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ISSN 2041-6539

EDGE ARTICLE

Takuya Hasegawa et al.
Ce(IV)-centered charge-neutral perovskite layers
topochemically derived from anionic [CeTa 2 O7]− layers

Volume 12
Number 45
7 December 2021
Pages 14955–15172

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Ce(IV)-centered charge-neutral perovskite layers
topochemically derived from anionic [CeTa2O7]
layers†Takuya Hasegawa,and Shu Yin a*a Naoki Yamasaki,b Yusuke Asakura,aTadaharu Uedabc

Layered perovskites have been extensively investigated in many research ﬁelds, such as electronics,
catalysis, optics, energy, and magnetics, because of the fascinating chemical properties that are
generated by the speciﬁc structural features of perovskite frameworks. Furthermore, the interlayers ofthese structures can be chemically modiﬁed through ion exchange to form nanosheets. To further
expand the modiﬁcation of layered perovskites, we have demonstrated an advance in the new structuralconcept of layered perovskite “charge-neutral perovskite layers” by manipulating the perovskite layer
itself. A charge-neutral perovskite layer in [CeIVTa2O7] was synthesized through a soft chemical oxidative
reaction based on anionic [CeIIITa2O7] layers. The Ce oxidation state for the charge-neutral [CeIVTa2O7]
layers was found to be tetravalent by X-ray absorption ﬁne structure (XAFS) analysis. The atomic
arrangements were determined through scattering transmission electron microscopy and extended XAFS
(EXAFS) analysis. The framework structure was simulated through density functional theory (DFT)
calculations, the results of which were in good agreement with those of the EXAFS spectra quantitative
analysis. The anionic [CeIIITa2O7] layers exhibited optical absorption in the near infrared (NIR) region at
approximately 1000 nm, whereas the level of NIR absorption decreased in the [CeIVTa2O7] chargeneutral layer due to the disappearance of the Ce 4f electrons. In addition, the chemical reactivity of thecharge-neutral [CeIVTa2O7] layers was investigated by chemical reduction with ascorbic acid, resulting in
the reduction of the [CeIVTa2O7] layers to form anionic [CeIIITa2O7] layers. Furthermore, the anionic[CeIIITa2O7] layers exhibited redox activity which the Ce in the perovskite unit can be electrochemicallyReceived 7th June 2021
Accepted 13th October 2021

oxidized and reduced. The synthesis of the “charge-neutral” perovskite layer indicated that diversefeatures were generated by systematically tuning the electronic structure through the redox control of

DOI: 10.1039/d1sc03053a

Ce; such diverse features have not been found in conventional layered perovskites. This study could

rsc.li/chemical-sciencedemonstrate the potential for developing innovative, unique functional materials with perovskite structures.

Introduction
Extensive attention has been given to perovskite compounds inelectronic, catalytic, optical and magnetic research elds due totheir diverse chemical properties. Many functional inorganic
compounds based on perovskite structures have been reported.1–3 Among various types of derived perovskite structures,aInstitute of Multidisciplinary Research for Advanced Material (IMRAM), Tohoku
University, 2-1-1 Katahira, Aoba-ku, Sendai, 980-8577, Japan. E-mail: hase@tohoku.
ac.jp; Fax: +81-22-217-5598; Tel: +81-22-217-5598

b

Department of Marine Resource Science, Faculty of Agriculture and Marine Science,
Kochi University, Nankoku, 783-8502, Japanc
Center for Advanced Marine Core Research, Kochi University, Nankoku 783-8520,
Japan† Electronic supplementary information (ESI) available: Detailed experimental
procedures and analyses including XANES and EXAFS spectra, electron
microscopy (SEM and STEM) images, XRD proles, solid-state NMR spectra andthermal analysis and chromatic measurements. See DOI: 10.1039/d1sc03053a

15016 | Chem. Sci., 2021, 12, 15016–15027

layered perovskites, particularly with a two-dimensional perovskite framework containing interlayer alkali-metal cations, canbe modied based on the precise design of the atomic
arrangements with post-treatment; such post-treatment
methods include ion exchange with other cations and exfoliation into a monolayer to add further functionality.4–6 A wide
variety of interlayer modications for layered perovskites havebeen carried out, and mixed-anion layered perovskites havebeen shown to induce fascinating functions and realize excellent levels of performance;7–10 nevertheless, limited studies onperovskite layer post-modications, which could change thecorresponding functionalities, have been conducted throughanion substitutions that are fundamentally based on conventional processes similar to inorganic compound substitutionsin other structures. It is quite diﬃcult to substitute the cation
parts containing multivalent ions in the perovskite framework
through post-treatment. To expand the potential functionalities© 2021 The Author(s). Published by the Royal Society of ChemistryView Article Online

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Edge Articleof layered perovskites, novel approaches have been needed for
perovskite modications beyond substitution-type techniques.
We focused on a strategy for valence tuning of perovskite
layers utilizing the oxidation/reduction ability of transitionmetal ions in the perovskite framework as a new modication
approach. This material concept can be used to expand thesemiconductive properties, as electron and hole doping can be
freely tuned by controlling the charges of the layers. Additionally, precise xation of the interlayer ion density could be
utilized to apply layered perovskites in ionic material elds. A
novel material concept for layered perovskites is based on theconstruction of a “charge-neutral layer” with a zero-valentperovskite layer. Many layered compounds with neutral layer
charges, such as graphene,11 boron nitrides,12 transition-metal
dichalcogenides (TMDs)13 and transition-metal carbides(MXenes),14 exhibit excellent performances as electronic andnanomaterials. However, 2D materials with charge-neutral
layered perovskite units have not been reported until now.
Poor redox activity species, such as Ti4+, Nb5+ and Ta5+, are oenintroduced at the B-site of most layered perovskites,4,15,16 while
the A-site contains relatively large ions of alkaline earth metals
and lanthanides that also show poor activity to redox. Even forlayered perovskite compounds with a variety of chemicalcompositions, suitable parent materials containing redox active
elements that can be used to realize novel material concepts
have not been discovered. Thus, charge-neutral layered perovskites could be used to dene a new material family.
Recently, we succeeded in discovering a new layered perovskite by introducing the trivalent cerium ion17 RbCeTa2O7 intoa Dion–Jacobson-type layered perovskite structure denoted as M
[An1BnO3n+1] (M: alkaline metals; n: number of octahedronsheets).16 This was the rst report of a compound containing Ceions in a layered perovskite capable of interlayer modication.
The trivalent Ce ion exhibits the reversible redox performanceof Ce(III/IV) (E ¼ 1.44 V vs. SHE).18 Trivalent Ce-containing
compounds have been applied to automotive catalysis.19 The
1 charge ([CeIIITa2O7]) in the perovskite layer of the Dion–
Jacobson-type RbCeTa2O7 could be neutralized to zero by theoxidation of trivalent Ce to realize a charge-neutral perovskite
layer [CeIVTa2O7] through topotactic chemical reactions fromthe anionic [CeIIITa2O7] layer as well as the occupation of the Asite of the perovskite structure by Ce4+. This chargeneutralization capability had never been reported throughout
the long history of perovskite studies. The electronic structureof RbCeTa2O7 is completely diﬀerent from that of RbLnTa2O7(Ln ¼ La, Pr, Nd and Sm), although both have similar ionexchange abilities and structural features. The Ce in theperovskite lattice has one 4f electron due to the trivalent Ce; thiselectron exhibits metal-to-metal charge transfer (MMCT) transitions between Ce 4f and Ta 5d, forming the conduction band.
Such MMCT transitions have rarely been observed in theconventional perovskite materials and complex oxides, indicating a unique electron transition. The electronic structure ofa cerium-containing layered perovskite must be tuned to
produce a unique electron transition through the control of theCe oxidation state at the A-site and interlayer modications.

