Structural and electrochemical characterization of metallo-porphyrins intercalated into ZnCr-layered double hydroxides: Some evidence of dimer formation

Article abstract of DOI:10.1039/c1nj20400a

In this study, inorganic-organic hybrids were prepared by intercalation of metal complexes (Zn and Fe) of 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin (TSPP) into Zn₂Cr layered double hydroxides (LDH) by coprecipitation method at constant pH. The resulting materials were characterized by powder X-ray diffraction, UV-Vis and FTIR spectroscopies showing the intercalation of porphyrins between the ZnCr layers. While monomer species are intercalated in the case of Zn₂Cr-ZnTSPP, the intercalation of dimers is likely to occur in the case of Zn₂Cr-FeTSPP. In both materials, a perpendicular orientation of the porphyrin ring against the hydroxide layers is proposed. Of particular interest is the good stability of ZnCr-LDH host over a wide pH range. The electrochemical behavior of these hybrid materials in both neutral (pH 7.0) and acid media (pH 4.5) was investigated by cyclic voltammetry. Zn ₂Cr-FeTSPP exhibits an electrocatalytic activity for the reduction of oxygen, hydrogen peroxide and nitrite.

Full text of DOI:10.1039/c1nj20400a

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PAPER
Structural and electrochemical characterization of metallo-porphyrins
intercalated into ZnCr-layered double hydroxides: some evidence
of dimer formationw
a
b
a
a
´
Christine Taviot-Gueho, Matilte Halma, Khaled Charradi, Claude Forano and
Christine Mousty*a
Received (in Montpellier, France) 9th May 2011, Accepted 21st June 2011
DOI: 10.1039/c1nj20400a
In this study, inorganic–organic hybrids were prepared by intercalation of metal complexes
(Zn and Fe) of 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin (TSPP) into Zn2Cr layered
double hydroxides (LDH) by coprecipitation method at constant pH. The resulting materials
were characterized by powder X-ray diffraction, UV-Vis and FTIR spectroscopies showing the
intercalation of porphyrins between the ZnCr layers. While monomer species are intercalated
in the case of Zn2Cr-ZnTSPP, the intercalation of dimers is likely to occur in the case of
Zn2Cr-FeTSPP. In both materials, a perpendicular orientation of the porphyrin ring against the
hydroxide layers is proposed. Of particular interest is the good stability of ZnCr-LDH host over a
wide pH range. The electrochemical behavior of these hybrid materials in both neutral (pH 7.0)
and acid media (pH 4.5) was investigated by cyclic voltammetry. Zn2Cr-FeTSPP exhibits an
electrocatalytic activity for the reduction of oxygen, hydrogen peroxide and nitrite.
Functionalization of inorganic host solids by porphyrins
1. Introduction
and metalloporphyrin complexes is under intensive investigations
due to their unique catalytic, photochemical and electro-
chemical properties, which make porphyrin based hybrid
materials potential candidates for applications in the develop-
ment of novel catalysts,5 light-harvesting photovoltaic systems,6
optoelectronic devices,7 and sensors.8
Layered double hydroxides (LDH) are synthetic solids with
positively charged brucite-like layers of mixed metal hydroxides
separated by interlayer hydrated anions, defined by the general
III
formula [MII
M
1ꢀx
x(OH)2]x+ [(Anꢀ
)x/nꢁyH2O] (abbreviated
as MIIIMII-A, where MII and MIII are respectively divalent
and trivalent metals, R corresponds to the MII/MIII mole ratio
(R = (1 ꢀ x)/x), and Anꢀ is the interlayer anion compensating
the positive charge of the metal hydroxide layers). The increasing
interest in LDH as host matrix arises from their versatile
properties in terms of chemical composition of both layer
and interlayer, their high and tuneable layer charge density
resulting in adaptable anion exchange capacity. Hence, these
materials are often used as immobilization matrices of organic
anions, macromolecules or biomolecules.1–3 These organic-
inorganic hybrid materials have been developed for various
application fields,4 including pharmaceutical, cosmetic, sensors,
photoreactive devices, etc.
