CC BY
40
УДК 621.793.18
SYNTHESIS, STRUCTURE AND OPTICAL PROPERTIES OF TIO2 AND TIO2/AL2O3 THIN FILMS DEPOSITED ON INDIUM TIN OXIDE SUBSTRATES PREPARED BY CHEMICAL BATH DEPOSITION
SAMEERAH S. S. AL-QADASY1, S. Q. CHISHTY1, HAKIM Q. N. M. AL-ARIQUE2, SAMI M. A. AL-ARIKI3, NIYAZI A. S. AL-AREQI2
department of Physics, Dr. Rafiq Zakaria College for Women, Aurangabad University, the Republic of India
2Department of Chemistry, Taiz University, the Republic of Yemen
3Department of Physics, Thamar University, the Republic of Yemen
TiO2 and TiO2 /Al2O3 thin films were applied on glass substrates containing indium and tin oxides (ITO) from a solution containing Ti[OCH(CH3)2]4 and AlCl3 • 6H2O at room temperature using a simple and economical method of chemical bath deposition (CBD-method). Then, the obtained thin films were annealed at 500 °C for two hours. The structural, morphological and optical properties of TiO2 and TiO2/Al2O3 thin films were analyzed by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), Fourier-transform infrared spectroscopy (FT-IR), Raman spectroscopy and UV-visible spectrophotometry. The XRD results showed that the average size of TiO2 crystallites is 58.6 and 22.8 nm before and after annealing, whilst the average size of TiO2/Al2O3 before annealing is 56 nm with the anatase phase and then after annealing crystallites with tetragonal symmetry are formed with a size of ~ 17 nm. The anatase peaks appeared at an annealing temperature of500 °C indicating an increase in crystallinity during the annealing process. However, FT-IR analysis showed the presence of absorption peaks for various functional groups, such as O—H, CH, Ti-O—Ti, Al-O). The appearance of the anatase peak at 144 cm-1 and sharp clear peaks after annealing also indicates an increase in crystallinity. FE-SEM images revealed an elongated spherical shape for TiO 2 nanocrystals and a nanoplatelet shape for TiO2/Al2O3. The optical band gap width for TiO2 thin films was 3.28 and 3.07 eV before and after annealing and for TiO2/Al2O3 thin films it decreased from 3.32 to 3.14 eV after annealing.
Keywords: thin films, TiO2/Al2O3, CBD-method, annealing, optical band gap width.
СИНТЕЗ, СТРУКТУРА И ОПТИЧЕСКИЕ СВОЙСТВА ТОНКИХ ПЛЕНОК TIO2 И TIO2/AL2O3, НАНЕСЕННЫХ НА ПОДЛОЖКИ ИЗ ОКСИДА ИНДИЯ И ОЛОВА, ПРИГОТОВЛЕННЫЕ МЕТОДОМ ХИМИЧЕСКОГО ОСАЖДЕНИЯ В ВАННЕ
САМЕЕРА С. С. АЛЬ-КАДАСИ1, С. К. ЧИШТИ1, ХАКИМ К. Н. М. АЛЬ-АРИКИ2, САМИ М. А. АЛЬ-АРИКИ3, НИВАЗИ A. С. АЛЬ-АРИКИ2
1 Кафедра физики Рафия Закария женского колледжа, Аурангабадский университет, Республика Индия
2Кафедра химии, Таизский университет, Республика Йемен
3Кафедра физики, Дамарский университет, Республика Йемен
Тонкие пленки TiO2 и TiO2/Al2O3 осаждались на стеклянных подложках содержащих оксиды индия и олова (ITO) из раствора, содержащего Ti[OCH(CH3)2]4 и AlCl3 • 6H2O при комнатной температуре с использованием простого и экономичного метода химического осаждения в ванне (CBD-метод). Затем по-
лученные тонкие пленки отжигали при 500 °C в течение двух часов. Структурные, морфологические и оптические свойства тонких пленок TiO2 и Ti02/Al203 исследовались методами рентгеновской дифракции (XRD), полевой эмиссионной сканирующей электронной микроскопией (FE-SEM), инфракрасной Фурье-спектрометрией (FT-IR), спектроскопией комбинационного рассеяния и УФ-видимой спектрофотометри-ей. Результаты XRD показали, что средний размер кристаллитов Ti02 составляет 58,6 и 22,8 нм до и после отжига соответственно, тогда как средний размер Ti02/Al203 до отжига составляет 56 нм с фазой анатаза, а затем после отжига формируются кристаллиты с тетрагональной симметрией с размером ~ 17 нм Появление пиков анатаза при температуре отжига 500 °Суказывает на увеличение кристалличности при отжиге. Однако FT-IR анализ показали присутствие пиков поглощения для различных функциональных групп, таких как 0-H, CH, Ti-0-Ti, Al-0). Появление пика анатаза при 144 см-1 и резких четких пиков после отжига также свидетельствует об усилении кристалличности. Изображения FE-SEM выявили удлиненную сферическую форму для нанокристаллов Ti02 и форму нанопластинок для Ti02/Al203. Ширина оптической запрещенной зоны для тонких пленок Ti02 составила 3,28 и 3,07 эВ до и после отжига соответственно, а для тонких пленок Ti02/Al203 после отжига уменьшилась с 3,32 до 3,14 эВ.