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Chemical Science
In this work, we tested the preparation of a charge-neutral[CeIVTa2O7] layered perovskite by so chemical oxidation of
RbCeTa2O7 as a parent compound with a permanganate ion(MnO4). The average and local structures of the obtained
samples were investigated by X-ray diﬀraction and X-ray
absorption spectroscopy, respectively, and the electronic structures were evaluated experimentally and theoretically. Furthermore, the chemical reactivity of the perovskite [CeIII/IVTa2O7]
layers was tested by XRD, XAFS and electrochemical analyses.

Experimental section
Materials
High-purity reagents were used as follows: TaCl5 (99.99%) was
purchased from Kojundo Chem. Lab. Co., Ltd. (Japan); RbNO3(99%), Ce(NO3)3$6H2O (99.9%), citric acid anhydrous (CA,C6H8O7; >98.0%), methanol (MeOH, CH3OH; >99.8%), ethylene
glycol (EG, C2H6O2; >99.5%), ethylamine aq. solution (EA,C2H5NH2; 70%), tetrabutylammonium hydroxide aq. solution(TBAOH, [(n-C4H9)4N]OH; 10%), sulfuric acid (H2SO4; >95.0%)
and KMnO4 (>99.3%) were purchased from Fujilm Wako Pure
Chem Co. (Japan); graphite powder and chloric acid (HCl; 35.0–
37.0%) were purchased from Kanto Chemical Co., Inc.
Synthesis of the RbCeTa2O7 parent material
The RbCeTa2O7 parent (Rb-form) was synthesized through thepolymerized complex (PC) method according to the literature(see the detailed procedure in the ESI†).17 Aer the Rb-form (ca.
1.5 g) was added to 50 mL of 1 M HCl solution, the solution was
stirred at room temperature for 4 days to produce HCeTa2O7$nH2O (H-form). The H-form was collected by ltration, washedwith deionized water, and dried at 60  C in a vacuum. For theintercalation reaction, the 1.0 g H-form compound was reactedwith 7 wt% EA aqueous solution for 5 days at room temperature.
The solution was centrifuged at 10 000 rpm to collect the EAintercalated form sediment (EA-form). Then, 1.0 g of EA-form
was reacted with 150 mL of 1 wt% TBAOH aqueous solution
for 7 days at room temperature to induce delamination into thecolloidal suspension containing the [CeIIITa2O7] sheets. The
spontaneous undelaminated precipitates formed aer centrifugation at 2500 rpm for 20 min were removed by using thesupernatant as a suspension. The suspension was centrifuged
at 15 000 rpm for 15 min to obtain pale green delaminated[CeIIITa2O7] powders (TBA-forms).
Oxidation of the perovskite delaminated sheets
The charge-neutral [CeIVTa2O7] layered perovskite (Ox-form)
was prepared through a so chemical reaction at room
temperature based on a chemical oxidation process usinga KMnO4 oxidizing agent. The [CeIIITa2O7] suspension (20 mL)
was added dropwise to 20 mL of 0.2 M H2SO4 solution containing 2 mM KMnO4. The transparent purple solution thenstarted to appear cloudy. The obtained suspension was centrifuged at 12 000 rpm to obtain the precipitants, which were
washed with diluted H2SO4 solution and then deionized water
more than 3 times until the ltrate became transparent. Finally,
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the collected pale-yellow powder was dried at 60  C overnight ina vacuum.

Results and discussion

Chemical reactivity test

It is diﬃcult to control an atomic charge valence for cerium
atoms in the crystal lattice because of the susceptibility tovalence changes. However, this control is very important for thesynthesis of Ce-based compounds. An accurate determinationof the compound valence is also valuable. In this work, theoxidation states of Ce atoms for all forms of [CeIIITa2O7] and[CeIVTa2O7] were precisely evaluated through an analysis of the
X-ray absorption near edge structure (XANES) spectra for the Ce
L3-edge (Fig. 1A). Specic XANES spectra of the Ce L3-edge,
which are analogous spectra for the CeCl3 standard sample,
were observed with a slightly asymmetric peak at 5726 eV for the
Rb-, H- and TBA-forms, indicating that trivalent Ce (Ce3+) could
be introduced into the main crystal lattice. Similar spectra wereobserved for the reported RbCeTa2O7 as well as other Ce-based
compounds.20,21 The Ox-form exhibited two evident absorption
peaks at 5729 and 5737 eV. The XANES spectrum of typical Ce4+containing compounds such as CeO2 exhibits two peaks, as
shown at the bottom of Fig. 1A, demonstrating that the Ce4+ ion
should dominantly exist in the Ox-form. The quantitative
analysis using CeCl3 and CeO2 as standard samples indicatedthat more than approximately 80% of Ce3+ or Ce4+ should becontained in the corresponding forms (Fig. S1†). The oxidation
state of Ce on the particle surface was analyzed based on Ce 3d
XPS spectra for all samples (Fig. 1B). Two main asymmetricbands appeared at approximately 880–890 and 895–910 eV in
the TBA-forms, which were attributed to the 3d3/2 and 3d5/2
levels, respectively. Since such characteristic main bands arealso observed in Ce3+-containing compounds such as CePO4,22
Ce must exist as a trivalent ion on the surface of the TBA-forms.
In the case of the Ox-forms, a new band appeared at approximately 917 eV with two specic bands corresponding to Ce3+;
these results are very similar to the characteristics of tetravalent
Ce compounds such as CeO2.22 In all forms, the chemical statesof Ce on the surface corresponded to the results of the XANES
analysis.
This evidence indicated that the anionic layer [CeIIITa2O7]
was converted to a charge-neutral [CeIVTa2O7] layer through theoxidation of the Ce3+ ions in the TBA-form with MnO4 ions toform Ce4+. The valence states of the tantalum in all forms werealso conrmed to be pentavalent through the analysis of Ta L3edge XANES spectra (Fig. 1C). The tantalum in the as-made
RbLaTa2O7 was conrmed to exist in Ta5+ (Fig. S2†) based on
the Ta L3-edge XANES spectra generated by utilizing Ta2O5 and
TaS2 as the reference samples for Ta5+ and Ta4+, respectively.
The same Ta L3-edge XANES spectra were obtained for all forms
presented without any signicant diﬀerences in the spectra.
The average oxidation state in the mixed-valence compounds
could be estimated from the peak position of the derivative
XANES spectra, namely, the shi of the absorption edge.23,24
Generally, the absorption edge should shi to a higher energy
as the oxidation state increases. Indeed, the derivative peak
position of RbLaTa2O7 (Ta5+) was observed at higher energy
than that of TaS2 (Ta4+) (Fig. 1D). On the basis of the

