Preparation and characterization of TiO2 thin film by thermal oxidation of sputtered Ti film

Titanium dioxide (TiO₂) thin films were successfully prepared on quartz substrate by thermal oxidation of sputtered titanium film in air. The structure, composition, morphology and optical properties of oxidized TiO₂ films were characterized by Raman spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, atomic force microscopy and UV-visible spectroscopy. Meanwhile, the photocatalytic activity of the films was evaluated on the basis of the degradation of methyl orange solution under UV irradiation. Ti films after oxidation present mainly in TiO₂ form with a larger amount of adsorbed O₂, and oxidation temperature has a strong impact on the crystal structure and properties of the films. A phase transformation of anatase to rutile for oxidized TiO₂ films occurred in the temperature range of 700–800 °C. The energy band gap of oxidized TiO₂ films decreased first and then increased with annealing temperature. Furthermore, TiO₂ film oxidized at 600 °C exhibited the best photocatalytic activity due to suitable crystal phase and size. These results might contribute to the synthesis of metal oxide thin films with expectant structural morphology and properties by thermal oxidation methods.     

Influence of Ti film thickness and oxidation temperature on TiO2 thin film formation via thermal oxidation of sputtered Ti film

Single-phase rutile TiO₂ films with good crystallinity were obtained by thermal oxidation of sputtered Ti films on Si and quartz substrates. The influence of the Ti film thickness on oxidation was systematically investigated. A temperature of 823 K was sufficient to fully oxidize Ti films of <0.2 μm in thickness, but 923 K was required for complete oxidation of thicker films. The crystal structure, phase, composition, and optical properties of the TiO₂ films were investigated using X-ray diffraction (XRD), Raman spectroscopy, energy-dispersive X-ray analysis (EDAX), and UV-vis-NIR spectroscopy. XRD and Raman analyses showed that the TiO₂ films are rutile phase. The bandgap of the TiO₂ films decreased with increasing thickness. A growth mechanism for TiO₂ thin films due to thermal oxidation of sputtered Ti films is proposed. Oxidation commences from the surface and proceeds inside the bulk and Ti→TiO₂ phase transformation occurs via different intermediate phases. We found that the oxidation temperature rather than the duration is the dominant factor in the growth of TiO₂ thin films.     

Sol–gel-synthesized mesoporous-assembled TiO2–ZrO2 mixed oxide nanocrystals and their photocatalytic sensitized H2 production activity under visible light irradiation

The purpose of this work was to investigate, in the first study of its kind, hydrogen production by photocatalytic water splitting under visible light irradiation using Eosin Y-sensitized mesoporous-assembled TiO₂–ZrO₂ mixed oxide nanocrystal photocatalysts. The mesoporous-assembled TiO₂–ZrO₂ mixed oxide nanocrystals, with various TiO₂-to-ZrO₂ molar ratios, were synthesized by a sol–gel method with the aid of a structure-directing surfactant. The synthesized nanocrystals were characterized by thermogravimetric and derivative thermogravimetric analyzer, N₂ adsorption–desorption, X-ray diffraction, UV–visible spectroscopy, scanning electron microscope–energy-dispersive X-ray analyzer, and transmission electron microscope analyses. Parameters affecting the photocatalytic activity, including calcination conditions and phase composition, were mainly discussed. Experimental results showed that the incorporation of ZrO₂ with suitable contents could preserve the mesoporous-assembled structure of TiO₂ at high calcination temperatures and enhance its thermal stability significantly. Results of the photocatalytic-sensitized hydrogen production revealed that the TiO₂–ZrO₂ mixed oxide photocatalyst, with a TiO₂-to-ZrO₂ molar ratio of 95:5, calcined at 800 °C for 4 h, provided maximum photocatalytic hydrogen production activity. The optimized TiO₂–ZrO₂ mixed oxide photocatalyst can be considered as a potential photocatalyst for hydrogen production under solar light irradiation.     