Ключевые слова: тонкие пленки, ТЮ2/А120з, CBD-метод, отжиг, ширина оптической запрещенной зоны.
Introduction
Titanium dioxide (Ti02) is a widely used material for photovoltaic and protective applications due to its high visible-region transparency, excellent mechanical properties and chemical stability in aqueous solution [1], [2]. Ti02 films are also useful for many other applications such as catalysis [3], photonic coatings, gas sensors [4], and other electronic devices [5]. Ti02 films have been prepared by a variety of chemical and physical deposition techniques, such as the sol-gel process [6]-[8], chemical vapor deposition [9], various reactive sputtering methods [10], [11], ion beam assisted processes [12], pulse laser deposition [13], chemical bath deposition [14], [15] atomic layer deposition [16] and evaporation [17]. It has been found that the physical properties of Ti02 strongly depend on the deposition method applied and calcination temperatures. Generally, Ti02 films change from amorphous to anatase and afterward to rutile based on the calcination temperature.
The dependences of structural and optical properties on annealing temperature have been reported [18]-[20]. The doping of Ti02 with more foreign elements such as Zn, Mn, Fe, Ni, Al and Co affects Ti02 nanoparticles. Its structural and optical properties allow it to be adapted for a variety of applications [21].
Al203 also known as alumina, is commercially produced from bauxite, an abundant ore, via the Bayer process. Al203 is similar to white powder (amorphous and polycrystals) or colorless hexagonal single crystals with a melting point about 2020 °C [22]. It is insoluble in water and organic liquids but very slightly soluble in acids and alkalis, it is a wonderful material that can show chemical resistance with good thermal stability.
Coating of materials such as thin films can impart these properties to protect other materials. Al203 it is widely used due to its high hardness, good chemical inertness, zero electrical conductivity and excellent optical transparency. A number of chemical and physical techniques have been reported for the fabrication of Al203 thin films [23]-[28]. Compared with conventional particles, nano-sized particles (e. g., iron, titanium, and alumina nanoparticles) definitely have a larger specific surface area which improves their super absorbent properties [29], [30]. So, in the present work, we have reported the successful utilization of the simple, cost-effective chemical bath deposition (CBD) method for preparation of nanocrystalline Ti02 and Ti02/Al203 thin films and studied the effect of annealing temperature on structural, morphological and optical properties of as- prepared thin films.
Experimental
Materials
Titanium isopropoxide Ti [0CH(CH3)2]4 (TTIP), (98 %), purchased from Sigma-Aldrich Chemie, India was used as Ti02 source. Aluminum chloride hexahydrate (AlCl3 • 6H20), (97 %), from Merck, Germany was used as Al203 source. Isopropanol
(CH3CHOHCH3) (99 %) was obtained from Molychen. Mumbai, (India), and absolute ethanol (C2H5OH), (99.9 %) from Changshu Hongsheng Fine Chemical Co. Ltd., China. The conducting Indium tin oxide (ITO) coated glass (dimensions 75 mm x 25 mm x 1.1 mm, surface resistivity < 10 Q/sq, was purchased from SHILP ENT and used as a substrate. Before the deposition, the substrates were cleaned by using chromic acid, distilled water, acetone, ethanol, distilled water and finally allowed to air dry to remove the negligible amount of surface residues. All solvents and chemicals were of analytical grade and were used without further purification.