To conrm the oxidative capacity for the [CeIVTa2O7] perovskite
layer, 20 mg of Ox-form sample was weighed and dispersed into
5 mL de-ionized water by sonication. Then, 5 mL of 10 mM L(+)ascorbic acid was added into the [CeIVTa2O7] solution drop bydrop with stirring at room temperature. Aer the [CeIVTa2O7]
solution in the presence of L(+)-ascorbic acid was kept stirringfor 10 min, the reacted powder was collected by centrifugation
at 10 000 rpm for 10 min, washed with de-ionized water anddried at 60  C in vacuo.
Electrochemical measurements
For the electrochemical analysis, the TBA-form-deposited
working electrode was fabricated as follows. The Au electrodewith a diameter of 6 mm was rstly cleaned with acetone, EtOH
and water by sonication for 20 min, respectively. Aer drying,
the electrode was immersed in 2.5 wt% PEI solution for 20 min,and then the TBA-form suspension was added dropwise onto
the PEI-coated Au electrode. Aer standing for 20 min, the TBAform modied Au electrode was rinsed with water, and subsequently it was immersed in 20 wt% PDDA solution for 20 min.
The PDDA/CeIIITa2O7/PEI-Au electrode was nally washed using
water and then dried in an ambient atmosphere.
Voltammetric measurements were carried out in acetonitrile
containing 100 mM TBAPF6 used as the electrolyte with anelectrochemical workstation (BAS 50W; Bioanalytical Systems,
BAS). A standard three-electrode electrochemical system wasutilized with the PDDA/CeIIITa2O7/PEI-Au electrode, Ag and Pt
wires as working, reference and counter electrodes, respectively.
The raw potential measured using Ag reference electrode wascalibrated by the ferrocene/ferrocenium ion (Fc/Fc+) scale using
data derived from voltammograms for the oxidation of 1.0 mM
Fc. The electrolyte was purged with Ar gas for more than 5 min.
in advance to remove the dissolved oxygen in the solvent. The
measurement was made at 25.0  2  C and the scan rate was set
to 100 mV s1.
Material characterization
The crystal phase was determined by X-ray diﬀraction (XRD).
The morphologies of the bulk samples and delaminated sheet
were observed with electron microscopy techniques (scanningelectron microscopy (SEM), transmission electron microscopy(TEM) and scanning transmission electron microscopy(STEM)) and atomic force microscopy (AFM), respectively. Ce
and Ta L3-edge X-ray absorption ne structure (XAFS) and Xray photoelectron spectroscopy (XPS) were employed tounderstand the chemical states and local structures of theobtained [CeTa2O7] derivative samples. The optical absorption
properties of the samples were measured using an ultravioletvisible-infrared (UV-vis-IR) spectrometer. The detailed characterization and theoretical calculation methods are also
described in the ESI.†

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Fig. 1 Normalized XANES spectra for the (A) Ce and (C) Ta L3-edges of
the Ce-containing layered perovskite derivatives, Rb-, H-, TBA- andOx-forms, with the reference compounds comprising the charged
cations Ce3+ (CeCl3) and Ce4+ (CeO2) for the Ce L3-edge in (A) andTa4+ (TaS2) and Ta5+ (RbLaTa2O7) for the Ta L3-edge in (C), respectively. (B) Ce 3d XPS spectra for the TBA- and Ox-forms. (D) The
derivative Ta L3-edge XANES spectra for all forms and references.

relationship between the derivative peak positions and theoxidation states of transition metals, the oxidation states oftantalum in all forms should be pentavalent corresponding to
the result of a similar absorption edge for RbLaTa2O7.
Morphology and atomic arrangement observations
RbCeTa2O7, as a parent compound, has a plate-like morphology
similar to the commonly found morphology in Dion–Jacobsontype layered perovskites.25,26 Similarly, based on the SEM
observations reported previously, the synthesized Rb- and Hforms were determined to have plate particles (Fig. S3†)
without remarkable size diﬀerences. On the other hand, theparticles of the TBA- and Ox-forms decreased in size comparedto those of the parent forms (Fig. S3†), although fewer diﬀerences in the particle morphologies in both samples wereobserved from the SEM images. The TEM images of the TBAforms show that the particles were well dispersed (Fig. 2A andB) due to less interaction between the bulky TBA cation and theanionic layer. These images also show that the TBA-forms haveplate-like morphologies with a lateral size of 500 nm, which issmaller than the parent Rb- and H-forms with a lateral size of
2 mm. In the case of the Ox-form, similar particle sizes andmorphologies to the TBA-form were found, although the particles were agglutinated due to the strong van der Waals interactions in the charge-neutral layer. Because the TEM image
cannot provide quantitative information on the thickness of the© 2021 The Author(s). Published by the Royal Society of Chemistry

Chemical Science
sheet particles, we utilized AFM images to determine thethickness details of the crystals (Fig. 2C–F). The thicknesses ofthe plates in the TBA- and Ox-forms were 30 and 20 nm,
respectively. Since the theoretical thickness of the [CeTa2O7]
perovskite monolayer was estimated to be 1 nm based on the
crystal structure of RbCeTa2O7,17 the plate particle should havea laminated structure composed of 10–30 layers. The crystal
thicknesses of the Rb- and H-forms were determined to be on
the order of 100 nm based on SEM images (Fig. S3†). Thus, theTBA- and Ox-forms should be partially delaminated comparedto the parent compound. Since the thickness of the exfoliatedperovskite nanosheets with a single layer has been shown to beon the single nanometer scale, the [CeTa2O7] nanoplates in thiswork should be relatively bulky.
The atomic arrangement of the Ox-form particles was directlyconrmed by high-angle annular dark-eld (HAADF) scanningtransmission electron microscopy (STEM) observations (Fig. 2G
and H). Flat and standing particles were acquired for the sample
holder STEM grid by carefully searching the sample to successfully observe the out-of-plane and in-plane atomic arrangements(Fig. S4†). The TEM and AFM images in Fig. 2A–F indicate thatthe nanoplate particles are 25 nm thick, which is in good
agreement with the AFM image results. The atomic arrangement
could be clearly observed in the out-of-plane direction because
the incident electron beams can be suﬃciently transmitted to thedetector. The atomic arrangements observed in the TEM and
AFM images were in good agreement with the atomic positions of
the Ce and Ta heavy atoms in the crystal structure based on theperovskite structure in the [001] direction (Fig. 2G). On the other
hand, the layered structure was evidently conrmed based on the
STEM image of the in-plane direction, although the obtained
STEM images were slightly blurred due to the insuﬃcienttransmission of the electron beam from the reduced crystallinity
and increased depth of the particle (Fig. 2H). Atoms witha similar number of electrons, namely, Ce and Ta atoms, should
exist throughout the layer because atomic shadows of similar
brightness were observed both on the surface and at the center ofthe layer; however, it is diﬃcult to distinguish the diﬀerence
between Ce and Ta due to the ambiguous atomic boundary. In
addition, the annular bright-eld (ABF) STEM projected images
had the opposite contrast of the HAADF-STEM image (Fig. S5†).
Energy-dispersive X-ray spectroscopy (EDS) mappings provided
evidence that the constituent elements of Ce, Ta and O were
homogeneously dispersed over a single particle, but nonlinear
column mapping indicated that Ce atoms certainly existed in thelayer structure (Fig. S6†). Such HAADF-STEM images were also
observed for the parent material of RbCeTa2O7,17 supporting thestrong evidence regarding the presence of Ce atoms at the centerof the perovskite layer.