Characterization of cadmium sulfide/titania heterostructure films by utilizing microstructure and optical characteristics

Engineering and controlling the bandgap of semiconducting metal oxide (TiO₂) to enhance photoactivity under visible light is challenging. Impact of the changing CdS thickness (50–150 nm) on the structure and optical properties of the CdS/TiO₂ heterostructure films (HSFs) which fabricated by pulsed laser deposition (PLD) was observed. XRD, FE-SEM, AFM, UV–vis and PL spectroscopy measurements were utilized to characterize structural and optical behaviors of the films. XRD measurement shows gradual increments of the lattice constants of the films with the increase of CdS thickness. The mean values of the calculated lattice constants and cell volume (V) were a=b=0.3785 nm, c=0.9475 nm and V=13.58 nm³ respectively. The average of crystallite sizes estimated for TiO₂ and CdS/TiO₂ at various CdS thickness is 12.20, 13.49, 24.24 and 43.10 nm. FESEM images prove the high quality nanocrystalline nature of the films without cracks and dislocation. The root means square roughness of the films was increased with the increase of CdS thickness as showed by AFM images. UV–vis measurement reveals an improvement in the optical absorbance of HSFs in the range of 380–550 nm due to presence of CdS. Interestingly, the PL intensity was enhanced by a factor of nineteen compare to pure TiO₂ attributed to the charge carrier recombination in the band gap. The current results suggest that possibility to improve the optical and structural properties of the TiO₂ films and also it possible to fabricate high quality CdS/TiO₂ HSFs by variation of the CdS thickness.     

Zero-valent nanophase iron and nitrogen co-modified titania nanotube arrays: Synthesis,characterization, and enhanced visible-light photocatalytic performance

We prepared nano-zero-valent iron (nZVI) and N co-modified TiO₂ (nZVI/N–TiO₂) nanotube arrays as an enhanced visible-light photocatalyst. The TiO₂ nanotube arrays were synthesized by electrochemical anodization of Ti foil in a two-electrode system. Amorphous TiO₂ nanotube arrays were immersed in ammonia and then annealed to produce crystalline N-doped TiO₂ (N–TiO₂) nanotube arrays. nZVI spheres were directly deposited on the N–TiO₂ nanotube arrays by borohydride reduction. The photocatalysts were characterized by field emission scanning electron microscopy (FESEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), UV–visible diffuse reflectance spectroscopy (UV–vis DRS), and electrochemical impedance spectroscopy (EIS). The environmental applicability and photocatalytic activity of the proposed nZVI/N–TiO₂ nanotube arrays were tested by phenol degradation in an aqueous system under UV and visible light irradiation. The phenol degradation rate constants of each sample under visible light irradiation were in the following order: nZVI/N–TiO₂ (k_obs=0.006 min⁻¹⁾>N–TiO₂ (k_obs=0.002 min⁻¹) ⪢ nZVI/TiO₂ (k_obs=0.0003 min⁻¹)>TiO₂ (k_obs=0.0001 min⁻¹). This result can be attributed to the synergistic effect of the N–TiO₂ nanotubes with lower energy band gap and the electron transfer from the conduction band (CB) of N–TiO₂ to nZVI spheres highly-dispersed on the N–TiO₂ for enhanced separation of photogenerated electrons and holes.     

WO3/TiO2 nanocomposites: Salt–ultrasonic assisted hydrothermal synthesis and enhanced photocatalytic activity

A series of WO₃/TiO₂ composite photocatalysts were fabricated via a facile salt–ultrasonic assisted hydrothermal process. The obtained samples were characterized by X-ray diffraction, scanning eletron microscopy, energy dispersive X-ray spectroscopy and UV–vis diffused reflectance spectroscopy. It was confirmed that anatase TiO₂ and monoclinic WO₃ coexisted in the composites. The photocatalytic activity of as-prepared WO₃/TiO₂ composites for degradation of Rhodamin B (RhB) under visible light irradiation was investigated. The results showed that WO₃/TiO₂ composites have a higher photocatalytic activity than those of pure TiO₂ and pure WO₃. First-principle calculations based on density functional theory were performed to explore the electronic structure and illustrate the photocatalytic mechanism of WO₃/TiO₂. The calculated energy gap was 2.53 eV, which was close to the experimental observation (2.58 eV). Due to the combination of WO₃/TiO₂, the photoinduced electrons and holes transfer between the WO₃ and TiO₂ in opposite directions, thus providing sufficient charge separation, which contributed to the photocatalytic activity enhancement.     

