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Water disinfection through photoactive modified titania
Journal of Photochemistry and Photobiology B: Biology 130 (2014) 310–317
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology B: Biology journal homepage: www.elsevier.com/locate/jphotobiol
Water disinfection through photoactive modified titania DiptiPriya Sethi 1, Ajoy Pal 1, Ramasamy Sakthivel ⇑, Sony Pandey, Tapan Dash, Trupti Das, Rohit Kumar CSIR-Institute of Minerals and Materials Technology, Bhubaneswar 751013, India
a r t i c l e
i n f o
Article history: Received 18 July 2013 Received in revised form 20 November 2013 Accepted 6 December 2013 Available online 14 December 2013 Keywords: Titania Doping Absorption Reactive oxygen species Antibacterial property
a b s t r a c t TiO2, N-TiO2 and S-TiO2 samples have been prepared by various chemical methods. These samples were characterized by X-ray diffractometer (XRD), X-ray photoelectron spectroscopy (XPS), Laser Raman spectrometer, UV–Visible spectrophotometer, field emission scanning electron microscope (FE-SEM) and transmission electron microscope (TEM). X-ray powder diffraction study reveals that all three samples are single anatase phase of titania and the crystallinity of titania decreases with sulphur doping whereas nitrogen doping does not affect it. UV–Visible (diffuse) reflectance spectra shows that doping of titania with nitrogen and sulphur shift the absorption edge of titania from ultraviolet to visible region. XPS study confirms that both nitrogen and sulphur are well doped in the titania lattice. It is observed that nitrogen occupies at both substitutional and interstitial position in the lattice of titania. FE-SEM and TEM studies demonstrate that the particles are below 50 nm range. It is found that S and N doping of titania increased its water disinfection property in the order TiO2 < S-TiO2 < N-TiO2 under 8 W UV/UV–Visible light irradiation. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction The conventional water disinfection method involves chemical treatment such as hydrogen peroxide treatment, and chlorination; but, it has disadvantage due to its toxicity and other environmental problems. Hence, there is a need for development of non-toxic, environment friendly and cost effective method for efficient water disinfection. Photocatalytic approach using titania has been the subject of study for the last two decades for various environmental applications [1–7]. Titania is a semiconductor and well studied system due to its low cost, nontoxicity and stability. Photo-excitation of titania leads to generation of electrons in the conduction band and holes in the valence band. These holes help in the splitting of water to produce hydroxyl radicals (OH). These OH radicals are extremely reactive and have redox potential as high as 2.7 V. They are non-selective and readily attack pathogenic microorganisms. This photoexcitation phenomenon will happen only if titania is excited with ultraviolet radiation because titania (anatase) band gap energy is around 3.25 eV which is corresponding to 385 nm wavelength. Among various phases of titania such as anatase, rutile and brookite, anatase phase of titania is preferred due to its high surface area and performance in the photocatalytic activity [8,9]. Since the solar radiation contains only 5% UV radiation, complete use of it is not possible for photocatalytic activity. Therefore, band gap of titania needs to be reduced below 3.2 eV. Numerous efforts have been made to dope ⇑ Corresponding author. Tel.: +91 6742379414. E-mail addresses: [email protected], [email protected] (R. Sakthivel). Academy of Scientific and Innovative Research (AcSIR), Anusandhan Bhawan, 2 Rafi Marg, New Delhi 110 001, India 1
1011-1344/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jphotobiol.2013.12.003
TiO2 with certain metals to shift its absorption to the visible range, but metal-doped TiO2 shows poor photoactivities due to the detrimental effect of metal ions acting as favorable sites for charge recombination [10]. However, it has been reported that Ag/TiO2 powder shows good antibacterial property which further improves with the increase in crystallinity of titania [11]. The same material in a different form such as Ag/TiO2 nanofiber membrane exhibits good photocatalytic disinfection [12]. Pt/TiO2 photocatalyst has also been found to show absorption in the visible light for elimination of soil-borne pathogens [13]. Recent reviews by Zaleska [14] and Manoj et al. [15] discuss the role of various dopants in titania and their effects in shifting of absorption from ultraviolet to visible region for water treatment applications. Asahi et al. [16] reported that Ndoped TiO2 absorbs visible light due to narrowing of band gap induced by nitrogen doping and they demonstrated the photocatalytic activity in the visible light. Since the anion doped titania brought major changes in both the electronic and photocatalytic activities, now major focus has been given on these materials in order to understand the photocatalytic behaviour. In this investigation, we present the N- and S-doped TiO2 photoactive materials characterization and their effect on the bactericidal properties are explained on the basis of ultraviolet and visible light absorption properties.
2. Materials and methods 2.1. Preparation of TiO2, N-TiO2 and S-TiO2 Titania (TiO2) was prepared by the hydrolysis of titanium isopropoxide with water at room temperature, followed by separation
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Fig. 3. FTIR spectra of titania and doped titania samples. Fig. 1. X-ray powder diffraction patterns of titania and doped titania samples.
Fig. 4. UV–Visible (diffuse) reflectance spectra of titania and doped titania samples. Fig. 2. Laser Raman spectra of titania and doped titania samples.
