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Structural, optical and photoconductivity of Sn and Mn doped TiO2 nanoparticles
Journal of Alloys and Compounds 622 (2015) 37–47
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Structural, optical and photoconductivity of Sn and Mn doped TiO2 nanoparticles Anand Kumar Tripathi a,⇑, Mohan Chandra Mathpal a,⇑, Promod Kumar b, Manish Kumar Singh c, M.A.G. Soler d, Arvind Agarwal a a
Department of Physics, Motilal Nehru National Institute of Technology, Allahabad 211004, India Department of Physics, National Institute of Technology, Hazratbal, Srinagar 190006, India Department of Physics, The LNM Institute of Information Technology, Jaipur 302 031, India d Instituto de Fisica, Universidade de Brasilia, Brasilia, DF 70910-900, Brazil b c
a r t i c l e
i n f o
Article history: Received 19 June 2014 Received in revised form 25 September 2014 Accepted 30 September 2014 Available online 8 October 2014 Keywords: Semiconductors Sol–gel processes Nanoparticles TiO2 Photoconductivity Photoluminescence
a b s t r a c t The Sn and Mn doped TiO2 nanostructures were synthesized by sol–gel method. The Sn doped TiO2 nanoparticles exhibit anatase–rutile mixed phase, while the Mn doped TiO2 nanoparticles exhibit anatase phase. The Sn and Mn doped TiO2 nanoparticles are spherical in shape and show tensile strain in the host lattice. The optical band gap for these nanoparticles indicates the red shift. It has been observed that the PL intensity decreases after doping of Sn in TiO2 but it starts increasing with increase in Sn content. The pure TiO2 exhibits all the possible emission bands while Sn and Mn doped TiO2 nanoparticles show blue– green emission bands. The doping behavior of Sn and Mn on crystal phase, particle size, XRD patterns, absorption spectra, photoluminescence and photoconductivity of TiO2 nanoparticles have been described. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction In the last many years researchers have systematically tried to improve the performance of titania photocatalyst. There are several ways for improving the performance of titania photocatalysts such as doping of the elements. Vinodgopal et al. observed that UV-activated degradation of an azo dye by a physical mixture of SnO2 and TiO2 is by an order of magnitude faster than pure oxide components and attributed this effect to the existence of physical contact between the two semiconductors [1]. TiO2 has been considered as the most promising semiconductor photocatalyst because of high stability and low cost [2]. It is only sensitive to UV light due to its large band gap (3.2 eV) [3], along with the relatively high electron–hole pair recombination rate [4]. In order to use TiO2 for solar energy more efficiently, most of the investigations are focused on the preparation of TiO2 which is sensitive to visible light [5]. Many ionic dopants in different valance state have been investigated, including metallic and nonmetallic ions.
⇑ Corresponding authors. Tel.: +91 532 2271263; fax: +91 532 2545342 (M.C. Mathpal). E-mail addresses: [email protected] (A.K. Tripathi), mohanatnpl@ gmail.com (M.C. Mathpal). http://dx.doi.org/10.1016/j.jallcom.2014.09.218 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
Particularly TiO2 doped with Sn has proved to be an effective and widely used method [6]. The positive effect of Sn on titania catalytic efficiency was attributed to a change of the anatase electronic structure which decreases the band gap (shift of absorption edge towards visible light) and to the introduction of specific Sn related surface sites [7,8]. The studies on Sn-modified titania show that it can improve a charge-hole separation [1], increase photo excited electron–hole life time within a single particle [9], alter the electronic structure of the metal oxide photocatalysts [7,8]. Generally, the presence of a dopant in TiO2 matrices can decrease the band gap of TiO2 materials, which can expand the optical response region of TiO2 materials. Metal ions such as iron, nickel, vanadium, chromium, platinum, ruthenium, copper, manganese, cerium and tin have been investigated as potential dopant of TiO2 to promote the photocatalytic activity of TiO2. Metal ion doping is able to enhance interfacial charge-transfer reactions. In this article Sn and Mn doped TiO2 nanoparticles were synthesized which formed anatase and anatase–rutile mixed phase by using different doping percentages. The prepared samples after calcination at 400 °C have been employed to study the structural and optical properties. The aim of this study is to understand the electronic band structure for enhancing the photoluminescence and absorption in visible range of light.
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A.K. Tripathi et al. / Journal of Alloys and Compounds 622 (2015) 37–47 2. Experimental Titanium tetra isopropoxide (TTIP), ethanol, SnCl4 and MnCl2 were procured from Merck (Analytical grade). Sol–gel method was used to synthesize the anatase, and anatase–rutile mixed phase of TiO2 nanoparticles. As a dopant chlorides were dissolved in distilled water (used in different weight percentage of SnCl4 and MnCl2) while the main precursor titanium tetra isopropoxide (TTIP) was dissolved in ethanol. The desired product was obtained by drop-wise addition of different concentration (0.0, 0.02, 0.04, 0.06 and 0.08 wt%) of dopant precursor into the main precursor. The reaction was performed at room temperature under stirring. The obtained gel was dried in an oven for 10 h and then crushed to get the fine powder. The prepared powder samples were calcinated for 1 h in a box furnace at a temperature of 400 °C.
3. Results and discussions 3.1. X-ray diffraction
Fig. 1. XRD pattern of TiO2 nanoparticles with different Sn content annealed at 400 °C (a) 0 wt%, (b) 0.02 wt%, (c) 0.04 wt%, (d) 0.06 wt% and (e) 0.08 wt%.
The impact of Sn and Mn proportions on the crystal phase of TiO2 was investigated by XRD whose patterns are shown in Figs. 1 and 2 respectively. The Sn doped TiO2 nanoparticles exhibit anatase and anatase–rutile mixed phase while Mn doped TiO2 nanoparticles are anatase in phase. This indicates that the doping of Sn will reduce the anatase to rutile phase transformation temperature. The phase transition from anatase to rutile can take place around 400 °C for Sn-doped TiO2 which is about 150 °C or 200 °C lower than that for undoped TiO2 (550, 600 °C) [3]. The average crystallite size of Sn and Mn doped TiO2nanoparticles varies from 14 to 8 nm and for undoped TiO2 is 27 nm as estimated by Scherer formula (Tables 1 and 2). The corresponding percentages (WR) of rutile in the mixed crystal phase were 30.7%, 39.8%, 45.4%, and 43.8% calculated by formula WR = (1 + 0.81IA/IR)1 [10], where IA and IR are diffraction peak intensity of anatase (2h = 25.3°) and rutile (2h = 27.4°) respectively. The presence of Sn4+ facilitates the transformation of TiO2 from anatase to rutile under the same temperature. The lattice strain has been calculated by using the relation for Williamson–Hall plot [11].
b cos h 1 g sin h ¼ þ r k k Fig. 2. XRD pattern of Mn doped TiO2 nanoparticles annealed at 400 °C (a) 0.0wt%, (b) 0.02wt%, (c) 0.04 wt%, (d) 0.06 wt% and (e) 0.08 wt%.
