Nanostructured TiO2 thin films by chemical bath deposition method for

Materials Research Express

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Nanostructured TiO2 thin films by chemical bath deposition method for high photoelectrochemical performance To cite this article before publication: S V KITE et al 2018 Mater. Res. Express in press https://doi.org/10.1088/2053-1591/aaed81

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Photoelectrochemical Performance

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Nanostructured TiO2 Thin Films by Chemical Bath Deposition Method for High S.V. Kite1, D.J. Sathe2, S.S. Patil3, P.N. Bhosale3, K.M. Garadkar1*

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1: Nanomaterials Research Laboratory, Department of Chemistry, Shivaji University, Kolhapur, India. 2: Department of Chemistry, KIT's College of Engineering (Autonomous), Kolhapur, India.

3: Materials Research Laboratory, Department of Chemistry, Shivaji University, Kolhapur, India.

Abstract

TiO2 thin films were deposited by simple chemical bath deposition method onto conducting and non-conducting glass substrates. The resulting films were annealed at 300°, 400°, and 500 °C for 3 hr. In

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XRD studies, crystallite size and microstrain from X-ray line broadening profile were estimated by Scherrer’s and Williamson-Hall plot method. Also, structural parameters such as unit cell volume, lattice constants, dislocation density, and stacking fault probability have been calculated. Optical absorption spectra illustrate that the band gap energy has been decreased from 3.45 to 3.19 eV with increasing

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annealed temperatures. Furthermore, the photoluminescence (PL) spectra exhibited two luminescence centers, one is for near-band-edge emission (NBE) and another is for deep level emission (DLE). From FT-IR spectrum of 400 °C annealed film, it was found that strong band around at 609 cm-1 is associated with the characteristic vibrational mode of anatase TiO2. The morphology and elemental composition of the 400 °C annealed film were investigated by field emission scanning electron microscopy (FESEM) and energy dispersive X-ray (EDS) analysis respectively. Present work deals with a significant

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photoelectrochemical cell performance of TiO2 thin films annealed at 400 °C and explored the cell parameters like conversion efficiency (η) and fill factor (FF). Finally, it was found that the deposited TiO2 films showed the outstanding PEC performance with supreme photoconversion efficiency 1.94 %.

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Keywords: Thin films; Anatase TiO2; Chemical bath deposition; Microstrain; Photoconversion efficiency.

*Corresponding information K.M. Garadkar

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AUTHOR SUBMITTED MANUSCRIPT - MRX-110058.R1

E-mail: [email protected]

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1. Introduction Globally the energy is a fundamental factor in the progress of any nation. The energy demand everywhere is mounting day by day because of population growth and rapid industrialization. To resolve

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the energy requirement problem is one of the biggest challenges to all nations. In addition to that, nonrenewable energy sources like fossil fuel, coal, and many others are restricted sources. These limited energy sources, mainly responsible for global warming and dangerous penalty in the environment through pollution. In contrast, non-conventional, most abundant, clean, natural, non-polluting, and sustainable source of solar energy can prove to have an enormous amount of optimistic and beneficial impact on the environment. Solar energy is one of the key sources, which is on the way of minimizing globally elevated energy problem. Solar energy is to converts a useful energy form by using

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photovoltaic, photothermal and photoelectrochemical processes.

TiO2 is a promising semiconducting material play a crucial role converts solar energy into electrical energy. The properties of the TiO2 thin films mainly depend on the deposition method and annealing temperatures. A wide range of chemical and physical deposition methods has been used for

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the synthesis of TiO2 thin films, such as the chemical bath deposition [1, 2], chemical vapor deposition [3, 4], hydrothermal [5, 6], spin coating [7, 8], pulsed-DC magnetron sputtering [9], dip coating [10, 11], sol-gel [12-14], spray pyrolysis [15], successive ionic layer adsorption and reaction method [16, 17]. In general, TiO2 exists three crystalline polymorphs inclusive of orthorhombic brookite (space group = Pcab), tetragonal anatase (space group = I41/amd), and tetragonal rutile (space group = P42/mnm) that is relying on the annealing temperatures [18]. In addition, the semiconducting anatase phase of TiO2

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was always in focus due to their outstanding chemical, optical, electrical, and electronic properties. Moreover, the ability of TiO2 to absorb UV light energy can provide a significant improvement in the chemical stability, mechanical durability. This property has established the use of TiO2 potential applications in various fields. At present, the whole world is facing a serious pollution problem because of an enormous amount of harmful gases, substances, chemicals released into the environment. Due to

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high photocatalytic activity of anatase TiO2 play an important role to overcome environmental pollution problem by photocatalysis [19-22], using TiO2 thin film as an active photoelectrode the photoelectrochemical (PEC) cells were fabricated [23, 24], Waveguiding applications [25, 26], gas sensor applications due to the TiO2 layers show a better surface interaction with reducing or oxidizing

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gases, which influences at the conductivity of the film [27, 28], and alcohol microsensor [29].

