Optical and electrochemical behavior of transparent glass electrodes coated with in 2O 3:Sn and in 2O 3 thin films Текст научной статьи по специальности «Химические науки»

Хамаганова Татьяна Николаевна, кандидат химических наук, старший научный сотрудник, лаборатория оксидных систем, Байкальский институт природопользования СО РАН, e-mail: khamaganova@binm.bscnet.ru

Хумаева Туяна Гатыповна, аспирант, лаборатория оксидных систем, Байкальский институт природопользования СО РАН, e-mail: tuyana8383@mail.ru

Khamaganova Tatyana Nikolaevna, candidate of chemistry, senior researcher, laboratory of oxide systems, Baikal Institute of Nature Management SB RAS, e-mail: khamaganova@binm.bscnet.ru

Khumaeva Tuyana Gatypovna, postgraduate student, laboratory of oxide systems, Baikal Institute of Nature Management SB RAS, e-mail: tuyana8383@mail.ru

УДК 544 © A. Delgerjargal, B. Battsengel, J. Oyunjargal,

B. Delgertsetseg, C. Ganzorig

OPTICAL AND ELECTROCHEMICAL BEHAVIOR OF TRANSPARENT GLASS ELECTRODES

COATED WITH In2O3:Sn AND I^O3 THIN FILMS

This work was supported by the Asia Research Center in Mongolia and the Korea Foundation for Advanced Studies within the framework of the Projects №3 (2006-2007) and №1 (2007-2008). This work was also supported in part by the

Mongolian National Nanotechnology Center

This paper describes UV-Visible spectroscopic and electrochemical behavior of In2O3 and In2O3:Sn in thin films on the glass substrates of n-type transparent conductive oxide (TCO) electrodes. The methods of TCO thin films manufacture have been outlined. The optical absorption and electrochemical properties of In2O3 thin films in pure and doped forms have been studied. The mechanisms of electrical conductivity and optical transmission are interdependent. The information in this paper may help in choosing the most appropriate TCO electrode materials for new applications in inorganic and organic solar cells and other optoelectronic devices.

Keywords: thin films, oxygen-conducting electrodes, mechanisms of electrical conductivity and optical transmission

А. Дэлгэржаргал, Б. Баттсэнгэл, Ж. Оюунжаргал, Б. Дэлгэрцэцэг, С. Ганзориг

ОПТИЧЕСКОЕ И ЭЛЕКТРОХИМИЧЕСКОЕ ПОВЕДЕНИЕ ЭЛЕКТРОДОВ ИЗ ПРОЗРАЧНОГО СТЕКЛА, ПОКРЫТЫХ ТОНКИМИ ПЛЕНКАМИ In2Oз:Sn И In2Oз

Описывается видимое и ультрафиолетовое спектроскопическое и электрохимическое поведение 1п2Р3 и In203 в тонких пленках на прозрачных стеклянных субстратах п-типа кислородпроводящих электродов (TCO). Кратко описаны методы изготовления TCO тонких пленок. Изучены оптическая абсорбция и электрохимические свойства тонких пленок In203 в чистой и измененных формах. Механизмы электропроводимости и оптической трансмиссии независимы. Данные могут помочь в выборе наиболее подходящих материалов для изготовления TCO электродов с целью поиска новых путей применения в неорганических и органических солнечных ячейках и других оптоэлектронных приборах.

Ключевые слова: тонкие пленки, кислородпроводящие электроды, механизмы электропроводимости и оптической трансмиссии

The first report on transparent conductive oxide (TCO) was published, when Badeker [1] stated that thin films of Cd metal deposited on the glass in a glow discharge chamber could be oxidized to become transparent while remaining conductive. Since then, most researchers have focused on n-type semiconductors consisting of metal (Sn4+, Ti2+, In3+, Zn2+ and Cd2+) oxides and their corresponding alloys to develop highly transparent and electrically conductive thin films. The previously published before 2006 material in this field has been reviewed by Ganzorig and Fujihira [2].

