Physical properties of the delafossite LaCuO2

ARTICLE IN PRESS
Journal of Physics and Chemistry of Solids 70 (2009) 1132–1136
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Journal of Physics and Chemistry of Solids
journal homepage: www.elsevier.com/locate/jpcs
Physical properties of the delafossite LaCuO2
B. Bellal, S. Saadi, N. Koriche, A. Bouguelia , M. Trari
Laboratoire de Stockage et de Valorisation des Energies Renouvelables, Faculte de Chimie BP 32, 16111 Algiers, Algeria
a r t i c l e in f o
a b s t r a c t
Article history:
Received 16 June 2008
Received in revised form
25 April 2009
Accepted 18 June 2009
High-quality LaCuO2, elaborated by solid-state reaction in sealed tube, crystallizes in the delafossite
structure. The thermal analysis under reducing atmosphere (H2/N2: 1/9) revealed a stoichiometric
composition LaCuO2.00. The oxide is a direct band-gap semiconductor with a forbidden band of 2.77 eV.
The magnetic susceptibility follows a Curie–Weiss law from which a Cu2+ concentration of 1% has
been determined. The oxygen insertion in the layered crystal lattice induces p-type conductivity.
The electrical conduction occurs predominantly by small polaron hopping between mixed valences
Cu+/2+ with an activation energy of 0.28 eV and a hole mobility (m300 K ¼ 3.5 107 cm2 V1 s1),
thermally activated. Most holes are trapped in surface–polaron states upon gap excitation. The
photoelectrochemical study, reported for the first time, confirms the p-type conduction. The flat band
potential (Vfb ¼ 0.15 VSCE) and the hole density (NA ¼ 5.8 1017 cm3) were determined, respectively, by
extrapolating the curve C2 versus the potential to their intersection with C2 ¼ 0 and from the slope of
the linear part in the Mott–Schottky plot. The valence band is made up of Cu-3d orbital, positioned at
4.9 eV below vacuum. An energy band diagram has been established predicting the possibility of the
oxide to be used as hydrogen photocathode.
& 2009 Elsevier Ltd. All rights reserved.
Keywords:
A. Oxides
C. Thermogravimetric analysis (TGA)
D. Electrochemical properties
D. Semiconductivity
D. Transport properties
1. Introduction
Papers on the physical properties of the delafossites Cu+M3+O2
(where M denotes a rare earth) are rarely encountered in the
literature [1]. So far, the only practical application, made in our
laboratory, concerns the conversion of solar energy to chemical
energy [2]. This was undertaken to circumvent the deep valence
band (VB) of oxygen parentage of most oxides owing to its high
electronegativity. Cu+ has a closed shell configuration and CuMO2
is expected to be a semiconductor [3]. By contrast, the transport
properties can be modulated to some extent by either oxygen
insertion [4] or by doping with altervalent ions onto the
M-sublattice [5] and they are usually available as p-type specimen. Moreover, VB is made up of Cu-3d character and slows down
the corrosion process considerably in aqueous electrolytes. Under
irradiation and in the presence of appropriate electroactive
species, the delafossites are remarkably stable particularly in
basic media. However, some of them dissolve in acids and the
oxidation state of copper was determined chemically.
This work introducing LaCuO2 is part of our research program
on the photoelectrochemistry and to our knowledge, no previous
physical study has been reported. Here, for the first time, we
report the synthesis in sealed tube, the physical properties and
photoelectrochemical (PEC) characterization with the main
Corresponding author. Tel.: +213 21 24 79 50; fax: +213 08263 24602.
E-mail address: [email protected] (A. Bouguelia).
0022-3697/$ - see front matter & 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jpcs.2009.06.011
features (Eg, DE, Vfb, ND, etcy) of p-LaCuO2 on the basis of
absorption spectra, polarization curves and capacitance measurements. It seemed attractive to study this ternary oxide, since it is
the only one among the isotypic CuMO2 congeners that was not
characterized photoelectrochemically because of the impossibility
to dope it onto the La-sublattice even with ions of similar size like
calcium.
2. Experimental
LaCuO2 was synthesized by heating stoichiometric amounts of
extra pure La2O3 and Cu2O (purity499.9%). The reactants were
ground in an agate mortar, compacted into disks under a pressure
of 4 kbar and sealed under dynamic vacuum (o1 mbar) in silica
tube. The tube was heat treated at 960 1C and air quenched. The
same procedure was repeated twice to get a single phase.
