Cd2NF, an analogue of CdO

Article abstract of DOI:10.1039/c8dt01137k

Cd₂NF, isoelectronic with CdO, has been prepared by ammonolysis of CdF₂. Cd₂NF has the rock salt structure of CdO and shows electronic properties similar to CdO. First principles calculations shed light on the electronic structure and properties.

Full text of DOI:10.1039/c8dt01137k

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Cd NF, an analogue of CdO†
2
a
b
b
Cite this: DOI: 10.1039/c8dt01137k
Krishnappa Manjunath, Suchitra Prasad, Umesh V. Waghmare and
a
C. N. R. Rao
*
Received 24th March 2018,
Accepted 6th May 2018
2 2 2
Cd NF, isoelectronic with CdO, has been prepared by ammonolysis of CdF . Cd NF has the rock salt
DOI: 10.1039/c8dt01137k
structure of CdO and shows electronic properties similar to CdO. First principles calculations shed light
on the electronic structure and properties.
rsc.li/dalton
obtained is isostructural with the CdO rock salt structure and
exhibits interesting electronic properties as well as visible-light
Introduction
Aliovalent anion substitution in inorganic materials gives rise induced water splitting to generate hydrogen. We have studied
to major changes in electronic properties. Thus, substitution the crystal and electronic structure as well as the properties of
of N and F− in place of O in metal oxides results in Cd NF both experimentally and from first principles
3
−
2−
2
changes in the band gap as well as other properties. It is calculations.
necessary to substitute a trivalent and a monovalent anion to
avoid the formation of defects. For example, exclusive substi-
3
−
tution of N anion gives rise to changes in the electronic
structure of ZnO and other oxides, but the product will have a
large amount of oxygen vacancies. Cosubstitution of N and Initially CdF was prepared by heating cadmium powder and
Experimental
1
–4
2
F in oxides such as ZnO is free of vacancies. In the literature, ammonium fluoride in a tube furnace at 400 °C for 4 h under
−
1
there have been reports in which Andersson et al. have prepared a continuous flow of N gas with a heating rate of 5 °C min
2
5
−1
Mg N F compounds with the rock salt structure of MgO.
2
and a cooling rate of 2 °C min . In the next step, CdF was
x
y
y
Computational studies revealed that Mg
direct and indirect band gap semiconductors. Materials such 400 °C for 5 h under a continuous flow of high purity NH gas
3 3 2
NF and Mg NF were taken in an alumina boat, and heated in a tube furnace at
6
3
−
1
as Ca NF, Sr NF and Ba NF have been reported but no interest- with a heating rate of 4 °C min
and a cooling rate of
2 °C min−1. After the completion of reaction, the furnace was
NF an analogue of ZnO was prepared. It is a fasci- cooled down to room temperature. An orange colored product
nating material with a different band gap and significant (Cd NF) was obtained which is stable at room temperature.
2
2
2
7
–9
ing properties have been observed in these materials.
Recently, Zn
2
2
1
0
photocatalytic activity. We considered it important to study This was examined by X-ray diffraction, X-ray photoelectron
3
−
−
2−
Cd
2
NF by substituting N and F for O . CdO with the rock spectroscopy and other methods. The XRD data were subjected
salt structure is usually nonstoichiometric with cation intersti- to Rietveld profile analysis.
1
1–14
tials (Cd excess) or anion vacancies,
which results in a
Photocatalytic hydrogen evolution experiments have been
−
3
high carrier concentration (1019–1021 cm ) which causes carried out by using 0.1 M Na S and 0.1 M Na SO . 20 mg of
2
2
3
degenerate behavior at high temperatures. Due to excess the obtained photocatalyst was dispersed in 70 ml of distilled
carrier concentration in CdO, the impurity band may overlap water containing sacrificial agents in a photocatalytic reactor.
with the conduction band making it quasimetallic or metal- The solution was purged with N for nearly 20 min prior to the
2
1
1
lic. We have successfully obtained Cd NF by the ammonoly- reaction and then the reactor was irradiated with a 400 W Xe
2
sis of CdF
2
in a tube furnace at 400 °C. The material so lamp. A cut-off filter λ > 395 nm was used to remove UV light
and a water filter was used to remove IR radiation. Evolved gas
samples were analysed using
a
gas chromatograph
a
New Chemistry Unit, International Centre for Materials Science, Sheikh Saqr
Laboratory, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur P.O.,
Bangalore-560064, India. E-mail: cnrrao@jncasr.ac.in
(
PerkinElmer, Clarus-580) for the quantification of hydrogen.
