Metathetic synthesis of lead cyanamide as a p-type semiconductor

Article abstract of DOI:10.1039/d0dt02677h

Lead cyanamide PbNCN was synthesized by solid-state metathesis between PbCl2 and Na2NCN in a 1?:?1 molar ratio, and its structure was confirmed from Rietveldrefinement of X-ray data. Electronic-structure calculations of HSE06 density-functional type revealPbNCN to be an indirect semiconductor with a band gap of 2.4 eV, in remarkable quantitative agreement with the measured value. Mott-Schottky experiments demonstrate PbNCN to be a p-type semiconductor with a flat-band potential of 2.3 eV vs. the reversible hydrogen electrode (RHE) which is commonly used to estimate the value of the valence band edge position. Moreover, thin films of powderous PbNCN were assembled into a photoelectrode for photoelectrochemical water splitting. On the example of p-type PbNCN, this study provides the first experimental evidence that MNCN compounds can be applied as photocathodes for reductive reactions in photoelectrochemical cells.

Full text of DOI:10.1039/d0dt02677h

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Metathetic synthesis of lead cyanamide as a p-type
semiconductor†
Cite this: DOI: 10.1039/d0dt02677h
a
a
a,b
a
c
Xianji Qiao,‡ Zili Ma, ‡ Dongbao Luo, Alex J. Corkett,
Adam Slabon,
d
d
a,b
Anna Rokicinska,
Piotr Kuśtrowski
and Richard Dronskowski
*
Lead cyanamide PbNCN was synthesized by solid-state metathesis between PbCl
2
2
and Na NCN in a
1
: 1 molar ratio, and its structure was confirmed from Rietveld refinement of X-ray data. Electronic-struc-
ture calculations of HSE06 density-functional type reveal PbNCN to be an indirect semiconductor with a
band gap of 2.4 eV, in remarkable quantitative agreement with the measured value. Mott–Schottky
experiments demonstrate PbNCN to be a p-type semiconductor with a flat-band potential of 2.3 eV vs.
the reversible hydrogen electrode (RHE) which is commonly used to estimate the value of the valence
band edge position. Moreover, thin films of powderous PbNCN were assembled into a photoelectrode for
photoelectrochemical water splitting. On the example of p-type PbNCN, this study provides the first
experimental evidence that MNCN compounds can be applied as photocathodes for reductive reactions
in photoelectrochemical cells.
Received 30th July 2020,
Accepted 11th September 2020
DOI: 10.1039/d0dt02677h
rsc.li/dalton
quently adopt layered structures in which metal cations alter-
nate with symmetric D∞h carbodiimide or lower symmetry cya-
Introduction
Solid-state metathesis (SSM) reactions have been reported in namide forms depending on the nature of the bonding metal.
the synthesis of many inorganic solids including metal These structures are often remarkably similar to those of
1
–6
borides, nitrides, and carbides.
These reactions exploit the their analogous oxides with a propensity for [NiAs] and [NaCl]
2
−
intrinsic energy of the reactants to promote an exothermic derived motifs in which close-packed layers of NCN anions
exchange of ions under relatively mild conditions yielding the alternate with octahedrally coordinated metal cations.
1
6,17
desired reaction product and a co-produced salt. As such, SSM However, the introduction of larger cations or those with
reactions are well-suited for the preparation of thermally labile specific coordination preferences yields alternate topologies
2
−
18,19
compounds. Since 2004 a large number of metal carbodiimide that are often unique to solid-state NCN chemistry.
and cyanamide compounds have been prepared via SSM reac- This is also the case in lead cyanamide, PbNCN, which one
tions between metal halides (MX
n
–11
) and alkali–metal carbodii- might expect to crystallize in either an α-SrNCN-type (i.e.,
More recently, the electro- NaSCN) or a β-SrNCN-type rock-salt derived structure, given
7
mides (A (NCN), A = Li, Na).