Unreacted Cu2O was removed by soaking the powder in NH4OH
(5 N). La2O3 was fired at 800 1C in air and Cu2O dried at 500 1C
under vacuum just before use. The starting oxides were handled in
an argon-filled box. Because of its strong moisture sensitivity,
La2O3 converts to La(OH)3 under wet air atmosphere. We were
unsuccessful in obtaining CuLa0.98Ca0.02O2 by a similar method.
The X-ray diffraction pattern, taken with a monochromatized
CuKa radiation, revealed a homogeneous single phase. The data
were recorded for 10 s at each 0.021 step over 2y interval 10–751.
The lattice constants were refined by the least-square method
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B. Bellal et al. / Journal of Physics and Chemistry of Solids 70 (2009) 1132–1136
are determined by Cu-3d levels; the conduction band CB (LUMO)
consists of empty antibonding s* hybridized 3dz2/4s wave
functions whereas the non-bonding Cu–t2g orbital constitutes
VB (HOMO). The electro positive character of lanthanum promotes
a high degree of covalency for Cu–O bond, which tends to
decrease the Cu–O length (0.1825 nm) in LaCuO2 with respect to
CuAlO2 (0.18611 nm) [7]. This raises the band s* leading to an
increase in the optical gap of LaCuO2, which possesses a large Eg
among delafossites (see below). This energetic scheme confers to
LaCuO2 its semiconductivity and is compatible with the chemical
inertness. The latter is corroborated by the semilogarithmic plot
(Fig. 2), where an exchange current density (0.07 mA cm2) and a
corrosion potential (0.858 V) have been determined.
Copper exhibits three oxidation states and the Cu+–Cu+
distance is large enough to accommodate oxygen species up to
0.62 ions per unit formula because of the large channels parallel
to the basal plans [8]. The DSC plot (insert, Fig. 3) gives the
recording in air of the thermal heat flow (dH/dT), where one can
easily detect the presence of intermediate compounds during the
heating process. The area enclosed by the peak is directly
proportional to the enthalpy change. The endothermic peak is
indicated by upward direction, i.e. an increase in enthalpy. By
contrast, in the TG plot under reducing atmosphere (H2/N2:1/9),
012
1000
800
600
116
024
010
202
110
113
107
104
015
006
101
200
018
400
003
Intensity (a.u)
using silicon as internal standard. The density was determined by
the displacement method in toluene because of its wetting
property. The compactness of the pellets was approximately 95%
with good mechanical properties. The infrared spectrum was
obtained by employing the KBr disk technique in a Perkin–Elmertype spectrum (resolutiono2 cm1). The formal oxidation state of
copper was evaluated by iodometry and ~60 mg of powder was
dissolved under nitrogen atmosphere in HCl (3 N) solution
containing KI in excess. The liberated iodine was accurately
titrated by a standard Na2S2O3 solution using starch as indicator.
The elemental analysis was performed by inductively coupled
plasma emission (Vista-Pro CCD ICP-OES), the La/Cu was equal to
unity within the experimental errors. The oxygen content was
determined by thermogravimetric analysis (TGA) with an accuracy of 70.01 under reducing atmosphere (10% H2/N2). In this way
copper was reduced to Cu0 whereas La3+ kept the same oxidation
state La2O3. For corrosion tests, Cu2+ was analyzed by atomic
absorption (Pye Unicam SP9). The diffuse reflectance spectrum
was recorded with a Jasco V-530 spectrophotometer attached to a
reflectance accessory that goes from 200 to 800 nm.
The magnetic susceptibility was measured down to liquid
helium under a field of 18,000 G. The w(T) values were corrected
by subtracting the core diamagnetism of relevant ions [6].
The conductivity s was determined on sintered pellets by using
the two-probe method. All the temperature measurements are
referred to a digital calibrated chromel–alumel (type-K) thermocouple. The thermopower S ( ¼ DV/DT) was determined on
sintered pellets, thanks to a household equipment. Electric
contact on pellets was made with silver paint, a copper wire
was soldered to the contacting surface and the electrode was
mounted in a glass holder using epoxy resin. The PEC characterization was performed in a standard three electrode cell equipped
with a loose cover that positions the electrodes. The potential of
the working electrode (geometric area, 0.5 cm2) was monitored by
a Voltalab PGP 201 potentiostat. A large Pt sheet served as an
emergency electrode and all the potentials were scaled with
respect to a saturated calomel electrode (SCE). The support
electrolyte KOH (0.5 M) was deoxygenated by continuous bubbling with nitrogen. The light source was a 600 W halogen lamp
equipped with a water filter to remove IR light.