http://www.jncasr.ac.in/cnrrao/
Computational methods
b
Theoretical Sciences Unit, Jawaharlal Nehru Centre for Advanced Scientific
Our first-principles calculations are based on density func-
tional theory (DFT) as implemented in the Quantum
ESPRESSO (QE) package, with local density approximation
Research, Jakkur P.O., Bangalore-560064, India
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
1
7
c8dt01137k
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(LDA) to treat the exchange–correlation energy of electrons. We structures of Cd
2
NF and CdO. Standard deviations are given
used ultrasoft pseudopotentials to represent the interaction in Table S2.†
between ionic cores and valence electrons, and an energy cut-
off of 45 Ry (450 Ry) to truncate the plane-wave basis for repre- of Cd
sentation of Kohn–Sham wave functions (density). Crystal
structures were relaxed to minimize the total energy with
respect to atomic positions until the Hellmann Feynman
Fig. 3 shows the high-resolution X-ray photoelectron spectra
2
NF which describes the nature and compositions of Cd,
2
Table 1 Comparison of crystallographic parameters for Cd NF and
−
1
forces on each atom are less than 0.01 eV Å . We used the
CdO
1
8
Heyd–Scuseria–Ernzerhof (HSE-06) hybrid functional (that
mixes short range exact exchange energy) to obtain accurate Name
estimates of electronic band gaps. We considered anionic dis-
order through various configurations of N and F decorating
anionic sites.
Cd NF
2
CdO standard
Cubic
Fm3m (no. 225)
Dark brown
Crystal system
Space group
Colour
Cell parameters
Cell volume
Calculated density
Cd–N
Cubic
Fm3m (no. 225)
Orange
ˉ
ˉ
a = 4.7052 Å
3
3
104.17 Å
8.315 g cm−
2.3524
2.3564
—
3
Results and discussion
Cd–F
Cd–O
Cd NF was obtained by heating CdF under a continuous flow
2
2
of NH
3
gas. The XRD pattern of Cd
2
NF is shown in Fig. 1. The
XRD pattern was similar to that of CdO. The structure of
Cd NF was solved by Rietveld refinement and the best fit of
2
the diffraction data gave the space group as Fm ˉ3 m (no. 225)
and a unit cell parameter of a = 4.7052 Å.
In the structure, each Cd atom is coordinated with six N/F
atoms. The bond distances of Cd–N and Cd–F are found to be
2
.3524 and 2.3564 Å, since the anions are disordered and the
average bond distance of Cd–N/F is similar to that of CdO.
−
3
2
The calculated density of Cd NF was 8.315 g cm and the
3
cell volume was 104.17 Å . The results obtained from
Rietveld analysis are listed in Table 1. In Fig. 2, we show the
Fig. 2 Structural representations of CdO and Cd
2
NF structures (rock
salt type).
2
Fig. 1 Rietveld refinement profiles for (a) Cd NF and (b) CdO from XRD
data. Black patterns indicate the observed XRD data, the red dots indi-
cate the calculated profiles and the blue lines below are the difference
profiles.
Fig. 3 X-ray photoelectron spectra of (a) Cd 3d, (b) N 1s, and (c) F 1s
and (d) survey spectra of Cd NF. Actual ratios of the elements are
Cd
2
2
1.0155 0.974
N F .
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Fig. 6 Photocatalytic hydrogen evolution using (a) CdO and (b) Cd
2
NF
as
2
Fig. 4 Optical photon energy spectra of (a) CdO and (b) Cd NF.
under visible light irradiation in the presence of Na
scavengers.
2 2 3
S and Na SO
N, and F atoms. The signals at 410 eV and 416.6 eV in Fig. 3a
show the core spectra of Cd5/2 and Cd3/2 states respectively.
