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Manipulation of charge carrier flow in Bi4 NbO8 Cl nanoplate
photocatalyst with metal loading
Separation of photoexcited charge carriers in a semiconductor
particle is crucial for efficient photocatalysis such as water splitting.
Here, a spatial charge separation in Bi4 NbO8 Cl nanoplates via
site-selective deposition of cocatalyst is demonstrated. Althoughboth photoexcited electrons and holes travel along the in-plane
direction in the layered crystal structure and recombine at theedge of the nanoplate without a cocatalyst, Rh particles loaded on
the edge extract electrons selectively from the photocatalyst andthereby enable the remaining holes to move along the out-of-plane
direction, affording efficient charge separation for H2 evolution.

See Hiroshi Kageyama,
Ryu Abe et al.,Chem. Sci., 2022, 13, 3118.

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Manipulation of charge carrier ﬂow in Bi4NbO8Cl
nanoplate photocatalyst with metal loading†Kanta Ogawa, a Ryota Sakamoto, a Chengchao Zhong, a Hajime Suzuki,
Kosaku Kato, b Osamu Tomita, a Kouichi Nakashima,c Akira Yamakata, b
Takashi Tachikawa, d Akinori Saeki, e Hiroshi Kageyama *a and Ryu Abea*a

Separation of photoexcited charge carriers in semiconductors is important for eﬃcient solar energy
conversion and yet the control strategies and underlying mechanisms are not fully established. Althoughlayered compounds have been widely studied as photocatalysts, spatial separation between oxidation
and reduction reaction sites is a challenging issue due to the parallel ﬂow of photoexcited carriers along
the layers. Here we demonstrate orthogonal carrier ﬂow in layered Bi4NbO8Cl by depositing a Rh
cocatalyst at the edges of nanoplates, resulting in spatial charge separation and signiﬁcant enhancementof the photocatalytic activity. Combined experimental and theoretical studies revealed that lighter
photogenerated electrons, due to a greater in-plane dispersion of the conduction band (vs. valence
Received 2nd November 2021
Accepted 22nd January 2022band), can travel along the plane and are readily trapped by the cocatalyst, whereas the remaining holes
hop perpendicular to the plane because of the anisotropic crystal geometry. Our results propose

DOI: 10.1039/d1sc06054f

manipulating carrier ﬂow via cocatalyst deposition to achieve desirable carrier dynamics forrsc.li/chemical-sciencephotocatalytic reactions in layered compounds.

Introduction
Semiconductor photocatalysis is a promising solar energy
conversion method. Therein, photogenerated charge carriers
migrate from the bulk to the surface to induce redox reactions –water splitting being the primary target.1–3 For practical solar-tohydrogen conversion systems via photocatalytic water splitting,both the eﬀective absorption of photons in the visible lightregion and the eﬃcient charge separation of photoexcitedcharge carriers (electrons and holes) are crucial. Thus, semiconductor materials with narrow band gaps such as non-oxides
and mixed-anion materials have increasingly been investigatedas photocatalysts.4–9 Although an extraordinarily high quantum
eﬃciency of nearly 100% has recently been demonstrated under
UV light on an Al-doped SrTiO3 particulate photocatalyst,3 suchhighly eﬃcient charge separation has not yet been achieved onaDepartment of Energy and Hydrocarbon Chemistry, Graduate School of Engineering,
Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japanb

Graduate School of Engineering, Toyota Technological Institute, 2-12-1 Hisakata,Tempaku, Nagoya 468-8511, Japanc
Department of Materials Science and Engineering, College of Engineering, Ibaraki
University, 4-12-1, Nakanarusawa, Hitachi, Ibaraki 316-8511, Japand

Molecular Photoscience Research Center, Kobe University, 1-1 Rokkodai-cho, Kobe
657-8501, Japane

Department of Applied Chemistry, Graduate School of Engineering, Osaka University,
2-1 Yamadaoka, Suita, Osaka 565-0871, Japan† Electronic supplementary information (ESI) available: Photocatalytic activity,
SEM, and TEM images, PL images. See DOI: 10.1039/d1sc06054f

3118 | Chem. Sci., 2022, 13, 3118–3128

visible-light-responsive
photocatalysts.
Recenteﬀorts,including the Al-doped SrTiO3 and some visible-responsive
materials (e.g., BiVO4, BaTaO2N), have focused on facet engineering to expose crystal faces suitable for redox reactions,3,10–14
where the energy level of each facet determines the charge
separation. However, it is not always possible to expose
a desired facet of the targeted semiconductors. Moreover, thisstrategy relies on the intrinsic characteristics of photocatalysts'
facets and does not answer the fundamental question of how
charge separation is achieved in the bulk of photocatalysts; thecarrier ow inside the photocatalyst particle is still not fully
understood.
Layered semiconductors such as UV-responsive K4Nb6O17
have shown promise in photocatalysis4,15,16 with a couple ofadvantages, including their controllability of the chemicalcomposition through ion exchange17–19 and large surface-tovolume ratios.20 However, from the viewpoint of eﬃcientcharge separation inside the photocatalysts, two-dimensional
crystal structures may be unfavorable because both photoexcited electrons and holes travel mainly along the in-plane
direction, as observed theoretically21–24 and experimentally.25–27
Such parallel carrier ows based on the crystal and band
structure make it diﬃcult to separate the oxidation and reduction reaction sites in the photocatalyst particles, oen resulting
in charge recombination. Although the reduction and oxidationsites on several layered photocatalysts such as BaLa4Ti4O15 wereseparated,13,28 the underlying carrier ows in the bulk of thephotocatalysts remain elusive.

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Fig. 1 (a) Crystal structure of the layered perovskite oxyhalide Bi4NbO8Cl. (b) Time courses of H2 evolution over Bare-Bi4NbO8Cl(squares), Rh-Bi4NbO8Cl (triangles) and Pt-Bi4NbO8Cl (circles) ina methanol aqueous solution (20 vol%) under photoirradiation (l > 300nm).