Deposition of TiO2 thin films
The procedure for preparing of TiO2, TiO2/Al2O3 thin films by CBD method consists of three main steps, including the solution preparation, formation of thin films and annealing the films, TTIP (0.2M) was dissolved in Ethanol C2H5OH (40 ml) and isopropanol (10 ml) to constitute a solution (A). Solution (A) was then magnetically stirred at room temperature for 10 min for homogeneity, using a water bath to start the formation of TiO2 nanoparticles. The pH of the solution was adjusted at 5. Two ITO coated glass used as substrates which were immersed into the solution (A) for 1 h at room temperature. Thin films of TiO2 material were deposited onto glass plates, then dried in air for 24 h. One of these as-prepared film was annealed at 500 °C.
Deposition of TiO2/Al2O3 thin films
AlCl3 • 6H2O (0.05M) was dissolved in ethanol (10 ml) and the stirred for 2 h, constituting a solution (B) Then, the both solutions (A) (freshly prepared) and (B) were thoroughly mixed together under magnetic stirring for 1 h at 40 °C. After room temperature cooling, ITO coated glass substrates were immersed into the resulting solution with adjusted pH 4.2 for 1 h at room temperature. The deposited films were then dried in air for 24 h. Similarly, one of them was annealed at 500 °C. High-quality and highly transparent TiO2/Al2O3 nano structured thin films were obtained and subjected for further characterizations.
Characterization techniques
The deposited TiO2 thin films were characterized for their structural, morphological, and compositional properties. The crystal structure and crystallographic data were obtained from X-ray diffractometer (XRD) (Ultima IV of Rigaku Corporation, Japan) with CuKa (k = 1.54056 A).
Fourier transform-infrared (FT-IR) spectra (FT-IR JASCO-4600) were scanned from 4000 to 400 cm-1 using the transmittance mode.
Raman spectra were recorded on a Jobin Yvon Horibra LABRAM-HR instrument within a range of 200-1800 cm-1 by a scanning resolution of 1 cm-1, applying a back scattering geometry.
The surface morphology of TiO2 and TiO2/Al2O3 films were investigated by a Field emission-scanning electron microscope (FE-SEM) using FESEM-JEOL JEM-6360 Mira-3, Tascan, Republic of Czech.
Values of absorption coefficient (a) and band-gap energy (Ea) were calculated from the data obtained from UV-visible absorption spectra measured using a U-V Double Beam Spectrophotometer V-750 (Jasco Corp., Tokyo, Japan) at wavelengths scanned in the range of250-700 nm.
Results and discussion
Structure Analysis
The XRD patterns of TiO2 and TiO2/Al2O3 thin films deposited on the ITO coated glass substrate using the CBD method were presented in Fig. 1. The crystal structure and phase of the deposited TiO2 thin films before annealing (Fig. 1, a) are predominantly ana-tase and rutile as clearly evidenced by the appearance of the following sublattice peaks: 25.28°, 37.80°, 48.06°, 53.92°, 55.08°, 62.72°, 68.80°, 70.32° and 75.10° which were in-
dexed consequently as (101) A, (004) A, (200) A, (106) A, (211) R, (116) A, (301) R, (107) A, (216) A, where A for anatase phase and R for rutile phase [31], [32]. However, after addition of AlCl3 • 6H2O, the XRD pattern of TiO2/Al2O3 thin film before annealing (Fig. 1, b) shows additional three broad sublattice peaks within the diffraction angle range 20° and 80°. Where no obvious crystalline phase, indicating that this thin film is present in a defect crystal structure (amorphous-like) [33]. After annealing at 500 °C for 2 h, the XRD pattern of TiO2 thin film (Fig. 1, c) exhibits sublattice peaks at 25.28°, 37.88°, 48.02°, 54°, 55.06°, 62.18°, 62.78° and 75.16°, indexed as (101), (004), (200), (105), (211), (204), and (215), respectively, which are clearly indicative for the existence of a tetragonal anatase symmetry [34]. It is also clear that no other peaks for brookite or rutile phases were appeared, suggesting the high purity of annealed TiO2 thin film. These results were agreed well with JCPDS card number 21-1272. The most intense reflection corresponding to the (101) plane, whose intensity decreases with addition of AlCl3 • 6H2O) (Fig. 1, c), also indicates the presence of the only anatase phase for the TiO2/Al2O3 film [35]. The estimated lattice parameters were found to be a = b = 0.3785 nm and c = 0.9513 nm. The average crystallite size was computed using the Debye-Scherrer's equation:
D =
Kl b sin e:
(1)
where D is the average crystallite size, K is the Debye Scherrer's constant (K = 0.94), X is the wavelength of the CuKa-radiation (X = 1.5406 A), P is the full width half maximum (FWHM) of the peak, and 0 is the Bragg's angle [36]. The average crystallite size (D) for TiO2 was found to be 58.6 and 22.8 nm before and after annealing, respectively. Whereas that for TiO2/Al2O3 is 56 nm before annealing, which then decreases to 17 nm when the film annealed at 500 °C, indicating an improved crystallinity upon annealing. Moreover, the average of spacing between the diffracted planes was calculated by the Bragg law:
d =
nl 2sin e:
(2)
where d is the average of spacing between diffracting planes, and found to be 2.99 nm and thereafter remarkably reduces to 2.38 nm for annealed film at 500 °C.