Preparation and photocatalytic activity of Ag/bamboo-type TiO2 nanotube composite electrodes for methylene blue degradation

Photocatalysis phenomena in TiO₂ have been intensively investigated for its potential application in environmental remediation. The present work reports improved photocatalytic degradation of methylene blue dye in aqueous solution by using bamboo-type TiO₂ nanotubes deposited with Ag nanoparticles via electrochemical deposition. The photocatalytic processes are performed on Ag-modified TiO₂ bamboo-type nanotube arrays, Ag-modified smooth-walled nanotube arrays, and bare smooth-walled nanotube arrays. Both Ag-modified bamboo-type and smooth-walled nanotube arrays show improved photocatalytic degradation efficiencies (64.4% and 52.6%) compared to smooth-walled TiO₂ nanotubes of the same length (44.4%), due to the enhanced electron–hole seperation and more surface area provided by bamboo ridges. The photocatalytic activity and kinetic behavior of Ag-modified bamboo-type nanotube arrays are also optmized by tuning pulse deposition time of Ag nanoparticles. Bamboo-type nanotubes deposited with Ag nanoparticles via pulse deposition time of 0.5 s/1.5 s shows the highest methylene blue degradation efficiency of 78.5%, which represents 21.9% and 76.8% enhancement of efficiency compared to those of bare bamboo-type and smooth-walled nanotubes, respectively, indicating that a proper amount of Ag nanoparticles on TiO₂ can maximize the photocatalytic processes. In addition, overly long pulse deposition time will not further increase photocatalytic activity due to agglomeration of Ag paticles. For example, when the pulse deposition time is increased to 2 s/6 s, Ag-modified bamboo-type nanotube array exhibits a lower photocatalytic degradation efficiency of 62.9%.     

Sol gel synthesis of cobalt doped TiO2 and its dye sensitization for efficient pollutant removal

This study was concentrated on the synthesis of new cobalt doped (Co-TiO₂) photocatalyst via sol-gel method in the presence of surfactant Triton X100 or Tween 20. Surfactant was used to obtain smaller particles and enhance the photocatalytic action of the catalyst. Co-TiO₂ catalyst was further modified with a phthalocyanine for dye sensitization that facilitates electron transfer. A peripherally tetra 12-crown-4 substituted metal free phtalocyanine (Pc) derivative was first time synthesized and used as dye sensitizer employing same sol-gel method in the presence and absence of above surfactants (Pc/Co-TiO₂). The as-synthesized photocatalysts were characterized by XRD, TGA, SEM, EDX, TEM, BET and UV–vis diffuse reflectance spectroscopy (DRS). The results revealed that the both cobalt and Pc had been successfully immobilized on the TiO₂ nanoparticles and the nanocomposite had the anatase structure. Incorporation of Pc into TiO₂ structure caused significant shift of absorption edge into visible range due to strong absorbance of Pc. Apart from this particle size of Pc immobilized TiO₂ was dramatically decreased particularly in the presence of surfactant. The photocatalytic degradation of methyl orange (MO) dye solution (10 mg/L) and the photocatalytic reduction of Cr(VI) ions (10 mg/L) using Pc/Co-TiO₂ (2 g/L) was investigated under UV light irradiation (λ=365 nm). Photocatalytic studies revealed that the Pc/Co-TiO₂ prepared in the presence of Tween 20 has shown much higher photocatalytic activity than the other catalysts. After 150 min of irradiation, the photocatalytic activity of the catalysts follows the order: Pc/Co-TiO₂-T20>Pc/Co-TiO₂-TX100>Pc/Co-TiO₂>Co-TiO₂-T20>Co-TiO₂-TX100>Co-TiO₂. It was found that about 60.3% of the MO was removed and 71.3% of the Cr(VI) ions were reduced by Pc/Co-TiO₂ after 150 min under 365 nm light irradiation.     

Synthesis of TiO2 thin films with highly efficient surfaces using a sol–gel technique

Three different strong acid catalysts were used in a simple sol–gel synthesis to produce TiO₂ thin films with increased homogeneity and enhanced photocatalytic activity on their mesoporous surfaces. Various techniques were used to characterize the samples, including UV–visible spectrophotometry, X-ray diffraction, micro-Raman spectrometry, photobleaching, scanning electron microscopy, transmission electron microscopy and high-resolution transmission electron microscopy. The band gaps varied from 3.73 to 3.75 eV and the transmittance was >80%. An anatase phase was obtained in all the samples and the crystal size varied from 20 to 45 nm as a function of the annealing temperature. The increase in the efficiency of the surface of the TiO₂ thin films was evaluated by photodegradation of methylene blue in water. The results showed that the acid catalysts used in the synthesis had an important effect on the morphology and photocatalytic activity of the thin films, resulting in more efficient surfaces. Synthesis with hydrofluoric acid produced thin films with a homogenous mesoporous structure and improved the photodegradation of the methylene blue dye to 92% in 2.5 h.     