2
2
+
2+
chemical and photochemical properties of this family have the similar ionic radii of Pb (r = 1.13 Å) and Sr (r =
been explored with encouraging results.
1
2–15
20
1.19 Å). The structure of PbNCN is, however, somewhat dis-
Somewhat simplified, metal carbodiimides may be tinct and presents two structural features, which may imbue
2
1
regarded as pseudo-oxides or “divalent nitrides” and fre- PbNCN with potential photochemical properties. The first is
the highly distorted square pyramidal coordination environ-
ment of PbN , augmented by two more N atoms further apart,
which is a long way from the regular PbS octahedra seen in
6
5
a
Chair of Solid-State and Quantum Chemistry, Institute of Inorganic Chemistry,
RWTH Aachen University, 52056 Aachen, Germany.
rock-salt PbS and more akin to the four-coordinate Pb environ-
E-mail: drons@HAL9000.ac.rwth-aachen.de
22
ment in the PbO litharge structure. This infers the presence
b
Hoffmann Institute of Advanced Materials, Shenzhen Polytechnic,
2
23
of stereochemically active Pb 6s lone pairs, mediated by
N 2p orbitals, which may help to promote hole conduction as
7
c
098 Liuxian Blvd, Nanshan District, Shenzhen, China
Department of Materials and Environment Chemistry, Stockholm University,
Svante Arrhenius väg 16 C, 106 91 Stockholm, Sweden
2
4
in PbO.
d
Faculty of Chemistry, Jagiellonian University, Gronostajowa 2, 30-387 Krakow,
In addition, the cyanamide anion in PbNCN exhibits a sig-
nificant deviation from the symmetric carbodiimide form,
with distinct single (1.30(3) Å) and triple (1.16(3) Å) bonds,
Poland
†
Electronic supplementary information (ESI) available: Further structural, spec-
troscopic and photoelectrochemical details of PbNCN. See DOI: 10.1039/
d0dt02677h
2
5
similar to those in molecular cyanamide (H
2
NCN). This cya-
NCN
‡
These authors equally contributed to this work.
namide character is also expressed in HgNCN and Ag
2
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binaries, which is attributed to the chemically soft acid–base
The Raman spectroscopy was performed on a WITec alpha
nature of these cations that bind more covalently to one end of 300R confocal Raman microscope setup using a 532 nm diode
2
−
2−
the NCN unit thereby engendering less symmetric NuC–N
pumped solid state laser. The Raman spectrum was recorded
2
1,25
cyanamide forms.
In recent years, the photocatalytic pro- on powder samples on glass slides.
perties of the n-type semiconductor Ag NCN have been
2
SEM
studied, revealing promising water photooxidation and photo-
2
6
current generation capabilities. This has been attributed, in
part, to the presence of localized dipoles on the asymmetric
cyanamide anion, which promote the separation of electron–
Scanning electron microscopy (SEM) images were acquired on
a Leo Supra 35VP SMT (Zeiss).
hole pairs, thereby enhancing the photo-generation of carriers. X-ray photoelectron spectroscopy (XPS)
On the basis of the above considerations PbNCN appears an
XPS analysis was done in a hemispherical VG SCIENTA R3000
analyzer with a monochromatized aluminum source Al K (E =
486.6 eV) at constant pass energy of 100 eV. The adventitious
C 1s line at 284.6 eV was used as a charge-referencing stan-
dard. A fitting of high resolution spectra was obtained through
the Casa XPS software.
excellent model compound for photocatalytic investigations.
