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Effect of zinc doping on the bandgap and photoluminescence of Zn2+-doped TiO2 nanowires
Author’s Accepted Manuscript Effect of zinc doping on the bandgap and photoluminescence of Zn2+-doped TiO2 nanowires Trinh Thi Loan, Vu Hoang Huong, Vu Thi Tham, Nguyen Ngoc Long www.elsevier.com/locate/physb
PII: DOI: Reference:
S0921-4526(17)30252-1 http://dx.doi.org/10.1016/j.physb.2017.05.027 PHYSB309951
To appear in: Physica B: Physics of Condensed Matter Received date: 2 December 2016 Revised date: 3 May 2017 Accepted date: 14 May 2017 Cite this article as: Trinh Thi Loan, Vu Hoang Huong, Vu Thi Tham and Nguyen Ngoc Long, Effect of zinc doping on the bandgap and photoluminescence of Zn2+-doped TiO 2 nanowires, Physica B: Physics of Condensed Matter, http://dx.doi.org/10.1016/j.physb.2017.05.027 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Effect of zinc doping on the bandgap and photoluminescence of Zn2+-doped TiO2 nanowires Trinh Thi Loan1, Vu Hoang Huong, Vu Thi Tham and Nguyen Ngoc Long Faculty of Physics, Hanoi University of Science, Vietnam National University, 334 Nguyen Trai, Thanh Xuan, Hanoi, Vietnam
A R T I C L E
I N F O
A B S T R A C T
This study was focused on the effect of Zn2+ dopant concentration on the absorption edge and photoluminescence of anatase TiO2 nanowires synthesized Keywords:
by hydrothermal technique. For the undoped anatase TiO2 nanowires, the
TiO2:Zn2+ nanowires
indirect band gap of 3.26 eV and the direct band gap of 3.58 eV are assigned to
Optical materials
the indirect
Hydrothermal method
doping makes the absorption edge of TiO2:Zn2+ nanowires shift towards the
Absorption
lower energy side (red shift). On the other hand, the replacing Ti 4+ ions with
Photoluminescence
Zn2+ ions creates oxygen vacancies (VO) and shallow defects associated with
and direct
transitions, respectively. The Zn2+-
VO. Just these defects are responsible for the enhanced luminescence of Zn2+doped TiO2 nanowires.
1. Introduction Titanium dioxide (TiO2) is one of the most promising optical materials, which are widely used in different applications such as photocatalysis, photovoltaics, and photosensors, etc. [1]. In the past decades, TiO2 material with nanostructures, such as nanowires, nanorods, nanotubes, nanoparticles and so on has attracted a great deal of attention due to their novel photoelectrical properties and potential applications in various fields of technology and engineering [1,2]. However, pure TiO2 with the large band gap energy (>3 eV) absorbs only the UV part of solar radiation ( 5 % of the total solar radiation), which leads to some limitations for the practical applications [3,4]. Hence, various works have been devoted to improve the optical response of TiO2 under visible light irradiation by doping TiO2 with metal or non-metal ions such as C [3], Zn [4,5], Cu [6], Co [7], Ni [4,8,9], Cr [10], N [11] and Zn,S
______________
1 Corresponding author: [email protected] (Trinh Thi Loan)
co-doped [5]. It should be noted that the research results on Zn2+-doped TiO2 reported in very recent works of Chitra et al. [4] and Xu et al. [5] are inconsistent with each other. In this work, we synthesized TiO2 nanowires doped with different amounts of Zn2+ ions by hydrothermal technique. Our research was focused on the effect of Zn2+ dopant concentration on the absorption edge and photoluminescence of anatase TiO2 nanowires.
2. Experimental The TiO2 nanowires doped with different amounts of Zn2+ ions (from 0 to 10 mol%) were synthesized by hydrothermal technique using NaOH aqueous solution, anatase TiO2 powders, Zn(NO3)2 aqueous solution and urea powder as the precursors. The typical procedure was as follows: 0.6 g urea was dissolved in 80 ml of 10 M solution of NaOH. Then, 0.8 g of TiO2 was dispersed in the above solution followed by steady stirring for 15 min. Then, an appropriate quantity of 0.02 M solution of Zn(NO3)2 was added to the above mixed solution followed by continuous steady stirring. The solution was transferred into Teflon-lined steel autoclave. The autoclave then was put in a drying cabinet and kept at temperature of 200 oC for 36 h and then cooled naturally to room temperature. Thereafter, the precipitate was filtered and washed with HCl and distilled water, and then was poured back into Teflon-lined steel autoclave with 80 ml of distilled water and kept at temperature of 160 oC for 15 h. Finally, the precipitate was filtered and dried in air at 120 oC for 24 h. The crystalline phase of Zn2+-doped TiO2 was studied by a Siemens D5005 Bruker, Germany X-ray diffractometer (XRD) with Cu-Kα1 irradiation (λ = 1.54056 Å). Raman spectra were measured using LabRam HR800, Horiba spectrometer with 632.8 nm excitation. Nova Nano SEM 450, FEI field emission scanning electron microscope (FESEM) and Tecnai G2 transmission electron microscope (TEM) were used to observe the sample morphologies. Diffuse reflection spectroscopy measurements were carried out on a VARIAN UV-VIS-NIR Cary-5000 spectrophotometer. The Kubelka-Munk (K-M) function F(R) proportional to the absorption coefficient was calculated using the equation: F(R) = (1-R)2/(2R) = K/S, where R, K and S are the reflection, the absorption and the scattering coefficient, respectively. The photoluminescence (PL) spectra were measured at room temperature using a Fluorolog FL3-22 Jobin Yvon Spex, USA spectrofluorometer with a xenon lamp of 450 W being used as an excitation source.
