Inherent electrochemistry of layered post-transition metal halides: The unexpected effect of potential cycling of PbI2

Article abstract of DOI:10.1002/chem.201405204

The development of two-dimensional nanomaterials has expedited the growth of advanced technological applications. PbI₂ is a layered inorganic solid with important and unique properties suitable for applications in the detection of electromagnetic radiation. While the optical and electrical properties of layered PbI₂ have been generally established, its electrochemistry has remained largely unexplored. In this work, we examine the inherent electrochemistry of PbI₂ in relation to its morphological and structural properties. A direct comparison between commercially available and solution-grown PbI₂ showed high similarity in properties based on characterizations by X-ray photoelectron spectroscopy, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The respective layered PbI₂ materials also exhibited similar inherent electrochemistry. Electrochemical potential cycling of PbI₂ in phosphate buffer resulted in the dissolution of iodide ions from PbI₂ to form complex lead-phosphate-chloride with the oxygen groups of the phosphate ions while retaining the hexagonal structure. In the cas of KCl solution, the formation of PbO₂ was observed.

Full text of DOI:10.1002/chem.201405204

DOI: 10.1002/chem.201405204
Full Paper
&
Electrochemistry
Inherent Electrochemistry of Layered Post-Transition Metal
Halides: The Unexpected Effect of Potential Cycling of PbI2
[a]
[b]
[a]
[a]
Chun Kiang Chua, Zden eˇ k Sofer, Chee Shan Lim, and Martin Pumera*
Abstract: The development of two-dimensional nanomateri-
als has expedited the growth of advanced technological ap-
based on characterizations by X-ray photoelectron spectros-
copy, scanning electron microscopy, and energy-dispersive
X-ray spectroscopy. The respective layered PbI2 materials
also exhibited similar inherent electrochemistry. Electro-
plications. PbI is a layered inorganic solid with important
2
and unique properties suitable for applications in the detec-
tion of electromagnetic radiation. While the optical and elec-
chemical potential cycling of PbI in phosphate buffer result-
2
trical properties of layered PbI have been generally estab-
ed in the dissolution of iodide ions from PbI to form com-
2
2
lished, its electrochemistry has remained largely unexplored.
In this work, we examine the inherent electrochemistry of
plex lead-phosphate-chloride with the oxygen groups of the
phosphate ions while retaining the hexagonal structure. In
the case of KCl solution, the formation of PbO2 was
observed.
PbI in relation to its morphological and structural proper-
2
ties. A direct comparison between commercially available
and solution-grown PbI showed high similarity in properties
2
Introduction
Two-dimensional materials have taken the scientific communi-
[1]
ty by storm ever since the successful isolation of graphene.
Existing as layered structures, these materials are held together
by strong in-plane chemical bonds and weak out-of-plane van
der Waals interactions. The two-dimensionality of such materi-
als endows them with unique physical, chemical, and electron-
ic properties that are generally not found for their bulk coun-
terparts. Indeed, these properties have expedited develop-
ments of applications for high-performance and clean energy
generation and conversion, next-generation electronics, and
[
2]
sensing devices. Following the success of graphene materials
2
Figure 1. Structure of lead(II) iodide with a CdI -based structure.
[
3]
in revolutionizing these applications, other two-dimensional
materials, such as hexagonal boron nitride, transition metal di-
chalcogenides, silicene, germanene, and III-VI and IV-VI semi-
conductors have been actively researched to challenge the
cally lead iodide (PbI ). PbI has a hexagonal layered structure
2
2
(CdI type), in which a plane of lead atoms is sandwiched be-
2
[
1a]
current technological limits.