However, whilst the preparation of lead cyanamide reported by
Liu et al., which utilizes the reaction of cyanamide with a solu-
α
1
2
1
tion of lead acetate in the presence of ammonia, does yield
single crystals, powder samples are prone to significant contri-
butions from a secondary PbO·xH O phase. Here, we report a
2
new SSM routine to synthesize monophasic polycrystalline
PbNCN that avoids the formation of such by-products. We then
go on to characterize PbNCN as a new p-type semiconductor,
with an indirect band gap of 2.4 eV, that can be assembled into
Density-functional theory (DFT) calculations
3
0
The Vienna Ab initio Simulation Package (VASP) code and the
3
1
Perdew–Burke–Ernzerhof functional in the generalized gradi-
3
2
2
7
ent approximation was adopted to calculate the electronic
structure within the framework of density-functional theory
a photoelectrochemical cell for water splitting.
(
(
DFT). In addition, the hybrid Heyd–Scuseria–Ernzerhof
3
3,34
HSE06) functional
was used to obtain a more accurate
Experimental section
Synthesis
band gap. The pseudopotentials were constructed by means of
the augmented plane-wave method with 5d 6s 6p , 2s 2p
and 2s 2p treated as the valence electrons of the Pb, C and N
atoms, respectively. A cutoff energy of 800 eV was adopted.
1
0
2
2
2
2
2
3
All synthetic steps were performed in an argon-filled glove box.
2
8
PbCl2 (Alfa) and Na NCN were reacted in a 1 : 1 stoichio-
2
metric ratio, according to eqn (1). The reactants were mixed
and ground using an agate pestle and mortar, and the reaction
UV-Vis spectroscopy measurements
mixture was loaded into a glass ampoule. The ampoule was UV-Vis spectra were collected using a Shimadzu UV-2600
then placed in a tube furnace under flowing argon and heated spectrophotometer. The band gap was determined from a
to 400 °C for 12 h, with heating and cooling rates of 2 °C Tauc plot by means of the Kubelka–Munk function F(R) = (1 −
−
1
2
29
min . The resulting yellow powder was subsequently air- R) /2R.
exposed and then washed using water to remove excess reac-
tants and the NaCl metathesis salt.
Photoelectrochemical (PEC) characterization
The PbNCN photoelectrode was fabricated via an electrophor-
PbCl2 þ Na2NCN ! PbNCN þ 2NaCl
ð1Þ
35
etic deposition (EPD) process. In short, a uniform suspen-
sion was made by sonicating the mixture of 30 mg PbNCN
powder and 10 mg iodine in 30 ml acetone. Two precleaned
PXRD analysis
Powder X-ray diffraction (PXRD) data were collected on a conductive fluorine-doped tin oxide (FTO) slides were
washed sample of lead cyanamide, PbNCN, at room tempera- immersed in the above suspension with 1 cm distance, and a
ture using a calibrated STOE STADI-MP powder diffractometer potential of 30 V was applied between them for 5 min. PbNCN
with a flat sample holder (Mo K , linear PSD, 2θ range from particles were deposited on the electrode. The electrodes were
α1
0
–85° with individual steps of 0.015°). Rietveld analysis was dried at 333 K for 12 hours.
2
9
undertaken using the Rietveld refinement suite GSAS2, using
the Pnma structural model of PbNCN described by Liu et al. as Venture) was used to carry out the measurements. The pre-
a starting point. In the final cycles of least-squares refine- pared PbNCN, platinum wire and a 1 M Ag/AgCl electrode were
ment, atomic positions and isotropic thermal displacement used as working, counter and reference electrodes, respect-
A conventional three-electrode configuration PEC cell (WAT
2
1
parameters were refined.
ively. All current values of the photoelectrodes were recorded
versus 1 M Ag/AgCl and converted vs. RHE according to ERHE
=
Infrared and Raman measurements
θ
E
+ 0.059 pH + EAg/AgCl. 0.1 M potassium phosphate
1
MAg=AgCl
The infrared spectrum of PbNCN was measured on a washed (KP
i
) buffer at pH 7 was used as supporting electrolyte for all
powder sample with no further pre-treatment using an experiments. The electrolyte was prepared with Milli-Q water
ALPHA II FT-IR-Spectrometer (Bruker).
(18.3 Ω cm).
