function F(R)2 vs energy in electron volts. The linear part of the curve was extrapolated to F(R)2 = 0 to get the direct energy bandgap. The optical band gap of the all samples was determined by the above method.
Fig. 5 shows the comparative the UV-vis diffuse reflectance spectrum of A-TNTs, B-TNTs taken at room temperature in the range of 200-800nm. It is clear from the above figure that doping transition metals into titania nanotubes tune the optical band gap in comparison to undoped TNTs, thus shifting UV absorption to visible-light absorption and band gap narrowing. The band gap is reduced with doping of TNTs with Fe and Cr (Kochkar et al., 2009; Quan et al., 2005; Tongpool et al., 2007) which was determined by using. K-Munk function. The narrower band gap is, the more easily an electron is excited from the valence band to the conduction band. The differences in the band gap and light absorption property lead to different photocatalytic behavior. The UV-vis diffuse reflectance spectra are in good agreement with the observed photocatalytic activity (Kubo et al., 2006).
Figure 6 (a&b) shows the bandgap calculation of A-TNTs, B-TNTs from K-Munk function, respectively. The direct band gap energies estimated from the intercept of the tangents to the plots are 3.25 eV and 1.85 eV for the samples A-TNTs and B-TNTs, respectively. The decrease in band gap energy of B-TNTs can be ascribed to Fe/Cr atom would be incorporated into the lattice of titania. Thus, it induces a larger red-shift and bandgap narrowing thus changing the crystalline and electronic structures (Kochkar et al., 2009; Tongpool et al., 2007).
It was also found, that constitution of the titania nanotubes (doped and undoped TiO2) also plays an important role in its photoactivity This change in energy bandgap is directly reflected in the photocatlytic activity studies, where appreciable change is observed due doping (Dmitry et al., 2005; Tongpool et al., 2007).
TiO2 nanoparticles were transformed to TNTs after hydrothermal treatment. However, the exact mechanism of formation and the morphology of TNTs is still subject of debate. There are few reports about the growth mechanism of titania (Kubo et al., 2006). Kesuga et al proposed that TNTs are formed during the acid washing step (Kubo et al., 2006; Chen et al., 2002). The nanotubes obtained after hydrothermal synthesis, showing very clear morphology and well defined structure.the nanotubes grow extensively with the length of several hundred nanometers with smooth tubular surfaces (Zhang et al., 2004). From the results of present work and previous reports we proposed a plausible mechanism of formation of nanotubes. certain Ti-O-Ti building units in titania are broken down via alkali hydrothermal treatment of titania with NaOH, resulting in the formation of new co-ordination i.e.,Ti-O-Na and Ti-OH , lateron, some of Na+ ions were expected to be exchanged with H+ ions during acid treatment and at this stage titanate sheets were exfoliated into layered nanosheets which subsequently scroll and form nanotubes at high temperature and high pressure to avoid electrostatic repulsions within the charges and to minimize the steric repulsions between the sheet edges while the tubular morphology is retained (Chen et al., 2002; Zhang et al., 2004; Liu et al., 2002). These H+ ions were then exchanged with Fe3+/Cr3+ while reaction of titania nanotubes with their respective salts to obtain doped titania nanotubes (Zhang et al., 2004).
To study the surface composition and chemical states of the synthesized nanotubes, the doped and undoped TNTs (A-TNTs & B-TNTs) was also analyzed by XPS and its XPS spectra are shown in Fig. 7 (I&II). The individual peaks of O1s at 529.5 eV and Ti 2p at 458.2 and 463.9 eV (Liu et al., 2002; Huogen et al., 2007) can be clearly seen in the high-resolution spectra (Fig. 7 I&II), which mean that chemical state of the sample is Ti4+ bonded with oxygen (Ti4+ -O), which has been modified by iron and Chromium doping. The binding energy of the Ti 2p3/2 and 2p1/2 band in case of doped Cr2O3 /Fe2O3 -TiO2 samples was found to be higher than that of the pure TiO2. This shift towards higher side could be attributed to an atomic dispersion of iron and chromium on TiO2, and the formation of a Ti-O-Cr/ Ti-O-Fe bond (Chen et al., 2002; Huogen et al., 2007; Kim et al., 2006).
4.1. Photocatalytic Degradation of Phenol
The TiO2 anatase is the main catalyst used in the contaminants photodegradation. However, the photocatalytic activity of TiO2 nanotubes has been little explored in the literature (Huogen et al., 2007; Kim et al., 2006). Thus, a detailed study of the photocatalytic properties of TiO2 nanotubes is the need of the day. In this study photocatalytic activity of the mesoporous titania nanotube composites was evaluated by photocatalytic phenol degradation, the results are shown in figure 8, under IR and visible light illumination. The degradation kinetics was computed by the change in phenol concentration (1 g/L) employing UV-visible spectrometry as a function of irradiation time. The phenol conversion is estimated using the following formulation (Hou et al., 2007).
Phenol Conversion (%) = [Phenol]0 - [Phenol]t / [Phenol]0 x100 (1)
The study of figure (8) indicates that Fe/Cr doped nanotubes decompose phenol more rapidly than does titania nanotubes alone under uv light .similar trend is observed In case of IR irradiation.
