Zinc Oxide Nanowire Films: Solution Growth, Defect States and Electrical Conductivity - Advanced Materials for Agriculture, Food and Environmental Safety - page 462

in the range of 300 C and the precursor solution is sprayed onto

the substrates by a nozzle. Commercial glass slides or TCOs are used as
substrates for deposition; however any arbitrary substrate can be used
which can be sustained at this temperature without any damage [101].
Compressed air is used as a carrier gas and generally few ml of precursor
solution is sprayed at slow rate [101]. h e growth of ZnO nanowire on
substrate requires slightly higher substrate surface temperature than tem-
perature required for thin i lm growth. h e substrate temperatures around
400

° C to 600 °

° C result in ZnO i lms, the substrate temperature 450 ° C leads to hazy

i lms, while 490 C results in oriented hexagonal rods [101]. Krunks at al.

showed that a spray of aqueous solution of zinc chloride (with concentra-
tion of 0.05 mol/l) on ITO at 550

°

C results in well-shaped hexagonal ZnO
nanorods with diameter of 100-150 nm and length of up to 1 μm [97].
An alcoholic precursor solution leads to high aspect ratio of nanorod at
lower substrate temperature compared to the spray of aqueous solution
[100]. An addition of thiocarbamide into the spray solution signii cantly
changes the size and shape and thinner nanorods are grown. h ough spray
pyrolysis method provides low cost, high yield of ZnO nanorod i lms, this
process has some limitations in control over nanowire diameter and length.

°

16.2.2.3 Electrochemical Growth

h e electrochemical growth process is performed in a three-electrode
electrochemical cell using potentiostat/galvanostat; the substrate on which
i lm has to be grown is taken as working electrode, while two other elec-
trodes named as counter and reference are used in this procedure. A con-
ducting substrate (TCO/metal deposited glass/conducting polymer coated
i lm, etc.) is required which acts as a working electrode. Electrolyte of the
experimental solution contains zinc salt (zinc nitrate or zinc chloride solu-
tion) with some supporting electrolyte such as KCl [102-104]. Deposition
reaction is generally carried out at a slightly higher temperature than room
temperature in the range of 60-90

C. h e ZnO posses intrinsic highly
anisotropic crystal structures, which can facilitate templateless growth of
one-dimensional nanostructure [105]. h us template-free electrochemical
growth of ZnO nanowire i lms is possible and does not require the compli-
cated pre- and post-treatments as in the case of template-assisted growth
and is relatively less expensive [104, 105]. Template grown NWs show poor
structural and surface properties and defects of host pore leads to polycrys-
talline ZnO nanowires [105]. Hence, to get single crystalline ZnO nanow-
ire, template-free electrochemical process is more suitable. Diameter of
nanowire can be controlled by pore diameter in template NW, however

°