3. Results and discussion 3.1. Structure characterization and morphology
2
X-ray diffraction (XRD) patterns of Zn2+-doped TiO2 nanowires with different doping concentration are shown in Fig. 1.
Fig. 1. XRD patterns of the Zn2+-doped TiO2 samples with different doping concentration. All samples exhibit the similar patterns assigned well to the anatase crystalline phase of TiO2 and there is no other phase in the samples. In each pattern, the nine peaks lying at 2θ angles: 25.3 o, 36.9o, 37.9o, 38.6o, 47.9o, 53.9o, 55.1o, 62.8o, and 68.9o were observed. These peaks correspond to the (101), (103), (004), (112), (200), (105), (211), (204), and (116) diffraction planes of anatase phase with tetragonal geometry, respectively (JCPDS card: 04-0477). Table 1. The lattice parameters of the TiO2:Zn2+samples with different doping concentration. Zn2+ content (mol%)
d101 (Å)
d004 (Å)
d200 (Å)
d204 (Å)
a=b (Å)
c (Å)
0.1
3.5150
2.3748
1.8944
1.4798
3.787 ± 0.017
9.495 ± 0.032
1.6
3.5170
2.3752
1.8921
1.4813
3.785 ± 0.011
9.494 ± 0.063
6.0
3.5150
2.3694
1.8917
1.4805
3.784 ± 0.006
9.495 ± 0.064
10.0
3.5130
2.3749
1.8914
1.4791
3.782 ± 0.008
9.495 ± 0.053
The average lattice parameters of samples calculated from the XRD patterns are shown in Table 1. The lattice parameters within error limits remain unchanged, independent on Zn2+ content. The ionic radius of Zn2+ (0.74 Å) is close to that of host Ti4+ ions (0.605 Å) [12]. Hence Zn2+ ions can easily substitute Ti4+ ions in TiO2 lattice without distorting the pristine crystal structure, thereby stabilizing the anatase phase over a range of dopant concentrations [13]. The average lattice parameters were calculated to be a = 3.785 ± 0.003 Å and c = 9.495 ± 0.001 Å in good agreement with standard values a = 3.783 Å and c = 9.510 Å (JCPDS card: 04-0477).
3
It is well known that Raman scattering is one of the most effective tools to research the crystallinity and defects in the materials. Anatase TiO2 has tetragonal structure, belongs to space group D4h19 (I4/amd), and has two TiO2 units per primitive cell. According to the factor group analysis, six modes of pure anatase TiO2, A1g + 2B1g + 3Eg, are Raman active and three modes, A2u + 2Eu, are infrared active. One vibration, B2u, will be inactive in both infrared and Raman spectra. Thus, group theory predicts six Raman active modes for the tetragonal anatase phase [14]. The Raman active modes of the anatase structure were observed at approximately 144 cm‒ 1 (Eg(1)), 197 cm‒ 1 (Eg(2)), 399 cm‒ 1 (B1g), 519 cm‒ 1 (A1g&B1g) and 639 cm‒ 1 (Eg(3)) [9,14]. In which the Eg modes are the symmetric stretching vibrations of O-Ti-O bonds; the B1g modes are the symmetric bending vibrations of O-Ti-O bonds and the A1g mode is the antisymmetric bending vibrations of O-Ti-O bonds [15-17].
Fig. 2. Raman spectra of the Zn2+-doped TiO2 samples with different doping concentration. In our case, the Raman spectra of Zn2+-doped TiO2 are shown in Fig. 2. The spectra exhibit modes at 142 cm‒ 1 (Eg(1)), 395 cm‒ 1 (B1g), 513 cm‒ 1 (A1g, B1g) and 637 cm‒ 1 (Eg(3)), with the Eg (1) mode is very intense. The scattering peaks of all the samples correspond to the anatase phase of TiO2. No secondary Raman peaks related to Zn or its oxide phases are detected which is well consistent with the XRD. Figs. 3a and 3b show the FESEM and TEM images of TiO2 sample doped with 1.6 mol% Zn2+. It is found that the wires with the diameter in the range of 60 - 80 nm and the length in the range of 10-15 µm are formed from many crystallites. The high resolution TEM (HRTEM) image and its fast Fourier transform pattern of a crystallite are presented in Fig. 3c and 3d. The spacing of the lattice fringes in the HRTEM image is found to be 0.47 nm in good agreement with the reported value of d002 of anatase TiO2 crystal [18].
