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Cite this:
Energy Environ. Sci.,
2016,
9
,1734
Effect of interlayer anions on [NiFe]-LDH
nanosheet water oxidation activity
†
B. M. Hunter,
a
W. Hieringer,
b
J. R. Winkler,
a
H. B. Gray
a
and A. M. Mu
̈
ller*
a
We synthesized nickel–iron layered double hydroxide ([NiFe]-LDH) nanosheets with different interlayer
anions to probe their role in water oxidation catalysis. In alkaline electrolyte in ambient air, carbonate
rapidly replaced other interlayer anions and catalytic activity was highest. Electrocatalytic water oxidation
in virtually carbonate-free alkaline electrolyte revealed that activity was a function of anion basicity. Our
[NiFe]-LDH nanosheets, prepared by pulsed laser ablation in liquids, were regenerated in carbonate-
containing aqueous KOH. Anion binding motifs were assessed by X-ray photoelectron spectroscopy in
combination with density functional theory calculations, suggesting that nitrite species bound to edge-
site Fe in the precatalyst correlated with higher water oxidation activity.
Broader context
There is an urgent need to develop carbon-neutral technologies to accelerate the transition to renewable energy for use on our planet. Much work has be
en done
on solar-driven water splitting to produce hydrogen fuel from water, but successful development of a scalable technology will depend critically on t
he
availability of highly active catalysts made from abundant elements. At the least, it will be absolutely essential to have structural and mechanisti
c
understanding of heterogeneous materials to facilitate the construction of robust catalytic photoanodes. In our work, we have discovered that inte
rlayer
anions play key roles in the catalytic performance of nickel–iron layered double hydroxide nanosheets, as incorporating anions with different basi
cities tunes
the water oxidation activities of these materials. By probing anion binding sites by X-ray photoelectron spectroscopy along with DFT calculations,
we have found
that water oxidation occurs mainly on nanosheet edges. Importantly, our work opens the way for rational design of a new generation of highly active wat
er
splitting catalysts.
1. Introduction
Global supply of sustainable fuels affects every aspect of human
life. Electrocatalytic water splitting (2H
2
O
-
2H
2
+O
2
)isa
promising approach towards transportable, carbon-neutral hydro-
gen fuel. The water oxidation h
alf reaction is more demanding
because it involves four electron and proton transfer steps,
1,2
for
which highly active, earth-abundant catalysts are needed.
Layered double hydroxides (LDHs), materials based on
mineral structures readily found in nature, have been shown
to be active for water oxidation.
3–22
We recently reported that a
[NiFe]-LDH nanomaterial synthesized by pulsed laser ablation
in liquids (PLAL) is among the best water oxidation catalysts
made of earth abundant elements.
20
The structures of these nanosheet precatalysts were analyzed
by X-ray diffraction (XRD) and Raman spectroscopy.
20
The
[NiFe]-LDH materials consist of sheets of edge-shared nickel
oxide octahedra, with varying amounts of ferric iron substituting
at nickel sites. Hydroxide ligands extend into the interlayer
space, which also contains water. The excess positive charges
of Fe
3+
substituting for Ni
2+
are balanced by interlayer anions
23
(Fig. 1).
Fig. 1
Schematic illustration of the [NiFe]-LDH structure. Anions and
water are present in the interlayer space, and Ni
2+
or Fe
3+
ions are surrounded
by six hydroxides in distorted octa
hedral coordination (upper left).
a
Beckman Institute and Division of Chemistry and Chemical Engineering,
California Institute of Technology, M/C 139-74, Pasadena, California 91125, USA.
E-mail: astridm@caltech.edu
b
Theoretical Chemistry and Interdisciplinary Center for Molecular Materials,
Department of Chemistry and Pharmacy, Friedrich-Alexander Universita
̈
t Erlangen-
Nu
̈
rnberg, Egerlandstrasse 3, 91058 Erlangen, Germany
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ee00377j
Received 5th February 2016,
Accepted 17th March 2016
DOI: 10.1039/c6ee00377j
www.rsc.org/ees
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The affinities for various mono- and divalent anions were
established for [MgAl]-LDH hydrotalcite materials, which are
structurally similar to our [NiFe]-LDHs. Data derived from
anion-exchange isotherms revealed that divalent anions generally
had higher ion selectivity than monovalent ones. The ion-
exchange equilibrium constants followed the sequence CO
3
2
4
SO
4
2
4
OH
4
F
4
Cl
4
I
.