tween two planes of iodine atoms (Figure 1). Stacking of the I-
An increasing emphasis on the exploration of new layered
materials with improved performances for these important ap-
plications has motivated us to look into metal halides, specifi-
Pb-I layers gives rise to several polytypes of PbI , the most
2
common being the 2H-polytype, which belongs to the D
3
d
[4]
symmetry group. It is also a direct band gap semiconductor
of 2.4 eV with room temperature p-type conduction controlled
[5]
[
a] Dr. C. K. Chua, C. S. Lim, Prof. M. Pumera
Division of Chemistry & Biological Chemistry
School of Physical and Mathematical Sciences
Nanyang Technological University
by holes. Typically synthesized to a high purity by a combina-
[6]
tion of zone melting and Bridgman–Stockbarger methods,
micro- and nano-sized PbI have also been produced by solu-
2
[
7]
[8]
[9]
[10]
2
1, Nanyang Link, Singapore 637371
tion growth, sol–gel, colloidal, reverse-micelle,
sono-
[11]
[12]
[13]
E-mail: pumera@ntu.edu.sg
chemical, vapor deposition, and hydrothermal methods.
[
b] Prof. Z. Sofer
Synthesized PbI is typically used in photoconductors, X-ray de-
2
Department of Inorganic Chemistry
Institute of Chemical Technology
Technickꢀ 5, 166 28 Prague 6 (Czech Republic)
[4,14]
tectors, and low-energy g-ray detectors.
The optical and electrical properties of single-crystal and
polycrystalline PbI have been widely researched. However, its
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201405204.
2
electrochemistry has hitherto not been reported. Herein, we
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report an investigation of the inherent electrochemistry of two
with 0.05 mm alumina on a polishing cloth. The working electrode
À1
was then modified by applying 5 mL of 5 mgmL PbI material dis-
PbI materials, namely one obtained commercially and another
2
2
persed in H O and allowed to dry. The cyclic voltammetry measure-
synthesized by a solution-growth method. The structural and
morphological properties of these materials have been exam-
ined by X-ray photoelectron spectroscopy (XPS), scanning elec-
tron microscopy (SEM), energy-dispersive X-ray spectroscopy
2
ments were performed with zero equilibration time.
2
.3.3 Conductivity measurements: Current–voltage measure-
ments (I–V curves) were conducted with an interdigitated gold
electrode (IDE) platform by depositing 2 mL of the material suspen-
(EDS), and X-ray diffraction analysis (XRD). In addition, the ef-
À1
sion (dispersion of 5 mgmL in water) onto the electrode surface
fects of applied electrochemical potential on the structures
with 10 mm gold spacings. The electrode was then dried under
a lamp for 20 min, leaving a randomly deposited material film on
the interdigitated area bridging the two Au electrode bands. The
I–V curves were obtained by linear sweep voltammetric measure-
and morphologies of both PbI materials have been examined.
2
À1
Experimental Section
ments at a scan rate of 20 mVs . The slope of the I–V curve corre-
sponded to the conductivity of the material. The reported conduc-
tivity values are averages from three independent scans. The con-
ductivity between two points on the gold strip on the IDE was
2.1 Materials
Lead(II) iodide beads (99.999%) and N,N-dimethylformamide (DMF)
were obtained from Sigma-Aldrich, Singapore. Lead nitrate
À1
around 194 mAV .
2
.3.4 X-ray diffraction: X-ray powder diffraction data were collect-
(
99.5%), potassium iodide (99.8%), and methanol (99.9%) were ob-
ed at room temperature with an X’Pert PRO q-q powder diffrac-
tometer with parafocusing Bragg–Brentano geometry using CuKa
tained from Lach-Ner, Czech Republic. A glassy carbon (GC) elec-
trode with a diameter of 3 mm was obtained from Autolab, The
Netherlands. Milli-Q water (resistivity: 18.2 MWcm) was used
throughout the experiments.
radiation (l=1.5418 ꢁ, U=40 kV, I=30 mA). Data were scanned
with an X’Celerator ultrafast detector over the angular range 5–808
(
2
2q) with a step size of 0.01678 (2q) and a counting time of
0.32 sstep . Data were evaluated with the software package
À1
2.2 Apparatus
HighScore Plus.