where b is FWHM in radians, k is the wavelength of X-ray, h is the diffraction angle, r is the effective particle size and g is the effective
Table 1 Structural parameters of Sn doped TiO2 nanoparticles. Structural parameters
0.0 wt%
0.02 wt%
0.04 wt%
0.06 wt%
0.08 wt%
FWHM (°) Lattice constant (Å)
0.2991 a = b = 3.786 c = 9.504 3.5280 136.08 27.2 3.89 5.67 105
0.5983 a = b = 3.792 c = 9.468 3.5032 136.14 13.5 3.89 11.34 105
0.8974 a = b = 3.788 c = 9.490 3.4970 136.17 9.08 3.89 16.98 105
1.0470 a = b = 3.764 c = 9.421 3.4902 133.47 7.78 3.97 19.42 105
1.0470 a = b = 3.764 c = 9.463 3.5163 134.06 7.77 3.95 19.54 105
d-spacing (Å) Unit cell volume, a2c (Å3) Average crystallite size (nm) Density, q (g/cm3) Specific surface area, Sa (cm2/g)
Table 2 Structural parameters of Mn doped TiO2 nanoparticles. Structural parameters
0.0 wt%
0.02 wt%
0.04 wt%
0.06 wt%
0.08 wt%
FWHM (°) Lattice constant (Å)
0.2992 a = b = 3.786 c = 9.504 3.5282 136.18 27.4 3.90 5.76 105
0.5983 a = b = 3.784 c = 9.492 3.5174 135.93 13.6 3.90 11.31 105
1.0471 a = b = 3.804 c = 9.468 3.5096 137.00 7.78 3.87 19.92 105
0.6731 a = b = 3.760 c = 9.440 3.5231 133.46 12.09 3.97 12.50 105
0.6731 a = b = 3.780 c = 9.428 3.5441 134.73 12.09 3.93 12.62 105
d-spacing (Å) Unit cell volume, a2c (Å3) Average crystallite size (nm) Density, q (g/cm3) Specific surface area, Sa (cm2/g)
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strain. Figs. 3 and 4 show the Williamson–Hall plot between bcos h/k and sin h/k for the prepared samples. Negative slope in the plot indicates the presence of compressive strain [12] whereas the positive slope indicates the presence of tensile strain [13]. Positive slopes were obtained for all the samples which indicate the tensile strain in each sample. When compared with bulk TiO2 (a = b = 3.784 Å and c = 9.514 Å), and synthesized TiO2 a variation in lattice constants has been observed for all the prepared samples (Tables 1 and 2); may be due to tensile strain. The X-ray density of the samples has been calculated by the formula [14]:
q¼ Fig. 3. Williamson–Hall plots of TiO2 nanoparticles with different Sn content annealed at 400 °C (a) 0 wt%, (b) 0.02 wt%, (c) 0.04 wt%, (d) 0.06 wt% and (e) 0.08 wt%.
where M is the molecular weight, N is the Avogadro’s number and V is the volume of unit cell. For anatase phase n is four and for rutile phase n is two [15,16]. The specific surface area is calculated by formula [14]:
Sa ¼
Fig. 4. Williamson–Hall plots of TiO2 nanoparticles with different Mn content annealed at 400 °C (a) 0 wt%, (b) 0.02 wt%, (c) 0.04 wt%, (d) 0.06 wt% and (e) 0.08 wt%.
nM NV
6 Dq
where D is the crystallite size and q is the density. The density of the samples vary very closely while the specific surface area increases with increase in Sn and Mn content but it decreases for Mn content 0.04 and 0.06 wt% (Tables 1 and 2). The similar ionic radius of Ti4+ (0.605 Å) [17] and Sn4+ (0.690 Å) [18] allow easy substitution of Ti4+ by Sn4+ in TiO2 lattice. It is observed that the value of FWHM increases with the increase in doping percentage which may leads to the decrease in crystallinity and crystallite size. Nanoparticles with small crystallite size have less thermal stability. In the case of solid particles, the total free energy is determined by volume energy Gv, surface energy Gs, and surface stress induced energy Gf in comparison with bulk materials. The last two terms are very large in nanoparticles so they cannot be ignored. The surface energy is increased by the
Fig. 5. FESEM images of prepared samples with different Sn content annealed at 400 °C (a) 0.02 wt%, (b) 0.04 wt%, (c) 0.06 wt% and (d) 0.08 wt%.
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Fig. 6. FESEM images of prepared samples with different Mn content annealed at 400 °C (a) 0.02 wt%, (b) 0.04 wt%, (c) 0.06 wt% and (d) 0.08 wt%.
Fig. 7. TEM micrographs at the 50 nm scale of TiO2 having Sn content: (a) 0.04 wt%, (c) 0.08 wt%, (b) SAED pattern of Sn content 0.04 wt% and (d) SAED pattern of Sn content 0.08 wt%.
construction of new surface which is due to nanoparticles formation. The Gibbs free energy of phase transformation for anatase nanoparticles is as follows [19]:
DGA!R ¼ DGV;R ðTÞ DGV;A ðTÞ þ 2MfR 3MfA þ qR rR qA rA
3M cR 3McA qR rR qA rA
where c is the surface free energy, r is the particle radius, M is the molecular weight, q is the phase density and f is coefficient of surface tension related to excess pressure with a value of 2fr1. Only a large negative DGA ? R is an adequate driving force for anatase to rutile transformation. The higher surface energy and surface stress energy have more contribution to the non-equilibrium transition from anatase to rutile. Particle with smaller size have large surface areas and higher surface energy. Therefore, in anatase with smaller
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Fig. 8. TEM micrographs at the 50 nm scale of TiO2 having Mn content: (a) 0.04 wt%, (c) 0.08 wt%, (b) SAED pattern of Mn content 0.04 wt% and (d) SAED pattern of Mn content 0.08 wt%.
particle size it is easier to start the phase transformation at lower temperature than in the large particle under similar conditions. The higher negative DGA ? R value corresponds to lower activation energy which is needed for anatase to rutile transition [19]. The Gibbs free energy of rutile (888.8 kJ/mol at 298 K) is less than that of anatase at all temperatures [20] and activation energy of 418– 753 kJ/mol was reported for millimeter-sized anatase single crystal in the temperature range 900–950 °C, while a much lower value of 166 ± 1 kJ/mol was found for nanocrystalline anatase prepared by the sol–gel method in the temperature range 465–525 °C [21]. Therefore the increase in the surface area leads to decrease in the activation energy and the phase transformation takes place at low temperature. The other important factor affecting the phase transformation from anatase to anatase–rutile mix phase is due to tetrahedral distortion in Ti4+ sites and the presence of defects on the oxygen sublattice. The excess of dopant concentration, changes Ti–O and O–Ti–O bond angles, resulting in the anatase phase with the formation of rutile TiO2 [20,22].
decreases and the agglomeration between the particles also reduces. The selected area electron diffraction (SAED) indicates polycrystalline form of samples and show that the crystallinity decreases with increase in dopant concentration. 3.4. TGA curves of samples TGA profiles of Sn and Mn doped TiO2 are presented in Figs. 9 and 10 respectively. The weight loss in the temperature range between 90 °C and 200 °C can be attributed due to the loss of adsorbed water in the titanium oxide [23]. The combustion of residual organic species, including the dehydroxylation of the gel and decomposition of Cl ions are responsible for the weight loss between 300 °C and 550 °C [24]. In the Fig. 9 the weight loss of 6% for Sn proportion 0.02% (curve (a)), loss of 5% for 0.04% (curve (b)), and loss of 4% for 0.06% (curve (c)) have been observed. This
3.2. Field emission scanning electron microscopy The FESEM images for Sn and Mn doped TiO2 nanoparticles calcinated at 400 °C have been presented in Figs. 5 and 6 respectively, which show the smaller particle size at higher doping percentage and less agglomeration has occurred with increase in doping proportion of Sn and Mn. The shape and surface morphology of doped nano-powders have a critical role in the phase transformation from anatase to rutile. 3.3. Transmission electron microscopy (TEM) TEM results for Sn and Mn doped TiO2 nanoparticles are shown in Figs. 7 and 8 respectively. This is clear from the TEM images, as the concentration of dopant increases average particle size
Fig. 9. TGA of samples with different Sn proportions annealed at 400 °C (a) 0.02 wt%, (b) 0.04 wt%, (c) 0.06 wt% and (d) 0.08 wt%.