24
Some hydroxide ions are likely
always present in the interlayer galleries because OH
partakes
in strong interlamellar hydrogen bonding networks with the
interlayer water, the ionic radius of hydroxide is very similar to
that of water, and hydroxide is required to form the basic layers.
25
Moreover, given the large slab hy
droxide and intercalated water
content of LDHs, the experimental determination of minute
amounts of interlayer OH
ions is not practical. Nevertheless, we
were interested in the identity of non-hydroxide interlayer anions
in [NiFe]-LDHs during turnover in aqueous base in ambient air
and their effects on water oxidation activity.
We exploited the benefits of PLAL as a synthetic method to
investigate the role of interlayer anions in [NiFe]-LDH water
oxidation catalysts. PLAL produces small, mono-dispersed,
surfactant-free nanoparticles.
26
The small size (
o
25 nm)
allowed for the complete metathesis of interlayer anions simply
by soaking in aqueous solutions. In addition, by modifying the
anion content of the ablation liquid, we were able to synthesize
species with different interlayer ions.
2. Experimental section
Materials and methods
Nanomaterial synthesis by pulsed laser ablation in liquids was
performed in the Beckman Institute Laser Resource Center at
California Institute of Technology. X-ray photoelectron spectro-
scopy was carried out at the Molecular Materials Research Center
(Beckman Institute at California Institute of Technology).
All chemicals were used as received. Deionized water was
obtained from a Barnstead Diamond Nanopure system and had
a resistivity of
Z
16 M
O
cm
1
. Data analysis and graphing was
performed with Igor Pro 6.37 (Wavemetrics).
Synthesis
PLAL
26
was used to synthesize [NiFe]-LDH nanosheets.
20
Sus-
pensions of 0.5 g iron (Alfa,
200 mesh) powder were stirred in
10 mL aqueous nickel salt solutions, using a magnetic stirrer in
a 30 mL glass beaker at room temperature in ambient air. Metal
salt solutions consisted of 3.0 M nickel nitrate (Ni(NO
3
)
2
6H
2
O,
Alfa), or 2.5 M nickel chloride (NiCl
2
6H
2
O, JT Baker) and 0.5 M
nickel nitrate, or 1.5 M each nickel sulfate (NiSO
4
6H
2
O, EMD)
and nickel nitrate. The limited solubilities of nickel chloride
and sulfate required addition of nickel nitrate to keep the total
nickel concentration of all three solutions at 3.0 M. The
material resulting from use of 3.0 M aqueous nickel nitrate
solution is henceforward called [NiFe]–(NO
3
)-LDH. Beakers
and stir bars were thoroughly cleaned with
aqua regia
before
use. A 355 nm, 8 ns pulse length, 90 mJ per pulse laser beam,
provided by the third harmonic of a 10 Hz Q-switched Nd:YAG
laser (Spectra-Physics Quanta-Ray PRO-Series), was focused
0.5 mm below the surface of the liquid with a 100 mm focal
length plano-convex quartz lens. Each sample was irradiated
for 60 min. After synthesis, unreacted iron ablation target
powder was separated from the nanoparticle suspensions using
a rare-earth magnet. Solid nanopowders were obtained by
centrifugation and washing with water until the supernatant
did not show any nickel salt absorption. The nanoparticles were
then washed twice with 3 mL acetone (EMD, OmniSolv
s
) and
dried under vacuum.
Anion exchange was effected by suspending 10 mg [NiFe]–
(NO
3
)-LDH nanosheets in 2 mL 1.0 M aqueous alkali salt
solutions and letting the rigorously vortexed nanopowder soak
for 45 min; the soaking time was 10 min for aqueous KOH
solutions. Th
esaltswereK
2
CO
3
,KOH,KCl,NaF(allMallinckrodt),
Na
2
SO
4
(EMD), NaClO
4
1H
2
O (Fisher Scientific), KI, KBF
4
,K
3
PO
4
,
and K
2
C
2
O
4
1H
2
O (all Sigma-Aldrich). For exchange as a function
of solution pH, 1.5 mg [NiFe]–(NO
3
)-LDH were dispersed in
1.0 mL 1.0 M (pH 14.0), 1.0
10
3
M (pH 11.0), or 1
10
6
M
(pH 8.0) aqueous KOH solutions for 5 min. Anion-exchanged
[NiFe]-LDH powders were obtained by centrifugation and wash-
ing first three times with 5 mL water and then twice with 3 mL
acetone; the powders were dried under vacuum.