We have recently reported that Bi4NbO8Cl, with a layered
Sillén–Aurivillius structure comprising [Bi2O2], [NbO4], and [Cl]modules (Fig. 1a), exhibits the highest activity among layered
compounds, as an O2 evolution photocatalyst in visible-light Zscheme water-splitting.29 When prepared by a ux method,
Bi4NbO8Cl yields as nanoplates with good crystallinity.30
However, despite its band positions being suitable for both H2and O2 production from water, the H2 evolution activity of Bi4NbO8Cl was negligible, and surprisingly, the situation did notchange when Pt, an established H2 evolution cocatalyst,4 wasintroduced.31,32 This result suggests that charge carriers are noteﬀectively separated in Bi4NbO8Cl.
Here, we demonstrate an eﬃcient spatial charge separationin Bi4NbO8Cl nanoplates by controlling the direction of carrierows with Rh loading. In the absence of a cocatalyst, bothphotoexcited electrons and holes travel along the in-plane
direction based on the band dispersions, resulting in recombination at the edge of the nanoplate. However, site-selective Rh
deposition at the edge allows the electrons to migrate in the inplane direction and to be captured by the cocatalyst, while theholes hop along the out-of-plane direction. The resultant spatial
charge separation signicantly improves the H2 evolution
activity. Based on experimental and theoretical investigations,
we discuss the carrier ow dynamics and the associated photocatalytic properties of Rh-loaded Bi4NbO6Cl nanoplates incomparison to unloaded- and Pt-loaded ones.

Results and discussion
Improvement of Bi4NbO8Cl photocatalytic H2 evolution
activity
As stated above, the H2 evolution activity of bare Bi4NbO8Cl(hereaer denoted as Bare-Bi4NbO8Cl) is low even though it has
a conduction band (CB) potential suﬃcient for H+ reduction,31
and little improvement is seen when the Pt cocatalyst (5 wt%) is
loaded by a photodeposition (PD) method (Pt-Bi4NbO8Cl;
Fig. 1b). In contrast, Rh loading (5 wt%) via PD signicantly
enhances photocatalytic H2 production, as displayed in the timecourse of H2 evolution (Rh-Bi4NbO8Cl; Fig. 1b). We additionally© 2022 The Author(s). Published by the Royal Society of Chemistry

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investigated H2 production activities for samples with various
metal cocatalysts (Pt, Ru, Ir, Pd, Au, Rh) deposited by PD and
impregnation methods, the most frequently used methods forparticulate photocatalysts. The results highlight the superiority
of Rh-Bi4NbO8Cl (Fig. S1†). H2 evolution is observed at
a remarkably high rate under visible-light irradiation (l > 400nm) on Rh-Bi4NbO8Cl (Fig. S2†). This result conrms thatvisible-light water splitting is possible and that photocatalytic
H2 evolution is based on the bandgap excitation of Bi4NbO8Cl(Fig. S3†). Although the activity decreased during long-term
irradiation due to physical detachment of the cocatalyst(Fig. S4†), the stability will be improved further by elaborated
loading procedures.

Site-selective deposition of the metal cocatalyst
We conducted detailed investigations using mainly electron
microscopic tools (Fig. 2) to understand why Rh-Bi4NbO8Cl
showed the excellent H2 evolution activity. Fig. 2a and S5†
represent a typical SEM image of Bare-Bi4NbO8Cl synthesized bythe ux method, which has nanoplate morphology with large
aspect ratios; the typical lateral length and thickness areapproximately 1 mm and 50 nm, respectively.30 The TEM/SAED
pattern (Fig. S6†) indicated that the dominant facet is the(001) plane. HAADF-STEM images recorded along the [110] and[100] directions are shown in Fig. 2f, g and S7,† consistent withthe reported crystal structure.33 Notably, the STEM/EDX line
scan analysis and elemental maps suggest that the outermost
layer is the [Bi2O2] layer (Fig. 2h–l and S7†).
A major diﬀerence between Rh-Bi4NbO8Cl and Pt-Bi4NbO8Cl
is the location of the cocatalyst, as seen in SEM and HRTEM
images; the Rh cocatalyst, deposited as agglomerations, wasfound only at the edges of the Bi4NbO8Cl nanoplate (Fig. 2b andd), whereas the Pt cocatalyst was observed as nanoparticulates
dispersed mainly on the (001) facet (Fig. 2c and e). Absorption
spectroscopy (Fig. S8†) ensured that precursors for the Rh and
Pt cocatalysts, RhCl63 and PtCl42, were fully consumed during
the PD process. The analysis of HRTEM (Fig. 2d and e) and XPS(Fig. S9†) conrmed that the Rh and Pt species were reduced tozero-valence by photogenerated electrons. The crystal phaseand the valence state of Bi4NbO8Cl were not changed by the
cocatalyst loading (Fig. S10†). For Rh-Bi4NbO8Cl, no Rh species
were observed on the (001) facet (Fig. S11†), showing selective
deposition at the edges. Direct photoreduction of Rh3+ was
ruled out by a control experiment (Fig. S12†). We also veriedthat the p/n type, doping level, and zeta potentials were notaﬀected by the Pt and Rh loading (Fig. S13†). The H2 evolution
rate of Rh-Bi4NbO8Cl reached 222.4 mmol h1 at 20 wt% loading(Fig. S14†), showing an apparent quantum eﬃciency (AQE)
value of 2.6% at 405 nm (Table S1†); note that there was room
for improvement as the Rh particles grew into aggregates(Fig. S15d–f, 16c and d†).
As in the case of Rh and Pt, photoreduction of Au3+ and Ag+
resulted in preferential deposition as elemental metals on theedges and dominant facet, respectively (Fig. S17†). Note that
although Au was deposited at the edge as Rh, the Au-loaded
Bi4NbO8Cl showed negligible activity (Fig. S1†), due to theChem. Sci., 2022, 13, 3118–3128 | 3119