X-ray photoelectron spectroscopy was performed with a Phoi-
bos 100 spectrometer and a non-monochromatic Mg X-ray radia-
tion source (SPECS, Germany). Wide survey scans and high-resolu-
tion spectra for Pb4f, I3d, and C1s were collected. The peak posi-
tions were calibrated with respect to adventitious carbon at
Results and Discussion
3.1 Morphological and structural properties
2
84.5 eV. XPS measurements were made on SPE electrodes with
Two samples of PbI , one obtained commercially in 99.999%
drop-cast samples of the studied materials. A JEOL-7600F semi-in-
lens FE-SEM, operating in lower secondary electron image (LEI)
mode at 5 kV, was used to acquire the SEM images. EDS analysis
2
purity (designated as S-PbI , S=Sigma) and the other synthe-
2
sized by slow crystallization from solution (designated as C-
PbI , C=crystallized) were investigated in this work. These PbI
(
Oxford Instruments) was performed in LEI mode at 15 kV. The
2
2
solid samples were transferred to a carbon tape attached to an
SEM holder for analysis. The samples were analyzed at an average
working distance of 15 mm. All voltammetric experiments were
performed on a mAutolab type III electrochemical analyzer (Eco
Chemie, The Netherlands) connected to a personal computer and
controlled by General Purpose Electrochemical Systems Version 4.9
software (Eco Chemie).
materials were investigated in parallel to compare and deter-
mine any possible effects of source on their morphological,
structural, and electrochemical properties based on X-ray pho-
toelectron spectroscopy, scanning electron microscopy,
energy-dispersive X-ray spectroscopy, and cyclic voltammetry
analyses.
The morphologies of S-PbI2 and C-PbI2 were analyzed by
scanning electron microscopy. Figure 2A, C show SEM images
of S-PbI in powder form and C-PbI in crystal flake form, re-
2
.3 Procedures
2
2
2
.3.1 Synthesis of PbI : Lead(II) iodide powder was synthesized by
2
spectively (see Figure 2E for optical images for both PbI mate-
2
a
metathesis reaction in aqueous solution. A stoichiometric
rials). S-PbI was seen to be made up of highly compact multi-
2
amount of 5 wt% aqueous KI was added to 5 wt% Pb(NO3)2
100 mL) under vigorous stirring. The PbI produced was separated
layer PbI sheets with a typical size of less than 100 mm, along
2
(
2
with smaller scattered sheets. On the other hand, C-PbI con-
by vacuum suction and washed repeatedly with deionized water
2
sisted of large sheets of more than 100 mm. In order to extend
and methanol. The PbI powder was then dried at 1008C for 24 h.
2
Large crystals of PbI were prepared by slow cooling of a saturated
the investigation to electrochemical studies, the PbI
2
materials
2
À1
solution. For this, PbI (5 g) was dispersed in water and heated to
were dispersed in water at 5 mgmL . It became apparent that
dispersing S-PbI and C-PbI in water (see Figure 2F for optical
2
1
008C. The solution was filtered and slowly cooled to room tem-
2
2
perature in a Dewar flask. The PbI crystals thus formed were sepa-
2
images of both PbI materials) only resulted in partial dissolu-
2
rated by vacuum suction and washed repeatedly with deionized
water and methanol. The crystals were dried at 1008C for 24 h.
tion, despite extended ultrasonication treatment, and the ma-
terials subsequently recrystallized upon concentration. Disper-
sions of S-PbI and C-PbI in water were subsequently drop-
2
.3.2 Electrochemical measurements: The electrochemical experi-
2
2
ments were carried out in a 10 mL voltammetric cell at room tem-
perature using a three-electrode configuration. A platinum elec-
trode served as the auxiliary, an Ag/AgCl electrode as the refer-
ence, and glassy carbon was utilized as the working electrode.
Prior to a measurement, the glassy carbon electrode was polished
cast onto screen-printed electrodes to examine their morphol-
ogies, which are shown in Figure 2B, D, respectively. The re-
crystallized PbI flakes of S-PbI and C-PbI consisted of varying
2 2 2
mixtures of hexagonal crystals and amorphous material. Rec-
&
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Figure 3. High-resolution XPS for S-PbI
2 2
and C-PbI at (A) Pb4f and (B) I3d
core levels. Calibrated according to the C1s core-level peak at 284.5 eV.
