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Origin of the overall water splitting activity of Ta3N5
revealed by ultrafast transient absorption
spectroscopy†
Dharmapura H. K. Murthy, a Hiroyuki Matsuzaki, *a Zheng Wang, b
Yohichi Suzuki,a Takashi Hisatomi, b Kazuhiko Seki, a Yasunobu Inoue,cd
Kazunari Domen *bc and Akihiro Furube *ae
Tantalum nitride (Ta3N5) is one of the few visible light absorbing photocatalysts capable of overall water
splitting (OWS), by which the evolution of both H2 and O2 is possible. Despite favourable energetics,
realizing the OWS or eﬃcient H2 evolution in Ta3N5 prepared by the nitridation of tantalum oxide (Ta2O5)
or Ta foil remains a challenge even after 15 years of intensive research. Recently our groupdemonstrated OWS in Ta3N5 when prepared by the short time nitridation of potassium tantalate (KTaO3).
To obtain a mechanistic insight on the role of Ta precursor and nitridation time in realizing OWS,
ultrafast dynamics of electrons (3435 nm probe) and holes (545 nm probe) is investigated using transientabsorption spectroscopy. Electrons decay majorly by trapping in Ta3N5 prepared by the nitridation of
Ta2O5, which do not show OWS. However, OWS activity in Ta3N5 prepared by 0.25 hour nitridation of
KTaO3 is particularly favoured by the virtually absent electron and hole trapping. On further increasing
the nitridation time of KTaO3 from 0.25 to 10 hour, trapping of both electron and hole is enhanced

Received 15th January 2019
Accepted 25th April 2019

which concurrently results in a reduction of the OWS activity. Insights from correlating the synthesis
DOI: 10.1039/c9sc00217k

conditions—structural defects—carrier dynamics—photocatalytic activity is of importance in designingrsc.li/chemical-science

novel photocatalysts to enhance solar fuel production.

1. Introduction
Considering ever increasing energy demand, there is an urgent
need to promote and enhance the production of renewableenergy sources. This approach will pay dividends in protecting
our environment from a serious crisis. In this direction, using
solar energy to generate H2 from water using a photocatalytic
process constitutes one of the methods.1–9 Over the past
decades, this approach has been intensively investigated toenhance the solar to hydrogen (STH) conversion eﬃciency. For
the commercial use of the solar H2, a stable photocatalystaNational Institute of Advanced Industrial Science and Technology (AIST), Tsukuba
Central 2, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan. E-mail:hiroyuki-matsuzaki@aist.go.jpbCentre for Energy & Environmental Science, Shinshu University, 4-17-1 Wakasato,
Nagano-shi, Nagano 380-8553, Japanc
Department of Chemical System Engineering, School of Engineering, The University ofTokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan. E-mail: domen@chemsys.t.
u-tokyo.ac.jpdJapan Technological Research Association of Articial Photosynthetic Chemical
Process (ARPChem), 2-11-9 Iwamotocho, Chiyoda-ku, Tokyo 101-0032, Japane

Department of Optical Science, Tokushima University, 2-1 Minamijosanjima-cho,
Tokushima 770-8506, Japan. E-mail: furube.akihiro@tokushima-u.ac.jp† Electronic supplementary
10.1039/c9sc00217k

information(ESI)

This journal is © The Royal Society of Chemistry 2019available.

See

DOI:

consistently demonstrating a higher STH eﬃciency is desired.
To address this challenge, novel visible light absorbing photocatalysts are being synthesized and tested for photocatalytic
water splitting activity.
Tantalum nitride (Ta3N5) is an n-type, visible light absorbing
nitride-based photocatalyst. The conduction band (CB) andvalence band (VB) of Ta3N5 are characterized by the Ta 5d and N
2p orbitals, respectively.10,11 Ta3N5 is commonly prepared by thenitridation reaction of tantalum (Ta) precursors such as Ta foil
or tantalum oxide (Ta2O5). The VB and CB positions in Ta3N5 arethermodynamically suitable to drive water oxidation andreduction to generate O2 and H2, respectively. Hence, Ta3N5 iscapable of overall water splitting (OWS) upon one-step photoexcitation. However, Ta3N5 is usually known as a photoanodematerial with eﬃcient O2 evolution,10,12–17 but not for either H2evolution or OWS ability.
Recently, OWS activity with an apparent quantum eﬃciency(AQE) of 70% is demonstrated in an ultraviolet light absorbing
SrTiO3 photocatalyst.18 As a further advancement, Ta3N5 forms
one of the few promising visible light absorbing photocatalysts
capable of OWS. Nevertheless, achieving OWS in Ta3N5
prepared by nitriding Ta foil or Ta2O5 still remains a challenge.
Recently our group employed potassium tantalate (KTaO3) asa Ta precursor for the nitridation reaction to prepare Ta3N5. In
this approach, Ta3N5 prepared by 0.25 hour nitridation ofChem. Sci., 2019, 10, 5353–5362 | 5353

