Detailed Description of Application of Transient Absorption Spectrometry in Photoelectrochemical Splitting of Water for studying the electron-hole pair recombination in semiconductor. [Illustrated with examples (Reference: Research Papers)]

1. Application of Transient Absorption
Spectrometry in Photoelectrochemical
Splitting of Water
--------------------------By Runjhun Dutta

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The performance of PECs is considered in
terms of:
• Excitation of electron–hole pair in photo-
electrodes.
• Charge separation in photo-electrodes.
• Electrode processes and related charge
transfer within PECs.
• Generation of the PEC voltage required for
water decomposition.
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3. Water photo-electrolysis using a PEC involves several processes within photo-
electrodes and at the photo-electrode electrolyte interface, including:
 Light-induced intrinsic ionization of the semiconducting material (the photo-anode), resulting in
the formation of electronic charge carriers (quasi-free electrons and electron holes).
 Oxidation of water at the photo-anode by electron holes.
 Transport of H+ ions from the photo-anode to the cathode through the electrolyte and transport of
electrons from photo-anode to the cathode through the external circuit.
 Reduction of hydrogen ions at the cathode by electrons.
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 However, rather than proceeding to partake in redox reactions, photoexcited electrons and holes could,
instead, recombine, which results in a loss of useful energy. Recombination is commonly identified as a
key factor in limiting efficiency.
 It may also be inferred that higher energy conversion efficiencies require longer charge carrier
lifetimes, as the probability of an excited charge carrier partaking in redox catalysis increases relative
to the probability of recombination.
 For the continuous advancement of rationally designed photo- catalysts, it is necessary to characterize
and understand the dynamics of photoexcited charge carriers.

5. Transient Absorption Spectroscopy (TAS)
▸ TAS is generally used to study the time scale of fundamental processes in solar energy conversion, including
formation, trapping, transfer, and recombination of photogenerated charge carriers in semiconductors.
▸ TAS is based on the pump-probe technique, where the pump initiates the photoexcitation of a sample and the
probe measures the optical absorption of the sample at various time following the excitation, thus tracing the
time evolution from excited states to ground states.
▸ In situ TAS measurement under PEC conditions enables the direct observation of the photogenerated carrier
kinetics in photoelectrodes, and the spectra should be collected in the time range matching the time scale of the
kinetic process of interest. Generally, charge transfer at solid-liquid interfaces exhibits slower kinetics (longer
time scales) than that at solid-solid interfaces.
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μs-s TAS For solid-solution interfacial carrier dynamics Weakness: Intense excitation
pulse is required to generate
detectable signals, which
deviates from continuous solar
irradiation
Ultrafast TAS For solid-solid interfacial carrier dynamics

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FIG.2.Schematic illustrating thebasicprinciplebehind transmission-mode transientabsorption spectroscopy,withpanels (a)and (b),respectively,showing measurements
ofthesampleinitsgroundandexcitedstates.Panel(b)alsoillustratesthattime-zerooftheexperimentisdefinedasthemomentofsampleexcitation,andthetimedifference
betweenthepumpandprobepulsesdefinesthetime-delay(τ),whichisvariedtoachievetime-resolution.I0 andI,respectively,representtheincidentandtransmittedprobe
I0 = I0
∗
.
intensities through the ground-state sample, whilst I0
∗
and I∗
, respectively, represent the incident and transmitted intensities through the excited sample. In the ideal case,

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ADVANTAGE OF TAS IN PEC
TAS allows us to directly monitor the presence of photogenerated holes by the observation of their optical
absorption. This ability to directly monitor photogenerated holes is particularly important for photoanodes as it
is the photogenerated holes which drive the key electrode function of water oxidation.
DISADVANTAGE OF TAS IN PEC
TAS has a limited sensitivity and therefore requires the use of relatively intense excitation pulses to generate
sufficiently high hole density to be detectable.