Figure 2. SEM and optical images of S-PbI
2
and C-PbI
2
. SEM images of (A) S-
PbI
2
powder, (B) drop-cast S-PbI
2
, (C) C-PbI
2
crystal, and (D) drop-cast C-PbI .
2
À1
Optical images of S-PbI
dispersions in water.
2
2
and C-PbI (E) as-received solids and (F) 5 mgmL
tangular and rod-like structures of PbI were also observed to
2
a small extent in both dried materials. Despite this, the ele-
mental compositions derived from EDS and XPS analyses sug-
gested the major presence of lead and iodine. The ratios of
Pb:I obtained from EDS on S-PbI and C-PbI were 1:1.98 and
2
2
2 2
Figure 4. Cyclic voltammograms of S-PbI , C-PbI , and bare GC for probing
1
:1.97, respectively, close to the stoichiometric proportion of
PbI2.
Subsequently, the structural properties of the drop-cast S-
PbI and C-PbI dispersions were examined by X-ray photoelec-
their inherent electrochemistry. Conditions: measured with GC electrode,
À1
50 mm PBS at pH 7.2, purged with nitrogen, scan rate of 100 mVs
.
2
2
3
.2 Inherent electrochemistry, electrical properties, and
tron spectroscopy, which confirmed the prominent presence of
lead and iodine, with negligible impurities in the materials
based on XPS survey scans. Figure 3 shows high-resolution
scans of the Pb4f and I3d core levels, which provide informa-
tion on the bond types in the materials. The Pb4f core-level
spectra in Figure 3A show spin-orbit doublets for 4f5/2 and
electrochemical applications
Next, we investigated the inherent electrochemistry of S-PbI2
and C-PbI2 by cyclic voltammetry (CV), cycling the electro-
chemical potential from À1.4 to +1.4 V as shown in Figure 4.
4
f7/2 (splitting value ꢀ 4.86 eV) at binding energies of 143.5
Since PbI is covalent but with a certain degree of ionic charac-
2
[4]
and 138.6 eV for S-PbI and at 143.9 and 139.0 eV for C-PbI .
ter, it was expected to display an inherent electrochemistry
2
2
2
+
À
On the other hand, the I3d core-level spectra in Figure 3B indi-
cate spin-orbit doublets of 3d3/2 and 3d5/2 (splitting value
due to the presence of Pb and I ions. Indeed, a small oxida-
tion peak was observed at a potential of À0.45 V in the initial
2
+
ꢀ
11.4 eV). Doublets are seen at 630.2 and 618.7 eV for S-PbI2
forward sweep. This peak was ascribed to the redox pair Pb
and at 630.5 and 619.1 eV for C-PbI . The positions of these
/Pb, based on a similar observation in an independent cyclic
voltammetry analysis of lead(II) nitrate, a precursor of PbI2.
With an extended equilibration time of up to 60 s prior to the
CV scan, an increment in the peak area was observed (see Fig-
ure S1A, Supporting Information). Subsequently, two further
oxidation peaks at anodic potentials of +0.5 and +0.8 V as
well as reduction peaks at +0.6 and +0.2 V were observed.
The anodic peaks at +0.5 and +0.8 V were attributed to the
2
peaks fall within the range for previously reported PbI materi-
2
[
13a,15]
als.
The sharp peaks in the spectra suggest the presence
of only lead-iodide bonds.
À
À
oxidation of I to I3 and finally to I . The corresponding reduc-
2
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À
3
tion peak at +0.6 V was assigned to the reduction of I to I
2
À
À [16]
and the peak at +0.2 V to the reduction of I3 to I . The less
prominent reduction peak at +0.6 V was due to the loss of
iodine to form ion pairs with the surrounding water mole-
[
16a]
cules.