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TiO2 has been considered as most suitable working photoanode for photoelectrochemical performance. According to the literature survey, photoconversion efficiency of TiO2 photoanode has been reported by various deposition techniques. Recently, Dubal and co-workers prepared the TiO2

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electrode with a maximum energy conversion efficiency of 0.037% by controlled precipitate route [30]. Bhat et al. [31] fabricated mesoporous architecture of TiO2 microspheres photoelectrode via chemical bath deposition method with a photocurrent 531 µA/cm2 and the efficiency of 0.35%. Also, Bhat et al. fabricated photoelectrochemically active TiO2 by the hydrothermal method, which exhibited power conversion efficiency of 0.05% [32]. Efforts have also been made by Pawar et al. [33], implemented to grow crystalline TiO2 nanorods on FTO substrate by the hydrothermal technique with reported energy conversion efficiency of 0.10%. Desai et al. [34] investigated the improved PEC behavior of TiO2 via

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single-step hydrothermal technique which is attributed to the increases deposition temperature results photoconversion efficiency increases from 0.33-0.47%. Patil et al. [35] investigated extensively enhanced photoresponse of TiO2 by using a hydrothermal method. The earliest report on the photosensitive nanograined TiO2 thin films via successive ionic layer adsorption and reaction (SILAR)

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technique was published by Patil et al. [17], which achieved energy conversion efficiency of 0.047%. In a recent article, Wagh et al. [36] developed HgS sensitized TiO2 photoanode by sol-gel spin coating for PEC performance offering photoconversion efficiency of 0.39%. Sadikin et al. in 2017 synthesized TiO 2 photoanode on ITO substrate via sol-gel spin coating method and reported 0.70% conversion efficiency in PEC cell with the addition of N719 dye [37]. The Kikuchi et al. [38] reported TiO2 by RF reactive magnetron sputtering technique having 0.33% photoconversion efficiency. Recently, Ayal et al [39] and

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Aljohani et al. [40] successfully synthesized TiO2 thin film by electrodeposition method for solar cell application. The TiO2 photoanode showed a conversion efficiency of 0.02% and 1.12% respectively. Shinde et al. [41] obtained 71.24% Incident photon to current conversion efficiency (IPCE) for TiO2 thin films deposited by a spray pyrolysis technique. In this study, we investigated the PEC performance of

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TiO2 thin films prepared by chemical bath deposition method and annealed at 400 °C. The present work is based on the deposition of TiO2 thin films using non-aqueous chemical bath

deposition method. When precursors with strong reactivity towards water are used, the hydrolysis reaction can result in precipitation. Therefore, this problem can be overcome by using non-aqueous

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chemical deposition method for the deposition of TiO2 thin films onto the conducting and non conducting glass substrates, additionally annealing effect on the properties of the TiO2 films were reported. The employed method was very simple, low-temperature, inexpensive chemical bath 3


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deposition method for the synthesis of TiO2 thin films. This technique involves controlled precipitation of a compound from the solution on a suitable substrate with uniform, well adherent, large area deposition, and reproducible thin films.

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2. Experimental Details 2.1. Materials

All chemicals used in this work for the deposition of TiO2 thin films were analytical grade. Titanium tetra isopropoxide (TTIP) was purchased from Sigma-Aldrich Chemie (India); Propan-2-ol was obtained from Thermo Fisher Scientific Pvt. Ltd., (India), and Ethanol from Changshu Hongsheng fine chemical Co Ltd., Jiangshu Province (China). The conducting fluorine doped tin oxide coated (FTO) glass (dimensions 60 mm × 15 mm × 3.2 mm, surface resistivity ~8 Ω/sq, Transmittance 80-

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81.5%) was purchased from Sigma-Aldrich (USA) as well as commercial non-conducting microscopic glass (75mm × 25mm × 2 mm) has been used as a substrate. Before the deposition, the substrates were cleaned for proper nucleation by using chromic acid, detergent, distilled water, acetone, ethanol, and

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finally allowed air dry to remove the negligible amount of surface contaminants. 2.2. Deposition of TiO2 thin films

For the deposition of TiO2 thin films, simple chemical bath deposition method was used. The chemical bath was prepared by using titanium tetra isopropoxide (TTIP) as a precursor, propan-2-ol and ethanol (EtOH) as solvents in the volume ratio 1: 2: 8. Initially, TTIP mixed with propan-2-ol (ratio 1: 2) at constant stirring for 10 min. at room temperature to get a clear yellowish solution. After that, EtOH was added to the as-prepared solution, to maintain the ratio between TTIP and EtOH was 1: 8. The