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Fig. 2 (a–c) SEM images of Bare-Bi4NbO8Cl (a) Rh-Bi4NbO8Cl (b) and Pt-Bi4NbO8Cl (c). (d and e) Bright-ﬁeld TEM images of Rh-Bi4NbO8Cl (d)and Pt-Bi4NbO8Cl (e). The insets show HRTEM images of the cocatalysts. (f) HAADF-STEM image of a Bare-Bi4NbO8Cl nanoplate along the [110]direction. (g) Close-up view of the red rectangle in (f), together with the Bi4NbO8Cl crystal structure. (h–l) STEM-EDX line scan analysis (h) along
the yellow arrow in (f) and atomic resolution elemental maps (i–l) for Bi (i) Nb (j) O (k) Cl (l).

much inferior HER catalytic activity of Au to Rh.34 The observed
variation in metal deposition sites is unaccountable by the
known facet-dependent redox mechanism in which reductive
cocatalyst deposition should take place at xed sites suitable for
reduction, regardless of the metal species.11 A possible mechanism for the variation in the metal deposition site is describedin Fig. S18,† where the reduction rate of each metal may be
a key.
In general, dispersed nanoparticles with small overpotentials for target reactions are preferable as cocatalysts andcan oen enhance photocatalytic reactions.35–37 However, thecontrasting H2 evolution activities of Pt-Bi4NbO8Cl and RhBi4NbO8Cl suggest that the eﬀect of the deposition site ispivotal, rather than the intrinsic catalytic capability of the
cocatalyst, which is supported by the following ve additional
observations:(1) The impregnation method produced Rh particles mainly
dispersed on the (001) facet (with a minor fraction on the edge),

3120 | Chem. Sci., 2022, 13, 3118–3128

which did not serve as an eﬀective H2 evolution cocatalyst(Fig. S19†). (2) The Pt cocatalyst enhanced H2 evolution when
loaded on the edges of Bi4NbO8Cl nanoplates. With increased
loading (20 wt%), some Pt nanoparticles appeared on the edgesof Pt-Bi4NbO8Cl (Fig. S15a–c, 16a and b†). The Pt-Bi4NbO8Cl
sample with 20 wt% Pt exhibited a rate of H2 evolution of 65.9mmol h1 (Fig. S14†), which was greater than those of thesamples with 1 wt% and 5 wt% Pt mainly on the (001) facet (0.3
and 2.4 mmol h1). (3) When Pt was deposited on not only the(001) surface but also the edge at a high loading amount, the H2evolution activity of Pt–Bi4NbO8Cl was not higher than that of
Rh–Bi4NbO8Cl, regardless of whether the activity is plottedversus the loading amount in wt% or mol% (Fig. S14†), despite
better catalytic activity for the HER of Pt than Rh. This result
indicates that Pt on (001) aﬀords a negative contribution to theH2 evolution. (4) When additional Pt (1 wt%) was deposited onthe (001) facet of Rh-Bi4NbO8Cl with 5 wt% Rh, the photocatalytic activity deteriorated substantially (from 39.5 to 8.5© 2022 The Author(s). Published by the Royal Society of ChemistryView Article Online

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mmol h1, Fig. S20†). (5) Rh-Bi4NbO8Cl showed a higher photocatalytic activity for not only water reduction but also wateroxidation than Pt-Bi4NbO8Cl (Fig. S21a†), although the catalytic
activity for IO3 reduction of Rh is inferior to that of Pt(Fig. S21b and c†). These results clearly show that the deposition site, rather than the metal species, of the cocatalyst
signicantly aﬀects the photocatalytic activity.
Carrier ows in Bare-Bi4NbO8Cl
We hereaer elucidate the dynamics of photoexcited carriers in
Bi4NbO8Cl. Let us rst discuss the results of time-resolved
microwave conductivity (TRMC),38,39 which provides direct
insight into the carrier ow in Bare-Bi4NbO8Cl. Of note, we
adopted two measurement congurations (Fig. 3a) taking
advantage of the morphology of Bi4NbO8Cl nanoplates; theanisotropy of carrier mobilities perpendicular and parallel tothe (001) plane was evaluated by changing the direction of theincident microwave with respect to the nanoplates xed toa substrate (Fig. S22†). The conductivity transients for the twosettings are displayed in Fig. 3b, indicating an intense signal forthe in-plane direction, with a maximum value of photoconP
ductivity 4 m being 17 times higher. Provided that 4 isconstant (supported by the fact that the photoexcitation for
carrier generation is xed throughout the experiment), theChemical Science
TRMC results indicate that the Bi4NbO8Cl nanoplate exhibitsa large anisotropy in the charge carrier ow, with greater
mobility along the in-plane direction.
Theoretical investigations further support the anisotropic
nature of the carrier ow in Bi4NbO8Cl. Fig. 3c depicts theorbital distribution estimated by DFT calculations, indicatingthat Bi-6p orbitals in the [Bi2O2] layer mainly contribute to thebottom of the conduction band (CB, blue) and O-2p orbitals in
the [NbO4] layer to the top of the valence band (VB, orange). Theband structure of Bi4NbO8Cl (Fig. 3d and S23†) shows that both
the CB and VB have greater dispersion along the in-plane
directions (G–X and G–Y) than the out-of-plane direction (G–Z), in common with layered materials (e.g., PbBiO2Cl24 andSr2TaO3N40). However, a closer look reveals that the CB is moredispersive than the VB, and the eﬀective mass of electrons (0.25m0) is smaller than that of holes (1.4 m0). Thus, electrons havein-plane mobility much higher than holes.
Fate of the carriers
The fate of photogenerated carriers in Bi4NbO8Cl was visualizedby single-particle photoluminescence (PL) imaging,41 using
recombination of the electrons and holes, or redox reaction
products as probes. The PL image of a single Bare-Bi4NbO8Cl
nanoplate excited by a 405 nm pulse laser (Fig. 4a and S24†)