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Fig. 13. FTIR spectra of samples with different Sn content annealed at 400 °C (a) 0 wt%, (b) 0.02 wt%, (c) 0.04 wt%, (d) 0.06 wt% and (e) 0.08 wt%. Fig. 10. TGA of samples with different Mn proportions annealed at 400 °C (a) 0.02 wt%, (b) 0.04 wt%, (c) 0.06 wt% and (d) 0.08 wt%.
indicates that the increased loss in adsorbed water and residual organic species. 3.5. Raman spectra
Fig. 11. Raman spectra of samples with different Sn proportions (a) 0 wt%, (b) 0.02 wt%, (c) 0.04 wt%, (d) 0.06 wt% and (e) 0.08 wt% annealed at 400 °C. The inset shows the shifting towards lower wave number for highest intense peak in Raman bands.
weight loss decreases with increase in Sn proportion. But the weight loss of 7% has been observed in Sn proportion 0.08% (curve (d)) which indicates the increased loss of adsorbed water and higher residual organic species. In curve (d) there is a weight gain of 0.21% has been observed at temperature 282 °C which may be due to mixing of more number of Sn4+ in TiO2 matrix. In the Fig. 10 the weight loss increases from 5.3% to 7.7% with increase in Mn content from 0.02 to 0.08 wt% has been observed, which
The Raman bands were assigned in Raman spectra of Sn and Mn doped TiO2 nanostructures are presented in Figs. 11 and 12 [25– 27]. The specific vibrational modes are located at 147 cm1 (Eg), 199 cm1 (Eg), 399 cm1 (B1g), 518 cm1 (A1g + B1g) and 643 cm1 (Eg) [28–30], indicating the presence of the anatase phase in the un-doped TiO2 calcinated at 400 °C. The inset in Figs. 11 and 12 represents the exact peak position (in cm1) corresponding to the highest intense peak for the un-doped and doped TiO2 nanoparticles, which shows the corresponding peaks for doped samples shifted upto 140 cm1 from the position of 147 cm1. The main Raman bands at 490, 574, 636, and 776 cm1 due to the crystalline SnO2 were not detected [28], which indicates that the Sn does not exist as a separate crystalline oxide phase. These results are in good agreement with those obtained from the XRD analysis. The Raman spectrum for Mn-doped TiO2 nanoparticles indicates that the peaks of all doped samples shift towards the lower wave number side (red shift) and no indication of the presence of rutile or any secondary phases. All doped samples shows Raman shift from 147 cm1 to 140 cm1 which may be due to incorporation of Sn4+ and Mn2+ to titania matrix. 3.6. Fourier transform infrared spectroscopy FTIR spectra of Sn and Mn doped TiO2 nanostructures are presented in Figs. 13 and 14 respectively. The absorption bands
Fig. 12. Raman spectra of samples with different Mn proportions (a) 0 wt%, (b) 0.02 wt%, (c) 0.04 wt%, (d) 0.06 wt% and (e) 0.08 wt% annealed at 400 °C. The inset shows the shifting towards lower wave number for highest intense peak in Raman bands.
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Fig. 14. FTIR spectra of samples with different Mn content annealed at 400 °C (a) 0 wt%, (b) 0.02 wt%, (c) 0.04 wt%, (d) 0.06 wt% and (e) 0.08 wt%.
Fig. 16. Absorption spectra (i) and Tauc plot (ii) of prepared samples with different Mn content annealed at 400 °C (a) 0 wt%, (b) 0.02 wt%, (c) 0.04 wt%, (d) 0.06 wt% and (e) 0.08 wt%.
Table 3 Absorption edges for Sn doped TiO2 nanoparticles. Sn proportion (wt%)
Wavelength (nm)
Band gap (eV)
0.0 0.02 0.04 0.06 0.08
383 531 540 552 560
3.24 2.34 2.29 2.25 2.21
Table 4 Absorption edges for Mn doped TiO2 nanoparticles.
Fig. 15. Absorption spectrum (i) and Tauc plot (ii) of prepared samples with different Sn content annealed at 400 °C (a) 0 wt%, (b) 0.02 wt%, (c) 0.04 wt%, (d) 0.06 wt% and (e) 0.08 wt%.
between 400 cm1 and 800 cm1 are mainly ascribed to Ti–O and Ti–O–O bonds [10,29–31]. This vibration is dominant for Sn proportion 0.04 and 0.08 wt% respectively and in all the Mn doped TiO2 compare to undoped TiO2. The vibrational band between 1300 cm1 and 4000 cm1 indicates the adsorbed H2O and CO2 molecules on the surface [31]. The broad intense band between 1200 cm1 and 1022 cm1 present in all the samples which indicates the Ti–O–Ti vibrations [32]. The intensity of this band is higher for Sn and Mn doped TiO2 as compare to undoped TiO2 which indicates the more Ti–O–Ti vibration takes place. Here the bending vibration of the adsorbed water molecules are present between 1620 cm1 and 1630 cm1 [32] in all Sn and Mn doped TiO2. 3.7. UV–Vis absorption spectroscopy
Mn proportion (wt%)
Wavelength (nm)
Band gap (eV)
0.0 0.02 0.04 0.06 0.08
383 408 465 483 465
3.24 3.04 2.66 2.56 2.66
red shift takes place and the photo-activity greater than 400 nm exists. Earlier studies have shown that Sn-doping in TiO2 anatase shifts the absorption edge to higher wavelengths [8,33,34] and at higher concentration of Sn it produces new band absorbing roughly between 400 and 500 nm [8,33] probably the consequences of formation of new electron levels of Sn ions in titania band structure. Red shift in wavelength has been observed with the increase in Mn content from 0.02 to 0.06 wt% (Table 4), and the absorption intensity is higher for the sample having Mn content 0.02 wt%. The sample of Mn content 0.08 wt% the absorption edge is found to be 465 nm. The optical band gap of the prepared samples is calculated by Tauc plot. The absorption band gap energy can be determined by the following equation [35,36]. n
ðahmÞ ¼ Bðhm Eg Þ The optical absorption spectra of Sn and Mn doped TiO2 nanostructures are displayed in Figs. 15 and 16 respectively show red shift with increasing Sn and Mn proportions in TiO2 nanoparticles. The un-doped TiO2 has absorption edge at 383 nm (Table 3) which is similar to the anatase TiO2 nanoparticles (386 nm). The maximum shift has been observed for higher Sn percentage (0.08 wt%) at 560 nm. It has been observed that due to doping of Sn in TiO2
where hm is the photon energy, a is the absorption coefficient, B is a constant relative to the material and n is a value that depends on the nature of transition (n = 2 for direct band gap, 2/3 for direct forbidden gap and 1/2 for indirect band gap). The band gap energy for un-doped TiO2 has been observed 3.24 eV which is similar to the band gap of anatase TiO2. It has been observed that on doping the
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Fig. 17. Photoluminescence spectrum of prepared samples with different Sn content (a) 0.02 wt%, (b) 0.04 wt%, (c) 0.06 wt% and (d) 0.08 wt% annealed at 400 °C and inset represents the PL spectrum of pure TiO2.