4
Fig. 3. (a) FESEM, (b) TEM images of as-synthesized anatase TiO2:Zn2+ sample with 1.6 mol% Zn2+, (c) HRTEM image and (d) fast Fourier transform pattern of a TiO2:Zn2+ crystallite. 3.2. Absorption The diffuse reflectance spectra of TiO2:Zn2+ with different doping concentrations are depicted in Fig. 4a. Fig. 4b shows the Kubelka-Munk functions F(R) of the TiO2:Zn2+ samples obtained from the diffuse reflection data. From these figures it is noted that the absorption edge is shifted into the lower energy side (red shift) with increasing Zn2+ dopant content. The band gap energy ]
(
), where
can be determined by using the Tauc-Davis-Mott equation [19,20]: [ ( )
( ) is K-M function proportional to the absorption coefficient,
is the photon
energy, B is a constant and n is the exponential constant index which depends on the nature of transition (n = ½ and 2 for indirect allowed and direct allowed transitions, respectively). Figs. 4c and 4d show the [ ( ) [ ( )
] versus
]
and
for the indirect and direct allowed transitions, respectively. By extrapolating the straight
portion of the graph on hν axis at F(R) = 0 we found the band gaps of the anatase TiO2:Zn2+ nanowires and the results are given in Table 2. For the undoped TiO2 sample, we have obtained the indirect band gap of 3.26 eV and the direct band gap of 3.58 eV, which are in good agreement with the calculated values repored by Daude et al [21] for the
5
indirect
(3.19 eV) and direct
(3.59 eV) transitions, respectively, (Fig. 5). Our values are in good
agreement with the experimental values for TiO2 nanoparticles reported by Valencia et al. [22] as well.
Fig. 4. (a) Diffuse reflectance spectra of TiO2:Zn2+ with different concentrations, (b) Kubelka-Munk functions deduced from diffuse reflectance spectra, (c) plots of [ ( ) photon energy
]
and (d) plots of [ ( )
] versus
.
Table 2. Variations of the band gap energy for Zn2+-doped TiO2 samples versus Zn2+ concentration. Zn2+ content (mol%)
Indirect transitions
Direct transitions
0.0
3.26
3.58
0.1
3.22
3.57
1.6
3.20
3.57
10.0
3.19
3.52
(eV)
The results in Table 2 indicate that the band gap of the TiO2:Zn2+ nanowires is decreased with increasing Zn2+ dopant content. It is well known that the valence band edge of TiO2 material is dominated by O 2p, and the conduction band edge is formed from Ti 3d [23]. When Zn2+ ions were doped into TiO2 host, some of the new occupied molecular orbitals located below the conduction band of TiO2 would be formed. Therefore, the decreased band gap of the 6
TiO2:Zn2+ nanowires should be likely attributed to the charge transfer from the dopant energy level Zn2+ to the conduction band of TiO2 or O 2p to Zn 3d instead of Ti 3d [24]. A similar effect also was observed for different doping transition metal ions on TiO2 host such as Ni [9], Fe [25] and Cu [6]. Beside, the transition metals could also make significant changes on the electronic structure of a crystalline material and thus on the values of the gap energy [6, 10].
Fig. 5. Simplified energy level diagram based on calculations by Daude et al [21], which shows a few of the allowed indirect and direct transitions. 3.3. Photoluminescence The PL emission spectrum is useful to reveal the effciency of charge carrier trapping, immigration, and transfer [26]. In general, the PL spectrum of anatase TiO2 resulted from four origins: Band-to-band [27,28], self trapped excitons [27,29], surface states [27,30] and oxygen vacancies [27]. Fig. 6a shows the room-temperature PL spectrum for the undoped anatase TiO2 nanowires under 300 nm excitation. In the region from 2.4 to 3.6 eV the spectrum clearly exhibits peaks/shoulders at 3.29 eV (376.9 nm) 3.14 eV (394.9 nm), 3.01 eV (411.9 nm), 2.84 eV (436.6 nm), 2.76 eV (449.2 nm), 2.65 eV (467.9 nm), 2.57 eV (482.4 nm) and 2.52 eV (492.0 nm), of which the peak at 3.29 eV is dominant. Basically, the PL spectra of pure anatase TiO2 materials can be divided into three regions. The first region including the emission peaks at 2.52, 2.57 and 2.65 eV is usually attributed to the PL from TiO2 surface states, in which the emission results from the radiative recombination of photogenerated hole with an electron occupying surface defect [27,30,31]. The second region including the emission peaks at 2.76 and 2.84 eV can be attributed to recombination of photoinduced electrons and holes via the oxygen vacancies with two trapped electrons (F-centers) 7
[27]. The PL peaks in the high-energy region 3.01, 3.14 and 3.29 eV can be ascribed to the near band edge emission. Namely, the peaks at 3.01 and 3.14 eV are attributed to the indirect [21]; whereas the peak at 3.29 eV is assigned to the indirect
and
transitions, respectively
transition [21].
