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Highly stable colloidal TiO2 nanocrystals with strong violet-blue emission
Author’s Accepted Manuscript Highly stable colloidal TiO 2 nanocrystals with strong violet-blue emission Morteza Sasani Ghamsari, Mohammad Reza Gaeeni, Wooje Han, Hyung-Ho Park www.elsevier.com/locate/jlumin
PII: DOI: Reference:
S0022-2313(15)30210-6 http://dx.doi.org/10.1016/j.jlumin.2016.05.036 LUMIN14007
To appear in: Journal of Luminescence Received date: 7 July 2015 Accepted date: 19 May 2016 Cite this article as: Morteza Sasani Ghamsari, Mohammad Reza Gaeeni, Wooje Han and Hyung-Ho Park, Highly stable colloidal TiO 2 nanocrystals with strong violet-blue emission, Journal of Luminescence, http://dx.doi.org/10.1016/j.jlumin.2016.05.036 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.
Highly stable colloidal TiO2 nanocrystals with strong violet-blue emission Morteza Sasani Ghamsaria,*, Mohammad Reza Gaeenia, Wooje Hanb, Hyung-Ho Parkb a
b
Laser & Optics Research School, NSTRI, 11155-3486 Tehran, Iran Department of Materials Science and Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 120-749, Korea
Abstract: Improved sol-gel method has been applied to prepare highly stable colloidal TiO2 nanocrystals. The synthesized titania nanocrystals exhibit strong emission in the violet-blue wavelength region. Very long evolution time was obtained by preventing the sol to gel conversion with reflux process. FTIR, XRD, UV-Vis absorption, photoluminescence and high resolution transmission electron microscope (HRTEM) were used to study the optical properties, crystalline phase, morphology, shape and size of prepared TiO2 colloidal nanocrystals. HR-TEM showed that the diameter of TiO2 colloidal nanocrystals is about 5 nm. Although the PL spectra show similar spectral features upon excitation wavelengths at 280, 300 and 350 nm, but their emission intensities are significantly different from each other. Photoluminescence quantum yield for TiO2 colloidal nanocrystals is estimated to be 49% with 280 nm excitation wavelength which is in agreement and better than reported before. Obtained results confirm that the prepared colloidal TiO2 sample has enough potential for optoelectronics applications.
Keywords: Sol-gel preparation; Nanocrystalline materials; Colloidal TiO2 nanoparticles; Violet-Blue emission. *Corresponding author: [email protected] [email protected]
1. Introduction 1
During the past two decades, light emitting materials have been one of the exhaustive research objects. to develop a synthesis method for producing high efficient, bright and stable light emitting materials with improved their optical properties was extensively attracted much attention [1,2]. From all emission bands in visible wavelength region, violet-blue emission line has especial position in generation of white light under ultra-violet or blue light excitation [3]. On the other hand, the biocompatible and stable violet-blue emission source is very important for in vivo cancer imaging. Consequently, the colloidal semiconductor nanocrystals (NCs) with considerable optical properties including high efficiency, narrow emission band, enabling spectral purity and fine tunability are gaining prominence in recent years [4,5]. As the most promising semiconducting materials, titanium dioxide is biocompatible and as a key material for optoelectronic applications has been widely investigated. It is currently employed to make photovoltaic cells, batteries, chemical sensing, optical emissions, photonic crystals, optical waveguide, photo catalysis and nanomedicine[6-8]. The colloidal form of TiO2 nanocrystals has plentiful practical applications as self-cleaning glass, heat mirror, optical waveguide, and solar cell [9-12]. From technological point of view, the synthesis of highly stable colloidal TiO2 quantum dots (QDs) with high crystallinity is very important [13-17]. To prepare a TiO2 sol with strong violet-blue emission, the crystallinity of TiO2 QDs in the sol plays a critical role [18]. Despite several approaches for synthesis of highly crystalline TiO2 sol, controlling the crystallinity and physical properties of the sol-gel derived TiO2 nanocrystals is not easily [19-27]. It means that more research works must be done to establish a preparation method of highly crystalline TiO2 sol with wide potential in different applications [28]. In our last papers we have tried to improve a procedure for preparation of highly crystalline TiO2 nanocrystals. Some details can be found in our last papers [29.30]. Recently, room temperature synthesized TiO2 nanocrystal was also reported by our
2