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KTaO3 demonstrated the highest OWS activity. This is the rst
report on realizing visible light OWS activity without the need ofsacricial reagents for Ta3N5.19 However, the eﬃciency of OWS
activity was found to decrease with increase in the nitridation
time from 0.25 to 10 hour. These observations emphasize theneed to elucidate the role of the Ta precursor and the nitridation time in determining the eﬃciency of OWS.
As stated earlier, the H2 evolution in Ta3N5 (prepared by thenitridation Ta foil or Ta2O5) is apparently less known andtypically smaller when compared to O2 evolution. In a few
reports that demonstrate H2 evolution in Ta3N5, the size of
Ta3N5 particles was relatively small (between 5 to 8 nm (ref. 20)
and in one case it is 2 nm (ref. 21)). In addition to the smallersize, Ta3N5 particles were typically present in a composite form
with either SiO2 (ref. 20) or graphene oxide.21 In these reports,
the eﬀect of size on the band bending/optical properties and the
expected electronic interaction between the Ta3N5 with thecounterpart in the composite is not considered. In anotherreport showing H2 evolution, the surface of the Ta3N5 wasmodied with magnesium layer.22 Thus, without the modication of either structural/surface properties of Ta3N5 or by
inducing electronic interaction with another material, H2evolution was not realized. Based on these observations, it isagreeable to state that eﬃcient generation of H2 in Ta3N5 is
fundamentally less pronounced compared to O2.10,21,23 However,
the origin of this observation is unclear.
A key approach to examine the role of Ta precursor and thenitridation time on the eﬃciency of OWS is investigating the
decay dynamics of photogenerated charge carriers. Whilst there
has been extensive research/progress in the development of
Ta3N5 from the materials perspective, the dynamics of photogenerated charge carriers and its impact on the eﬃciency of thephotocatalytic reaction have not been explored. The fundamental process in OWS in Ta3N5 is the generation of electron–
hole pairs upon absorption of light. Then, these carriers must
migrate towards the surface of the Ta3N5 to get involved in thereaction with water. However, due to structural/electronic
defects in Ta3N5, eﬃcient charge transport towards thesurface is aﬀected by the undesirable trapping and/or recombination. To unveil these photoinduced processes in detail,femtosecond time-resolved diﬀuse reectance (fs-TDR) spectroscopy is employed, which is a pump–probe method. Here,
excitation of Ta3N5 with an ultrafast laser pulse leads to theformation of holes and electrons in the VB and CB, respectively.
The dynamics of these transient species can be followed bymonitoring their transient absorption spectrum using an
optical probe pulse from visible to infrared. This technique has
been widely used to examine charge carrier dynamics in various
types of photocatalysts and to correlate the obtained photophysical parameters with that of photocatalytic eﬃciency.24–29
Note that TDR is essentially similar to the transient absorption
spectroscopy (TAS) except that the TA signal in the former ismonitored in diﬀuse reection mode because of the opaquenature of the Ta3N5 powder samples. The ultrafast dynamics ofelectrons and holes and their decay pathway was found to bestrongly inuenced by the Ta precursor used to prepare Ta3N5
and the nitridation time. Results from this systematic

5354 | Chem. Sci., 2019, 10, 5353–5362

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investigation help in understanding the structure–property
relationship and provide rational insights on Ta3N5 material
design to improve the eﬃciency of OWS. As mentioned earlier,this work provides mechanistic insight on explaining OWS
through charge carrier dynamics, while a comprehensive studyof the morphological characterization and photocatalytic
performance is reported elsewhere.19

2.

Results and discussion

Fig. 1 shows the ground-state optical absorption spectra for the
Ta3N5 prepared by nitridation of Ta2O5 or KTaO3 in both visible
and NIR region. The absorption peak at 545 nm (Fig. 1A) is
attributed to the fundamental transition (N 2p to Ta 5d) in
Ta3N5. Though the magnitude of absorption at 545 nm
increases with nitridation time, the energetic position was notinuenced by the choice of Ta precursor or the nitridation time.
As shown in Fig. 1B, an enhancement in the magnitude of NIR
absorption is noticed with an increase in the nitridation timefrom 0.25 to 10 hour indicating the presence of defect states
within the bandgap of Ta3N5. For Ta3N5 prepared by 0.25 hournitridation of KTaO3, noticing a virtually absent NIR absorption
suggests a signicant reduction in the concentration of defectstates. This conclusion is in good agreement with our previous
study using scanning transmission electron microscopy(STEM), which revealed the single-crystal nature of thissample.19 Previous reports have diﬀerent opinions in relating
the NIR absorption with the electronic nature of these defectstates.17,30–33 Hence, we consider that the NIR absorption in
Ta3N5 is due to the formation of defects related to charged
nitrogen vacancies and/or reduced Ta vacancy.
In the following parts, Section 2.1 will correlate the charge
carrier dynamics in Ta3N5 synthesized by the nitridation of
Ta2O5 and KTaO3 (0.25 hour). In Section 2.2, the eﬀect of KTaO3
nitridation time on the charge carrier dynamics in Ta3N5 will bepresented. In the last Section 2.3, plausible approaches toenhance the eﬃciency of OWS in Ta3N5 will be outlined.
2.1 Origin of overall water splitting in Ta3N5 prepared by the0.25 hour nitridation of KTaO3
The eﬃciency of OWS was found to be highest in Ta3N5
prepared by 0.25 hour nitridation of KTaO3. On the other hand,
0.25 hour nitridation of Ta2O5 neither forms Ta3N5 phase norshows the OWS. To understand the origin of OWS activity,
charge carrier dynamics is compared between the Ta3N5
prepared by 0.25 hour nitridation of KTaO3 and long-time
nitridation of Ta2O5. We selectively monitor the dynamics offree electrons and VB holes at 3435 and 545 nm wavelengths,
respectively. The responsible optical transitions probed atthese wavelengths are depicted in Scheme 1. It has been
consistently demonstrated that probing at 3435 nm (IR region)
typically monitors the intra-band transition close to the CB
levels. Thus, 3435 nm probe oﬀers unique information on the
dynamics of free/shallowly trapped electrons.24,29,34,35 With
regard to the hole dynamics, 545 nm probe is employed. As
depicted in Fig. 1A, the absorption peak observed at 545 nm is