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obtained solution was kept at room temperature with constant stirring to get a clear and homogeneous solution. To prepare TiO2 thin films, the substrate was dipped vertically inside the resulting chemical bath for 5 minutes and then substrates pulled out and allowed to dry. Following this route for 2 to 3 times and finally, thin uniform films were obtained. After completion of the process, the films were

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withdrawn from the bath, sufficiently rinsed with the distilled water and dried in the oven at 100 °C for 2 hr. and subsequently annealed at 300°, 400°, and 500 °C for 3 hr. as shown in Fig. 1. 2.3. Characterization Techniques The deposited TiO2 thin films were characterized for their structural, optical, morphological, and

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compositional properties. The crystalline structure and crystallographic data obtained by using X-ray diffractometer (Ultima IV of Rigaku Corporation, Japan) with Cu (Kα) (λ=1.54056 Å). Optical 4

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absorbance spectra of TiO2 were obtained using (UV-VIS (DRS)-NIR V-77O, JASCO, Japan spectrophotometer) in the wavelength range from 200 to 800 nm. The PL spectra were obtained under 340 nm excitation and the signal was recorded using (Spectrofluorometer-FP-8200, JASCO, Japan) in

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the wavelength range from 200 to 800 nm. Fourier transform-infrared (FT-IR) spectrum was taken using (FT-IR JASCO-4600) scanning from 4000 to 400 cm-1 in the % transmittance mode. The surface morphology of films was studied using (FESEM-JEOL JEM-6360 Mira-3, Tascan, Czech Republic). Energy Dispersive X-ray Spectroscopy (EDS) was used to determine the chemical composition of the deposited TiO2 films. The PEC measurement was carried out by irradiating the TiO2 photoanode with UV light recorded at the electrochemical workstation (Autolab PGSTAT 100 Potentiostat).

3.1.

Results and discussions

X-ray diffraction (XRD) analysis of TiO2 thin films

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3.

The crystal structure and phase of the deposited TiO2 thin films were investigated using X-ray diffractometer. XRD pattern of TiO2 thin film annealed at 300 °C for 3 hr. as shown in Fig. 2(a). It

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shows presence of diffraction peaks at 25.29°, 36.91°, 37.78°, 38.51°, 48.02°, 53.92°, 55.01°, 62.73°, 68.81°, 70.27° and 75.12° were indexed to (101), (103), (004), (112), (200), (105), (211), (204), (116), (220), and (215) (hkl) planes respectively [42]. The strong and sharp diffraction peaks indicate the materialization of well crystallized TiO2 thin films. These peaks correspond to the crystal structure of tetragonal anatase phase, no peaks analogous to the brookite and rutile phase indicating the high purity of the film. Correspondingly carried out the XRD patterns of TiO2 thin films annealed at 400 °C Fig.

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2(b), annealed at 500 °C Fig. 2(c). There are appreciable similarities in their structures, which showed that the three samples are of similar material. For further details, the interplanar spacing between atoms (d-spacing) for TiO2 thin films annealed at different temperatures is calculated using Bragg’s law(equation 1): nλ = 2d sin θ

(1)

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The observed d-values of the deposited TiO2 thin film were compared with JCPDS Card No. 21-1272, which is good agreement with the standard d-values of tetragonal anatase phase shown in Table 1. Also, microstructural properties of some (hkl) planes of TiO2 thin films annealed at various temperatures were summarized in Table 2. The lattice constants (a, c) and unit cell volume of the TiO2 films have been

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determined from interplanar spacing (d-spacing) of different (hkl) planes by using equations (2) and (3) respectively:

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Unit Cell Volume = a²c

(2)

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1 h² + k² l² = + … . . (For lattice constant a, c) d² a² c²

(3)

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The unit cell parameters were obtained for TiO2 thin film annealed at 300 °C, a = 3.7850 Å, c = 9.5157Å and unit cell volume = 136.32 Å3, these results were good concurrence with JCPDS card number 211272. Furthermore, lattice constants and unit cell volume of TiO2 thin films annealed at 400 °C and 500 °C are summarized in Table 3.