Synthesis of cysteine,cobalt and copper-doped TiO2 nanophotocatalysts with excellent visible-light-induced photocatalytic activity

Charge separation is a vital process in order to achieve high performance TiO₂-based photocatalysts. In the current study, to enhance the charge separation and suppress the charge recombination, novel nanocomposites of metal (Co and Cu) and nonmetal (C, N, S) doped TiO₂ nanophotocatalyst (labeled as Co@C,N,S-doped TiO₂ and Cu@C,N,S-doped TiO₂) were synthesized via photochemical deposition-assisted sol–gel technique. The crystalline structures and characteristic of the prepared catalysts were analyzed by XRD, SEM, TEM, EDAX, PL, and UV–vis DRS spectroscopies. The photocatalytic performance of the samples was tested through decomposition of methyl orange (MO) aqueous solutions under UV and visible light irradiation. It was found that photodeposition of metals onto TiO₂ slowed charge recombination compared to pure TiO₂ in addition to formation of higher concentration of OH radicals on the TiO₂ photocatalyst surface during light irradiation compared to pure TiO₂. Beyond the optimum content of 2 wt% Cu and Co, a decline in photocatalytic performance was observed which was ascribed to metal agglomeration and blockage of the photosensitive TiO₂ surface.     

High performance dye-sensitized solar cells (DSSCs) achieved via electrophoretic technique by optimizing of photoelectrode properties

This paper describes a simple method utilizing electrophoretic deposition (EPD) of commercial P25 nanoparticles (NPs) films on fluoride-doped tin oxide (FTO) substrate. In this process, voltage and the number of deposition cycles are well controlled to achieve TiO₂ film thickness of around 1.5–26 μm, without any mechanical compression processing. The experimental results indicate that the TiO₂ film thickness plays an important role as the photoelectrode in DSSCs because it adsorbs a large number of dye molecules which are responsible for electrons supply. Furthermore, it was found that effects of the bulk traps and surface states within the TiO₂ films on the recombination of the photo-injected electrons (electron–hole pairs) strongly depend on the TiO₂ electrode annealing temperature. Finally, a DSSC with a 24 μm thick TiO₂ film and annealed at 500 °C produced the highest conversion efficiency (η=6.56%, I_SC=16.4, V_OC=0.72, FF=0.55) with an incident solar energy of 100 mW/cm².     

Hydrothermal synthesis of WO3 films on the TiO2 substrates and their photochromic properties

Tungsten oxide (WO₃) films have been prepared on the synthesized TiO₂ substrates from a sodium tungsten precursor via a hydrothermal method. X-ray diffraction, scanning electron microscopy and transmission electron microscopy analyses were used to investigate the effect of precursor concentrations on the structures and morphologies of the films. Ordered WO₃ films were successfully synthesized on the as-grown TiO₂ substrates. With the concentrations increasing from 0.001 M to 0.024 M, the morphologies of the films changed from multi-layer laminated structure to ladder-shaped lamellar structure finally columnar structure. The results also showed that with an increase in precursor concentration, the observed absorptions at 365 nm of the films increased until precursor concentration of 0.016 M, and then decreased with higher concentration. The film obtained with precursor concentration of 0.016 M on the TiO₂ substrate had the best photochromic properties.     

Polymer films with multilayer low-E coatings

The paper presents the experimental results on depositing a multilayer low-emissivity (low-E) coating with oxide–metal–oxide structure on polyethylene terephthalate (PET) and polyethylene (PE) films by magnetron sputtering. The TiO₂/ZnO:Ga/Ag/ZnO:Ga/TiO₂ coating on the PET film with high water-resistance and capability to be used outside of sealed double-glazed panes was proposed. The optimal thickness of coating layers was experimentally determined. The coating with the optimal structure has 82% transmittance over the visible spectrum and 91% reflection over the infrared spectrum. The window with a PET film and low-E coating was investigated in terms of heat engineering. It was revealed that heat transfer resistance increased up to 0.73 m² °C W⁻¹ for the windows with a PET film and low-E coating. Heat transfer resistance of the windows without a polymer film was 0.38 m² °C W⁻¹. The water-resistant ZnO:Ga/Ag/ZnO:Ga/SiO₂ coating on a PE film with 77% transmittance and 91–92% reflection in the IR range was proposed to be used as greenhouse covering material. The possibility of using the PE film with a low-E coating to reduce heat loss in greenhouses and enhance yielding capacity was demonstrated.     