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Ground-state optical absorption spectra measured in the diﬀuse reﬂection mode in the (A) visible and (B) NIR region for KTaO3, Ta3N5
nitrided from KTaO3 for diﬀerent times as indicated and Ta3N5 nitrided from Ta2O5. The arrow in the (B) indicates an enhancement in the NIR
absorption with an increase in the nitridation time of KTaO3.
Fig. 1due to the bandgap transition from N 2p (VB) to Ta 5d (CB).
Monitoring the dynamics at 545 nm yields direct informationon the occupancy of holes in the VB, where the recovery of theground-state bleaching signal oﬀers information on the holedynamics.
2.1.1 The eﬀect of Ta precursor on the free electron
dynamics probed at 3435 nm. Fig. 2A shows the transients
corresponding to free electrons (3435 nm probe) in Ta3N5
prepared by the nitridation of KTaO3 for 0.25 hour. In the Ta3N5
prepared by the nitridation of KTaO3 for 0.25 hour, there still
exists a large amount of unreacted KTaO3 phase. Hence, it isessential to disregard the possibility of free electron generationfrom the unreacted KTaO3. The onset in the optical absorptionof KTaO3 is z345 nm (z3.6 eV). The visible light excitation
employed in this study does not allow bandgap transition in
KTaO3 to generate free electrons (Fig. S1†). Hence, the freeelectron dynamics presented in Fig. 2A originates from theTa3N5 phase formed aer nitridation, not from the unreacted
KTaO3.

Considering the TA signal is monitored in the diﬀuse
reection mode in the current study, a direct comparison of thesignal magnitude is not straightforward due to the likely
diﬀerences in the scattering/surface properties of diﬀerent
types of powder samples under study. Hence, we primarily relyon the decay dynamics rather than the signal magnitude while
comparing between two diﬀerent samples. The TA signal
magnitude at 3435 nm probe for Ta3N5 prepared by 0.25 hournitridation of KTaO3 can be found in ESI (Fig. S2).† To understand the decay pathways of free electrons in Ta3N5 (0.25 hournitridation of KTaO3), the eﬀect of pump uence on the half
lifetime, s1/2, (i.e., the time at which initial concentration ofphotogenerated electrons reach to half of its initial value) is
compared. The s1/2 value decreases from 200 to 5 ps uponincreasing the uence from 0.014 to 0.12 mJ per pulse. A qualitatively similar observation is noticed also for hole dynamics
probed at 545 nm (Fig. S3†). Thus, the dynamics of both electron and hole is very sensitive to pump uence from early pstime. In addition, matching of both free electron and hole

Scheme 1 Schematics illustrating the nature of electronic transitions responsible for the transient absorption signal when probed at 545 and3435 nm, corresponding to VB holes and free electrons, respectively. When the VB holes are formed due to light absorption, a ground-statebleaching signal is expected at 545 nm probe. The redox potentials of H2 and O2 evolution reaction15,36 with respect to CB and VB positions of
Ta3N5 shows the capability of Ta3N5 to participate in overall water splitting.