The crystallite size (D) of TiO2 thin films annealed at 300, 400, and 500 °C have been estimated from the full-width at half-maximum(FWHM) of the diffraction peaks by using Scherrer’s equation [43, 44]: 0.9λ β cos θ

(4)

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D=

Where D is the crystallite size (nm), β is the full-width at half-maximum (FWHM) diffraction peaks (radians) and λ is the wavelength of Cu (Kα) (λ=1.54056 Å). The average crystallite size (D) found to be

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21.77, 22.28, and 22.53 nm for film annealed at 300, 400, and 500 °C respectively (Table 3). It indicates the increase in crystallite size by increasing the annealing temperature of the TiO2 thin films, which might be due to the recrystallization of the film [43].

A dislocation is a crystallographic defect or irregularity within a crystal structure which strongly influences on the properties of the material. The dislocation density is the length of dislocation lines per

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unit volume of the crystal. The dislocation density (δ) was calculated using the relation [44, 45]: n δ= (5) D² Where n is the factor which is equal to unity for minimum dislocation density, δ is the dislocation density in ‘line/ m²’, and D is particle size in ‘nm’. The effect of crystallite size of different (hkl) planes on dislocation density of TiO2 thin films shown in Fig.3 (a-c) as well as the influence of the annealing temperature on average dislocation density of films shown in Fig. 4. Moreover, at the low annealing

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temperature, surface atoms had much less energy, so the surface mobility of atoms turned into low, resulting in the generation of defects in TiO2 films. On the contrary, as increasing annealing temperature, since those atoms gained as well as supplied enough kinetic energy for crystallite growth and surface mobility to occupy stable positions inside the crystals with lowering the lattice imperfection

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(defects, impurities) present in the TiO2 films. This helped in the enhancement of crystallite size and decrease dislocation density of TiO2 thin films with increasing annealing temperature [46]. (Table 3) 6

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Lattice microstrain arises from displacements of the unit cells about their normal positions. The presence of three types of imperfections like dislocation, stacking fault probability and lattice distortion in the TiO2 crystal, which can be responsible for the formation of strain in crystals. Microstrain is

Ɛ =

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calculated by using equation [45]: β cos θ 4

(6)

The microstrain of thin films is decreased with increasing annealing temperature (see Table 3). In addition, for more elucidation, the crystallite size and microstrain are calculated using, a WilliamsonHall plot method [47]. β cos θ =

0.89λ + 4Ɛ sin θ D

(7)

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The βcosθ is plotted with respect to 4sinθ for the preferred orientation peaks of TiO2 thin films shown in Fig. 5(a-c). The slope and intercept of the linear fit give microstrain (Ɛ) and crystallite size (D in Å) respectively. The plots showed a negative microstrain for the films due to the lattice shrinkage that was observed in the lattice parameters and indicates the broadening from a small internal strain, therefore,

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result suggest for the compressive strain in the crystal lattice of the film [47]. The feasible reason for the decrease in microstrain is that the increasing annealed temperature will increase crystallite size which impacts on grain size consequently the grain boundary decreases [46]. A comparison of the standard 2θ value with the observed 2θ value shows the occurrence of the slight peak shift due to the stacking fault. The following equation that gives stacking fault probability (α) can be given as follows [48]. 2π2

45√3 tan θ

]β

(8)

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α=[

The stacking fault probability of the TiO2 thin films are highlighted in Table 2. Stacking fault probability (α) is the fraction of layers that undergo sequential stacking faults in a crystal. The stacking fault is found to be decreased with increasing temperature. The variation of microstructural parameters

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like lattice constant, unit cell volume, crystallite size, microstrain, dislocation density and stacking fault with annealed temperature is given in Table 3.

3.2. UV-Visible absorbance spectra of TiO2 thin films UV-Visible absorbance spectra of TiO2 thin films were obtained in the wavelength ranged from

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200-800 nm. The spectra of films exhibit strong absorption peaks in the range of 310 to 340 nm, which is due to the excitation of electrons from the valence band to the conduction band by absorbing light 7


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shown in Fig. 6. Confirmation of the synthesized TiO2 films was exhibited by the blue shift absorption at 316.53, 336.40, 335.42, and 333.76 nm for as-deposited film, films annealed at 300°, 400°, and 500 °C respectively. TiO2 films show absorbance in the UV region, which is the characteristic of TiO2 thin

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films. Thus, the optical absorption decreases effectively with an increase in annealing temperature as well as the shift of the absorption peak observed due to the change in band gap energy (see Fig. 6). The value obtained for the optical band gap is 3.45 eV, 3.24 eV, 3.21 eV and 3.19 eV for as-deposited film, films annealed at 300°, 400°, and 500 °C respectively as represented in Fig. 7. After annealing of films, it was found that the optical band gap decreases from 3.45 to 3.19 eV after annealing. It occurs due to the quantum-size effect (quantum confinement phenomenon) and increases in the particle or grain size of TiO2 on the surface of the substrate [49, 50].

