Surface morphology engineering of metal-oxide films by chemical spray pyrolysis

1. Surface morphology engineering of metal-oxide films by chemical spray pyrolysis Juan Rodríguez, José Solís , Mónica Gómez and Walter Estrada Universidad Nacional de Ingeniería, Facultad de Ciencias, Casilla 31-139, Lima, Perú

2. I. Introduction • Thin film technology is nowadays a widespread technique for materials fabrication, and numerous materials have been prepared in the form of thin films. • The fabrication techniques are based on different physical-chemical principles, but generally speaking most procedures can be distinguished as either vacuum-based or chemical-based techniques.

3. Owing to their simplicity and inexpensiveness, chemical techniques have been studied extensively for the preparation of thin films. • Those techniques facilitate the desing of materials at the molecular level. • The chemical spray pyrolysis technique (SPT) is one of the major techniques to deposit a wide variety of thin films. It can use a variety of atomization techniques such as ultrasonic nebulisation, spray hydrolysis, corona spray pyrolysis, electrostatic spray pyrolysis

4. I.1 Conventional Spray Pyrolysis Technique

5. Gas exhausting Air compressor temperature heater controller substrate Fluxmeter hose Pressure gauge Chamber spraying peristaltic pump gel I.2 Novel Spray Pyrolysis Technique(SPT)

6. This spray system has the following advantages:- Good size selectivity of the droplets- Vorticity suppression of the spray gas in the spraying chamber - Reduced convection flow of the spraying gas in the depositionchamber.

7. I.2 Sol-Gel Technique • The sol-gel process constitute an important part of so-called “soft-chemistry”. It has a low cost and is rather simple: a network is progressively built through inorganic polymerization reactions at room or moderate temperature. • Depending on the regularity of the macromolecular structure, crystalline or amorphous materials may be prepared.

8. 4 11 5 1 2 3 solvent 2 extraction Uniform particles Aerogel Sol Gel gelefication 5 evaporation 6 6 7 fibres solvent evaporation Xerogel film Xerogel 11 10 8 heat heat 9 Thin film Ceramic Sol-Gel Process

9. Sol-Gel Process • A ceramic is formed by reactions between molecular precursors, then homogeneity of the material is perfect at the atomic scale. • This makes the sol-gel technique, for instance, very suitable for producing optical coatings. • A common process for coatings is by dipping the substrate in a colloidal solution; however one of the major drawbacks of this method is to produce films of limited thickness.

10. I.3 Morphology engineering • •According to the kind of application, a specular or porous thin film is pursued • • SPT has proved to be a versatile technique in order to control the morphology of the coating • • Specular coatings are required mainly for optical applications:selective surfaces, mirrors, electrochromic devices, etc.

11. Surface roughness can be monitored from rough to specularbycontrolling the precursor solution: type of solvent and pH,and keeping fixed optimal spraying conditions: substrate temperature, gas carrier flux and pressure,. • In this work we shall discuss the morphology of NiOx and ZnO films.

12. Porous materials are of interest in a variety of devices such as electrochromic ‘smart’ windows, nanocrystalline solar cells, batteries, photocatalytic reactors, and gas sensors.•Porosity in thin films can be achieved in many ways. Standard procedure relies on atomistic deposition of species under conditions giving a low ad-atom mobility so that a fine-grained crystalline or amorphous structure is grown. •In this work we take an alternative route by means of soft chemistry.

13. • When SPT and sol-gel are properly combined the resulting spray-gel technique (SGT) can be very useful for large area applications, letting the film´s morphology be monitored by controlling the precursors and deposition conditions. • The technique basically consists in producing an aerosol from a gel which is sprayed over a hot substrate where the film will grow. • In this work we shall discuss the morphology of tungsten-oxide and tin-oxide-based coatings using the SGT.

14. Morphology and related optical properties of coatings obtained by SPT • •Schematic representation of an improved spray system associated to an optical system for in-situ measurement of the film thickness. • • The optical system also allows the determination of the roughness of the films.

15. Gas out Movingplate Heater Substrate Air compress pipe Beam splitter Spray nozzle He-Ne Laser Pressure gauge Photodetectors and signal amplifiers Spray system with an optical set-up for in-situ film thickness measurement.