In 1989, Park et al. have prepared for the first time a
porphyrin-LDH intercalation compound.9 Since then, several
publications have been focused on the intercalation of metallo-
porphyrins, bearing anionic groups, particularly in LDH hosts,
pointing out the advantages of this type of hybrid materials for
oxidation catalysis10–14 and photochemistry.15–20 Hence various
free bases and metal-coordinated porphyrins were immobilized
in LDH, such as 5,10,15,20-tetrakis(carboxyphenyl)porphyrins
(pTCPP and oTCPP),14,21 5,10,15,20-tetrakis(4-sulfonatophenyl)-
porphyrin (TSPP17,22) ant its metal complexes (ZnTSPP,23
PdTSPP,16,24
MnTSPP,13,25,26
CoTSPP,27
FeTSPP,14,28
OQTiIVTSPP18), iron(III) complexes of 5,10,15,20-tetrakis(2,6-
difluoro-3-sulfonatophenyl)porphyrin (FeTDFSPP), 5,10,15,20-
tetrakis(2,6-dichloro-3-sulfonatophenyl)porphyrin (FeTDCSPP)
and 5,10,15,20-tetrakis(2-chloro-6-fluoro-3-sulfonatophenyl)-
porphyrin (FeTCFSPP).12,29
a
´
Clermont Universite, Universite Blaise Pascal, Laboratoire des
Materiaux Inorganiques, CNRS UMR 6002, F-63171 AUBIERE,
´
´
France. E-mail: Christine.Mousty@univ-bpclermont.fr;
Fax: +33 473407108; Tel: +33 473407598
The LDH host compositions used for the intercalation of
porphyrins have so far mostly been MgAl and ZnAl.23 The
intercalation of these negatively charged macrocycles into the
LDH structures is generally achieved by conventional anion
exchange of pre-formed LDH,9,26,30 calcination/restacking
method,12,13,31,32 direct coprecipitation procedure.11,18,21,23
b Universite´ Joseph Fourier, De´partement de Chimie Mole´culaire,
CNRS UMR 5250, F-38041 GRENOBLE, France
w Electronic supplementary information (ESI) available: UV-Vis spectra
of FeTSPP in solutions; X-ray powder diffraction patterns in the 2y
range 5–651 (inset 1–301) of Zn2Cr-ZnTSPP and Zn2Cr-FeTSPP; FTIR
spectra of Zn2Cr-ZnTSPP and Zn2Cr-FeTSPP and cyclic voltammo-
grams in different buffer solutions. See DOI: 10.1039/c1nj20400a
c
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More sophisticated methods using exfoliated hybrid LDH
phases (MgAl-glycinate and MgAl-dodecylsulfate) were also
reported for exchange or chemical grafting reactions with
porphyrins.33,34 Moreover, nano-ordered hybrid films of
LDH nanosheets, TCPP and pyrenetetrasulfonic acid (PTSA)
were prepared by a layer-by-layer deposition method.35
The interlayer confinement of porphyrinic macrocycles has
been investigated as regard to guest ordering, microporosity
and inner site accessibility, three key structural properties that
determine material performances. It has been reported
that free base porphyrins are intercalated with a selective
orientation depending on the isomeric position of the anionic
groups, pTCPP versus oTCPP for instance.21 A perpendicular
arrangement of macrocycles with respect to the hydroxide layer
is obtained with the para carboxylate and para sulfonate tetra-
phenyl porphyrinic guest molecules.11 More recently, molecular
modeling coupled with powder X-ray diffraction (XRD) has
suggested an alternation of nitrate and porphyrin containing
interlayer domains for partially exchanged MgAl-NO3/TSPP
samples.22 The [TSPP]4ꢀ anions are tilted with respect to the
hydroxide layer of about 50–601.
porphyrin (FeTSPP) was obtained by iron insertion into the
free-base porphyrin ligand using ferrous chloride tetrahydrate
in DMF. Purification was performed by colum chromato-
graphy on sephodex, using deionized water as eluent.12
Zinc(II) porphyrin (ZnTSPP) was prepared by stirring pTSPP
with an equimolar amount of Zn2+ for 1 h.23
Hybrid LDH materials, noted as Zn2Cr-ZnTSPP and
Zn2Cr-FeTSPP, were prepared by coprecipitation by adapting
the method for small quantities of materials. Typically, a mixed
aqueous solution of Zn(NO3)2ꢁ6H2O and Cr(NO3)3ꢁ9H2O,
with a Zn2+/Cr3+ molar ratio R = 2, and a total concen-
tration of metallic cations of 0.1 M, was introduced with a
constant flow (0.007 mL minꢀ1) into a reactor containing
metalloporphyrins (ZnTSPP or FeTSPP), noted MTSPP here-
after, in aqueous solution stirred under nitrogen atmosphere.
The amount of MTSPP corresponded to twofold the number
of mole required for a stoichiometric 100% exchange reaction
(6–8 mM in 10 ml). The pH was maintained constant during
the coprecipitation process at a value of 6.25 by simultaneous
addition of a 0.2 M NaOH solution. After an aging time of
24 h, Zn2Cr-MTSPP solid products were recovered by centri-
fugation and washed twice with decarbonated water.