group [31]. In this approach, we have successfully synthesized highly stable colloidal TiO2 nanocrystals with a strong violet-blue emission that has not been reported before. 2. Experimental Materials and Methods The analytical grade of Titanium (IV) Isopropoxide (TTIP, 97% Aldrich), 2-propanol (CH3CH(OH)CH3, 99.5% Merck), and distilled water, were used as precursor materials. In this work, hydrolysis and precursor solutions were prepared separately to control the sol-gel process as well as colloidal particle size. At first, 2-propanol and titanium (IV) isopropoxide was mixed and used as precursor solution. A mixed solution of 2-propanol and water (hydrolysis solution) was prepared and heated at temperatures 60–80°C under vigorous stirring. Then the precursor solution was dropwisely added to the hydrolysis solution. The resultant mixture was kept standing for 2 h (Refluxing process) and a transparent (Bluish) TiO2 sol was finally obtained. Fourier Transform Infrared spectroscopy (Bruker Vector 22 FT-IR) was used for detection of functional groups within the compounds. The photoabsorption characteristic of prepared sol was studied by photospectrometer (Perkin Elmer). Photoluminescence spectroscopy was carried out to identify the emission spectrum and quantum yield of TiO2 colloidal nanocrystals using GILDEN PHOTONICS FLUOROSENS spectrometer. In order to study the crystalline phase of sol, it was dried and characterized by XRD (Bruker AXS-D8). Particle size of the colloidal sol was characterized by using high resolution transmission electron microscopy (JEM-2010F, JEOL). 3. Results and Discussion Fourier transform infrared (FTIR) was carried out to detect the presence of functional groups in the compound of synthesized colloidal TiO2 sol in the range 400–4000 cm-1 wavenumbers. The FTIR spectrum has been shown in Figure 1. This spectrum is to identify the chemical bonds, as well as functional groups, in the compound. The source of these bands was individually identified in our last
3
paper[11]. The absorption band at 1037 cm−1 is due to the stretching vibration of the O–C–C bands of the TTIP isopropyl groups [11]. Sharp absorption band at 1651 cm−1 shows the nitrate group and is a result of nitric acid addition. The OH stretching frequencies of alcohols lead to large absorption peak in the region 3200–3600 cm−1. The deformation vibration bands of the C–H, C–O and Ti–O can be observed in the 900–400 cm−1 region. The low energy region (below 1000 cm-1) indicated the bands due to stretching modes of Ti-O and Ti-O-Ti bonds of a titanium dioxide network. Absorption band at 603 cm−1 is due to the Ti–O band. The particle size and crystallinity of colloidal sample were confirmed by further characterizations. The formation of all TiO2 nanoparticles and their size were identified by the high magnification HR-TEM image. The high resolution images of colloidal TiO2 sample and their lattice image were gotten in figure 2a, b. The obtained results confirm that the prepared sample has a narrow size distribution and their size to be equal to 5nm. The UV-Vis absorption spectrum of the fresh colloidal sample was shown in figure 3a. For fresh prepared colloidal TiO2 nanocrystals, the strong absorption at about 340 nm wavelength is attributed to the band-to-band transition. From figure 3a it can be found that the onset point of absorption peak is 370 nm. Therefore, the band gap of TiO2 colloidal nanoparticles can be determined by: Egnano ≈1240/λonset. It is about 3.36 eV which is 0.26 eV higher than that of bulk anatase TiO2 (~3.1eV). This blue shift of the absorption edge is attributed to the quantum confinement effect. Size quantization effect has been observed in TiO2 nanocrystals [32-40]. Anpo et al [33] studied the change of TiO2band gap over a wide range of particle sizes (e.g., 3.8–200 nm). When particle size was less than 12 nm, the significant blue shift of the absorption edge by 0.093 and 0.156 eV for rutile and anatase crystalline was respectively found by them. The quantum confinement effect of illuminated TiO2 colloids with particle size less than 3 nm was observed by Kormann et al [34]. They reported a blue-shift by 0.150.17eV in absorption spectrum. However, Monticone et al [39], examined the quantum size effect in 4
anatase TiO2 nanoparticles and the band gap energy shift has not been observed for sizes 2R1.5 nm. Their results indicate the considerable effect of the size on the electronic band structure, which disables the EMA in TiO2 crystallites. Recently, Satoh et al [40], observed the quantum size effects in the TiO2 nanoparticles. They reported a blue shift corresponded to particle size reduction for band gap energy of TiO2 nanoparticles. Therefore, it can easy conclude that earlier reports of blue-shifted absorption thresholds, taken as evidence for Q-size effects in very small TiO2 particles, are not in fact direct (Franck-Condon type) transitions in an otherwise indirect band gap semiconductor. In order to evaluate the experimentally obtained optical band gap energy of TiO2 colloidal nanocrystals, band-toband transitional relation was employed. The optical band gap energy (Eg) of the nanocrystalline titanium dioxide sol was also determined by extrapolating a straight line to [αhν]n. In the high absorption region, the energy of incident photons, hν, is related to absorption coefficient, α(λ), by band-to-band transition relation [41]: αhν = β(hν - E gnano )n
(1)
