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Metal Oxide Thin Film Deposition By Low Pressure Spray Pyrolysis Manohar Dangat1 and J. D. Desai2* 2
Sharadchandraji Pawar College, Jejuri Tal:Purandar Dist: Pune 412303 (MS) India Physics Dept. Arts, Commerce & Science College, Indapur Dist:Pune-413106. India
*
* Corresponding author: J. D. Desai; E-mail:
[email protected] +919763422279. Abstract
In era of nanotechnology, there is dominance of metal oxides. The metal oxides in thin film form can be obtained by a chemical method Spray Pyrolysis (SP) also called as Chemical Solution Spray; is a Chemical method for deposition of thin films. It can be defined as a “material synthesis, in which the constituents of the vapour phase react to form a solid film at some surface” thus; the occurrence of the chemical reaction is an essential characteristic of the pyrolysis method. Precursor solution droplets are produced, spread onto preheated substrates undergo pyrolytic decomposition for formation of thin film. In fact, we start with laboratory chemicals, their solution, fine droplets/aerosol formation by atomization through nozzle and decomposition at higher temperature and final product thin film. Thus thin film deposition by SP involves three stages as: a) Atomization of the precursor solution b) Aerosol transport of the droplet c) Decomposition of the precursor to initiate film growth. Since each drop has the same composition, it is possible to produce complex compounds reliably and in a homogeneous manner. The particle morphology varies and is difficult to control. However, with careful choice of reactants and process conditions, dense or hollow, porous or non-porous, agglomerated or non-agglomerated, coated or non-coated particles can be produced with sizes ranging from a few nanometres up to micrometers. A variety of atomization techniques have been used for solution aerosol formation, including pneumatic, ultrasonic, and electrostatics. Depending on the atomization technique there are three methods: a) Air Blast/Pressure Spray Pyrolytic Deposition(PSPD): solution is atomized by passing compressed air through nozzle b) Ultrasonic Spray Pyrolysis Deposition (USPD):precursor solution is agitated by piezoelectric generator and c) Electro spray Pyrolysis deposition (EPD):High DC voltage is applied between nozzle and surface. Some of advanced spray atomization techniques include (a) Improved spray pyrohydrolysis, (b) Microprocessor based Spray Pyrolysis, (c) Electrostatic Spray Pyrolysis, (d) Corona Spray Pyrolysis, (e) Ultrasonic nebulized atomization technique etc. In spite of all the encouraging developments there is lack of information that relates preparative parameters and desired product thin films, the various aspects have been discussed in relation to metal oxide thin films such as Copper oxide, Nickel oxide etc. Keywords-Chemical method, spray pyrolysis, metal oxides, thin films
1
INTRODUCTION
The metal oxides due to stability and abundance have been attractive in catalytic applications. Typical example of widely used oxides are TiO2, Vanadium oxide VOx[1] It is well known that Spray Pyrolysis has been employed for thin films for variety semiconducting, ceramic thin films such as dense or
porous oxide films [2], ceramic coatings, and powders [3] also. The applications of deposited films ranges from Solar Cells[4,5], Solid Oxide Fuel Cells, [6], antireflection coatings, superconducting , to cathode materials for lithium ion battery. Due to versatility the chemical technique has been refined over the years for industrial applications due to the direct implementation
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of large area deposition using low cost equipment [7]. Only very crude models about the mechanism of spray deposition and film formation have been developed up to now. There are too many processes that occur either sequentially or simultaneously during film formation by spray [8] this article is intended to highlight preparative parameters and their influence on thin film properties [6], in case of PSPD.
2.0 Preparative Parameters: Usually inexpensive precursors such as metal nitrate, chloride, acetate are typically used for forming solutions to sprayed. The following preparative parameters are important in order to obtain desired final product thin/thick film.
2.1 Substrate surface temperature: The pyrolytic decomposition behavior of precursor salt solution is decided by substrate surface temperature. Decomposition of salt plays an important role in film formation. It is always preferred to understand decomposition behavior by performing thermo gravimetric (TG) analysis of salt; before selecting desired substrate surface temperature. When a droplet hits the surface of the substrate multiple processes occur at the same time, evaporation of solvent, spreading of the droplet, and salt decomposition. Various schemes have been proposed. Most of the investigators suggest that only a kind of CVD process gives high quality films by spray pyrolysis. At higher temperatures the solvent evaporates completely during the flight of the droplet and dry precipitate reaches the substrate, where decomposition occurs. At still higher temperature, non adherent, films with rough surface have been obtained [6].