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dynamics probed at 3435 nm for Ta3N5 nitrided from (A) KTaO3 for0.25 hour and (B) from Ta2O5. 440 nm pump is used to conduct thesemeasurements.

dynamics is noticed (Fig. S4†). Fitting the transients in Fig. 2A toa second-order type recombination model showed good agreement (Fig. S5†). Taking these observations collectively, it isinferred that photogenerated carriers in Ta3N5 (0.25 hournitridation of KTaO3) decay primarily via second-order type
recombination of free (untrapped) electrons with holes. If thecharge carriers decay via trapping to defect states (generally
related to rst-order type recombination), uence-independent
dynamics is expected, which is however not observed.
Fig. 2B displays the normalized electron kinetics in Ta3N5
prepared by the nitridation of Ta2O5. The TA signal magnitudeat 3435 nm probe for Ta3N5 prepared by nitridation of Ta2O5 canbe found in ESI (Fig. S2).† Despite increasing the pump uence
by a factor of approximately nine, the electron dynamics in early
5 ps (Fig. 2B inset) is virtually unaﬀected suggesting theoccurrence of electron trapping. A similar observation isnoticed also at 580 nm pump (Fig. S6†). The s1/2 value in Fig. 2B
decreases from 50 to 7 ps upon increasing the uence from
0.014 to 0.12 mJ per pulse. A matching between the free electron
and hole dynamics was not observed (Fig. S7†) and the transients could not be tted to the second-order type electron–holerecombination model (Fig. S5†). Combining these observations,
it is concluded that the major part of electrons in Ta3N5
prepared by the nitridation of Ta2O5 decays via trapping rather
than second-order type direct electron–hole recombination.

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2.1.2 Electron transfer process from Ta3N5 prepared by
0.25 hour nitridation of KTaO3 to the Rh cocatalyst. To reveal
the mechanism of OWS, the eﬀect of Rh cocatalyst loading andthe water interface on charge carrier dynamics in Ta3N5
prepared by 0.25 hour nitridation of KTaO3 is carried out.
Typically, metal cocatalyst particles are loaded on the surface ofthe photocatalyst to enhance the overall eﬃciency of photocatalytic reaction. In the case of Ta3N5, Rh metal cocatalyst
particles are photodeposited on the surface by the photodeposition process. The Rh cocatalyst acts as an electron
acceptor to facilitate the reduction reaction with water ingenerating H2. A faster electron (3435 nm probe) decay startingfrom z5 ps was noticed for Ta3N5 prepared by 0.25 hournitridation of KTaO3 aer Rh cocatalyst loading (Fig. S8†)suggests electron transfer process to the Rh cocatalyst. To
further corroborate this process, the eﬀect of water interface on
the electron transfer to Rh cocatalyst is investigated. Considering water is prone to absorb IR probe light, 545 nm probe
which corresponds to VB holes is employed. Fig. 3 compares the
VB hole dynamics (545 nm probe) in the presence of waterinterface for Ta3N5 prepared by 0.25 hour nitridation of KTaO3
with and without Rh cocatalyst loading. Clearly, a longer hole
lifetime is noticed from z5 ps aer loading Rh cocatalyst. As
concluded from Fig. 2A, electrons decay majorly via recombination with VB holes. However, when electrons are transferred
to Rh cocatalyst present on the surface, the number of electrons
that recombine with VB holes is decreased. As a result, theeﬃciency of electron–hole recombination is reduced eventually
resulting in a prolonged lifetime for VB holes due to slow
recovery of the ground-state bleaching. A longer hole lifetimes isparticularly benecial to promote water oxidation reaction to
generate O2 and to realise OWS. Observing electron transfer
process corroborates the notion that electron trapping by the
structural/electronic defects is an ineﬃcient process, which isin agreement with the virtually absent ground-state NIR
absorption (Fig. 1B) and the single-crystal nature of the Ta3N5

The fs-TDR time proﬁles corresponding to VB holes probed at
545 nm for 0.25 hour nitridation of KTaO3 with and without Rh
cocatalyst loading. Both these transients are recorded in the presenceof water under the same pump ﬂuence (440 nm pump; 0.23 mJ per
pulse). Note that these experiments are conducted in water media andsigns of these transients are inverted for easy comparison.
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formed upon nitriding KTaO3 for 0.25 hour as reported previously by using STEM.19
2.1.3 On the inadequate H2 evolution in Ta3N5 prepared by
the nitridation of Ta2O5. For Ta3N5 prepared by the nitridationof Ta2O5, due to electron trapping within the bulk, eﬃcientmigration of electrons towards the surface cannot be anticipated.
As a result, electron transfer to the Rh was indeed not observed(Fig. S8†). Instead, hole transfer to Rh cocatalyst was observed as
evidenced by the longer electron lifetime. We do not completely
understand why electron transfer to the Rh cocatalyst was notobserved here. From XPS study, the Rh cocatalyst was conrmed
to be metallic but not as an oxide (Fig. S9†). One of the plausible
mechanisms is due to the occurrence of faster/eﬃcient electron
trapping is expected to reduce the electron mobility and thenumber of free electrons available for transfer to the Rh cocatalyst. This is the likely reason behind noticing inadequate H2evolution (compared to O2) and consequently the absence of
OWS activity in Ta3N5 prepared by the nitridation of Ta2O5. Thisobservation is in contrast to the Ta3N5 prepared by 0.25 hournitridation of KTaO3 which shows OWS. This comparison further
corroborates the eﬀect of Ta precursor on the electron dynamics
and on the electron transfer to the Rh cocatalyst.