Similar voltammetric peak profiles were observed in
another independent cyclic voltammetry analysis performed
on potassium iodide, which is also a precursor of PbI . More-
2
over, increasing the scan rate resulted in a shift in the anodic
peak to more positive values, suggesting electrochemical irre-
versibility of the system (see Figure S1B, Supporting
Information).
The conductivities of S-PbI and C-PbI were measured using
2
2
an interdigitated gold electrode. The results showed low con-
ductivities, with the measured slopes of the I–V curves being
À1
only 0.4 and 0.2 mAV , respectively. Efforts to probe the use-
fulness of PbI for oxygen reduction, hydrogen evolution, and
2
oxygen evolution reactions only proved its lack of electrocata-
lytic properties (see Figure S2, Supporting Information). Further
attempts to anneal both materials at 2008C did not provide
any positive effects.
3.3 Consequences of electrochemical potential cycling
The prominent inherent electrochemistry of PbI2 suggested
possible structural changes occurring within the material, spe-
cifically in terms of elemental composition and morphology.
2 2
Figure 5. Cyclic voltammograms of (A) S-PbI and (B) C-PbI upon four po-
tential cycles. Conditions: measured with screen-printed electrodes, 50 mm
À1
À
PBS at pH 7.2, purged with nitrogen, scan rate of 100 mVs
.
The drastic dissolution of I ions upon potential cycling sug-
gested possible loss of the hexagonal PbI structure. In order
2
to further investigate this, S-PbI2 and C-PbI2 materials were
each subjected to four cycles of cyclic voltammetry scans from
À1.4 to +1.4 V on screen-printed electrodes (the resulting PbI
2
materials are designated as S-PbI -R and C-PbI -R, respectively)
2
2
in phosphate buffer, as shown in Figure 5. The diminishing in-
tensities of the anodic and cathodic peaks with each consecu-
tive scan implied rapid decreases in the amounts of electro-
2
+
À
chemically active elements, specifically Pb and I from the
2
+
PbI materials. Moreover, the lower height of the Pb peak in
2
À
comparison to the I peak indicated a lower extent of lead dis-
solution from the parent structure. In fact, analysis of the ele-
mental composition by XPS, as shown in Figure 6, indicated
a prominent absence of iodine. Even though lead was still
present, the peak positions of the Pb4f core level were shifted
by approximately 1.6 eV to higher binding energies of 145.1
and 140.2 eV.
Whereas the absence of iodine peaks in the XPS analysis
suggested that possible morphological changes could have oc-
curred in S-PbI and C-PbI , SEM images of S-PbI -R and C-PbI -
2 2
Figure 6. High-resolution XPS spectra for S-PbI -R and C-PbI -R at (A) Pb4f
and (B) I3d core levels.
2
2
2
2
R showed negligible changes in their morphologies (Figure 7A,
B). However, several random cracks were observed in S-PbI -R
trolyte). Considering the drastic decrease in iodine content, the
shift of Pb4f5/2,7/2 to higher binding energies, and the minimal
2
and C-PbI -R as a result of the consecutive potential cycles.
2
EDS analyses of S-PbI -R and C-PbI -R indicated comparable
changes in the hexagonal structure of PbI , we hypothesize
2
2
2
distributions of Pb, I, O, P, and Cl at approximately 20, 2, 57,
0, and 9 at%, respectively, for both S-PbI -R and C-PbI -R. The
iodide replacement by electronegative oxygen groups of the
phosphate ions. An independent control study, in which the
electrochemical potential cycling was performed in potassium
chloride as electrolyte, resulted in a breakdown of the hexago-
1
2
2
distribution of these elements remained constant following im-
mediate rinsing of S-PbI -R and C-PbI -R with ultrapure water
2
2
after electrochemical potential cycling in an effort to remove
any adsorbed phosphate or chloride ions (from the buffer elec-
nal PbI structure to needle-like structures, as shown in Fig-
2
ure 7C, D. EDS analyses indicated an elemental distribution of
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Figure 7. SEM images of (A) S-PbI
2
2
-R and (B) C-PbI -R after potential cycling
in PBS. SEM images of (C) S-PbI -R and (D) C-PbI
2
2
-R after potential cycling in
KCl electrolyte (ꢂ5000 magnification).
