Zinc Oxide Nanowire Films: Solution Growth, Defect States and Electrical Conductivity - Advanced Materials for Agriculture, Food and Environmental Safety - page 479

h e ZnO-based electrodes are utilized for hydrogen generation via photo-
electrochemical process and are found to be suitable candidates for water
splitting. Nanowire i lm-based photoanode comes with some promising
properties, which may enhance the performance of electrochemical cell.
High band gap (3.37 eV) and high electrochemical stability of ZnO make
it an attractive material for PEC water splitting [13]. Further, the level of
conduction and valence band are apart from H 2 O/H 2 and OH-/O 2 redox
level and satisfy a mandatory requirement for spontaneous photo splitting
of water [197]. Zhang et al. have studied ZnO nanostructured photoanode
and observed that hydrogen generation ei ciency is a function of the mor-
phology, semiconductor electrolyte interaction and defect density [13].
Nitrogen-doped ZnO nanowire arrays have shown signii cant enhance-
ment in conversion ei ciency in the visible region. An order of magnitude
increase in photoresponse with an overall hydrogen generation ei ciency
of 0.15% is reported in nitrogen doping [12]. Quantum dot monolayer
sensitized ZnO nanowire-array has shown enhanced ei ciency for water
splitting. Metal nanoparticles-coated ZnO nanowire shows improved
charge transport due to inhibition of ZnO surface charge recombination
and enhanced light trapping (harvesting) due to plasmonic ef ects, which
improved the overall ei ciency [198]. A three-dimensional branched ZnO/
Si heterojunction nanowire-based PEC has been recently demonstrated
which has very high ei ciency of solar water splitting [196].
16.6.3 PiezoelectricNanogenerators
Due to piezoelectric nature, mechanical stress in ZnO can produce electri-
cal voltage. Wang et al ., utilized this property and demonstrated a piezo-
electric nanogenerator based on ZnO nanowire [15]. h ey proposed that
the coupling of piezoelectric and semiconducting properties in zinc oxide
created a strain i eld and resulted in charge separation across the NW
at er bending. A typical device design consists of a long ZnO nanowire,
with one end i xed on silicon substrate using silver paste and the other
end let free. When ZnO nanowire is del ected using conductive AFM
tip, a change in device output voltage is observed. h us they concluded
that defection of nanowire generates electrical power in the device [14].
A more robust and simpler device design has been promoted by the Wang
group to generate direct current (DC) by using ZnO nanowire array [199].
Figure 16.10 shows a schematic cartoon of a direct-current nanogenera-
tor driven by ultrasonic waves. h e device consists of vertically aligned
zinc oxide nanowire array that is covered by a zigzag silicon electrode on
top. Ultrasonic waves create bending and/or vibration on ZnO nanowires