here 𝛽 is a constant nearly equal to one at absorption edge, n is an index that characterizes the optical absorption process being theoretically equal to 0.5, 1.5, 2 or 3 for direct allowed, direct forbidden, indirect allowed, and indirect forbidden transitions, respectively. From the different Eg obtained values, Madhusudan Reddy [42], showed that the estimated shift of band gap for the synthesized TiO2 nanoparticles is too large if they considered as indirect transition material and therefore it could be inferred that the direct transition material are more appropriate. This could be one reason to suggest that the direct, and not indirect transition, is more favorable in anatase TiO2 nanoparticles. Figure 3b illustrates (αhν)2 versus energy of the photon for TiO2 colloidal nanocrystals. The obtained value for direct allowed band gap energy ( E gnano ) is about 3.44 eV. In order to study the optical properties resulted from the photogenerated electron/ hole pairs in colloidal TiO2 nanocrystals the 5
photoluminescence (PL) spectroscopy was used. Usually, the recombination of electron/hole pair is contributing in the origination of semiconductor nanoparticles photoluminescence. Figure 4a shows the PL spectra of fresh and 1 year aged colloidal TiO2 nanocrystals, which were taken at 280 nm excitation wavelengths. The spectra upon excitation wavelengths at 280, show similar spectral features but their emission intensities are a little different from each other (Figure 4a). The emission spectra of fresh sample excited at 300 and 350 nm show similar spectral features but their emission intensities are significantly different from each other. Also, aged sample exhibited similar behavior. The variation of PL spectra peak intensity of aged and fresh samples vs excitation wavelength is shown in figure 4b. It is seen that titanium oxide colloidal nanocrystals showed a strong PL band peaking at 420 nm and a broad one ranging from 400 to 500 nm. There are two different theories about photoluminescence source in TiO2 nanocrystals. In the first hypothesis, the surface defects are sources of TiO2 nanocrystals photoluminescence behavior. For example, Xu et al. described that the light radiation in the UV region is due to the radiative annihilation of excitons (band-to-band recombination) [43]. While the visible light emission is attributed to the electron transition mediated by defect levels such as oxygen vacancies in the band gap of TiO2 nanopowder [44]. Also, Mathew et al. believed that the photoluminescence of TiO2 nanocrystals is mostly a surface phenomenon and a change in the surface environment would have a significant effect on the photoluminescence process. They concluded that 420 nm, 491 nm, 530 nm emissions are assigned to the surface state emissions and are due to the recombination of trapped electron–hole arising from dangling bonds in the surface of TiO2 nanoparticles [38]. On the basis of the difference between emission peaks, it has been concluded that the visible emissions is from the deexcitation of lower vibronic levels in Ti3+ 3d states of TiO2 lattice to the deep trap levels (acceptor) created by (OH-) [38]. They have also explained that the emission wavelengths 533 nm and 612 nm, which are also separated by nearly 80 nm, are due to deexcitation
6
from lower vibronic levels in deexcitation from lower vibronic levels in the oxygen vacancies of TiO2 lattice to the ground state. In addition, Knorr et al. explained that the red PL emission can be originated from the electron traps. While hole traps result the observed green luminescence for all oxide nanoparticles. It was concluded that the surface oxygen vacancies are playing the role of hole trap states [45]. In all mentioned conclusions, the surface state was considered as source of visible emission. If we accept that all visible emission peaks are due to the surface state, a red-shifted emission must be observed when different excitation wavelength higher than band edge wavelength to be used. These emission peaks shifted to the larger wavelengths are distinguishing from the band edge emission attributed to the surface state emissions [46]. Figure 3a, confirms that the emission spectrum of sample excited by different wavelengths did not show any shift. In the second theorem, the intrinsic unit cell defects are sources of TiO2 nanocrystals photoluminescence behavior [47,48]. It means that the observed visible emission is probably generated from self-trapped excitons (electron–hole pairs) localized on the TiO6 octahedron and/or transition from charge-transfer excited states (Ti3+-O−) in trap sites [48]. Consequently, observed strong blue emission centered at 420 nm of prepared sample is caused from intrinsic unit cell defects. On the other hand, red PL emission was not observed in the luminescence spectra and a weak green luminescence at 530 nm was recorded. According to the standard method the photoluminescence quantum yield (QY) of TiO2 colloidal nanocrystals can be determined by [49]: QY QYr
P ODr n 2 Pr OD nr2
(2),
here, P is the photoluminescence integrated intensity, OD is the optical density at the excitation wavelength, and n is the linear refractive index of TiO2 QDs. The subscript “r” refers to the Fluorescein with known quantum yield. The linear refractive indexes (n) of TiO2 QDs were measured by equation (3): 7
n 1
bulk 1