The signal at 399.5 eV is a characteristic feature of N 1s and
that at 689 eV corresponds to F 1s. The actual ratios of the
compositions of the respective elements such as Cd, N, and F
have been calculated and found to be 2 : 1.0155 : 0.974 as
above 125 K. The resistivity of CdO gradually decreases down
to 150 K, behaves as a semiconductor first and then trans-
forms to the metallic state above 150 K. Such a metal–insulator
transition is also exhibited by CdO (Fig. 5) which is a degener-
ate material with its impurity band in the vicinity of the con-
2
required by the composition Cd NF (2 : 1 : 1).
duction band. This behavior of Cd
due to unintentional doping.
2
NF (and CdO) could arise
Fig. 4 shows the optical photon energy spectra of Cd NF
2
and CdO. Both Cd NF and CdO have characteristic absorption
2
Hydrogen evolution experiments were carried out to test the
activities of Cd NF and CdO because they exhibit suitable
at 500 nm and 563 nm in the visible region. The band gaps
have been calculated by following Kulbeka–Munk equations
2
band positions for photocatalytic hydrogen evolution as well
as absorb visible light. During the experiments, 0.1 M each of
Na S and Na SO have been added as sacrificial agents. In
2 2 3
which show that both Cd NF and CdO exhibit direct band gap
2
2
semiconductors. Cd NF has a band gap of 2.48 eV which is
slightly higher than the band gap of CdO (2.2 eV).
In Fig. 5, we present the variation of electrical resistivity
2
with temperature in the range of 0–300 K for Cd NF. We
Fig. 6, we show the photocatalytic hydrogen evolution results
of Cd NF and CdO. Both Cd NF and CdO exhibit H evolution
2
2
2
under visible light irradiation. Cd
2
NF is a good photocatalyst
observed the decrease in the resistivity of Cd NF initially down
2
for the hydrogen evolution reaction with four times higher
activity compared to CdO.
We shall now discuss the results of our first principles cal-
2
culations. As Cd NF crystallizes in the cubic rock salt structure,
to 125 K with the increase in temperature, above 125 K, the res-
istivity starts increasing gradually with respect to temperature
up to 300 K. Cd NF material behaved as a semiconductor until
2
1
25 K and then progressively transformed to the metallic state
we simulated disorder in site occupancy by N and F by con-
structing the 1 × 1 × 1, √2 × √2 × 1 and 2 × 2 × 2 periodic
supercells (shown in Fig. 7), where the 1 × 1 × 1 unit cell
corresponds to conventional cubic cells with four Cd sites. In
the 1 × 1 × 1 configuration shown in Fig. 7(a), each Cd is con-
nected to four N/(F) atoms and two F/(N) atoms. As a result of
such N and F ordering, there is a contraction in the lattice con-
stant a and expansion in the b and c. The Cd–N/F bond
lengths are 2.26 Å and 2.41 Å. In the √2 × √2 × 1 supercell of
Cd NF, we considered three symmetry inequivalent configur-
2
ations (Fig. 7(b)–(d)). The relative energies of these configur-
ations and their associated calculated lattice constants have
been tabulated in Table 2. From the energies of these configur-
2
ations in the √2 × √2 × 1 supercell of Cd NF (see Table 2), it
is clear that configuration III (shown in Fig. 7(d)) is the most
stable. In this, lattice constants a and b expand, whereas c con-
tracts. The in-plane Cd–N (Cd–F) bond lengths are 2.41 Å and
2
.24 Å (2.92 Å and 2.39 Å), and the out-of-plane Cd–N (Cd–F)
Fig. 5 Variation of resistivity with temperature (0–300 K) of (a) CdO
and (b) Cd NF.
2
bond lengths are 2.22 Å (2.29 Å). We have constructed a 2 × 2 × 2
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Fig. 8 Coordination of Cd atoms in configurations with (a) 1 × 1 × 1, (b)
√
2
2 × √2 × 1, and (c) 2 × 2 × 2 supercells of Cd NF.
Cd–N and Cd–F bond lengths are 2.28 Å and 2.50 Å respect-
ively. In this, the Cd atoms come closer to each other forming
a long Cd–Cd bond of length 3.03 Å. The structural parameters
of these configurations are given in Table 3. For further
analysis, we have considered three chemically ordered con-
figurations of N and F atoms, in the 1 × 1 × 1, √2 × √2 ×
1
(configuration III being the energetically low structure), and
2
× 2 × 2 periodic supercells. The coordination of Cd atoms in
all the three configurations is shown in Fig. 8 and is given in
Table 3.
