Titanium dioxide (TiO2) is a wide band-gap n-type semiconductor. Anatase TiO2 is the most common structure used in high performance dye-sensitized solar cell (DSSC). Nanoporous TiO2 serves double-duty as an electron acceptor and a scaffold to hold large numbers of dye molecules in DSSC. The porosity of TiO2 is a key feature as it has roughly a thousand times greater a surface area than the equivalent flat area.  In this work, TiO2 crystallite size (43.55 nm) has been calculated by using XRD data, and the morphology and the grain-size of TiO2 (average grain size of 0.2 mm ~ 0.3 mm) with different solvents have been also studied by SEM

Khin Phyu Phyu Tun

Kyaw San Oo

San Nyein Khine

Soe Myat

Than Htay

Tin Hla Yin

Tin Tin Mya

English

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International Research Journal of Advanced Engineering and Science ISSN (Online): 2455-9024   103  Soe Myat, Than Htay, San Nyein Khine, Kyaw San Oo, Tin Hla Yin, Tin Tin Mya, and Khin Phyu Phyu Tun, ―XRD and SEM Analysis, and Semiconductor Type Determination of TiO2 for Dye-sensitized Solar Cell,‖ International Research Journal of Advanced Engineering and Science, Volume 4, Issue 2, pp. 103-107, 2019. XRD and SEM Analysis, and Semiconductor Type Determination of TiO2 for Dye-sensitized Solar Cell  Soe Myat*, Than Htay*, San Nyein Khine**, Kyaw San Oo*, Tin Hla Yin*, Tin Tin Mya*, Khin Phyu Phyu Tun* *Department of Physics, Sittway University **Faculty of Computer System and Technology, University of Computer Studies-Sittway   Abstract— Titanium dioxide (TiO2) is a wide band-gap n-type semiconductor. Anatase TiO2 is the most common structure used in high performance dye-sensitized solar cell (DSSC). Nanoporous TiO2 serves double-duty as an electron acceptor and a scaffold to hold large numbers of dye molecules in DSSC. The porosity of TiO2 is a key feature as it has roughly a thousand times greater a surface area than the equivalent flat area.  In this work, TiO2 crystallite size (43.55 nm) has been calculated by using XRD data, and the morphology and the grain-size of TiO2 (average grain size of 0.2 m ~ 0.3 m) with different solvents have been also studied by SEM.   Index Terms— Anatase, DSSC, SEM, TCO, TiO2, XRD. I. INTRODUCTION Large amount of carbon dioxide emission by the combustion of fossil fuel leads to serious environmental problem such as the global warming. Solar energy is regarded as one of the perfect clean energy resources. Because the amount of solar energy reaching the surface of the earth is about 10,000 times larger than worldwide energy consumption. So, there would be no worry for the energy supply line even if a small fraction of sunlight could be converted to alternative and usable energy forms. Devices that will permit direct conversion of sunlight to electricity are called photovoltaic solar cells. But the price of the first and second generation photovoltaic cells exceeds that from the traditional fossil fuel. So, dye-sensitized solar cell (DSSC) has been regarded as the most prospective clean energy source. DSSC consists of four main parts such as TiO2 (electron acceptor), dye sensitizer (photochemical pumping device), redox electrolyte (electron donor) and carbon layer (catalyst)[1][3][5-7]. Titanium dioxide (TiO2) is cheap, abundant, nontoxic and biocompatible, and anatase TiO2 is the low temperature stable form and it gives mesoscopic films that are transparent and colorless[11]. DSSCs are able to work under cloudy skies and non-direct sunlight, and also used indoor. Dr. blade technique, screen printing, spin coating, spray pyrolysis, electrochemical deposition and gas phase deposition can be used to deposit TiO2 colloidal on transparent conducting oxide (TCO) layer of glass electrode. Nanoporous TiO2 used in this work, has double-duty: (i) a scaffold to hold large numbers of dye molecules and (ii) an electron acceptor. ZnO, Fe2O3, SnO2, CdSe, CdS, etc can be used as electron acceptor in DSSC[4][11][12]. X-ray diffractometry is mainly used for the identification and qualification of the samples by their diffraction patterns. The sample is placed in the centre of goniometer. After that it is scanned through an angle 2θ from 10o to 70o and the diffracted X-rays enter the detector, and then the data are recorded. These data produce their respective diffraction patterns which are identified by JCPDS (Joint Committee on Powder Diffraction Standards) files. So, XRD is a useful technique to investigate the crystal