1133
0
10
3. Results and discussion
30
40
50
2θ (degree)
60
70
Fig. 1. X-ray diffraction pattern of LaCuO2.
-0.8
log J (μA cm-2)
Very pure LaCuO2 has been synthetized under oxygen-free
atmosphere to preclude the formation of La2CuO4. Standard
analysis showed the product composed of 70.13 wt% of La2O3
and 29.87 wt% of Cu2O, a composition very close to the theoretical
one ( 69.48 wt% and 30.51 wt% respectively). The X-ray pattern
(Fig. 1), virtually free from impurties, is indexed in a hexagonal
unit structure where all the peaks are assigned to the delafossite
phase. The refined lattice constants ah ¼ 0.3833(3) nm
ch ¼ 1.7109(6) nm are in perfect agreement with the published
data [7]. The experimental density dexp (5.10 g cm3) agrees with
that calculated on the basis of three formula weights per unit cell
(5.36 g cm3). The formal oxidation state, determined by
iodometry (1.02), corresponds to oxygen over stoichiometry of
~0.01 and corroborates the formulation found from magnetism.
The crystal structure consists of sequences of layers of
Cu–O–La–O–Cu, where the oxygen is surrounded by a tetrahedron
of 3 La and 1 Cu. It belongs to the centro-symmetric space group
R3̄m (D53d), visualized as a close packed layers of La3+ octahedra
sharing common edge infinite [LaO6]N layers. Octahedra, belonging to two adjacent layers, are linked to each other by Cu+ in linear
coordination forming {CuO3
2 } ‘‘dumb-bell’’ parallel to the c-axis.
As mentioned in the introduction, the electronic bands of CuMO2
20
-1.0
-1.2
-1.4
-1.2
-1.0
-0.8
-0.6
Volt (Vsce)
Fig. 2. Semilogarithmic plots for Pt electrode in KOH at a scan rate of 5 mV/s.
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B. Bellal et al. / Journal of Physics and Chemistry of Solids 70 (2009) 1132–1136
the first weight loss at 310 1C is originated from the removal of
oxygen excess (de-intercalation process, arrow D in Fig. 3. The
second one starts at ~440 1C and ends at 630 1C after which a
plateau region is reached. It corresponds to the reduction process
60
2LaCuO2+H2-2Cu+La2O3+H2O.
40
w ¼ w0 þ Cc =T
0.01
0.00
100
-0.02
-0.03
98
DTG (%)
TGA (%)
-0.01
D
-0.04
97
-0.05
96
-0.06
200
400
600
800
T (°C)
1000
1200
1400
Fig. 3. TGA/TDA plots of LaCuO2 under reducing atmosphere (H2/N2:1/9).
Differential scanning calorimetry of LaCuO2 (insert).
χ = 50x10-6 (emu CGS mol)-1
12
8
105 χ (emu CGS/mol)-1
103 χT (emu CGS/ mol)-1 °C
100
20
50
10
λ0 = 448 nm
4
0
-4
-8
30
20
10
0
0
100
400
450
500
λ (nm)
550
600
0
650
Fig. 5. Diffuse reflectance spectrum of LaCuO2.
for the green emission (550 nm) upon UV excitation [9].
The diffuse reflectance spectrum is given in Fig. 5; the precise Eg
value (2.77 eV) was obtained from the derivative curve.
Some authors have proposed a critical interionic distance in
transition metal oxides, above which a localized electron behavior
is expected [10]. The Cu+–Cu+ separation ( ¼ a-parameter) is
greater than the interatomic length in copper metal (0.255 nm)1
[6] and LaCuO2 is predicted to be a semiconductor. However,
the semiconducting properties of delafossites depend on the
deviation from stoichiometry. The impossibility to synthesize
CuLa0.98Ca0.02O2 in spite of the closeness of ionic radii of Ca2+
(0.100 nm) and La3+ (0.103 nm) in six-fold coordination [11]
cannot be explained by the consideration of size. The conversion
of LaCuO2 to p-type SC is achieved through oxygen insertion and
would induce mixed valence states Cu+/2+ [12]. In delafossites,
the conduction process occurs predominantly by small polaron
hopping [13] and is dominated mainly by holes as the thermo
power S remains positive (Fig. 6). The decrease of S with
temperature up to 390 K indicates that the mobility m is
thermally activated rather than the hole density NA. The
acceptors are no longer completely ionized and in such a case, S
is given by [14]
S ¼ k=efDE=kT þ Bg ¼ ðk=eÞlnðNo =NA Þ
ð2Þ
The dimensionless constant B can be neglected when EgbkT.