Fig. 3 (a) Schematic illustration of the experimental setup for the anisotropic TRMC measurement. (b) Conductivity transients for Bare-Bi4NbO8Cl. Blue and green proﬁles are those observed along the in-plane and out-of-planeP
direction against P
the (001) plane corresponding to theblue and green arrows in (a), respectively. The vertical axis represents photoconductivity 4 m, where 4 and m are the quantum eﬃciency of thecharge carrier photogeneration and sum of the carrier mobilities, respectively. (c) Orbital distribution of the lower parts of the conduction band
(blue) and the upper parts of the valence band (orange) of Bi4NbO8Cl estimated by DFT calculation. (d) Band structure of Bi4NbO8Cl, where bothCB minimum and VB maximum are located at the G point.

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Fig. 4 (a and b) PL (a) and transmission images (b) of a single Bare-Bi4NbO8Cl nanoplate immobilized on a cover glass in air under 405 nm laser
irradiation. The PL intensity was integrated over all monitored wavelengths. (c) Reductive turn-on conversion of the ﬂuorescent molecular probe
used in single-molecule ﬂuorescence imaging. (d and e) Fluorescence (d) and transmission (e) images of a single Bare-Bi4NbO8Cl nanoplate
immobilized on a cover glass in an Ar-saturated aqueous methanol solution containing MS-DN-BODIPY (0.3 mM), wherein methanol served asa hole scavenger. For (d), the image was taken under 488 nm laser irradiation. The red dots in (e) represent ﬂuorescence bursts observed during
the 180 s irradiation. (f and g) SEM images of Rh-Bi4NbO8Cl after the photocatalytic deposition of PbO2 (f) and MnO2 (g).

shows strong emission on the edges of the nanoplate whenreferring to a concurrent optical transmission image of thenanoplate (Fig. 4b). Herein we conclude that the optical waveguide eﬀect does not play a chief role in the strong emission on
the edge, because PL from the bulk and that from the edge
possess lifetimes diﬀerent from each other, and the waveguideeﬀect is negligible when the particle is irradiated with a 532 nm
laser that Bi4NbO8Cl does not absorb (Table S2 and Fig. S24†).
Combined with the TRMC results, the single-particle PL
imaging result of Bare-Bi4NbO8Cl indicates that both photogenerated carriers move in the in-plane direction, leading torecombination at the nanoplate edges.
We then performed two measurements in the presence ofreactants to study the dynamics of either carrier, using thesingle-molecule uorescence imaging technique for electrons42,43 and oxidative PD of metal oxides for holes. Fig. 4dshows the uorescence image of a single Bare-Bi4NbO8Cl
nanoplate excited at 488 nm in an Ar-saturated aqueous methanol solution containing an MS-DN-BODIPY molecular probe
for electrons (Fig. 4c), wherein methanol served as a hole
scavenger. Green and yellow spots showing intense uorescencewere observed mostly at the edge of the Bi4NbO8Cl nanoplate.
These spots arose from the luminescent MS-HN-BODIPY converted from the non-luminescent MS-DN-BODIPY upon reduction by photogenerated electrons,44 as supported by the time
evolution of uorescence mapping (Fig. S25†). Several uorescence bursts were detected over 180 s of photoirradiation, which
are displayed in Fig. 4e as red spots overlaid with a concurrent

3122 | Chem. Sci., 2022, 13, 3118–3128

optical transmission image of the nanoplate, with the reproducibility checked on other nanoplates (Fig. S26†). Overall, thesingle-molecule uorescence experiments show that photoexcited electrons predominantly move along the in-plane direction and accumulate at the nanoplate edge.
Turning to the holes, we conducted the photocatalyticoxidation of Pb2+ and Mn2+ in an oxygen-saturated aqueous
solution. Here, the metal ions are oxidized by photogenerated
holes to the corresponding metal oxides (PbO2 and MnO2)
deposited on the photocatalyst surface, thereby acting as probes
for the oxidation sites of photocatalysts, where Pb2+ and Mn2+
have oen been employed.11 On the other hand, photoexcitedelectrons are trapped by electron acceptors (O2 and/or H2O). For
Bare-Bi4NbO8Cl nanoplates, negligible metal oxide deposition
was observed under visible light irradiation (l > 400 nm)(Fig. S27†), which suggests that holes recombine with electrons
before participating in the oxidation. In contrast, Rh-Bi4NbO8Cl
experienced PbO2 and MnO2 deposition (Fig. S28–S31†);
a number of particles and sponge-like deposits, in the cases of
Pb2+ and Mn2+, respectively, were scattered on the (001) facet(Fig. 4f and g). The result for Rh-Bi4NbO8Cl is particularly
important as it indicates that photogenerated holes are allowed
to migrate along the out-of-plane direction and reach the (001)surface, despite the dispersionless VB structure along thisdirection. Together with the hydrogen evolution experiments(Fig. 1b), we conclude that the carrier dynamics of Rh-Bi4NbO8Cl is characterized by a unique charge separation, where
electrons and holes are spatially separated and migrate,© 2022 The Author(s). Published by the Royal Society of ChemistryView Article Online

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respectively, toward Rh at the nanoplate edge and toward thebasal plane, triggering their respective redox reactions.

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Carrier decay dynamics

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was less prominent in Rh-Bi4NbO8Cl. These observations
suggest that Pt on the (001) facet traps both electrons and holes,
while Rh at the edge captures only electrons, leaving holes inthe bulk of Bi4NbO8Cl on picosecond timescales.