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Linear sweep voltammetry (LSV) with a scan rate of 10 mV
s− and chronoamperometry (CA) curve at different constant
bias versus RHE were recorded with a potentiostat (PalmSens4,
PalmSens BV). The photoelectrodes were illuminated from the
1
−
2
back side with 100 mW cm
simulated visible light
(
(
AM 1.5G), which was generated by a solar light simulator
class-AAA 94023A, Newport) equipped with an ozone-free 450
W xenon short-arc lamp. Mott–Schottky measurements were
conducted using the Gamry INTERFACE 1010 T Potentiostat/
Galvanostat/ZRA workstation in a dark setting at ac amplitude
of 5 mV and a frequency of 1000 Hz.
Results and discussion
Structural characterization of PbNCN
The surface morphology of PbNCN is shown in Fig. 1. The
scanning electron microscopy images reveal that PbNCN’s crys-
tallites are in the micrometer range and strongly resemble
flowers petal.
Fig. 2 Rietveld fit of PbNCN to Mo Kα1 PXRD data, showing observed
(red), calculated (black), and difference (blue) intensities. Bragg positions
of PbNCN (pink) are denoted by vertical markers.
The PXRD pattern of PbNCN was indexed to an ortho-
rhombic unit cell with lattice parameters (a ≈ 5.54 Å, b ≈
3
.86 Å, c ≈ 11.72 Å) in good agreement with those reported by
2
1
Liu et al. This Pnma model of PbNCN was therefore used as a
starting point for Rietveld analysis, with additional C–N bond
and N–C–N angle restraints for a more stable refinement
based on the superior single-crystal data. This resulted in an
excellent agreement between the observed and calculated
intensities (Fig. 2), and a chemically sensible model with
reasonable agreement factors (Table S1†).
2
1
The resultant structural model is very close to that reported
by Liu et al., characterized by corrugated double layers of
2
−
2+
NCN anions and Pb cations (Fig. 3). The divalent Pb cation
is coordinated by five terminal nitrogen atoms with one short
apical Pb–N1 bond of 2.311(16) Å almost parallel to the c axis
and four longer basal bonds (2× Pb–N1 = 2.607(10) Å; 2×
Pb–N2 = 2.615(10) Å) in a distorted square-pyramidal environ-
ment, augmented by two nonbonding Pb–N distances of
3
.472(17) Å. This reflects the presence of stereochemically
2
active 6s lone pairs that are orientated away from the PbN
polyhedra into the interlayer voids, as seen in litharge PbO.
5
7
1
A comparison of the derived bond lengths from this work and
those from the single-crystal analysis and DFT calculations is
given in Table S2.†
Fig. 3 Projection of (a) the crystal structure of PbNCN along b, (b) the
2+
coordination environment of Pb with dashed bonds representing the
nearest contact towards the neighboring double layer, and (c) the crystal
structure of PbNCN along c.
The cyanamide character is, however, impressively validated
through IR measurements (Fig. 4), which evidence asymmetric
(ν
as = 1983 cm− ) and deformation vibrations (δ = 618 cm ),
1
−1
as well as strong symmetric vibrations (ν
s
= 1265 and
1185 cm− ) that are IR-forbidden for symmetric D∞h carbodi-
1
Fig. 1 SEM micrograph of PbNCN powder with different magnification.
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Fig. 4 Infrared and Raman spectra of PbNCN.
7
,36,37
imide anions.
The asymmetric and deformation vibrations
are observable in the Raman spectra (Fig. 4), while the sym-
metric vibrations are extremely weak.
