Characterization of Chemical-Bath-Deposited TiO2 Thin Films

In this study, a series of titania thin films was prepared by chemical bath deposition (CBD) of TiCl3 on indium tin oxide (ITO) glass at room temperature, followed by calcinations at 500°C for 4 h. The effect of cyclic deposition on phase composition, microstructure, and electrical resistivity of Ti02 thin films was characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), and four- point probe, respectively. Results showed that Ti02 films produced by single deposition cycle were amorphous. In contrast, those produced by five and six deposition cycles were partly amorphous and partly crystalline with the formation of rutile. Both the film thickness and electrical resistivity increased with an increase in the number of deposition cycles.

Introduction

Thin films are important components of various devices widely used in electronic, protective, membrane, and sensor applications. The most common preparation method of thin films is chemical deposition since it offers several advantages such as low cost and the ability to produce samples with large surface area [1]. Some authors [1, 2] have reported various aspects of chemically deposited

Nanostructured Titanium Dioxide in Photocatalysis

It-Meng Low, Hani Manssor Albetran, Victor Manuel de la Prida Pidal, and Fong Kwong Yam Copyright © 2021 Jenny Stanford Publishing Pte. Ltd.

ISBN 978-981-4877-07-7 (Hardcover), 978-1-003-14853-1 (eBook) www.jennystanford.com thin films, while others [3, 4] have studied the effects of varying the growth parameters such as deposition rates, bath compositions, and bath temperature on the microstructures and properties of thin films.

Ti02 exists in three main phases: anatase, brookite, and rutile where the latter is a stable phase. This oxide is an attractive material due to its wide applications, such as in paints, sunscreens, and as photo catalyst under ultraviolet light [5-7]. Hitherto, the synthesis of Ti02 thin films or bulk has led to the emergence of nanoscience and nanotechnology by virtue of their superior physical and chemical properties [8, 9].

In this study, we have used chemical bath deposition to synthesize a series of titania thin films on ITO glass and investigated the effect of cyclic deposition on phase composition, microstructure, and electrical resistivity of Ti02 thin films.

Results and Discussion

XRD Analysis

It was found that the color of Ti02 thin films is clear or transparent and adheres well to the ITO glass substrate. Visually the films are homogenous with smooth surface appearance. The XRD spectra were collected for samples A, D, and R The XRD data were also collected on ITO glass as a control. The diffraction pattern of sample F is shown in Fig. 8.1, which depicts the existence of rutile only in the thin film of Ti02. In all samples, only rutile was detected and no peaks due to anatase (101) were detected probably due to their transformation to rutile during sintering at 500°C for 4 h.

It should be mentioned that the normal XRD is not ideal for characterizing the phase compositions of thin films due to relatively high penetrating (>30 pm) depth of X-rays in symmetric mode. Since the Ti02 thin films produced in this study are less than 5 pm thick, the X-rays will penetrate into the ITO substrate and produce an undesirable strong background noise due to the amorphous nature of ITO. Hence, it will be more accurate to use grazing-incidence XRD to probe their near-surface phase compositions of Ti02 thin films. The critical angle (ac) for grazing-incidence diffraction can be approximated by [10]:

where ac is in radians, p is the density of Ti02 (rutile = 4.25 g/cm3), and A is wavelength in A. Hence, the calculated value of ac for Ti02 thin films is 0.010472 or 0.6°. Above the critical angle, the penetration depth (of) can be calculated from [11]:

where p is the linier absorption coefficient of matter. Since the value of p for rutile is 907.0775/cm [12] and the normal XRD uses an incidence angle of 8°, the penetration depth of X-rays will be ~30.8 pm, which is deeper than the thickness of the thin films. As a result, the X-rays will also detect the ITO glass and give rise to undesirable large amorphous background.

The XRD plot for sample F with six cycles of deposition

Figure 8.1 The XRD plot for sample F with six cycles of deposition.

By using the Scherrer equation [13], one can determine the crystallite size of this sample. By taking the width of diffraction peak into consideration, the crystallite size of Ti02 in sample F was determined to be ~10 nm. This value is much less than the particle size of ~200 nm indicated by the SEM image [Fig. 8.2(d)], Since particles or grains are made up of small crystallites, this suggests that there are about 20 crystallites in each Ti02 particle on sample F.

The PDF file number 21-1276 for rutile from the database was used to identify the phases formed [14]. According to Lokhande [15], there is no difference between ITO glass and Ti02 having low grade of crystallinity. In fact, it is not necessary for Ti02 to have a high grade of crystallinity when it is used in applications such as a photo detector [16].