Table 1 Raman spectral assignments for various samples. Wavenumber (cm1)
Spectral assignment
TiO2
STiO2
NTiO2
144 195 396 516 638
146 – 397 517 638
144 195 397 516 638
Eg Eg B1g A1g + B1g Eg
of precipitate by filtration and drying at room temperature. The dried sample was calcined at 450 °C for 3 h. In order to obtain nitrogen doped titania (N-TiO2), titanium isopropoxide (Sigma–Aldrich, reagent grade 97%) was hydrolyzed with an aqueous solution of ammonia (Analytical reagent grade) at room temperature for 4 h. The sample thus prepared was separated by filtration, dried at room temperature, and then calcined at 450 °C for 3 h. The sample was designated as N-TiO2. Similarly, sulphur doped titania (S-TiO2) was prepared by hydrolysis in the presence of Na2SO4
(Fischer Chemicals, India, Analytical reagent grade). The weight ratio of TiOSO4/Na2SO4/ethanol/water was maintained at 13.8:7.1:55.2:77.4 and the solution were stirred, followed by aging at 80 °C for 5 h. The final precipitate was filtered and dried at room temperature. The dried powders were calcined at 450 °C for 3 h to get a white colour powder. The sample was designated as S-TiO2. Phase identification of these powder samples were determined by X-ray diffraction (diffractometer: X’PERT PRO, PANalytical, Netherlands) using Cu Ka radiation source (wavelength 0.154056 nm). Surface morphology and elemental composition were studied by the field emission scanning electron microscope (FESEM, model ZEISS SUPRA55) and Energy dispersive specroscopy (EDS) attached to it. The transmission electron microscopic (TEM) images and the selected area electron diffraction (SAED) pattern were obtained with a TECHNAI G2, FEI operated at 200 kV. To get the TEM images, the powdered samples were dispersed in isopropanol by ultrasonication for 20 min. Then one drop of the dispersed solution was deposited on the carbon coated copper grid. The sample was dried completely under an IR lamp before taking the observation. The UV–Visible diffuse reflectance spectra (DRS) were recorded from 200 to 800 nm by a Varian Cary UV–Vis spectrophotometer. The Raman spectra were taken in a dispersive type
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Fig. 5. FE-SEM images (left side) and their corresponding EDX spectra (right side) of titania and doped titania samples.
micro-Raman spectrometer (Renishaw in Via Reflex, UK). Fourier Transform Infrared (FTIR) spectra were recorded in the range of 400–4000 cm1 wavenumber on a Fourier Transform Infrared spectrophotometer supplied by Perkin Elmer India Pvt Ltd., Kolkata and KBr was used as a reference. The X-ray photoelectron spectroscopy (XPS) spectra of various samples were carried out using VG Scienta hemispherical energy analyser source having resolution of 44.1 meV at 50 pass energy and Al Ka (hm = 1486.6 eV) source (S/N:10001, Prevac, Poland). Instrument base pressure of 5 1010 mbar was maintained during data acquisition.
under UV and UV–Visible light. One ml of sample was drawn from the reaction vessel at various intervals of time and serially diluted up to 104 times. This was followed by plating 0.1 ml of the sample on Liquid Broth Agar (LBA) plates by adopting spread plate method, followed by incubation at 37 °C without illumination for 18–24 h. After incubation the number of colonies were counted in all the plates, and The Colony Forming Unit (CFU) per ml was calculated for each sample at different time intervals by using the following formula:
2.2. Biocidal activity of titania and doped titania
where dilution factor is the reciprocal of the dilution in which the plate count was taken, and volume inoculated is 0.1 ml.
Biocidal activities of various samples under UV and Visible light conditions were examined with 107 cells/ml of Escherichia coli in tap water with 0.85% NaCl (to maintain the osmotic balance of bacterial cells during the experiment). A bioreactor was set up with 8 W UV and visible light with a magnetic stirrer on which the sample was placed in a beaker with 25 ml of water spiked with E. coli as mentioned above. For each sample, the biocidal activity was tested
CFU=ml ¼ No: of colonies Dilution factor=volume inoculated
3. Results and discussion X-ray diffraction patterns of TiO2, N-TiO2 and S-TiO2 samples are shown in Fig. 1. It reveals the presence of single anatase phase of titania in all three samples. However, peak broadness has been observed in case of sulphur doped titania which indicates that
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Fig. 6. FE-SEM images (left side) and their corresponding EDX spectra (right side) of titania and doped titania samples.
sulphur brings down the crystallinity as in case of SO4/ZrO2 [17]. The peak corresponding to h1 0 1i plane shows highest intensity among others peaks corresponding to 0 0 4, 2 0 0, 1 0 5, 2 1 1, 2 0 4 planes. The observed d values are matched with standard powder diffraction data of anatase phase of titania (JCPDS File No. 01-0894921). Laser Raman spectra of these samples are shown in Fig. 2. It shows bands around 144, 195, 396, 516, 638 cm1 and these are ascribed for the Eg, B1g, A1g + B1g, modes of vibrations [18]. The observed spectral bands position of all three samples and their corresponding vibrations are given in Table 1. The intensity of spectral bands drastically reduced when sulphur is doped in titania indicating poor crystallinity of sample, however, when nitrogen is doped in titania the intensity of these bands increases due to increase in crystallinity of sample. This observation is in accordance with the XRD results. The change in crystallinity of the samples also affects
the bactericidal properties as discussed below. FTIR spectra of all three samples (Fig. 3) show stretching vibration band for hydroxyl group of adsorbed water molecules at 3000–3550 cm1 and bending vibration band at 1630 cm1. The bands observed at 3695 and 800 cm1 are corresponding to respective stretching and bending vibration of surface hydroxyl group present in all three samples [19,20]. Diffuse (UV–Visible) reflectance spectra of all three samples have been shown in Fig. 4. It shows that the spectra of TiO2, NTiO2 and S-TiO2 samples are differing from each other. Nitrogen and sulphur doping brought drastic change in the spectrum of titania. It has been reported that N or S doping decreases the band gap energy of titania leading to red shift in UV–Vis absorption band [21,22]. Titania shows absorption around 390 nm wavelength where as N-TiO2 shows very strong absorption around 450 and
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Fig. 7. XPS spectra of TiO2, N-TiO2 and S-TiO2 samples.
Fig. 7b. XPS spectra of N-TiO2. Fig. 7a. XPS spectra of TiO2.
390 nm wavelength region. S-TiO2 sample shows a broad absorption in the visible range between 400 and 800 nm wavelength and narrow absorption (390 nm) in the UV region. FE-SEM images and their corresponding EDS spectra of all three samples are shown in Fig. 5. It reveals that all these samples have particles size below 50 nm, and they are distributed in very narrow range. This observation is in accordance with the reported results where titania
deposited on silicon support by plasma enhanced chemical vapour deposition also show the formation of nanosized titania [23]. EDS spectrum of titania shows peaks for titanium and oxygen. In addition to these peaks, other peaks for nitrogen and sulphur are observed in case of N-TiO2 and S-TiO2 samples respectively. Presence of aluminum peaks in all three samples is due to aluminum foil support used in the study. TEM pictures of these samples are depicted in Fig. 6. It shows that both TiO2 and N-TiO2 have similar morphology close to the spherical shape but not fully spherical.