Physical characterisation
X-ray photoelectron spectra (XPS) were collected using a Surface
Science Instruments M-probe surface spectrometer. Mono-
chromatic Al K
a
radiation (1486.6 eV) was used to excite elec-
trons from the samples, which had either been deposited as dry
powders on double-sided adhesive carbon tape (EM Sciences) or
drop-cast from aqueous suspension on clean Cu foil and dried in
ambient air at room temperature; we did not find different
results for the two sample mounting methods. The sample
chamber was maintained at
o
5
10
9
Torr. Survey scans from
0 to 1000 eV were performed to identify the elements present in
the nanoparticles. Binding energies were referenced to the C 1s
peak arising from adventitious carbon, taken to have a binding
energy of 284.8 eV.
27
High-resolution spectra were collected for
all materials for the Fe 2p, Ni 2p, C 1s, N 1s, and O 1s regions.
In addition, depending on the elemental composition of the
material, high-resolution spectra were taken in the Cl 2s, S 2s,
I 3d, B 1s, F 1s, and P 2s regions. Quantitative peak areas were
derived after Shirley background subtraction
28
and using relative
sensitivity factors. Binding energies were obtained from the
same peak fits. Quantitative XPS analysis was performed with
CasaXPS (Version 2.3.16 PR 1.6).
XRD data were collected with a Bruker D2 PHASER diffracto-
meter. Monochromatic Cu K
a
radiation (1.5418 Å; tube power
30 kV, 10 mA) was used; the instrument was equipped with 0.1
1
divergence, 1.5
1
Soller, and 0.6 mm detector slits, and had a
3 mm secondary anti-scatter screen. Diffracted radiation was
collected with a Lynxeye detector. The instrument resolution was
0.030
1
in 2
y
, and the counting time was 4.5 seconds per step,
resulting in a total scan time of about 3.5 hours for each sample.
Solid samples were deposited with vaseline (X-Alliance GmbH)
on a zero-diffraction silicon plate (MTI Corporation). XRD
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background subtraction and Scherrer analysis
29
were performed
with the Bruker DIFFRAC.SUITE software. Reflections were
analyzed using a Scherrer constant
K
of 0.89 and integral
breadth, defined as the total area under the diffraction max-
imum divided by the peak intensity.
30
Basal spacings
d
were
derived using Braggs law,
31
n
l
=2
d
sin(
y
), where the integer
n
equals one,
l
is the wavelength of the incident light, and
y
is
the angle of incidence. The (003) peaks of background-
subtracted XRD data were fitted with Gaussians (ESI
†
), whose
centers at 2
y
were divided by two to obtain the angle of
incidence required to calculate basal spacings.
Attenuated total reflectance infrared (IR) spectra of dry
nanoparticulate powders were collected with a Thermo Nicolet
iS50 FT-IR spectrometer, equipped with a Pike Technologies
GladiATR accessory plate, an uncooled pyroelectric deuterated
triglycine sulfate (DTGS) detector, and a KBr beamsplitter.
Spectra of the solid nanoparticulate powders were collected at
room temperature in ambient air, and 132 scans were averaged
for each sample.
Electrochemical characterisation
Nanosheet powders were weighed with a high precision balance
(Sartorius CPA225D), and aqueous 2 mg mL
1
suspensions
were prepared. A catalyst loading of 40
m
g was used throughout
this work and was obtained by drop-casting 20
m
L of these
suspensions on highly-ordered pyrolytic graphite (HOPG) disks,
which were dried in ambient air under a heat lamp at 50
1
C.
The electrode disk had stabilizing epoxy around its side and a
surface area of 0.20 cm
2
. HOPG electrodes were cleaned by
soaking for 5 min in concentrated hydrochloric acid, washed
with water, and their surfaces were polished using 400 and
600 grit sandpaper, after which the graphite was cleaved with
adhesive tape to obtain a fresh HOPG surface for each catalyst.