A new optical approach was demonstrated by Kuwana and coauthors [3], that is: the use of n-type TCO electrode in which the product of an electrochemical reaction is monitored spectroscopically. There has been considerable interest in the fundamental electrochemical properties of SnO2 semiconductive electrodes deposited on the glass or quartz substrate [4-9]. In this form, electrodes have been prepared that can be used to observe electroactive species near the electrode-solution interface by internal reflectance spectroscopy in the visible region of the spectrum [5-7]. The electrochemical and surface characteristics of doped SnO2 and In2O3 film electrodes [10] were discussed and compared to Pt and Au ones [10, 11]. Kuwana [12] has reviewed some earlier studies of the effect of light irradiation on electrochemical cells and those in which light emission is a consequence of the electrochemical process. Gerischer [13, 14] has discussed the photoeffects in electrochemical systems under illumination associated with semiconductive electrodes. Spectral photosensitization was also discussed from electrochemical point of view, as it was caused by electron transfer from or to excited molecules [14]. Little attention, however, has been given until the early 1970s to the application of these effects for energy conversion.

Fujishima and Honda [15] have discussed the electrochemical photocell based on n-type TiO2 electrode in contact with the aqueous electrolyte and platinum as counter electrode. The photoreaction in their system is the decomposition

of water due to the oxygen evolution at the irradiated TiO2 electrode and hydrogen evolution at the Pt electrode. Unfortunately, TiO2 has a low quantum yield for the photochemical conversion of solar energy. High-surface-area TiO2 films deposited on the conductive glass sheet from colloidal suspensions were reported by O'Regan and Gratzel [16]. The use of these films with the addition of dye molecules has been shown to improve solar cells [16].

Most of the useful TCOs with a wide bandgap (>3 eV) studied until nowadays have been chemically inert conductive oxides, such as TiO2 [17], SnO2 [18], In2O3 [19, 20] and ZnO [13]. The bandgaps of these «-type semiconductors correspond to the energy photons in the UV region. Schematic band diagram of these oxides [2] is shown in Fig. 1. Solar energy reaching the Earth surface has, however, spectrum distribution in the longer wavelength range and cannot, therefore, be used effectively by these electrodes [21]. Spectral sensitization would solve this problem. Osa and Fujihira [22] have developed a new type of electrochemical photocell, using a TCO electrode which surface was modified by the chemical binding of sensitizing dyes. The solar cells developed by Gratzel and his colleagues [16] described above are in this category.

The material properties of In2O3 are generally characterized by the high optical transparency for visible light and the high electrical conductivity [23]. To increase the conductivity of In2O3 , it is generally doped with Sn and further known as ITO (or In2O3:Sn). The conductivity of ITO films can be as high as 0.5-1.0x104 S/cm. In ITO films, the electron concentration of ITO is ~1021 cm-3 [23], with optical transparency still existing. Such unique characteristics have led to the exclusive utilization of ITO films in flat-panel displays and solar cells.

Vacuum level

3.2 eV

3.8 eV

3.75 eV

3.4 eV

4.0

ZnO

no

OB

SnO

5.0

6.0

7.0

VB

8.0

Fig. 1. Schematic diagram illustrating energy levels of conduction band (CB) and valence band (VB) of some «-type undoped semiconductive oxides TiO2 [17], SnO2 [18], In2O3 [19, 20] and ZnO [13]

The electrical, optical, and chemical properties as well as physical properties of TCO materials can be controlled by altering their chemical compositions [24]. The physical properties shown in Table are the bandgap energy and work function for various types of pure and doped TCO semiconductors [2]. Bandgap widening associated with doping has been observed in earlier work on doped In2O3 films [23].

The optical data [23] indicated that a gradual shift of the band gap to warded higher energy as the electron density was increased. Values for the work function ($,) are defined, as the minimum energy required for an electron to escape into vacuum from the Fermi level. The work function of TCO electrodes can be controlled after chemical modification [2]. The work function of TCO electrode surfaces also depend upon cleaning method.

