Effect of Cu-doping on the visible photocatalytic performance of Cu 2 O immersion-plated on silicon nanoporous pillar array
2017
In the past decades, semiconductor photocatalytic degradation was
considered as one of the primary means to deal with the organic pollutants
in water, because its final products, water and carbon dioxide, are
harmless to environment. Owing to the high photocatalytic activity
and corrosion stability, titanium dioxide (TiO 2 ) has been
widely studied and much promising progress has been made in the past
years. However, TiO 2 is a wide bandgap semiconductor (~3.2
eV), which decides that the generation of the active holes needed
for photocatalysis can only be realized under the ultraviolet illumination
accounting for only ~5% of the total solar energy. Considering that
the solar energy distributed in the visible region is over 45%, the
exploration on high-efficiency visible photocatalysts is of special
significance. Actually, great efforts have been made to expand the
active light wavelength of TiO 2 from ultraviolet to visible
regime, such as through doping or co-doping techniques or modifying
semiconductor surfaces with other substances. Although the results
from theoretical calculations is rather encouraging, the actual effectiveness
through either element doping or surface modifying on tuning the energy
band structure is far from satisfactory in experiments. Furthermore,
the unstability of the semiconductors after doping or co-doping treatment
is still a problem to be solved. Cuprous oxide (Cu 2 O) is a p-type and direct bandgap
(2.17 eV) semiconductor material, which imply that it can directly
utilize the visible light regime accounting for 45% of the solar energy.
As a visible light photocatalyst, Cu 2 O has been studied
and exhibited given photocatalytic activity. Meanwhile, the photocatalyst
existed in nanopowder form would not be helpful for realizing cyclic
utilization. So, preparing a high-efficiency, easy to be recycled
photocatalyst under the visible light irradiation is of great importance. In the previous study, we reported the preparation and characterization
of silicon nanoporous pillar array (Si-NPA), a silicon hierarchical
structure featured by an array of regularly aligned, quasi-identical
and highly nanoporous silicon pillars with strong light absorption
across the whole visible region. These structural and optical properties
imply that Si-NPA might be an effective photocatalyst to degrade organic
pollutants under visible light irradiation. Its large specific surface
area could provide sufficient space or active sites for pollutant
adsorption, its strong visible region light absorption would be beneficial
to fully utilize the sunlight energy, and its solid-state form would
be helpful for realizing cyclic utilization compared with nanopowder
photocatalysts. Considering the special structure and physical properties of Si-NPA,
this work selected Si-NPA as substrates and the samples were obtained
with different preparing parameters. Cu 2 O/Si-NPA, Cu 2 O:Cu/Si-NPA (with Cu 2 O as the main component),
Cu:Cu 2 O/Si-NPA (with Cu as the main component) and Cu/Si-NPA
were prepared by an immersion-plating method. The surface morphologies,
microstructures and visible photocatalytic degradation performances
to methyl orange (MO) were characterized. It is disclosed that for
samples specified by 1.8 cm×2.0 cm, a photocatalytic degradation
rate of ~53.6% to MO for Cu 2 O:Cu/Si-NPA was achieved under
visible light (400–800 nm) with an optical power density of
10 mW/cm 2 , which was about 20% higher than Cu 2 O/Si-NPA. The efficiency promotion is attributed to the Cu-doping
in Cu 2 O, which would be beneficial to decrease the probability
of the photon-generated electrons captured by defects or recombined
with holes and enhance their transport process from internal to material
surface. Such an enhancement would produce the reactive oxygen species
with a concentration that higher than pure Cu 2 O. These
results might provide a promising way for further promoting the visible
photocatalytic degradation rate of Cu 2 O to organic pollutant.
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