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Fig. 8. Antibacterial property of titania and doped titania samples under UV light source.
Fig. 9. Antibacterial property of titania and doped titania samples under UV and visible light source.
However, S-TiO2 exhibits spherical morphology indicating the role of sulphur. It is seen from SAED patterns that both TiO2 and N-TiO2 have shown bright circular spots for various hh k li planes reflection of titania, whereas in case of S-TiO2 sample, the intensity of those spots drastically diminished and clear circular lines are observed due to poor crystallinity. This observation commensurates with the observed XRD data. XPS spectra of TiO2, N-TiO2 and S-TiO2 samples are shown in Figs. 7 and 7a–c. Interpretation of the peaks of XPS spectra have been done according to the available literature [24,25]. All three samples show well-defined spin-coupled peaks at the binding energy around 458.4 eV and 464.1 eV (Table 2) for Ti 2P3/2 and Ti 2p/ Ti 2p1/2 respectively which confirms the existence of Ti in the state of Ti4+. Oxygen peak at around 530 eV is observed for O 1S and this
Fig. 7c. XPS spectra of S-TiO2.
Table 2 XPS spectral assignment. Sample name
TiO2 N-TiO2 S-TiO2
Ti
O2
N
S
2P 3/2 (eV)
2P 1/2 (eV)
Lattice oxygen (eV)
Surface OH (eV)
Substitutional
Interstitial
Substitutional
Interstitial
458.4 458.5 458.8
464.1 464.2 464.5
530.2 530.1 530.1
532.0 532.6 532.3
– 399.7 eV –
– 400.7 eV –
– – 168.7 eV
– – –
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peak is de-convoluted to give two peaks, one observed at the binding energy of 530.2 eV and the other at 532.6 eV which are assigned to the lattice oxygen of TiO2 and oxygen due to surface hydroxyl (OH) group respectively. In addition to the above peaks, the N-TiO2 sample shows a peak for nitrogen at the binding energy of 400 eV (Fig. 7b), when this peak was further de-convoluted it has given two peaks; one at 399.7 eV and another at 400.7 eV for the substitutional N-doping (substitution of N by O) and interstitial N-doping in the lattice of TiO2 respectively. In this study, it is interesting to observe that both types of N-doping (substitution and interstitial) take place in the titania lattice structure. However, the peak intensity of 399.7 eV is much stronger than the other one observed at 400.7 eV peak indicating that more amount of nitrogen occupy at the interstitial position rather than substitutional position in the lattice of TiO2. In the case of S-TiO2 (Fig. 7c), a peak for S 2p is observed at the binding energy of 168.7 eV which might be due to the partial substitution of S4+ for the Ti4+ in the TiO2 lattice. Antibacterial property of TiO2, N-TiO2 and S-TiO2 samples examined separately under ultraviolet (UV) alone and UV–Visible lights are shown in Figs. 8 and 9 respectively. These figures illustrate that with increasing light exposure time the CFU counts of bacteria decreases. There is a drastic decrease in viable cell counts of the E. coli during the initial period of light exposure because the reactive oxygen species (ROS) generated due to photocatalysis probably plays a significant role in arresting the bacterial defense system, whereas at a later period of time it decreases very slowly. This could possibly be due to the competition between the residual active bacterial cells and cell materials (released during the photocatalytic process) for ROS [26]. However, antibacterial property differs from sample to sample, it shows the following trend: TiO2 < STiO2 < N-TiO2, irrespective of the light source used in this study, either UV light alone or UV–Visible light. However, it is very interesting to note that N-TiO2 and S-TiO2 samples show better antibacterial activity than TiO2 when both UV and Visible lights are used compared to UV light alone and these changes are due to change in their optical absorption properties. A comparative study of the antibacterial property of N-TiO2 and S-TiO2 samples, revealed that N-TiO2 shows better antibacterial activity due to its high crystallinity. This observation very well supports the results obtained by Kedziora et al. [11], where they reported that the antibacterial activity of Ag/TiO2 improves with the increase in crystallinity. Further, N-TiO2 shifts the absorption edge of titania relatively stronger than S-TiO2 from UV to visible region which helps in improving the photocatalytic activity to yield high antibacterial activity. N-TiO2 synthesized by Liu et al. [26] using ethylediamine as nitrogen source shows complete killing of 109 CFU/mL of E. coli cells in 120 min with photocatalyst concentration of 0.1 mg/mL. But, in this study, it has been observed that catalyst concentration of 1 mg/mL has taken 10 min for complete killing of 107 CFU/mL of E. coli cells. So our results are in good agreement with the literature [26]. Mechanism involved in antibacterial property of these materials is based on the following; when light interact with titania based materials present in the water system, it produces OH radicals, hydrogen peroxide (H2O2) and superoxide ions (O2). These are known as reactive oxygen species (ROS) which attacks the phospholipids of the cell membrane of bacteria in suspension [27]. As a consequence cell membrane gets ruptured due to lipidperoxidation which subsequently leads to cell death [28].