Measurements were carried out in ambient air or argon
atmosphere in 100 mL three-neck r
ound-bottom flasks, filled with
25 mL electrolyte. An Hg/HgO reference electrode (CH Instru-
ments) and a Ni gauze (Alfa) counter electrode were used. All data
were collected at room temperature, using a rotating disk elec-
trode (RDE) setup at 1500 rpm. Ex
periments in Ar atmosphere
were carried out in a glove box, whose catalyst tolerates water;
during oxygen evolution the c
atalyst was shut off, and after
experiments the box was thoroughly flushed with Ar. In ambient
air, a Pine MSR variable speed rotator and a Gamry Reference 600
potentiostat were used. In Ar atmosphere, a Pine WaveNow
potentiostat and a (smaller) ho
me-built RDE apparatus were
employed; the rotation speed was determined to be 1500 rpm
with a laser tachometer.
In ambient air, the electrolyte was aqueous 1.0 M (pH 14.0)
KOH. The electrolyte took up CO
2
from ambient air, which was
dissolved as carbonate at this high pH. Even highest purity
reagent grade alkali hydroxide solutions contain a minimum of
1% (
B
0.1 M) alkali carbonate.
32
Measurements in virtually
carbonate-free electrolyte were performed under Ar atmo-
sphere in a glove box. The electrolyte was prepared in the
box from thoroughly degassed water, 1.0 M high purity KOH
(Alfa, 99.98%, packaged under Ar), and 0.43 M barium
hydroxide (Ba(OH)
2
1H
2
O, Sigma-Aldrich). The barium hydro-
xide was added in excess to precipitate all dissolved carbonate
as BaCO
3
, rendering the supernatant electrolyte virtually free
of carbonate (see IR data below). Before use, an Hg/HgO refer-
ence electrode was equilibrated for one week in this solution.
The pH of the virtually carbonate-free electrolyte was measured
to be 14.6. Electrolyte from the same stock solution was used for
all experiments requiring carbonate-free electrolyte.
Chronopotentiometry was performed for 3.5 hours at
1mAcm
2
. Observed fluctuations in the data were due to
formation and release of oxygen bubbles from the electrode
surface. Cyclic voltammograms were measured at 0.1 V s
1
scan
rate and after the working electrode had been held for 10 min at a
current density of 1.0 mA cm
2
. The current density
versus
potential
data were post-measurement corrected for uncompensated
resistance losses (details are described elsewhere).
20
All polar-
ization potentials reported here are relative to the normal
hydrogen electrode (NHE), and current densities are per geo-
metric area. Overpotentials
Z
were calculated from polarization
potentials
E
p
as
Z
=
E
p
(1.23 V
0.059 V pH), taking into
account the different pH values of electrolytes with and without
carbonate.
Computational methods
Density-functional theory calculations were performed with the
Turbomole program package.
33
The PBE0 hybrid functional
34–36
was used throughout this work. The SV(P) basis set
37
was
employed for geometry optimizations as well as for N 1s core
level energy calculations. Solvent screening effects were approxi-
mately included
via
the conductor-like screening model
(COSMO)
38,39
with a solvent permittivity of 80. All calculations
were spin-unrestricted. In this work, we imposed the lowest total
spin projection
S
z
= 0.5 possible for the cluster models used in
this work (one excess spin-up spin orbital in the Slater determi-
nant; see ESI
†
for further details). No point group symmetries
were assumed. The model clusters (Fig. 12 and 13; ESI
†
)were
subjected to constrained geometry optimizations, where the
atomic Ni, O, H positions of the parent LDH structure were kept
fixed at the experimental positions, whereas the positions of all
other atoms (Fe, additional OH groups, nitrate/nitrite anions)
were optimized (more details and Cartesian atomic positions are
in the ESI
†
). Standard convergence criteria (Turbomole defaults)
were used in general. Only for the calculation of the core-level
binding energies, SCF convergence criteria were tightened to
10
7
a.u. for the energy and density matrix.
The calculation of N 1s core-level binding energies (BEs) was
performed using N 1s orbital energies and with the so-called
Slater transition state (STS) method.
40
Initial state contribu-
tions to core level shifts (is-CLS) were estimated as orbital
energy differences of the N 1s orbital energies
e
in the chosen
SCF solutions of the clusters without any core hole. A limited
overview of the performance of the approach for nitrate and
nitrite is given in Table S2 (ESI
†
). The adequacy of the SV(P) basis
set for N 1s core level calculations was verified (Table S3, ESI
†
),
and the influence of additional explicit water molecules on N 1s
binding energy shifts was discussed (Table S4, ESI
†
). Only shifts
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in core-level binding energies (core-level shifts, CLS) were relevant
in this work, absolute binding energies should not be compared
to experimental XPS data. Further detail on the calculations and
additional results can be found in the ESI.