Crystal phase determination and its theoretical investigation
The parent layered perovskite compound RbCeTa2O7 was
discovered in our previous study.17 The crystal structure wasdetermined to be in the space group of P4/mmm (No. 123) with
the Dion–Jacobson type (Fig. S7†). The XRD pattern of the
protonated form (H-form) is also shown in Fig. S8.† The XRD

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Edge Article(A and B) TEM images and (C and D) AFM images with (E and F) thickness proﬁles for the (A, C and E) anionic TBA- and (B, D and F) chargeneutral Ox-forms. HAADF-STEM images of the Ox-form in the charge-neutral [CeIVTa2O7] from (G) basal and (H) vertical views for the layer; (left)
lower and (right) higher magniﬁcations.
Fig. 2peaks of the H-form were broader than those of the Rb-form
because of the lower ordering in the interlayer hydrogen bondarrangement found through 1H magic-angle spinning nuclear
magnetic resonance (MAS NMR) measurements (Fig. S8†). Ina tetragonal structure, the lattice constants for each crystal axis
can be determined based on the d-spacing changes in [110] forthe a- and b-axes and [001] for the c-axis. The lattice constantsfor the H-form calculated from the XRD pattern were estimatedto be a ¼ 3.8766(2) and c ¼ 10.785(2) A, which are slightly
shorter than those of the La analog (HLaTa2O7; a ¼ 3.885(2) andc ¼ 11.106(6) A)27 due to the diﬀerence in the ionic radius (La3+:3+
1.16 A; Ce : 1.143 A, for 8-fold).28 Many protonated Dion–
Jacobson-type layered perovskites easily accept water molecules
into the interlayer under acidic conditions.26,29,30 The H-form
sample in this work was also compatible with such watermolecule introduction. The amount of water (H2O) was determined to be approximately 0.5, denoted as HCeTa2O7$0.5H2O,
based on thermogravimetric (TG) analysis (Fig. S9A†). In the Hform reacted with EA, a new XRD peak appeared at a lower angle
than the (001) diﬀraction peak of the H-form; thus, EA wasshown to intercalate into the interlayer of HCeTa2O7 (Fig. S7†).
A similar intercalation reaction occurs in the La analog of
HLaTa2O7 through an acid–base reaction during the nanosheet
synthesis process.31 By reacting the H-forms with a bulkyorganic cation such as a tetrabutylammonium ion, (n-C4H9)4N+(TBA+), the perovskite stacks could be stretched and exfoliatedto form a sheet morphology at the nano-order level.31 The fully
exfoliated monolayer nanosheets produce strong XRD peaks
from (00l) stacking planes.32 The XRD prole of the prepared
TBA-form was similar to those of Dion–Jacobson structure types
HCeTa2O7 and RbCeTa2O7, implying that exfoliation did not
fully occur and laminations with several layers remained in thesample (Fig. 3A–C). However, the TBA+ cation could also be
introduced into the interlayers, and the structures were partiallyexfoliated to form monolayer nanosheets. This result wasdetermined because a strongly sharp XRD diﬀraction peak was
observed due to the stacking direction of the (00l) plane and the15020 | Chem. Sci., 2021, 12, 15016–15027

other XRD peaks related to the in-plane directions werebroadened. In addition, an XRD peak with a large d-spacing (ca.
19 A) for the TBA-form was observed at 4.65 in the low-angle
XRD pattern (Fig. 3D). The XRD patterns of the Ox-form werequite similar to those of the TBA-form, whereas interestingly,
anisotropic diﬀraction peak shis were observed in the[CeIVTa2O7] due to the oxidation of Ce3+. Particularly, the XRD
peak position of the in-plane direction (hk0) on the a- and b-axes
shied to the higher angle side, while no evident peak shi wasobserved for the stacking (00l) plane on the c-axis. Therefore,

Fig. 3 XRD patterns of the TBA- and Ox-forms: (A) overall pattern, (B)enlarged patterns with a range of 6–10 and (C) 31–35 and (D) lowerangle side.

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the anisotropic lattice change indicates the occurrence of thechemical oxidation of the anionic [CeIIITa2O7] to chargeneutral [CeIVTa2O7] layers. In addition, some XRD peaks of
the Ox-form were broadened and asymmetric, which may be
due to the decrease of crystallinity with the chemical treatment
as well as the existence of the chemical species, e.g. H2O, in theinterlayer.
Almost the same selected-area electron diﬀraction (SAED)
patterns of the TBA- and Ox-forms were observed to have clear
spot patterns (Fig. 4A), indicating that the plate particles of bothforms are individual single crystals. In addition, both SAED
patterns were quite similar to the simulation pattern of the [001]direction in the parent RbCeTa2O7 (Fig. S10†), as the lateralplane of the plate particle was oriented in the direction of the{100} planes. In the two-layer Dion–Jacobson type layered
perovskites, such as [LnM2O7] layers (Ln: lanthanides; M: Ta
and Nb), it is well known that the B-site octahedron tilts byintroducing lanthanide ions with diﬀerent sizes.33 For example,
RbNdTa2O7 layered perovskites can change the octahedrontilting depending on the temperatures.34 At room temperature,
it can take the alternated stacking structure with the tilting
polar perovskite layers of the space group of I2cm, which can bepresented by aac+/(aac+) tilting as the Glazer notation,35
whereas, as the temperature increases, the structure take ananti-polar (Cmca), non-polar (I4/mcm) and then nally a nontilting P4/mmm phase. Such layered perovskite with octahepﬃﬃﬃﬃﬃ pﬃﬃﬃﬃﬃ
dral tilting takes the supercell of 2a  2a or 2a  2a forviewing from c-axis; however, a displacement can be observed
due to the tilting of the oxygen ligand around the B atoms
between two layers. Thus, the presence/absence of the octahedron tilting can be determined by SAED pattern analysis
because if the material has some tilting, i.e. symmetry is lower,
the diﬀraction spot corresponding to the displacement of theoxygen should be observed. Therefore, the SAED patterns of the