(3)
1.2
3 1 8R
where R refers to the radius of TiO2 QDs in nm and ε is the relative permittivity of TiO2 [50]. The gotten result for linear refractive index of TiO2 with R=2.3nm was 1.92. The photoluminescence quantum yield of TiO2 colloidal nanocrystals was compared to Fluorescein as a reference emission material with a quantum yield of 95%. Optical Density (OD) of TiO2 nanocrystals can be calculated using Beer-Lambert Law: I A log 0 I
OD
(4),
here I is the light intensity at the center of the cuvette, I0 is the intensity of the incident light to the cuvette and A is the absorbance. Photoluminescence quantum yield for TiO2 colloidal nanocrystals is estimated to be 49% with 280 nm excitation wavelength which is in agreement and better than reports in refs 51 and 52. A PL peak with full width at half maximum (FWHM) of around 65 nm is obtained, which infers a homogeneous and narrow size distribution of TiO2 colloidal nanocrystals. An example of the photoluminescence decay profile for colloidal TiO2 is shown in Fig. 5. This decay profile was well traced using the extended exponential function: t I I 0 exp
(5)
where τ is the lifetime. The decay profile of the emission from colloidal TiO2 nanocrystyals, was also well traced using this equation. As it has been confirmed by Fujihara, et al [53], the decay profiles cannot be fitted by the second order kinetics, which has frequently been examined for analysis of the charge carrier dynamics of TiO2 particles. The origin of the nonexponential decay has also been attributed to a distribution of recombination rates [54]. Good fitting of the decay profiles of the luminescence of TiO2 particles by Eq. (5) suggests the existence of carrier trapping sites with different 8
energy levels, which lead to a distribution of the carrier transport rates. To our knowledge, however, there have been no previous reports on the lifetime (59 ns) of emission from TiO2 powders by this equation. As it has been described by Gordon et al [55], the long lifetime of biocompatible luminescent nanocrystals provides a significant advantage in discriminating against background autofluorescence. In addition to the long lifetime, other considerations for applying luminescent nanoparticles in multi-spectral immunoassays include: (1) finding multiple reporters with distinct emission wavelengths to attach to different antibodies, (2) having a common excitation wavelength for the different reporters, and (3) keeping the particles sufficiently small or soluble not to alter the activity or solubility of the antibodies [55]. In order to determine the size of TiO2 nanocrystals, the hyperbolic band model (HBM) was directly used. On the basis of maximum absorption spectrum peak, the size of nanoparticles can be estimated by [56]:
Egnano
Eg2
2 2 Eg 2
(6)
m * R2
In the above equation, E gnano is the energy band of the nanoparticle (3.44 eV), Eg is the bulk energy band (~3.1 eV), ℏ is the Planck’s constant, m* is the effective mass of the TiO2 and R is the radius of titanium dioxide nanoparticles. By placing these values in equation (6), the radius of TiO2 colloidal nanoparticles was obtained to be equal to 2.5 nm which is in good agreement with HETEM result. It means that the hypothesis of direct band gap transition is correct for TiO2 colloidal nanocrystals. In order to identify the crystalline phase of TiO2, the sol was dried and the crystallinity of precipitated TiO2 nanopowders was evaluated by X-ray diffraction technique. The pattern of XRD result was shown in figure 6 and confirms that the dried sol has anatase crystalline phase.
9
4. Conclusion Improved sol-gel method has been applied to prepare highly crystalline colloidal TiO2 nanocrystals. Some analytical techniques such as UV-Vis, PL, FT-IR, XRD and HRTEM were used to characterize the synthesized colloidal TiO2 QDs. Experimental results have shown that the prepared sample has strong violet-blue emission that has not been reported before. It has a narrow size distribution which has been confirmed by HRTEM. Upon aging at room temperature, there are no considerable changes in the intensity of UV-Vis and photoluminescence spectrums of TiO2 sol. Photoluminescence quantum yield for TiO2 colloidal nanocrystals is estimated to be 49% with 280 nm excitation wavelength which is in agreement and better than reported before. Obtained results show that the prepared colloidal TiO2 sample has enough potential to be used in preparation of nanostructured thin films, which are useful for optoelectronic applications.
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Figure Captions Figure 1 : FT-IR spectrum of colloidal TiO2 nanocrystals. Figure 2 : (a) HRTEM image (b) 1 year aged colloidal TiO2 nanocrystals samples. Figure 3 : (a) Absorption spectra of fresh and 1 year aged colloidal TiO 2 nanocrystals samples. (b) Tauc plots of prepared TiO2 colloidal nanocrystals. Figure 4 : (a) Photoluminescence spectra of fresh and 1 year aged samples (b) The peak intensity of PL spectra vs excitation wavelength. Figure 5 : Decay profile of photoluminescence from colloidal TiO2 nanocrystals samples. Figure 6 : XRD spectrum of dried TiO2 sol.
