Received Date: October 17, 2014; Accepted Date: January 06, 2015; Published Date: January 15, 2015
Citation: Rajkumar R, Singh N (2015) To Study the Effect of the Concentration of Carbon on Ultraviolet and Visible Light Photo Catalytic Activity and Characterization of Carbon Doped TiO2. J Nanomed Nanotechnol 6:260. doi:10.4172/2157-7439.1000260
Copyright: © 2015 Rajkumar R, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
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Photo catalytic degradation of pollutants in water or air using titanium dioxide has been considered one of the efficient methods but it shows poor absorption of visible light and requires ultraviolet light for its activation. Which increase the cost of system, So that we moved toward the doping of non-metal in TiO2 which gave good result in visible and UV light, carbon doped TiO2 is prepared using air heating method at lowest temperature and minimum time by taking glucose and ethanol as carbon source. The experimental results showed that carbon was successfully doped into TiO2 .The physical properties of carbon doped TiO2 were studies by XRD, TG-TGA and UV spectroscopy which showed that the photo catalytic efficiency increase in UV and visible lights by creation of oxygen vacancies and Ti3+ in C-TiO2 catalyst. UV spectroscopy results showed the optical response of TiO2 was shifted from UV to the visible light region.
C-doped TiO2 nano particles; Photocatalysis; TiO2; Doping with non-metals; Preparation; Low temperature
Semiconductor titanium dioxide (TiO2) is a promising photocatalyst for air purification, water disinfection, hazardous waste remediation and water purification . It exists as three different polymorphs; anatase, rutile and brookite . It is typically an n-type semiconductor due to oxygen deficiency . The band gap is 3.2 eV for anatase and 3.0eVfor rutile [4,5]. Unfortunately, because of its large band gap of 3.2 eV, it can be activated only under UV light irradiation of wavelength (<387 nm). However, UV light constitutes only a small fraction (about 3-5%) of the solar spectrum; it will have a positive effect for improving the photo catalytic efficiency of TiO2 by shifting its optical response to the visible range. Hence scientists are exploring methods to modify TiO2 to absorb visible light. For example, carbondoped TiO2 was found to be effective for water splitting under visible light. Most of these investigations had been made to convert the TiO2 so that its absorption power shift from the UV to the visible region by doping of TiO2 with transition metals such as Cr, Fe, Mn, V [6-11], etc. However the photo catalytic activity of transition metals doping is poor because of their thermal instability . The non-metal doping has been considered for about ten years  as a best way to change the photoabsorption properties of TiO2for using of solar light with applications in photocatalysis and photo electrochemistry [12-15]. Due to doping, the dopants of non-metals in the lattice might affect the band edges or introduce impurity states in the band gaps of the system [16,17]. Carbon-doped titania with high surface area (204 m2/g) was prepared by temperature programmed carbonization of K-contained anatase titania under a flow of cyclohexane at temperatures between 450 and 500°C. This carbon-doped titania has much better photo catalytic activity for gas-phase photo-oxidation of benzene under irradiation of artificial solar light than pure titania. Obviously, these non-metals doping in TiO2 were all synthesized at high temperature. So it is still a challenge to prepare non-metal doped TiO2 at a low temperature, especially for the energy-saving production of visible-light driven photo catalyst in a large scale for pollutants removal. They prepared carbon doped TiO2 by hydrothermal treatment at 160°C which displayed stronger absorption in both UV and visible range. These papers gave me the knowledge to develop a low-temperature method to prepare carbon-doped TiO2, by using ethanol and titanium oxide Degussa (P25) as the carbon and TiO2 sources, respectively. The carbon doping could take place at the same time under a new method by using as heating medium except water vapor treatment at 180°C for 4 h at constant heating in hot air oven. To the best of our knowledge, this is the lowest temperature to prepare carbon-doped TiO2 using air as heating medium. It is found that the resulting carbon-doped TiO2 by this new method exhibits much higher photo catalytic activity than the undoped counterpart and Degussa P25 on the degradation of phenol under visible and UV light. The focus of this work is on the determination of the true concentration of raw, exfoliated HiPCO SWNTs in the supernatant phase. A UV-visible absorption-based approach is suggested for a direct measurement of the SWNT and the surfactant concentration in the supernatant. UV-visible absorbance spectra of SWNTs-surfactant dispersions and surfactants alone reveal that the intensity of a certain peak, attributed to the π-plasmon resonance absorption, is unaffected by the presence of most surfactants. A calibration plot is then made by monitoring the intensity of the peak as a function of the true concentration of the exfoliated SWNTs. Thus, we are able to determine the unknown concentration of surfactant-dispersed HiPCO SWNTs in the supernatant solution, simply by measuring its optical absorbance. Moreover, we can now calculate the surfactant efficiency in dispersing SWNTs. Cryogenic-transmission electron microscopy and thermo gravimetric analysis techniques are used for the characterization of these dispersions and to complement the UV-visible measurements.