Chemical ScienceOx-form were carefully analyzed by adjusting the contrast andbrightness; however, we could not conrm even the ultraweakreections originating from the octahedral tilting. The result ofthe ne analysis of SAED patterns provides evidence that thebright spots in Fig. 4A is only (001) direction as well as the
structure does not have octahedral tilting, meaning that it has
tetrahedral symmetry. As is known from the estimation of thestability of the perovskite using the tolerance factors, in general,
the tilting is aﬀected by the ratio of the ionic radius of A- and Bsite cations. The ionic radius of Ce4+ (1.14 
A, CN ¼ 12)28 issmaller than that of Ce3+ (1.34 
A, CN ¼ 12)28 as well as smaller
than that of Nd3+ (1.27 
A, CN ¼ 12)28 ions; thus, the Ox-formshould take the more signicant tilting than the RbNdTa2O7
from the viewpoint of solid-state chemistry. However, the resultof the SAED analysis indicated no tilting in the Ox-form, whichis quite irregular for the perovskite material, suggesting that the
charge neutral [CeTa2O7] layer synthesized in this study hassome special atomic arrangement.
We presumed that the irregularity of such tetragonal structure with no-tilting originated from the anisotropic latticechange from the TBA- to the Ox-form as mentioned in the XRD
analysis section. To reveal the anisotropic lattice changes in[CeIVTa2O7] indicated by XRD results, theoretical calculations
based on density functional theory (DFT) were carried out for
both anionic [CeIIITa2O7] and charge-neutral [CeIVTa2O7]
forms. Perovskite mono-layers with vacuum layers of 12 
A wereemployed as structural models to simplify the calculation,
unlike the multilayer perovskite particles in the real sample,
and the perovskite unit of RbCeTa2O7 was utilized for the initialstructure. Through lattice relaxation optimization, the crystal
structure was maintained as layered perovskite units (Fig. 4B,
which is illustrated by using the VESTA program36). The optimized lattice parameters and bond lengths of the anionic[CeIIITa2O7] and charge-neutral [CeIVTa2O7] perovskites from

Fig. 4 (A) SAED patterns of the (left) anionic TBA- and (right) charge-neutral Ox-forms. (B) Schematic structures of the anionic [CeIIITa2O7] andA). (C) Simulated electron density
charge-neutral [CeIVTa2O7] perovskite layers optimized based on DFT calculations (the bond length unit is 
distributions on the (110) plane and (D) electron density proﬁles at lines a and b in F for the anionic [CeIIITa2O7] and charge-neutral [CeIVTa2O7]
perovskite sheets.

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the calculation by xing the alternative symmetry of the P4mm
(no. 99) space group are summarized in Table 1. The incorporation of smaller ions into the same crystal lattice generally
shrinks the whole lattice, depending on the introduced ion size,
according to Vegard's rule.37 Since the Ce4+ (1.14 
A, CN ¼ 12)28 in[CeIVTa2O7] is smaller than the Ce3+ (1.34 
A, CN ¼ 12)28 in[CeIIITa2O7], the lattice constants for the in-plane directions of
the a- and b-axes shrank 3.27%, whereas that in the stacking
direction of the c-axis clearly expanded ca. +2.99%. This calculation result is consistent with the experimental XRD analysis
results. The anisotropic lattice shrinkage/expansion for chargeneutral [CeIVTa2O7] can be mainly explained by the geometricchange in the TaO6 octahedron in the perovskite framework.
Such changes are quantied according to the diﬀerences inthree types of Ta–O bond lengths between apical (O1), equatorial (O2) and bridging oxygens (O3) and centered Ta ions.
Compared with the DFT calculation, the bond length of Ta–O2
in the in-plane direction was 0.94% shorter than that of[CeIIITa2O7]; this diﬀerence is related to the change in thelattice parameter. In contrast, in the stacking direction, thebond length of Ta–O1 shrank 1.92%, while the length of Ta–
O3 expanded by +7.00%. On the other hand, two types of Ce–O
bonds that form CeO12 icosahedrons shrank 2.22% for Ce–O2
and 3.27% for Ce–O3, respectively. The distance between Ce
and Ta was also slightly separated by 0.71%. The relative position of the Ta ion in the perovskite framework was displaced
toward the apical direction, resulting in a longer bond lengthfor Ta–O3. On the basis of the above bonding change results,
the anisotropic lattice change should be attributed to thefollowing reasons: shrinkage in the in-plane direction due toa decrease in the ionic radius of Ce generated via Ce oxidationfrom +III to + IV and expansion in the stacking direction due toan increase in the bond length of Ta–O3 (Fig. 4B and Table 1).
This irregular expansion toward the stacking direction irrespective of the ionic radius order can be understood on thebasis of electrostatic interactions between the cations in theanionic/charge-neutral layer frameworks. The electron density
maps of each perovskite were simulated through DFT calculations (Fig. S11† and 4C). Although no apparent diﬀerences canbe observed from the 2D electron density distributions in the

Table 1 Lattice parameters of a (¼b), thicknesses of the perovskitemonolayer and atomic distances of the anionic [CeIIITa2O7] andcharge-neutral [CeIIITa2O7] perovskite layers optimized from DFT
calculationsa (¼b) axis
Thicknessa
Ce–O2 8
Ce–O3 4
Ta–O1
Ta–O2 4
Ta–O3
Ce–Ta
a

[CeIIITa2O7] (A)[CeIVTa2O7] (A)Diﬀerences(%)

3.94932
7.855292.62275
2.79259
1.76286
2.02279
2.16476
3.533393.82032
8.09015
2.564512.70137
1.72898
2.00371
2.31623
3.55841

3.27
+2.99
2.22
3.27
1.92
0.94
+7.00
+0.71

Thickness of the perovskite monolayer and O1–O1 atomic distance.

15022 | Chem. Sci., 2021, 12, 15016–15027(110) plane, diﬀerences in the electron density between Ce andTa were clearly observed from the corresponding line proles(Fig. 4D). There was no clear diﬀerence in the electron density of
the Ta atom in the two perovskite layers, while that of Ce in the
charge-neutral [CeIVTa2O7] layer exhibited a noticeable positiveshi, namely, a positive charge bias, indicating that the electrostatic repulsion between the Ce and Ta atoms should
increase with an increasing positive shi of the Ce atom. The Bsite (Ta atom) in the 2D perovskite layer had fewer degrees of
freedom in the in-plane direction because of crystallographic
constraints, but it had more degrees of freedom in the stacking
direction due to the lack of crystallographic constraints. Thus,
the Ta atom signicantly shied in the direction of the layer
surface through Ce–Ta electrostatic repulsion, resulting in the
expansion of the bond length of Ta–O3. On the other hand, theCe atoms formed strong bonds with surrounding oxygens, andthe Ce–O bond lengths became isotopically short. Therefore,
the anisotropic lattice change based on chemical oxidation was
concluded as follows: the shrinkage in the in-plane direction
originated from the contraction of the ionic radius of Ce andelectrostatic attractive forces between Ce and O, and the
expansion of the stacking direction was attributed to theincrease in the electrostatic repulsion between Ce and Ta.
Indeed, the centered metals in the octahedron, namely, B-site
atoms, can be displaced toward the stacking directions bychanging the charge of the layers in layered perovskite
compounds.38 For example, in Ruddlesden–Popper-type LixLaTa2O7, the charges of the perovskite layer are equal to 1 for x¼ 1 and 2 for x ¼ 2. The Ta–O3 bond length (2.262(6) Å) in
LiLaTa2O7 is ca. 5% longer than that in Li2LaTa2O7 (2.154(3)Å).38 In other words, the displacement of the B-site should
depend on the sum of the ion charge at the A- and B-sites,
namely, the magnitude of the electrostatic repulsions. In
general, it is well known that the introduction of smaller ions
into the A site leads to tilting of the B-site octahedron. However,
although the smaller ion of Ce4+ (1.14 
A; CN ¼ 12) was introduced into the A-site of the perovskite lattice, it was formed as
the tetragonal structure, which is irregular from the viewpointof solid-state chemistry. It can be suggested that the exibility
due to the stacking direction shi of the Ta position promotes
the stabilization of the tetragonal structure. Additionally, thedelamination of the [CeTa2O7] perovskite layer should accelerate the displacement of Ta to contribute to the stabilization of
the tetragonal structure, i.e. relaxing the tilting. This uniquefeature should be realized in other layered perovskites; however,it might be necessary to devise the chemical compositions that
increase the positive repulsion between A- and B-sites.