The density of polaron sites No is assimilated to Cu+ concentration
(1.32 1022 m3), deduced from the density dexp. The activation
energy DE (0.28 eV) is evaluated from S300 K (930 mV K1). The
ratio NA/No (2 105) determined from relation (2) indicates that
most holes are trapped in surface hole polarons, localized on Cu2+
sites. Low polarons have high concentration and low mobility and
behave as heavy pseudoparticles that polarize the surrounding
coordination and enhance the electron effective mass. At
room temperature, the low mobility (7.4 107 cm2 V1 s1),
calculated from the relation s ¼ (NA mh e), is inferred to the
narrow valence band of Cu-3d-character, which does not exceed
2 eV. This leads to an enhanced effective mass m* (~2mo)
calculated from DE.2
The intensity potential J(V) relationship is traced in an effort to
elucidate the electrochemical processes in LaCuO2. Fig. 7 shows
C = 3.25x10-3
16
30
350
101
0
150
ð1Þ
with w0~50 106 (emu CGS mol)1. The susceptibility can be
regarded as a magnetic titration owing to the Cu2+ dilution effect.
The Curie constant (Cc ¼ 3.25 103) comes from the odd number
of Cu2+ ion (3d9) generated by oxygen insertion to achieve the
charge balance. Cc is ~1/100 of the value for of free spins Cu2+
(S ¼ 12, Cc ¼ 0.374, ~1%) leading to LaCuO2.00, a formulation close to
that obtained from the chemical analysis.
LaCuO2 has a yellowish color and the photoluminescence
involves 3d94s-3d10 inter-configurational transitions responsible
99
T (%)
A total weight loss of 3.46% allowed us to evaluate the oxygen
content at exactly 2.00 per unit formula.
Cu+ ion has a filled inner shell (3d10) and stoichiometric
LaCuO2 is expected to be diamagnetic. However, the thermal
variation of the magnetic susceptibility (Fig. 4) is well described
by the Curie–Weiss law:
200
50
Differential (a.u.)
1134
200
T (°C)
-12
0
50
100
150
T (°C)
200
250
300
Fig. 4. Thermal variation of magnetic susceptibility (wT) of LaCuO2.
1
Calculated from the lattice constant of copper, which crystallizes in a body
centred cubic.
2
DE ¼ e4m*/8(eeoh)2, e being the electron charge, h the Planck constant and eo
the dielectric constant of free space.
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1135
0.2
1.2
J (mA/cm2 )
S (mVK-1)
1.0
0.8
0.6
-0.5
0.4
0.1
Vfb = 0.14 V/sce
0.0
0.0
0.2
-0.1
0.5
V (Volt/sce)
1.0
In the dark
Under illumination
0.0
0
30
60
90
120
Temperature (°C)
150
180
Fig. 6. Thermal dependence of Thermopower S of LaCuO2.
Ox
A value of 0.14 V was obtained in KOH solution, very close to that
of the isotopic CuYO2 [15], because of the similarities of
electronegativity of lanthanum and yttrium [6]. Over time, the
decrease of Jph is due to the photocorrosion of LaCuO2, since the
solution does not contain any redox couple.
The exchange of majority carriers between the oxide and the
solution occurs in a direct way and the interface is approximated
by a plate condensator:
0.2
0.0
0
J (mA/cm2 )
-1
Fig. 8. The J(V) curve of LaCuO2 in the dark and under illumination in 1 M KOH
solution scan rate of 5 mV/s.
1
2
Volt (V/ecs)
-0.2
C ¼ eeo A=d
1
1
þ CSCR
C 1 ¼ Cdl
Red
-0.4
Fig. 7. Cyclic voltammetry in the dark of LaCuO2 in KOH solution. Scanning rate
5 mV/s.
the cyclic J(V) curves plotted in KOH media; the use of NaOH gave
a similar results. The potential was initiated from the cathodic
side and the curve exhibits an oxidation wave at ~ 0.54 V (Ox).
Because of the layered crystal lattice, oxygen can be intercalated
electrochemically and electrons are coming out from the basal
Cu2O plans. The electroneutrality condition implies a partial
oxidation of Cu+ to Cu2+ and the reaction can be written as
LaCuO2+2eOH-LaCuO2+e+eH2O+2ee
where e stands for the amount of oxygen incorporated. On the
reverse scan, the oxide undergoes reversible reduction at 0.98 V
(red) corresponding to oxygen de-intercalation. Below 1.4 V, the
current shoots up considerably owing to H2-evolution.
