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Synthesis and characterization of TiO2 nanorods by hydrothermal method with different pH conditions and their photocatalytic activity
Applied Surface Science 500 (2020) 144058
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Full Length Article
Synthesis and characterization of TiO2 nanorods by hydrothermal method with different pH conditions and their photocatalytic activity
T
⁎
K. Santhia, M. Navaneethana,b, S. Harisha,c, S. Ponnusamya, , C. Muthamizhchelvana a
Functional Materials and Energy Devices Laboratory, Department of Physics and Nanotechnology, SRM Institute of Science and Technology, Kattankulathur, Chennai 603203, Tamil Nadu, India b Nanotechnology Research Center (NRC), Faculty of Engineering and Technology, SRM Institute of Science and Technology, Chennai 603203, Tamil Nadu, India c Graduate School of Science and Technology, Shizuoka University, 3-5-1 Johoku, Naka-Ku, Hamamatsu, Shizuoka 432-8011, Japan
A R T I C LE I N FO
A B S T R A C T
Keywords: Titanium dioxide Hydrothermal Photocatalysis Methyl orange
Surfactant free Titanium dioxide (TiO2) nanorods under different pH conditions were synthesized by a hydrothermal method. The structure, morphology and framework substitution of the as – prepared nanorods was characterized by X-ray diffraction (XRD), Field Emission Scanning Electron Microscopy (FESEM), UV–Visible, Fourier Transform Infrared (FTIR) and X-ray Photon Spectroscopy (XPS). XRD analysis confirms the formation of anatase phase of TiO2 with tetragonal structure. FESEM micrograph confirms the formation of nanorods. The calculated band gap values of TiO2 nanorods was found to decrease with increasing pH from the optical absorption spectra. XPS spectra confirm the presence of Ti 2p and O 1s states. The photocatalytic activity of the nanorods against methyl orange (MO) was examined and their results have shown highest degradation (51%) of MO was achieved within 150 min.
1. Introduction Over the recent years, investigation on environmental protection from pollutions has become a crucial area of interest. The major causes of pollution are due to industrial waste generated and disposed into large water bodies like river and lakes. These pollutants can affect the food chain and cause unfavorable effects on plants and animals. Most of the pollutants are organic molecules and these can be destroyed by the process of photocatalysis [1]. Removal of such organic compounds in waste water using photocatalytic oxidation is an important area in present research. In this regard, variety of semiconductor nanostructures such as ZnO, CuO, CdS, TiO2, ZnS and iron oxides have been utilized as photocatalyst and studied their photocatalytic properties. Of the various semiconductor metal oxides, TiO2 is found to be the suitable material because of its quite interesting properties such as chemical stability, low toxicity, high photoactive, refractive index and also transparent in the visible region with its band gap around 3.2 eV [2–6]. TiO2 can mineralize organic pollutants into carbon dioxide and water as harmless end products under irradiation. The inherent properties of TiO2 depend on many factors such as crystal structure, size, shape and the synthesis procedure. TiO2 exists in three crystallographic phases namely anatase, rutile and brookite [7,8]. In general, anatase TiO2 is found to be active and possess high photocatalytic efficiency compared
⁎
to rutile phase due to electron – hole pair recombination rate [9,10]. Photocatalytic process involves the electron in the valance band (VB) were excited towards the conduction band (CB) by illuminating the UV light [11]. The holes were generated in the valance band. Thereafter the electron and holes were transferred to the surface of the crystal and react with the molecular oxygen (O2) and H2O for generating the super oxide radicals anion (%O2−) and hydroxyl radicals (OH%). Many synthesis methods have been reported for the preparation of Nano Titania with varying pH, include sol-gel reaction [12], chemical precipitation [13], microwave irradiation [14], hydrothermal reaction [15], etc. These methods provide various morphologies like nanosphere, nanorods, nanocubes, nanowires and have different photocatalytic efficiency. For instance, Huaming Yang et al. [16] synthesized pure TiO2 with pH-3 using sol-gel method, 65% of MO degradation was obtained by anatase titania. Fabien Dafour et al. [17], synthesized nanopowder with different pH conditions using hydrothermal method and observed particles with different morphologies like rod, bipyramide, cube. Their results demonstrated that 95% of RhB degradation was achieved with anatase TiO2 nanobipyramide compared to other stuctures. T. Suprabha et al. [18], synthesized nano powder with varying pH conditions using hydrothermal method and observed different morphology like rod, sphere, cube. Among them, nanocube like particles have shown more efficiency compared to others. Xiuzhen Wei et al.