Fig. 6. The room temperature photoluminescence spectra of TiO2:Zn2+ nanowires with different concentrations: (a) Undoped TiO2 sample, (b) TiO2 samples doped with 0.1, 0.4, 1.6 and 10 mol% Zn2+ compared with undoped sample. The PL spectra of TiO2 nanowires doped with 0.1, 0.4, 1.6 and 10 mol% Zn2+ are shown in Fig. 6b, in which the PL spectrum of undoped TiO2 sample is also depicted for comparison. From Fig. 6b it is noted that the incorporation of Zn2+ ions into the lattice of TiO2 leads to a strong enhancement in PL intensity compared to pure TiO2. It is clear that compared with undoped TiO2 sample, the TiO2 samples doped with 0.1 mol% Zn2+ displays a new strong peak located at 2.96 eV. Besides, the fluorescence peaks at 2.65, 2.76 and 2.83 eV significantly increase in intensity. Moreover, the intensity of the peaks at 2.65, 2.76, 2.83 and 2.96 eV is dominant in comparison with that of the peak at 3.29 eV (Fig. 6b line b and line c).
Fig. 7. The room temperature photoluminescence spectrum of TiO2:0.1mol%Zn2+ nanowires with Gaussian peak fitting. 8
In order to clear up the origin of the peaks in the range from 2.7 eV to 3.1 eV, the PL spectrum of TiO2:0.1mol%Zn2+ sample was fitted with five Gaussian bands centered at 2.73, 2.75, 2.82, 2.97 and 3.29 V (Fig.7). The band at 3.29 eV is assigned to the indirect
transition [21]. The three bands at 2.73, 2.82 and 2.97 eV are
usually related to the oxygen (O) vacancies (VO) [28], F-centers associated with VO [32,33], Ti3+ [34], the O vacancies associated with Ti3+ [35,36]. The O vacancies are intrinsic defects, which usually appear in the synthesized TiO2 samples. Number of VO can be increased by heat-treating TiO2 samples in hydrogen atmosphere [35]. In our case Ti4+ ions are replaced by Zn2+ ions, the O vacancies are created to maintain the charge imbalance. The formation of the VO can be accompanied by the generation of F-centers and Ti3+ ions [32-34]. Indeed, according to Serpone [32] some shallow traps associated with the O vacancy such as F-, F+-, F2+-centers and Ti3+ ions are formed by the following reactions:
{ (
)
} (
)
Additionally, the O vacancy can be associated with the nearest Ti3+ ion, forming the [VO-Ti3+] defects. Just the mentioned above defects result in the emission bands centered at 2.73, 2.82 and 2.97 eV. As seen from Fig. 6b, with further increasing Zn2+ concentration the intensity of the mentioned above emission bands decreases in comparison with that of the band at 3.29 eV. This is well-known concentration quenching phenomenon. Indeed, when the concentration of an emission center is higher than an appropriate value, the luminescence of the materials is usually lowered. This is because the emission centers are paired or coagulated and are changed to quenching centers [37]. Ultimately, it is worth noting that very recently a number of studies have been focused on Zn 2+-doped TiO2 [4,5]. When studying effect of divalent metal dopant on the structural and optical properties of TiO2:Zn2+ quantum dots Chitra et al. [4] revealed that the doping TiO2 with Zn2+ makes the excitonic absorption peak shift into the higher energy side (blue shift) and leads to strong enhancement in photoluminescence (PL) yield compared to pure TiO2. Whereas while studying the photocatalytic properties of TiO2 nanomaterials doped with Zn2+ and co-doped with
9
Zn2+,S2‒ Xu et al. [5] found that in comparison with pure TiO2, there is a red-shift of absorption edge into the visible light region for the Zn2+-doped TiO2 and these samples exhibit much lower PL intensity than pure TiO2. As mentioned above, our experimental results showed that there is a red-shift of absorption edge into the lower energy side for the Zn2+-doped TiO2 in comparison with pure TiO2. Moreover, the TiO2:Zn2+ samples exhibit much stronger PL intensity than pure TiO2. Thus, our results are different from that reported by both Chitra et al. [4] and Xu et al. [5].
4. Conclusion Undoped and Zn2+-doped TiO2 nanowires were successfully synthesized by hydrothermal method. The XRD and Raman analysis indicated that all the synthesized nanowires exhibit anatase single phase. The synthesized TiO2:Zn2+ nanowires are formed from many crystallites. For the undoped anatase TiO2 nanowires, the indirect band gap of 3.26 eV and the direct band gap of 3.58 eV are attributed to the indirect
and direct
transitions,
respectively. When doping anatase TiO2 nanowires with Zn2+ ions, the absorption edge of TiO2:Zn2+ nanowires shifts towards the lower energy side (red shift). On the other hand, the incorporation of Zn 2+ ions into TiO2 host lattice leads to a strong enhancement in PL intensity in the range of 2.73-2.97 eV. The reason of this is that the replacing Ti4+ ions with Zn2+ ions generates oxygen vacancies and shallow defects associated with VO (F-centers, Ti3+ ions, etc). Just these defects are responsible for the enhanced luminescence of Zn2+-doped TiO2 nanowires.