band gap energy decreases from 3.24 eV (pure TiO2) to 2.21 eV with the increase in Sn content in TiO2 nanoparticles. The lowest band gap energy of 2.21 eV has been observed when Sn proportion is 0.08 wt% is very similar to rutile phase attributes the existence of mixed phase of rutile and anatase TiO2 nanoparticles [32]. The band gap decreases from 3.24 eV to 2.56 eV with increase in Mn content from 0.0 to 0.06 wt% (Table 4). These results indicate that the doping of Mn in TiO2 nanoparticles gives the red shift and decreases the band gap. Reduction in band gap can be attributed to charge transfer from bulk to surface of the nanocrystals and supports the photogeneration after photo-excitation [14]. 3.8. Photoluminescence Figs. 17 and 18 illustrates the photoluminescence features of prepared TiO2 nanoparticle doped with Sn and Mn respectively calcinated at 400 °C and inset figure represents the PL spectra of undoped TiO2 nanoparticles calcinated at the same temperature. The PL spectrum of prepared samples is obtained as a result of the electron–hole separations, electron–phonon scattering and electron–hole recombination. TiO2 has a direct band gap but is subjected to dipole-forbidden transition [37]. PL spectra of anatase TiO2 materials are attributed to three kinds of physical origins: self-trapped excitons [38,39], oxygen vacancies [39,40] and surface states (defects) [41]. Most of the surface states are oxygen vacancies or the Ti4+ ions adjacent to oxygen vacancies [42,43]. PL measurements of pure TiO2 nanoparticles show the clear emission bands (inset Fig. 17). It has been observed that the PL intensity decreases after doping and then increases with increase in Sn content from 0.02 to 0.06 wt%. It has been observed that the PL intensity of doped TiO2 for Sn contents 0.06 and 0.08 wt% are very similar. The results show that crystallite size decreases with increase in Sn content from 0.02 to 0.06 wt%, the size of 0.06 and 0.08 wt% is similar. The blue–green emission of 485 nm (2.55 eV) in undoped TiO2 (inset Fig. 17) and 482 nm in Sn-doped TiO2 (Fig. 17) has been observed, which can be attributed to the charge transfer from Ti3+ to oxygen anion in a [TiO6]8 complex associated with oxygen vacancies on the surface, it indicates that
band is originating from the intrinsic state rather than the surface state. Therefore the 485 nm band can be assigned to self-trapped excitons localized on TiO6 octahedral [39]. The intensity of these bands increases with increase in Sn content which indicates the rapid transfer of charge of Ti3+ to oxygen anion. PL bands at the longer wavelength side of TiO2 nanoparticles have been reported by some workers and attributed to the oxygen vacancies [39,40,44,45]. The blue–green emission of 485 nm (2.55 eV) in Mn-doped TiO2 having Mn content 0.02, 0.08 (Fig. 18) has been observed but in the sample having Mn content 0.04 wt% this emission band has shifted to 438 nm (2.83 eV) with higher PL intensity. In the sample having Mn content 0.06 wt% only one emission band of 436 nm (2.84 eV) has been observed with high PL intensity, this indicates that samples having Mn content 0.04 and 0.06 wt% exhibit quite same luminescence. It is observed that visible emission obtained in PL spectra of all doped samples can be attributed to the oxygen vacancies and Ti vacancies introduced after Sn and Mn doping. 3.9. Photoconductivity measurements 3.9.1. Rise and decay time response photoconductivity measurements The rise and decay time transient photoconductivity response measurements help to study the photoconductivity dynamics of TiO2 nanoparticles during which the light was abruptly switched on and off at room temperature. The time-resolved rise and decay of the photocurrent spectra for the Sn, Mn doped nanoparticles under visible illumination of 370 nm are shown in Figs. 19 and 20. The surface related phenomenon, which is primarily governed by adsorption and desorption processes, plays an important role in the photoconductivity properties of the nanostructures due to large surface-to-volume ratio [46]. The rise and decay of photocurrent spectrum is useful in determining the nature of trap and recombination centers present inside the materials. When the field is applied, the initial dark current is very high in all prepared samples. This may be attributed to the presence of oxygen vacancies on the surface and other native defects acting as donors as well as the process of adsorption of water molecules, thereby releasing charge
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Fig. 18. Photoluminescence spectrum of prepared samples with different Mn content (a) 0.02 wt%, 0.04 wt%, 0.08 wt% and (b) 0.06 wt% annealed at 400 °C.
Fig. 19. Photoconductivity rise and decay time spectra: photoconductive response due to visible light excitation for TiO2 nanoparticles containing Sn weight percentage (a) 0.02, (b) 0.04, (c) 0.06 and (d) 0.08.
carriers. After this process the dark current starts decreasing and attains a minimum value in the prepared samples. When visible illumination is switched on, the photocurrent rises in all the samples, but this rise in photocurrent is higher in undoped TiO2 as compare to Sn and Mn doped TiO2 nanoparticles. Slow
photoconductive rise and decay response may be attributed to a large amount of recombination centers and presence of trap levels and defect states within the band gap [47,48]. When illumination is switched off, electronhole recombination process dominates and the conductivity decreases in the samples.
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Fig. 20. Photoconductivity rise and decay time spectra: photoconductive response due to visible light excitation for TiO2 nanoparticles containing Mn weight percentage (a) 0.02, (b) 0.04, (c) 0.06 and (d) 0.08.
3.9.2. Trap depth determination Trap depth is defined as the required energy to remove an atom from the trap. Trap depths can be calculated from decay curves. The decay of photocurrent can be represented by the equation I = I0exp (pt), according to Bube model [49] where I0 is the current at the time when light is switched off, I is photocurrent at any instant of time for growth and decay function and the p = S exp (E/kT), probability of escape of an electron from trap per second. The trap depth (E) can be calculated by using the following equation:
"
ln Io E ¼ kT ln S ln I t
#
where E denotes trap depth, k is Boltzmann constant, T is the absolute temperature and S is the frequency factor [14] defined as the number per second that the quanta from the lattice vibrations (phonons) attempt to eject the electron from the trap multiplied by the probability of transition of the ejected electron to the conduction band and is of the order of 109 at room temperature. Here the trap depth calculated for undoped TiO2 is 0.67 eV. The trap depth is 0.72 eV, 0.69 eV, 0.67 eV, 0.65 eV obtained for Sn doped TiO2 while 0.75 eV, 0.68 eV, 0.67 eV, 0.66 eV obtained for Mn doped TiO2 containing weight percentage 0.02, 0.04, 0.06, 0.08
respectively. These are much greater than the reported value for anatase TiO2 between 0.10 and 0.27 eV [50–52].
4. Conclusions The XRD result shows that the average crystallite size decreases from 27 nm to 8 nm with increase in Sn and Mn content in TiO2. All the Sn doped TiO2 nanoparticles are spherical in shape and exhibit anatase–rutile mixed phase while the Mn doped TiO2 nanoparticles exhibit anatase phase with tensile strain in the host lattice. The optical band gap decreases from 3.24 eV to 2.21 eV indicates the red shift for Sn and Mn doped anatase and anatase–rutile mixed phase of TiO2 nanoparticles. XRD spectra confirm that Ti atom has been successfully replaced by Sn and Mn atom in all the samples and it is not present on the surface in the metal–oxide form. Thus depending on the nature of chemical dopant and how they are introduced into the titania lattice the adsorption of visible light can be tuned easily. It has been observed that the PL intensity decreases after doping of Sn in TiO2 and then the intensity increases with increase in Sn content. The synthesized TiO2 exhibits all the possible emission bands while Sn doped TiO2 samples having blue–green emission bands.