Corresponding author. E-mail address: [email protected] (S. Ponnusamy).
https://doi.org/10.1016/j.apsusc.2019.144058 Received 22 March 2019; Received in revised form 23 August 2019; Accepted 16 September 2019 Available online 01 October 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
Applied Surface Science 500 (2020) 144058
K. Santhi, et al.
Fig. 1. XRD pattern of the TiO2 samples.
Fig. 3. FT-IR spectra of TiO2 samples.
[19], reported that 95% of reactive brilliant red X-3B degradation was observed in 80 min. Sunderishwary et al. [20] TiO2 nanopowder synthesized using precipitation technique with pH-5,7,9 and observed that TiO2 nanosphers with pH-9 has higher photocatalytic activity. Muhsin A. et al. [21] synthesized donut-like nanopowder using sol-gel method and observed 67% of MB degradation in 200 min of irradiation. The above research works clearly demonstrated that the pH played a crucial role in the formation of TiO2. Herein we report synthesis of anatase TiO2 with pH-7 (neutral) and pH-9 (base) using hydrothermal method and its photocatalytic activity was examined. Effect of NaOH concentration on the structural, elemental composition and morphology has been studied. The photocatalytic effect of TiO2 nanorods were studied on the degradation of Methyl orange in aqueous solution.
2. Experimental section Titanium tetra-isopropoxide [Ti{OCH(CH3)}4], 98% ; Merck], Sodium hydroxide [ NaOH, 98% ; Merck], and Methyl Orange [C14H14N3NaO3S, 99% ; Merck] was used without any further purification.
2.1. Synthesis of pure TiO2 nanorods In a typical synthesis, 5 mL of titanium tetra isopropoxide was taken in 100 mL of deionized water and stirred for 3 h to get complete dissolution. To this solution, 0.2 mL of NaOH was added drop wise and allowed to stir for 30 min until a white precipitate is formed. The pH
Fig. 2. FE-SEM of sample S1(a) and S2(b) and HRTEM of sample S1(c and d). 2
Applied Surface Science 500 (2020) 144058
K. Santhi, et al.
Fig. 5. XPS spectra of (a) Ti 2p and (b) O 1s of TiO2 samples.
2.2. Characterization The structure and phase of the synthesized products were characterized using an X’ Pert PRO (PANalaytical diffractometer using Kα (1 = 1.5405 Å) radiation at a scan rate of 0.02°/S. The morphology was measured by FEI Quanta FEG200 field emission scanning electron microscope (FE-SEM). High Resolution Transmission electron microscopy (HRTEM) images were recorded using a JEOL JEM 2100F microscope at an accelerating voltage of 200 kV. Functional group analysis was carried out by Bruker IFS 88 Fourier transform infrared (FT-IR) spectrometer equipped with a MCT cryodetector. Optical absorption studies were done by shimadzu UV-2600 spectrophotometer. X-ray photoelectron spectra (XPS) were measured by shimadzu ESCA 3400.
2.3. Photocatalytic measurement Fig. 4. UV–visible spectra of TiO2 (a), (b) and (c).
The photocatalytic activity of the as prepared TiO2 nanorods was examined by employing immersion type double layered quartz tube photo reactor under UV irradiation using mercury lamp at 365 nm wavelength (intensity is ~650 lux). 10 ppm of 100 mL of methyl orange (MO) in aqueous solution was prepared. To this, 0.05 g of synthesized nanopowder was added and continuously stirred for 30 min to analyze adsorption and desorption equilibrium between TiO2 and MO under dark condition. The stable aqueous dye solution was exposed to UV light irradiation for 30 min. for every interval of 30 min, the UV absorption spectrum was recorded and continued for a total period of
was maintained to 7 and 9. Finally the mixture was transferred to Teflon lined stainless steel autoclave and heated to 180 °C for 24 h. The obtained precipitate was washed several times with distilled water and ethanol by centrifugation and then dried at 80 °C for 3 h. The dried sample was annealed at 400 °C for 5 h. The resulting sample was labeled as S1 for pH-7and S2 for pH-9.
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150 min. At the end of the cycle, the solution was centrifuged to separate TiO2 particles. The photodegradation percentage of MO was calculated from the following Eq. (1) [22].