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3.3. Photoluminescence spectra (PL) of TiO2 thin films

Photoluminescence spectra of TiO2 thin films are obtained in the wavelength range 200-800 nm with an excitation wavelength of 340 nm (see Fig. 8). The PL spectra of TiO2 films showed two

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emission peaks, which can be located at UV and visible region. In the UV region, the Ist sharp emission peak was observed between 317 to 338 nm for different annealed temperatures. This emission peak can be recognized as near-band-edge emission (NBE) because of the recombination of free exciton by the exciton-exciton collision process. The presence of IInd weak emission peak in the visible region was observed between 636 to 676 nm (red) corresponds to the deep level emissions (DLE) for different annealed temperatures of TiO2 thin films could be attributed to the crystalline defects and impurities

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present in the films [51]. The UV emission peak is shifted from 318 to 338 nm with increasing annealing temperature, which is due to the relaxation of strain and an increase in the grain size. Moreover, the deep level emission (DLE) peaks position shifted from 636 to 676 nm after annealing because of doublyionized and single-ionized interstitial Ti vacancies and oxygen vacancies in the TiO2. 3.4. Fourier Transform-Infrared (FT-IR) spectrum of TiO2 thin films

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The FTIR spectrum of TiO2 thin film annealed at 400 °C was shown in Fig. 9. The observed

strong band around 609 cm-1 is associated with the Ti-O-Ti stretching vibration, which indicates the formation of anatase TiO2 [52-54]. The band at 3440 cm-1 due to the stretching vibration of O-H groups. The band centered at 1626 cm-1 corresponds to the bending vibrations of H-OH of adsorbed water. The

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bands at 1346 cm-1, 2340 cm-1, 2918 cm-1 are attributed to the presence of organic residues [54].

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3.5. Field Emission Scanning Electron Micrographs (FE-SEM) of TiO2 thin films The surface morphology of TiO2 thin film was studied by field emission scanning electron microscopy (FE-SEM) with the magnified images (magnifications at 25KX and 50 KX). Fig. 10 shows

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the FE-SEM images of TiO2 thin films annealed at 400 °C for 3 hr. The result revealed that the glass substrate is entirely covered with spherical nanograins with uniform in size TiO2 particles [15]. Due to the larger surface-to-volume ratio, the surface activity of the film enhanced which leading to a higher degree of UV light absorption takes place.

3.6. Energy Dispersive Analysis by X-ray (EDS) spectrum of TiO2 thin films

The chemical composition of 400 °C annealed TiO2 thin film was determined by energy dispersive spectrometer (EDS) in the binding energy region of 0-10 keV. In Fig. 11, the observed peaks

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at about 0.5 and 4.5 keV, which has assigned to oxygen and titanium respectively [15, 34]. The occurrence of the peak at 1.8 keV, due to the presence of silicon in glass substrate. Also, spectrum demonstrated some secondary impurities like Na, Mg and Ca peaks, which are the most common

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elements presented in the glass substrate. From the EDS spectrum, it is observed that the TiO2 has a nearer stoichiometric ratio of Ti (24.54 %) and O (51.66 %) which is indicated or evidence of oxygen deficiency.

3.7. Photoelectrochemical cell performance of TiO2 thin films The method of chemical bath deposition is used to assemble the TiO2 photoelectrode on Fluorine-doped tin oxide (FTO) substrates. The PEC performance of the sample was measured by an

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electrochemical workstation using the two probe electrode system. TiO2 thin films annealed at 400 °C photoelectrode with an active surface area of 1 cm2 was used as the working electrode and graphite as the counter electrode. Based on these materials, the PEC performances of the TiO2 photoelectrode were measured in 0.1 and 0.5 M NaOH electrolytes. The electrochemical workstation was used to measure dark and illuminated current with a 5mW/cm2 light intensity which is shown in Fig. 12. In PEC cell two

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components are playing a significant role, the first one is a light absorber TiO2 semiconductor which generates electron-hole pairs in the depletion region and in diffusion layer during illumination. Second is the electrolyte that facilitates the charge transfer (usually a redox couple), which forms a junction in contact with the semiconductor and produces an electric potential difference across the interface. This

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generates photovoltage under open circuit (Voc) and photocurrent under short circuit (Isc).

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In this work, we have studied the effect on the PEC performance of nanostructured TiO2 photoanode by varying concentrations of electrolyte. So we first fabricated two electrode cell configurations, Glass/FTO/TiO2/NaOH (0.5M)/Graphite, and the corresponding photocurrent density(J-V)

characteristics

curves

are

shown

in

Fig.

13.