The photocatalytic decomposition of phenol is remarkably accelerated by the doped-TNTs photocatalysts prepared in the present work. This may be due to the inhibition of a spontaneous recombination between the hole and electron occurring on the surface of excited TNTs (Xu et al., 2005) by the metals deposited on the TNTs surface. The photodegradation efficiency of titanate nanotubes is 35%, while after doping the efficiency increased to 75% under IR irradiation. Metal ion doping influence the photoactivity of TiO2 by electron or hole traps. Where the trap causes the formation of some active species that benefit degradation of phenol. Here the electron scavenger effect of Cr3+/Fe3+ prevent the recombination of electron and hole pairs thus resulting in increase of the efficiency of photodegradation process (Kim et al., 2006; Hou et al., 2007). In Fe/Cr-doped samples, electrons are either directly trapped at Ti(IV) surface sites (form Ti3+) or in deeper Fe(III)/ Cr(III) sites (form Cr2+/Fe2+). In this case, the trapped electron can be easily transferred from Cr2+/Fe2+ to a neighboring surface Ti4+ because of the proximity of the energy levels (Dmitry et al., 2005; Huogen et al., 2007; Hou et al., 2007). As a result, the photocatalytic activity is improved after the doping. The proposed mechanism of process is presented in figure (9). The degradation process is enhanced with the presence of Fe & Cr because of the decrease bandgap. These materials provide sufficient negative and positive redox potential, which accelerate the degradation process.
4.2 Conversion of CO2 + H2O by solar energy
The conversion of CO2+H2O into alcohol by the synthesized undoped and doped titania nanotubes was evaluated under UV and IR radiation, respectively and presented in figure 10(a&b). The comparative stdudy of figure 10(a&b) reveal that the doped TNTs show considerably higher conversion of CO2 + H2O in comparison with the undoped nanotubes under both IR and UV irradiation. CO2 conversion is calculated using the following formula .
CO2 conversion (%) = [CO2]t/[CH3OH]0 x 100 --------- (2)
When titanium dioxide absorbs UV radiation from sunlight or illuminated light source, it will produce pair of electrons and holes. The electrons of valence band are excited when illuminated by light. The excess energy of this excited electro promoted the electron to the conduction band of titanium dioxide therefore producing the negative electron (e-) and positive hole (h+) pair within the catalyst (Xu et al., 2005). A portion of this photo-excited electron-hole pairs diffuse to the surface of the catalytic particle (electron hole pairs are trapped at the surface) and take part in the chemical reaction with the adsorbed donor (D) or acceptor (A) molecules (Syoufian et al., 2007). The positive holes can oxidize donor molecules i.e., break apart water molecules to form hydrogen gas and hydroxyl radicals which combines with carbon to form alcohol whereas the negative electrons can reduce appropriate electron acceptor molecules i.e., react with oxygen molecules to form super oxide anion [Fig. 9], illuminated TiO2 photocatalysts can decompose and mineralize organic compounds by participating in a series of oxidation reactions leading to carbon dioxide to harmless substance that can be released to the environment (Xu et al., 2005; Syoufian et al., 2007). This cycle continue when the sun light is available. The energy needed to activate TNTs is 3.2 eV or more, which nearly corresponds to UV radiation wavelength of 380 nm or less. This makes it possible to use the sun as illumination source, since about 4-6% of the solar energy that reaches the earth's surface is less than 400 nm. This process is referred to as heterogeneous photocatalysis or more specifically, photocatalysis oxidation (PCO). The energy difference between valence band and conduction band is referred as band gap (Jin et al., 2004).
We propose here that the Fe3+ and Cr3+ presence inside the lattice of TiO2 nanotubes induces a delocalized electron density within CO2+H2O structure that promotes the adsorption of the molecule on to the surface. This effect could support the reaction pathway to the mineralization on to TiO2 surface with consequent yields of intermediate species and creates extra energy due to presence of dopand which results in the breakage of H-OH and C-O bonds. We speculate that molecules can adsorb through the formation of Ti+4(surface) Ï€ electron or OH (surface) Ï€ electron type complex and latter is related to the production of products. For both type of adsorption, the nature of promoter related to their withdrawing or donor capacity plays a major role. All types of Lewis and Bronsted-Lowry sites are generally present at various extents on the lattice of TiO2 nanotubes [29, 30]. In the CO2 + H2O conversion to alcohol, two important species are involved in the process, H+ (hydrogen radical) and CO2- (carbon dioxide anion radical). The incident photons are observed by TiO2 and photoexcited electron (e-) and positive hole (h+) are formed in the catalyst due to presence of Fe3+ and Cr3+ ( i.e., by a charge transfer to the excited site of Fe-Cr-Ti-O). Furthermore the photoexcited electrons and holes in the lattice are separated and trapped by appropriate sites of Fe-TiO2 and Cr-TiO2. The whole react with H2O adsorbed on the surface of the catalyst giving rise to oxygen and H+. The H+ ions interact with excited electrons, resulting the formation of H+ radical. This is enhanced by the presence of multiple active sites of Fe and Cr coupled with TiO2. The interaction of all these species results in the formation of alcohol. The role that which types of sites are responsible for the photocataytic reaction is difficult to assess. This study is under way at present and will be published later.
In this work, we have synthesized mesoporous nanotubes by hydrothermal treatment in aqueous sodium hydroxide solution. Direct hydrothermal synthesis method is easy and efficient to synthesize pure and titanate nanotubes doped with transition metals.
The use methanol and sodium hydroxide mixture, the considerable decrease in reaction time , reaction temperature was achieved.
High aspect ratio of TiO2 undoped and doped TiO2 nanotubes of homogenous length are synthesized.
XRD result suggests that cryatallanity enhanced after annealing while the band gap and BET surface area increased after doping.
TNTs revealed a larger specific surface area and a higher pore volume, than that of nanoparticles, leading to an obvious enhancement of the photocatalytic activity for the photocatalytic degradation of pollutants (CO2 and phenol) under UV and IR irradiation.
Doping of metals, such as Iron and chromium on the surface of TNTs resulted in enhanced rate of the photocatalytic decomposition of organic compounds in aqueous solutions.
In conclusion, TiO2 nanotubes are found to be feasible and attractive for use in further investigation of CO2 reduction for CO2 environment management.