D% = (1 − Ct /Co) × 100
(1)
Were C0 and Ct are the concentration of MO at time 0 and t respectively, and t is irradiation time in seconds. 3. Result and discussion 3.1. Structural analysis Fig. 1 illustrates the powder X-ray diffaraction (XRD) pattern of synthesized TiO2 nanorods prepared under different pH conditions. The peaks obtained at 2θ values 25.3, 37.8, 48.1, 53.9, 55.1, 62.8, 70.8, 75.1 corresponds to (1 0 1), (0 0 4), (2 0 0), (1 0 5), (2 1 1), (2 0 4), (1 1 6), (2 2 0) and (2 1 5) crystal planes indicating the formation of anatase phase TiO2. The observed results are in good agreement with JCPDS Card No 21-1272. No diffraction peaks due to other impurities were detected, implying high purity of the samples [23]. The average crystallite size of TiO2 nano powder (S1 and S2) was estimated from the Scherrer’s formula [24].
D = k.λ/β. cos θ
(2)
where λ is the X-ray wavelength (0.154 nm), θ is the diffraction angle and β is the full width at half maximum (FWHM). The calculated average crystallite size of TiO2 nanoparticles was found to be 14 nm and 16 nm for S1 and S2. This shows that the particle sizes increases as pH increased from 7 to 9. 3.2. Morphological analysis The FESEM and TEM micrographs of the synthesized nanoparticles were taken to analyze the morphology and precise determination of particle size. Fig. 2(a&b) shows the FESEM image of TiO2 nanopowder synthesized under pH 7 and 9 conditions. It can be seen from the image 2a that the nanoparticles synthesized under pH 7 exhibit rod like morphology with length of around 300–350 nm and diameter of 70–100 nm, respectively. TEM and HR-TEM images further confirms the rod like structure of the S1 nanoparticles synthesized under pH 7 as shown in Fig. 2(c&d). When the pH was increased to 9 the morphology changes from rod to nanoplatelet like structure as shown in Fig. 2b. This shows that increasing pH suppresses the growth of nanorods to nanoplatelets. Similar morphologies of TiO2 were reported by few researchers under different synthesis process [25,26]. Growth mechanism was illustrated by graphical abstract.
Fig. 6. UV absorbance spectra of MO samples.
3.3. FT-IR spectra The FT-IR spectra of TiO2 nanopowder is shown in Fig. 3. It is believed that the broad absorption peak at 3000 cm−1–3600 cm−1 is attributed to the fundamental OeH stretching vibration of hydroxyl group [27]. The weak band occurs at 1615 cm−1–1635 cm−1 correspond to OeH bending vibration of hydroxyl group due to chemically adsorbed water molecules [28]. The peak around 800 cm−1 was assigned to the TieO stretching band [29] confirms the formation of TiO2. 3.4. Optical properties The optical absorbance spectra of TiO2 nanoparticles taken at room temperature are shown in Fig. 4a. It is seen that the particles exhibit maximum absorbance in the UV region. It is reported that the absorption band at 400 nm for TiO2 is due to the charge-transfer from the valence band (mainly formed by 2p orbitals of the oxide anions) to the conduction band (mainly formed by 3d orbitals of the Ti4+ cations) [30]. The broad absorbance edge of the spectra is the result of anatase
Fig. 7. Time Vs C/C0 of TiO2 samples under MO degradation.
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(O2%−), which further indirectly turn into highly reactive hydroxide radicals (OH%). Moreover, holes by interacting with OH− form highly reactive hydroxyl radicals. Reactive hydroxide radicals with high oxidation ability, generate either photogenerated electrons or holes which finally oxidize the dye molecules. The possible mechanism of photocatalytic reactions of MO dye as follows TiO2 + hν → TiO2 + h+ + e− −
h + H2O → OH + H +
−
h + OH → OH +
−
Fig. 8. Plausible mechanism for the MO degradation by using the TiO2.
nanoparticle. The energy band gap of the prepared nanoparticles is determined as 3.2 eV for S1 and 3.15 eV for S2 samples [31]. The optical band gap is found to be pH dependent and there is a decrease in the bandgap with increase in pH. The decreased in band gap with increasing pH may be due to base nature [32], variation of the crystallite size, morphology, quantum confinement and dislocation density will affect the band gap [33].