Fig. 5 Band structure of PbNCN with orbital contributions of C 2s, C
2p, N 2s, N 2p, Pb 6s, and Pb 6p represented by green, blue, light blue,
To investigate the stability of PbNCN to the PEC water orange, black and red colours, respectively.
reduction process, XRD and IR measurements were performed
on a portion of separate PbNCN photoelectrodes removed
from the thin film after PEC at 0.5 V vs. RHE and 0 V vs. RHE between −4 eV and 0 eV, the N 2p levels dominate, with a little
for 1 h in potassium phosphate electrolyte at pH 7, respect- admixture of dispersed Pb 6s and Pb 6p levels, while the CBM
ively. The Rietveld refinements shown in Fig. S1† indicate the consists of emptied Pb 6p levels. This picture is reminiscent of
2
3
phase purity of PbNCN and its structural bulk stability during that in litharge PbO, and alludes to an (antibonding) inter-
PEC water reduction. Furthermore, the IR spectrum of PbNCN action between the Pb 6s and N 2p orbitals or, in an alternative
after PEC water reduction is consistent with that of the as- wording, indicates the formation of a stererochemically active
2
41
made product (Fig. S2†), confirming that the cyanamide char- 6s lone pair, which may be beneficial to hole conduction.
acter is also maintained. Fig. S3† illustrates the CA at 0.5 V vs.
RHE and 0 V vs. RHE recorded for 1 h.
Photoelectrochemistry
Electronic structure
In order to verify or falsify the above expectations, photoelec-
trochemical experiments were performed on as-deposited
PbNCN thin film electrodes prepared from PbNCN powder via
an electrophoretic deposition (EPD) process. The Mott–
Schottky (MS) plot was performed at 1000 Hz frequency in the
dark, Fig. 6(a). The curve exhibits a negative slope, indicating
The optical band gap of a yellow PbNCN powder was deter-
mined by means of UV-vis spectroscopy, yielding a value of
2
.4 eV (Fig. S4†) with the slope of the Tauc plot fitting well for
3
8
the exponent r = 2, indicating an indirect transition. This
value is similar to the experimental indirect band gaps of 1.9
eV, 2.8 eV and 2.0 eV as determined for α-PbO, β-PbO and
3
9,40
PbO , respectively.
2
In order to furnish more details of the electronic structure
of PbNCN, the band structure was calculated (Fig. 5). The DFT
calculation yields an indirect zero-temperature band gap of
approximately 2.4 eV between VBM (at −0.14 eV) and CBM (at
1
2
2
.26 eV) located at (0.154, 0, 0) and U ( , 0, 0), respectively.
This value is practically identical with that determined from
UV-Vis measurements, see above.
The band structure analysis clarifies that the levels occu-
pied by the Pb 6s electrons are the dominant contribution in
the region between approximately −11 eV and −8 eV. The
region above, between −8 eV and 0 eV, is primarily composed
Fig. 6 (a) The Mott–Schottky (M–S) plot of the PbNCN photocathode
of N 2p levels. Additionally, in the vicinity of the VBM, at 1000 Hz in the dark and (b) band-edge positions of PbNCN.
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lead cyanamide to be a p-type semiconductor. This is excep-
tional, the second report only of p-type semiconductivity in a
4
2
solid-state metal cyanamide or carbodiimide after NiNCN,
with n-type behavior typically predominating, as in analogous
8
,26
metal oxides.
NiNCN as a p-type semiconductor has been
previously integrated into a p-type dye-sensitized solar cell. It
should be noted that PbO, structurally and chemically related
to the pseudo-oxide PbNCN, has been reported as a p-type
4
3
semiconducting behavior by photocurrent measurements.
Although PbO could show anodic photocurrent (n-type behav-
ior) at relative higher potential, the surface of PbO will be oxi-
2
dized to higher oxides, i.e. PbO or intermediates due to
4
4
photogenerated holes.
The flat-band potential was estimated from the extrapol-
ation of the MS plot to be about 2.3 eV relative to RHE, which
is commonly used to estimate the VBM position in p-type
semiconductors. It should be noted that the absolute values of
the VBM are generally negative, ca. 0.1–0.3 V from the flat
band potential. The VBM of PbNCN was estimated between 2.2
and 2.0 eV, and the CBM position was estimated to be between
–0.2 and −0.4 eV on the RHE scale, Fig. 6(b). The CBM posi-
tion and p-type behavior would render PbNCN a possible can-
didate for photoelectrochemical water reduction. Furthermore,
we note that the conduction band and valence band positions
straddle the water redox potentials, indicating the potential of
4
5
splitting water without external bias.