Then

we

fabricated

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voltage

cell,

Glass/FTO/TiO2/NaOH (0.1M)/Graphite, and photocurrent density-voltage characteristics curves are shown in Fig. 14. The dark photocurrent density of TiO2 photoelectrode in both the electrolytes is negligible, indicating no apparent change. In Table 4 short-circuit current density (Jsc), open circuit potential (Voc), fill factor (FF), and the power conversion efficiency of both cells obtained under the illumination are summarized. The fill factor and conversion efficiency (ɳ) are calculated by using equation (9) and (10) respectively [35].

η (%) =

JMax × VMax × 100 JSC × VOC

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Fill Factor (%) =

JSC × VOC × Fill Factor PInput

(9) (10)

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Where JMax is the maximum current density, VMax is the maximum voltage and PInput is the input light intensity. The J-V characteristics are mainly dependent on the series resistance (Rs) and shunt resistance (Rsh) was estimated from the slopes of power output characteristic curves. The ideal solar cells have shunt resistance value approaching infinity and series resistance near to zero. The series resistance estimated from the inverse slope at a positive voltage, where the current density-voltage curves become linear. The shunt resistance can be resulting through taking the inverse slope of the

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current density-voltage curves near to zero voltage (see Table 4). TiO2 has been an excellent semiconductor material for the PEC cell due to high thermal and chemical stability as well good light absorption capacity. The larger surface-to-volume ratio, surface activity of the film enhanced which leading higher degree of UV light absorption. The interlinked spherical TiO2 nanostructured aggregates have confirmed high conversion efficiencies, due to their ability to generate effective light scattering

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which allows a high internal surface area. Under the illumination, photocurrent density of TiO2 electrodes increases with decreasing electrolyte concentration, which indicates that both J sc and Voc parameters strongly depend on the concentration of electrolyte (Fig. 15). Comparing to that photoelectrode exposed in both electrolytes, in 0.5 M NaOH electrolyte, the TiO2 thin film reaches only

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0.2451 mA/cm2 short-circuit current density with Voc of 821.68 mV. Obviously enhancement on the PEC performance has been achieved in the 0.1 M NaOH with Jsc of 0.3127 mA/cm2 and Voc of 846.86 mV. However, also the highest energy conversion efficiency (1.94%) and fill factor was found to be a 10

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TiO2 electrode with 0.1 M NaOH electrolyte. It may be due to the two reasons, firstly it allows a fast and efficient transfer of the photogenerated electrons through the compact layers to the conducting substrate with minimum loss, which can efficiently suppress the recombination of holes and electrons. On the

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other hand, generate a higher resistance to transfer excited electrons reverse to the electrolyte, and which exhibits better PEC performance. Finally, overall experimental outcomes, be a sign of there is a distinct dependency of the PEC performance on electrolyte and morphology of TiO2 thin films. On the basis of this aspect, 0.1 M NaOH was the best electrolyte for the TiO2 semiconductor and exhibited corresponding energy conversion efficiency of 1.94%. 4.

Conclusions

In this study, TiO2 thin film has been successfully deposited by simple, inexpensive, low-

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temperature chemical bath deposition method. The impacts on structural, optical, morphological, and photoelectrochemical cell properties were systematically were studied. XRD patterns of the films revealed the existence of polycrystalline in nature with tetragonal anatase phase and the crystallinity of

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the films improves with annealed temperature. The decreases dislocation density, microstrain, and stacking fault with an increase in crystallite size of the TiO2 thin film. The optical studies show that the band gap energy decreases from 3.45 to 3.19 eV after annealing due to quantum size effects. The PL spectra showed two emission peaks, which can be located at UV and visible region. In FTIR spectra, the existence of Ti-O-Ti stretching vibration band around 609 cm-1, which is a characteristic of anatase TiO2. The FESEM studies showed spherical morphology and uniform size distribution of TiO2

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nanoparticles. The EDS pattern confirms the presence of Ti and O in deposited samples. The PEC cell performance demonstrates the surface morphology and electrolytes responsible to improve conversion efficiency. The significant improvement in efficiency, due to the spherical morphology provides a higher surface area for charge separation and electron collection. Highest photoconversion efficiency for

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TiO2 thin film annealed at 400 °C is found to be 1.94 % by using 0.1 M NaOH electrolyte.

Acknowledgment

Authors are thankful to SERB-DST, New Delhi for the financial support under project NO.SB/FT/CS018/2013.