%
+
%
− −
(3) (4) (5)
O2 + e → ( O2 )
(6)
2H2O + O2 + e → 2OH + 2OH
(7)
OH + Dye (MO) → degradation products
(8)
4. Conclusions TiO2 nanorods were synthesized by the hydrothermal method. The crystalline nature and nano-rod structure of the TiO2 were confirmed by XRD and SEM. Structural analysis confirms the formation of anatase TiO2. The average crystallite size increases from 14 nm to 16 nm as the pH increases from 7 to 9. FTIR spectra confirms the formation of TieO stretching bond. UV visible absorption spectra report that the addition of NaOH restrict the growth of TiO2 nanorods and the absorption band edge red shifted. The band gap value decreased from 3.2 eV to 3.15 eV as the pH increased from 7 to 9. The highest degradation (51%) of MO was achieved by the irradiation of UV light for 150 min.
3.5. XPS analysis XPS analysis was carried out to analyze the surface electronic status of Ti and O elements in TiO2. Fig. 5(a) shown that Ti 2p spectrum presented two peaks at 458.8 eV and 457.9 eV for Ti 2p3/2 and 465 and 463.6 eV for Ti 2p1/2 which indicates the Ti4+ in TiO2 [34]. The observed binding energy for Ti 2p3/2 and Ti 2p1/2 decreases towards lower binding energy in S2. This shift confirms the presence of Ti3+. In addition, Ti3+ sites reduced the band gap of TiO2. Ti4+ reduction to Ti3+ is usually appeared by calcination [20]. In this study, the synthesized TiO2 nanoparticles was calcined at 400 °C, hence Ti3+ ions were produced. The binding energy of O 1s of TiO2 nanostructures are shown in Fig. 5(b). The peaks at 530.4 (S1) and 529.5 eV (S2), which corresponded to the TieO, respectively. The secondary peak at 532.2 eV corresponded to the contributions from the surface adsorbed hydroxyl groups [35]. The interaction significantly modified the original chemical states and electronic properties in the samples. The degradation of methyl orange (MO) was determined from absorption spectra by the irradiation of UV light. The absorbance spectra was measured with respect to the UV light irradiation with time interval from 0 to 150 min. The temporal evolution of spectral changes accompanying the photodecomposition of MO over as-prepared TiO2 nanostructures are shown in Fig. 6(a and b). The initial absorption peak gradually decreases under UV light irradiation. The MO decompose to form CO2, H2O and other inorganic molecules as a harmless product. The time-dependent UV absorption spectra of sample S1 and S2 with an irradiation time of 150 min. The maximum degradation efficiency for TiO2 nanorods (S1) exhibit 51% degradation of MO in 150 min. While TiO2 nanoplatelet (S2) exhibit 10% degradation of MO in 150 min. Which can be concluded that TiO2 nanorods showed the better degradation due to a greater number of vacant sites when compared to TiO2 platelet. Without catalyst condition, the degradation of dye (4% for 150 mins) was very less, and the sample attained a stable value after 35 mins as shown in Fig. 7. By above discussion, the photocatalytic mechanism of TiO2, are shown in Fig. 8. The electron in the valance band (VB) were excited towards the conduction band (CB) by illuminating the UV light and the holes were generated in the valance band. There after the electron and holes were transferred to the surface of the crystal and react with the molecular oxygen (O2) and H2O for generating the super oxide radicals anion (%O2−) and hydroxyl radicals (OH%). These electrons, upon reacting with dissolved oxygen molecules, form super oxide radical anion