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2.1.4 The eﬀect of Ta precursor on the hole trapping
process. Fig. 4 displays the TA spectra of Ta3N5 prepared by thenitridation of KTaO3 (for 0.25 hour) and Ta2O5 in the visible
region at diﬀerent delay times starting from 0.6 to 200 ps. The
TA spectra provide useful information on the presence of defectstates within the bandgap. Before discussing these results, it isimportant to associate the spectral features with the possible
transitions and/or defect states. Typically, the TA spectrum of
Ta3N5 shows three prominent features: (i) a bleaching (negative)
signal around 545 nm, (ii) a second bleaching signal at 590 nm,
and (iii) a broad positive TA signal between 650 to 750 nm. Let
us discuss each one of this contribution.
The beaching signal at 545 nm is due to the depopulation of
the ground state due to band gap transition (Fig. 1A and Scheme1) and is correlated to the VB free hole dynamics. The second
bleaching (negative) signal at 590 nm probe cannot be
explained unless considering the possibility of an optical transition from a lled defect state. To this end, the electronictransition from a positively charged nitrogen vacancies ðVN Þ to
the CB is tentatively proposed. A previous prediction using rst
principles density functional theory calculations is in good
agreement with this assignment.30,37 However, this transition is
not discernible in Fig. 1A, which is likely due to low absorption
coeﬃcient typical to transitions involving defect states. By
correlating the energetic positions of bleaching at 590 nm with
545 nm, the VN states are situated at 0.17 eV above the VB
maxima. When holes from the VB are trapped to VN states, VN isconverted to VN or VN , which reduces the density of VN states.
As a result, possibility of the VN to the CB optical transition at
590 nm reduces and eventually shows bleaching signal(Fig. S10†). We observe the 590 nm bleaching already at 0.6 psdelay time which indicates the ultrafast hole trapping processfrom the VB to VN . Thus, the presence of hole trapping process
can be tentatively related to 590 nm bleaching. In an earlier
report from Ziani et al. also observed the bleaching signal at
590 nm for Ta3N5 (nitrided from Ta foil) thin lms.38 In a recent
microsecond TAS study, trapped holes are found to be probed at
590 nm in Ta3N5 (nitrided from Ta2O5).34 Combining theseobservations, the presence of bleaching signal at 590 nm can becorrelated to the formation of hole trapping nitrogen vacancies
above the VB. The broad positive TA signal between 650 to
750 nm is tentatively assigned to shallowly trapped electrons(Fig. S11†) and will be discussed in Section 2.2.
Fig. 4A shows the TA spectra of Ta3N5 prepared by thenitridation of KTaO3 for 0.25 hour. The free VB hole dynamics
shows time-dependent change as observed at 545 nm bleaching. The virtually absent second bleaching at 590 nm indicatesthat the density of VN defect states is too low to promote holetrapping. This conclusion is in good agreement with noticing
a pronounced pump uence dependent free VB hole dynamics
probed at 545 nm (Fig. S3†). The occurrence of hole trapping isfound to be an incompetent process in Ta3N5 prepared by 0.25
hour nitridation of KTaO3.
Fig. 4B shows the TA spectra of Ta3N5 prepared by thenitridation of Ta2O5, which is diﬀerent compared to Fig. 4A in
terms of the second bleaching signal at 590 nm in addition to




















Fig. 4 TA spectra in the visible region at diﬀerent delay times for Ta3N5

prepared by the nitridation of (A) KTaO3 for 0.25 hour and (B) Ta2O5.
440 nm pump with a ﬂuence of 0.48 mJ per pulse is used while
conducting these measurements.

