Photon Factory Activity Report 2019 #37 (2020)

BL-12C, NW10A/2018G078

NaTaO photocatalysts doubly doped with metal cations
3Hanggara SUDRAJAT , Nobuyuki ICHIKUNI and Hiroshi ONISHI
Department of Chemistry, School of Science, Kobe University,
Rokko-dai, Nada, Kobe, 657-8501, Japan
School of Engineering, Chiba University, Tokyo, 123-4567, Japan
121*1

21 Introduction
Currently, the use of solar energy for hydrogenproduction via the water splitting reaction is of widespread
importance as it is regarded as one of the most promisingroutes for the conversion of solar energy into chemicalenergy [1]. Such a conversion route mimics photosynthesis
in nature, and it demonstrates immense potential to be thepinnacle of sustainable energy. Only light, water, and acatalyst are required. Thus, the selection of an appropriate
photocatalyst is key to achieving a high solar energy
conversion efficiency.
Among the photocatalyst materials investigated in thelast two decades, alkali tantalates, particularly sodiumtantalate (NaTaO ), are excellent materials for thephotocatalytic overall water splitting [2–5]. When properly
doped with La cations through the solid-state route,
NaTaO shows a quantum efficiency of as high as 56%
upon ultraviolet exposure at 270 nm [6]. Following theirbreakthrough, a lot of efforts have been made in the world
to build a library of highly efficient photocatalysts basedon metal-doped NaTaO .
While La-doped NaTaO (La-NTO) has showeddevelopments in engineering for artificial photosynthesis,
questions in science remain to be answered. The
mechanism behind enhancement of water splitting
efficiency induced by the La doping is unknown. The
decisive properties of the dopant in relation withphotoexcited electrons and holes remain nearly unexplored.
Without understanding science of the highly efficientphotocatalysts, how and why water efficiently splits on LaNTO, proper research direction can never be achieved.
Another important issue to be addressed is sensitizing
La-NTO to visible light. Visible light accounts forapproximately 45% of total solar radiation reaching theEarth's surface. Doping metals with half-filled d states is awell-known method to produce filled or empty electronicstates in the band gap for visible light sensitization. Double
doping with half-filled transition metal (TM) together with
La is suggested to be a suitable way of visible lightsensitization of NTO. With this double doping system,
cationic charge is balanced without need for oxygen
vacancy. Therefore, undesired, negative contribution
coming from oxygen vacancy formation can be avoided.
33332 Methodology
The first purpose to be achieved in this research projectis to understand the detailed bulk structure of La-NTO andits photocatalytic behavior for water splitting reaction
under ultraviolet irradiation. The insights obtained isexpected to be transferable to design visible-light-

sensitized NTO through double doping with La and TMs.
To achieve the first purpose, three topics are studied. The
first topic is the mechanism of enhanced water splitting
activity upon La doping into NTO. Here, a plausible
mechanism of photoactivity enhancement is proposed. The
occupation preference of La cations in the host NTO latticeis also elucidated. The second topic is the role of core andshell structure in dictating the water splitting activity of LaNTO. Core-shell structure forms in the highly active LaNTO prepared through the solid-state route [6].
Understanding the role of such a unique nanoarchitectureis thus important for designing highly active perovskitestructured tantalate photocatalysts which are likely
featured with such a core-shell structure. The third topic is
the role of spatial La distribution in controlling the water
splitting activity of La-NTO. When prepared with thesolid-state method, the distribution of La dopants is also
different in space from the surface to the bulk region. Thewater splitting activity is potentially sensitive to the spatial
dopant distribution in the solid-state prepared La-NTO.
Herein, the detailed structure and photocatalytic
behavior of La-NTO are investigated with a number ofmicroscopy and spectroscopy techniques, such as scanningtransmission electron microscopy, X-ray absorption
spectroscopy,
light-induced
infraredabsorption
spectroscopy, microwave conductivity, and photoacoustic
spectroscopy to unravel the key insights into the structurephotoactivity relationship.
The second purpose is visible-light sensitization ofLa-NTO with TMs, including Cr, Mn, Fe, Co, Ni, Rh, andIr. Seven topics are thus studied according to the TM being
employed for the sensitization. In most cases, the dopant
concentrations are systematically varied to know thesensitization behavior with respect to the extent of doping(doping level). TMs in the same group and TMs in thesame row are to be compared to get hidden insights into
the sensitization behavior of TMs to La-NTO. X-ray
absorption spectroscopy is employed to investigate thelocal environments around dopants, while infraredabsorption spectroscopy induced by visible light is used toconfirm whether electrons are photoproduced by thedoubly doped samples upon visible light exposure.
3 Results and Discussion
A series of La-NTO samples have been successfully
prepared through the solid-state method to reproduce thehighly photoactive La-NTO reported earlier [6]. In LaNTO, the La cations sit in the Na site of the host lattice. At
a low concentration of less than 5 mol%, La dopants
effectively restrict electron-hole recombination to increase