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[26] E. Filippo, C. Carlucci, A.L. Capodilup, P. Perulli, F. Conciauro, G.A. Corrente, G. Gigli, G. Ciccarella, Enhanced photocatalytic activity of pure anatase TiO2 and Pt-TiO2 nanoparticles synthesized by green microwave assisted route, Mat. Res. 18 (3) (2015). [27] T. Prakash, M. Navaneethan, J. Archana, S. Ponnusamy, C. Muthamizhchelvan, Y. Hayakawa, Synthesis of TiO2 nanoparticles with mesoporous spherical morphology by a wet chemical method, Mater. Lett. 82 (2012) 208–210. [28] S. JanitabarDarzi, A.R. Mahjoub, A. Bayat, Synthesis and characterization of visible light active S-doped TiO2 nanophotocatalyst, Int. J. Nano Dimens. 7 (1) (2016) 33–40 Winter 2016. [29] M.M. Ba-Abbad, A.A.H. Kadhum, A.B. Mohamad, M.S. Takriff, K. Sopian, Synthesis and catalytic activity of TiO2 nanoparticles for photochemical oxidation of concentrated chlorophenols under direct solar radiation, Int. J. Electrochem. Sci. 7 (2012) 4871–4888. [30] K. Mani Rahulan, S. Ganesan, P. Aruna, Synthesis and optical limiting studies of Audoped TiO2 nanoparticles, Adv. Nat. Sci.: Nanosci. Nanotechnol. 2 (2011) 025012. [31] J. Xu, L. Li, Y. Yan, H. Wang, X. Wang, X. Fu, G. Li, Synthesis and photoluminescence of well-dispersible anatase TiO2 nanoparticles, J. Colloid Interface Sci. 318 (2008) 29–34. [32] T. van Buuren, L.N. Dinh, L.L. Chase, W.J. Siekhaus, L.J. Terminello, Changes in the electronic properties of Si nanocrystals as a function of particle size, Phys. Rev. Lett. 18 (17) (1998). [33] F.G. Hone, F.B. Dejene, Cationic concentration and pH effect on the structural, morphological and optical band gap of chemically synthesized lead sulfide thin films, J. Mater. Res. Technol. 8 (1) (2019) 467–474. [34] D. Wang, B. Yu, F. Zhou, C. Wang, W. Liu, Synthesis and characterization of anatase TiO2 nanotubes and their use in dye-sensitized solar cells, Mater. Chem. Phys. 113 (2009) 602–606. [35] G. Li, L. Li, J.B. Goates, B.F. Woodfield, High purity anatase TiO2 nanocrystals: near room-temperature synthesis, grain growth kinetics, and surface hydration chemistry, J. Am. Chem. Soc. 127 (2005) 8659–8666.
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6
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Full Length Article
Synthesis and characterization of TiO2 nanorods by hydrothermal method with different pH conditions and their photocatalytic activity
T
⁎
K. Santhia, M. Navaneethana,b, S. Harisha,c, S. Ponnusamya, , C. Muthamizhchelvana a
Functional Materials and Energy Devices Laboratory, Department of Physics and Nanotechnology, SRM Institute of Science and Technology, Kattankulathur, Chennai 603203, Tamil Nadu, India b Nanotechnology Research Center (NRC), Faculty of Engineering and Technology, SRM Institute of Science and Technology, Chennai 603203, Tamil Nadu, India c Graduate School of Science and Technology, Shizuoka University, 3-5-1 Johoku, Naka-Ku, Hamamatsu, Shizuoka 432-8011, Japan
A R T I C LE I N FO
A B S T R A C T
Keywords: Titanium dioxide Hydrothermal Photocatalysis Methyl orange
Surfactant free Titanium dioxide (TiO2) nanorods under different pH conditions were synthesized by a hydrothermal method. The structure, morphology and framework substitution of the as – prepared nanorods was characterized by X-ray diffraction (XRD), Field Emission Scanning Electron Microscopy (FESEM), UV–Visible, Fourier Transform Infrared (FTIR) and X-ray Photon Spectroscopy (XPS). XRD analysis confirms the formation of anatase phase of TiO2 with tetragonal structure. FESEM micrograph confirms the formation of nanorods. The calculated band gap values of TiO2 nanorods was found to decrease with increasing pH from the optical absorption spectra. XPS spectra confirm the presence of Ti 2p and O 1s states. The photocatalytic activity of the nanorods against methyl orange (MO) was examined and their results have shown highest degradation (51%) of MO was achieved within 150 min.