Carbon doped TiO2 nanoparticles were prepared by air heating method by taking 4 g of TiO2 and 40 ml ethanol. The mixture of ethanol and TiO2 was constantly stirred by magnetic stirrer for 10 min and then the heated for 4h at 180°C in hot air oven. The catalyst was kept in dissector for cooling to obtain white powder of carbon doped TiO2. For calcinations half of catalyst was kept in muffle furnace for 1 hour at 350°C. Similarly this method is used for making of different concentration of ethanol C-TiO2 at are 92.20% ethanol TiO2 at 94.04%, ethanol TiO2, 95.17% ethanol TiO2, 95.94% ethanol TiO2 etc.
Characterization of C-doped TiO2 nano particles
X-ray powder diffraction (XRD) analysis was performed using Goniometer=PW3050/60 X-ray diffract meter (CuKa k = 0.154 nm). The working voltage and current of the X-ray tube were 45 kV and 40 mA, AVarian Cary 100 Scan UV-visible system equipped with a lab sphere diffused reflectance accessory was used to obtain the absorbance spectra of the catalysts over a range of 200-600 nm. Lab sphere USRS- 99-010 was employed as a absorbance standard.
Photocatalytic degradation experiments
The photocatalytic degradation experiments of methyl orange by the TiO2 photocatalysts were performed under visible light irradiation. The photocatalytic degradation was monitored by use of a UV-Vis spectrometer at λ max = 468 nm. In all experiments a 100 mg of catalyst was suspended in 100 ml of 5 ppm methyl orange solution in a conical flask. The flask was wrapped with aluminum foil then placed in the dark for an hour for adsorption to take place. The system was then irradiated with visible light from a lamp that was fixed in the middle of the system and 12 cm above the surface of the solution. To detect changes in concentration, aliquots of methyl orange (5 mL) were taken after every 30 minutes and centrifuged. The absorbance of the clean solution was measured by a UV-Vis spectrometer.
To characterize the crystalline structure of the carbon doped and undoped TiO2 samples, for that XRD patterns in Figures 1 and 2 were obtained. The pattern of undoped TiO2 showed the presence of anatase and bookie However, only anatase is present in the XRD pattern of C-doped TiO2 indicate the doping of carbon prohibit the formation of brookite. It can be also found that the crystal size of anatase decrease with the carbon doping by comparing the (101) peak of anatase in Figures 1 and 2. This decrease on crystal size can be attributed to dissimilar boundaries created by carbon doping. Meanwhile, the significant decrease on the intensity of XRD pattern of TiO2. The crystal size decrease can be attributed to dissimilar boundaries created by carbon doping, because of the significant decrease on the intensity of XRD pattern of TiO2. The diffraction peaks of XRD of carbon doped TiO2 were not observed which showed that carbon was highly dispersed on TiO2 or XRD was not sensitive enough to detect such minor changes to TiO2.
The recently started R&D activity of growth and characterization of NLO single crystals has been continued in view of (i) growing good quality crystals (ii) growing big size crystals suitable for device applications related to photonics, (iii) to enhance the efficiency of second harmonic generation (SHG) by using different dopants and functional groups, (iv) to search new NLO materials and grow their single crystals, (v) to evaluate their crystalline perfection by highresolution XRD and (vi) other studies like powder XRD, FTIR, UV-Vis. etc. In this endeavor and with the help of various collaborators working in this upcoming area we have achieved several important R&D results leading to twenty one articles in leading SCI journals and one article in non SCI journal. Some of these important results are briefly described in Figures 1 and 2.
