Tuning electron transfer rates via systematic shifts in the acceptor state density using size-selected ZnO colloids

Article abstract of DOI:10.1021/ja104482t

We report direct measurements of the influence of the available density of acceptor states on the rate of near-barrierless electron transfer between a dye sensitizer and an oxide semiconductor. The electron donor was the excited state of a zinc porphyrin, and the acceptors were a series of size-selected ZnO nanocrystals. The available density of states was tuned by controlling the relative position of the ZnO band edge using quantum confinement. The resulting change in the rate was consistent with a simple model of the state density as a function of energy above the ZnO band edge.

Full text of DOI:10.1021/ja104482t

Published on Web 09/21/2010
Tuning Electron Transfer Rates via Systematic Shifts in the Acceptor State
Density Using Size-Selected ZnO Colloids
Adam S. Huss, Andrew Bierbaum, Raghu Chitta, Darren J. Ceckanowicz, Kent R. Mann,
Wayne L. Gladfelter, and David A. Blank*
Department of Chemistry, UniVersity of Minnesota, 207 Pleasant Street SE, Minneapolis, Minnesota 55455
Received June 3, 2010; E-mail: blank@umn.edu
Abstract: We report direct measurements of the influence of
the available density of acceptor states on the rate of near-
barrierless electron transfer between a dye sensitizer and an
oxide semiconductor. The electron donor was the excited state
of a zinc porphyrin, and the acceptors were a series of size-
selected ZnO nanocrystals. The available density of states was
tuned by controlling the relative position of the ZnO band edge
using quantum confinement. The resulting change in the rate
was consistent with a simple model of the state density as a
function of energy above the ZnO band edge.
Electron transfer (ET) processes in dye-sensitized solar cells
made from nanocrystalline semiconductors have received con-
siderable attention. Rates for ET are typically found to be
multifaceted and complicated by film heterogeneity.1-3 This has
motivated measurements of ET in somewhat better defined
systems comprising dye molecules bound to colloidal nano-
crystals (NCs) dispersed in solution.4-7 These systems can
Figure 1. (a) Absorption and emission spectra of ZnP. (b) Absorption
spectra of the size-selected colloidal ZnO NC samples.
exhibit single-exponential ET kinetics and provide a platform
for more detailed investigations of the ET reaction.5 One example
is the ability to use quantum confinement to tune the energetic
alignment of the donor and acceptor without altering the
chemistry. By tuning the relative donor potential via size
selection of CdSe NCs, Robel et al.8 demonstrated control over
the classical Marcus barrier, thereby changing the ET rate. Here
we present the use of size-selected semiconductor electron
acceptors to measure the influence of the density of acceptor
states on the ET rate. The donor was a zinc porphyrin, and the
acceptors were a series of size-selected ZnO NCs. To our
knowledge, this is the first reported evidence for modulation of
ET rates using quantum confinement to tune the acceptor state
density systematically.
The electron donor, 5-(4-carboxyphenyl)-10,15,20-tris(2,4,6-
trimethylphenyl)porphyrinatozinc(II) (abbreviated below as ZnP),
and size-selected colloidal dispersions of ZnO NCs were
synthesized as described elsewhere.9,10 All of the experiments
were performed in methanol at room temperature, and the ZnP
concentration was kept under 0.1 mM. Absorption and emission
spectra for ZnP are shown in Figure 1a. The absorption spectra
for the four size-selected distributions of ZnO NCs used in this
study are shown in Figure 1b, and the average NC diameter for
each sample, indicated in the figure legend, was assigned on
the basis of the absorption onset.11
The electron transfer kinetics were measured using a combina-
tion of pump-probe and time-resolved fluorescence spec-
troscopies. Figure 2a shows the pump-probe spectrum for ZnP
alone in solution, and Figure 2b presents the same experiment
for a ∼1:1 ratio of ZnP and 3.7 nm diameter ZnO NCs. ZnP
Figure 2. Transient spectra following 2.2 eV excitation of (a) ZnP and
(b) a ∼1:1 mixture of ZnP and 3.7 nm diameter ZnO NCs. (c) TCSPC at
1.9 eV following excitation at 3.3 eV of ∼1:1 ZnP/ZnO NC mixtures. (d)
Relative electron injection rates.
has a broad excited-state absorption that shows very little time
evolution over the first nanosecond, consistent with the singlet
lifetime of 2.3 ns. By 11 ns, the spectrum has shifted to that of
the lower-energy triplet absorption. Overlapped with the transient
absorption is stimulated emission that appears as a negative
contribution to the transient spectrum corresponding to the
spontaneous emission spectrum, which is shown as a solid black
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J. AM. CHEM. SOC. 2010, 132, 13963–13965 13963