1. Introduction Over the recent years, investigation on environmental protection from pollutions has become a crucial area of interest. The major causes of pollution are due to industrial waste generated and disposed into large water bodies like river and lakes. These pollutants can affect the food chain and cause unfavorable effects on plants and animals. Most of the pollutants are organic molecules and these can be destroyed by the process of photocatalysis [1]. Removal of such organic compounds in waste water using photocatalytic oxidation is an important area in present research. In this regard, variety of semiconductor nanostructures such as ZnO, CuO, CdS, TiO2, ZnS and iron oxides have been utilized as photocatalyst and studied their photocatalytic properties. Of the various semiconductor metal oxides, TiO2 is found to be the suitable material because of its quite interesting properties such as chemical stability, low toxicity, high photoactive, refractive index and also transparent in the visible region with its band gap around 3.2 eV [2–6]. TiO2 can mineralize organic pollutants into carbon dioxide and water as harmless end products under irradiation. The inherent properties of TiO2 depend on many factors such as crystal structure, size, shape and the synthesis procedure. TiO2 exists in three crystallographic phases namely anatase, rutile and brookite [7,8]. In general, anatase TiO2 is found to be active and possess high photocatalytic efficiency compared
⁎
to rutile phase due to electron – hole pair recombination rate [9,10]. Photocatalytic process involves the electron in the valance band (VB) were excited towards the conduction band (CB) by illuminating the UV light [11]. The holes were generated in the valance band. Thereafter the electron and holes were transferred to the surface of the crystal and react with the molecular oxygen (O2) and H2O for generating the super oxide radicals anion (%O2−) and hydroxyl radicals (OH%). Many synthesis methods have been reported for the preparation of Nano Titania with varying pH, include sol-gel reaction [12], chemical precipitation [13], microwave irradiation [14], hydrothermal reaction [15], etc. These methods provide various morphologies like nanosphere, nanorods, nanocubes, nanowires and have different photocatalytic efficiency. For instance, Huaming Yang et al. [16] synthesized pure TiO2 with pH-3 using sol-gel method, 65% of MO degradation was obtained by anatase titania. Fabien Dafour et al. [17], synthesized nanopowder with different pH conditions using hydrothermal method and observed particles with different morphologies like rod, bipyramide, cube. Their results demonstrated that 95% of RhB degradation was achieved with anatase TiO2 nanobipyramide compared to other stuctures. T. Suprabha et al. [18], synthesized nano powder with varying pH conditions using hydrothermal method and observed different morphology like rod, sphere, cube. Among them, nanocube like particles have shown more efficiency compared to others. Xiuzhen Wei et al.
Corresponding author. E-mail address: [email protected] (S. Ponnusamy).
https://doi.org/10.1016/j.apsusc.2019.144058 Received 22 March 2019; Received in revised form 23 August 2019; Accepted 16 September 2019 Available online 01 October 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
Applied Surface Science 500 (2020) 144058
K. Santhi, et al.
Fig. 1. XRD pattern of the TiO2 samples.
Fig. 3. FT-IR spectra of TiO2 samples.
[19], reported that 95% of reactive brilliant red X-3B degradation was observed in 80 min. Sunderishwary et al. [20] TiO2 nanopowder synthesized using precipitation technique with pH-5,7,9 and observed that TiO2 nanosphers with pH-9 has higher photocatalytic activity. Muhsin A. et al. [21] synthesized donut-like nanopowder using sol-gel method and observed 67% of MB degradation in 200 min of irradiation. The above research works clearly demonstrated that the pH played a crucial role in the formation of TiO2. Herein we report synthesis of anatase TiO2 with pH-7 (neutral) and pH-9 (base) using hydrothermal method and its photocatalytic activity was examined. Effect of NaOH concentration on the structural, elemental composition and morphology has been studied. The photocatalytic effect of TiO2 nanorods were studied on the degradation of Methyl orange in aqueous solution.
2. Experimental section Titanium tetra-isopropoxide [Ti{OCH(CH3)}4], 98% ; Merck], Sodium hydroxide [ NaOH, 98% ; Merck], and Methyl Orange [C14H14N3NaO3S, 99% ; Merck] was used without any further purification.
2.1. Synthesis of pure TiO2 nanorods In a typical synthesis, 5 mL of titanium tetra isopropoxide was taken in 100 mL of deionized water and stirred for 3 h to get complete dissolution. To this solution, 0.2 mL of NaOH was added drop wise and allowed to stir for 30 min until a white precipitate is formed. The pH
Fig. 2. FE-SEM of sample S1(a) and S2(b) and HRTEM of sample S1(c and d). 2
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Fig. 5. XPS spectra of (a) Ti 2p and (b) O 1s of TiO2 samples.
2.2. Characterization The structure and phase of the synthesized products were characterized using an X’ Pert PRO (PANalaytical diffractometer using Kα (1 = 1.5405 Å) radiation at a scan rate of 0.02°/S. The morphology was measured by FEI Quanta FEG200 field emission scanning electron microscope (FE-SEM). High Resolution Transmission electron microscopy (HRTEM) images were recorded using a JEOL JEM 2100F microscope at an accelerating voltage of 200 kV. Functional group analysis was carried out by Bruker IFS 88 Fourier transform infrared (FT-IR) spectrometer equipped with a MCT cryodetector. Optical absorption studies were done by shimadzu UV-2600 spectrophotometer. X-ray photoelectron spectra (XPS) were measured by shimadzu ESCA 3400.