Local structure determination by EXAFS analysis
To support the validity of the structural optimization throughDFT calculations and determine the local structures of Ce andTa, EXAFS analysis based on the crystal structure optimized by
DFT calculations was carried out. This method was chosenbecause the TBA- and Ox-forms do not have suﬃciently highcrystallinities to identify the structures from the XRD results.
The EXAFS vibrations for each edge extracted from the overall© 2021 The Author(s). Published by the Royal Society of ChemistryView Article Online

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XAFS spectra were weighted to the third power with the k3function (Fig. S12†). The diﬀerent k3-weighted EXAFS spectra in
the TBA- and Ox-forms for the Ce L3-edge suggested that there
was a signicant change in the coordination conformation
around Ce. No clear diﬀerences were observed in the Ta L3-edge
spectra, although slight diﬀerences were observed at higher
wavenumbers. The bond length details for Ce–O and Ta–O were
estimated with the Artemis program using the Ce and Ta L3edge XAFS spectra. The EXAFS analysis was conducted with theinverse-transform EXAFS spectra, so-called q-spacing(Fig. S13†). The Ce and Ta L3-edges Fourier transformed (FT)EXAFS spectra for the R-space for the TBA- and Ox-forms areshown in Fig. 5A and B, respectively. In the Ce L3-edge, one
major peak was observed at a radial distance of 2–3 
A for eachform, and some smaller peaks were observed at larger
distances. Since the main peak of the nearest-neighbor bonding
Ce–O appeared at approximately 2 
A in CeO2,39 the major peakat 2–3 
A and minor peaks should be assigned to the nearest Ce–
O bond and the next-nearest-neighbor bonding species such as
Ce–Ce, Ce–Ta and Ce–O. Since the long-distance information in
the Ce L3-edge XAFS spectrum is susceptible to other L2-edge
absorptions, we investigated only the nearest peak to quantitatively rene the EXAFS spectra. The local structure

Chemical Sciencesurrounding the Ce in the TBA-form was not diﬀerent from thelocal structures in the Rb- and H-forms (Fig. S14†). The FTEXAFS spectrum for the Ox-form exhibited a nearest-neighbor
Ce–O peak shi towards the shorter distance and a signicantchange in the next-nearest-neighbor region compared with thatof the TBA-form; these results indicated that the bond lengths
around Ce in the Ox-form were shortened signicantly. Dion–
Jacobson-type double-layered perovskites such as RbCeTa2O7
have two oxygen atoms with diﬀerent environments coordinated that are with the central A-site, i.e., the Ce3+ ion is coordinated with two oxygen sites to form 12-fold A-sites. The Ce–O2
and Ce–O3 bond lengths were estimated to be 2.646(4) and2.749(6) A for the TBA-form and 2.53(1) and 2.70(5) A for the Oxform, respectively (Table 2), as well as the all tting parameters
were summarized in Table S1.† This shortening of the bonddistance was similar to the corresponding result in the DFT
calculation. The Ce–O bond length in CeO2 is estimated to be
2.35(2) A based on EXAFS tting,40 a value that is reasonably
similar to the Ce–O bond length in the Ox-form. A ghost peak
and the nearest Ta–O major peak were observed at 1 andapproximately 1.5 A, respectively, for the Ta L3-edge EXAFS
spectra in each form (Fig. 5B). The peak position due to thenearest Ta–O in the Ox-form slightly shied in the shorter
bonding direction more than that in the TBA-form. The nextnearest-neighbor peaks slightly changed through the shortening of the nearest Ta–O bond distance; this shortening
occurred due to the contraction of the entire lattice associatedwith the shrinkage of the Ce–O bonds and the disturbance ofthe long-range order. The quantitatively analyzed local Ta–O
bonds are also summarized in Table 2. The fractional change inthe bond lengths of Ce–O and Ta–O between the TBA- and Oxforms was obtained based on DFT calculation and EXAFS
analysis (Fig. 5C). The three types of Ta–O bond lengths thatwere experimentally determined from the EXAFS measurements
were in good agreement with those obtained from the theoretical calculation. In particular, the experimentally determined
Ta–O3 bond length was almost the same as the bond length
determined based on the structure optimized in the DFT
calculations. Therefore, the local structures of Ce and Ta obtained from the quantitative EXAFS analysis were validated
based on DFT calculations for the TBA- and Ox-forms. In thesummary of the results of DFT calculations and EXAFS analyses,
the c-axis displacement of Ta atoms can work to relax theoctahedral tilting, which would otherwise occur, to take a highsymmetric tetragonal structure with no tilting. This new
knowledge is one of the notable ndings in this work because it
is not based on conventional solid-state chemistry and is expected to further develop perovskite structures.