LaCuO2 works as photosensitive electrode. The shape of Jph(V)
characteristic (Fig. 8) is clearly that of p-type semiconductor, i.e.
an increase of the photocurrent Jph along the cathodic direction.
Jph increases slowly and saturates at ~0.16 V, the magnitude of
which is found to depend only on light intensity, a behavior
ascribed to a zero electron–hole (e/h+) pair recombination.
The potential Von corresponds to the onset photocurrent; it was
taken as the potential above which no photocurrent could be
observed and can be assimilated to the flat band potential Vfb.
ð3Þ
where A is the surface of the electrode and d the thickness. The
total capacitance C consists of a double layer (Cdl) in the
electrolyte side and SCR (CSCR):
ð4Þ
The density NA (see below) leads to a depletion width Wo
expressed by [16]:
Wo ¼ f2eeo ðVfb VÞ=eNA g1=2
ð5Þ
A value of ~63 nm is deduced for a band bending (VfbV) equal to
0.3 V. Hence, the value of CSCR is much larger and CECdl. The
Mott–Schottky plot gives the differential capacitance dependence
of SCR on the electrode potential [17]:
1
2
kT
¼
7
V
V
ð6Þ
fb
AeNA ee0
e
C2
The linear characteristic C2V over the potential range
(0.35 V) implies a uniform density NA, whereas the negative
slope confirms the extrinsic p-type conductivity (Fig. 9).
The permittivity e is not available in the literature and its value
(~10) was determined independently from the dielectric
measurement. The extrapolation of the fitted line at C2 ¼ 0
corresponds to the potential Vfb from which a value of 0.15 V was
determined. This value is very close to the potential Von and this
indicates a quasi-absence of surface states within the gap.
The density NA, evaluated from the slope, was found to be
5.8 1017 cm3 in agreement with that determined from the
thermopower (2.8 1017 cm3). The position of VB is given by
P ¼ eVfb þ DE þ eVh þ 4:74 ¼ 5:1870:05 eV
ð7Þ
This value is typical of oxides in which VB derives from Cu-3d
character, instead O2; 2p orbital as is the case for most oxides.
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B. Bellal et al. / Journal of Physics and Chemistry of Solids 70 (2009) 1132–1136
confirmed by the semilogarithmic plot. LaCuO2 can be made
p-type conductor by hosting oxygen in the two-dimensional
lattice followed by oxidation of copper to maintain the charge
neutrality. The optical transition is directly allowed and the
valence band is made up of Cu-3d wave function. The results of
Mott–Schottky plot and the cyclic voltammetry showed that the
electrochemical response of the moderately doped electrode is
characteristic of a classical semiconductor. LaCuO2 will serve as a
starting material for further heat and chemical treatments to
synthesize two new phases to be used for H2 photogeneration.
5
10-10 C-2 (F-2 cm4)
4
3
2
Acknowledgements
1
Vfb = 0.15 V/sce
0
0.00
0.04
0.08
0.12
0.16
V (V/sce)
Fig. 9. Mott–Schottky plot of LaCuO2.
Hence CB (7.95 eV/3.2 V/s) is positioned far below the H2O/H2
level and this should lead to a spontaneous H2-evolution in strong
alkaline media. The PEC field is open to further explorations into
the photointercalation mechanism. LaCuO2 will serve as a starting
material to prepare a new copper lanthanum oxide hardly
distinguishable trough XRD patterns from those prepared previously; the results are under way and will be reported in the next
paper.
4. Conclusion
Delafossite LaCuO2 of high quality has been prepared by direct
reaction in silica tubes. LaCuO2 is a low-mobility, hopping-type
semiconductor with a hole mobility thermally activated. The
electropositive character of lanthanum leads to a stronger
covalency of Cu–O bond resulting in a destabilization of
antibonding CB of hybridized Cu-3d/4s orbital and an increase
of Eg-value. The magnetic susceptibility comes from localized Cu2+
moments. The low deviation from stoichiometry results from a
slight variation in the transport properties. The material possesses
a high degree of corrosion resistance in aqueous electrolytes
This work was financially supported by the Faculty
of Chemistry (Algiers). The authors would like to thank
Mr. D. Bouteldja for his technical assistance and are also grateful
to Ms. S. Omeiri for her help and fruitful comments.
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