We further compared the charge carrier decay proles using
time-resolved absorption spectroscopy (TRAS), as used forsemiconductor photocatalysts.45,46 A microsecond transient
spectrum for Bare-Bi4NbO8Cl, when excited by a 355 nm laser
pulse (Fig. 5a), shows three characteristic absorption peaks at2000 cm1, 13 000 cm1, and 20 600 cm1. Referring toprevious results (e.g., TiO2,47–49 a-Fe2O3 50 and LaTiO2N45), therst absorption is attributed to photogenerated free (or shallowly trapped) electrons in the CB,51,52 the second one to theexcitation of trapped electrons from the mid-gap state to the
CB46 (derived from halogen defects, see Fig. S32†), and the third
one to the photogenerated holes. The initial decay proles aer
photoexcitation were examined using femtosecond TARS,
focusing on the absorbance changes at 2000 and 20 600 cm1
(Fig. 5b–e). The decay of free electrons at 2000 cm1 wasaccelerated by the loading of Rh or Pt compared to Bare-Bi4NbO8Cl (Fig. 5b and c); especially, Pt-Bi4NbO8Cl decayed more
rapidly than Rh-Bi4NbO8Cl. Regarding hole absorption at
20 600 cm1 (Fig. 5d and e), the Pt cocatalyst accelerated
a decline in the absorption peak, while the decay acceleration

Fig. 6 illustrates the suggested carrier ow dynamics and charge
separation of the three Bi4NbO8Cl nanoplate samples. The
thorough and systematic study described above allows us toreveal how the deposition site of the cocatalyst aﬀects the
direction of the carrier ow in the bulk and inuences the H2evolution photocatalytic activity. Hereaer, the carrier ow ineach material is discussed from the viewpoints of solid-statechemistry, solid state physics, semiconductor engineering,and catalysis science.
In Bare-Bi4NbO8Cl, photogenerated carriers (both electrons
and holes) are likely to travel in-plane, as expected from thelayered crystal structure and band dispersion. More precisely,
the in-plane dispersion of the conduction band is greater thanthat of the valence band (Fig. 3d), suggesting that electrons arelighter than the holes and can migrate faster along this direction. However, since the bare surface of Bi4NbO8Cl has no
catalytic site for H+ reduction, the lighter electrons cannot ow
out of the nanoplate. As a result, the electrons are trapped by theFig. 5 (a) Microsecond TRAS of Bare-Bi4NbO8Cl measured in thevacuum. The sample was excited by 355 nm laser pulses (6 ns duration,
0.5 mJ, 5 Hz). (b–e), Femtosecond decay proﬁles of transientabsorptions at 2000 cm1 (b and c) and 20 600 cm1 (d and e) for
Bare-Bi4NbO8Cl, Rh-Bi4NbO8Cl, and Pt-Bi4NbO8Cl. The samples
were excited by 355 nm laser pulses (90 fs duration, 6 mJ, 500 Hz) in air
for the several pico-second (ps) region.

Fig. 6 (a–c) Suggested major carrier ﬂow in Bare-Bi4NbO8Cl (a), PtBi4NbO8Cl (b) and Rh-Bi4NbO8Cl (c). In Bare-Bi4NbO8Cl, both electrons and holes are prone to move in the in-plane direction according
to their band dispersions, resulting in recombination at the nanoplateedges. In Pt-Bi4NbO8Cl, both carriers are collected by the Pt cocatalyst based on the band bending mechanism, leading again torecombination (Fig. 6b). However, the edge-deposited Rh cocatalyst in
Rh-Bi4NbO8Cl can collect photogenerated “light” electrons predominantly, while “heavy” holes left in the photocatalyst hop towards the(001) facet thanks to the anisotropic crystalline shape (approximately
50 nm in thickness). The orthogonal carrier ﬂow leads to the highphotocatalytic hydrogen activity.

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Carrier ow engineering

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Chemical Scienceholes that subsequently reach the nanoplate edges, causing
recombination, as conrmed by PL imaging (Fig. 4).
Once Rh is loaded on the nanoplate edges (Rh-Bi4NbO8Cl),
the situation changes drastically. Herein, the “light” electrons,
which arrive at the edge rst, can be extracted from the nanoplate by Rh and then used in the reduction of H+ (or H2O),
instead of recombining with the holes, as demonstrated by
TRAS (Fig. 5). With the electrons being consumed, the photogenerated “heavy” holes must move to the surface of thenanoplate to react with methanol in order to satisfy charge
neutrality in the bulk Bi4NbO8Cl. Of importance here is that our
Bi4NbO8Cl crystals are in the form of nanoplates with a thickness of about 50 nm, and this forces the holes to hop towards
the nearest (001) facet (Fig. 6c) against the dispersionless out-ofplane VB. The resultant orthogonal carrier ow realizes thespatial separation of photoexcited electrons and holes, greatly
enhancing the photocatalytic activity.
In Pt-Bi4NbO8Cl, Pt captures both electrons and holes, incontrast to what the band dispersion of Bi4NbO8Cl tells us(Fig. 6b). Here, changes in the ows of both carriers areaccounted for by band bending. In general, when novel metals
such as Pt are brought into contact with an n-type semiconductor, upward band bending and Schottky barrier areformed in the space charge layer.53 In fact, such upward band
bending was reported to allow photogenerated holes to migratefrom n-type photocatalysts to Pt.54–56 In Pt-Bi4NbO8Cl, Pt nanoparticles are highly dispersed on the basal plane (i.e., (001) facet)of the nanoplate (Fig. 2c). Considering the fact that the typical
thickness of the space charge layer is a few tens of nanometers,57
the thickness of about 50 nm in the present nanoplates meansthat the scattered Pt and n-type Bi4NbO8Cl junctions give eﬀective upward band bending over the entire crystal, allowing thephotogenerated holes to move into Pt. On the other hand, thephotogenerated electrons are prone to move in the in-plane
direction toward the edge of Bi4NbO8Cl due to the highmobility along the in-plane direction. However, in Pt-Bi4NbO8Cl,
they cannot be consumed at the edge, because of the inertness to
H+ reduction in the absence of a cocatalyst there. As a result,electrons are eventually captured by Pt on the (001) plane, followed by recombination with holes therein, as shown by TRAS
(Fig. 5). The electron transfer mechanism from Bi4NbO8Cl to Pt
might be tunneling through the Schottky barrier,58–60 where thehigh dispersity of Pt on the (001) facet should increase theprobability of electron transfer to Pt via tunneling. Note that thepreceding hole transfer from Bi4NbO8Cl to Pt reduces the band
bending and barrier height, which may make the subsequent
electron transfer easier. In addition, the requirement for charge
neutrality in the bulk Bi4NbO8Cl aer the hole transfer to Pt may
also be at play in the electron transfer to Pt and subsequent
recombination with the holes. Another possible case is that, atthe edge, electrons are recombined with holes that are notcaptured by Pt as in Bare-Bi4NbO8Cl.
Recall that previous studies on photocatalytic systems reliedon facet engineering for eﬃcient charge separation, specically,in TiO2,10 BiVO4,11 SrTiO3 12 and Bi2MoO6,13 where the photooxidation and reduction sites are separated inherently on
diﬀerent facets. For example, in BiVO4,11 photo-oxidation and3124 | Chem. Sci., 2022, 13, 3118–3128