Fig. 1
14
a
b Fig.2
15
Fig.3
16
a
b Fig.4.
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Fig.5
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PII: DOI: Reference:
S0022-2313(15)30210-6 http://dx.doi.org/10.1016/j.jlumin.2016.05.036 LUMIN14007
To appear in: Journal of Luminescence Received date: 7 July 2015 Accepted date: 19 May 2016 Cite this article as: Morteza Sasani Ghamsari, Mohammad Reza Gaeeni, Wooje Han and Hyung-Ho Park, Highly stable colloidal TiO 2 nanocrystals with strong violet-blue emission, Journal of Luminescence, http://dx.doi.org/10.1016/j.jlumin.2016.05.036 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.
Highly stable colloidal TiO2 nanocrystals with strong violet-blue emission Morteza Sasani Ghamsaria,*, Mohammad Reza Gaeenia, Wooje Hanb, Hyung-Ho Parkb a
b
Laser & Optics Research School, NSTRI, 11155-3486 Tehran, Iran Department of Materials Science and Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 120-749, Korea
Abstract: Improved sol-gel method has been applied to prepare highly stable colloidal TiO2 nanocrystals. The synthesized titania nanocrystals exhibit strong emission in the violet-blue wavelength region. Very long evolution time was obtained by preventing the sol to gel conversion with reflux process. FTIR, XRD, UV-Vis absorption, photoluminescence and high resolution transmission electron microscope (HRTEM) were used to study the optical properties, crystalline phase, morphology, shape and size of prepared TiO2 colloidal nanocrystals. HR-TEM showed that the diameter of TiO2 colloidal nanocrystals is about 5 nm. Although the PL spectra show similar spectral features upon excitation wavelengths at 280, 300 and 350 nm, but their emission intensities are significantly different from each other. Photoluminescence quantum yield for TiO2 colloidal nanocrystals is estimated to be 49% with 280 nm excitation wavelength which is in agreement and better than reported before. Obtained results confirm that the prepared colloidal TiO2 sample has enough potential for optoelectronics applications.
Keywords: Sol-gel preparation; Nanocrystalline materials; Colloidal TiO2 nanoparticles; Violet-Blue emission. *Corresponding author: [email protected] [email protected]
1. Introduction 1
During the past two decades, light emitting materials have been one of the exhaustive research objects. to develop a synthesis method for producing high efficient, bright and stable light emitting materials with improved their optical properties was extensively attracted much attention [1,2]. From all emission bands in visible wavelength region, violet-blue emission line has especial position in generation of white light under ultra-violet or blue light excitation [3]. On the other hand, the biocompatible and stable violet-blue emission source is very important for in vivo cancer imaging. Consequently, the colloidal semiconductor nanocrystals (NCs) with considerable optical properties including high efficiency, narrow emission band, enabling spectral purity and fine tunability are gaining prominence in recent years [4,5]. As the most promising semiconducting materials, titanium dioxide is biocompatible and as a key material for optoelectronic applications has been widely investigated. It is currently employed to make photovoltaic cells, batteries, chemical sensing, optical emissions, photonic crystals, optical waveguide, photo catalysis and nanomedicine[6-8]. The colloidal form of TiO2 nanocrystals has plentiful practical applications as self-cleaning glass, heat mirror, optical waveguide, and solar cell [9-12]. From technological point of view, the synthesis of highly stable colloidal TiO2 quantum dots (QDs) with high crystallinity is very important [13-17]. To prepare a TiO2 sol with strong violet-blue emission, the crystallinity of TiO2 QDs in the sol plays a critical role [18]. Despite several approaches for synthesis of highly crystalline TiO2 sol, controlling the crystallinity and physical properties of the sol-gel derived TiO2 nanocrystals is not easily [19-27]. It means that more research works must be done to establish a preparation method of highly crystalline TiO2 sol with wide potential in different applications [28]. In our last papers we have tried to improve a procedure for preparation of highly crystalline TiO2 nanocrystals. Some details can be found in our last papers [29.30]. Recently, room temperature synthesized TiO2 nanocrystal was also reported by our
2