4. Conclusions Among TiO2, N-TiO2 and S-TiO2 samples, N-TiO2 shows good absorption in the visible in addition to ultraviolet region therefore, it yields high antibacterial property. Nitrogen/sulphur doping of
titania significantly improved the visible light absorption to yield high antibacterial activity and particularly this effect is seen more in case of N-TiO2. FE-SEM and TEM characterization revealed that the particles are below 50 nm range. XRD observation confirms the formation of anatase phase in all three samples. Sulphur doping decreases the crystallinity of titania whereas nitrogen doping does not affect it. XPS study confirms that nitrogen occupies the both substitutional and interstitial position in the lattice of titania. On the basis of the above observation, it is suggested that N-TiO2 could be a better choice material for water disinfection under combined UV and visible light exposure condition. Acknowledgments We acknowledge the Ministry of Environment and Forest, New Delhi for financial support and thanks to the Director of CSIR-Institute of Minerals and Materials Technology, Bhubaneswar for his encouragement. One of the authors Dipti Priya Sethi wishes to acknowledge the Rajiv Gandhi National Fellowship funded by Ministry of Social Justice and Empowerment Government of India. References [1] H.A. Foster, I.B. Ditta, S. Varghese, A. Steele, Photocatalytic disinfection using titanium dioxide: spectrum and mechanism of antimicrobial activity, Appl. Microbiol. Biotechnol. 90 (2011) 1847–1868. [2] D. Zhang, G. Li, J.C. Yu, Inorganic materials for photocatalytic water disinfection, J. Mater. Chem. 20 (2010) 4529–4536. [3] S. Sakthivel, M. Janczarek, H. Kisch, Visible light activity and photoelectrochemical properties of nitrogen doped TiO2, J. Phys. Chem. B 108 (2004) 19384–19387. [4] Y. Wang, Solar photocatalytic degradation of eight commercial dyes in TiO2 suspension, Water Res. 34 (2000) 990–994. [5] B. Naik, K.M. Parida, C.S. Gopinath, Facile synthesis of N-and S-incorporated nanocrystalline TiO2 and direct solar light driven photocatalytic activity, J. Phys. Chem. C 114 (2010) 19473–19482. [6] A. Mills, S.L. Hunte, An overview of semiconductor photocatalysis, J. Photochem. Photobiol. A 108 (1997) 1–35. [7] S. Devipriya, S. Yesodharan, Photocatalytic degradation of pesticide contaminants in water, Sol. Energy Mater. Sol. C 86 (2005) 309–348. [8] J. Zhu, J. Yang, Z.F. Bian, J. Ren, Y.M. Liu, Y. Cao, H.X. Li, H.Y. He, K.N. Fan, Nanocrystalline anatase TiO2 photocatalysts prepared via a facile low temperature nonhydrolytic sol–gel reaction of TiCl4 and benzyl alcohol, Appl. Catal. B: Environ. 76 (2007) 82–91. [9] F.D. Hardcastle, H. Ishihara, R. Sharma, A.S. Biris, Photoelectroactivity and Raman spectroscopy of anodized titania (TiO2) photoactive water-splitting catalysts as a function of oxygen-annealing temperature, J. Mater. Chem. 21 (2011) 6337–6345. [10] O. Carp, C.L. Huisman, A. Reller, Photoinduced reactivity of titanium dioxide, Progr. Solid State Chem. 32 (2004) 33–177. [11] A. Kedziora, W. Strek, L. Kepinski, G.B. Ploskonska, W. Doroszkiewicz, Synthesis and antibacterial activity of novel titanium dioxide doped with silver, J. Sol–Gel Sci. Technol. 62 (2012) 79–86. [12] L. Liu, Z. Liu, H. Bai, D.D. Sun, Concurrent filteration and solar photocatalytic disinfection/degradation using high-performance Ag/TiO2 nanofiber membrane, Water Res. 46 (2012) 1101–1112. [13] Y.L. Chen, Y.S. Chen, H. Chan, Y.H. Tseng, S.R. Yang, H.Y. Tsai, H.Y. Liu, D.S. Sun, H.H. Chang, The use of nanoscale visible light-responsive photocatalyst TiO2– Pt for the elimination of soil-borne pathogens, PLos One 7 (2012) e31212. [14] A. Zaleska, Doped-TiO2: a review, Recent Patents Eng. 2 (2008) 157–164. [15] M.A. Lazar, S. Varghese, S.S. Nair, Photocatalytic water treatment by titanium dioxide: recent updates, Catalysts 2 (2012) 572–601. [16] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Visible-light photocatalysis in nitrogen-doped titanium oxides, Science 293 (2001) 269–271. [17] R. Sakthivel, H.A. Prescotta, J. Deutsch, H. Lieske, E. Kemnitz, Synthesis, characterization, and catalytic activity of SO4/Zr1xSnxO2, Appl. Catal. A: Gen. 253 (2003) 237–247. [18] L. Dong, G.X. Cao, Y. Ma, X.L. Jia, G.T. Ye, K.S.K. Guan, Enhanced photocatalytic degradation properties of nitrogen-doped titania nanotube arrays, Trancs. Nonferrous Met. Soc. China 19 (2009) 1583–1587. [19] P.V. Kamath, G.N. Subbana, Electroless nickel hydroxide: synthesis and characterization, J. Appl. Electrochem. 22 (1992) 478–482. [20] F. Portemer, A.D. Vidal, M. Figlarz, Characterization of active material deposited at the nickel hydroxide electrode by electrochemical impregnation, J. Electrochem. Soc. 139 (1992) 671–678. [21] M. Xing, J. Zhang, F. Chen, New approaches to prepare nitrogen-doped TiO2 photocatalysts and study on their photocatalytic activities in visible light, Appl. Catal. B: Environ. 89 (2009) 563–569.
D. Sethi et al. / Journal of Photochemistry and Photobiology B: Biology 130 (2014) 310–317 [22] T. Umebayashi, T. Yamaki, H. Itoh, K. Asai, Band gap narrowing of titanium dioxide by sulfur doping, Appl. Phys. Lett. 81 (2002) 454–456. [23] K.M.K. Srivatsa, D. Chhikara, M.S. Kumar, Synthesis of anatase titania nanostructures at room temperature by PECVD technique, J. Mater. Sci. Technol. 27 (2011) 696–700. [24] Y. Xie, J. Kum, X. Zhao, S.O. Cho, Enhanced photocatalytic activity of mesoporous S-N-codoped TiO2 loaded with Ag nanoparticles, Semicond. Sci. Technol. 26 (2011) 085037. 6pp. [25] J.A.R. Herrera, E. Mielczarski, J. Mielczarski, N.C. Castilo, J. Kiwi, C. Pulgarin, Escherichia coli inactivation by N, S co-doped commercial TiO2 powders under UV and visible light, Appl. Catal. Environ. 84 (2008) 448–456.