Edge Article
photo-reduction take place preferentially on the (110) and (010)
facets, respectively, which is associated with the VBM/CBM
levels and the energy of each facet surface. This means thatthe direction of the carrier ow is determined by the inherent
properties of photocatalysts themselves. The choice of the facet
plane is a parameter to control. However, there are diﬃculties
and uncertainties in exposing the desired facet experimentally.
The present study proposes another strategy for the eﬃcientcharge separation: site-specic metal loading manipulates thecarrier ow of layered Bi4NbO8Cl; the intrinsic but unfavorable
parallel carrier ow was changed into the orthogonal one.

Conclusions
In summary, we successfully manipulated the carrier ow ina layered Bi4NbO8Cl nanoplate suﬀering from charge recombination by Rh deposition at the edge, which leads to the spatial
charge separation and signicantly improves the photocatalytic
activity. In conjunction with diﬀerences in the in-planedispersion between the conduction and valence bands, thesite-selective deposition allows the Rh cocatalyst to extract
photoexcited light electrons eﬃciently from the photocatalyst,
which then forces the remaining heavy holes to hop perpendicular to the plane with the aid of anisotropic crystal geometry.
We believe the present strategy is applicable not only to otherSillén(-Aurivillius) layered oxyhalide series,31,61,62 but also toa variety of semiconductor photocatalysts including mixedanion compounds with narrow band gaps;5,9,63 above all, thoseconsidered to be useless or ineﬃcient as photocatalysts can betransformed into eﬀective ones by metal-species deposition on
appropriate sites using the knowledge on band structures. The
present work provides a deeper understanding on, and a new
insight for charge separation in semiconductor photocatalysts.

Experimental section
Materials
CsCl (99.0%), NaCl (99.5%), Bi2O3 (99.99%), BiOCl (99.5%),
Nb2O5 (99.9%), H2PtCl6 (99.9%), CH3OH (99.8%), Pb(NO3)2(99.9%), and MnSO4$5H2O (99.9%) were purchased from FUJIFILM Wako Pure Chemical Corporation. Rh(NO3)3 and Na3RhCl6 were purchased from Kanto Chemical Corporation.
Sodium 2-sulfonate-1,3,5,7-tetramethyl-8-(3,4-dinitrophenyl)4,4-diuoro-4-bora-3a,4a-diaza-s-indacene(MS-DN-BODIPY)
was synthesized according to the literature. Water was puried using a Milli-Q purication system (Direct-Q UV S.).
Preparation of Bi4NbO8Cl nanoplates
Bi4NbO8Cl nanoplates were synthesized via the ux method
according to our previous report.30 A eutectic mixture (65 : 35) of
CsCl and NaCl was used as a ux. The ux was mixed witha stoichiometric mixture of Bi2O3, BiOCl, and Nb2O5 at a soluteconcentration of 1 mol%. The total mass was set to be 25 g. The
mixture was placed in an alumina crucible with a capacity of 30
cm3, and then heated at a rate of 50  C h1 to 650  C, being heldat the nal temperature for 10 h. Aer the natural cooling under© 2022 The Author(s). Published by the Royal Society of ChemistryView Article Online

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Chemical Scienceambient conditions, the product was washed thoroughly with
deionized water, collected by ltration, and air-dried.

eﬃciency (AQE) was evaluated using a 405 nm monochromatic
LED light source (ASAHI SPECTRA, CL-1501).

Cocatalyst loading

Single-molecule uorescence imaging

Rh and Pt and other cocatalysts (Ru, Ir, and Pd) were loaded on
Bi4NbO8Cl by photo-deposition (PD) and impregnation. In the
PD process, metal cations in the precursors are reduced byphotogenerated electrons and deposited on photocatalysts. The
following precursors were employed: H2PtCl6, RuCl3, Na3IrCl6,
H2PdCl4 and Na3RhCl6. In a typical procedure of photodeposition, 0.2 g of Bi4NbO8Cl powder and a calculated
amount of metal precursor (0.5–20 wt% for Pt and Rh, 0.5 wt%
for others) were mixed in 250 mL of 20 vol% methanol aqueous
solution. The suspension was then irradiated with visible light(l > 400 nm) for 5–12 h through a cutoﬀ lter from a 300 W Xearc lamp with continuous stirring in a Pyrex reaction vessel
connected to a closed circulation system in an Ar atmosphere ataround 298 K.
On the other hand, in the impregnation method, the deposition process is initiated by the “forced” adsorption of metal
cations onto photocatalyst surfaces by solvent evaporation,
followed by thermal reduction.5 First, 0.2 g of Bi4NbO8Cl powder
was immersed in an aqueous solution containing eachprecursor (0.5 wt%). In the case of 5 wt% of Rh deposition by theimpregnation method, Rh(NO3)3 was used. The suspension was
evaporated under constant stirring until complete dryness wasreached, followed by heating in an H2 ow at 200  C for 30 min.