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PII: DOI: Reference:
S0921-4526(17)30252-1 http://dx.doi.org/10.1016/j.physb.2017.05.027 PHYSB309951
To appear in: Physica B: Physics of Condensed Matter Received date: 2 December 2016 Revised date: 3 May 2017 Accepted date: 14 May 2017 Cite this article as: Trinh Thi Loan, Vu Hoang Huong, Vu Thi Tham and Nguyen Ngoc Long, Effect of zinc doping on the bandgap and photoluminescence of Zn2+-doped TiO 2 nanowires, Physica B: Physics of Condensed Matter, http://dx.doi.org/10.1016/j.physb.2017.05.027 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Effect of zinc doping on the bandgap and photoluminescence of Zn2+-doped TiO2 nanowires Trinh Thi Loan1, Vu Hoang Huong, Vu Thi Tham and Nguyen Ngoc Long Faculty of Physics, Hanoi University of Science, Vietnam National University, 334 Nguyen Trai, Thanh Xuan, Hanoi, Vietnam
A R T I C L E
I N F O
A B S T R A C T
This study was focused on the effect of Zn2+ dopant concentration on the absorption edge and photoluminescence of anatase TiO2 nanowires synthesized Keywords:
by hydrothermal technique. For the undoped anatase TiO2 nanowires, the
TiO2:Zn2+ nanowires
indirect band gap of 3.26 eV and the direct band gap of 3.58 eV are assigned to
Optical materials
the indirect
Hydrothermal method
doping makes the absorption edge of TiO2:Zn2+ nanowires shift towards the
Absorption
lower energy side (red shift). On the other hand, the replacing Ti 4+ ions with
Photoluminescence
Zn2+ ions creates oxygen vacancies (VO) and shallow defects associated with
and direct
transitions, respectively. The Zn2+-
VO. Just these defects are responsible for the enhanced luminescence of Zn2+doped TiO2 nanowires.
1. Introduction Titanium dioxide (TiO2) is one of the most promising optical materials, which are widely used in different applications such as photocatalysis, photovoltaics, and photosensors, etc. [1]. In the past decades, TiO2 material with nanostructures, such as nanowires, nanorods, nanotubes, nanoparticles and so on has attracted a great deal of attention due to their novel photoelectrical properties and potential applications in various fields of technology and engineering [1,2]. However, pure TiO2 with the large band gap energy (>3 eV) absorbs only the UV part of solar radiation ( 5 % of the total solar radiation), which leads to some limitations for the practical applications [3,4]. Hence, various works have been devoted to improve the optical response of TiO2 under visible light irradiation by doping TiO2 with metal or non-metal ions such as C [3], Zn [4,5], Cu [6], Co [7], Ni [4,8,9], Cr [10], N [11] and Zn,S
______________
1 Corresponding author: [email protected] (Trinh Thi Loan)
co-doped [5]. It should be noted that the research results on Zn2+-doped TiO2 reported in very recent works of Chitra et al. [4] and Xu et al. [5] are inconsistent with each other. In this work, we synthesized TiO2 nanowires doped with different amounts of Zn2+ ions by hydrothermal technique. Our research was focused on the effect of Zn2+ dopant concentration on the absorption edge and photoluminescence of anatase TiO2 nanowires.
2. Experimental The TiO2 nanowires doped with different amounts of Zn2+ ions (from 0 to 10 mol%) were synthesized by hydrothermal technique using NaOH aqueous solution, anatase TiO2 powders, Zn(NO3)2 aqueous solution and urea powder as the precursors. The typical procedure was as follows: 0.6 g urea was dissolved in 80 ml of 10 M solution of NaOH. Then, 0.8 g of TiO2 was dispersed in the above solution followed by steady stirring for 15 min. Then, an appropriate quantity of 0.02 M solution of Zn(NO3)2 was added to the above mixed solution followed by continuous steady stirring. The solution was transferred into Teflon-lined steel autoclave. The autoclave then was put in a drying cabinet and kept at temperature of 200 oC for 36 h and then cooled naturally to room temperature. Thereafter, the precipitate was filtered and washed with HCl and distilled water, and then was poured back into Teflon-lined steel autoclave with 80 ml of distilled water and kept at temperature of 160 oC for 15 h. Finally, the precipitate was filtered and dried in air at 120 oC for 24 h. The crystalline phase of Zn2+-doped TiO2 was studied by a Siemens D5005 Bruker, Germany X-ray diffractometer (XRD) with Cu-Kα1 irradiation (λ = 1.54056 Å). Raman spectra were measured using LabRam HR800, Horiba spectrometer with 632.8 nm excitation. Nova Nano SEM 450, FEI field emission scanning electron microscope (FESEM) and Tecnai G2 transmission electron microscope (TEM) were used to observe the sample morphologies. Diffuse reflection spectroscopy measurements were carried out on a VARIAN UV-VIS-NIR Cary-5000 spectrophotometer. The Kubelka-Munk (K-M) function F(R) proportional to the absorption coefficient was calculated using the equation: F(R) = (1-R)2/(2R) = K/S, where R, K and S are the reflection, the absorption and the scattering coefficient, respectively. The photoluminescence (PL) spectra were measured at room temperature using a Fluorolog FL3-22 Jobin Yvon Spex, USA spectrofluorometer with a xenon lamp of 450 W being used as an excitation source.