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[26] Y. Liu, J. Li, X. Qiu, C. Burda, Bactericidal activity of nitrogen-doped metal oxide nanocatalysts and the influence of bacterial extracellular polymeric substances (EPS), J. Photochem. Photobiol. A: Chem. 190 (2007) 94–100. [27] J.M. Slauch, How does the oxidative burst of macrophages kill bacteria? Still an open question, Mol. Microbiol. 80 (2011) 580–583. [28] P.C. Maness, S. Smolinski, D.M. Blake, Z. Huang, E.J. Wolfrum, W.A. Jacoby, Bactericidal activity of photocatalytic TiO2 reaction: toward an understanding of it killing mechanism, Appl. Environ. Microbiol. 65 (9) (1999) 4094–4098.
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology B: Biology journal homepage: www.elsevier.com/locate/jphotobiol
Water disinfection through photoactive modified titania DiptiPriya Sethi 1, Ajoy Pal 1, Ramasamy Sakthivel ⇑, Sony Pandey, Tapan Dash, Trupti Das, Rohit Kumar CSIR-Institute of Minerals and Materials Technology, Bhubaneswar 751013, India
a r t i c l e
i n f o
Article history: Received 18 July 2013 Received in revised form 20 November 2013 Accepted 6 December 2013 Available online 14 December 2013 Keywords: Titania Doping Absorption Reactive oxygen species Antibacterial property
a b s t r a c t TiO2, N-TiO2 and S-TiO2 samples have been prepared by various chemical methods. These samples were characterized by X-ray diffractometer (XRD), X-ray photoelectron spectroscopy (XPS), Laser Raman spectrometer, UV–Visible spectrophotometer, field emission scanning electron microscope (FE-SEM) and transmission electron microscope (TEM). X-ray powder diffraction study reveals that all three samples are single anatase phase of titania and the crystallinity of titania decreases with sulphur doping whereas nitrogen doping does not affect it. UV–Visible (diffuse) reflectance spectra shows that doping of titania with nitrogen and sulphur shift the absorption edge of titania from ultraviolet to visible region. XPS study confirms that both nitrogen and sulphur are well doped in the titania lattice. It is observed that nitrogen occupies at both substitutional and interstitial position in the lattice of titania. FE-SEM and TEM studies demonstrate that the particles are below 50 nm range. It is found that S and N doping of titania increased its water disinfection property in the order TiO2 < S-TiO2 < N-TiO2 under 8 W UV/UV–Visible light irradiation. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction The conventional water disinfection method involves chemical treatment such as hydrogen peroxide treatment, and chlorination; but, it has disadvantage due to its toxicity and other environmental problems. Hence, there is a need for development of non-toxic, environment friendly and cost effective method for efficient water disinfection. Photocatalytic approach using titania has been the subject of study for the last two decades for various environmental applications [1–7]. Titania is a semiconductor and well studied system due to its low cost, nontoxicity and stability. Photo-excitation of titania leads to generation of electrons in the conduction band and holes in the valence band. These holes help in the splitting of water to produce hydroxyl radicals (OH). These OH radicals are extremely reactive and have redox potential as high as 2.7 V. They are non-selective and readily attack pathogenic microorganisms. This photoexcitation phenomenon will happen only if titania is excited with ultraviolet radiation because titania (anatase) band gap energy is around 3.25 eV which is corresponding to 385 nm wavelength. Among various phases of titania such as anatase, rutile and brookite, anatase phase of titania is preferred due to its high surface area and performance in the photocatalytic activity [8,9]. Since the solar radiation contains only 5% UV radiation, complete use of it is not possible for photocatalytic activity. Therefore, band gap of titania needs to be reduced below 3.2 eV. Numerous efforts have been made to dope ⇑ Corresponding author. Tel.: +91 6742379414. E-mail addresses: [email protected], [email protected] (R. Sakthivel). Academy of Scientific and Innovative Research (AcSIR), Anusandhan Bhawan, 2 Rafi Marg, New Delhi 110 001, India 1
1011-1344/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jphotobiol.2013.12.003
TiO2 with certain metals to shift its absorption to the visible range, but metal-doped TiO2 shows poor photoactivities due to the detrimental effect of metal ions acting as favorable sites for charge recombination [10]. However, it has been reported that Ag/TiO2 powder shows good antibacterial property which further improves with the increase in crystallinity of titania [11]. The same material in a different form such as Ag/TiO2 nanofiber membrane exhibits good photocatalytic disinfection [12]. Pt/TiO2 photocatalyst has also been found to show absorption in the visible light for elimination of soil-borne pathogens [13]. Recent reviews by Zaleska [14] and Manoj et al. [15] discuss the role of various dopants in titania and their effects in shifting of absorption from ultraviolet to visible region for water treatment applications. Asahi et al. [16] reported that Ndoped TiO2 absorbs visible light due to narrowing of band gap induced by nitrogen doping and they demonstrated the photocatalytic activity in the visible light. Since the anion doped titania brought major changes in both the electronic and photocatalytic activities, now major focus has been given on these materials in order to understand the photocatalytic behaviour. In this investigation, we present the N- and S-doped TiO2 photoactive materials characterization and their effect on the bactericidal properties are explained on the basis of ultraviolet and visible light absorption properties.
2. Materials and methods 2.1. Preparation of TiO2, N-TiO2 and S-TiO2 Titania (TiO2) was prepared by the hydrolysis of titanium isopropoxide with water at room temperature, followed by separation
D. Sethi et al. / Journal of Photochemistry and Photobiology B: Biology 130 (2014) 310–317
311
Fig. 3. FTIR spectra of titania and doped titania samples. Fig. 1. X-ray powder diffraction patterns of titania and doped titania samples.
Fig. 4. UV–Visible (diffuse) reflectance spectra of titania and doped titania samples. Fig. 2. Laser Raman spectra of titania and doped titania samples.