Two additional separate electrodes were used to carry out
LSV measurements in different scan direction to avoid any (pH = 7) at a scan rate of 10 mV s under chopped AM 1.5G illumination
i
Fig. 7 LSV curves of a PbNCN photocathode in 0.1 M KP electrolyte
−1
100 mW cm− ). The difference between (a) and (b) is the scan direction.
2
(
reduction or oxidation of Pb affecting the photocurrent
response. The results are depicted in Fig. 7. A cathodic photo-
current was observed when sweeping negatively and positively,
thereby confirming the p-type nature of PbNCN inferred from
Mott–Schottky measurements. When sweeping in different
directions, a similar net photocurrent was achieved. The
noticeable dark current could be caused by the large crystal
size of PbNCN, which is inadequate to fully block the under-
lying FTO substrates.
The periodic on/off photocurrent response of the PbNCN
electrode is depicted in Fig. 8. Just like the LSV curves, the CA
curves also develop a cathodic current at 0.5 V vs. RHE. It
reconfirms the p-type character of semiconducting PbNCN.
The PEC performance demonstrates that PbNCN is a promis-
ing photo-active material which is worthy of further
investigation.
X-ray photoelectron spectroscopy (XPS) analysis
The surface of a PbNCN photocathode after PEC water
reduction at 0 V vs. RHE for 1 h was analyzed by XPS (Fig. 9).
The obtained results clearly confirm the stability of the PbNCN
phase. In the Pb 4f region, two dominant components at 138.4
eV (Pb 4f7/2) and 143.2 eV (Pb 4f5/2) are observed, corres-
Fig. 8 Periodic on/off photocurrent response of a PbNCN electrode in
0
.1 M KP
i
electrolyte (pH = 7) at 0.5 V vs. RHE under chopped AM 1.5G
−2
illumination (100 mW cm ).
2
+
46
ponding to Pb (PbNCN and PbO). No metallic lead was
formed during the photoelectrochemical operation. Some part 132.8 eV in the survey spectrum (not shown). On the other
of the active surface, however, was covered by phosphate hand, in the high-resolution XPS C 1s spectrum, apart from
species as evidenced by the low intense doublet at 139.8 eV the typical C–C component at 284.6 eV, two peaks at 285.8 eV
and 144.5 eV and additionally confirmed by the P 2p3/2 peak at and 287.8 eV appear, which are attributed to C–N and CuN
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Acknowledgements
We thank the China Scholarship Council for PhD stipends to
Xianji Qiao and Zili Ma, Tobias Storp for assistance with the
PXRD measurement, and Birgit Hahn for assistance with the
SEM measurement. The XPS measurements were carried out
with the equipment purchased with the financial support of
the European Regional Development Fund in the framework of
the Polish Innovation Operational Program (contract no.
POIG.02.01.00-12-023/08). In addition, we acknowledge the
provision of computing time by the IT center of RWTH Aachen
University.
Fig. 9 XPS Pb 4f and C 1s spectra of a PbNCN photocathode after PEC
water reduction at 0 V vs. RHE for 1 h.
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4
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Lead cyanamide, PbNCN, has been prepared by an SSM route
and structurally characterized through PXRD and IR measure-
ments. Electronic structure calculations indicate an indirect
band gap of 2.4 eV, in excellent agreement with the measured
value from UV-vis spectroscopy. Mott–Schottky experiments
demonstrate PbNCN to be a p-type semiconductor with a CBM
position theoretically suitable for application as a photo-
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photoactive properties encourage further exploration of cyana-
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Conflicts of interest
There are no conflicts to declare.
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