Enhanced properties for visible-light-driven photocatalysis of Ag nanoparticle modified Bi2MoO6 nanoplates

The visible light driven Bi₂MoO₆ photocatalyst doped with different contents of Ag nanoparticles was successfully synthesized by a combination of hydrothermal and sonochemical methods. The as-synthesized samples were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning and transmission electron microscopy (SEM and TEM) and UV–visible spectroscopy to investigate crystalline structure, morphology, composition and photocatalytic properties. XRD patterns and TEM images of the samples revealed pure phase orthorhombic Bi₂MoO₆ nanoplates without any detection of Ag dopant due to its low concentration and very tiny particle size. TEM images showed that Ag nanoparticles with the size of 10–15 nm were dispersed randomly on the surface of Bi₂MoO₆. The XPS analysis of Ag/Bi₂MoO₆ nanocomposites revealed the presence of additional metallic Ag. Photocatalytic activities of the Ag/Bi₂MoO₆ nanocomposites were evaluated by determining the degradation of rhodamine B (RhB) under visible light radiation. In this research, the 10 wt% Ag/Bi₂MoO₆ nanocomposites showed the best photocatalytic activity. The results suggest that the dispersion of Ag nanoparticles on the surface of Bi₂MoO₆ significantly enhances its photocatalytic activity.     

Effects of Cu/In ratio and annealing temperature on physical properties of dip-coated CuInS2 thin films

CuInS₂ thin films were prepared by sol–gel dip-coating method on glass substrates using 0.75, 1 and 1.25 ratios of Cu/In in the solution. The prepared films were annealed at 380 °C, 420 °C and 460 °C for 30 min under argon environment. The structural, optical, morphological and composition properties of those were investigated by X-ray diffraction (XRD), UV–vis transmittance spectroscopy and scanning electron microscopy with an energy dispersive X-ray spectrometer. The XRD results showed that the films exhibit polycrystalline tetragonal CuInS₂ phase with (112) orientation. According to the EDX results the Cu/In ratios of the films were respectively 0.65, 0.92 and 1.35 for the Cu/In ratios of 0.75, 1 and 1.25 in the solutions. The optical band gap was found to be between 1.30 eV and 1.43 eV, depending on Cu/In ratio.     

Copper-incorporated titania nanotubes for effective lead ion removal

Small copper (Cu²⁺) dopant levels were successfully diffused into titanium dioxide (titania or TiO₂) nanotube lattice via an incipient wet impregnation technique. This study investigated the optimum Cu²⁺ dopant content to be incorporated into the TiO₂ nanotubes to achieve an effective lead ion (Pb(II)) removal system. The exciton states of the PL intensities varied in the following order: pure TiO₂>0.6 M Cu–TiO₂>0.1 M Cu–TiO₂>0.06 M Cu–TiO₂>0.01 M Cu–TiO₂. The significant quenching of the PL intensity indicates that incorporation of the appropriate amount of Cu²⁺ dopants into the TiO₂ lattice markedly enhanced the charge-carrier separation and transport. The photocatalytic ability of the samples was evaluated by the removal of the Pb(II) ions under UV illumination. The results show that the Cu dopants in the TiO₂ lattice at the optimum concentration (0.8 at%) acted as photoinduced electron mediators and thus increased the Pb(II) ion removal efficiency. The maximum Pb(II) ion removal rates for the 0.01 M Cu–TiO₂ nanotubes and after five hours of UV illumination were approximately 56.3% and 79.5% at pH 5 and pH 11, respectively. The generation of strong oxidizing agents (OH radicals) effectively reduces the toxic Pb(II) ions into PbO/PbOH.     