8. ▸ TA signals are usually directly reported as changes in the absorbance of a material.
▸ The transient absorption signal (ΔA) is defined as the difference between the absorbance of the
excited and ground-state samples.
ΔA(λ, τ) = A∗(λ, τ) − A0(λ)
▸ where A∗(λ, τ) and A0(λ), respectively, represent the absorbance of the excited and ground-state
samples.
▸ The parameters λ (wavelength) and τ (time-delay) have been explicitly stated to emphasize that
the transient signal is a function of both the (probe) wavelength and the pump–probe time-delay,
defined as time elapsed after excitation.
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TA SIGNALS

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There are three electronic contributions to the TA signal:
The final observed TA signal is a sum of all three contributions.
 Excited-state absorption
Positive
 Stimulated emission
Negative
 Ground-state bleach
Negative

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• Schematic illustrating the origins of excited-state absorption, stimulated emission, and ground-state bleach in
the TA signal of a semiconductor.
• The ground-state absorption is always subtracted as a reference, so any absorption of the ground-state
sample contributes as negative ground-state bleach in the final signal.
• Probe photons are shown as green arrows. Photoexcited electrons and holes are, respectively, shown as
green and hollow circles.

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 In negative regions of the TA spectrum, it may be deduced that ground-
state bleach and/or stimulated emission dominates over contributions
from excited-state absorption, and vice versa for positive regions of the
spectrum.
 In semiconductor systems, ground-state bleach can be expected to
dominate the TA signal at probe energies above the bandgap.
 As the behavior of photoexcited charges is often the subject of interest,
contributions to the TA signal from ground-state bleach and stimulated
emission are often unwanted factors that complicate the analysis of TA
data.

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To record the spectra in the presence of electron and hole scavengers:
 These scavengers capture photoexcited electrons/holes thus, suppressing recombination, allowing
long-lived counterparts to be observed. Depending on the nature of the scavenger used, the TA
spectrum can be accordingly assigned.
Electrical bias can also be used to assign spectral features in TA spectra:
 Positive bias creates “excess” holes; therefore, the TAS signal of photoexcited holes can be identified.
 Negative bias creates “excess” electrons, therefore, allowing electron signals to be identified.
Method of distinguishing between electron and hole contributions

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 It has been previously noted that TA spectra can be very sensitive to conditions such as pH, surface
conditions, and properties such as particle size.
 Experimental parameters such as excitation wave length may also contribute to some apparent
discrepancies between different works.
 Extra caution must be taken when assigning TA signals near and above the bandgap of a material.
Thermal effects that manifest as lattice expansions result in a slight bandgap reduction. This shifts
the absorption edge to longer wavelengths, which shows up in difference spectra as a distinct
positive peak near the bandgap.

14. Experimental Procedure 01
Charge generation dynamics in hematite photoanodes decorated with gold nanostructures
under near infrared excitation
{ OKAZAKI etal (2020) }
▸ Femtosecond pump–probe transient absorption spectroscopy was employed with an amplified Ti:sapphire
laser (800 nm wavelength, 130 fs FWHM pulse width, 0.8 mJ/pulse intensity, 1 kHz repetition, Spectra Physics,
Hurricane).
▸ Used the fundamental beam or second harmonic light from an amplified Ti:sapphire laser at a 500 Hz
modulation frequency as a pump beam.
▸ A typical pump power of 1.0 mW corresponds to 1.0 mJ/cm2. In contrast, the white-light continuum generated
by focusing the fundamental beam onto a sapphire plate (2 mm thick) was used as a probe beam.
▸ The probe beam was focused at the center of the pump light (∼0.3 mm diameter) on the specimen, and the
transmitted probe beam was detected by a Si photodiode after passing through a monochromator (Acton
Research, SpectraPro-150).
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TA spectra of
(a) hematite excited at 400 nm (b) AuNRs/hematite excited at 800 nm.
TA decays of
(c) hematite excited at 400 nm (d) AuNRs/hematite excited at 800 nm.