Optical absorption properties

Fourier transformed (FT) EXAFS spectra in the R-space at the (A)Ce and (B) Ta L3-edges for the TBA- and Ox-forms. (C) Percentages ofthe bond length changes between the TBA- and Ox-forms obtained
from DFT calculations and EXAFS analysis.
Fig. 5© 2021 The Author(s). Published by the Royal Society of Chemistry

A cerium-introduced layered perovskite has unique opticalabsorption properties based on the location of the Ce 4f orbitalin the bandgap at the mid-state level.17 In particular, RbCeTa2O7
powder exhibits a specic pale green color due to the twoabsorption types due to diﬀerent electron transitions that occur
in the blue regions; these transitions include the ligand-to-

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Table 2 Bond lengths, coordination numbers, and Debye–Waller factors obtained through ﬁrst-shell EXAFS ﬁtting at the Ce and Ta L3-edges in
the TBA- and Ox-formsa

Derivatives

Path

N

R (
A)

s2

Path

N

R (
A)

s2

TBA-form

Ce–O2
Ce–O3

8
4

2.646(4)2.749(6)0.0071(7)0.0001(9)Ox-form

Ce–O2
Ce–O3

8
4

2.53(1)2.70(5)0.012(3)0.017(8)

Ta–O1
Ta–O2
Ta–O3
Ta–O1
Ta–O2
Ta–O3

1
4
1
1
4
1

1.78(3)2.002(6)2.26(3)1.75(8)1.95(1)2.36(10)0.012(6)0.009(1)0.007(5)0.01(2)0.008(2)0.01(2)a

R: bond length; N: coordination number; s: Debye–Waller factor; parenthesis: standard deviation.

metal charge transfer (LMCT) transition from the O 2p (valenceband) to the Ce 4f mid-state and the near infrared (NIR) regions
through the MMCT transition from the Ce 4f mid-state to Ta 5d(conduction band). No reports have indicated that such
multiple transitions, including the MMCT transition, occur
among any well-known layered perovskites except for
RbCeTa2O7. Therefore, we measured the diﬀuse reectance(DR) spectra in the UV-visible-NIR region for the TBA- and Oxforms with a Tauc plot (Fig. 6A). The specic NIR absorption
was observed at approximately 1–2 eV in the TBA-forms, which
is similar to the DR spectrum of RbCeTa2O7. However, the NIR
absorption in the Ox-form decreased in intensity. To visualize
the decrease in NIR absorption, valence band (VB) XPS spectra
were observed, as shown in Fig. 6B. In the TBA-form, due to theCe 4f level, an evident peak was observed at approximately 2 eVwith a broad shoulder band due to the O 2p level at 3–10 eV.

This shoulder band forms a valence band, which is similar tothat in the RbCeTa2O7 parent material, while the corresponding
Ce 4f peak disappeared in the Ox-form. Therefore, the decrease
in NIR absorption should be ascribed to the lack of the midstate Ce 4f electrons located at the near-Fermi level. Theenergy gaps of the LMCT and MMCT transitions were estimatedto be 2.47 and 1.40 eV for the TBA-form and 2.54 and 1.12 eV for
the Ox-form, respectively. Additionally, the optical absorption
diﬀerences in the chemical oxidation from [CeIIITa2O7] to[CeIVTa2O7] induced drastic coloring changes from pale green topale yellow. To quantitatively compare the hue changes in bothforms, the chromatic properties were investigated in terms of
the Commission Internationale de l'Éclairage (CIE) L*a*b*
system based on the DR spectra, in which each L*a*b* value issummarized in Table S2.† The chromatic values of the Ox-form
increased compared to those of the TBA-form; in particular, theb* value drastically changed from 11.84 to 18.65, which indicates an increase in the yellow color composition and a decreasein the blue color composition.
Electronic structure calculation

Fig. 6(A) Tauc plots and (B) VB XPS spectra for the TBA- and Ox-

forms.

15024 | Chem. Sci., 2021, 12, 15016–15027

The total and partial density of states (DOS) plots for each
[CeIIITa2O7] and [CeIVTa2O7] layered perovskite were calculated
from the same optimized structure with the DFT method to
obtain a reasonable understanding of the optical absorptionchanges aer chemical oxidation from [CeIIITa2O7] to[CeIVTa2O7] (Fig. 7A). The VB and the conduction band (CB) for
the [CeIIITa2O7] layer were mainly formed from the O 2p
bonding orbitals and from Ta 5d empty orbitals that werepartially hybridized with the O 2p antibonding orbital, respectively; they are not related to the Ce 4f orbitals. Moreover, the Ce
4f state was located at the interband between the VB and CB as
the mid-state. Each of the band positions of the VB, CB and midstate in the [CeIVTa2O7] layer was similarly due to the O 2p, Ta 5d
and Ce 4f levels, respectively. Although the energy gap between
the VB maximum (VBM) and CB minimum (CBM) could befound to be similar in the [CeIIITa2O7] and [CeIVTa2O7] layers,
the Ce 4f orbital levels were observed in the diﬀerent positions,which is a lower energy shi with the disappearance of the Ce 4f
electron due to oxidation. The Fermi level in the [CeIVTa2O7]
layer was formed from the O 2p orbitals in conjunction with theCB, meaning that the 4f electrons are not fully occupied in theCe 4f level. The empty Ce 4f level was located at a 1 eV higher
energy potential than the VBM (Fermi level). The position of the© 2021 The Author(s). Published by the Royal Society of ChemistryView Article Online

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Chemical ScienceFig. 7 (A) Total (gray solid area) and partial DOSs (lines) for the Ce 4f(pink), Ta 5d (cyan) and O 2p (yellow) orbitals in the (top) anionic[CeIIITa2O7] and (bottom) charge-neutral [CeIVTa2O7] perovskite
layers. The VBM of the [CeIIITa2O7] layer was drawn by shifting thepeak top of the O 2p occupied orbital to be equal to that of [CeIVTa2O7]
layers. (B) Schematic band diagram comparison for the anionic[CeIIITa2O7] and charge-neutral [CeIVTa2O7] perovskite layers.

Ce 4f level in the [CeIVTa2O7] layer is lower than the Ce4+-doped
zirconates and cerates, which has the empty Ce 4f levels at
2 eV higher than the VBM.41–43 Since the Ta 5d orbital forming
CB is lower than that of Zr 4d, the Ce 4f orbital position may bedecreased by its repulsion. Based on these DOS results, a schematic diagram of each band position and the electrontransition mechanisms of the two forms is provided inFig. 7B. The [CeIIITa2O7] layer exhibits multiple absorptions
from two types of LMCT transitions: from the VB to the CB andthen the VB to Ce 4f and from one MMCT transition and then Ce
4f to the CB, which results in NIR absorption. In contrast, the[CeIVTa2O7] layer exhibited absorption from only two electronictransitions of LMCT transitions without the MMCT transition
because of the empty Ce 4f level. Therefore, the [CeIVTa2O7]
layer absorbed only two regions of UV and blue light with weak
NIR absorption and was colored pale yellow.