3. Results and discussion 3.1. Structure characterization and morphology
2
X-ray diffraction (XRD) patterns of Zn2+-doped TiO2 nanowires with different doping concentration are shown in Fig. 1.
Fig. 1. XRD patterns of the Zn2+-doped TiO2 samples with different doping concentration. All samples exhibit the similar patterns assigned well to the anatase crystalline phase of TiO2 and there is no other phase in the samples. In each pattern, the nine peaks lying at 2θ angles: 25.3 o, 36.9o, 37.9o, 38.6o, 47.9o, 53.9o, 55.1o, 62.8o, and 68.9o were observed. These peaks correspond to the (101), (103), (004), (112), (200), (105), (211), (204), and (116) diffraction planes of anatase phase with tetragonal geometry, respectively (JCPDS card: 04-0477). Table 1. The lattice parameters of the TiO2:Zn2+samples with different doping concentration. Zn2+ content (mol%)
d101 (Å)
d004 (Å)
d200 (Å)
d204 (Å)
a=b (Å)
c (Å)
0.1
3.5150
2.3748
1.8944
1.4798
3.787 ± 0.017
9.495 ± 0.032
1.6
3.5170
2.3752
1.8921
1.4813
3.785 ± 0.011
9.494 ± 0.063
6.0
3.5150
2.3694
1.8917
1.4805
3.784 ± 0.006
9.495 ± 0.064
10.0
3.5130
2.3749
1.8914
1.4791
3.782 ± 0.008
9.495 ± 0.053
The average lattice parameters of samples calculated from the XRD patterns are shown in Table 1. The lattice parameters within error limits remain unchanged, independent on Zn2+ content. The ionic radius of Zn2+ (0.74 Å) is close to that of host Ti4+ ions (0.605 Å) [12]. Hence Zn2+ ions can easily substitute Ti4+ ions in TiO2 lattice without distorting the pristine crystal structure, thereby stabilizing the anatase phase over a range of dopant concentrations [13]. The average lattice parameters were calculated to be a = 3.785 ± 0.003 Å and c = 9.495 ± 0.001 Å in good agreement with standard values a = 3.783 Å and c = 9.510 Å (JCPDS card: 04-0477).
3
It is well known that Raman scattering is one of the most effective tools to research the crystallinity and defects in the materials. Anatase TiO2 has tetragonal structure, belongs to space group D4h19 (I4/amd), and has two TiO2 units per primitive cell. According to the factor group analysis, six modes of pure anatase TiO2, A1g + 2B1g + 3Eg, are Raman active and three modes, A2u + 2Eu, are infrared active. One vibration, B2u, will be inactive in both infrared and Raman spectra. Thus, group theory predicts six Raman active modes for the tetragonal anatase phase [14]. The Raman active modes of the anatase structure were observed at approximately 144 cm‒ 1 (Eg(1)), 197 cm‒ 1 (Eg(2)), 399 cm‒ 1 (B1g), 519 cm‒ 1 (A1g&B1g) and 639 cm‒ 1 (Eg(3)) [9,14]. In which the Eg modes are the symmetric stretching vibrations of O-Ti-O bonds; the B1g modes are the symmetric bending vibrations of O-Ti-O bonds and the A1g mode is the antisymmetric bending vibrations of O-Ti-O bonds [15-17].
Fig. 2. Raman spectra of the Zn2+-doped TiO2 samples with different doping concentration. In our case, the Raman spectra of Zn2+-doped TiO2 are shown in Fig. 2. The spectra exhibit modes at 142 cm‒ 1 (Eg(1)), 395 cm‒ 1 (B1g), 513 cm‒ 1 (A1g, B1g) and 637 cm‒ 1 (Eg(3)), with the Eg (1) mode is very intense. The scattering peaks of all the samples correspond to the anatase phase of TiO2. No secondary Raman peaks related to Zn or its oxide phases are detected which is well consistent with the XRD. Figs. 3a and 3b show the FESEM and TEM images of TiO2 sample doped with 1.6 mol% Zn2+. It is found that the wires with the diameter in the range of 60 - 80 nm and the length in the range of 10-15 µm are formed from many crystallites. The high resolution TEM (HRTEM) image and its fast Fourier transform pattern of a crystallite are presented in Fig. 3c and 3d. The spacing of the lattice fringes in the HRTEM image is found to be 0.47 nm in good agreement with the reported value of d002 of anatase TiO2 crystal [18].