16. ▸ Two spectra at 500 ps are similar in shape with each other, which suggests that the injected
electron transfers backward from hematite to AuNRs and recombination is completed within 500
ps.
▸ These spectra reached nearly zero after 100 ps. It is known that electrons in the semiconductor
give broad absorption due to intra-band transition within the conduction band.These results denote
that the electrons injected from AuNRs to hematite underwent a recombination with
photogenerated holes in AuNRs within 100 ps.
▸ Electron injection time seems to be within 250 fs (the time-resolution), judging from the prompt
rise observed at all probe wavelengths in Fig. (d).
▸ Photogenerated electrons at AuNRs transfer to hematite within 250 fs, followed by a
recombination with photo- generated holes at hematite as back electron transfer within 100 ps.
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17. ▸ In order to discuss electron transfer dynamics from gold nanorods to hematite, differences in
spectra were calculated by subtracting TA spectra of pristine hematite excited at 400 nm from
those of AuNRs/hematite excited at 800 nm.
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18. Conclusions Drawn-
▸ The similar decay processes at different pump intensities indicate that photogenerated
carriers recombine through geminate electron–hole recombination so that second order bulk
electron–hole recombination is not observed in our pump power range.
▸ In general, more mobile charges allow absorption in the longer wavelength region, so the
observed decay behavior being faster at longer wavelength should include the charge trapping
process.
▸ A large decrease in the amplitude to 10 ps suggests significant loss of the activity of
photogenerated carriers.
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19. Experimental Procedure 02
 Charge carrier dynamics of Fe2O3 photoanodes in a complete photoelectrochemical cell were measured using
microsecondsecond transient absorption spectroscopy (TAS).
 A three-electrode configuration was used, with a Pt gauze counter electrode, Ag/AgCl/3M NaCl reference
electrode, a Ministat 251 (Thompson Electrochemical) and 0.1M NaOH electrolyte (pH12.8).
 Potentials are primarily reported versus the Ag/AgCl/3M NaCl (SSC) reference electrode; these were converted to
those versus the reversible hydrogen electrode (RHE) using the Nernst equation:
ERHE= E0SSC + ESSC + 0.059pH
where E0SSC is the standard potential of the SSC reference (0.21 VRHE at 25 C)
ESSC is the potential versus SSC.
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Correlating long-lived photogenerated hole populations with photocurrent
densities in hematite water oxidation photoanodes
{ PENDLEBURY etal (2011) }

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Electrolyte solutions were prepared from NaOH (as received from Sigma-Aldrich, reagent
grade) and Milli-Q-water (Millipore Corp., 18.2MU cm at 25 C) and degassed with argon (BOC,
pure shield grade) prior to measurement.
Degradation of the reference electrode by the alkaline electrolyte was prevented by the use of
a double junction configuration with a 0.5 M NaClO4 junction electrolyte (pH 6) and a 3 A
molecular sieve porous junction frit.
The transient absorption spectrometer consists of a 75 W Xe lamp probe beam which passes
though monochromators before and after the sample and is detected by a silicon PIN
photodiode.
 The third harmonic of a Nd:YAG laser is used as the excitation source (355nm, 6 ns pulse
width). Laser intensities were adjusted for absorption by the substrate (for SE
measurements), such that approximately the same number of photons was incident on the a-
Fe2O3 in all measurements described.
 Reasonably low laser intensities were used (0.17–0.39 mJ cm2, except for excitation intensity
studies). Laser repetition rates (0.25–0.33 Hz) were chosen such that signals due to
photogenerated charge carriers had decayed to zero before the next laser pulse; each trace is
the result of averaging over 300–1000 laser pulses.

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Transient photocurrent (TPC)
 Measurements were made on a timescale of microseconds milliseconds using the three-electrode cell and
TAS apparatus but with the probe beam blocked. The voltage drop across a 47 U resistor in series between
the working and counter electrodes was recorded using an oscilloscope (Tektronix TDS220), averaging 500–
750 measurements per trace.
Photoelectrochemistry
 Current/voltage curves (20 mV s1 scan rate) were recorded using the three-electrode set-up in a home-made
PTFE cell with quartz windows and using an Autolab potentiostat (PGSTAT12).
 A monochromated 75 W ozone-free Xe lamp was used as the light source. The lamp output was adjusted with
neutral density filters such that the white light incident on the cell was equivalent to 1 Sun in intensity, although
there are some differences between the spectral distributions of the Xe lamp and the solar spectrum.
 The illuminated area of the Fe2O3 working electrodes was 0.12–0.25 cm2; approximately the same part of
the Fe2O3 film was illuminated as probed during TAS measurements.
 For EE (‘‘front-side’’ illumination) measurements, a piece of blank FTO glass substrate was used as a filter to
ensure that the same light intensity was incident on the Fe2O3 for both EE and SE (‘‘back-side’’ illumination)
measurements.

