The SnO2 films that I have sprayed always have powder formation whenever at high temperature(>320℃) or low temperature(room temperature~150℃). The films are not smooth and like to be formed by many small pieces. Which parameters should I adjust and how should I try? Thank you very much.
You have not probably reached the substrate temperature at which the heterogeneous reaction for the formation of SnO2 is overcome. Adequate temperature is required for this and to decompose and volitize the reaction by products. So you need to increase the deposition temperature further, say from 380oC and see whether there would be adhesion to subsrate or not. Check the literature regarding the range of temperature for SnO2 film deposition. Also, check your precursor solution composition and contact senior colleague in the field for possible assistance. I wish you goodluck.
To achieve the thin film formation at the substrate, you need to ensure that the reaction occurs on the substrate only. If due to heat radiation the solution vapors are undergoing the reaction in the pathway (from spray nozzle to the heated substrate) you will always find powder/particles formation at the substrate. If you keep the temperature low, than the solution does not remains at the substrate for a few seconds, that enhance the droplet or pool formation at the substrate. On evaporation or completion of the reaction, these droplet gives non-uniform film.
All you need to ensure that the solution flow rate should be in such a way that the moment it reaches at the substrate, the reaction should be completed. That you can do by changing the carrier gas/solution vapor ratio. I suggest you work in the range of 270-320 degC temperature and see the effect of changing the carrier gas flow (keeping the amount of solution vapor constant).
Good Luck
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