30
50 20
70
25.28
b)
37.8 48
-ТЮ2-А12О3 - - TiO2
546 62 8 69'5 753 829
-1— 30
—I—
40
~~1— 50
~~1— 60
—1— 70
~~1— 80
29
Fig. 1. XRD patterns of (a) TiO2, (b) TiO2/Al2O3 before annealing and (c) annealed TiO2
and T^/AUO, at 500 °C
40
60
80
Fourier transform-infrared (FT-IR) analysis
FT-IR spectroscopy can be successfully applied to study the formation of as-prepared nanocrystalline sized oxides and characterize the surface state of deposited nanoparticles by CBD. FT-IR results complements the information obtained from XRD analysis. It represents the integration of all data that is used to understand and refine the structure of films more efficiently [37]-[39]. The obtained absorption peak frequencies indicate the type of functional group present in the material.
Figure 2 depicts many absorption bands within wide ranges of 450-1000 cm-1, and 3400-4000 cm- , while no apparent bands found in the range of 1000-3400 cm-1. In the first range, there are many sharp peaks can be attributed to characteristic to Ti-O and Ti-O-Ti stretching and bending vibrational modes for rutile TiO2 [40], whereas the absorption band appeared at 1400 cm-1 is attributed to C-H bonds of -CH3 group [41] These bands become sharper by addition of AlCl3, particularly in the region 3400-4000 cm-1 assigned to the stretching O-H bond [42], [43].
D
ra
4000
3500
3000
2500
2000
1500
1000
500
wavenumber cm 1
Fig. 2. FT-IR spectra of TiO2 and TiO2-Al2O3 before annealing
ra
befor after
TiO2-Al2O3
-NLJ
4000 3500 3000 2500 2000 1500 1000 500
wavenumber cm-1 Fig. 3. FT-IR spectra of TiO2/Al2O3 before and after annealing
The FT-IR spectrum of the annealed TiO2 thin film at 500 °C is shown in Fig. 3, the strong bands located around 460-640 cm-1 are associated with the Ti-O-Ti stretching vibration, indicating the formation of TiO2 anatase phase [44]-[46]. There is a clear shifting in the band around 3640-3870 cm-1 due to the O-H stretching vibration. However, the band due to the Al-O stretching vibrations of the octahedral coordinated Al site can be seen in the range of 500-880 cm-1 [47]-[49].
Raman Spectroscopy
Raman shifts are affected by the vibration of the electronic polarization of components in the films, which depend on bonding properties such as atomic distance and bonding angle for the product structure changed when the chemical bond was rearranged. According to the data of Raman spectroscopy, anatase phase was formed mostly in the film at 140, 386, 504, and 627 cm-1, and it is clearly evidenced by the appearance of an anatase peak at 144 cm-1 as shown in Fig. 4. This peak is well known as a probe, and is very sensitive to the anatase phase in TiO2 films [50], [51]. A large distortion of the curve occurs by the addition of Al2Cl3, indicating a change in the composition of the sample. A new and wide peak for the aluminum and titanium phases is also observed. After annealing many peaks of TiO2 are observed in the ring of 0-1000 cm-1 (Fig. 5) with a sharp peak assigned to anatase phase. Figure 6 present Raman spectra of TiO2/Al2O3 before and after annealing. Sharp and clear peaks shown after annealing the film are clear evidence for the improve-
Raman shift cm 1 Raman shift cm 1
Fig. 4. Raman spectra of (a) TiO2, and (b) TiO2-Al2O3 before annealing
Fig. 5. Raman spectra of TiO2 and TiO2/Al2O3 after annealing at 500 °C
Fig. 6. Raman spectra of TiO2/Al2O3 before and after annealing
Field emission-scanning electron microscopy (FE-SEM)
SEM analysis is a powerful tool for examining and characterizing nanoparticles, fracture surfaces, surface morphologies, constituents and microstructures of prepared materials. Figure 7, a shows spherical, uniform sized TiO2 nanoparticles which seem to be completely covering the ITO glass substrate [52]. Due to the larger surface-to-volume ratio, the surface activity of the film without doubt results in a higher degree of UV light absorption. While, an irregular shaped nanoparticles of the TiO2/Al2O3 are clearly observed in Fig. 7, b without significantly homogenous distribution [53], which are entirely changed to nano-sheets.