†
3. Results and discussion
We synthesized twelve materials with different interlayer anions,
either by anion exchange from [NiFe]-LDH nanosheets made by
PLAL from nitrate-containing ablation liquid,
20
denoted [NiFe]–
(NO
3
)-LDH, or with Cl
or SO
4
2
in the PLAL ablation liquid.
Aqueous solutions for anion exchange contained 1.0 M BF
4
,
Cl
,ClO
4
,CO
3
2
,C
2
O
4
2
,F
,I
,PO
4
3
,orSO
4
2
.
We discovered that all interlayer anions quickly exchanged
for carbonate in 1.0 M aqueous KOH in ambient air, as
evidenced by IR, XPS, and XRD data (see below). Strongly
alkaline aqueous electrolyte exposed to ambient air is self-
buffered regarding carbonate content, as dissolved carbonate
is in steady-state equilibrium with gaseous CO
2
.
41
The active
catalyst was therefore the carbonate-containing species, regardless
of the precatalyst composition.
Physical characterisation of materials
X-Ray photoelectron spectra.
We collected XPS data to
identify nanoparticle compositions by peak integrations of
high-resolution spectra of the Ni 2p, Fe 2p, B 1s, C 1s, N 1s,
O 1s, B 1s, F 1s, Cl 2s, I 3d, P 2s, and S 2s regions, where
applicable (ESI
†
). The regions were chosen as to collect data on
transitions with the highest X-ray ionization cross-sections,
42
and the data were background-subtracted.
28
We deliberately
did not attempt to quantify oxygen content from XPS data
because its amounts are regularly overestimated; oxygen occurs
in many adventitious sources.
The Ni 2p core-level binding energies of all catalysts were
consistent with assignment to hydrated Ni(OH)
2
,
43
with Ni 2p
3/2
binding energies close to 855.5 eV. After exposure to 1.0 M
aqueous KOH in ambient air, all [NiFe]-LDH materials featured
an additional, lower binding-energy peak in the Ni 2p
3/2
core
level region, consistent with NiO.
44,45
The Fe 2p core level
spectra of all catalysts showed peaks attributable to iron oxides
and oxyhydroxides,
13,46
with Fe 2p
3/2
binding energies close to
711.9 eV. Various iron oxides and oxyhydroxides, such as FeO,
Fe
2
O
3
,Fe
3
O
4
, and FeOOH, have similar Fe core-level binding
energies and spectral shapes.
46
Therefore, it is impossible to
distinguish different Fe phases in our materials from Fe 2p XPS
data. All materials contained 22% Fe relative to the total metal
content.
The N 1s core level spectra of [NiFe]–(NO
3
)-LDH, [NiFe]–
(Cl
)-LDH made by PLAL and [NiFe]–(SO
4
2
)-LDH made by
PLAL showed peaks with binding energies around 407.3 eV,
consistent with nitrate.
47
In addition, [NiFe]–(SO
4
2
)-LDH made
by PLAL exhibited an N 1s peak at 403.5 eV, attributable to nitrite.
48
As reported by us before, PLAL-synthesized [NiFe]–(NO
3
)-LDH
featured in addition to the nitrate signal an N 1s peak centered
at 405.1 eV.
20
After exposure to 1.0 M aqueous KOH in ambient
air, we could no longer detect any peaks in the N 1s core level
region, indicating that all nitrogen species were readily exchanged
by other anions. The O 1s spectra were consistent with Fe or
Ni oxide and hydroxide species.
44
Contributions attributable to
oxygen-containing anions
44
and from adventitious sources were
also present. The C 1s spectra of as-synthesized [NiFe]-LDH
materials with different interlayer anions showed mostly the
presence of adventitious carbon, whereas those of nanosheets
that had been exposed to 1.0 M aqueous KOH in ambient air
exhibited additional peaks, which were consistent with more
highly oxidized carbon.
