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

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

References

1. Natsume Y and Sakata H. Zinc oxide films prepared by sol-gel spin coatings. Thin Solid Films, 2000; 372: 30 – 36.
2. Chang JF, Kuo HH, Leu IC and Hon MH. The effects of thickness and operation temperature on ZnO:Al thin film CO gas sensor. Sensors and Actuators B, 2002; 84: 258 – 264.
3. Ohyama M, Kozuka H and Yoko T. Sol-gel preparation of ZnO films with extremely preferred orientation along (002) plane from zinc acetate solution. Thin Solid Films, 1997, 306: 78 – 85.
4. Moulson AJ and Herbert JM. Electroceramics, Charpman & Hall, 1990, p. 163.
5. Bae HY, Choi GM. Electrical and reducing gas sensing properties of ZnO and ZnO - CuO thin films fabricated by spin coating method. Sensors and Actuators B, 1999; 55: 47 – 54.
6. Ray SC. Preparation of copper oxide thin film by the sol-gel-like dip technique and study of their structural and optical properties. Solar Energy Materials & Solar Cells, 2001; 68: 307 – 312.
7. Shi C, Fu Z, Guo C, Ye X, Wei Y, Deng J, Shi J and Zhang G. UV luminescence and spectral properties of ZnO films deposited on Si substrates. J. Electron Spectroscopy and Related Phenomena, 1999; 101 – 103: 629 – 632.
8. Kamalasanan MN and Chandra S. Sol-gel of ZnO thin films. Thin Solid Film, 1996; 288: 112 – 115.