16. II.1 Pyrolytic zinc-oxide-based coatings • Zinc-oxide-based coatings have interesting potential applications: thermoelectric and gas sensor devices, transparent electrodes, selective surfaces and piezo-electric devices, etc. •Using the novel system we have grown undoped zinc oxide thin films. In a previous work we analyzed their growing characteristics, atomic composition, optical properties, photocatalitic response and morphology. • In this work the influence of spraying solution on morphology is analyzed.

17. • Droplets of the spraying solution was obtained with a medical nebulizer and compressed air was used as gas carrier. • In all experiments the gas carrier pressure was kept at 1.7x105 Pa during deposition. • The fog of the spraying solution was brought to the hot substrate (350 °C) where the solvent evaporates taking place the pyrolytic reaction and the film starts to grow. •The pH of the spraying solution was kept constant at 5. Deposition was stopped when the number of interference fringes measured in-situ during deposition were three.

18. In-situ normal reflectance as a function of time for pyrolytic ZnO-based films. Each curve corresponds to the shown ethanol / water proportion in the precursor solution. Diffuse transmittance, TD, of ZnO - films obtained with different ethanol / water proportion in the spraying solution.

19. 2 mm a) b) c) d) e) f) g) Morphology of ZnO-based films obtained by SEM from pyrolytic solutions at different etanol / water proportions: (a) 0/1, (b) 0.1/1, (c) 0.2/1, (d) 0.4/1, (e) 0.6/1, (f) 1/1, and (g) 3/1.

20. • ZnO film prepared at an ethanol / water ratio of 3/1, well defined interference fringes are observed to decrease in intensity as the film becomes thicker. • As the amount of ethanol decreases in the precursor solution, the films become rougher as deduced from the fast dimming of their interference fringes • This trend is observed up to a ratio of ethanol to water of 0.2/1. For films prepared at a lower ethanol / water ratio the surface becomes smoother. Defined interference fringes are observed in a film prepared at a ethanol / water ratio of 0/1.

21. • Similar behavior is founded by measuring the diffuse transmittance, TD, at 550 nm wavelength for ZnO-based films obtained at different proportions of ethanol / water. Films obtained at ratios of 0.1/1, 0.2/1 and 0.4/1 show optical diffusing surfaces, while films obtained with either pure water or pure ethanol show specular surfaces.

22. II.2 Pyrolytic Nickel-oxide-based coatings •Nickel-oxide-based coatings have been largely studied for many kind of applications, like gas sensors, batteries, fuel cells, electrochromism, etc. • They are particularly promising as electrochromic materials possessing anodic coloration

23. Glass or plastic substrates Transparent conducting electrodes Electrochromic materials are characterized by a reversible and persistent change of the optical properties under the action of an applied electric field. V Ion storage Electrochromic Ion conductor/electrolyte

24. • Many techniques has been used to get a nickel-oxide thin film and most of them are based on vacuum processes. However, spray pyrolysis is also a good alternative for growing electrochromic-nickel-oxide-based coatings. • • Smooth nickel-oxide-based coatings with specular optical properties were produced in two ways: • using an alcoholic solution (ethanol) of Ni(NO3)26H2O at 0.25 M • using aqueous solutions of the mixture Ni(NO3)2.6H2O / Co(NO3)2.6H2O at 90 / 10M ratio.

25. •Those solutions were sprayed on to substrates at 300 C during 30 minutes giving rise to coatings with 0.5 - 1 m thickness. •Electrochromism was studied by cyclic voltammetry using a potentiostat connected to a three electrodes cell, where a platinum foil and a saturated calomel electrode were used as counter and reference electrode, respectively. • Aqueous solution of 0.1 M KOH was used as electrolyte for cation insertion/extraction. In-situ optical transmittance measurements together with electrochemical measurements were carried out using a He-Ne laser beam ( = 632.8 nm)

26. Films obtained from alcoholic solutions (b) Ex-situ optical transmittance a) b) (a) Cyclic voltammetric curve for a NiOx film obtained from an alcoholic solution (bottom) and the in-situ transmittance at  = 632.8 nm (top). The scan rate was 10 mV/s and the electrolyte was aqueous 0.1 M KOH.