group [31]. In this approach, we have successfully synthesized highly stable colloidal TiO2 nanocrystals with a strong violet-blue emission that has not been reported before. 2. Experimental Materials and Methods The analytical grade of Titanium (IV) Isopropoxide (TTIP, 97% Aldrich), 2-propanol (CH3CH(OH)CH3, 99.5% Merck), and distilled water, were used as precursor materials. In this work, hydrolysis and precursor solutions were prepared separately to control the sol-gel process as well as colloidal particle size. At first, 2-propanol and titanium (IV) isopropoxide was mixed and used as precursor solution. A mixed solution of 2-propanol and water (hydrolysis solution) was prepared and heated at temperatures 60–80°C under vigorous stirring. Then the precursor solution was dropwisely added to the hydrolysis solution. The resultant mixture was kept standing for 2 h (Refluxing process) and a transparent (Bluish) TiO2 sol was finally obtained. Fourier Transform Infrared spectroscopy (Bruker Vector 22 FT-IR) was used for detection of functional groups within the compounds. The photoabsorption characteristic of prepared sol was studied by photospectrometer (Perkin Elmer). Photoluminescence spectroscopy was carried out to identify the emission spectrum and quantum yield of TiO2 colloidal nanocrystals using GILDEN PHOTONICS FLUOROSENS spectrometer. In order to study the crystalline phase of sol, it was dried and characterized by XRD (Bruker AXS-D8). Particle size of the colloidal sol was characterized by using high resolution transmission electron microscopy (JEM-2010F, JEOL). 3. Results and Discussion Fourier transform infrared (FTIR) was carried out to detect the presence of functional groups in the compound of synthesized colloidal TiO2 sol in the range 400–4000 cm-1 wavenumbers. The FTIR spectrum has been shown in Figure 1. This spectrum is to identify the chemical bonds, as well as functional groups, in the compound. The source of these bands was individually identified in our last
3
paper[11]. The absorption band at 1037 cm−1 is due to the stretching vibration of the O–C–C bands of the TTIP isopropyl groups [11]. Sharp absorption band at 1651 cm−1 shows the nitrate group and is a result of nitric acid addition. The OH stretching frequencies of alcohols lead to large absorption peak in the region 3200–3600 cm−1. The deformation vibration bands of the C–H, C–O and Ti–O can be observed in the 900–400 cm−1 region. The low energy region (below 1000 cm-1) indicated the bands due to stretching modes of Ti-O and Ti-O-Ti bonds of a titanium dioxide network. Absorption band at 603 cm−1 is due to the Ti–O band. The particle size and crystallinity of colloidal sample were confirmed by further characterizations. The formation of all TiO2 nanoparticles and their size were identified by the high magnification HR-TEM image. The high resolution images of colloidal TiO2 sample and their lattice image were gotten in figure 2a, b. The obtained results confirm that the prepared sample has a narrow size distribution and their size to be equal to 5nm. The UV-Vis absorption spectrum of the fresh colloidal sample was shown in figure 3a. For fresh prepared colloidal TiO2 nanocrystals, the strong absorption at about 340 nm wavelength is attributed to the band-to-band transition. From figure 3a it can be found that the onset point of absorption peak is 370 nm. Therefore, the band gap of TiO2 colloidal nanoparticles can be determined by: Egnano ≈1240/λonset. It is about 3.36 eV which is 0.26 eV higher than that of bulk anatase TiO2 (~3.1eV). This blue shift of the absorption edge is attributed to the quantum confinement effect. Size quantization effect has been observed in TiO2 nanocrystals [32-40]. Anpo et al [33] studied the change of TiO2band gap over a wide range of particle sizes (e.g., 3.8–200 nm). When particle size was less than 12 nm, the significant blue shift of the absorption edge by 0.093 and 0.156 eV for rutile and anatase crystalline was respectively found by them. The quantum confinement effect of illuminated TiO2 colloids with particle size less than 3 nm was observed by Kormann et al [34]. They reported a blue-shift by 0.150.17eV in absorption spectrum. However, Monticone et al [39], examined the quantum size effect in 4
anatase TiO2 nanoparticles and the band gap energy shift has not been observed for sizes 2R1.5 nm. Their results indicate the considerable effect of the size on the electronic band structure, which disables the EMA in TiO2 crystallites. Recently, Satoh et al [40], observed the quantum size effects in the TiO2 nanoparticles. They reported a blue shift corresponded to particle size reduction for band gap energy of TiO2 nanoparticles. Therefore, it can easy conclude that earlier reports of blue-shifted absorption thresholds, taken as evidence for Q-size effects in very small TiO2 particles, are not in fact direct (Franck-Condon type) transitions in an otherwise indirect band gap semiconductor. In order to evaluate the experimentally obtained optical band gap energy of TiO2 colloidal nanocrystals, band-toband transitional relation was employed. The optical band gap energy (Eg) of the nanocrystalline titanium dioxide sol was also determined by extrapolating a straight line to [αhν]n. In the high absorption region, the energy of incident photons, hν, is related to absorption coefficient, α(λ), by band-to-band transition relation [41]: αhν = β(hν - E gnano )n
(1)