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Fig. 3. (a) FESEM, (b) TEM images of as-synthesized anatase TiO2:Zn2+ sample with 1.6 mol% Zn2+, (c) HRTEM image and (d) fast Fourier transform pattern of a TiO2:Zn2+ crystallite. 3.2. Absorption The diffuse reflectance spectra of TiO2:Zn2+ with different doping concentrations are depicted in Fig. 4a. Fig. 4b shows the Kubelka-Munk functions F(R) of the TiO2:Zn2+ samples obtained from the diffuse reflection data. From these figures it is noted that the absorption edge is shifted into the lower energy side (red shift) with increasing Zn2+ dopant content. The band gap energy ]
(
), where
can be determined by using the Tauc-Davis-Mott equation [19,20]: [ ( )
( ) is K-M function proportional to the absorption coefficient,
is the photon
energy, B is a constant and n is the exponential constant index which depends on the nature of transition (n = ½ and 2 for indirect allowed and direct allowed transitions, respectively). Figs. 4c and 4d show the [ ( ) [ ( )
] versus
]
and
for the indirect and direct allowed transitions, respectively. By extrapolating the straight
portion of the graph on hν axis at F(R) = 0 we found the band gaps of the anatase TiO2:Zn2+ nanowires and the results are given in Table 2. For the undoped TiO2 sample, we have obtained the indirect band gap of 3.26 eV and the direct band gap of 3.58 eV, which are in good agreement with the calculated values repored by Daude et al [21] for the
5
indirect
(3.19 eV) and direct
(3.59 eV) transitions, respectively, (Fig. 5). Our values are in good
agreement with the experimental values for TiO2 nanoparticles reported by Valencia et al. [22] as well.
Fig. 4. (a) Diffuse reflectance spectra of TiO2:Zn2+ with different concentrations, (b) Kubelka-Munk functions deduced from diffuse reflectance spectra, (c) plots of [ ( ) photon energy
]
and (d) plots of [ ( )
] versus
.
Table 2. Variations of the band gap energy for Zn2+-doped TiO2 samples versus Zn2+ concentration. Zn2+ content (mol%)
Indirect transitions
Direct transitions
0.0
3.26
3.58
0.1
3.22
3.57
1.6
3.20
3.57
10.0
3.19
3.52
(eV)
The results in Table 2 indicate that the band gap of the TiO2:Zn2+ nanowires is decreased with increasing Zn2+ dopant content. It is well known that the valence band edge of TiO2 material is dominated by O 2p, and the conduction band edge is formed from Ti 3d [23]. When Zn2+ ions were doped into TiO2 host, some of the new occupied molecular orbitals located below the conduction band of TiO2 would be formed. Therefore, the decreased band gap of the 6
TiO2:Zn2+ nanowires should be likely attributed to the charge transfer from the dopant energy level Zn2+ to the conduction band of TiO2 or O 2p to Zn 3d instead of Ti 3d [24]. A similar effect also was observed for different doping transition metal ions on TiO2 host such as Ni [9], Fe [25] and Cu [6]. Beside, the transition metals could also make significant changes on the electronic structure of a crystalline material and thus on the values of the gap energy [6, 10].
Fig. 5. Simplified energy level diagram based on calculations by Daude et al [21], which shows a few of the allowed indirect and direct transitions. 3.3. Photoluminescence The PL emission spectrum is useful to reveal the effciency of charge carrier trapping, immigration, and transfer [26]. In general, the PL spectrum of anatase TiO2 resulted from four origins: Band-to-band [27,28], self trapped excitons [27,29], surface states [27,30] and oxygen vacancies [27]. Fig. 6a shows the room-temperature PL spectrum for the undoped anatase TiO2 nanowires under 300 nm excitation. In the region from 2.4 to 3.6 eV the spectrum clearly exhibits peaks/shoulders at 3.29 eV (376.9 nm) 3.14 eV (394.9 nm), 3.01 eV (411.9 nm), 2.84 eV (436.6 nm), 2.76 eV (449.2 nm), 2.65 eV (467.9 nm), 2.57 eV (482.4 nm) and 2.52 eV (492.0 nm), of which the peak at 3.29 eV is dominant. Basically, the PL spectra of pure anatase TiO2 materials can be divided into three regions. The first region including the emission peaks at 2.52, 2.57 and 2.65 eV is usually attributed to the PL from TiO2 surface states, in which the emission results from the radiative recombination of photogenerated hole with an electron occupying surface defect [27,30,31]. The second region including the emission peaks at 2.76 and 2.84 eV can be attributed to recombination of photoinduced electrons and holes via the oxygen vacancies with two trapped electrons (F-centers) 7
[27]. The PL peaks in the high-energy region 3.01, 3.14 and 3.29 eV can be ascribed to the near band edge emission. Namely, the peaks at 3.01 and 3.14 eV are attributed to the indirect [21]; whereas the peak at 3.29 eV is assigned to the indirect
and
transitions, respectively
transition [21].