27. Films obtained from aqueous solutions Optical transmittance for the bleached and colored state of the nickel - cobalt oxide film prepared by spray pyrolysis. Cyclic voltametric curve and associated transmittance at 632.8 nm.

28. 5 mm SEM micrographs of specular surface of sprayed nickel-oxide based coating obtained from alcoholic solution SEM micrographs of rough surface of nickel-oxide based coating obtained from aqueous solution

29. II.3 Morphology and related optical-electrical properties of coatings obtained by SGT •Porous materials are of interest in a variety of devices requiring a large interface between a solid and either a liquid or gas medium. Interesting applications are related to gas sensors. •Semiconductor gas sensors use changes in the electrical conductance of a polycrystalline sensing ceramic to detect gas components in air.

30. Al2O3 substrate Gold electrodes Heating resistor Nanocrystalline WO3 film Gas sensors

31. •In this work, we report highly porous WO3, CuWO4 and SnO2 coatings as gas sensing materials produced by SGT. •Mixed WO3 and CuWO4 films were prepared from a gel via acidification of 0.1 M sodium tungstate aqueous solution through a proton exchange resin.

32. Na WO .2H O 2 4 2 0,1M(ac) pH = 7,8 Acidificatión by ion exchange using a strongly acid resin o [WO (OH) ] 2 2 Acetone 2% Vol. Polymerizatión (t = 45min.) (WO .H O)n 3 2 pH = 1,1 Cu(SO ).5H O , in 4 2 (s) ~1mL H O.( molar ratio 2 Cu:W between 0,01 – 1,0) (WO .H O)n 3 2 2+ 2 - + (Cu , SO ) 4 pH = 1,5 – 2,0 spraying Film (CuWO4-x.H2O)n Flow Chart of Gel Preparation for CuWO4-x Film

33. Substrate temperature(°C) 220 Air flux (L/ min) 9 Spraying time (min.) 45 Air pressure (kPa) 30 Nozzle- Substrate distance (cm) - 0.5 Experimental conditions The obtained thin films were subjected to heat treatment at 600 °C for 3 h.Alumina slides were used as substrate.

34. 5 m 0.5 m Characterization - SEM SEM micrograph of WO3 film

35. 0.5m 0.5m 0.5m 0.5m 0.5m 0.5m Cu/W : 3% Cu/W : 7% Cu/W : 10% Cu/W : 20% Cu/W : 30% Cu/W : 100%

36. Gas-Sensing Properties

37. SnCl4.5H2O (C5H11O)4Sn + H2O + H2O SnO2(Sol) SnO2(Sol) + H2O + NH3 (T=600C) + H2O + NH3 (T=600C) (SnO2)n (ac) (SnO2)n (ac) Centrifugal Separation Centrifugal Separation Liquid (SnO2)n (s) Liquid (SnO2)n (s) Wash Wash (SnO2)n (ac) (SnO2)n (ac) +NH3 (15 M) or TEA (15M) PH=8,8 (SnO2)n (ac) (SnO2)n (ac) Spraying Spraying (SnO2)n film (SnO2)n film Flow Chart of Gel Preparation for SnO2 Film A I or T +NH3 (15 M) PH=8,8

38. Substrate temperature(°C) 130 Air flux (L/ min) 40 Spraying time (min.) 60 Air pressure (kPa) 150 Nozzle- Substrate distance (cm) - 0.1 Experimental conditions The obtained thin films were subjected to heat treatment at 500 °C for 2 h.Alumina slides were used as substrate.

39. 0.5 m 0.5 m 0.5 m Characterization - SEM I A T SEM micrograph of SnO2 film

40. Gas-Sensing Properties

41. III. Conclusions • SPT and SGT have proved to be a versatile technique in order to control the morphology of the coating • Combining the spray pyrolysis and the sol-gel techniques (spray-gel) gives the possibility to produce very rough and highly diffuse films, under appropriate conditions. • This technique brings new possibilities for fabricating highly porous films like WO3, CuWO4-x and SnO2.

42. Gracias