structure, the Miller indices (h k l) and the lattice parameters (a, b, c) of the sample studied. By using the Miller indices and the lattice parameters, the interplannar spacing can be evaluated and then the crystallite size can also be calculated by Scherrer formula[2][9]. Scanning Electron Microscope (SEM) is one of the most versatile instruments available for the examination and the analysis of the microstructural characteristics of solid objects. This is because it can provide high resolution image of the material. Sharply focused high-energy electrons scan over the surface of specimens, and signals can be classed as secondary electrons, backscattered electrons, characteristic X-rays, and auger electrons. Photographic images of various energies can be obtained. SEM can be directly used to observe the surface micro-structures. Its magnifications are from 10 to 200000 or more[8]. Fig 1.1 shows the structure of TiO2 based DSSC.  Fig. 1.1 Structure of TiO2 based dye-sensitized solar cell II. BACKGROUND THEORY 2.1. X-ray Diffractometry (XRD)  XRD can be used to investigate the crystal structure of the sample studied. By using the following formula, the interplannar spacing can be evaluated for tetragonal structure.  International Research Journal of Advanced Engineering and Science ISSN (Online): 2455-9024   104  Soe Myat, Than Htay, San Nyein Khine, Kyaw San Oo, Tin Hla Yin, Tin Tin Mya, and Khin Phyu Phyu Tun, ―XRD and SEM Analysis, and Semiconductor Type Determination of TiO2 for Dye-sensitized Solar Cell,‖ International Research Journal of Advanced Engineering and Science, Volume 4, Issue 2, pp. 103-107, 2019. 2222221clakhd   where  d = interplannar spacing             a, c = lattice parameters             (h k l) = Miller indices This is Bragg’s law formula. Bragg’s law action is shown in Fig 2.1.  nd sin2  where  n = 1             d = interplannar spacing             θ = diffraction angle of the peak              λ = wavelength of incident X-ray            The lattice parameters a and c for tetragonal structure can be calculated by 2222222224)(sincalakhc where  λ = wavelength of incident X-ray              θ = diffraction angle of the peak  X-ray diffractometer used in this work with the wavelength of 1.54056 Å of copper target, is shown in Fig 2.2[2].   Fig. 2.1 Bragg’s law action   Fig. 2.2 X-ray diffractometer (XRD)  The following is the Scherrer formula used to calculate the crystallite size. cos9.0Bt   where  t = thickness of the crystallites (nm)            λ = wavelength of incident X-ray (nm)            θ = diffraction angle of the peak under                  consideration at FWHM (°)             B = observed FWHM (radians) 2.2. Scanning Electron Microscopy (SEM) SEM is one of the more useful instruments to examine and analyze the microstructure of the materials. In the present work, morphological features of anatase TiO2 powders were investigated by using JEOL JSM-5610LV SEM with secondary electron imaging (SEI) and backscattered electron imaging (BEI), and 10,000 times of photo-magnification. Schematic diagram and scanning electron microscope (SEM) are shown in Fig 2.3 and Fig 2.4 respectively.   Fig. 2.3 Schematic diagram of SEM   Fig. 2.4 Scanning electron microscope (SEM) 2.3. Semiconductor Type Determination of TiO2 The hot point probe method has been used to determine the conductivity type of the semiconductor. In semiconductor, there are two types of majority carriers: electron and positive hole. In n-type semiconductor, the majority carrier is electron and hole for p-type. The hot point probe set-up is shown in Fig 2.5. Firstly, the hot soldering gun tip is connected to the positive probe of the multi-meter. The sign of the voltage polarity defines the type of the semiconductor whether n-type or p-type. Due to the hot positive point probe, electrons move away from the hot point and the polarity of the thermal voltage International Research Journal of Advanced Engineering and Science ISSN (Online): 2455-9024   105  Soe Myat, Than Htay, San Nyein Khine, Kyaw San Oo, Tin Hla Yin, Tin Tin Mya, and Khin Phyu Phyu Tun, ―XRD and SEM Analysis, and Semiconductor Type Determination of TiO2 for Dye-sensitized Solar Cell,‖ International Research Journal of Advanced Engineering and Science, Volume 4, Issue 2, pp. 103-107, 2019. shows positive value. Similarly, the majority carriers of p-type semiconductor: holes move away from the point due to the hot point. So, the polarity shows negative.  