C O M M U N I C A T I O N S
offset.7 However, in the case of an energetic distribution of
available acceptor states, when λ is smaller than ∆G0, acceptor
states with barrierless transfer (λ + E ) -∆G0) can dominate
the rate.15 Under such circumstances, it is the pre-exponential
terms that control the rate, and this is the case here. At 3.65 eV
below the vacuum level, the excited state of ZnP lies well above
the ZnO CB edge energetically.16 The ZnO CB edge ranges from
3.92 to 4.13 eV (below vacuum) for NCs larger than 2.5 nm
(see the Supporting Information). Under the assumption that the
electronic coupling and Fermi occupancy do not depend
significantly on E, changes in kET result primarily from changes
in the density of acceptor states for near-barrierless ET, F(E )
-∆G0 - λ). Decreasing the size of the ZnO NC acceptor shifts
the CB edge up and decreases F(E ) -∆G0 - λ). Using a simple
model in which F(E) is proportional to ꢀE,15 we compared the
prediction of eq 1 to our measured relative ET rates as a function
of ZnO NC diameter, as shown in Figure 2d. The best fit was
found using λ ) 0.3 eV, which is comparable to the values of
0.5-0.7 eV reported for analogous zinc porphyrin ET systems.17,18
The single-exponential ET kinetics and the good agreement with
eq 1 when a very simple model for F(E) was employed suggests
that states near and below the CB band edge, such as trap states,
likely do not play a significant role in the observed charge
injection. This reflects the fact that ET predominantly takes place
via barrierless transfer well above the CB edge, consistent with
the rather subtle influence of particle size on the ET rates.15
The single-exponential ET kinetics also reflects the relatively
homogeneous distribution of donor-acceptor interactions pro-
vided by the ∼1:1 ZnP/ZnO NC ratio in the colloidal dispersions.
We believe this to be the first experimental observation using
the shift in acceptor state density caused by the shift of a
semiconductor CB via NC size selection to control electron
injection rates in a model dye-sensitized solar cell system.
Chakrapani et al.19 have suggested the influence of F(E) on back-
ET with variation of the pH. The present results provide direct
experimental evidence to support the predicted influence of F(E)
on ET. As the acceptor NC size becomes small, injection
becomes heavily weighted by the higher rates at the larger end
of a given colloidal sample distribution. Future development of
methods to produce narrower distributions of oxide semiconduc-
tors within the quantum-confined size regime will improve the
prospects for detailed studies of F(E) and its influence on ET.
Table 1. Time Constants for Electron Injection
ZnO NC diameter (nm)
τ (ps)
2.8
3.7
4.9
5.4
369 ( 10
299 ( 10
258 ( 9
245 ( 9
line for reference. The ZnP/ZnO NC mixture exhibits an
accelerated decay of the stimulated emission feature, and there
is a correlated rise of a new transient absorption centered around
1.9 eV. This feature has been identified previously as absorption
by the radical cation form of the porphyrin,12 and our own
spectroelectrochemistry measurements provide additional support
for this assignment (see the Supporting Information). On the
basis of the pump-probe measurements, we have assigned the
primary mechanism for excited-state quenching of ZnP by ZnO
NCs as ET.
To eliminate complications from the overlapping spectral
contributions in the pump-probe transients, time-correlated
single photon counting (TCSPC) was used to measure the rate
of ET via quenching of spontaneous emission from ZnP. These
data were found to fit well to a sum of two exponential decays,
one with a time constant matching the unperturbed excited-state
dye lifetime of 2.3 ns, corresponding to noninjecting ZnP, and
a larger amplitude, faster decay component with a time constant
of ∼300 ps. Fitting parameters for the faster quenching are
shown in Table 1, and normalized TCSPC traces with the
noninjecting lifetime component subtracted for clarity are shown
in Figure 2c.
The faster fluorescence decay component, which we believe
to be dominated by ET, is slower than previous reports for
electron injection from ZnP on TiO2 films.12,13 The time
evolution of the pump-probe measurements at 1.9 eV agree well
with the TCSPC results and fail to show evidence for any faster
components despite the much higher time resolution (∆t < 100
fs). Slower ET from dyes attached to ZnO than from dyes
attached to TiO2 has been observed for many systems. This has
been attributed to a number of possible origins, including weaker
electronic coupling between the dye and ZnO.6,14 Rather than
on the physical basis of the quantitative value of the rate, the
discussion here will focus on the observation that the measured
ET rate exhibits a systematic dependence on the average diameter
of the ZnO NC acceptor.
Within the assumption of weak coupling (diabatic), the ET rate
coefficient kET can be estimated from the classical Marcus expres-
sion, as shown in eq 1.15 The pre-exponential factor has been
expanded to include explicitly the density of acceptor states, F(E),
as a function of energy above the acceptor band edge, E. The
electronic coupling is H(E), λ is the reorganization energy, f(E, EF)
is the Fermi occupancy factor, and ∆G0 is the energy difference
between the conduction band (CB) edge of the acceptor and the
excited state of the donor. Integration over all values of E accounts
for injection into all possible acceptor states.
Acknowledgment. This work was funded by a grant from the
Chemical Sciences, Geosciences, and Biosciences Division,
Office of Basic Energy Sciences, Office of Science, U.S.
Department of Energy, under Award DE-FG02-07ER15913.
Supporting Information Available: Synthesis, experimental and
analysis procedures, spectroelectrochemistry of ZnP, and additional
references. This material is available free of charge via the Internet
at http://pubs.acs.org.
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