44
High-resolution XP spectra of anionic
hetero-atoms (other than Ni, Fe, C, N, or O) showed that the
initially present anions were lost after exposure to pH 14 aqueous
KOHsolutioninambientair(ESI
†
). Quantification of non-metal
atom content in the as-synthesized [NiFe]-LDH materials is
summarized in Table 1. Additional hydroxide ions may have
been present to balance the overall charge in the materials.
Knowing that all nitrogen species of the as-synthesized
[NiFe]–(NO
3
)-LDH nanosheets were replaced by carbonate in
strong aqueous base in ambient air, we exposed [NiFe]–(NO
3
)-
LDH to aqueous KOH solutions with pH values of 8.0, 11.0, and
14.0 and took XP spectra (Fig. 2). For comparison, we also
collected XP spectra of commercial Ni(NO
3
)
2
6H
2
O, whose N 1s
region featured a single peak centered at 407.4 eV, consistent
with previous reports for metal nitrate.
48
The observed C 1s
signal for Ni(NO
3
)
2
6H
2
O was attributable to ad
ventitious carbon,
and the O 1s peak was consistent with a transition metal nitrate.
44
Its Ni 2p
3/2
peak was centered at 857.0 eV as expected,
44
and no
peaksintheFe2pcorelevelregionwereobserved.
We found that all nitrogen species of the original material were
gradually exchanged into carbon species with higher C 1s binding
energy than adventitious carbon. As the solution pH rose, the N 1s
peak centered at 405.1 eV disapp
eared before that at 407.3 eV.
After exposure to pH 14.0 solution in ambient air, no nitrogen
species were detectable (Fig. 2). Instead the material took up
carbonate from the aqueous base (see also IR spectra below).
QuantificationofXPSsignalsasafunctionofsolutionpHshowed
Table 1
Summary of quantitative XPS data analysis of [NiFe]–(A
m
)-LDH
materials with different interlayer anions A
m
, prepared by pulsed-laser
ablation in liquids (PLAL) or by anion exchange from [NiFe]–(NO
3
)-LDH
(exch.). Portion of anion elements with respect to total metal content; the
relative error is
10%
Material
XPS line
% Non-metal atoms
[NiFe]–(NO
3
)-LDH (PLAL)
N 1s
10
[NiFe]–(BF
4
)-LDH (exch.)
B1s
18
F1s
72
[NiFe]–(Cl
)-LDH (PLAL)
Cl 2s
16
N 1s
4.7
[NiFe]–(Cl
)-LDH (exch.)
Cl 2s
21
[NiFe]–(ClO
4
)-LDH (exch.)
Cl 2s
10
[NiFe]–(CO
3
2
)-LDH (exch.)
C 1s
20
[NiFe]–(C
2
O
4
2
)-LDH (exch.)
C 1s
20
[NiFe]–(F
)-LDH (exch.)
F 1s
18
[NiFe]–(I
)-LDH (exch.)
I 3d
19
[NiFe]–(PO
4
3
)-LDH (exch.)
P 2s
8.0
[NiFe]–(SO
4
2
)-LDH (PLAL)
S2s
10
N 1s
4.2
[NiFe]–(SO
4
2
)-LDH (exch.)
S 2s
17
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a linear decline of N 1s and a concomitant increase of C 1s signals
(ESI
†
). In aqueous KOH solutions at room temperature with pH
values above 10, the predominant species resulting from dissolu-
tion of ambient CO
2
is carbonate, whereas at pH 8.0 dissolved
inorganic carbon speciation favors HCO
3
over CO
3
2
ions.
41
The observed C 1s signals of [NiFe]-LDH materials after expo-
sure to different pH aqueous KOH solutions were consistent
with transition metal bicarbonates and carbonates.
44
X-Ray diffraction data.
XRD data were collected to determine
crystalline phases, basal spacings by Bragg’s law
31
analysis, and
crystallite sizes by Scherrer analysis.
29
XRD data of [NiFe]-LDH
nanosheets with different interlayer anions as synthesized and
after suspension in 1.0 M aqueous KOH in ambient air are
depicted in Fig. 3.
All materials showed XRD patterns characteristic for layered
double hydroxides.
23
The intensities of the basal (00
l
) reflec-
tions decrease as
l
increases. The intensity ratio of the (006) and
(003) reflections is a measure of the interlamellar electron
density. Loss of water in the interlayer galleries, associated
with reduction in interlamellar electron density, has been
reported to lead to less intense (006) reflections with respect
to (003) diffractions.