Photon Factory Activity Report 2019 #37 (2020)

the electron population. Energy gradient of the conduction
band edge (CBB) is proposed to be responsible for therestricted recombination. A concentration gradient of theLa dopants decreasing gradually from the surface to thebulk is suggested to exist. The concentration gradient of
dopants simultaneously induces an energy gradient of theCBB. The energy gradient of the CBB separates the excitedelectrons to the bulk away from holes populating thesurface to restrict recombination. Because the upward shiftof the CBB is small enough relative to the thermal energy
at room temperature, the separated electrons in the bulk canthen return to the surface.
Herein, electron population is measured with lightinduced infrared absorption spectroscopy. The order ofelectron population agrees with that of gas evolution rates.
Given this fact, light-induced infrared absorption
spectroscopy potentially offers a simple, straightforwardmanner to foresee the photocatalytic behavior of newly
developed NTO materials.
Furthermore, a La-rich shell with the thickness of about
3 nm forms heteroepitaxially over La-poor core when LaNTO is prepared through the solid-state route. The formedcore and shell structure present in the La-NTO particle playa role in governing the water splitting activity. In the caseof La-NTO, the presence of La-rich shell is detrimental to
the restriction of recombination. Systematic etching of LaNTO with HF is then carried out to verify the roles of coreand shell. Etching gradually diminishes the shell.
Increasing the etching time decreases the particle size andalters the particle shape to remove the typical step-edge
structure on the surface. Increased etching time leads to thedecreased electron population and hence the water splitting
rate. Systematic etching also confirms the presence of a La
concentration gradient, which agrees well with the resultsof element maps by the scanning transmission electron
microscopy coupled with energy dispersive X-ray
spectroscopy.
Given the fact that the La dopant distribution is spatiallydifferent across the particle; decreasing in the Laconcentration from the surface to the bulk region, thisfeature may importantly contribute to the water splitting
activity. If this is indeed the case, designing water splitting
activity through controlling the spatial dopant distribution
is thus possible and should be a convenient approach. To
confirm this supposition, two La-NTO samples with an
identical dopant concentration yet possessing different
spatial La distribution are prepared. Less homogeneous
sample shows a noticeably higher water splitting activity.
The higher water splitting activity of this sample is
proposed to be caused by the higher electron population
due to a better restriction of recombination. The better
restriction of recombination is suggested to be due to a
sufficiently larger energy gradient of the CBB that
separates electrons to the bulk region away from holes
populating the surface. The less homogeneous sample also
shows a considerably higher electron trap density aroundthe CBB. These shallow electron traps may beneficiallyaffect the water splitting activity. Specifically, the shallow
electron traps capture a large portion of photoexcitedelectrons. The shallowly trapped electrons remain reactive
to contribute to the higher hydrogen evolution rate [7].
Visible light sensitization is then performed on La-NTO
by coupling with TMs. It is found upon the double dopingthat not only is light absorption extended to visible light,
the doubly doped samples can also produce electrons undervisible light irradiation. These photoproduced electrons
should be usable to drive redox reaction on the surface
upon exposure to visible light. Low doping levels of lessthan 10 mol% generally give large electron population,although the light extensions to visible region are lower
than samples with high doping levels. This finding
supports the interpretation that extending light absorption
does not necessarily increase the electron population.
Furthermore, X-ray absorption fine structure confirmsthat in the doubly doped samples La cations occupy the Nasite while TM cations occupy the Ta site, most likely
forming LaTMO -NaTaO solid solutions. In this case,oxygen vacancy is not required to balance the cationiccharge. At an identical dopant concentrations, there is no
systematic trend recognized on the sensitization feature
among the TMs in the same group or those in the same row
in terms of electron population. The difference in thespatial dopant distributions at the same doping level can be
one reason to make finding the trend of the sensitization
behavior difficult.
It is to be noted that in the doubly doped samples, forinstance, La+Co co-doped NTO and La+Rh co-doped
NTO, the dopant distributions can also be spatially tuned
to result in varied electron population. Less homogeneous
samples show higher electron population, similar to thoseobserved with single doping La-NTO. Given this fact, an
identical mechanism of restricted electron recombination
may be the case for both the singly doped and doubly
doped NTO samples.
Themechanismofrestricted
electron-hole
recombination proposed here for NTO singly doped with
La and doubly doped with La and TMs may also be
applicable to interpret the restricted recombination andenhanced water splitting activity in metal-doped
perovskite metal oxides such as SrTiO . As is well known,
SrTiO is also an important water splitting photocatalyst.
A part of results of this research project have beenreported in peer-reviewed journals. The results of studying
La+Cr co-doped NTO has been published in [8]. Visible
light sensitization of La-NTO with Mn is reported in [9],while that with Fe is reported in [10]. Upon coupling withthose TMs, La-NTO is able to absorb visible light toproduce electrons. Restriction of electron-hole
recombination in La+TM co-doped NTOs is optimized ata low doping level of less than 8 mol%, similar to thoseobserved in La-NTO. Given this fact, the mechanism of therestricted recombination in those singly doped and doubly
doped NTOs may be identical.
Revealing mechanisms behind the world-record
efficiency for photocatalytic water splitting such as thatover La-NTO provides us rich knowledge about creation,transport, and reactivity of electronically excited states inmulti-element semiconductors. These issues are3333