To obtain isolated Bi4NbO8Cl particles, a well-dispersed methanol suspension of Bi4NbO8Cl in low concentration was spincoated onto a cleaned cover glass. The particle-coated coverglass was annealed at 363 K for 30 min to immobilize theparticles on the glass surface, and then placed in a chamberlled with an Ar-saturated aqueous methanol solution of MSDN-BODIPY (1 mM). A 488 nm CW laser (OBIS 488LX,
Coherent; 10 mW cm2) passing through an objective lens (CFI
Plan Apo l100  H, Nikon; NA 1.45) aer reection bya dichroic mirror (Di02-R488, Semrock) was reectedcompletely at the cover glass–solution interface to generate anevanescent eld, which made it possible to detect the uorescent products selectively on the bottom surface of the crystal.
The emission from the sample was collected by the same
objective lens, aer which it was magnied by a 1.5 built-in
magnication changer, and passed through a band-pass lter(FF01-535/50, Semrock) to remove undesired scattered light.
The emission images were recorded using an electronmultiplying charge-coupled device (EMCCD) camera (Evolve
512, Roper Scientic) using Micro-Manager (https://www.micromanager.org/). All experimental data were obtained at room
temperature.
Single-particle PL imaging

Characterization
Scanning electron microscopy (SEM) images were taken usinga Zeiss Nvision 40 microscope. High-angle annular dark-eldscanning transmission electron microscopy (HAADF-STEM)
and annular bright-eld scanning transmission electron
microscopy (ABF-STEM) images were collected using a JEMARM200CF (JEOL Ltd, Tokyo, Japan) operating at an accelerating voltage of 200 kV and equipped with a cold eld emission
gun and a Cs corrector to observe atomic columns of Bi4NbO8Cl.
Elemental analysis was carried out using a JEM-ARM200CF
equipped with energy dispersive X-ray spectroscopy (EDX).
The samples were prepared by grinding the material anddepositing a few drops of the suspension onto a holey copper
grid covered with a thin carbon lm. Transmission electron
microscopy (TEM) was carried out using a JEOL JEM-2100F
microscope. X-ray photoelectron spectroscopy (XPS) measurements were performed with an ULVAC-PHI 5500MT system.
Photocatalytic reaction
Photocatalytic reactions were performed in a gas closedcirculation system. The photocatalyst powder (0.1 g) was
dispersed in a methanol aqueous solution (20 vol%, 100 mL) ina Pyrex top-window cell. The photocatalyst was irradiated with
UV and visible light (l > 300 nm) or visible light (l > 400 nm)
through a cutoﬀ lter from a 300 W Xe-arc lamp. The quantity ofthe evolved gas was determined using an online gas chromatograph (thermal conductivity detector; molecular sieve 5 Åcolumn packing; Ar carrier gas). The apparent quantum© 2022 The Author(s). Published by the Royal Society of Chemistry

PL microscopy measurement was also conducted based ona Nikon Ti-E inverted uorescence microscope. For wide-eld
microscopy, the 405 nm CW laser (OBIS 405LX, Coherent; 30
mW cm2) was used to excite the Bi4NbO8Cl. The emission
images were recorded on an EMCCD at a rate of 30 frames s1. A
suitable dichroic mirror (Di02-R488, Semrock) and a long-passlter (BLP01-488R, Semrock) were used to improve the signalto-noise ratio. For confocal microscopy, the 405 nm pulsed
diode laser (Advanced Laser Diode System, PiL040X; 45 ps
FWHM, 1 MHz repetition rate) was used to excite the samples.
The emitted photons were passed through a 100 mm pinhole
and then directed onto a single-photon avalanche diode (Micro
Photon Devices, SPD-050). The signals from the detector weresent to a time-correlated single photon counting module(Becker & Hickl, SPC-130EM) for further analysis. A dichroic
mirror (Semrock, Di02-R405) and a longpass lter (Semrock,
BLP01-458R) were used to remove the scattering from the excitation light. PL spectra were obtained by directing the emission
into an imaging spectrograph (SOL instruments, MS3504i)
equipped with a CCD camera (Andor, DU416A-LDC-DD)
through a slit. All experimental data were obtained at room
temperature in air. The data were analyzed using the open
source image soware ImageJ (http://rsb.info.nih.gov/ij/) andOrigin 2015 (Origin-Lab).
Photocatalytic deposition of PbO2 and MnO2
Photo-deposition of PbO2 or MnO2 was conducted with
Pb(NO3)2 or MnSO4. Photocatalyst powders (20 mg) were

Chem. Sci., 2022, 13, 3118–3128 | 3125

View Article Online

Chemical Sciencedispersed in 10 mL of an aqueous Pb(NO3)2 or MnSO4 solution(5 wt% as Pb or Mn) in a test tube. Aer the suspension was
purged with O2 gas for 30 min, visible light (l > 400 nm) was
irradiated using a 300 W Xe-arc lamp. Aer the 3 h irradiation,
the suspension was ltered, washed with deionized water anddried at room temperature.