9. Fujihara S, Sasaki C and Kimura T. Crystallization behavior and origine of c-axis orientation in sol-gel derived ZnO: Li thin films on glass substrates. Applied Surface Science, 2001; 180: 341 – 350.
10. Purica M, Budianu E, Rusu E, Danila M and Gavrila R. Optical and structural investigation of ZnO thin films prepared by chemical vapor deposition (CVD). Thin Solid Film, 2002; 403 – 40: 485 – 488.
11. Bandyopadhyay S, Paul GK, Roy R, Sen SK and Sen S. Study of structural and electrical properties of grain-boundary modified ZnO films prepared by sol–gel technique. Materials Chemistry and Physics, 2002; 74: 83 – 91.
12. Pierre AC. Introduction to sol-gel processing. Kluwer Academic Publishers, 1998: 237.
13. Liu YL, Liu YC, Liu YX, Shen DZ, Lu YM, Zhang JY and Fan XW. Structural and optical properties of nanocrystalline ZnO films grown by cathodic electrodeposition on Si substrates. Physica B: Condensed Matter, 2002; 322: 31 – 36.
14. Bhuiyan MS, Paranthaman M and Salama K. Solution-derived textured oxide thin films-a review. Supercond. Sci. Technol., 2006; 19: R1 – R21.
15. Diaz-Parralejo A, Caruso R, Ortiz AL and Guiberteau F. Densification and porosity evaluation of ZrO2–3 mol% Y2O3 sol–gel thin films. Thin Solid Films, 2004; 458: 92 – 97.
16. Ech-chamikh E, Essafti A, Azizan M and Ijdiyaou Y. XPS study of amorphous carbon nitride (a-C:N) thin films deposited by reactive RF sputtering. Solar Energy Materials and Solar Cells, 2006; 90: 1424 – 1428.
17. Wilde FD and Gibs J. Turbidity. Geological Survey TWRI Book, 9: 1 – 30.
18. Xu Y and Li L. Thermoreversible and salt-sensitive turbidity of methylcellulose in aqueous solution. Polymer 46, 2005; 7410 – 7417.
19. Jerónimo PCA, Araújo AN, Conceição M and Montenegro BSM. Development of a sol–gel optical sensor for analysis of zinc in pharmaceuticals. Sensors and Actuators B: Chemical, 2004; 103: 169 – 177.
20. Sheffer M, Groysman A and Mandler D. Electrodeposition of sol–gel films on Al for corrosion protection. Corrosion Science, 2003; 45: 2893 – 2904.
21. Brinker CJ and Scherer GW. Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing, Academic Press, San Diego, 1990: 2, 656.
22. Pierre AC. Introduction to Sol-gel Processing, Kluwer Academic Publishers, Boston, 1998: 36.
23. Celik E, Avci E and Hascicek YS. Growth Characteristic of ZrO2 Insulation Coatings on Ag/AgMg Sheathed Bi-2212 Superconducting Tapes. Materials Science and Engineering B, 2004; 110: 213 – 220.
24. Phonthammachai N, Rumruangwong M, Gulari E, Jamieson AM, Jitkarnka S and Wongkasemjit S. Synthesis and rheological properties of mesoporous nanocrystalline CeO2 via sol–gel process. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2004; 247: 61 – 68.
25. Abel-Aal EA, Ismail AA, Rashad MM and El-Shall H. New approach for measuring gelation rate during synthesis of ZnO/SiO2 gel. Journal of Non-Crystalline Solids, 2006; 352: 399 – 403.
26. Baraton MI. Synthesis, Functionalization and Surface Treatment of Nanoparticles, American Scientific Publishers, 2003: 10.
27. Rudé Payró E and Llorens Llacuna J. Rheological characterization of the gel point in sol–gel transition. Journal of Non-Crystalline Solids, 2006; 352: 2220 – 2225.
28. Knoth K, Hühne R, Oswald S, Schultz L and Holzapfel B. Highly textured La2Zr2O7 buffer layers for YBCO coated conductors prepared by chemical solution deposition. Supercond. Sci. Technol., 2005; 18: 334 – 339.
29. Celik E, Yildiz AY, Tanoglu M, Ak Azem NF, Toparli M, Emrullahoglu OF and Ozdemir I. Preparation and Characterization of Fe2O3-TiO2 Thin Films on Glass Substrate for Photocatalytic Applications. Materials Science and Engineering B, 2006; 129: 1993 – 1997.
30. Celik E, Gokcen Z, Ak Azem NF, Tanoglu M and Emrullahoglu OF. Processing, characterization and photocatalytic properties of Cu doped TiO2 thin films on glass substrate by sol–gel technique. Materials Science and Engineering: B, 2006; 132: 258 – 265.
31. Ning W, Shen H and Liu H. Study of the effect preparation method on CuO-ZnO-Al2O3 catalyst. Applied Catalysis A: General, 2001; 211: 153 – 157. 32. Sha ZD, Wang J, Chen ZC, Chen AJ, Zhou ZY, Wu XM and Zhuge LJ. Initial study on the structure and optical properties of ZnO film on Si (111) substrate with a SiC buffer layer. Physica E: Low-dimensional Systems and Nanostructures, 2006; 33: 263 – 267.
32. Tahar RBH. Structural and electrical properties of aluminum-doped zinc oxide films by sol-gel process. J. of the European Ceramic Society, 2005; 25: 3301 – 3306.
33. Shinde VR, Gujar TP, Lokhande CD, Mane RS, Han S-H. Mn doped and undoped ZnO films: A comparative structural, optical and electrical properties study. Materials Chemistry and Physics, 2006; 96: 326 – 330.
34. Malzbender J, den Toonder JMJ, Balkenende AR and de With G. Measuring mechanical properties of coatings: a methodology applied to nano-particle-filled sol–gel coatings on glass. Materials Science and Engineering: R: Reports, 2002; 36: 47 – 103.
35. Celik E, Keskin I, Kayatekin I, Ak Azem F and Özkan E. Al2O3–TiO2 thin films on glass substrate by sol–gel technique. Materials Characterization, 2007; 58: 349 – 357.
36. Fallah HR, Ghasemi M, Hassanzadeh A and Steki H. The effect of deposition rate on electrical, optical and structural properties of tin-doped ITO films on glass at low substrate temperature. Physica B, 2006; 373: 274 – 279.
37. Papadimitropoulos G, Vourdas N, Em Vamvakas V and Davazoglou D. Optical and structural properties of copper oxide thin films grown by oxidation of metal layers. Thin Solid Films, 2006; 515: 2428 – 2432.