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Nano Gold Rush: On the Origin of the Photocurrent Enhancement in Hematite Photoanodes Decorated with Gold Nanoparticles
journal contribution
posted on 2016-06-24, 00:00 authored by Moran Gross Koren, Hen Dotan, Avner RothschildHematite (α-Fe2O3) photoanodes are
widely studied as candidates for solar water splitting. Recent reports
suggest that the photocurrent of thin film hematite photoanodes could
be enhanced by using gold nanoparticles (Au NPs) that give rise to
plasmonic light trapping close to the hematite/electrolyte interface.
This work examines the effect of Au NPs on the optical, electrochemical
(in the dark), and photoelectrochemical (under illumination) properties
of thin film (20–115 nm thick) Ti-doped hematite photoanodes.
Au NPs were obtained by annealing 2.5–15 nm thick Au layers
which led to dewetting and formation of 15–150 nm Au NPs, respectively.
Au NPs on glass substrates displayed broad and shallow plasmonic peaks
in the visible range, commensurate with the size distribution of the
Au NPs. Two photoanode configurations with Au NPs decorating the surface
or embedded under the hematite films were examined. Photoanodes of
the first configuration displayed smaller photocurrents compared to
counterpart photoanodes without Au NPs, most likely due to wasted
absorption by the Au NPs and scattering into inactive parts of the
device. The Au NPs underwent a redox reaction (Au + 3OH– ⇌ Au(OH)3 + 3e–) that gave rise
to spurious contribution to the current. In addition, they also reduced
the onset potential of water oxidation by ∼200 mV due to an
electrocatalytic effect. Photoanodes of the second configuration displayed
considerable enhancement (up to 92%) in absorption with respect to
counterpart photoanodes without Au NPs. The enhancement was broadband,
as expected for Mie scattering. The plasmonic resonances in their
absorption spectra were red-shifted to wavelengths above the hematite
absorption edge (600 nm); therefore, no plasmonic peaks were observed
in the photocurrent action (IPCE) spectra. Small (<18%) enhancement
in the plateau photocurrent (at 1.53 VRHE) was observed
in some cases, whereas in other cases the photocurrent was smaller
than that of counterpart photoanodes without Au NPs. The photocurrent
enhancement was considerably higher close to the onset potential,
reaching up to 124% at 1.23 VRHE, but the photocurrents
were quite low (<0.5 mA/cm2) at these low potentials.
No obvious correlation was observed between the absorption and photocurrent
gains in photoanodes with different hematite film thicknesses and
Au NP sizes, except for 4 photoanodes (out of 14 specimens that were
examined) with a hematite film thickness of 75 nm. Therefore, we conclude
that the photocurrent enhancement observed in our hematite photoanodes
with embedded Au NPs was largely due to electrochemical effects rather
than optical ones. This study highlights the intricate nature of several
effects, both optical and electrochemical, of the Au NPs on hematite
photoanodes, which come together to produce different contributions
to the photocurrent. Depending on the device structure, some effects
may enhance the water photo-oxidation current but other ones may not,
and some effects may even suppress it. Therefore, careful design and
optimization must be carried out in order to take advantage of the
beneficial effects and mitigate the deleterious ones.