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TRMC measurements
X-band microwave (9.1 GHz) and third harmonic generation(THG; 355 nm) of a Nd:YAG laser (Continuum Inc., Surelite II,
5–8 ns pulse duration, 10 Hz) were used as the probe and bandgap excitation (4.6  1015 photons per cm2 per pulse), respectively. A powdery Bi4NbO8Cl sample was xed onto a quartz
substrate using optically clear adhesive tape that does notinterfere with any TRMC signal. The photoconductivity Ds wascalculated using the following formula: Ds ¼ DPr/(APr), where
DPr, A, and Pr are the transient power change of the reected
microwave, the sensitivity factor, and the power of reected
microwave, respectively. The obtained Ds values were thenconverted into the product of the quantum yield (4) and theP
sum of the carrier mobilities ( m ¼ m+ + m) using the followingP1
formula: 4 m ¼ Ds(eI0Flight) , in which e and Flight are the unit
charge of a single electron and a correction (or lling) factor,
respectively. All TRMC measurements were performed in an
ambient atmosphere at 25  C.
TRAS measurements
A set of custom-built TRAS spectrometers was employed as
described previously.45 In the femtosecond to nanosecond
regions, experiments were performed using a conventional
pump–probe method based on a Ti:sapphire laser system(Spectra Physics, Solstice & TOPAS Prime; duration, 90 fs;
repetition rate, 1 kHz). In this experiment, a 355 nm laser pulse
was used as the pump pulse. The experiments were performed
in air to prevent heating of the sample and to minimize theaccumulation of electrons in the photocatalyst due to the highfrequency pump pulse irradiation (500 Hz). In the microsecond
to seconds region, transient absorption spectra were measured
from 25 000 to 1 000 cm1. 355 nm light (Continuum, SureliteII, 6 ns, 355 nm, repetition rate of 5–0.01 Hz) was used as thepump pulse. The spectra were obtained at intervals of 200 cm1and averaged over 300 scans per spectrum. The measurements
were performed under vacuum at room temperature. The
powder photocatalyst was xed on a CaF2 plate with a density of1 mg cm2, and the sample plate obtained was placed ina stainless steel cell. For fair comparison with PD-Rh-Bi4NbO8Cl
and PD-Pt-Bi4NbO8Cl, Bare-Bi4NbO8Cl was irradiated using
visible light for 5 h in a MeOH solution prior to use.
Density functional theory calculation
The band structure calculation of Bi4NbO8Cl was performed
within the framework of Density Functional Theory (DFT) usinga plane-wave pseudopotential method as implemented in the
Cambridge Serial Total Energy Package (CASTEP) code of BIOVIA's Material Studio 2019.64 The interaction between the ioniccore and valence electrons was treated with the OTFG ultra-so

3126 | Chem. Sci., 2022, 13, 3118–3128

Edge Article
pseudopotential using the scalar relativistic Kolling-Harmon
approximation. The non-scalar relativistic eﬀects and spin–
orbital coupling (SOC) were not considered. The Perdew–Burke–
Ernzerhof (PBE) function of generalized gradient approximation (GGA) was employed as the exchange–correlation functional. A plane wave basis set with an energy cut oﬀ of 630 eV
and the Monkhorst-pack 3  3  1 k-point mesh was used. The
minimization algorithm of Broyden–Fletcher–Goldfarb–Shanno(BFGS) was employed for geometry optimizations with a totalenergy convergence tolerance of 106 eV per atom. The separation between k-points in the band structure calculations was
0.001 Å. Other convergence parameters are as follows: a selfconsistent eld tolerance of 1  105 eV per atom,a maximum stress of 0.05 GPa, and the maximum ionicdisplacement of 1  103 Å.
Eﬀective mass m* was calculated according to the obtained
band structure. m* is dened by the following equation:
m0
m0 d2 3
¼ 2
m*
ħ dk2

(1)

where m0 is the free electron mass, k is the reciprocal lattice
vector in the direction of interest, and d23/dk2 is the curvature of
the band at a maximum or a minimum, respectively. Assuming
the band around their minima/maxima to be parabolic, weestimate the curvature of the band using the nite diﬀerences
approximation:
d2 3
2½3ðk þ DkÞ  3ðkÞ
z
2
dk
Dk2

(2)

where Dk ¼ 0.05 Å.

Author contributions
Kanta Ogawa: conceptualization, investigation, methodology,
writing – original dra. Ryota Sakamoto: conceptualization,
writing – review & editing. Chengchao Zhong: writing – review &editing. Hajime Suzuki: writing – review & editing. Kosaku Kato:
investigation. Osamu Tomita: methodology. Kouichi Nakashima: resources. Akira Yamakata: resources, data curation.
Takashi Tachikawa: resources, investigation, data curation,
writing – review & editing. Akinori Saeki: resources, data curation. Hiroshi Kageyama: supervision, writing – review & editing.
Ryu Abe: resources, supervision, writing – review & editing.

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

Acknowledgements
This work was nancially supported by JST-CREST(JPMJCR1421, JPMJCR18R3), the JSPS KAKENHI in Grant-inAid for Scientic Research (A) (JP20H00398), the JSPS
KAKENHI Grant Numbers 17H06439, JP16H06438, JP16H06440
in Scientic Research on Innovative Areas “Innovations forLight-Energy Conversion (I4LEC)”, “Synthesis of Mixed Anion
Compounds toward Novel Functionalities”, JSPS Core-to-Core© 2022 The Author(s). Published by the Royal Society of ChemistryView Article Online

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Program “International Core-to-Core Project on Mixed Anion
Research for Energy Conversion”, and the JSPS Research Fellow(Grant Number 19J23357). This work was also supported by the
Advanced Characterization Platform of the Nanotechnology
Platform Japan (JPMXP09A20KU0341) sponsored by the
Ministry of Education, Culture, Sports, Science and Technology(MEXT), Japan. We are grateful to Dr Takaaki Toriyama of
Kyushu University for his helpful support in STEM analysis.

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