2.2: Carrier gas: Ordinarily, readily commercially available gases have been employed by many investigators. In many cases air is used as carrier gas. In our report, by employing oxygen as carrier gas pure phase of iron oxide i. e. alpha iron oxide/hematite thin films have been obtained in nanocrystalline form [9]. Ruiz et al. [10] deposited nanocrystalline zirconia films and observed that the crystallite size increased when the carrier gas was lighter. 2.3: Pressure of carrier gas: In PSPD carrier gas pressure needs to sufficient of order of several KPa to atomize the liquid for forming aerosols, normally higher the pressure, smaller the fine droplet size. The smaller the nozzle orifice, smaller is pressure required for production of atomized mist. 2.4: Solution flow rate: The solution flow rate has to be optimum, although it affects film morphology neglibly. It should not exceed the optimum value otherwise rapid
cooling of substrate may lead to cracking of substrate in case of glass substrate. The optimum value depends on the type of atomizer, deposition temperature, nozzle to substrate distance, etc. Typically, several mL/min. flow rate is desirable [9]. 2.5: Nature of salt: Ordinarily available inexpensive precursor salts is great advantage in SP. The additional requirements of salt are viz. it should be aqueous soluble or alcoholic solvents with low decomposition temperature. Usually chlorides, nitrates, acetates or metal-organic salts have been used. Amongst them chlorides provide high solubility in water and ethyl alcohol, but chemical corrosion of the in the SP set up and to some extent surroundings. 2.6: Solvent: The physical properties of the solution such as boiling point, solubility of salts, spreading behavior of droplets on the substrate are bound to change due to solvent. It is reported that it is necessary to use a solvent with a high boiling point in order to deposit a dense film [10]. By employing methanolic and ethanolic solution mixture of maghemite, haematite and haematite phases have been reported [12,13]. This has been attributed to be mainly due to Under the identical conditions of deposition, heat of combustion of solvent influences crystalline structure and morphology also. 2.7: Additives in precursor solutions: There are scant reports regarding role of additives in precursor solutions[6]. The lower surface roughness has been obtained by adding polyethylene glycol (PEG) as additive in case of yttrium-stabilized zirconia (YSZ) thin film deposition. The polymer improves the spreading behavior of the droplets on the substrate. The results demonstrate a significant role of additives on the microstructure of the films [6]. 2.8: Deposition time: Deposition time does not have a remarkable effect on the film morphology. More the deposition time more is the film formed remaining on hot substrate surface may be one of the reasons for change in the morphology. 2.9: Duration of spray single spray: In the laboratory scale experimental work spray may be intermittent. Therefore duration of single spray play a role which may be precursor as well as PSPD set up dependant, it influences film thickness. 2.10: Time between two consecutive sprays: This parameter provides sufficient time for agglomerization of the particles at high temperature that are formed due to decomposition; otherwise undesirable crack formation on film surface due to thickness is obvious [14]. Sufficient time
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between two successive sprays enables substrate surface to attain temperature enough for decomposition of precursor particle. 2.11: Substrate to nozzle separation:
3.1 Preparative Parameters for Cu and Ni Oxide Thin Films have been tabulated in following Table:1
1
Spray parameter for Cu and Ni Oxide thin films Precursors CuCl2.2H2O and
2
Substrate to nozzle distance
3
Substrate temperatore
2700C and 3900C
3.12: Deposition rate:
4
Air carrier gas pressure
1 kg/cm2
Deposition rate measurement in terms of film thickness is rather difficult. Thickness measurement by profilometer requires sharp steps between a film and a substrate - which is very difficult to achieve in the spray pyrolysis process. SEM analysis is the most reliable of all the techniques mentioned, but is time consuming. A quick but approximate estimate of deposition rate in terms of film thickness is possible by gravimetric weight difference method [14].
5
Duration of spray single spray
100 S
Each nozzle has a spraying cone formed in PSPD set up. The larger the distance of the atomizer to the substrate, the smaller the deposition rate and the larger the area of coating covered. Ordinarily, 20-30 cm distance between substrate and nozzle are typical.
3.0 Spray Pyrolysis System: By taking into account the facts stated above the fabrication of spray pyrolysis system was carried out by Sahyadri Engineering Works, Lonand; under the guidance and supervision of authors. Modification in substrate heating arrangements were by using a boiler quality steel as heating plate with a centrally hole drilled in it. A 1000W cylindrical high energy density cartridge heater of appropriate size (200 mm long and 15 mm diameter) is enclosed in it. This arrangement is capable of producing temperature of the order of 4500 C, sufficient for spray deposition of metal oxide thin films. Due to high density cartridge heater the required time for heating substrate has been found to be around 10 min. The Fig.1 following shows 3 D schematic of spray pyrolysis system employed for oxide thin film depositions.
Sr. No
6 7 8
Optimized value 0.1M
NiCl2. 6H2O
Time between two consecutive sprays Number of cycles of spray Solution flow rate
43 cm
15 Min. 04 8ml/min.
Both the films obtained were found to be adherent to ordinary glass and ITO substrates. Cu oxide thin films have been tested for photoelectrochemical (PEC) response in white light by forming two electrode system, graphite as counter electrode. The Cu oxide thin films were exhibit photoelectrochemical response. Further characterizations (viz. structural, optical, morphological) of Cu oxide and Ni oxide are in progress. 4.0 Conclusions: The PSPD technique is suitable for thin film deposition of metal oxides. Immense opportunities do exist to understand, modify and utilize the deposited films for various applications. There appears a relationship amongst the preparative parameters, which when optimized may give desired thin metal oxide film with desired physicochemical properties. ACKNOWLEDGMENT
Exhaust
A
H
ELGI VAYU AIR COMPRESSOR
B
D
C
D
E F G I
A:-Exhaust Fan. B:-Reservoir. C:-nozzle. D:-Creative controller. E:-pressure gauss. F:-Substrate. G: - Temperature controller (SELEC TC30). H:-Air Compressor (ELGI VAYU). I:-Heater
Figure 1:3D Schematic Spray System
AUTHORS ARE THANKFUL TO BCUD, SAVITRIBAI PHULE PUNE UNIVERSITY FOR FINANCIAL ASSISTANCE FOR RESEARCH PROPOSAL NO. 14SCI001452, OFFICE BEARERS OF ACAHRYA ATRE VIKAS PRATISTHAN, PURANDAR; SASWAD DIST:PUNE FOR ENCOURAGEMENT AND AVAILING FACILITIES FOR INVESTIGATIONS, SAHYADRI ENGG. WORKS LONAND, DIST:SATARA FOR FABRICATION OF EQUIPMENT, ARC AND COLLEAGUES IN COLLEGES FOR SUPPORTIVE ROLE PLAYING DURING INVESTIGATIONS.
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