impressive overall performance for water oxidation, reaching a photocurrent density of 2.8 T
0.2 mA/cm2 at 0.6 V versus RHE (Fig. 3A and
table S2), which is markedly better than those of
BiVO4/FeOOH and BiVO4/NiOOH and is almost comparable with the performance of bare
BiVO4 for sulfite oxidation.
When NiOOH was first deposited on the
BiVO4 surface and FeOOH was added as the
outermost layer to form BiVO4/NiOOH/FeOOH
(reversed OEC junction), the resulting EFBs determined by sulfite photocurrent onset (fig. S9A
and table S3) and Mott-Schottky plot (fig. S10
and table S4) are comparable with those of
BiVO4/FeOOH, again confirming that the EFB of
the BiVO4 photoanode is affected by the pHPZZP
of the outermost OEC. Also, the J-V curve for
sulfite oxidation by BiVO4/NiOOH/FeOOH was
comparable with that by BiVO4/NiOOH, confirming that a BiVO4/NiOOH junction is not favorable for interface recombination (Fig. 3, C and
E). As a result, BiVO4/NiOOH/FeOOH shows the
lowest photocurrent for water oxidation. These
results prove that the photocurrent enhancement
achieved by the BiVO4/FeOOH/NiOOH photoanode for photoelectrolysis of water is truly due
to the simultaneous optimization of the BiVO4/
OEC and OEC/electrolyte junctions, using an optimum dual OEC structure.
The applied bias photon-to-current efficiency
(ABPE) of the BiVO4/FeOOH/NiOOH electrode
calculated by using its J-V curve, assuming 100%
Faradaic efficiency, is plotted in Fig. 4A (35).
The maximum ABPE of 1.75% achieved by the
system is impressive because it is obtained by
using unmodified BiVO4 as a single photon absorber. Moreover, this efficiency is achieved at a
potential as low as 0.6 V versus RHE, which is a
highly favorable feature for assembling a tandem
cell or a photoelectrochemical diode (12, 36, 37).
The ABPE obtained by using a two-electrode
system (working electrode and a Pt counter
electrode), which achieves the maximum ABPE
of 1.72%, is also shown in fig. S11 (35). The long-term stability of BiVO4/FeOOH/NiOOH was
tested by obtaining a J-t curve. A photocurrent
density of 2.73 mA/cm2, obtained by applying
0.6 V between the working and counter electrodes,
was maintained for 48 hours without showing
any sign of decay, proving its long-term stability (Fig. 4B). The O2 measurement made by using
a fluorescence O2 sensor confirmed that the
photocurrent generated at 0.6 V versus counter-electrode was mainly associated with O2
production (> 90% photocurrent-to-O2 conversion efficiency) (Fig. 4C). The same results were obtained
when the measurement was performed at 0.6 V
versus RHE. H2 production at the Pt counter
electrode was also detected with gas chromatography
(GC) (Fig. 4C). The molar ratio of the produced H2:
O2 was 1.85:1. The slight deviation from the stoichiometric ratio of 2:1 is due to our imperfect
manual sampling method of H2 for GC analysis.
Because this outstanding performance was
achieved by using simple, unmodified BiVO4
(no extrinsic doping and no composition tuning)
as the only photon absorber, further improvement of the cell efficiency is expected when various strategies of tuning compositions or forming
heterojunctions and tandem cells are used to
enhance photon absorption and electron-hole
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Acknowledgments: We acknowledge support from the
Center for Chemical Innovation of the National Science
Foundation (POWERING THE PLANET: grant CHE-1305124).
We thank M. A. Woo for electrodepositing FeOOH and NiOOH
on FTO substrates and testing their electrochemical water
Materials and Methods
Figs. S1 to S11
Tables S1 to S4
7 October 2013; accepted 31 January 2014
Published online 13 February 2014;
A Direct Quantitative Measure of
Surface Mobility in a Glassy Polymer
Y. Chai,1 T. Salez,3 J. D. McGraw,4 M. Benzaquen,3 K. Dalnoki-Veress,3,4
E. Raphaël,3 J. A. Forrest1,2†
Thin polymer films have striking dynamical properties that differ from their bulk counterparts.
With the simple geometry of a stepped polymer film on a substrate, we probe mobility above and
below the glass transition temperature Tg. Above Tg the entire film flows, whereas below Tg only
the near-surface region responds to the excess interfacial energy. An analytical thin-film model for
flow limited to the free surface region shows excellent agreement with sub-Tg data. The system
transitions from whole-film flow to surface localized flow over a narrow temperature region
near the bulk Tg. The experiments and model provide a measure of surface mobility in a simple
geometry where confinement and substrate effects are negligible. This fine control of the glassy
rheology is of key interest to nanolithography among numerous other applications.
The last decades have seen a considerable interest in the dynamical and rheological properties of glassy materials (1, 2). Re- cent efforts (1, 3–5) have focused on elucidating the nature of glassy dynamics both in the bulk and in systems, such as thin films or colloids,