Fig. 6. The room temperature photoluminescence spectra of TiO2:Zn2+ nanowires with different concentrations: (a) Undoped TiO2 sample, (b) TiO2 samples doped with 0.1, 0.4, 1.6 and 10 mol% Zn2+ compared with undoped sample. The PL spectra of TiO2 nanowires doped with 0.1, 0.4, 1.6 and 10 mol% Zn2+ are shown in Fig. 6b, in which the PL spectrum of undoped TiO2 sample is also depicted for comparison. From Fig. 6b it is noted that the incorporation of Zn2+ ions into the lattice of TiO2 leads to a strong enhancement in PL intensity compared to pure TiO2. It is clear that compared with undoped TiO2 sample, the TiO2 samples doped with 0.1 mol% Zn2+ displays a new strong peak located at 2.96 eV. Besides, the fluorescence peaks at 2.65, 2.76 and 2.83 eV significantly increase in intensity. Moreover, the intensity of the peaks at 2.65, 2.76, 2.83 and 2.96 eV is dominant in comparison with that of the peak at 3.29 eV (Fig. 6b line b and line c).
Fig. 7. The room temperature photoluminescence spectrum of TiO2:0.1mol%Zn2+ nanowires with Gaussian peak fitting. 8
In order to clear up the origin of the peaks in the range from 2.7 eV to 3.1 eV, the PL spectrum of TiO2:0.1mol%Zn2+ sample was fitted with five Gaussian bands centered at 2.73, 2.75, 2.82, 2.97 and 3.29 V (Fig.7). The band at 3.29 eV is assigned to the indirect
transition [21]. The three bands at 2.73, 2.82 and 2.97 eV are
usually related to the oxygen (O) vacancies (VO) [28], F-centers associated with VO [32,33], Ti3+ [34], the O vacancies associated with Ti3+ [35,36]. The O vacancies are intrinsic defects, which usually appear in the synthesized TiO2 samples. Number of VO can be increased by heat-treating TiO2 samples in hydrogen atmosphere [35]. In our case Ti4+ ions are replaced by Zn2+ ions, the O vacancies are created to maintain the charge imbalance. The formation of the VO can be accompanied by the generation of F-centers and Ti3+ ions [32-34]. Indeed, according to Serpone [32] some shallow traps associated with the O vacancy such as F-, F+-, F2+-centers and Ti3+ ions are formed by the following reactions:
{ (
)
} (
)
Additionally, the O vacancy can be associated with the nearest Ti3+ ion, forming the [VO-Ti3+] defects. Just the mentioned above defects result in the emission bands centered at 2.73, 2.82 and 2.97 eV. As seen from Fig. 6b, with further increasing Zn2+ concentration the intensity of the mentioned above emission bands decreases in comparison with that of the band at 3.29 eV. This is well-known concentration quenching phenomenon. Indeed, when the concentration of an emission center is higher than an appropriate value, the luminescence of the materials is usually lowered. This is because the emission centers are paired or coagulated and are changed to quenching centers [37]. Ultimately, it is worth noting that very recently a number of studies have been focused on Zn 2+-doped TiO2 [4,5]. When studying effect of divalent metal dopant on the structural and optical properties of TiO2:Zn2+ quantum dots Chitra et al. [4] revealed that the doping TiO2 with Zn2+ makes the excitonic absorption peak shift into the higher energy side (blue shift) and leads to strong enhancement in photoluminescence (PL) yield compared to pure TiO2. Whereas while studying the photocatalytic properties of TiO2 nanomaterials doped with Zn2+ and co-doped with
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Zn2+,S2‒ Xu et al. [5] found that in comparison with pure TiO2, there is a red-shift of absorption edge into the visible light region for the Zn2+-doped TiO2 and these samples exhibit much lower PL intensity than pure TiO2. As mentioned above, our experimental results showed that there is a red-shift of absorption edge into the lower energy side for the Zn2+-doped TiO2 in comparison with pure TiO2. Moreover, the TiO2:Zn2+ samples exhibit much stronger PL intensity than pure TiO2. Thus, our results are different from that reported by both Chitra et al. [4] and Xu et al. [5].
4. Conclusion Undoped and Zn2+-doped TiO2 nanowires were successfully synthesized by hydrothermal method. The XRD and Raman analysis indicated that all the synthesized nanowires exhibit anatase single phase. The synthesized TiO2:Zn2+ nanowires are formed from many crystallites. For the undoped anatase TiO2 nanowires, the indirect band gap of 3.26 eV and the direct band gap of 3.58 eV are attributed to the indirect
and direct
transitions,
respectively. When doping anatase TiO2 nanowires with Zn2+ ions, the absorption edge of TiO2:Zn2+ nanowires shifts towards the lower energy side (red shift). On the other hand, the incorporation of Zn 2+ ions into TiO2 host lattice leads to a strong enhancement in PL intensity in the range of 2.73-2.97 eV. The reason of this is that the replacing Ti4+ ions with Zn2+ ions generates oxygen vacancies and shallow defects associated with VO (F-centers, Ti3+ ions, etc). Just these defects are responsible for the enhanced luminescence of Zn2+-doped TiO2 nanowires.
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