49
All XRD peaks were significantly broadened, owing to small
crystallite size and stacking faults. The XRD data of [NiFe]–
(ClO
4
)-LDH (exch., as synthesized) and [NiFe]–(SO
4
2
)-LDH
(PLAL, as synthesized) materials showed non-uniform (‘‘saw-
tooth’’) broadening of (
h
0
l
) reflections, which are indicative of
turbostratic disorder.
25
The observed reflections were indexed
in a three-layer 3R polytype with rhombohedral symmetry, such
as in synthetic hydrotalcite
50
(ESI
†
).
We obtained crystallite sizes from Scherrer analysis
29
of our
XRD data (Table 2). Crystalline domain sizes in the
a
and
c
unit
cell directions were derived from broadening of the (110) and
(00
l
)(
l
= 3, 6) reflections, respectively.
25
In case of [NiFe]–
(ClO
4
)-LDH (exch., as synthesized) and [NiFe]–(SO
4
2
)-LDH
(PLAL, as synthesized) materials, only (003) reflections were
used to determine nanosheet thicknesses, as the asymmetric
line shape of the (006) reflections hampered proper analysis.
The unit cell directions
a
and
c
correspond to lateral size
(diameter) and nanosheet thickness, respectively. Most materials
had very similar size. However, the [NiFe]–(Cl
)-LDH (PLAL)
materials, both as synthesized and after suspension in 1.0 M
aqueous KOH in ambient air, exhibited significantly narrower
peaks, indicating larger crystallite sizes.
Gaussian fits of the (003) peaks (ESI
†
) were performed to
obtaintheangleofincidencerequiredtocalculatebasalspacings.
The fits had an average error in 2
y
of
0.05
1
, resulting in a
basal spacing error of
0.03 Å. The derived basal spacings of
[NiFe]–(A
m
)-LDH materials with different intercalated anions
A
m
, prepared by PLAL or by anion exchange from [NiFe]–
(NO
3
)-LDH (exch.), are depicted in Fig. 4.
We correlated the basal spacings of [NiFe]-LDH materials
with the ionic radii of the anions. Non-spherical anions occupy
interlamellar galleries of LDH materials at various angles,
and their arrangement depends on many factors.
24,25,51
There-
fore, we limited our analysis to the spherical halogen anions.
Fig. 2
XPS data of [NiFe]–(NO
3
)-LDH (as synthesized, a) and exchanged
in different pH aqueous KOH solutions (b, pH 8.0; c, pH 11.0; d, pH 14.0).
Depicted in grey are data of commercial Ni(NO
3
)
2
6H
2
O (e). The blue line
in the C 1s panel indicates the binding energy of adventitious carbon. Open
circles, data; lines, peak fits.
Fig. 3
XRD data of [NiFe]-LDH nanosheets with different interlayer anions
as synthesized (blue) and after suspension in 1.0 M aqueous KOH in
ambient air (red).
Table 2
Crystalline domain sizes from Scherrer analysis of [NiFe]–(A
m
)-
LDH materials with different intercalated anions A
m
, prepared by PLAL or
by anion exchange from [NiFe]–(NO
3
)-LDH (exch.), as synthesized and
after exposure to 1.0 M aqueous KOH. The error in all dimensions is
13%
A
m
(prep.)
Thickness
(nm) as
synth.
Thickness
(nm) after
base
Diameter
(nm) as
synth.
Diameter
(nm) after
base
NO
3
(PLAL)
4
4.4
13
13
BF
4
(exch.)
3.9
3.9
14
12
Cl
(PLAL)
12
13
21
23
Cl
(exch.)
4.8
4.7
13
14
ClO
4
(exch.)
3.2
4.5
12
12
CO
3
2
(exch.)
4.8
4.6
12
13
C
2
O
4
2
(exch.)
3.6
4.6
10
13
F
(exch.)
3.9
4.7
11
14
I
(exch.)
4.7
4.5
12
13
PO
4
3
(exch.)
4.5
4.8
11
14
SO
4
2
(PLAL)
3.8
5.2
12
14
SO
4
2
(exch.)
3.8
4.7
10
13
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The Royal Society of Chemistry 2016
Energy Environ. Sci.,
2016,
9
, 1734--1743 |
1739
We found a linear correlation of measured basal spacing with
anionic radii. Our observation indicates that the anions did
indeed reside in the interlayer galleries of our LDH materials
(Fig. 5).
