Low-Cost Nanomaterials for Photoelectrochemical Water Splitting - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 264

measurement is independent of the power and model of the light source. Moreover,
by integrating the IPCE spectrum with the standard solar spectrum, a simulated
photocurrent density can be obtained. The simulated value should be close to the
experimentally determined value if the light coming from the solar simulator is
closely matched with the standard solar spectrum.
Furthermore, STH efficiency can be estimated based on the simulated photo-
current density using the following equation:

g ¼ I ð 1 : 23 ffi V bias Þ
J light

100 %

where I is the photocurrent density under AM 1.5G light illumination; 1.23 is the
theoretical value of voltage is needed for water splitting; V bias is the applied
external bias versus RHE and J light is the power density of the AM 1.5G white
light. This calculation is based on the assumption of faradic efficiency of water
splitting is 100 %, which is practically difficult to be achieved. Therefore, the most
precise way to calculate the STH efficiency is based on the hydrogen production
rate and the power density of solar light.

1.1 Low-Cost Metal Oxide Nanomaterials: Promising
Photoelectrodes for Water Splitting

Nanostructured electrodes have potential advantages over their bulk counterparts.
Nanomaterials provide not only extremely large surface area for oxidation and
reduction reactions, but also a short diffusion length for minority carriers that can
improve the charge separation efficiency and reduce the loss through electron-hole
recombination. Moreover, nanostructured electrodes could increase light scattering
and absorption, compared to planar structure. With such potentials, nanostructured
materials could open up new opportunities in improving the performance of
photoelectrodes.
A number of semiconductor nanomaterials have been studied as photoelectrodes
for PEC water splitting, including metal oxides, metal chalcogenides, and III-V
compounds. Figure 2 shows the band edge positions of common semiconductors in
aqueous electrolyte with pH 1 [ 23 ]. However, not a single semiconductor material
can meet all the technical requirements discussed above. For example, metal
chalcogenides such as CdS and CdSe are photocorrosive. Hole scavengers such as
Na 2 S and Na 2 SO 3 have to be added to protect the photoelectrode from self-oxidation
[ 28 , 50 , 94 , 102 , 103 ]. Likewise, electrochemical instability is also a major concern
for III-V semiconductor materials [ 112 ]. Additionally, the cost of III-V materials
such as GaN ($1,900 for 2 inch bulk GaN wafer) and GaP ($50/g) is relatively high,
which could limit their applications [ 87 , 93 ]. In terms of chemical stability and
materials cost, metal oxides hold great promise as photoelectrode materials for
PEC water splitting. Metal oxides such as TiO 2 ($2-3/Kg), ZnO ($1-3/Kg),