here 𝛽 is a constant nearly equal to one at absorption edge, n is an index that characterizes the optical absorption process being theoretically equal to 0.5, 1.5, 2 or 3 for direct allowed, direct forbidden, indirect allowed, and indirect forbidden transitions, respectively. From the different Eg obtained values, Madhusudan Reddy [42], showed that the estimated shift of band gap for the synthesized TiO2 nanoparticles is too large if they considered as indirect transition material and therefore it could be inferred that the direct transition material are more appropriate. This could be one reason to suggest that the direct, and not indirect transition, is more favorable in anatase TiO2 nanoparticles. Figure 3b illustrates (αhν)2 versus energy of the photon for TiO2 colloidal nanocrystals. The obtained value for direct allowed band gap energy ( E gnano ) is about 3.44 eV. In order to study the optical properties resulted from the photogenerated electron/ hole pairs in colloidal TiO2 nanocrystals the 5
photoluminescence (PL) spectroscopy was used. Usually, the recombination of electron/hole pair is contributing in the origination of semiconductor nanoparticles photoluminescence. Figure 4a shows the PL spectra of fresh and 1 year aged colloidal TiO2 nanocrystals, which were taken at 280 nm excitation wavelengths. The spectra upon excitation wavelengths at 280, show similar spectral features but their emission intensities are a little different from each other (Figure 4a). The emission spectra of fresh sample excited at 300 and 350 nm show similar spectral features but their emission intensities are significantly different from each other. Also, aged sample exhibited similar behavior. The variation of PL spectra peak intensity of aged and fresh samples vs excitation wavelength is shown in figure 4b. It is seen that titanium oxide colloidal nanocrystals showed a strong PL band peaking at 420 nm and a broad one ranging from 400 to 500 nm. There are two different theories about photoluminescence source in TiO2 nanocrystals. In the first hypothesis, the surface defects are sources of TiO2 nanocrystals photoluminescence behavior. For example, Xu et al. described that the light radiation in the UV region is due to the radiative annihilation of excitons (band-to-band recombination) [43]. While the visible light emission is attributed to the electron transition mediated by defect levels such as oxygen vacancies in the band gap of TiO2 nanopowder [44]. Also, Mathew et al. believed that the photoluminescence of TiO2 nanocrystals is mostly a surface phenomenon and a change in the surface environment would have a significant effect on the photoluminescence process. They concluded that 420 nm, 491 nm, 530 nm emissions are assigned to the surface state emissions and are due to the recombination of trapped electron–hole arising from dangling bonds in the surface of TiO2 nanoparticles [38]. On the basis of the difference between emission peaks, it has been concluded that the visible emissions is from the deexcitation of lower vibronic levels in Ti3+ 3d states of TiO2 lattice to the deep trap levels (acceptor) created by (OH-) [38]. They have also explained that the emission wavelengths 533 nm and 612 nm, which are also separated by nearly 80 nm, are due to deexcitation
6
from lower vibronic levels in deexcitation from lower vibronic levels in the oxygen vacancies of TiO2 lattice to the ground state. In addition, Knorr et al. explained that the red PL emission can be originated from the electron traps. While hole traps result the observed green luminescence for all oxide nanoparticles. It was concluded that the surface oxygen vacancies are playing the role of hole trap states [45]. In all mentioned conclusions, the surface state was considered as source of visible emission. If we accept that all visible emission peaks are due to the surface state, a red-shifted emission must be observed when different excitation wavelength higher than band edge wavelength to be used. These emission peaks shifted to the larger wavelengths are distinguishing from the band edge emission attributed to the surface state emissions [46]. Figure 3a, confirms that the emission spectrum of sample excited by different wavelengths did not show any shift. In the second theorem, the intrinsic unit cell defects are sources of TiO2 nanocrystals photoluminescence behavior [47,48]. It means that the observed visible emission is probably generated from self-trapped excitons (electron–hole pairs) localized on the TiO6 octahedron and/or transition from charge-transfer excited states (Ti3+-O−) in trap sites [48]. Consequently, observed strong blue emission centered at 420 nm of prepared sample is caused from intrinsic unit cell defects. On the other hand, red PL emission was not observed in the luminescence spectra and a weak green luminescence at 530 nm was recorded. According to the standard method the photoluminescence quantum yield (QY) of TiO2 colloidal nanocrystals can be determined by [49]: QY QYr
P ODr n 2 Pr OD nr2
(2),
here, P is the photoluminescence integrated intensity, OD is the optical density at the excitation wavelength, and n is the linear refractive index of TiO2 QDs. The subscript “r” refers to the Fluorescein with known quantum yield. The linear refractive indexes (n) of TiO2 QDs were measured by equation (3): 7
n 1
bulk 1
(3)
1.2
3 1 8R