If a sample contains anatase and rutile, the mass fraction of anatase (WA) and futile (WR) can be calculated from the equations 1 and 2:
Where AA and AR are the integrated intensity of the anatase (101) and rutile (110) peaks, respectively, WA and WR represents the mass fraction of anatase and rutile, respectively, and KA is a coefficient and its value is 0.886. The average crystallite sizes of anatase were determined as follows:
Where λ is the wavelength of characteristics X-ray applied and β is the half-value width of anatase (101) peak obtained by XRD and is X-ray diffraction angle. According to the Scherrer equation using the data of each different phase after correcting the instrumental broadening in Table 1.
|88.75% ethanol and 4 g TiO2||41.74|
|92.20% ethanol and 4 g TiO2||43.68|
|94.04% ethanol and 4 g TiO2||60.20|
|95.17% ethanol and 4 g TiO2||61.01|
|95.94% ethanol and 4 g TiO2||62.50|
Table 1: Size comparisons between TiO2 and its carbon TiO2.
UV–vis diffuse reflectance spectra
UV vis absorption of undoped TiO2, 88.75%, 92.20%, 95.94% ethanol doped TiO2 are shown in Figures 3 and 4 respectively. Carbon-doped TiO2 sample absorbs more light in the range of 400-450. Obviously, the optical absorption edges of these non-metal doped samples shift to the lower energy region compare to the undoped TiO2. This showed that our method is effective to extend the absorption to visible light range. A plot of the (ahv) 1/2 versus the energy of absorbed light affords the band gap energy approximately 2.90 and 3.1 eV for the carbon-doped TiO2 and pure TiO2, respectively. This result obviously reveals the band gap of the carbon-doped TiO2 is narrower than that of the pure TiO2.
To see the variation in weight of C-TiO2, TGA measurements were carried out. The results are shown in Figure 5. As expected, the C-TiO2 powders can be oxidized by oxygen in the air at an elevated temperature of From 300°C the total weight increase is about 10%. If some amount of carbon were oxidized, the weight increase would be 31.25% as calculated from the reaction: C-TiO2 + 4O2 - TiO2 + CO2. The amounts of residual carbon and are dependent on the hot air oven temperature.
The effect of catalyst concentration on photo catalytic degradation efficiency of TiO2 was investigated by keeping all other experimental parameter constant. It is observed that there is steady increase in the degradation efficiency up to 5 g/l of the catalyst beyond which the degradation efficiency decrease. Within the range of catalyst dose from 2.5 to 5.0 g/l, observed phenomena enhancement in degradation may be due to an increase in available absorption and catalyst sites on the surface of carbon doped TiO2. A further increase in concentration of catalyst however, may cause light scattering, blocking of UV light and particles of agglomeration which decrease the number of active sites. The result indicates that an optimized catalyst concentration 5 g/l is necessary for enhancing the degradation efficiency, as shown in Figures 6 and 7.
Figures 8 and 9 shows the photocatalitic degradation curves of 100ppm over the P25, 88.75%, 92.20%, 95.94% ethanol doped TiO2 photocatalysts with different carbon concentration and got maximum degradation near about 97% of 100 ppm phenol solution at 88.75% ethanol doped TiO2 under UV/Vis light. The photocatalytic degradation of 100 ppm phenol solution reaction times is first order with K from 0.0144 to 0.0058 min^-1 in concentration range of 88.75% -95.94% ethanol in TiO2 as confirmed by the linear transformation of ln (Co/C)~t shown in Figures 10 and 11. Apparent rate constants in UV light were obtained with various catalyst samples with different ethanol concentration from Figure 12 and graph shows that K decrease with increase in the initial concentration of ethanol and maximum K is 0.0144 for 88.75% ethanol doped in TiO2 because XRD and UV result shows that 88.75% C-TiO2 have minimum partical size (41.74 nm) and maximum shift in visible region (450 nm). When ethanol concentration added greater than 88.75% that decrease photocatalytically active sites on the surface of catalyst particals and reduing the intreaction of photons with these sites.which can reduced the photocatalytic activity.