Useful descriptions of the most widely used preparative deposition techniques for thin films of TCO electrodes were presented in detail [2]. To investigate the optical and electrochemical TCO electrodes, we report here the use of In2O3 and In2O3:Sn thin films prepared on the glass substrate to measure the absorption in the UV and visible regions and the electrochemical conductivity behavior of these TCO electrodes studied.

Table

Bandgap energy and work function for some «-type transparent conductive pure or doped semiconductive oxides

Material Bandgap (eV) Work function (eV)

SnO2 3.8 4.7

SnO2:F - 4.4

In2O3 3.75 4.4 ± 0.1

In2O3:Sn 3.8 - 4.5 4.4 - 4.8

ZnO 3.4 4.3

ZnO:F - 4.2

ZnO:Al 3.9 4.4

TiO2 3.0 - 3.2 4.2 - 4.3

SrTiO3 3.4 4.0

Experimental details

ITO coated glass (Sanyo Vacuum Industries) and quartz substrates were cut into 15x20 mm2 stripes. The substrates were cleaned and rinsed by sonication successively in two detergents (Extran MA 03, pH 6.8, MERCK and Kontaminon O, pH 10, WAKO) and distilled water and were stored under ethanol until being required. Prior to use, the substrates were further cleaned and ultrasonicated successively in acetone and ethanol, and then were transferred to boiled ethanol.

In2O3 coated glass was made by the vacuum evaporation process under the pressure of ~5-7x10-6 Torr, (Fig. 2). To study optical properties of ITO and In2O3 the absorption spectral data for all the thin films were taken using an UV-visible spectrophotometer (UV-265FW, Shimadzu) at room temperature in ambient atmosphere.

Fig. 2. Vacuum deposition system

To study PV properties, the ITO (160 nm)/KCl (0.5M)/Pt and M2O3 (60 nm) KCl (0.5M)/Pt cells were prepared. The current density-voltage (J-V) curves were measured under the sun light conditions. Electric data were taken with a hand-made voltage current source unit.

Results and discussion

To study the optical and electrochemical behaviour of transparent glass electrodes coated with the thin films of ITO and In2O3-inorganic semiconductor, we compared its absorption and transmittance properties with glass substrates. Transmittance of glass along the whole wavelength is 100%, so that glass can not cut poisonous ray. Fig. 3 shows the wavelength dependence on the optical transparency of 200-900 nm range.

Fig. 3. Transmittance of ITO-electrodes

^Vavelen gth / n^n

Fig. 4. Absorption spectrum of ITO-electrode

ITO electrode has full transmittance in visible wavelength range and no transmittance in UV (200-300nm), that allows wide practical use. Absorption spectrum of ITO -electrodes on glass was shown in Fig. 4.

We have recorded the UV-visible absorption spectra and transmittance of In2O3 on glass, (Fig. 5 and Fig.6). In visible range ITO as well as In2O3 have shown high transparency.

In Fig.7, the transmittance of ITO and In2O3 is compared with glass. ITO has significantly higher absorption and broader wavelength range compared to In2O3 in the visible wavelength range. Electrochemical properties of ITO electrode were measured by the current density-voltage (J-Vbias) characteristics.

Electron displacement from ITO electrodes to electrolyte solution has high barrier to overcome and current density amounts to zero value at voltage lower 1.3 V (Fig. 8). From this point the current density increases significantly. This indicates ITO can be used as a semiconductor. Besides conductive properties of In2O3 deposited on the glass were measured. As is shown in Fig.9. In2i has wider bandgap than ITO that let electrons move into electrolyte solution easier. This electrode obeys Ohm Law.