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the one at 545 nm. Eﬀect of pump uence on the dynamics of
VB free holes (545 nm probe) is found to be less prominent
particularly in early 5 ps time scale (Fig. S12†). This observationsuggests the presence of hole trapping VN defect states above
the VB. Noticing the pronounced second bleaching signal at
590 nm with time indeed support the occurrence of hole trapping. In short, the presence of second bleaching at 590 nm
indicates an enhancement in the formation of hole trapping VN
defect states.
Conclusion from Section 2.1 is briey summarized asfollows. Depending on the Ta precursor employed for thenitridation reaction to prepare Ta3N5, a distinctive electron andhole dynamics is rationalized. In Ta3N5 prepared by the nitridation of Ta2O5, trapping of both electrons and holes isobserved. In particular, due to eﬃcient electron trapping,electron transfer to the Rh cocatalyst is not observed which isessential to realize H2 evolution and consequently OWS activity.
In the case of Ta3N5 prepared by the 0.25 hour nitridation of
KTaO3, charge carriers are free and the decay is primarily via
second-order recombination between CB electrons and VB
holes. The decay of both electrons and holes by trapping isfound to be ineﬀective. This conclusion is in good agreementwith noticing virtually absent ground-state NIR absorption(Fig. 1B) and with the single-crystal nature of the Ta3N5 without
surface defects.19 Hence, eﬃcient migration of both electrons
and holes toward the surface is ensured eventually prompting
the OWS activity.
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2.2 The eﬀect of nitridation time on the charge carrier
dynamics in Ta3N5 prepared by the nitridation of KTaO3
The eﬃciency of OWS is found to be high for 0.25 hour nitrided
Ta3N5. However, the OWS activity was reduced by a factor of
approximately ve with an increase in the nitridation time from0.25 to 10 hour.19 Hence, investigating the eﬀect of nitridation
time on the carrier dynamics is essential to understand theorigin of this phenomena.
2.2.1 Comparing the free electron dynamics in Ta3N5
prepared by 0.25 and 10 hour nitridation of KTaO3. The TA
signal magnitude at 3435 nm probe for Ta3N5 prepared by 10hour nitridation of KTaO3 can be found in ESI (Fig. S2).† As
depicted in Fig. 5A, despite changing the pump uence bya factor of approximately nine, the free electron dynamics(3435 nm probe) in early 5 ps for Ta3N5 prepared by 10 hournitridation of KTaO3 is barely aﬀected. Comparing between
Fig. 2A and 5A, a pronounced electron trapping in Ta3N5
nitrided from KTaO3 for 10 hour compared to 0.25 hour can beperceived. This notion is further corroborated by noticing: (i)a negligible inuence on the free electron dynamics uponloading Rh cocatalyst (Fig. S13†), and (ii) a slow recovery of the
VB free holes due to a reduction in the number of free electrons(because of trapping) available for recombination (Fig. S14†).
These observations indicate the occurrence of electron trappingin Ta3N5 nitrided from KTaO3 for 10 hour. This conclusion isfurther supported by noticing an enhancement in the groundstate NIR absorption (Fig. 1B) due to defect formation within
the bandgap.

5358 | Chem. Sci., 2019, 10, 5353–5362(A) The fs-TDR time proﬁles corresponding to free electron
dynamics (440 nm pump and 3435 nm probe) in Ta3N5 prepared by
the 10 hour nitridation of KTaO3. (B) TA spectra (440 nm pump; 0.48 mJ
per pulse) of Ta3N5 prepared by the 10 hour nitridation of KTaO3. (C)Eﬀect of nitridation time on the free electron dynamics (440 nm pump;
0.04 mJ per pulse) in Ta3N5 prepared by the nitridation of KTaO3.
Fig. 52.2.2 The eﬀect of nitridation time on the hole trapping. To
investigate the eﬀect of increasing the nitridation time on thehole dynamics, TA spectra of Ta3N5 prepared by 10 hour nitridation of KTaO3 is recorded (Fig. 5B). The presence of a second
bleaching signal at 590 nm (which is absent in Fig. 4A) indicatesthe formation of hole trapping VN defect states. A similar
observation is also noticed for Ta3N5 prepared by 4 hours
nitridation of KTaO3 (Fig. S15†). To rule out any potential
contribution to the bleaching signal at 590 from the unreacted
KTaO3, TA spectra of KTaO3 is recorded. As expected, no suchThis journal is © The Royal Society of Chemistry 2019

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distinct bleaching signal at 590 nm reminiscent to that of Ta3N5
is detected for KTaO3 (Fig. S16†).
To further rationalize the eﬀect of nitridation time on thehole trapping process, the electron lifetime probed at 3435 nm
is compared as a function of KTaO3 nitridation time. As shownin Fig. 5C, the electron lifetime gets longer with an increase in
the nitridation time. This observation can be explained asfollows. Due to hole trapping, fewer holes are available forelectrons to decay by recombination and hence a longer electron lifetime can be expected. Thus, from Fig. 5B and C, anenhancement in the eﬃciency of hole trapping process with anincrease in the nitridation time is inferred.
Note that, we have not considered the eﬀect of dark carrier(which depends on the relative position of Fermi level withrespect to the CB and the defect density) on the decay dynamics
of electrons and holes in Ta3N5 prepared by the nitridation of
KTaO3. This is due to experimental diﬃculty in determining the
precise concentration of dark carrier or Fermi level position inthese powder samples which is in a composite form with KTaO3.
In this regard, a further systematic investigation of the eﬀect ofnitridation time on the relative shi in the Fermi level (withrespect to the CB) and determining the corresponding dark
carrier density would be helpful.
To put the results from Fig. 5 in perspective, it is instructive
to understand the plausible mechanism by which defects areformed and their relation with the nitridation time. In thisregard, a tentative model is proposed. The temperature
employed during the nitridation reaction of KTaO3 is 1173 K.
The nitrogen source used for nitriding the Ta precursor is NH3gas, which decomposes into N2 and H2 under the experimental
condition. Ideally, the Ta 5d orbital of the CB is present in Ta5+
states, which ensures to maintain charge neutrality and avoid
electron trapping. However, during the nitridation reaction, thetime exposed to H2 increases with nitridation time. This H2 gas
acts as a reducing agent to convert the Ta5+ states to form Ta4+
and/or Ta3+, which are proposed to act as electrons trapping
centres. This notion is in agreement with a previous reportdemonstrating the formation of reduced Ta vacancies usingsynchrotron-excited X-ray photoelectron spectroscopy (XPS).39
Similarly, the formation of nitrogen ðVN Þ vacancies which areassociated to hole trapping centres becomes inevitable, particularly for nitridation time exceeding 0.25 hour. Maintaining theideal Ta3N5 stoichiometric ratio via synchronous substitution of
nitrogen to oxygen (present in Ta precursor) is the key to control
the defect formation.
Based on the conclusions drawn from Fig. 5, the formationof both electron and hole trapping centres within the bandgap
of Ta3N5 is pronounced with an increase in the nitridation time.
The eﬃciency of charge transport from the bulk to the surface isnegatively aﬀected by carrier trapping. As a consequence, inefcient electron transfer to the Rh cocatalyst is observed and thenumber of free carriers available at the surface of Ta3N5 forphotocatalytic reaction will be reduced. Due to these two
combined eﬀects, the eﬃciency of OWS is found to be reducedwith increase in the nitridation time from 0.25 to 10 hour
(Scheme 2). From our previous STEM study, the location of
charge trapping defects in long time (>0.25 hour) nitrided Ta3N5
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Proposed model to explain the eﬀect of nitridation time on
the formation of defects within the bandgap and its impact on theeﬃciency of OWS reaction. The Fermi level positions of Ta3N5 and Rh
metal are not considered while drawing the energetic position of Rh
metal cocatalyst with respect to the CB.
Scheme 2