а) b)
Fig. 7. FE-SEM images of (a) TiO2, and (b) ТЮ2/А12О3 thin films
Optical studies
Figure 8, a shows the optical absorption spectra of as-prepared and annealed TiO2 thin films in the wavelength range of 300-500 nm. Distinct strong absorption peaks are seen in the range of 310 to 340 nm, which is due to the excitation of electrons from the valence band to the conduction band of TiO2 under influence of light absorption. It can be observed that the absorption edge of annealed TiO2 slightly shifts to 350 nm (i. e., red shift), which can be attributed to grain size effects on thermal treatment.
On other hand, the absorption edge is found to shift to the visible region with addition of AhO3 as shown in Fig. 8, b. This implies that the presence of AhO3 can enhance the photocatalytic activity of TiO2 under visible-light excitation. Further, after annealing TiO2/Al2O3 thin film, the absorption edge shifts to a longer wavelength with the increased intensity [43], [54]. The absorbance data was also used to calculate the direct band-gap energy of the TiO2 thin film by equation (3):
A(hv - Eg )n
а =-:-— • (3)
hv
ÎM S£5
—i— «0
BOO
wavelength nm
wavelength nm
а)
b)
Fig. 8. Optical absorption spectra of TiO2 (a) and TiO2/Al2O3 (b) films before
and after annealing
Fig. 9. Determination of band-gap energy of TiO2 before and after annealing
Figure 9 illustrates plots of (ahv)2 vs. hv for the as-deposited and annealed TiO2 thin films, which are linear at the absorption edge, confirming a direct band-gap material. Values of the direct band-gap energy of the as-deposited and annealed TiO2 thin films are determined by extrapolating the linear portion of the curve. The optical band-gap energy of as-deposited TiO2 thin film is found to be 3.28 eV, which is slightly greater than that of the annealed TiO2 thin film (3.07 eV). Because of the slight improvement in the crystallization after annealing, the absorbance edge showed red shift to some extent.
However, the value of Eg for TiO2/Al2O3 thin film was estimated as 3.32 eV, that means it slightly increases on adding Al2O3, and similarly decreases upto 3.14 eV after annealed at 500 °C, due to the improved crystallization [55], [56], as shown in Fig. 10.
photon Energy (eV) photon Energy (eV)
a) b)
Fig. 10. Determination of band-gap energy of TiO2/Al2O3 before (a) and after annealing (b)
Conclusion
Despite of its simplicity and low cost, CBD was proven to be an effective and successful technique in depositing and preparing thin films of metals oxides. In the present work, CBD was successfully utilized for preparation of TiO2 and TiO2/Al2O3 thin films deposited on ITO glass substrates. The combined XRD and FE-SEM results revealed a spherical structure of anatase phase for TiO2 film and a tetragonal anatase structure for TiO2/Al2O3 film, and that the grain size goes on decreasing when doped with Al2O3 and annealed at 500 °C for 2 h. Interestingly, the spherically shaped nanoparticles of TiO2 turned to nanosheets after Al2O3 addition. However, FT-IR and Raman results confirmed the structures of prepared thin films and explore the advantages of annealing and Al2O3 addition in the enhancement of crystallinity. The optical band-gap energy of TiO2 thin film was found to be 3.28 eV, which then slightly decreased upto 3.07 eV after annealing. For TiO2/Al2O3 thin film, Eg was reduced from 3.32 to 3.14 eV after annealed. Generally, the structural, morphological and optical properties investigated in this study confer the deposited TiO2 and TiO2/Al2O3 thin films promising, near future applications in electronic and optical devices.
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Получено 18.08.2022