We obtained the electronic structures of the three configur-
ations along the high symmetry lines of the Brillouin zone
Fig. 7 Configurations of the different chemical ordering of N and F in
Cd
Cd,
respectively.
2
NF with (a) 1 × 1 × 1, (b)–(d) √2 × √2 × 1, and (e) 2 × 2 × 2 supercells.
(
Fig. 9). It is clearly seen that the different chemical ordering
of N and F in Cd NF greatly affects its electronic properties.
The configuration of Cd NF with the 1 × 1 × 1 unit cell exhibits
a metallic character (Fig. 9(a)). Similarly, the configurations of
NF in the √2 × √2 × 1 supercell also have an electronic
stants of various chemically ordered configurations of N and F atoms in structure with the bands crossing the Fermi level indicating
N
and are represented by purple, red and green spheres
F
2
2
Table 2 Relative energies and their associated calculated lattice con- Cd
2
the √2 × √2 × 1 supercell of Cd
2
NF. Configurations I, II and III have
their metallic nature. We note that there is a gap along the
Z–R–A–Z directions (Fig. 9(b)) giving rise to a pseudo gap in its
been shown in Fig. 7(b)–(d) respectively
2
electronic structure. We find that Cd NF in the 2 × 2 × 2 super-
Configurations
E
n
–EIII (eV per unit cell)
Lattice constants (Å)
cell also has an electronic structure with bands crossing
the Fermi level (Fig. 9(c)). From the projected density of
electronic states (PDOS), we note N 2p orbitals dominating
the states at the E of Cd NF in 1 × 1 × 1 and 2 × 2 × 2 super-
I
II
III
0.53
0.42
0.00
a = b = 4.67, c = 4.82
a = b = 4.79, c = 4.65
a = b = 4.95, c = 4.40
F
2
cells (Fig. 10(a) and (c)). In contrast, we do not find a signifi-
cant contribution of N 2p orbitals to the states at the Fermi
Table 3 Coordination of Cd, theoretical lattice constants, and
Cd–X (X = N, F) bond lengths of Cd NF in various configurations of
level of Cd NF configuration in the √2 × √2 × 1 supercells
2
2
anionic chemical ordering in Cd
2
NF
(Fig. 10(b)).
2
While Cd NF is a semiconductor as observed experi-
Coordination
of Cd atoms
Lattice
constants (Å)
mentally, our LDA-DFT calculations predict it to be metallic.
To corroborate this finding, we determined the electronic
structure with calculations based on the projected augmented
wave (PAW) using VASP. Indeed, we still find a metallic charac-
ter in all the configurations considered here. The structural
parameters obtained using this method have been tabulated
in Table 4, and the electronic structures obtained using PAW
pseudopotential have been shown in Fig. S1.† As we obtain
Supercell
× 1 × 1
Bond lengths (Å)
1
CdN
2
F
F
4
, CdN
4
F
F
2
a = 4.51 and
b = c = 4.82
a = b = 4.95
and c = 4.40
.22 (out of plane)
Cd–F: 2.92, 2.39
in-plane) and
.29 (out of plane)
a = b = c = 4.71 Cd–N: 2.28, Cd–F:
.50
Cd–N/F: 2.26
and 2.41
Cd–N: 2.41, 2.24
(in-plane) and
√
2 × √2 × 1 CdN
2
4
, CdN
4
2
2
(
2
2
× 2 × 2
3
CdN F
3
, CdN
3
F
3
2
tetragonal structures upon relaxation of Cd NF configurations
2
in the 1 × 1 × 1 and √2 × √2 × 1 supercells, in contrast to the
cubic structure observed experimentally, we also studied the
effects of variation in lattice constants. In all these simu-
supercell and arranged N and F atoms in such a way that lations, we found Cd NF to be weakly metallic. In all the
2
three N and three F atoms attached to the Cd atom are close to chemically ordered states of N and F atoms considered within
each other in the cis-position (see Fig. 7(e)). In this configur- the 1 × 1 × 1, √2 × √2 × 1 and 2 × 2 × 2 supercells, we find
ation, there is a contraction in the three lattice constants. configuration III of the √2 × √2 × 1 supercell to be energeti-
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Fig. 9 Electronic structure of chemically ordered configurations of N
and F atoms in (a) 1 × 1 × 1, (b) √2 × √2 × 1, and (c) 2 × 2 × 2 unit cells
of Cd NF.