Fig. 2.5 Hot point probe set-up III. EXPERIMENTAL DETAILS 3.1. XRD Study Powder X-ray diffraction patterns of TiO2 is shown in Fig 3.1. XRD data of diffraction angle (2θ), atomic spacing (d), Miller indices (hkl), Full Width at Half Maximum (FWHM) and Peak height (%) of the sample are tabulated in Table 3.1. The collected XRD lines of TiO2 are (101), (103), (004), (112), (200), (105), (211), (204) and (116). They are found to agree with the standard data of Joint Committee on Powder Diffraction Standards (JCPDS). The appearance of these diffraction peaks demonstrates that TiO2 possesses tetragonal (anatase), shown in Fig 3.1. The lattice parameters are a = b = 3.7577 Å and c = 9.5227 Å. By using FWHM (B), θ and λ , the crystallite size of TiO2 can be calculated. But FWHM(B)(degree) needs to be converted into FWHM(B)(radian).    Fig. 3.1 XRD pattern of TiO2  The crystallite size of TiO2 is 43.55 nm. The nanoporous structure of TiO2 layer suggests that the roughness factor of 1000 is achievable while the particle size of TiO2 is less than 50 nm. In other words, 1 cm2 coated area of the conductive transparent electrode with nanostructured TiO2 layer actually possessing a surface area of 1000 cm2. In this work, the crystallite size of TiO2 is about 43.55 nm and dye staining area of TiO2 is about 1000 cm2.  TABLE 3.1. XRD Data of TiO2 Line No: 2θ (o) (hkl) d (Å) FWHM (o) Height (%) 1 25.643 (101) 3.4711 0.185 100.0 2 37.265 (103) 2.4100 0.154 7.6 3 38.140 (004) 2.357 0.157 26.8 4 38.898 (112) 2.3134 0.184 8.1 5 48.397 (200) 1.8792 0.146 41.4 6 54.221 (105) 1.6903 0.159 22.7 7 55.364 (211) 1.6581 0.123 27.3 8 62.986 (204) 1.4745 0.140 20.2 9 69.074 (116) 1.3587 0.136 9.6 3.2. SEM Study SEM has been employed for the characterization of morphology of TiO2 powder. Fig 3.2, Fig 3.3 and Fig 3.4 show the SEM images of TiO2(raw), TiO2(H2O2) and TiO2(HNO3) respectively. The porosity of TiO2 is essential not only to ensure high roughness factor, but also to enhance the penetration of the redox couples into film. According to the SEM images, TiO2 with HNO3 has larger porosity than that with H2O2. TiO2 is composed of interconnected spherical shaped particles with an average grain size in the range 0.2 m ~ 0.3 m. Most of the TiO2 powders are uniform with clear grain boundary.   Fig. 3.2 SEM image of TiO2 (raw)   Fig. 3.3 SEM image of TiO2 (H2O2) International Research Journal of Advanced Engineering and Science ISSN (Online): 2455-9024   106  Soe Myat, Than Htay, San Nyein Khine, Kyaw San Oo, Tin Hla Yin, Tin Tin Mya, and Khin Phyu Phyu Tun, ―XRD and SEM Analysis, and Semiconductor Type Determination of TiO2 for Dye-sensitized Solar Cell,‖ International Research Journal of Advanced Engineering and Science, Volume 4, Issue 2, pp. 103-107, 2019.  Fig. 3.4 SEM image of TiO2 (HNO3) 3.3. n-type Semiconductor TiO2 According to the literature, TiO2 is wide band-gap n-type semiconductor with band-gap energy 3.2 eV. So the hot point probe method has been used to determine the type of the semiconductor whether p-type or n-type. On the other hand, the conductivity type (plus or minus) has been studied by the hot point probe method. The hot soldering gun tip is connected to the positive probe of the multi-meter first. And then the measurements can be made as shown in Fig 3.7. The polarity of TiO2 thermal voltage is positive in Table 3.2. It means that the majority carriers (electrons in n-type) moves away from the point in which the high temperature probe of voltmeter exists and the polarity of the thermal voltage shows positive value. So, TiO2 in this work is n-type semiconductor.   Fig. 3.8 Positive polarity of TiO2  TABLE 3.2. n-type semiconductor TiO2 Sample Thermal voltage (Vt) Semiconductor type TiO2 +9 mV n-type semiconductor IV. RESULTS AND DISCUSSION 4.1 XRD and SEM Results Anatase and rutile are two crystalline forms of TiO2 (the third form, brookite, is difficult to obtain). From XRD data, TiO2 is anatase form of tetragonal structure with crystallite size of 43.55 nm and the lattice parameters are a = b = 3.7577Å and c = 9.5227Å. To increase the dye adsorbing area on TiO2, the crystallite size of TiO2 needs to be as small as possible. Therefore, P-25 commercial TiO2 powder should be used to achieve small crystallite size. Moreover, TiO2 (rutile) has larger crystallite size than anatase. So, TiO2 (anatase) is suitable to use for the high performance of DSSC. The size of TiO2 with hydrogen peroxide (H2O2) becomes slightly greater than that of TiO2 with HNO3 as shown in SEM images. In