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Acknowledgments The authors are thankful to CIR staff at MNNIT and Director MNNIT for providing the characterization facility and funding support through TEQIP-II project for carrying out the research work. References [1] K. Vinodgopal, Prashant V. Kamat, Environ. Sci. Technol. 29 (1995) 841. [2] R.J. James, G. Andrie, M.P. Laurence, S. Patric, B.W. Alison, JACS 130 (40) (2008) 13364. [3] Y.Q. Cao, T. He, L.S. Jhao, E.J. Wang, W.S. Yang, Y.A. Cao, J. Phys. Chem. C 113 (42) (2009) 18121. [4] Y.Q. Cao, T. He, Y.M. Chen, Y.A. Cao, J. Phys. Chem. C 114 (8) (2010) 3627. [5] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Science 293 (5528) (2001) 269. [6] J. Li, H.C. Jhang, JACS 129 (51) (2007) 15839. [7] Y.Q. Cao, W.S. Yang, W.S. Zhang, P.L. Yue, New J. Chem. 28 (2) (2004) 218. [8] V.B.R. Boppana, R.F. Lobo, J. Catal. 281 (1) (2011) 156. [9] Jun Lin, Jimmy C. Yu, D. Lo, S.K. Lam, J. Catal. 183 (1999) 368. [10] Chandana Rath, P. Mohanty, A.C. Pandey, N.C. Mishra, J. Phys. D: Appl. Phys. 42 (2009) 205101. [11] G.K. Williamson, W.H. Hall, Acta Metal. 1 (1953) 22. [12] R.R. Prabhu, M. Abdul Khadar, Bull. Mater. Sci. 31 (2008) 511. [13] Shenthil kumar, P. Vickraman, M. Jayachandran, J. Mater. Sci: Mater. Electron. 21 (2010) 343. [14] A. Maurya, P. Chauhan, S.K. Mishra, R.K. Srivastava, J. Alloys Comp. 509 (2011) 8433. [15] A.I. Kingon, J.P. Maris, S.K. Steiffer, Nature 1032 (2000) 406. [16] W. Li, C. Ni, H. Lin, C.P. Huang, S.Ismat Shah, J. Appl. Phys. 96 (11) (2004) 6663. [17] R.C. Pullar, S.J. Penn, X. Wang, I.M. Reaney, N.M. Alford, J. Eur. Ceram. Soc. 29 (3) (2009) 419. [18] S. Wu, G. Wang, S. Wang, D. Liu, J. Mater. Sci. Technol. 21 (5) (2005) 773. [19] W. Li, C. Ni, H. Lin, C.P. Huang, S. Ismat Shah, J. Appl. Phys. 96 (2004) 6663– 6668. [20] M.C. Mathpal, A.K. Tripathi, P. Kumar, B. Rajagopalan, M.K. Singh, J.S. Chung, S.H. Hur, A. Agarwal, Phys. Chem. Chem. Phys. (2014), http://dx.doi.org/ 10.1039/C4CP02982H. [21] J.G. Li, T. Ishigaki, Acta Mater. 52 (2004) 5143–5150. [22] M.C. Mathpal, A.K. Tripathi, P. Kumar, V. Agrahari, M.K. Singh, A. Agarwal, Chem. Phys. Lett. (2014), http://dx.doi.org/10.1016/j.cplett.2014.09.035.
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Structural, optical and photoconductivity of Sn and Mn doped TiO2 nanoparticles Anand Kumar Tripathi a,⇑, Mohan Chandra Mathpal a,⇑, Promod Kumar b, Manish Kumar Singh c, M.A.G. Soler d, Arvind Agarwal a a
Department of Physics, Motilal Nehru National Institute of Technology, Allahabad 211004, India Department of Physics, National Institute of Technology, Hazratbal, Srinagar 190006, India Department of Physics, The LNM Institute of Information Technology, Jaipur 302 031, India d Instituto de Fisica, Universidade de Brasilia, Brasilia, DF 70910-900, Brazil b c
a r t i c l e
i n f o
Article history: Received 19 June 2014 Received in revised form 25 September 2014 Accepted 30 September 2014 Available online 8 October 2014 Keywords: Semiconductors Sol–gel processes Nanoparticles TiO2 Photoconductivity Photoluminescence
a b s t r a c t The Sn and Mn doped TiO2 nanostructures were synthesized by sol–gel method. The Sn doped TiO2 nanoparticles exhibit anatase–rutile mixed phase, while the Mn doped TiO2 nanoparticles exhibit anatase phase. The Sn and Mn doped TiO2 nanoparticles are spherical in shape and show tensile strain in the host lattice. The optical band gap for these nanoparticles indicates the red shift. It has been observed that the PL intensity decreases after doping of Sn in TiO2 but it starts increasing with increase in Sn content. The pure TiO2 exhibits all the possible emission bands while Sn and Mn doped TiO2 nanoparticles show blue– green emission bands. The doping behavior of Sn and Mn on crystal phase, particle size, XRD patterns, absorption spectra, photoluminescence and photoconductivity of TiO2 nanoparticles have been described. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction In the last many years researchers have systematically tried to improve the performance of titania photocatalyst. There are several ways for improving the performance of titania photocatalysts such as doping of the elements. Vinodgopal et al. observed that UV-activated degradation of an azo dye by a physical mixture of SnO2 and TiO2 is by an order of magnitude faster than pure oxide components and attributed this effect to the existence of physical contact between the two semiconductors [1]. TiO2 has been considered as the most promising semiconductor photocatalyst because of high stability and low cost [2]. It is only sensitive to UV light due to its large band gap (3.2 eV) [3], along with the relatively high electron–hole pair recombination rate [4]. In order to use TiO2 for solar energy more efficiently, most of the investigations are focused on the preparation of TiO2 which is sensitive to visible light [5]. Many ionic dopants in different valance state have been investigated, including metallic and nonmetallic ions.
⇑ Corresponding authors. Tel.: +91 532 2271263; fax: +91 532 2545342 (M.C. Mathpal). E-mail addresses: [email protected] (A.K. Tripathi), mohanatnpl@ gmail.com (M.C. Mathpal). http://dx.doi.org/10.1016/j.jallcom.2014.09.218 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
Particularly TiO2 doped with Sn has proved to be an effective and widely used method [6]. The positive effect of Sn on titania catalytic efficiency was attributed to a change of the anatase electronic structure which decreases the band gap (shift of absorption edge towards visible light) and to the introduction of specific Sn related surface sites [7,8]. The studies on Sn-modified titania show that it can improve a charge-hole separation [1], increase photo excited electron–hole life time within a single particle [9], alter the electronic structure of the metal oxide photocatalysts [7,8]. Generally, the presence of a dopant in TiO2 matrices can decrease the band gap of TiO2 materials, which can expand the optical response region of TiO2 materials. Metal ions such as iron, nickel, vanadium, chromium, platinum, ruthenium, copper, manganese, cerium and tin have been investigated as potential dopant of TiO2 to promote the photocatalytic activity of TiO2. Metal ion doping is able to enhance interfacial charge-transfer reactions. In this article Sn and Mn doped TiO2 nanoparticles were synthesized which formed anatase and anatase–rutile mixed phase by using different doping percentages. The prepared samples after calcination at 400 °C have been employed to study the structural and optical properties. The aim of this study is to understand the electronic band structure for enhancing the photoluminescence and absorption in visible range of light.
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A.K. Tripathi et al. / Journal of Alloys and Compounds 622 (2015) 37–47 2. Experimental Titanium tetra isopropoxide (TTIP), ethanol, SnCl4 and MnCl2 were procured from Merck (Analytical grade). Sol–gel method was used to synthesize the anatase, and anatase–rutile mixed phase of TiO2 nanoparticles. As a dopant chlorides were dissolved in distilled water (used in different weight percentage of SnCl4 and MnCl2) while the main precursor titanium tetra isopropoxide (TTIP) was dissolved in ethanol. The desired product was obtained by drop-wise addition of different concentration (0.0, 0.02, 0.04, 0.06 and 0.08 wt%) of dopant precursor into the main precursor. The reaction was performed at room temperature under stirring. The obtained gel was dried in an oven for 10 h and then crushed to get the fine powder. The prepared powder samples were calcinated for 1 h in a box furnace at a temperature of 400 °C.
3. Results and discussions 3.1. X-ray diffraction
Fig. 1. XRD pattern of TiO2 nanoparticles with different Sn content annealed at 400 °C (a) 0 wt%, (b) 0.02 wt%, (c) 0.04 wt%, (d) 0.06 wt% and (e) 0.08 wt%.