2
cally stable (see Table S1†). For more accurate determination Fig. 10 Projected density of states of configurations of Cd NF with varied
2
of electronic structures, we used hybrid functional calculations N and F ordering in (a) 1 × 1 × 1, (b) √2 × √2 × 1, and (c) 2 × 2 × 2 supercells.
as implemented in the Vienna ab initio simulation package
1
5
16
(
VASP) with the projected augmented wave method (PAW).
Table 4 Theoretical lattice constants and Cd–X (X = N, F) bond lengths
of Cd NF in various configurations of anionic chemical ordering in
Cd NF using PAW pseudopotentials
In the Cd NF configurations with the √2 × √2 × 1 super-
2
2
cells with the experimental lattice constants, we relaxed the
structure internally maintaining the cubic lattice constants.
We obtained an indirect band gap of 0.79 eV (Fig. 11(a)),
which is close to the experimental band gap (0.84–0.90 eV) of
2
Supercell
× 1 × 1
Lattice constants (Å)
Bond lengths (Å)
1
√
a = 4.68 and b = c = 4.99
a = b = 5.10 and c = 4.59
Cd–N/F: 2.50 and 2.34
Cd–N: 2.47, 2.31 (in-plane)
and 2.32 (out of plane)
Cd–F: 2.99, 2.48 (in-plane)
and 2.38 (out of plane)
Cd–N: 2.35, Cd–F: 2.61
1
4
CdO.
Fig. 11(a)) of Cd
structures of Cd
cells with theoretical lattice constants exhibit bands crossing
the Fermi level indicating the metallic nature of Cd NF
Fig. 11(b)). We also performed LDA + U calculations taking
The direct band gap in the electronic structure
NF is 0.92 eV at the Γ-point. The electronic
NF configurations with the 2 × 2 × 2 super-
2 × √2 × 1
(
2
2
2
× 2 × 2
a = b = c = 4.95
2
(
U = 3 eV to estimate the effects of onsite correlations on the the 2 × 2 × 2 supercells of Cd NF, we find a band splitting at
2
electronic structure using VASP and PAW potentials. The U cor- the Γ-point (Fig. 12(b)), accompanied by a band just below the
rection does not open up a gap in √2 × √2 × 1; we have bands Fermi level shifting closer to it, than that obtained from simu-
crossing the Fermi level (Fig. 12(a)). In the configurations with lations based on the HSE functional.
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Conclusions
2
Cd NF, isoelectronic with CdO, has the same crystal structure.
The electronic properties are also similar. The electronic struc-
ture of Cd NF is greatly affected by the arrangement of N and
2
F atoms in a unit cell. The DFT calculations predict weakly
2
metallic character of Cd NF, and a small gap opening in the
electronic structure of Cd NF with the √2 × √2 × 1 supercell.
2
2
We find that the most stable configuration of Cd NF contains
one Cd atom connected to four N atoms and two F atoms (and
another Cd to two N atoms and four F atoms). Inclusion of the
Hubbard U correction did not open up a gap in the electronic
structure of Cd NF in the √2 × √2 × 1 and 2 × 2 × 2 supercells.
2
Cd NF and related materials could be useful as transparent
2
conductors.
Conflicts of interest
There are no conflicts of interest to declare.
Acknowledgements
K. M. is grateful to Sheikh Saqr Laboratory, JNCASR for the fel-
lowship and research facilities. U. V. W. acknowledges support
from SSL and the JC Bose National Fellowship. Suchitra is
thankful to TUE-CMS, JNCASR for computational facility.
Fig. 11 Electronic structure of various ordered configurations of N and
F atoms in (a) √2 × √2 × 1 and (b) 2 × 2 × 2 supercells of Cd NF,
2
obtained from calculations based on the hybrid functional.
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