2.3. Photocatalytic measurement Fig. 4. UV–visible spectra of TiO2 (a), (b) and (c).
The photocatalytic activity of the as prepared TiO2 nanorods was examined by employing immersion type double layered quartz tube photo reactor under UV irradiation using mercury lamp at 365 nm wavelength (intensity is ~650 lux). 10 ppm of 100 mL of methyl orange (MO) in aqueous solution was prepared. To this, 0.05 g of synthesized nanopowder was added and continuously stirred for 30 min to analyze adsorption and desorption equilibrium between TiO2 and MO under dark condition. The stable aqueous dye solution was exposed to UV light irradiation for 30 min. for every interval of 30 min, the UV absorption spectrum was recorded and continued for a total period of
was maintained to 7 and 9. Finally the mixture was transferred to Teflon lined stainless steel autoclave and heated to 180 °C for 24 h. The obtained precipitate was washed several times with distilled water and ethanol by centrifugation and then dried at 80 °C for 3 h. The dried sample was annealed at 400 °C for 5 h. The resulting sample was labeled as S1 for pH-7and S2 for pH-9.
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150 min. At the end of the cycle, the solution was centrifuged to separate TiO2 particles. The photodegradation percentage of MO was calculated from the following Eq. (1) [22].
D% = (1 − Ct /Co) × 100
(1)
Were C0 and Ct are the concentration of MO at time 0 and t respectively, and t is irradiation time in seconds. 3. Result and discussion 3.1. Structural analysis Fig. 1 illustrates the powder X-ray diffaraction (XRD) pattern of synthesized TiO2 nanorods prepared under different pH conditions. The peaks obtained at 2θ values 25.3, 37.8, 48.1, 53.9, 55.1, 62.8, 70.8, 75.1 corresponds to (1 0 1), (0 0 4), (2 0 0), (1 0 5), (2 1 1), (2 0 4), (1 1 6), (2 2 0) and (2 1 5) crystal planes indicating the formation of anatase phase TiO2. The observed results are in good agreement with JCPDS Card No 21-1272. No diffraction peaks due to other impurities were detected, implying high purity of the samples [23]. The average crystallite size of TiO2 nano powder (S1 and S2) was estimated from the Scherrer’s formula [24].
D = k.λ/β. cos θ
(2)
where λ is the X-ray wavelength (0.154 nm), θ is the diffraction angle and β is the full width at half maximum (FWHM). The calculated average crystallite size of TiO2 nanoparticles was found to be 14 nm and 16 nm for S1 and S2. This shows that the particle sizes increases as pH increased from 7 to 9. 3.2. Morphological analysis The FESEM and TEM micrographs of the synthesized nanoparticles were taken to analyze the morphology and precise determination of particle size. Fig. 2(a&b) shows the FESEM image of TiO2 nanopowder synthesized under pH 7 and 9 conditions. It can be seen from the image 2a that the nanoparticles synthesized under pH 7 exhibit rod like morphology with length of around 300–350 nm and diameter of 70–100 nm, respectively. TEM and HR-TEM images further confirms the rod like structure of the S1 nanoparticles synthesized under pH 7 as shown in Fig. 2(c&d). When the pH was increased to 9 the morphology changes from rod to nanoplatelet like structure as shown in Fig. 2b. This shows that increasing pH suppresses the growth of nanorods to nanoplatelets. Similar morphologies of TiO2 were reported by few researchers under different synthesis process [25,26]. Growth mechanism was illustrated by graphical abstract.
Fig. 6. UV absorbance spectra of MO samples.
3.3. FT-IR spectra The FT-IR spectra of TiO2 nanopowder is shown in Fig. 3. It is believed that the broad absorption peak at 3000 cm−1–3600 cm−1 is attributed to the fundamental OeH stretching vibration of hydroxyl group [27]. The weak band occurs at 1615 cm−1–1635 cm−1 correspond to OeH bending vibration of hydroxyl group due to chemically adsorbed water molecules [28]. The peak around 800 cm−1 was assigned to the TieO stretching band [29] confirms the formation of TiO2. 3.4. Optical properties The optical absorbance spectra of TiO2 nanoparticles taken at room temperature are shown in Fig. 4a. It is seen that the particles exhibit maximum absorbance in the UV region. It is reported that the absorption band at 400 nm for TiO2 is due to the charge-transfer from the valence band (mainly formed by 2p orbitals of the oxide anions) to the conduction band (mainly formed by 3d orbitals of the Ti4+ cations) [30]. The broad absorbance edge of the spectra is the result of anatase
Fig. 7. Time Vs C/C0 of TiO2 samples under MO degradation.