The impact of Sn and Mn proportions on the crystal phase of TiO2 was investigated by XRD whose patterns are shown in Figs. 1 and 2 respectively. The Sn doped TiO2 nanoparticles exhibit anatase and anatase–rutile mixed phase while Mn doped TiO2 nanoparticles are anatase in phase. This indicates that the doping of Sn will reduce the anatase to rutile phase transformation temperature. The phase transition from anatase to rutile can take place around 400 °C for Sn-doped TiO2 which is about 150 °C or 200 °C lower than that for undoped TiO2 (550, 600 °C) [3]. The average crystallite size of Sn and Mn doped TiO2nanoparticles varies from 14 to 8 nm and for undoped TiO2 is 27 nm as estimated by Scherer formula (Tables 1 and 2). The corresponding percentages (WR) of rutile in the mixed crystal phase were 30.7%, 39.8%, 45.4%, and 43.8% calculated by formula WR = (1 + 0.81IA/IR)1 [10], where IA and IR are diffraction peak intensity of anatase (2h = 25.3°) and rutile (2h = 27.4°) respectively. The presence of Sn4+ facilitates the transformation of TiO2 from anatase to rutile under the same temperature. The lattice strain has been calculated by using the relation for Williamson–Hall plot [11].
b cos h 1 g sin h ¼ þ r k k Fig. 2. XRD pattern of Mn doped TiO2 nanoparticles annealed at 400 °C (a) 0.0wt%, (b) 0.02wt%, (c) 0.04 wt%, (d) 0.06 wt% and (e) 0.08 wt%.
where b is FWHM in radians, k is the wavelength of X-ray, h is the diffraction angle, r is the effective particle size and g is the effective
Table 1 Structural parameters of Sn doped TiO2 nanoparticles. Structural parameters
0.0 wt%
0.02 wt%
0.04 wt%
0.06 wt%
0.08 wt%
FWHM (°) Lattice constant (Å)
0.2991 a = b = 3.786 c = 9.504 3.5280 136.08 27.2 3.89 5.67 105
0.5983 a = b = 3.792 c = 9.468 3.5032 136.14 13.5 3.89 11.34 105
0.8974 a = b = 3.788 c = 9.490 3.4970 136.17 9.08 3.89 16.98 105
1.0470 a = b = 3.764 c = 9.421 3.4902 133.47 7.78 3.97 19.42 105
1.0470 a = b = 3.764 c = 9.463 3.5163 134.06 7.77 3.95 19.54 105
d-spacing (Å) Unit cell volume, a2c (Å3) Average crystallite size (nm) Density, q (g/cm3) Specific surface area, Sa (cm2/g)
Table 2 Structural parameters of Mn doped TiO2 nanoparticles. Structural parameters
0.0 wt%
0.02 wt%
0.04 wt%
0.06 wt%
0.08 wt%
FWHM (°) Lattice constant (Å)
0.2992 a = b = 3.786 c = 9.504 3.5282 136.18 27.4 3.90 5.76 105
0.5983 a = b = 3.784 c = 9.492 3.5174 135.93 13.6 3.90 11.31 105
1.0471 a = b = 3.804 c = 9.468 3.5096 137.00 7.78 3.87 19.92 105
0.6731 a = b = 3.760 c = 9.440 3.5231 133.46 12.09 3.97 12.50 105
0.6731 a = b = 3.780 c = 9.428 3.5441 134.73 12.09 3.93 12.62 105
d-spacing (Å) Unit cell volume, a2c (Å3) Average crystallite size (nm) Density, q (g/cm3) Specific surface area, Sa (cm2/g)
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strain. Figs. 3 and 4 show the Williamson–Hall plot between bcos h/k and sin h/k for the prepared samples. Negative slope in the plot indicates the presence of compressive strain [12] whereas the positive slope indicates the presence of tensile strain [13]. Positive slopes were obtained for all the samples which indicate the tensile strain in each sample. When compared with bulk TiO2 (a = b = 3.784 Å and c = 9.514 Å), and synthesized TiO2 a variation in lattice constants has been observed for all the prepared samples (Tables 1 and 2); may be due to tensile strain. The X-ray density of the samples has been calculated by the formula [14]:
q¼ Fig. 3. Williamson–Hall plots of TiO2 nanoparticles with different Sn content annealed at 400 °C (a) 0 wt%, (b) 0.02 wt%, (c) 0.04 wt%, (d) 0.06 wt% and (e) 0.08 wt%.
where M is the molecular weight, N is the Avogadro’s number and V is the volume of unit cell. For anatase phase n is four and for rutile phase n is two [15,16]. The specific surface area is calculated by formula [14]:
Sa ¼
Fig. 4. Williamson–Hall plots of TiO2 nanoparticles with different Mn content annealed at 400 °C (a) 0 wt%, (b) 0.02 wt%, (c) 0.04 wt%, (d) 0.06 wt% and (e) 0.08 wt%.
nM NV
6 Dq
where D is the crystallite size and q is the density. The density of the samples vary very closely while the specific surface area increases with increase in Sn and Mn content but it decreases for Mn content 0.04 and 0.06 wt% (Tables 1 and 2). The similar ionic radius of Ti4+ (0.605 Å) [17] and Sn4+ (0.690 Å) [18] allow easy substitution of Ti4+ by Sn4+ in TiO2 lattice. It is observed that the value of FWHM increases with the increase in doping percentage which may leads to the decrease in crystallinity and crystallite size. Nanoparticles with small crystallite size have less thermal stability. In the case of solid particles, the total free energy is determined by volume energy Gv, surface energy Gs, and surface stress induced energy Gf in comparison with bulk materials. The last two terms are very large in nanoparticles so they cannot be ignored. The surface energy is increased by the
Fig. 5. FESEM images of prepared samples with different Sn content annealed at 400 °C (a) 0.02 wt%, (b) 0.04 wt%, (c) 0.06 wt% and (d) 0.08 wt%.
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Fig. 6. FESEM images of prepared samples with different Mn content annealed at 400 °C (a) 0.02 wt%, (b) 0.04 wt%, (c) 0.06 wt% and (d) 0.08 wt%.
Fig. 7. TEM micrographs at the 50 nm scale of TiO2 having Sn content: (a) 0.04 wt%, (c) 0.08 wt%, (b) SAED pattern of Sn content 0.04 wt% and (d) SAED pattern of Sn content 0.08 wt%.
construction of new surface which is due to nanoparticles formation. The Gibbs free energy of phase transformation for anatase nanoparticles is as follows [19]:
DGA!R ¼ DGV;R ðTÞ DGV;A ðTÞ þ 2MfR 3MfA þ qR rR qA rA
3M cR 3McA qR rR qA rA
where c is the surface free energy, r is the particle radius, M is the molecular weight, q is the phase density and f is coefficient of surface tension related to excess pressure with a value of 2fr1. Only a large negative DGA ? R is an adequate driving force for anatase to rutile transformation. The higher surface energy and surface stress energy have more contribution to the non-equilibrium transition from anatase to rutile. Particle with smaller size have large surface areas and higher surface energy. Therefore, in anatase with smaller
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Fig. 8. TEM micrographs at the 50 nm scale of TiO2 having Mn content: (a) 0.04 wt%, (c) 0.08 wt%, (b) SAED pattern of Mn content 0.04 wt% and (d) SAED pattern of Mn content 0.08 wt%.
particle size it is easier to start the phase transformation at lower temperature than in the large particle under similar conditions. The higher negative DGA ? R value corresponds to lower activation energy which is needed for anatase to rutile transition [19]. The Gibbs free energy of rutile (888.8 kJ/mol at 298 K) is less than that of anatase at all temperatures [20] and activation energy of 418– 753 kJ/mol was reported for millimeter-sized anatase single crystal in the temperature range 900–950 °C, while a much lower value of 166 ± 1 kJ/mol was found for nanocrystalline anatase prepared by the sol–gel method in the temperature range 465–525 °C [21]. Therefore the increase in the surface area leads to decrease in the activation energy and the phase transformation takes place at low temperature. The other important factor affecting the phase transformation from anatase to anatase–rutile mix phase is due to tetrahedral distortion in Ti4+ sites and the presence of defects on the oxygen sublattice. The excess of dopant concentration, changes Ti–O and O–Ti–O bond angles, resulting in the anatase phase with the formation of rutile TiO2 [20,22].
decreases and the agglomeration between the particles also reduces. The selected area electron diffraction (SAED) indicates polycrystalline form of samples and show that the crystallinity decreases with increase in dopant concentration. 3.4. TGA curves of samples TGA profiles of Sn and Mn doped TiO2 are presented in Figs. 9 and 10 respectively. The weight loss in the temperature range between 90 °C and 200 °C can be attributed due to the loss of adsorbed water in the titanium oxide [23]. The combustion of residual organic species, including the dehydroxylation of the gel and decomposition of Cl ions are responsible for the weight loss between 300 °C and 550 °C [24]. In the Fig. 9 the weight loss of 6% for Sn proportion 0.02% (curve (a)), loss of 5% for 0.04% (curve (b)), and loss of 4% for 0.06% (curve (c)) have been observed. This
3.2. Field emission scanning electron microscopy The FESEM images for Sn and Mn doped TiO2 nanoparticles calcinated at 400 °C have been presented in Figs. 5 and 6 respectively, which show the smaller particle size at higher doping percentage and less agglomeration has occurred with increase in doping proportion of Sn and Mn. The shape and surface morphology of doped nano-powders have a critical role in the phase transformation from anatase to rutile. 3.3. Transmission electron microscopy (TEM) TEM results for Sn and Mn doped TiO2 nanoparticles are shown in Figs. 7 and 8 respectively. This is clear from the TEM images, as the concentration of dopant increases average particle size
Fig. 9. TGA of samples with different Sn proportions annealed at 400 °C (a) 0.02 wt%, (b) 0.04 wt%, (c) 0.06 wt% and (d) 0.08 wt%.