Infrared data.
IR spectra were collected to shed more light
on the identity of interlayer anions in our [NiFe]-LDH water
oxidation catalysts. Layered double hydroxides have a huge
affinity towards carbonate incorporation.
53,54
Given the ubiqui-
tous presence of carbonate in aqueous base in ambient air,
spectroscopic evidence for virtually carbonate-free materials is
paramount. The IR bands in the 1200–1600 cm
1
region are
characteristic for alkali carbonate (one narrow peak at 1400 cm
1
)
or nitrate (two broader bands centered around 1400 cm
1
).
55
Our [NiFe]–(NO
3
)-LDH nanosheets turned into [NiFe]–
(CO
3
2
)-LDH materials in strong aqueous base in ambient air
(Fig. 6). We confirmed the identity of interlayer carbonate by
collecting IR data after soaking [NiFe]–(NO
3
)-LDH in 0.1 M
aqueous K
2
CO
3
solution.
One way of de-carbonation is precipitation of dissolved
carbonate with Ba
2+
as BaCO
3
.
56
We used this method in inert
atmosphere (Fig. 7), using Ba(OH)
2
, as to not introduce anions
other than hydroxide. All attempts to achieve carbonate-free
electrolyte in ambient air failed (ESI
†
). Virtually carbonate-free
electrolyte was prepared under Ar as described above. In the
glove box, we dispersed [NiFe]–(NO
3
)-LDH in this electrolyte,
let it settle overnight, and removed the supernatant. Still in Ar
atmosphere, we dispersed the resulting material in water, let it
settle overnight, removed the supernatant, and repeated this
procedure twice. After the last supernatant removal, we let the
material dry in the glove box. The IR spectrum (Fig. 7) of this
material clearly shows the characteristic redshift and broad-
ening of the nitrate with respect to the carbonate band,
indicating that we succeeded to exclude carbonate from our
[NiFe]–(NO
3
)-LDH and electrolyte in the glove box.
Thenitratepeakintheinfraredspectrumoftheas-synthesized
[NiFe]–(NO
3
)-LDH nanosheets was broader than that observed
for the [NiFe]–(NO
3
)-LDH nanosheets which had been soaked in
carbonate-free aqueous base. We
attributed this to a change in the
morphology of the material at high pH. This is in agreement with
the dramatic changes observed in the OH and Ni–O lattice regions
upon exposure to highly alkali
ne solutions (see Fig. S12, ESI
†
).
Electrochemical characterisation
We prepared 1.0 M aqueous KOH electrolyte in a virtually
CO
2
-freeatmosphere(glovebox)andscrubbedthesolutionof
any residual dissolved carbonate by saturating with barium
hydroxide. IR data confirmed that the [NiFe]-LDH catalysts did
not take up carbonate under these conditions (see above). Con-
stant current electrolysis was used to assess long-term water
oxidation activity; measured potentials were converted to over-
potentials
Z
to facilitate comparison between electrolytes with and
virtuallywithoutcarbonate.InFig.8,theoverpotentialsasa
function of time are shown for [NiFe]–(NO
3
)-LDH in alkaline
electrolytes with and without carbonate. The [NiFe]–(CO
3
2
)-LDH
Fig. 4
Basal spacings of [NiFe]-LDH nanosheets with different interlayer
anions as synthesized (blue) and after suspension in 1.0 M aqueous KOH in
ambient air (red).
Fig. 5
Correlation of ionic radii
52
of intercalated halogen anions A
m
with
basal spacings of [NiFe]–(A
m
)-LDH materials. Open circles, data; line,
linear fit.
Fig. 6
Infrared spectra of [NiFe]–(NO
3
)-LDH nanosheets as synthesized
(black), after being suspended in 0.1 M aqueous K
2
CO
3
solution (grey), and
after being suspended in 0.1 M aqueous KOH (red); (a) full spectra, (b)
magnification of the region characteristic for NO
3
and CO
3
2
, Lorentzian
fits are depicted as thin lines.
Fig. 7
Infrared spectra of [NiFe]–(NO
3
)-LDH nanosheets as synthesized
(black), after being suspended in carbonate-free strong base electrolyte in
Ar atmosphere as described above (blue), and after being suspended in
0.1 M aqueous KOH in ambient air (red).
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