Fig. 5. Absorption spectrum of In2O3-electrode

Wavelength / i

Fig. 6. Transmittance of In2O3-electrodes

E

ITO, In2O3 эпектроfl

-10 -15

,-----7"

y

/

/

----In2O3/glass

---ITO/glass

- I

ITO (160 nm)

| In2O3 (60 nm)

| Glass (1 || Glass (1 Glass (1

400 500 600 Wavelength

Fig. 7. Transmittance of ITO and In2O3 compared with glass

• ITO O In2O3 » w (Ox)

/- AHru^pax 0 (Red)

on 80 -

Glass

60

40 *

20

800

-2

2

3

V / V

Fig. 8. Current density-voltage behavior of electrical conduction for ITO, In2O3 electrodes in 0.5 M KCl solution

Conclusion

In this study, we have described the UV-Visible spectroscopic and electrochemical behavior of In2O3 and In2O3:Sn thin films on the glass substrates. The optical absorption and electrochemical properties of In2O3 thin films in pure and doped forms have been studied. Further studies on the preparation and characterization of TCO electrodes such as In2O3 and ITO will be expanding our understanding and advancement of the science and applications of inorganic and organic optoelectronic materials and devices.

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Delgerjargal A., lecturer, Department of Chemical Technology, School of Chemistry and Chemical Engineering, National University of Mongolia, e-mail: delgerjargala@num.edu.mn

Battsengel B., doctor of chemical sciences, professor, Department of Chemical Technology, School of Chemistry and Chemical Engineering, National University of Mongolia, e-mail: b_battengel@web.de

Oyunjargal J., researcher, Center of Nanoscience and Nanotechnology, Department of Chemical Technology, School of Chemistry and Chemical Engineering, National University of Mongolia

Delgertsetseg B., researcher, Center of Nanoscience and Nanotechnology, Department of Chemical Technology, School of Chemistry and Chemical Engineering, National University of Mongolia

Ganzorig C., doctor of engineering, professor, Department of Chemical Technology, School of Chemistry and Chemical Engineering, National University of Mongolia, e-mail: ch_ganzorig@num.edu.mn

Дэлгэржаргал А., преподаватель, отделение химической технологии, школа химии и химической инженерии, национальный универитет Монголии, e-mail: delgerjargala@num.edu.mn

Баттсэнгэл Б., доктор химических наук, профессор, отделение химической технологии, школа химии и химической инженерии, национальный универитет Монголии

Оюунжаргал Ж., научный сотрудник, Центр нанонаук и нанотехнологий, отделение химической технологии, школа химии и химической инженерии, национальный универитет Монголии

Дэлгэрцэцэг Б., научный сотрудник, Центр нанонаук и нанотехнологий, отделение химической технологии, школа химии и химической инженерии, национальный универитет Монголии

Ганзориг С., доктор технических наук, профессор, отделение химической технологии, школа химии и химической инженерии, национальный универитет Монголии, e-mail: ch_ganzorig@num.edu.mn

УДК 666.9 © B. Tserenkhand, R. Sanjaasuren, P. Solongo

COMPOSITION OF SOME IRON ORES AND POSSIBILITY TO USE THEM IN THE CEMENT PRODUCTION (WESTERN REGION OF MONGOLIA)

Chemical and mineral compositions of some iron ores in the Mongolian western region were studied. Also the effect of calcium fluoride on decomposition temperatures of calcite in the raw mix to obtain cement clinker was investigated. The results showed that iron oxide in iron ores of western Mongolian constitutes 87.23% in Uvgondatsan ( Khovd), 85.00% in Suul Лhar (Khovd) and 89.29% in Khargant (Uvs). Iron ores of "Kharganat" and "Uvgundatsan" mostly contain magnetite while iron ore of "Suul Khar" -hematite. The decomposition temperature of calcite was reduced at 5, 10 and 15oC when calcium fluoride from the raw mix obtaining cement clinkers of "Shokhoit" limestone, "Shal" clay and "Kharganat" iron ore with 0.5%, 1.0% and 1.5% additives.

Keywords: iron ore, magnetite, magnetite, saturation coefficient, cement clinker.