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Results
Transient absorption studies of photogenerated holes
 Photocurrent generation is dependent on the generation of long lived holes.
 On the timescales of these measurements (microseconds), the transient absorption decays exhibit two
phases:
 A fast phase is observed between 1 ms and ca 20 ms, with a bias dependent median lifetime (t50%),
assigned to electron-hole recombination.
 This fast phase is followed by a slower phase with a lifetime on the hundreds of milliseconds to seconds
timescale that exhibits a bias-dependent amplitude.

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• The quantitative correlation between the amplitude of the long-lived hole signal at various applied bias
and the photocurrent/ voltage curve was observed for all the semiconductor photoanodes that they have
examined.
• There is a clear common correlation between the amplitude of the long-lived photogenerated hole signal
and the photocurrent density. This correlation provides further evidence for the importance of long-lived
holes in driving water photo-oxidation.

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 Electron-hole recombination is a bimolecular process dependent
upon both electron and hole densities. This is supported by both the
bias and excitation density dependence of the fast transient
absorption decay phase.

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Excitation density studies
 The excitation density range for the transient (laser) measurements is much greater than for the steady-
state (continuous wave) measurements; only the very lowest laser intensities are likely to correspond to
the charge density regime of the steady-state photocurrent measurements.
 As the excitation density is increased, the fast transient absorption decay phase becomes faster and
increasingly dominant.

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 The TPC signals are strikingly similar to the ‘fast phase’’ of the transient absorption decay curves for
both bias conditions.
 Under bias conditions where water oxidation is observed, electron extraction occurs on a timescale
much faster than the kinetics of water oxidation.
 These transient photocurrent decays can be most easily understood as monitoring the recovery of
electron density towards dark equilibrium following the pulsed excitation. This recovery in electron
density will result from both electron-hole recombination and electron extraction by the external circuit.
 Given that electron-hole recombination will also be dependent upon this excess electron density, this is
consistent with observation that the TPC decays track the fast phase of our transient absorption
decays. Note that an additional factor likely to impact upon the kinetics of the TPC decays may be the
time taken for electrons to move through the hematite film to the back contact.
 At 0.3 to 0 VSSC, both the TPC and transient absorption kinetics were independent of the direction of
excitation, consistent with these decays being dominated by electron-hole recombination in the
absence of significant electron collection.
 However at strongly anodic biases, where considerable photocurrent occurs, both the TPC and
transient absorption kinetics are dependent upon the direction of excitation, indicating that electron
transport dynamics to the back contact become significant relative to electron-hole recombination.
ELECTRON HOLE RECOMBINATION

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 The fast decay phase is assigned to electron-hole recombination. This electron-hole recombination
becomes slower as the steady state electron density in the film is reduced under positive electrical
bias.
 The slow transient absorption decay phase is assigned to residual photogenerated holes which have
not recombined with electrons.
 There is a quantitative correlation between the amplitude of this slow decay phase (i.e. the
population of long-lived holes) and the electrode photocurrent monitored as a function of applied
bias.
 The onset of photocurrent generation at positive bias is attributed to the pronounced retardation in
electron-hole recombination with applied positive potential, allowing electron extraction to the
external circuit and leading to the generation of long-lived holes. These long-lived holes then go on to
drive water oxidation on a timescale of hundreds of milliseconds to seconds.
 Actual hole lifetime required for water oxidation depends on the timescale of the oxidation reaction on
a given material. strategies to increase the yield of long lived holes and therefore to optimise the
performance of hematite photoanodes should focus upon decreasing the level of electron hole
recombination, either through material design to directly retard the kinetics of electron-hole
recombination, or by accelerating the kinetics of electron extraction by the external circuit.

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