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Fig. 13. FTIR spectra of samples with different Sn content annealed at 400 °C (a) 0 wt%, (b) 0.02 wt%, (c) 0.04 wt%, (d) 0.06 wt% and (e) 0.08 wt%. Fig. 10. TGA of samples with different Mn proportions annealed at 400 °C (a) 0.02 wt%, (b) 0.04 wt%, (c) 0.06 wt% and (d) 0.08 wt%.
indicates that the increased loss in adsorbed water and residual organic species. 3.5. Raman spectra
Fig. 11. Raman spectra of samples with different Sn proportions (a) 0 wt%, (b) 0.02 wt%, (c) 0.04 wt%, (d) 0.06 wt% and (e) 0.08 wt% annealed at 400 °C. The inset shows the shifting towards lower wave number for highest intense peak in Raman bands.
weight loss decreases with increase in Sn proportion. But the weight loss of 7% has been observed in Sn proportion 0.08% (curve (d)) which indicates the increased loss of adsorbed water and higher residual organic species. In curve (d) there is a weight gain of 0.21% has been observed at temperature 282 °C which may be due to mixing of more number of Sn4+ in TiO2 matrix. In the Fig. 10 the weight loss increases from 5.3% to 7.7% with increase in Mn content from 0.02 to 0.08 wt% has been observed, which
The Raman bands were assigned in Raman spectra of Sn and Mn doped TiO2 nanostructures are presented in Figs. 11 and 12 [25– 27]. The specific vibrational modes are located at 147 cm1 (Eg), 199 cm1 (Eg), 399 cm1 (B1g), 518 cm1 (A1g + B1g) and 643 cm1 (Eg) [28–30], indicating the presence of the anatase phase in the un-doped TiO2 calcinated at 400 °C. The inset in Figs. 11 and 12 represents the exact peak position (in cm1) corresponding to the highest intense peak for the un-doped and doped TiO2 nanoparticles, which shows the corresponding peaks for doped samples shifted upto 140 cm1 from the position of 147 cm1. The main Raman bands at 490, 574, 636, and 776 cm1 due to the crystalline SnO2 were not detected [28], which indicates that the Sn does not exist as a separate crystalline oxide phase. These results are in good agreement with those obtained from the XRD analysis. The Raman spectrum for Mn-doped TiO2 nanoparticles indicates that the peaks of all doped samples shift towards the lower wave number side (red shift) and no indication of the presence of rutile or any secondary phases. All doped samples shows Raman shift from 147 cm1 to 140 cm1 which may be due to incorporation of Sn4+ and Mn2+ to titania matrix. 3.6. Fourier transform infrared spectroscopy FTIR spectra of Sn and Mn doped TiO2 nanostructures are presented in Figs. 13 and 14 respectively. The absorption bands
Fig. 12. Raman spectra of samples with different Mn proportions (a) 0 wt%, (b) 0.02 wt%, (c) 0.04 wt%, (d) 0.06 wt% and (e) 0.08 wt% annealed at 400 °C. The inset shows the shifting towards lower wave number for highest intense peak in Raman bands.
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Fig. 14. FTIR spectra of samples with different Mn content annealed at 400 °C (a) 0 wt%, (b) 0.02 wt%, (c) 0.04 wt%, (d) 0.06 wt% and (e) 0.08 wt%.
Fig. 16. Absorption spectra (i) and Tauc plot (ii) of prepared samples with different Mn content annealed at 400 °C (a) 0 wt%, (b) 0.02 wt%, (c) 0.04 wt%, (d) 0.06 wt% and (e) 0.08 wt%.
Table 3 Absorption edges for Sn doped TiO2 nanoparticles. Sn proportion (wt%)
Wavelength (nm)
Band gap (eV)
0.0 0.02 0.04 0.06 0.08
383 531 540 552 560
3.24 2.34 2.29 2.25 2.21
Table 4 Absorption edges for Mn doped TiO2 nanoparticles.
Fig. 15. Absorption spectrum (i) and Tauc plot (ii) of prepared samples with different Sn content annealed at 400 °C (a) 0 wt%, (b) 0.02 wt%, (c) 0.04 wt%, (d) 0.06 wt% and (e) 0.08 wt%.
between 400 cm1 and 800 cm1 are mainly ascribed to Ti–O and Ti–O–O bonds [10,29–31]. This vibration is dominant for Sn proportion 0.04 and 0.08 wt% respectively and in all the Mn doped TiO2 compare to undoped TiO2. The vibrational band between 1300 cm1 and 4000 cm1 indicates the adsorbed H2O and CO2 molecules on the surface [31]. The broad intense band between 1200 cm1 and 1022 cm1 present in all the samples which indicates the Ti–O–Ti vibrations [32]. The intensity of this band is higher for Sn and Mn doped TiO2 as compare to undoped TiO2 which indicates the more Ti–O–Ti vibration takes place. Here the bending vibration of the adsorbed water molecules are present between 1620 cm1 and 1630 cm1 [32] in all Sn and Mn doped TiO2. 3.7. UV–Vis absorption spectroscopy
Mn proportion (wt%)
Wavelength (nm)
Band gap (eV)
0.0 0.02 0.04 0.06 0.08
383 408 465 483 465
3.24 3.04 2.66 2.56 2.66
red shift takes place and the photo-activity greater than 400 nm exists. Earlier studies have shown that Sn-doping in TiO2 anatase shifts the absorption edge to higher wavelengths [8,33,34] and at higher concentration of Sn it produces new band absorbing roughly between 400 and 500 nm [8,33] probably the consequences of formation of new electron levels of Sn ions in titania band structure. Red shift in wavelength has been observed with the increase in Mn content from 0.02 to 0.06 wt% (Table 4), and the absorption intensity is higher for the sample having Mn content 0.02 wt%. The sample of Mn content 0.08 wt% the absorption edge is found to be 465 nm. The optical band gap of the prepared samples is calculated by Tauc plot. The absorption band gap energy can be determined by the following equation [35,36]. n
ðahmÞ ¼ Bðhm Eg Þ The optical absorption spectra of Sn and Mn doped TiO2 nanostructures are displayed in Figs. 15 and 16 respectively show red shift with increasing Sn and Mn proportions in TiO2 nanoparticles. The un-doped TiO2 has absorption edge at 383 nm (Table 3) which is similar to the anatase TiO2 nanoparticles (386 nm). The maximum shift has been observed for higher Sn percentage (0.08 wt%) at 560 nm. It has been observed that due to doping of Sn in TiO2
where hm is the photon energy, a is the absorption coefficient, B is a constant relative to the material and n is a value that depends on the nature of transition (n = 2 for direct band gap, 2/3 for direct forbidden gap and 1/2 for indirect band gap). The band gap energy for un-doped TiO2 has been observed 3.24 eV which is similar to the band gap of anatase TiO2. It has been observed that on doping the
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Fig. 17. Photoluminescence spectrum of prepared samples with different Sn content (a) 0.02 wt%, (b) 0.04 wt%, (c) 0.06 wt% and (d) 0.08 wt% annealed at 400 °C and inset represents the PL spectrum of pure TiO2.