Table 1 Raman spectral assignments for various samples. Wavenumber (cm1)
Spectral assignment
TiO2
STiO2
NTiO2
144 195 396 516 638
146 – 397 517 638
144 195 397 516 638
Eg Eg B1g A1g + B1g Eg
of precipitate by filtration and drying at room temperature. The dried sample was calcined at 450 °C for 3 h. In order to obtain nitrogen doped titania (N-TiO2), titanium isopropoxide (Sigma–Aldrich, reagent grade 97%) was hydrolyzed with an aqueous solution of ammonia (Analytical reagent grade) at room temperature for 4 h. The sample thus prepared was separated by filtration, dried at room temperature, and then calcined at 450 °C for 3 h. The sample was designated as N-TiO2. Similarly, sulphur doped titania (S-TiO2) was prepared by hydrolysis in the presence of Na2SO4
(Fischer Chemicals, India, Analytical reagent grade). The weight ratio of TiOSO4/Na2SO4/ethanol/water was maintained at 13.8:7.1:55.2:77.4 and the solution were stirred, followed by aging at 80 °C for 5 h. The final precipitate was filtered and dried at room temperature. The dried powders were calcined at 450 °C for 3 h to get a white colour powder. The sample was designated as S-TiO2. Phase identification of these powder samples were determined by X-ray diffraction (diffractometer: X’PERT PRO, PANalytical, Netherlands) using Cu Ka radiation source (wavelength 0.154056 nm). Surface morphology and elemental composition were studied by the field emission scanning electron microscope (FESEM, model ZEISS SUPRA55) and Energy dispersive specroscopy (EDS) attached to it. The transmission electron microscopic (TEM) images and the selected area electron diffraction (SAED) pattern were obtained with a TECHNAI G2, FEI operated at 200 kV. To get the TEM images, the powdered samples were dispersed in isopropanol by ultrasonication for 20 min. Then one drop of the dispersed solution was deposited on the carbon coated copper grid. The sample was dried completely under an IR lamp before taking the observation. The UV–Visible diffuse reflectance spectra (DRS) were recorded from 200 to 800 nm by a Varian Cary UV–Vis spectrophotometer. The Raman spectra were taken in a dispersive type
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Fig. 5. FE-SEM images (left side) and their corresponding EDX spectra (right side) of titania and doped titania samples.
micro-Raman spectrometer (Renishaw in Via Reflex, UK). Fourier Transform Infrared (FTIR) spectra were recorded in the range of 400–4000 cm1 wavenumber on a Fourier Transform Infrared spectrophotometer supplied by Perkin Elmer India Pvt Ltd., Kolkata and KBr was used as a reference. The X-ray photoelectron spectroscopy (XPS) spectra of various samples were carried out using VG Scienta hemispherical energy analyser source having resolution of 44.1 meV at 50 pass energy and Al Ka (hm = 1486.6 eV) source (S/N:10001, Prevac, Poland). Instrument base pressure of 5 1010 mbar was maintained during data acquisition.
under UV and UV–Visible light. One ml of sample was drawn from the reaction vessel at various intervals of time and serially diluted up to 104 times. This was followed by plating 0.1 ml of the sample on Liquid Broth Agar (LBA) plates by adopting spread plate method, followed by incubation at 37 °C without illumination for 18–24 h. After incubation the number of colonies were counted in all the plates, and The Colony Forming Unit (CFU) per ml was calculated for each sample at different time intervals by using the following formula:
2.2. Biocidal activity of titania and doped titania
where dilution factor is the reciprocal of the dilution in which the plate count was taken, and volume inoculated is 0.1 ml.
Biocidal activities of various samples under UV and Visible light conditions were examined with 107 cells/ml of Escherichia coli in tap water with 0.85% NaCl (to maintain the osmotic balance of bacterial cells during the experiment). A bioreactor was set up with 8 W UV and visible light with a magnetic stirrer on which the sample was placed in a beaker with 25 ml of water spiked with E. coli as mentioned above. For each sample, the biocidal activity was tested
CFU=ml ¼ No: of colonies Dilution factor=volume inoculated
3. Results and discussion X-ray diffraction patterns of TiO2, N-TiO2 and S-TiO2 samples are shown in Fig. 1. It reveals the presence of single anatase phase of titania in all three samples. However, peak broadness has been observed in case of sulphur doped titania which indicates that
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Fig. 6. FE-SEM images (left side) and their corresponding EDX spectra (right side) of titania and doped titania samples.
sulphur brings down the crystallinity as in case of SO4/ZrO2 [17]. The peak corresponding to h1 0 1i plane shows highest intensity among others peaks corresponding to 0 0 4, 2 0 0, 1 0 5, 2 1 1, 2 0 4 planes. The observed d values are matched with standard powder diffraction data of anatase phase of titania (JCPDS File No. 01-0894921). Laser Raman spectra of these samples are shown in Fig. 2. It shows bands around 144, 195, 396, 516, 638 cm1 and these are ascribed for the Eg, B1g, A1g + B1g, modes of vibrations [18]. The observed spectral bands position of all three samples and their corresponding vibrations are given in Table 1. The intensity of spectral bands drastically reduced when sulphur is doped in titania indicating poor crystallinity of sample, however, when nitrogen is doped in titania the intensity of these bands increases due to increase in crystallinity of sample. This observation is in accordance with the XRD results. The change in crystallinity of the samples also affects
the bactericidal properties as discussed below. FTIR spectra of all three samples (Fig. 3) show stretching vibration band for hydroxyl group of adsorbed water molecules at 3000–3550 cm1 and bending vibration band at 1630 cm1. The bands observed at 3695 and 800 cm1 are corresponding to respective stretching and bending vibration of surface hydroxyl group present in all three samples [19,20]. Diffuse (UV–Visible) reflectance spectra of all three samples have been shown in Fig. 4. It shows that the spectra of TiO2, NTiO2 and S-TiO2 samples are differing from each other. Nitrogen and sulphur doping brought drastic change in the spectrum of titania. It has been reported that N or S doping decreases the band gap energy of titania leading to red shift in UV–Vis absorption band [21,22]. Titania shows absorption around 390 nm wavelength where as N-TiO2 shows very strong absorption around 450 and
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Fig. 7. XPS spectra of TiO2, N-TiO2 and S-TiO2 samples.