addition, it was found that TiO2 layer with H2O2 can easily peel off from TCO glass after sintering at 450oC. So, HNO3 (pH 3-4 in deionized water) should be used as the solvent for TiO2 suspension. According to the hot point probe method, TiO2 is n-type semiconductor. So TiO2 is a wide band-gap n-type semiconductor with cheap, abundant, nontoxic and biocompatible. And then TiO2 gives transparent and colourless mesoscopic film. So TiO2 based DSSC can be able to work under cloudy skies and non-direct sunlight, and also used indoor. According to SEM data, the average grain size of TiO2 is in the range of 0.2 m ~ 0.3 m[10]. And then TiO2 have clear grain boundary and uniform shape. V. CONCLUSION In XRD pattern, (hkl) planes: (101), (103), (004), (112), (200), (105), (211), (204) and (116) show that TiO2 has tetragonal (anatase) structure. The crystallite size of less than 100 nm is suitable for a scaffold to hold large number of dye molecules.  As shown in SEM images, TiO2 with solvent HNO3 has larger porosity than that with solvent H2O2. Large porosity gives high roughness factor to stain the large number of dye molecules and to penetrate the redox couples into the film. According to the hot point probe method, positive polarity of TiO2, in Fig 3.8, shows n-type semiconductor.  ACKNOWLEDGEMENT I would like to thank Mr. Htay Win for helping with this research work. REFERENCES [1] Alex, B. F. 2005. New Architectures for Dye-sensitized Solar Cells (New York: Wiley) [2] Cullity, B. D. 1978. Elements of X-Ray Diffraction (Reading: Wesley) [3] Graetzel, M. 2004. Conversion of Sunlight to Electric Power by Nanocrystalline Dye-sensitized Solar Cells (Lausanne: Elsevier) [4] Hedbor, S. & Klar, L. 2005. Plant Extract Sensitised Nanoporous Titanium Dioxide Thin Film Photoelectrochemical Cells (Department of Engineering Sciences: Uppsala University) [5] Jasim, K. E., Al-Dallal, S. & Hassan, A. M. 2012. Henna (Lawsonia inermis L.) Dye-Sensitized Nanocrystalline Titania Solar Cell (Bahrain: Hindawi) [6] Kalyanasundaram, K. 2010. Dye-sensitized Solar Cells (Lausanne: EPFL) [7] Lynn, P. A. 2010. Electricity from Sunlight: an Introduction to Photovoltaic (U K: Wiley) [8] Spilde, M. N. & Adcock, C. 1999. Scanning Electron Microscope: Operator’s Manual: Department of Earth and Planetary Science (Mexico: University of New Mexico) [9] Suryanarayana, C. & Norton, M. G. 1998. X-Ray Diffraction: A Practical Approach (New York: Plenum) [10] Travino, M. R. 2012. Dye-sensitized Solar Cells and Solar Cell Performance (New York: Nova Science Publishers, Inc) [11] Wang, H. H. et al., 2011. ―Preparation of nano-porous TiO2 electrodes for dye-sensitized solar cells,‖ Journal of Nanomaterials. [12] http://en.wikipedia.org.wiki/Titanium dioxide  International Research Journal of Advanced Engineering and Science ISSN (Online): 2455-9024   107  Soe Myat, Than Htay, San Nyein Khine, Kyaw San Oo, Tin Hla Yin, Tin Tin Mya, and Khin Phyu Phyu Tun, ―XRD and SEM Analysis, and Semiconductor Type Determination of TiO2 for Dye-sensitized Solar Cell,‖ International Research Journal of Advanced Engineering and Science, Volume 4, Issue 2, pp. 103-107, 2019. AUTHORS First Author – Soe Myat, Associate Professor in Physics, Sittway University, Myanmar and email address- soemy atsu1  @gmail.  com. Second Author – Than Htay, Professor, Head of the Department of Physics, Sittway University, Myanmar and email address- drthanhtay 44@ email. com. Third Author – San Nyein Khine, Associate Professor, Faculty of Computer System and Technology, University of Computer Studies-Sittway, Myanmar and email address- sannyeinkhaing 969 @ gmail. com. Fourth Author – Kyaw San Oo, Lecturer, Department of Physics, Sittway University, Myanmar and email address- kyaw sanoo 1431 @ email. com.  Fifth Author – Tin Hla Yin, Lecturer, Department of Physics, Sittway University, Myanmar and email address- dawt  inhlayin  11 @ gmail. com. Sixth Author – Tin Tin Mya, Lecturer, Department of Physics, Sittway University, Myanmar and email address- tint inmya  19@ gmail. com. Seventh Author – Khin Phyu Phyu Tun, Lecturer, Department of Physics, Sittway University, Myanmar and email address- phyu  4952 @ gmail. com.  

XRD and SEM Analysis, and Semiconductor Type Determination of TiO2 for Dye-sensitized Solar Cell

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XRD and SEM Analysis, and Semiconductor Type Determination of TiO2 for Dye-sensitized Solar Cell

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