H2S sensing using in situ photo-polymerized polyaniline–silver nanocomposite films on flexible substrates

We demonstrate the preparation of flexible polyaniline–silver (PANI–Ag) nanocomposite films via an in situ facile UV induced polymerization of aniline in presence of AgNO₃. The flexible substrates used were (3-aminopropyl)trimethoxysilane (APTMS) modified biaxially oriented polyethylene terephthalate (BOPET) substrates. The APTMS modification of BOPET surface has two advantages: (i) improved adhesion of the films, and (ii) directional growth of polymer perpendicular to the substrate plane, leading to nanobrush-like morphology. The PANI–Ag films have been characterized by various techniques, such as, UV/Vis, FTIR, Raman, SEM and XPS. These films were found to be highly selective and sensitive to the H₂S i.e. chemiresistive response of ∼100% at 10 ppm with a reasonably fast response time of 6 min. PANI–Ag films prepared on pristine BOPET exhibits chemiresistive response of ∼67% at 10 ppm of H₂S exposure. In contrast pure PANI films did not exhibit any response on exposure to H₂S. The plausible mechanism(s) of H₂S sensing have been discussed. This study highlights the importance of surface modification and the role of Ag in PANI matrix for H₂S sensing.     

Effect of substrates on epitaxial PZT films by a coating photolysis process

Preparation of Pb(Zr,Ti)O₃ (PZT) films on single crystalline STO(0 0 1), LAO(0 0 1) and MgO(0 0 1) substrates was investigated by a coating photolysis process using excimer laser. The effects of the substrate on the product films were examined by FT-IR, UV, XRD (theta–2theta, pole-figure analysis). In the case of using STO and LAO substrates, c-axis oriented films were successfully obtained by an ArF laser irradiation without heat treatment. Crystallinity of the PZT films prepared by a coating photolysis was found to strongly depend on the substrate used compared to conventional thermal process. Using XRD pole-figure analysis for the PZT films on STO substrates, it was found that the films were highly in-plane aligned. The formation mechanisms by a coating photolysis process are also discussed by a photothermal reaction due to both substrate materials and MO starting materials.     

Nb-doped TiO2 sol–gel films for CO sensing applications

Nb doped titania (TiO₂:Nb) multilayered films (1–10 layers) with anatase structure were obtained by the low-cost sol–gel and dipping method on microscope glass substrates, followed by thermal treatment at 450 °C for 1 h. After each layer deposition, an intermediate annealing step was performed at 300 °C for 30 min. Doping TiO₂ sol–gel films with a low amount of Nb (0.8 at%) allows obtaining an improved CO sensor able to operate under environmental atmosphere (air). It was found that the sensor sensitivity is less dependent on the film thickness but is significantly influenced by Nb doping at the optimal working temperature of 400 °C. Good recovery characteristics were obtained for a wide CO detection range, between 0 and 2000 ppm. The gas-sensing behavior of the films was correlated with the structural, chemical and morphological properties of the multi-layered structures.     

Effect of the film thickness on the crystal structure and ferroelectric properties of (Hf0.5Zr0.5)O2 thin films deposited on various substrates

In this work, the effect of the film thickness on the crystal structure and ferroelectric properties of (Hf_0.5Zr_0.5)O₂ thin films was investigated. The thin films were deposited on (111) Pt-coated SiO₂, Si, and CaF₂ substrates with thermal expansion coefficients of 0.47, 4.5, and 22×10⁻⁶/°C, respectively. From the X-ray diffraction measurements, it was found that the (Hf_0.5Zr_0.5)O₂ thin films deposited on the SiO₂ and CaF₂ substrates experienced in-plane tensile and compressive strains, respectively, in comparison with the films deposited on the Si substrates. For films deposited on all three substrates, the volume fraction of the monoclinic phase increased with increasing film thickness, with the SiO₂ substrate having the lowest monoclinic phase volume fraction at all film thicknesses tested. The grain size of the films, which is an important factor for the formation of the ferroelectric phase, remained almost constant at about 10 nm in diameter regardless of the film thickness and type of substrate utilized. Ferroelectricity was observed for the 17 nm-thick films deposited on SiO₂ and Si substrates, and the maximum remanent polarization (P_r) value of 9.3 µC/cm² was obtained for films deposited on the SiO₂ substrate. In contrast, ferroelectricity with P_r=4.4 µC/cm² was observed only for film on SiO₂ substrate in case of 55 nm-thick films. These results suggest that the films under in-plane tensile strain results in the larger ferroelectricity for 17 nm-thick films and have a ferroelectricity up to 55 nm-thick films.