is found to be predominantly on the surface of Ta3N5.19 Hence,
removing these surface defect states either by chemical treatment or by surface modication may further enhance the eﬃciency of OWS activity for long-time nitrided Ta3N5. A future
work is necessary to unravel the electronic nature of the bulk
and surface defect states in detail.
2.3 How to further enhance the OWS activity of Ta3N5 (0.25hour nitrided from KTaO3) under the visible light?
The AQE value at 500 nm is ten times smaller compared to
420 nm.19 Hence, one of the challenges is to enhance the AQE
under visible light excitation. To discern whether the low AQE invisible light is due to ineﬃcient charge generation, free electron
dynamics (3435 nm probe) is compared between 580 and
440 nm excitation (Fig. S17†). At 580 nm excitation, a similarity
in terms of both carrier lifetime and pump uence dependence
is noticed to that of 440 nm pump. In addition, electron transfer
to the Rh cocatalyst is also noticed at 580 nm pump (Fig. S18†).
Besides that, Ta3N5 is widely considered to have an indirect
bandgap. Note that 580 nm pump is very close to the band edgeof the Ta3N5, however, neither free electron lifetime nor electron
photogeneration is aﬀected. Hence, indirect bandgap nature of
the Ta3N5 does not suﬃciently explain the lower activity underthe visible light excitation. Alternatively, the absorption coeﬃcient value of Ta3N5 at 530 nm is lower by a factor of vecompared to 400 nm.31,40,41 Upon 0.25 hour nitridation of
KTaO3, the actual amount of Ta3N5 phase formed on KTaO3 is
z2%.19 Thus, a low AQE in the visible region is attributed to
a combination of the lower absorption coeﬃcient and thepresence of only 2% of Ta3N5 which can potentially absorb
visible light, but not due to ineﬃcient charge photogeneration.
Thus, increasing the eﬀective density of Ta3N5 on KTaO3 is expected to further improve the AQE under visible light excitation.
To this end, multiple nitridation of KTaO3 (while keeping theChem. Sci., 2019, 10, 5353–5362 | 5359

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nitridation time as 0.25 hour) to expose the unreacted KTaO3
surface in the direction of NH3 ow forms one of theapproaches.

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1173 K for 0.25, 4 and 10 hour under a ow of NH3 gas at 100
mL min1.

4.2

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3.

Conclusions

A detailed ultrafast TAS study allowed us to elucidate a correlation between the synthesis procedure – structural defects –carrier dynamics – eﬃciency of OWS. Selectively probing the
dynamics of electrons and holes allowed us to develop
a phenomenological model that describes the role of Ta
precursor employed to prepare Ta3N5 and the eﬀect of nitridation time in the context of OWS.
In addition to H2 evolution being fundamentally low
compared to O2, the OWS has not been realized for Ta3N5
prepared by nitriding Ta2O5. The answer to this longstanding
question was found to originate from the absence of electrontransfer to the Rh cocatalyst by the virtue of eﬃcient electron
trapping.
When Ta3N5 is prepared by employing KTaO3 as a Ta
precursor for nitridation reaction, OWS was observed for therst time. For 0.25 hour nitrided Ta3N5, which shows thehighest OWS activity, charge carriers decayed by second-ordertype electron–hole recombination but not by trapping. Due tosingle-crystal and defect-free nature of 0.25 hour nitrided Ta3N5,
eﬃcient migration of both electrons and holes towards thesurface was achieved. Thus, to realize OWS, avoiding carrier
trapping process was found to be indispensable.
When the nitridation time of KTaO3 was increased from 0.25
to 10 hour, the eﬃciency of OWS reduced by a factor of
approximately ve. This was attributed to the formation of
defects which promoted both electron and hole trapping eventually aﬀecting the eﬃcient migration of charge carriers to thesurface. These transient optical studies yielded key insights into
the factors determining the eﬃciency of solar driven OWS by
Ta3N5.