The photocatalytic degradation of 100ppm phenol solution reaction times is psedue first order with K from 0.0154 to 0.0078 min^-1 in concentration range of 88.75% - 95.94% ethanol in TiO2 as confirmed by the linear transformation of ln (Co/C)~t shown in Figure 11. We analyze that when we increase concentration of ethanol in TiO2 degradation of phenol under solar light increases because rate constant increase and varies in between 0.0158 to 0.0078 min^-1. As shown in Figure 13 the rate constant in solar light is higher than that of UV light so degradation of phenol solution in solar light is higher than UV light. The increased photoactivity of carbon-doped TiO2 may be associated with more-complex phenomena than the simple presence of the impurity atoms, such as, for instance, concomitant changes in the stoichiometry of the sample. According to laser flash photolysis experiments, 15 the holes formed upon visible light excitation are less reactive than those formed upon UV excitation in pure TiO2, because they are trapped at midgap levels induced by carbon doping. Lesser mobility is expected only for the holes but not for the electrons, which are excited into the conduction band. This means that carbon-doped systems exhibit decreased direct oxidation ability. On the other hand, reduction properties or indirect oxidation properties (via reactions with surface intermediates of water oxidation or oxygen reduction) are much improved upon irradiation with visible light, also because the holes trapped at the carbon doping site do not serve as effective charge recombination centers  and carbon doping favors the formation of oxygen vacancies. This feature makes nonmetal doping of TiO2 much more appealing than transition-metal doping. Analysis of the computed electronic structure shows that carbon doping in anatase results in a series of localized occupied states in the band gap whose density and nature are dependent on the interaction with the oxide matrix, the dopant concentration, and the presence of oxygen vacancies. It was observed that rate constant of 40 ml ethanol doped have higher in UV light than visible. Under UV irradiation, the best degradation of phenol was achieved in the presence of 88.75% ethanol in TiO2 and, it was observed that at relatively high ethanol concentrations, nanoparticles agglomerated which resulted in decreased surface area and the corresponding decrease photocatalytic activity in sun light. Therefore, increasing the dopant concentration has two opposite effects on the photocatalytic activity of the carbon doped TiO2 catalysts; increasing light absorption capacity and decreasing surface area. The photocatalytic activity depends on which one of these is the dominant factor. A lesser amount of surface hydroxyl groups was observed in the case of undoped TiO2 compared with the ethanol doped TiO2. This could be one of the reasons for the lower activity of the undoped TiO2 catalysts. Most of the non metal doped TiO2 samples absorb in the visible region was no direct correlation between the light absorption capacity of the carbon doped TiO2 catalysts and the rate of phenol solution degradation. It should be noted that the degradation rates in the presence of carbon doped TiO2 catalysts under UV were in general less than that of the undoped TiO2. Recombination of photo generated electrons and holes are one of the most significant factors that deteriorate the catalytic properties.
Intensity of sunlight
This was clearly explained in Figure 13.
UV-Vis absorption of the TiO2 nanoparticles
The UV-visible absorption spectra of the C-doped. TiO2 were shifted by between 7 nm to 36 nm depending on the doping level as shown in Figure 14. The highest shift of 36 nm was due to TiO2 doped with the lowest concentration of dopant (0.093 M glucose) while the lowest shift of about 7 nm was recorded for the sample doped with the highest concentration of dopant (0.278 M glucose). The absorbance increased with a decrease in the loading of the glucose, indicating that a higher concentration of the glucose led to a smaller particle size. The band gap was therefore increased as particle size decreased.
A visible-light-active carbon TiO2 nano particles acts as a photocatalyst was prepared by a new approach through carbon doping by using ethanol as carbon source at a lowest temperature by heating with air. Carbon-doped TiO2 possessed a high surface area and exhibited obvious absorption in the visible light range than P25 and undoped counterpart. It was found that carbon-doped TiO2 showed significantly higher absorbance in visible than P25 and undoped TiO2 under visible light irradiation. Our method is potential for industrial production of visible-light driven photocatalyst because of its convenience and energy-saving.