Fig. 7b. XPS spectra of N-TiO2. Fig. 7a. XPS spectra of TiO2.
390 nm wavelength region. S-TiO2 sample shows a broad absorption in the visible range between 400 and 800 nm wavelength and narrow absorption (390 nm) in the UV region. FE-SEM images and their corresponding EDS spectra of all three samples are shown in Fig. 5. It reveals that all these samples have particles size below 50 nm, and they are distributed in very narrow range. This observation is in accordance with the reported results where titania
deposited on silicon support by plasma enhanced chemical vapour deposition also show the formation of nanosized titania [23]. EDS spectrum of titania shows peaks for titanium and oxygen. In addition to these peaks, other peaks for nitrogen and sulphur are observed in case of N-TiO2 and S-TiO2 samples respectively. Presence of aluminum peaks in all three samples is due to aluminum foil support used in the study. TEM pictures of these samples are depicted in Fig. 6. It shows that both TiO2 and N-TiO2 have similar morphology close to the spherical shape but not fully spherical.
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Fig. 8. Antibacterial property of titania and doped titania samples under UV light source.
Fig. 9. Antibacterial property of titania and doped titania samples under UV and visible light source.
However, S-TiO2 exhibits spherical morphology indicating the role of sulphur. It is seen from SAED patterns that both TiO2 and N-TiO2 have shown bright circular spots for various hh k li planes reflection of titania, whereas in case of S-TiO2 sample, the intensity of those spots drastically diminished and clear circular lines are observed due to poor crystallinity. This observation commensurates with the observed XRD data. XPS spectra of TiO2, N-TiO2 and S-TiO2 samples are shown in Figs. 7 and 7a–c. Interpretation of the peaks of XPS spectra have been done according to the available literature [24,25]. All three samples show well-defined spin-coupled peaks at the binding energy around 458.4 eV and 464.1 eV (Table 2) for Ti 2P3/2 and Ti 2p/ Ti 2p1/2 respectively which confirms the existence of Ti in the state of Ti4+. Oxygen peak at around 530 eV is observed for O 1S and this
Fig. 7c. XPS spectra of S-TiO2.
Table 2 XPS spectral assignment. Sample name
TiO2 N-TiO2 S-TiO2
Ti
O2
N
S
2P 3/2 (eV)
2P 1/2 (eV)
Lattice oxygen (eV)
Surface OH (eV)
Substitutional
Interstitial
Substitutional
Interstitial
458.4 458.5 458.8
464.1 464.2 464.5
530.2 530.1 530.1
532.0 532.6 532.3
– 399.7 eV –
– 400.7 eV –
– – 168.7 eV
– – –
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peak is de-convoluted to give two peaks, one observed at the binding energy of 530.2 eV and the other at 532.6 eV which are assigned to the lattice oxygen of TiO2 and oxygen due to surface hydroxyl (OH) group respectively. In addition to the above peaks, the N-TiO2 sample shows a peak for nitrogen at the binding energy of 400 eV (Fig. 7b), when this peak was further de-convoluted it has given two peaks; one at 399.7 eV and another at 400.7 eV for the substitutional N-doping (substitution of N by O) and interstitial N-doping in the lattice of TiO2 respectively. In this study, it is interesting to observe that both types of N-doping (substitution and interstitial) take place in the titania lattice structure. However, the peak intensity of 399.7 eV is much stronger than the other one observed at 400.7 eV peak indicating that more amount of nitrogen occupy at the interstitial position rather than substitutional position in the lattice of TiO2. In the case of S-TiO2 (Fig. 7c), a peak for S 2p is observed at the binding energy of 168.7 eV which might be due to the partial substitution of S4+ for the Ti4+ in the TiO2 lattice. Antibacterial property of TiO2, N-TiO2 and S-TiO2 samples examined separately under ultraviolet (UV) alone and UV–Visible lights are shown in Figs. 8 and 9 respectively. These figures illustrate that with increasing light exposure time the CFU counts of bacteria decreases. There is a drastic decrease in viable cell counts of the E. coli during the initial period of light exposure because the reactive oxygen species (ROS) generated due to photocatalysis probably plays a significant role in arresting the bacterial defense system, whereas at a later period of time it decreases very slowly. This could possibly be due to the competition between the residual active bacterial cells and cell materials (released during the photocatalytic process) for ROS [26]. However, antibacterial property differs from sample to sample, it shows the following trend: TiO2 < STiO2 < N-TiO2, irrespective of the light source used in this study, either UV light alone or UV–Visible light. However, it is very interesting to note that N-TiO2 and S-TiO2 samples show better antibacterial activity than TiO2 when both UV and Visible lights are used compared to UV light alone and these changes are due to change in their optical absorption properties. A comparative study of the antibacterial property of N-TiO2 and S-TiO2 samples, revealed that N-TiO2 shows better antibacterial activity due to its high crystallinity. This observation very well supports the results obtained by Kedziora et al. [11], where they reported that the antibacterial activity of Ag/TiO2 improves with the increase in crystallinity. Further, N-TiO2 shifts the absorption edge of titania relatively stronger than S-TiO2 from UV to visible region which helps in improving the photocatalytic activity to yield high antibacterial activity. N-TiO2 synthesized by Liu et al. [26] using ethylediamine as nitrogen source shows complete killing of 109 CFU/mL of E. coli cells in 120 min with photocatalyst concentration of 0.1 mg/mL. But, in this study, it has been observed that catalyst concentration of 1 mg/mL has taken 10 min for complete killing of 107 CFU/mL of E. coli cells. So our results are in good agreement with the literature [26]. Mechanism involved in antibacterial property of these materials is based on the following; when light interact with titania based materials present in the water system, it produces OH radicals, hydrogen peroxide (H2O2) and superoxide ions (O2). These are known as reactive oxygen species (ROS) which attacks the phospholipids of the cell membrane of bacteria in suspension [27]. As a consequence cell membrane gets ruptured due to lipidperoxidation which subsequently leads to cell death [28].