4. Experimental section
4.1

Synthesis of Ta3N5 by the nitridation of KTaO3

KTaO3 particles were fabricated by a conventional solid-state
reaction method. Ta2O5 (99.9%; Kojundo Chemical LaboratoryCo., Ltd.) and K2CO3 (99.5%; Kanto Chemical Co., Inc.) weremixed at a Ta : K molar ratio of 1 : 1.05. Excess K was added tocompensate for losses by volatilization at high temperatures.
The mixture was thoroughly ground in an agate mortar for
90 min in the presence of a small amount of ethanol asa dispersant. Aer drying, the resulting mixture was transferred
into an alumina crucible and calcined at 1173 K for 1 hour andthen at 1423 K for 10 hour in static air. The KTaO3 obtained inthis manner was washed with ultrapure water at 343 K for
2 hour and centrifuged twice to remove any residual K2CO3. The
powder was then completely dried by heating at 343 K overnight. Subsequently, the as-prepared KTaO3 was subjected toa nitridation process to obtain Ta3N5 grown on KTaO3. KTaO3(0.5 g) was transferred into an alumina tube and nitrided at5360 | Chem. Sci., 2019, 10, 5353–5362

Synthesis of Ta3N5 by the nitridation of Ta2O5

The commercially available Ta2O5 (99.9%; Kojundo Chemical
Laboratory Co., Ltd.) was subjected to the nitridation at 1173 K
for 20 hours under a ow of NH3 gas at 100 mL min1.
4.3

Photodeposition of a Rh cocatalyst on Ta3N5

A Rh cocatalyst for H2 evolution was loaded on Ta3N5 photocatalysts by a previously-reported photodeposition process.19 A
Rh core was photodeposited using RhCl3$3H2O (Kanto Chemical Co., Inc.) as the metal precursor. This was accomplished bydispersing the photocatalyst powder in 150 mL of an aqueous
methanol solution (10 vol%) containing the metal precursor.
The pH of this solution was not adjusted but the temperature
was maintained at 288 K by circulating cooling water. The
suspension was evacuated to completely remove dissolved air
and then exposed to visible light (l $ 420 nm) with continuous
stirring. The photodeposition of Rh was conducted approximately for 3 hours.

4.4 Femtosecond time-resolved diﬀuse reectance (TDR)
spectroscopy
In fs-transient diﬀuse reectance (fs-TDR) measurements,
a femtosecond Ti:sapphire laser with a regenerative amplier(Spectra-Physics, Solstice, wavelength of 800 nm, pulse width of
100 fs, pulse energy of 3.5 mJ per pulse, and repetition rate of 1
kHz) was used as a light source. The output from the laser was
split into four paths for the excitation of two optical parametricampliers (OPAs: Spectra-Physics, TOPAS Prime), the whitelight-continuum generation by focusing the fundamental light(800 nm) into a sapphire plate, and the second and third
harmonic generations of the fundamental light (800 nm) byusing BBO (b-BaB2O4) crystals. For generating the 440 or 580 nm
pump pulse, light from one of the OPA was used. For the probe
pulse, a white light continuum covering from 500 nm to1600 nm and a 3435 nm probe light generated from the other
OPA with a diﬀerence-frequency generation crystal was used.
The time resolution of the system was about 140 fs. The powder
samples are taken in 1 mm quartz cuvettes. The diameter of thepump beam on the sample was about 0.5 mm as observed witha charge-coupled-device (CCD) camera. Amplied Si photodetector is used to measure the TA spectra in the visible region.
Liquid-nitrogen-cooled mercury-cadmium-telluride (HgCdTe)
photodetector is used for IR probe (3435 nm) experiments. The
diﬀusely reected light from the sample was passed througha grating monochromator (Princeton Instruments, Acton
SP2150) for data acquisition. The transient absorption intensityof the TDR measurements is presented as percentage absorption, where absorption (%) ¼ 100(1  R/R0), using R and R0 as
the intensities of the diﬀusely reected light with and without
excitation, respectively. A more detailed description of the fsTDR setup is available elsewhere.42,43

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4.5

X-ray photoelectron spectroscopy (XPS)The XPS spectra were obtained using X-ray photoelectron
ULVAC-PHI, INC PHI Quantera 2 spectrometer.

Conﬂicts of interest
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There are no conicts of interest to declare.

Acknowledgements
This work is supported by the “Research Project for Future
Development: Articial Photosynthetic Chemical Process(ARPChem)” (METI/NEDO, Japan: 2012-2022).

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