Photon Factory Activity Report 2019 #37 (2020)

undoubtedly in a frontier of chemistry today. Moreover,physical insights yielded in this research can potentially betransferred to innovate highly efficient, visible-lightsensitized photocatalysts for artificial photosynthesis in apractical scale. On broader impact, this research cancontribute to science in Indonesia, since photocatalysis ishighly relevant for tropical countries blessed with sunlight
all year round.
AcknowledgementH.S. is an International Research Fellow of the Japan
Society for the Promotion of Science (JSPS). The authors
acknowledge funding from JSPS KAKENHI (Grants
JP18F18029,
JP16H02250,
JP18KK0161,and
JP19H00915).
References[1] Y. Goto et al., Joule 2, 509 (2018).
[2] H. Kato and A. Kudo, J. Phys. Chem. B 105, 4285
(2001).
[3] A. Iwase et al., ChemSusChem 2, 873 (2009).
[4] A. Iwase et al., Chem. Lett. 33, 1260 (2004).
[5] H. Kato and A. Kudo, Catal. Today 78, 561 (2003).
[6] H. Kato et al., J. Am. Chem. Soc. 125, 3082 (2003).
[7] H. Sudrajat et al., J. Phys. Chem. C in press (2020).
[8] H. Sudrajat et al., Phys. Chem. Chem. Phys. 21, 5148
(2019).
[9] H. Sudrajat et al., ACS Appl. Energy Mater. 2, 7518
(2019).
[10] H. Sudrajat et al., Chem. Phys. 531, 110648 (2020).
* oni@kobe-u.ac.jp