where R refers to the radius of TiO2 QDs in nm and ε is the relative permittivity of TiO2 [50]. The gotten result for linear refractive index of TiO2 with R=2.3nm was 1.92. The photoluminescence quantum yield of TiO2 colloidal nanocrystals was compared to Fluorescein as a reference emission material with a quantum yield of 95%. Optical Density (OD) of TiO2 nanocrystals can be calculated using Beer-Lambert Law: I A log 0 I
OD
(4),
here I is the light intensity at the center of the cuvette, I0 is the intensity of the incident light to the cuvette and A is the absorbance. Photoluminescence quantum yield for TiO2 colloidal nanocrystals is estimated to be 49% with 280 nm excitation wavelength which is in agreement and better than reports in refs 51 and 52. A PL peak with full width at half maximum (FWHM) of around 65 nm is obtained, which infers a homogeneous and narrow size distribution of TiO2 colloidal nanocrystals. An example of the photoluminescence decay profile for colloidal TiO2 is shown in Fig. 5. This decay profile was well traced using the extended exponential function: t I I 0 exp
(5)
where τ is the lifetime. The decay profile of the emission from colloidal TiO2 nanocrystyals, was also well traced using this equation. As it has been confirmed by Fujihara, et al [53], the decay profiles cannot be fitted by the second order kinetics, which has frequently been examined for analysis of the charge carrier dynamics of TiO2 particles. The origin of the nonexponential decay has also been attributed to a distribution of recombination rates [54]. Good fitting of the decay profiles of the luminescence of TiO2 particles by Eq. (5) suggests the existence of carrier trapping sites with different 8
energy levels, which lead to a distribution of the carrier transport rates. To our knowledge, however, there have been no previous reports on the lifetime (59 ns) of emission from TiO2 powders by this equation. As it has been described by Gordon et al [55], the long lifetime of biocompatible luminescent nanocrystals provides a significant advantage in discriminating against background autofluorescence. In addition to the long lifetime, other considerations for applying luminescent nanoparticles in multi-spectral immunoassays include: (1) finding multiple reporters with distinct emission wavelengths to attach to different antibodies, (2) having a common excitation wavelength for the different reporters, and (3) keeping the particles sufficiently small or soluble not to alter the activity or solubility of the antibodies [55]. In order to determine the size of TiO2 nanocrystals, the hyperbolic band model (HBM) was directly used. On the basis of maximum absorption spectrum peak, the size of nanoparticles can be estimated by [56]:
Egnano
Eg2
2 2 Eg 2
(6)
m * R2
In the above equation, E gnano is the energy band of the nanoparticle (3.44 eV), Eg is the bulk energy band (~3.1 eV), ℏ is the Planck’s constant, m* is the effective mass of the TiO2 and R is the radius of titanium dioxide nanoparticles. By placing these values in equation (6), the radius of TiO2 colloidal nanoparticles was obtained to be equal to 2.5 nm which is in good agreement with HETEM result. It means that the hypothesis of direct band gap transition is correct for TiO2 colloidal nanocrystals. In order to identify the crystalline phase of TiO2, the sol was dried and the crystallinity of precipitated TiO2 nanopowders was evaluated by X-ray diffraction technique. The pattern of XRD result was shown in figure 6 and confirms that the dried sol has anatase crystalline phase.
9
4. Conclusion Improved sol-gel method has been applied to prepare highly crystalline colloidal TiO2 nanocrystals. Some analytical techniques such as UV-Vis, PL, FT-IR, XRD and HRTEM were used to characterize the synthesized colloidal TiO2 QDs. Experimental results have shown that the prepared sample has strong violet-blue emission that has not been reported before. It has a narrow size distribution which has been confirmed by HRTEM. Upon aging at room temperature, there are no considerable changes in the intensity of UV-Vis and photoluminescence spectrums of TiO2 sol. Photoluminescence quantum yield for TiO2 colloidal nanocrystals is estimated to be 49% with 280 nm excitation wavelength which is in agreement and better than reported before. Obtained results show that the prepared colloidal TiO2 sample has enough potential to be used in preparation of nanostructured thin films, which are useful for optoelectronic applications.
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Figure Captions Figure 1 : FT-IR spectrum of colloidal TiO2 nanocrystals. Figure 2 : (a) HRTEM image (b) 1 year aged colloidal TiO2 nanocrystals samples. Figure 3 : (a) Absorption spectra of fresh and 1 year aged colloidal TiO 2 nanocrystals samples. (b) Tauc plots of prepared TiO2 colloidal nanocrystals. Figure 4 : (a) Photoluminescence spectra of fresh and 1 year aged samples (b) The peak intensity of PL spectra vs excitation wavelength. Figure 5 : Decay profile of photoluminescence from colloidal TiO2 nanocrystals samples. Figure 6 : XRD spectrum of dried TiO2 sol.
Fig. 1
14
a
b Fig.2
15
Fig.3
16
a
b Fig.4.
17
Fig.5
Fig. 6
18