Chemical reactivity tests
The tetravalent Ce ion, Ce4+, containing inorganic compounds,
typically CeO2 and (NH4)2[Ce(NO3)6] (CAN), exhibit excellent
oxidative performance.19,44 Similar oxidative capacity was expected to be found in this unique perovskite layer with Ce4+ of[CeIVTa2O7]. Firstly, the reduction behavior of Ce4+ with ascorbic
acid was investigated to evaluate the oxidic reactivity of the[CeIVTa2O7] perovskite layer. The XRD pattern of the red-formshows no obvious change from before the reduction test
(Fig. 8A–C), indicating that the layered perovskite unit was
retained. Furthermore, the slight expansion and shrinkage ofthe lattice for the red-form could be observed in the in-plane[110] and stacking [001] directions, respectively. This phenomenon is an opposite trend compared with the change in the lattice
due to the oxidation reaction of the TBA-form to the Ox-form. Two
X-ray absorption peaks at 5729 and 5737 eV were observed in theCe L3-edge XANES spectrum of the Ox-form (Fig. 8D), whereas the
red-form also shows a tiny peak at 5737 eV due to Ce4+ witha dominant peak at 5725 eV in which it was almost same peak
position in the Ce L3-edge XANES spectrum of the parent forms of
Rb-, H- and TBA-forms (5726 eV). In the multilayered perovskite,© 2021 The Author(s). Published by the Royal Society of ChemistryFig. 8 (A-C) XRD patterns of the TBA-, Ox- and red-forms. (D) Ce L3edge XANES spectra of the Ox- and red-forms. (E) Cyclic voltammograms of the fabricated electrode at a scan rate of 100 mV s1. The CV
measurement was conducted in acetonitrile containing 100 mM
TBAPF6.

the reduction does not proceed suﬃciently, and a part of Ce in
the perovskite unit could not be reduced, resulting that it is
supposed that the Ce4+ state was kept. These results of the
reductive reduction tests for the [CeIVTa2O7] perovskite layer
could indicate that the [CeIVTa2O7] perovskite layer can bereversibly reduced to the original [CeIIITa2O7] anionic layer. The
oxidative capacity of the [CeIVTa2O7] perovskite layer wasdemonstrated by voltammetric behavior in acetonitrile. Although
the cyclic voltammogram of the fabricated PDDA/CeIIITa2O7/PEIAu electrode in the rst cycle showed several undened waves,a couple of waves were observed in the second and third cycles(Fig. S16A†). The cyclic voltammograms of the fabricated PDDA/
CeIIITa2O7/PEI-Au electrode in the 4th to 14th cycles are shown in
Fig. 8E. The oxidation wave appeared at around 1000 mV, and thecorresponding reduction wave at around 600 mV. Since thesewaves were not observed in similar measurements using bare Au
and PEI-coated Au electrodes (Fig. S16B†), they are originatedfrom the redox of the [CeTa2O7] perovskite layers. It is well known
that the reduction potential of Ce4+ is a +1.44 V vs. SHE.18 The
reduction potential of [CeIVTa2O7] perovskite layers can be converted to be +1300 mV (+1.3 V) vs. SHE from the redox potential of
Fc/Fc+ vs. SHE (+0.7 V), which is comparable to the standard
potential of Ce4+. These redox waves should correspond to theredox reaction of Ce inside the perovskite unit. This electrochemical redox reaction is chemically reversible because a coupleof waves were observed for at least 14 potential cycles. However,
the magnitude of waves decreased in current as the potential
cycle proceeded. This may be attributed to the fact that the
reductive reaction is not completely proceeded due to small
amount of Ce4+ residue, as evidenced by the above XANES result

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Chemical Scienceof the reduction behavior using ascorbic acid. To the best of ourknowledge, the electrochemical behavior of layered perovskite
materials via charge-neutral layers has been investigated for therst time.

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Conclusion
In this work, we rst proved the existence of a layered perovskite
tantalate containing tetravalent cerium (Ce4+) at the A-sitethrough experimental and theoretical approaches based on
DFT calculations. The charge-neutral layered perovskite of[CeIVTa2O7] was obtained through so chemical oxidation of ananionic [CeIIITa2O7] perovskite layer sheet with MnO4 ions.
TEM and AFM images were used to elucidate that the[CeIVTa2O7] particle had a nanoplate morphology with 10–30
stacked perovskite layers, similar to the anionic perovskite thatwas used as the precursor. The Ce and Ta L3-edge XANES
spectra of [CeIVTa2O7] indicated that the cerium ion in theperovskite layer fully changed to a tetravalent ion and tantalum
was maintained as a heptavalent ion due to the oxidation of[CeIIITa2O7]. The detailed analysis of the SAED patternprovides evidence that the Ox-form takes the higher symmetryof the tetragonal structure with no octahedral tilting, which is
unique in conventional solid-state chemistry. It was presumed
that this irregular stabilization of the tetrahedral symmetry in[CeTa2O7] is caused by the relaxation of the obvious displacement of Ta atoms to the outside of the perovskite layer. The
detailed local structures around the Ce and Ta atoms were obtained based on the EXAFS spectra of each elemental absorption edge, and all bond lengths in the [CeIIITa2O7] and[CeIVTa2O7] perovskites were found to be in good agreementwith those obtained from the DFT calculations. Although the[CeIVTa2O7] perovskite maintained strong optical absorptions in
the UV and blue-light regions, the strength of the NIR absorption at ca. 1 eV exhibited in the anionic [CeIIITa2O7] perovskite
was weakened. These changes in absorption properties should
be ascribed to the fact that no MMCT transition of Ce 4f / Ta
5d occurred because, based on DOS calculations, the 4f electron
associated with the oxidation of cerium disappeared. Furthermore, the obtained [CeIVTa2O7] perovskite layer can be reduced
by reaction with ascorbic acid solution to form the anionic[CeIIITa2O7] perovskite layer, which was conrmed by XRD and
XANES analyses. In addition, cyclic voltammetry in acetonitrile
revealed that the [CeIIITa2O7] perovskite exhibits a chemicallyreversible redox behavior. The redox ability in the layered
perovskite can form a new material group for catalyst science,and the Ce4+-based charge-neutral perovskite could be applied
to polar materials based on their structural features. This work
successfully dened the possibility of achieving extraordinary
redox performance in layered perovskites, which could lead tothe denition of an innovative material family that contributes
to the further development of perovskite compounds throughprecise control of the electronic structure associated with
optical performance. Particularly, the new concept of layer
charge control and charge-neutral layered perovskite materials
will be eﬀectively applied to the materials science eld, such as15026 | Chem. Sci., 2021, 12, 15016–15027

Edge Article
electrochromic materials utilizing color change by redox reactions and polar materials using BO6 octahedral distortion.

Data availability
All experimental data, computational data and detail experimental procedure are provided in the ESI.†

Author contributions
T. Hasegawa: all experiments except for the electrochemical
analysis, writing-original dra, supervision, conceptualization.
N. Yamasaki: measurement of the electrochemical analysis. T.
Ueda supervision for the electrochemical analysis. Y. Asakura
and S. Yin: writing-review and editing.

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

Acknowledgements
This work was supported by JSPS KAKENHI Grant Number
JP20K15106, Nippon Sheet Glass Foundation for Materials
Science and Engineering, The Dynamic Alliance for Open Innovations Bridging Human, Environment and Materials, and the
Cooperative Research Program of “Network Joint Research Center
for Materials and Devices”. XAFS measurements were performed
with the BL12C at the Photon Factory (PF) with the approval of theHigh Energy Accelerator Research Organization (KEK) (Proposal
No. 2020P001). We thank Dr M. Nagasako for the STEM observations, which were supported by the Tohoku University Microstructural Characterization Platform in Nanotechnology Platform
Project sponsored by The Ministry of Education, Culture, Sports,
Science And Technology (MEXT), Japan (Proposal No. A-20-TU0034), and Dr K. Hongo (JAIST, Japan) for the DFT calculation.

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