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(O2%−), which further indirectly turn into highly reactive hydroxide radicals (OH%). Moreover, holes by interacting with OH− form highly reactive hydroxyl radicals. Reactive hydroxide radicals with high oxidation ability, generate either photogenerated electrons or holes which finally oxidize the dye molecules. The possible mechanism of photocatalytic reactions of MO dye as follows TiO2 + hν → TiO2 + h+ + e− −
h + H2O → OH + H +
−
h + OH → OH +
−
Fig. 8. Plausible mechanism for the MO degradation by using the TiO2.
nanoparticle. The energy band gap of the prepared nanoparticles is determined as 3.2 eV for S1 and 3.15 eV for S2 samples [31]. The optical band gap is found to be pH dependent and there is a decrease in the bandgap with increase in pH. The decreased in band gap with increasing pH may be due to base nature [32], variation of the crystallite size, morphology, quantum confinement and dislocation density will affect the band gap [33].
%
+
%
− −
(3) (4) (5)
O2 + e → ( O2 )
(6)
2H2O + O2 + e → 2OH + 2OH
(7)
OH + Dye (MO) → degradation products
(8)
4. Conclusions TiO2 nanorods were synthesized by the hydrothermal method. The crystalline nature and nano-rod structure of the TiO2 were confirmed by XRD and SEM. Structural analysis confirms the formation of anatase TiO2. The average crystallite size increases from 14 nm to 16 nm as the pH increases from 7 to 9. FTIR spectra confirms the formation of TieO stretching bond. UV visible absorption spectra report that the addition of NaOH restrict the growth of TiO2 nanorods and the absorption band edge red shifted. The band gap value decreased from 3.2 eV to 3.15 eV as the pH increased from 7 to 9. The highest degradation (51%) of MO was achieved by the irradiation of UV light for 150 min.
3.5. XPS analysis XPS analysis was carried out to analyze the surface electronic status of Ti and O elements in TiO2. Fig. 5(a) shown that Ti 2p spectrum presented two peaks at 458.8 eV and 457.9 eV for Ti 2p3/2 and 465 and 463.6 eV for Ti 2p1/2 which indicates the Ti4+ in TiO2 [34]. The observed binding energy for Ti 2p3/2 and Ti 2p1/2 decreases towards lower binding energy in S2. This shift confirms the presence of Ti3+. In addition, Ti3+ sites reduced the band gap of TiO2. Ti4+ reduction to Ti3+ is usually appeared by calcination [20]. In this study, the synthesized TiO2 nanoparticles was calcined at 400 °C, hence Ti3+ ions were produced. The binding energy of O 1s of TiO2 nanostructures are shown in Fig. 5(b). The peaks at 530.4 (S1) and 529.5 eV (S2), which corresponded to the TieO, respectively. The secondary peak at 532.2 eV corresponded to the contributions from the surface adsorbed hydroxyl groups [35]. The interaction significantly modified the original chemical states and electronic properties in the samples. The degradation of methyl orange (MO) was determined from absorption spectra by the irradiation of UV light. The absorbance spectra was measured with respect to the UV light irradiation with time interval from 0 to 150 min. The temporal evolution of spectral changes accompanying the photodecomposition of MO over as-prepared TiO2 nanostructures are shown in Fig. 6(a and b). The initial absorption peak gradually decreases under UV light irradiation. The MO decompose to form CO2, H2O and other inorganic molecules as a harmless product. The time-dependent UV absorption spectra of sample S1 and S2 with an irradiation time of 150 min. The maximum degradation efficiency for TiO2 nanorods (S1) exhibit 51% degradation of MO in 150 min. While TiO2 nanoplatelet (S2) exhibit 10% degradation of MO in 150 min. Which can be concluded that TiO2 nanorods showed the better degradation due to a greater number of vacant sites when compared to TiO2 platelet. Without catalyst condition, the degradation of dye (4% for 150 mins) was very less, and the sample attained a stable value after 35 mins as shown in Fig. 7. By above discussion, the photocatalytic mechanism of TiO2, are shown in Fig. 8. The electron in the valance band (VB) were excited towards the conduction band (CB) by illuminating the UV light and the holes were generated in the valance band. There after the electron and holes were transferred to the surface of the crystal and react with the molecular oxygen (O2) and H2O for generating the super oxide radicals anion (%O2−) and hydroxyl radicals (OH%). These electrons, upon reacting with dissolved oxygen molecules, form super oxide radical anion
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