2 5 4 3
Figure 9. Crystal structures of (A) PbI and (B) Pb (PO ) Cl along the [001] di-
rection with four unit cells.
that of PbI and thus could not be distinguished by SEM (Fig-
2
ure 7A, B). Crystal structures of PbI2 and Pb (PO ) Cl in the
5
4 3
[
001] direction are shown in Figure 9. The transition of PbI to
2
Pb (PO ) Cl is accompanied by significant changes in proper-
5
4 3
ties, PbI being a wide band-gap semiconductor (ca. 2.3 eV)
2
and the mixed phosphate-chloride being an insulator. Both
materials belong to the hexagonal lattice system, albeit with
slight differences in symmetry group (P3m1 for PbI and P63/
2
m for Pb (PO ) Cl). The needle-like crystals obtained by poten-
5
4 3
tial cycling in KCl solution (Figure 7C, D) were subsequently
identified by X-ray diffraction analysis as PbO , as shown in Fig-
2
ure 8B. The spontaneity of PbO formation was greater on the
2
edge plane than on the basal plane of the PbI crystal (see Fig-
2
2
+
4+
ure S3, Supporting Information). The oxidation of Pb to Pb
Figure 8. X-ray diffraction analysis of PbI
2
after 10 potential cycles in
was accompa-
Cl (peaks marked with lines); (B) 0.1m KCl solution; the for-
2
was observed.
may arise from the in situ formation of chlorine during the po-
tential cycling.
(A) 50 mm PBS containing NaCl and KCl; the dominating PbI
2
5 4 3
nied by Pb (PO )
mation of PbO
Conclusion
Pb, I, O, and Cl of approximately 38, 2, 52, and 8 at%, respec-
tively. The EDS analyses suggested the possibility of lead-
oxygen bond formation, given the high atomic percentage of
oxygen. In all likelihood, the presence of phosphate ions
Layered lead(II) iodide has been examined as a new two-di-
mensional material for potential usage in electrochemical ap-
plications. In this study, we have investigated the morphologi-
cal and structural properties of PbI obtained from two differ-
2
allows preservation of the hexagonal structure of PbI even
ent sources, namely a commercial sample and material ob-
2
after electrochemical potential cycling. No obvious changes
can be seen in Figure 7A, B.
tained by a solution-growth method. The respective PbI mate-
2
rials did not show drastic structural and morphological
differences according to X-ray photoelectron spectroscopy and
scanning electron microscopy analyses. More importantly, both
exhibited similar inherent electrochemistry profiles and were
equally susceptible to a potential cycling effect that resulted in
the extensive oxidation of iodide ions. Whereas the loss of
X-ray diffraction was subsequently used to analyze the struc-
ture of PbI after several potential cycles in phosphate buffer
2
solution (PBS). The diffractograms shown in Figure 8A indicat-
ed the formation of Pb (PO ) Cl with hexagonal symmetry
5
4 3
(
P63/m, apatite structure type). The crystal habit was similar to
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2
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2
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Received: September 10, 2014
Published online on && &&, 0000
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Chem. Eur. J. 2014, 20, 1 – 7
www.chemeurj.org
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ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

Full Paper
FULL PAPER
Unexpected effects of potential cy-
cling: The inherent electrochemistry of
a layered post-transition metal halide,
&
Electrochemistry
C. K. Chua, Z. Sofer, C. S. Lim, M. Pumera*
& – &&
lead iodide (PbI ; see graphic), has been
2
&
examined to reveal a transformation of
the material under electrochemical po-
Inherent Electrochemistry of Layered
Post-Transition Metal Halides: The
Unexpected Effect of Potential Cycling
of PbI2
tential cycling. Conversion of PbI to
2
Pb (PO ) Cl or PbO occurred under cy-
5
4 3
2
cling in phosphate buffer and KCl solu-
tion, respectively.
Chem. Eur. J. 2014, 20, 1 – 7
www.chemeurj.org
7
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