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by the JNS pyrolysis technique for P-N junction diode application Mater. Sci. Semicond. Process. 43 104-13 [49]

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Chacko L and Aneesh P M 2018 Effect of growth techniques on the structural and optical properties of TiO2 nanostructures Mater. Res. Express 5 1-14

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an

magnetic property enhancement with Er3+ doping, of sol–gel TiO2 thin films Mater. Res. Express 4 1-10 [52]

Govindasamy G, Murugasen P and Sagadevan S 2016 Investigations on the synthesis, optical

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and electrical properties of TiO2 thin films by chemical bath deposition (CBD) method Mat. Res.

Sarode M T 2012 Effect of annealing temperature on optical properties of titanium dioxide thin films prepared by sol-gel method Int. J. Mod. Phys. Conf. Ser. 6 13-18 Bouachiba Y, Hanini F, Bouabellou A, Kermiche F and Taabouche A 2013 TiO2 thin films

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studied by FTIR, AFM and spectroscopic ellipsometry Int. J. Nanoparticles 6 169-77

ce

[54]

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Page 16 of 28

16

Figure Caption

Fig. 1 TiO2 thin films deposited on nonconducting glass substrate, (a) As- deposited, Annealed at (b)

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300 °C, (c) 400 °C, (d) 500 °C.

Fig. 2 X-ray diffraction patterns of TiO2 thin films deposited on nonconducting glass substrate and annealed at (a) 300 °C, (b) 400 °C, (c) 500 °C.

Fig. 3 Effect of crystallite size on dislocation density of TiO2 thin films annealed at, (a) 300 °C, (b) 400 °C, (c) 500 °C.

Fig. 4 Effect of annealing temperature on dislocation density of TiO2 thin films annealed at,

an

300 °C, (b) 400 °C, (c) 500 °C.

Fig. 5 Williamson- Hall plot of TiO2 thin films annealed at, (a) 300 °C, (b) 400 °C, (c) 500 °C. Fig. 6 UV-Visible absorption spectra of TiO2 thin films deposited on nonconducting glass substrate

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and annealed at various temperatures.

Fig. 7 Determination of the band gap energy from the variation of (αhν) 2 according to photon energy (hν) for TiO2 thin films, (a) As-deposited film, Annealed at (b) 300 °C, (c) 400 °C, (d) 500 °C. Fig. 8 Photoluminescence spectra of the TiO2 thin films deposited on nonconducting glass substrate, (a) As-deposited film, Annealed at (b) 300 °C, (c) 400 °C, (d) 500 °C.

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Fig. 9 Fourier transform infrared (FTIR) spectrum of 400 °C annealed TiO2 thin film deposited on nonconducting glass substrate.

Fig. 10 Field emission scanning electron microscopy (FE-SEM) images of 400 °C annealed TiO2 thin film deposited on nonconducting glass substrate.

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Fig. 11 Energy dispersive X-ray (EDS) analysis of 400 °C annealed TiO2 thin film deposited on nonconducting glass substrate.

Fig. 12 Schematic representation of 400 °C annealed TiO2 thin film as a working electrode in a photoelectrochemical cell.

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17


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Fig. 13 Current density-voltage (J-V) characteristics of 400 °C annealed TiO2 thin film deposited on FTO glass substrate under dark and illumination of light in 0.5M NaOH electrolyte.

Fig. 14 Current density-voltage (J-V) characteristics of 400 °C annealed TiO2 thin film deposited on

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FTO glass substrate under dark and illumination of light in 0.1M NaOH electrolyte.

Fig. 15 Comparatively studies of Current density-voltage (J-V) characteristics of 400 °C annealed TiO2

Fig. 1

ce

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an

thin film in, (a) 0.5M NaOH electrolyte, (b) 0.1M NaOH electrolyte.

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18

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Fig. 2

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Fig. 3

19

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Fig. 4

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Fig. 5

20

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Fig. 6

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Fig. 7

21

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Fig. 8

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Fig. 9

22

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Fig. 10

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Fig. 11 23

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Fig. 12

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Fig. 13 24

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Fig. 14

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Page 25 of 28

Fig. 15

25


pte

Annealed at 400 °C

ce

Annealed at 500 °C

25.271 36.932 37.786 38.560 48.030 53.869 55.038 62.663 68.732 70.279 74.997 25.271 36.932 37.786 38.560 48.030 53.869 55.038 62.663 68.732 70.279 74.997 25.271 36.932 37.786 38.560 48.030 53.869 55.038 62.663 68.732 70.279 74.997

26

d-spacing (Å) Observed Standard

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Annealed at 300 °C

25.29 36.91 37.78 38.51 48.02 53.92 55.01 62.73 68.81 70.27 75.12 25.29 36.91 37.80 38.5 48.04 53.9 55.03 62.74 68.81 70.27 75.14 25.28 36.9 37.77 38.51 48.01 53.92 55.03 62.73 68.81 70.27 75.12

( hkl )

(101) (103) (004) (112) (200) (105) (211) (204) (116) (220) (215) (101) (103) (004) (112) (200) (105) (211) (204) (116) (220) (215) (101) (103) (004) (112) (200) (105) (211) (204) (116) (220) (215)

an

TiO2 thin films

2θ ( Degree ) Observed Standard

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Samples

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Table 1 Crystallographic data of TiO2 thin films annealed at various temperatures.