4. Conclusions Among TiO2, N-TiO2 and S-TiO2 samples, N-TiO2 shows good absorption in the visible in addition to ultraviolet region therefore, it yields high antibacterial property. Nitrogen/sulphur doping of
titania significantly improved the visible light absorption to yield high antibacterial activity and particularly this effect is seen more in case of N-TiO2. FE-SEM and TEM characterization revealed that the particles are below 50 nm range. XRD observation confirms the formation of anatase phase in all three samples. Sulphur doping decreases the crystallinity of titania whereas nitrogen doping does not affect it. XPS study confirms that nitrogen occupies the both substitutional and interstitial position in the lattice of titania. On the basis of the above observation, it is suggested that N-TiO2 could be a better choice material for water disinfection under combined UV and visible light exposure condition. Acknowledgments We acknowledge the Ministry of Environment and Forest, New Delhi for financial support and thanks to the Director of CSIR-Institute of Minerals and Materials Technology, Bhubaneswar for his encouragement. One of the authors Dipti Priya Sethi wishes to acknowledge the Rajiv Gandhi National Fellowship funded by Ministry of Social Justice and Empowerment Government of India. References [1] H.A. Foster, I.B. Ditta, S. Varghese, A. Steele, Photocatalytic disinfection using titanium dioxide: spectrum and mechanism of antimicrobial activity, Appl. Microbiol. Biotechnol. 90 (2011) 1847–1868. [2] D. Zhang, G. Li, J.C. Yu, Inorganic materials for photocatalytic water disinfection, J. Mater. Chem. 20 (2010) 4529–4536. [3] S. Sakthivel, M. Janczarek, H. Kisch, Visible light activity and photoelectrochemical properties of nitrogen doped TiO2, J. Phys. Chem. B 108 (2004) 19384–19387. [4] Y. Wang, Solar photocatalytic degradation of eight commercial dyes in TiO2 suspension, Water Res. 34 (2000) 990–994. [5] B. Naik, K.M. Parida, C.S. Gopinath, Facile synthesis of N-and S-incorporated nanocrystalline TiO2 and direct solar light driven photocatalytic activity, J. Phys. Chem. C 114 (2010) 19473–19482. [6] A. Mills, S.L. Hunte, An overview of semiconductor photocatalysis, J. Photochem. Photobiol. A 108 (1997) 1–35. [7] S. Devipriya, S. Yesodharan, Photocatalytic degradation of pesticide contaminants in water, Sol. Energy Mater. Sol. C 86 (2005) 309–348. [8] J. Zhu, J. Yang, Z.F. Bian, J. Ren, Y.M. Liu, Y. Cao, H.X. Li, H.Y. He, K.N. Fan, Nanocrystalline anatase TiO2 photocatalysts prepared via a facile low temperature nonhydrolytic sol–gel reaction of TiCl4 and benzyl alcohol, Appl. Catal. B: Environ. 76 (2007) 82–91. [9] F.D. Hardcastle, H. Ishihara, R. Sharma, A.S. Biris, Photoelectroactivity and Raman spectroscopy of anodized titania (TiO2) photoactive water-splitting catalysts as a function of oxygen-annealing temperature, J. Mater. Chem. 21 (2011) 6337–6345. [10] O. Carp, C.L. Huisman, A. Reller, Photoinduced reactivity of titanium dioxide, Progr. Solid State Chem. 32 (2004) 33–177. [11] A. Kedziora, W. Strek, L. Kepinski, G.B. Ploskonska, W. Doroszkiewicz, Synthesis and antibacterial activity of novel titanium dioxide doped with silver, J. Sol–Gel Sci. Technol. 62 (2012) 79–86. [12] L. Liu, Z. Liu, H. Bai, D.D. Sun, Concurrent filteration and solar photocatalytic disinfection/degradation using high-performance Ag/TiO2 nanofiber membrane, Water Res. 46 (2012) 1101–1112. [13] Y.L. Chen, Y.S. Chen, H. Chan, Y.H. Tseng, S.R. Yang, H.Y. Tsai, H.Y. Liu, D.S. Sun, H.H. Chang, The use of nanoscale visible light-responsive photocatalyst TiO2– Pt for the elimination of soil-borne pathogens, PLos One 7 (2012) e31212. [14] A. Zaleska, Doped-TiO2: a review, Recent Patents Eng. 2 (2008) 157–164. [15] M.A. Lazar, S. Varghese, S.S. Nair, Photocatalytic water treatment by titanium dioxide: recent updates, Catalysts 2 (2012) 572–601. [16] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Visible-light photocatalysis in nitrogen-doped titanium oxides, Science 293 (2001) 269–271. [17] R. Sakthivel, H.A. Prescotta, J. Deutsch, H. Lieske, E. Kemnitz, Synthesis, characterization, and catalytic activity of SO4/Zr1xSnxO2, Appl. Catal. A: Gen. 253 (2003) 237–247. [18] L. Dong, G.X. Cao, Y. Ma, X.L. Jia, G.T. Ye, K.S.K. Guan, Enhanced photocatalytic degradation properties of nitrogen-doped titania nanotube arrays, Trancs. Nonferrous Met. Soc. China 19 (2009) 1583–1587. [19] P.V. Kamath, G.N. Subbana, Electroless nickel hydroxide: synthesis and characterization, J. Appl. Electrochem. 22 (1992) 478–482. [20] F. Portemer, A.D. Vidal, M. Figlarz, Characterization of active material deposited at the nickel hydroxide electrode by electrochemical impregnation, J. Electrochem. Soc. 139 (1992) 671–678. [21] M. Xing, J. Zhang, F. Chen, New approaches to prepare nitrogen-doped TiO2 photocatalysts and study on their photocatalytic activities in visible light, Appl. Catal. B: Environ. 89 (2009) 563–569.
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