band gap energy decreases from 3.24 eV (pure TiO2) to 2.21 eV with the increase in Sn content in TiO2 nanoparticles. The lowest band gap energy of 2.21 eV has been observed when Sn proportion is 0.08 wt% is very similar to rutile phase attributes the existence of mixed phase of rutile and anatase TiO2 nanoparticles [32]. The band gap decreases from 3.24 eV to 2.56 eV with increase in Mn content from 0.0 to 0.06 wt% (Table 4). These results indicate that the doping of Mn in TiO2 nanoparticles gives the red shift and decreases the band gap. Reduction in band gap can be attributed to charge transfer from bulk to surface of the nanocrystals and supports the photogeneration after photo-excitation [14]. 3.8. Photoluminescence Figs. 17 and 18 illustrates the photoluminescence features of prepared TiO2 nanoparticle doped with Sn and Mn respectively calcinated at 400 °C and inset figure represents the PL spectra of undoped TiO2 nanoparticles calcinated at the same temperature. The PL spectrum of prepared samples is obtained as a result of the electron–hole separations, electron–phonon scattering and electron–hole recombination. TiO2 has a direct band gap but is subjected to dipole-forbidden transition [37]. PL spectra of anatase TiO2 materials are attributed to three kinds of physical origins: self-trapped excitons [38,39], oxygen vacancies [39,40] and surface states (defects) [41]. Most of the surface states are oxygen vacancies or the Ti4+ ions adjacent to oxygen vacancies [42,43]. PL measurements of pure TiO2 nanoparticles show the clear emission bands (inset Fig. 17). It has been observed that the PL intensity decreases after doping and then increases with increase in Sn content from 0.02 to 0.06 wt%. It has been observed that the PL intensity of doped TiO2 for Sn contents 0.06 and 0.08 wt% are very similar. The results show that crystallite size decreases with increase in Sn content from 0.02 to 0.06 wt%, the size of 0.06 and 0.08 wt% is similar. The blue–green emission of 485 nm (2.55 eV) in undoped TiO2 (inset Fig. 17) and 482 nm in Sn-doped TiO2 (Fig. 17) has been observed, which can be attributed to the charge transfer from Ti3+ to oxygen anion in a [TiO6]8 complex associated with oxygen vacancies on the surface, it indicates that
band is originating from the intrinsic state rather than the surface state. Therefore the 485 nm band can be assigned to self-trapped excitons localized on TiO6 octahedral [39]. The intensity of these bands increases with increase in Sn content which indicates the rapid transfer of charge of Ti3+ to oxygen anion. PL bands at the longer wavelength side of TiO2 nanoparticles have been reported by some workers and attributed to the oxygen vacancies [39,40,44,45]. The blue–green emission of 485 nm (2.55 eV) in Mn-doped TiO2 having Mn content 0.02, 0.08 (Fig. 18) has been observed but in the sample having Mn content 0.04 wt% this emission band has shifted to 438 nm (2.83 eV) with higher PL intensity. In the sample having Mn content 0.06 wt% only one emission band of 436 nm (2.84 eV) has been observed with high PL intensity, this indicates that samples having Mn content 0.04 and 0.06 wt% exhibit quite same luminescence. It is observed that visible emission obtained in PL spectra of all doped samples can be attributed to the oxygen vacancies and Ti vacancies introduced after Sn and Mn doping. 3.9. Photoconductivity measurements 3.9.1. Rise and decay time response photoconductivity measurements The rise and decay time transient photoconductivity response measurements help to study the photoconductivity dynamics of TiO2 nanoparticles during which the light was abruptly switched on and off at room temperature. The time-resolved rise and decay of the photocurrent spectra for the Sn, Mn doped nanoparticles under visible illumination of 370 nm are shown in Figs. 19 and 20. The surface related phenomenon, which is primarily governed by adsorption and desorption processes, plays an important role in the photoconductivity properties of the nanostructures due to large surface-to-volume ratio [46]. The rise and decay of photocurrent spectrum is useful in determining the nature of trap and recombination centers present inside the materials. When the field is applied, the initial dark current is very high in all prepared samples. This may be attributed to the presence of oxygen vacancies on the surface and other native defects acting as donors as well as the process of adsorption of water molecules, thereby releasing charge
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Fig. 18. Photoluminescence spectrum of prepared samples with different Mn content (a) 0.02 wt%, 0.04 wt%, 0.08 wt% and (b) 0.06 wt% annealed at 400 °C.
Fig. 19. Photoconductivity rise and decay time spectra: photoconductive response due to visible light excitation for TiO2 nanoparticles containing Sn weight percentage (a) 0.02, (b) 0.04, (c) 0.06 and (d) 0.08.
carriers. After this process the dark current starts decreasing and attains a minimum value in the prepared samples. When visible illumination is switched on, the photocurrent rises in all the samples, but this rise in photocurrent is higher in undoped TiO2 as compare to Sn and Mn doped TiO2 nanoparticles. Slow
photoconductive rise and decay response may be attributed to a large amount of recombination centers and presence of trap levels and defect states within the band gap [47,48]. When illumination is switched off, electronhole recombination process dominates and the conductivity decreases in the samples.
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Fig. 20. Photoconductivity rise and decay time spectra: photoconductive response due to visible light excitation for TiO2 nanoparticles containing Mn weight percentage (a) 0.02, (b) 0.04, (c) 0.06 and (d) 0.08.
3.9.2. Trap depth determination Trap depth is defined as the required energy to remove an atom from the trap. Trap depths can be calculated from decay curves. The decay of photocurrent can be represented by the equation I = I0exp (pt), according to Bube model [49] where I0 is the current at the time when light is switched off, I is photocurrent at any instant of time for growth and decay function and the p = S exp (E/kT), probability of escape of an electron from trap per second. The trap depth (E) can be calculated by using the following equation:
"
ln Io E ¼ kT ln S ln I t
#
where E denotes trap depth, k is Boltzmann constant, T is the absolute temperature and S is the frequency factor [14] defined as the number per second that the quanta from the lattice vibrations (phonons) attempt to eject the electron from the trap multiplied by the probability of transition of the ejected electron to the conduction band and is of the order of 109 at room temperature. Here the trap depth calculated for undoped TiO2 is 0.67 eV. The trap depth is 0.72 eV, 0.69 eV, 0.67 eV, 0.65 eV obtained for Sn doped TiO2 while 0.75 eV, 0.68 eV, 0.67 eV, 0.66 eV obtained for Mn doped TiO2 containing weight percentage 0.02, 0.04, 0.06, 0.08
respectively. These are much greater than the reported value for anatase TiO2 between 0.10 and 0.27 eV [50–52].
4. Conclusions The XRD result shows that the average crystallite size decreases from 27 nm to 8 nm with increase in Sn and Mn content in TiO2. All the Sn doped TiO2 nanoparticles are spherical in shape and exhibit anatase–rutile mixed phase while the Mn doped TiO2 nanoparticles exhibit anatase phase with tensile strain in the host lattice. The optical band gap decreases from 3.24 eV to 2.21 eV indicates the red shift for Sn and Mn doped anatase and anatase–rutile mixed phase of TiO2 nanoparticles. XRD spectra confirm that Ti atom has been successfully replaced by Sn and Mn atom in all the samples and it is not present on the surface in the metal–oxide form. Thus depending on the nature of chemical dopant and how they are introduced into the titania lattice the adsorption of visible light can be tuned easily. It has been observed that the PL intensity decreases after doping of Sn in TiO2 and then the intensity increases with increase in Sn content. The synthesized TiO2 exhibits all the possible emission bands while Sn doped TiO2 samples having blue–green emission bands.
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