Synthesis, characterization and optical properties of ZnO-CuO-Al2O3 semiconducting films on glass substrates by sol-gel technique

Year 2012, Volume: 1 Issue: 2, 147 - 172, 27.03.2016

Erdal Çelik Funda Öztoprak M. Faruk Ebeoğlugil İşıl Birlik Recep Yiğit Metin Tanoğlu And Hasan Aslan - Hasan Aslan

Abstract

We systematically report synthesis, characterization and optical properties of ZnO-CuO-Al2O3 semiconducting thin films on glass substrates in terms of preparation condition such as Cu content for gas sensor applications. In this framework, the thin films were successfully fabricated by sol-gel dip-coating technique using Zn, Cu and Al based precursors as starting materials. Transparent solutions were prepared by dissolving the Zn acetate, Cu and Al chlorides in solutions of methanol and glacial acetic acid. Turbidity, pH values, wettability and rheological properties of the prepared solutions were measured by turbidimeter, pH meter, contact angle ganiometer and rheometer machines before coating process. Turbidity, pH and contact angle values of the solutions were found to be approximately 34 ntu, 4.5 and 18°, respectively. It was determined that viscosities of the solutions slightly changed with increasing Cu concentration. The measured gel points of ZnO-0, 5, 10 and 15% CuO-Al2O3 solutions were approximately determined as 4000, 3500, 3000 and 2500 sec, respectively. The ZnO-CuO-1 wt% Al2O3 composite films with 0, 5, 10 and 15 wt% CuO content were obtained on glass substrate by dipping and the gel films were dried at 300°C for 10 min and followed by heat treating at 500°C for 10 min in air. This process was repeated six cyclics to make thick films. The thick films were annealed at 600°C for 30 min in air. The thermal, structural, microstructural, adhesion and optical properties of the coatings were extensively characterized by using DTA-TG, FTIR, XRD, SEM-EDS, AFM, scratch tester, refractometer and spectrophotometer. The annealing temperature, 600°C, can completely remove all templates from Zn, Cu and Al networks and oxide crystalline structure is completely formed at this temperature. The spectrum of ZnO-CuO-Al2O3 precursor film heat-treated at 500°C and 600°C which shows an absence of absorption bands corresponding to organics and hydroxyls indicating complete removal of organics and hydroxyls. XRD study confirms that the composite films consist of ZnO and CuO phases after annealing process. SEM reveals that dense structures are obtained by increasing CuO content in the composite films. It is clear from EDS analysis that Zn, Cu and Al elements are present in the films and Al peak is very low because it has 1% Al2O3 in the films. It was found that surface roughnesses of the films changed between 90 and 200 nm. The films prepared from 15% CuO have better adhesion strength to the glass substrate among other coatings. Refractive indices of ZnO based sol–gel coatings usually lie within the interval 1.20–1.70. While pure ZnO films has a high porosity, the porosity values of ZnO-5, 10 and 15% CuO-Al2O3 films decrease as a function of CuO content. The film thicknesses for ZnO-0, 5, 10 and 15% CuO-Al2O3 were measured as 852 nm, 478 nm, 518 nm and 695 nm using optical method respectively. The optical direct band gap energies were found to be in the range of 3.18 to 3.15 eV for ZnO-CuO-1 wt% Al2O3 films including 0, 5, 10 and 15 wt% CuO content. The ZnO film shows the transmittance of the order of 44% while that of ZnO-CuO-Al2O3 films was of the order of 55%. It is noticed that the ZnO film has higher absorbance in visible range of spectrum.

Keywords

ZnO-CuO-Al2O3, films, sol-gel, semiconductivity, adhesion, optical direct band gap

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