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Page 26 of 28

3.5174 2.4324 2.3783 2.3349 1.8923 1.6983 1.6672 1.4793 1.3627 1.3379 1.2631 3.5174 2.4324 2.3771 2.3355 1.8916 1.6989 1.6667 1.4791 1.3627 1.3379 1.2628 3.5188 2.4330 2.3789 2.3349 1.8927 1.6983 1.6667 1.4793 1.3627 1.3379 1.2631

3.5200 2.4310 2.3780 2.3320 1.8920 1.6999 1.6665 1.4808 1.3641 1.3378 1.2649 3.5200 2.4310 2.3780 2.3320 1.8920 1.6999 1.6665 1.4808 1.3641 1.3378 1.2649 3.5200 2.4310 2.3780 2.3320 1.8920 1.6999 1.6665 1.4808 1.3641 1.3378 1.2649

Page 27 of 28

temperatures.

Annealed at 400 °C

FWHM (radians)

Crystallite Size (D) nm

Microstrain (Ɛ) (10-3)

Dislocation Density (δ) line/m²

Stacking Fault (α) (10-3)

25.29

(101)

3.5174

0.00711

19.97

1.734

2.505

3.797

37.78

(004)

2.3783

0.00829

17.66

1.961

3.204

3.586

48.02

(200)

1.8923

0.00653

23.22

1.491

1.853

2.476

53.92

(105)

1.6983

0.00663

23.43

1.478

1.821

2.353

55.01

(211)

1.6672

0.00635

24.57

1.409

1.655

2.229

25.29

(101)

3.5174

0.00707

20.08

1.725

2.478

3.777

37.80

(004)

2.3771

0.00775

18.88

1.835

2.803

3.354

48.04

(200)

1.8916

0.00648

23.39

1.481

1.826

2.458

53.9

(105)

1.6989

0.0065

23.92

1.448

1.747

2.306

55.03

(211)

1.6667

0.00621

25.13

1.379

1.583

2.180

25.28

(101)

3.5188

0.00715

19.87

1.743

2.531

3.818

37.77

(004)

2.3789

0.00712

20.58

1.683

2.359

3.078

48.01

(200)

1.8927

0.00704

21.54

1.608

2.153

2.669

53.92

(105)

1.6983

0.00654

23.77

1.458

1.769

2.321

55.03

(211)

1.6667

0.00581

26.88

1.289

1.383

2.038

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Interplaner Distance (d-spacing) (Å)

ce

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Annealed at 500 °C

(hkl)

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Annealed at 300 °C

2θ ( Degree )

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TiO2 thin films

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Table 2 Microstructural properties of some (hkl) planes of TiO2 thin films annealed at various

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27


a=b

Lattice constants

3.7850 (Å)

Unit Cell Volume

a2c

(Å3)

Scherrer’s Formula WilliamsonHall Plot Method

(nm)

Dislocation Density (δ)

n D2

Microstrain (Ɛ)

(nm)

line/m²

136.32

21.77

3.7837

Annealed at 500 °C

3.7857 9.5157

9.5103

15.52

2.20 ×1015

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Crystallite Size (D)

9.5157

Annealed at 400 °C

136.15

136.37

22.28

22.53

an

c

15.97

16.21

2.08 ×1015

2.03 ×1015

( β cos θ)/4)

1.61 ×10-3

1.574 ×10-3

1.55 ×10-3

WilliamsonHall Plot Method

-1.65 ×10-3

-1.60 ×10-3

-1.56 ×10-3

2π2

2.888 ×10-3

2.815 ×10-3

2.785 ×10-3

[

45√3 tan θ

]β

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Stacking Fault (α)

Annealed at 300 °C

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Units

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Table 3 Average microstructural properties of TiO2 thin films annealed at various temperatures.

Table 4 Parameters obtained from the photocurrent density-voltage (J-V) measurements of TiO2 thin film annealed at 400 °C. Jsc (mA/cm2)

Voc (mV)

Rs (Ω)

Rsh (Ω)

Fill Factor (%)

Efficiency (%)

0.2451

821.68

2281

5272

33.26

1.33

0.3127

846.86

1320

6282

36.72

1.94

ce

electrolyte

0.5 M NaOH

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Page 28 of 28

0.1 M NaOH

28