Microstructure Analysis

The SEM images of Ti02 thin films prepared by multiple depositions are shown in Fig. 8.2. These images clearly show distinct contrast in microstructures in films prepared between one and six deposition cycles. This indicates that the number of deposition cycles can lead to a significant change in the microstructures of Ti02 films.

SEM images of samples with various processing conditions

Figure 8.2 SEM images of samples with various processing conditions: (a) sample A (1 cycle @ 6 h), (b) sample В (2 cycles @ В h), (c) sample D (4 cycles @ l.S h), and (d) sample F (6 cycles @ 1 h).

The microstructures of the thin films shown in Fig. 8.2 are for samples with one, two, four, and six deposition cycles. It is interesting to note that the microstructures for thin films prepared from one to four cycles are quite similar although cracking can be seen in sample A with one deposition cycle. In films prepared from four and six deposition cycles, the cracks had disappeared and the amount of pores also decreased. It appears that multiple depositions had helped to fill the pores within the Ti02 layer, thus making the surface denser. If the film remains attached to the substrate and does not crack during sintering, shrinkage in the plane of the substrate can be inhibited. The stresses that arise during the sintering of constrained films are analogous to those in a sandwich seal caused by mismatch in the thermal expansion coefficients in the layers.

It is interesting to note that the films have pores distributed all over the surfaces. These pores can play the role of enhancing dye sensitization when the Ti02 films are used for solar cell application [17] as the dye can fill the pores. Dye can also absorb the incident ray and inject electrons to the Ti02 semiconductor.

SEM images showing the cross-sectional view of samples from top surface to the ITO glass substrate where the Ti0 layer is on the left and the ITO glass is on the right,

Figure 8.3 SEM images showing the cross-sectional view of samples from top surface to the ITO glass substrate where the Ti02 layer is on the left and the ITO glass is on the right, (a) Sample A, (b) Sample 8, and (c) Sample E.

Furthermore, samples were also observed using SEM in cross- sectional view from the top surface of the film to the ITO glass in order to estimate the film thickness. Microstructures of samples A, B, and E are shown in Fig. 8.3. Using the scale bar on each image, one can estimate the film thickness deposited on the substrate. The variation in film thickness as a function of deposition cycles is depicted in Fig. 8.4. By taking into account the two estimated standard deviations, the thickness of the films can be observed to increase with the number of deposition cycles. This is self-evident because when repeated depositions were done, the material deposited on the substrate also increased. From this figure, one can tailor-design the thickness of the film based on the number of deposition cycles.

Variations in film thickness as a function of deposition cycle

Figure 8.4 Variations in film thickness as a function of deposition cycle.

Electrical Resistivity

The electrical resistivity or conductivity of Ti02 films was measured using four-point probes. By knowing the resistivity, one can infer whether the film produced is a conductor or semiconductor. The application of Ti02 as a photoconductor must have properties of a semiconductor because a conductor has no band-gap energy. If the incident ray overcomes the band-gap energy, electrical conduction will occur. The variation in electrical resistivity as a function of deposition cycles is depicted in Fig. 8.5.

The resistivities of conductors and semiconductors normally range from 10~3 to 10s Qcm [18]. Based on this value, the resistivity of Ti02 thin films synthesized in this study can be classified as a semiconductor. Since the resistivity increases with deposition cycles, this indicates that multiple depositions serve to increase the resistivity by virtue of more pores and materials being deposited in the film as the thickness increases. When the deposited material increases, the resistivity of a layer increases with each deposition cycle. It has been reported that the resistivity value of Ti02 thin films prepared by DC magnetron sputtering ranged from 107 to Ю10 Qcm [19]. Thus, the resistivity results obtained in this work are comparable to results reported in the literature.

Variations in electrical resistivity of Ti0 films as a function of deposition cycle

Figure 8.5 Variations in electrical resistivity of Ti02 films as a function of deposition cycle.

Conclusion

Thin films of Ti02 produced by multiple depositions were more crystalline with rutile being the only phase formed. In contrast, Ti02 films produced by a single deposition cycle were amorphous. Both the film thickness and electrical resistivity of Ti02 increased with an increase in the number of deposition cycles.

Acknowledgments

The authors are particularly grateful to Prof. P. Manurung, Dr. Y. Putri, and Dr. W. Simanjuntak of the University of Lampungfor conducting this work using various analytical facilities.

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