Electrochemical behaviour of metalloprophyrins has also
been widely investigated because of their rich redox and
electrocatalytic properties.8,36 Such electrocatalytic properties
associated with the possibility of immobilizing on different
matrices have been exploited for the development of ampero-
metric and voltammetric sensors.37,38 However, to the best
of our knowledge, only two papers have been devoted to the
electrochemical study of porphyrins immobilized on LDH. So
far in 1993, Gaillon et al. were the first to study the electro-
chemical behaviour of MgAl-MnTSPP.25 More recently,
ordered ultrathin films were built on electrode surfaces by
the layer-by-layer self-assembled technique using FeTSPP and
CoAl LDH nanosheets.28 These modified electrodes exhibit
electrocatalytic behaviour with hydrogen peroxide. Finally,
Shich et al. in 2002 suggested that an electron transfer between
interlayer FeIII cations of MgFe-LDH and intercalated
CoTSPP could occurred spontaneously during the exchange
process.27
It is noteworthy that aqueous solutions of FeTSPP are
characterized by a pH-dependent equilibrium of monomers
and dimeric species.39–42 Indeed, this metalloporphyrin exists
as monomers in acidic solution and may form m-oxo dimers in
basic solutions with an apparent pK value of 7.8.42 The
equilibrium, which depends on the pH of the solution but also
on its ionic strength and on the porphyrin concentration, can
be written as follows:
2[Fe(H2O)TSPP]3ꢀ $ [TSPPFe-O-FeTSPP]8ꢀ + H2O + 2H+
(1)
where iron(III) is pentacoordinated in a high spin state. One
can also note that hexacoordinate iron Fe(H2O)2TSPP or
intermediate hydroxo species [Fe(OH)nTSPP]4ꢀ with n = 1
or 2 have been also reported.40–42 Since the coprecipitation of
Zn2Cr-FeTSPP LDH was carried out at pH 6.25 in an
aqueous solution of 8 mM FeTSPP, the formation of such
dimeric forms is highly probable. Therefore in order to specify
species present in the solution under our synthesis conditions,
the UV-Vis behaviour of an aqueous solution of FeTSPP
(60 mM) was examined as a function of the pH and the ionic
strength (see ESI, Fig. ESI1w). In water at pH 6.3, the
maximum adsorption is at 394 nm with a shoulder at 415 nm
(Soret band) and the smaller bands are at 500 and 529 nm
(Q bands). In 0.1 M buffer solutions, at pH 6.25, we observed
a red shift of the bands while at pH 7.5, the formation of the
deep green m-oxo dimer was observed characterized by a Soret
band at 407 nm and Q bands at 566 and 610 nm, as reported in
the literature.42 In the copreciptation medium, the porphyrin
concentration is ca. 130 times higher as compared to the
concentration used for these above UV-Vis measurements.
This concentration effect together with the increase in salt
concentration during the coprecipitation process are expected
to shift equilibrium (1) further towards dimer formation.40,41
Moreover, [TSPPFe-O-FeTSPP]8ꢀ exhibits a high negative
charge compared to [Fe(H2O)TSPP]3ꢀ which may also be in
In the present work, we describe the synthesis, the struc-
tural characterization and the electrochemical behaviour of
Zn2Cr-ZnTSPP and Zn2Cr-FeTSPP hybrid LDH materials.
The intercalation of porphyrins in ZnCr-LDH has never been
reported so far. Although, the particularly good stability of
Zn2Cr-LDH host over a wide pH range allows electrochemical
applications of these hybrid materials in both neutral and acid
buffer solutions. Electrocatalytic properties of Zn2Cr-FeTSPP
for the reduction of oxygen, hydrogen peroxide and nitrite
were investigated by cyclic voltammetry.
2. Experimental
2.1. Material
Free base 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin (TSPP)
was purchased from Sigma-Aldrich. Hydrogen peroxide
(H2O2) solutions (from Sigma-Aldrich) were freshly prepared
before being used. Na2HPO4ꢁ10H2O, NaH2PO4, Zn(NO3)2ꢁ6H2O
and Cr(NO3)3.9H2O were purchased from Acros. Iron(III)
c
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sample holder and the measurements were performed
in continuous scanning mode with the following setting
conditions: beam mask = 10 mm, divergence slits = 1/321,
detector active length = 2.1221, scan range = 5–701, steps size =
0.01671 and counting time = 850 s. Profile matching
refinements were carried out with the Fullprof program43
using the pseudo-Voigt profile function of Thompson, Cox
and Hastings,44 and included variation of lattice para-
meters and peak shape parameters. A LaB6 standard was
used to correct for instrumental broadening (U = 0.008(1),
V = -0.009(1), W = 0.0036(5), Y = 0.033(2)). Owing to the
platelet-like morphology of LDH particles, peak broadening
was mainly attributed to anisotropic size effects; it was
modeled in terms of spherical harmonics allowing the calcula-
tion of the coherent domain apparent size along different [hkl]
crystallographic directions.45
FTIR-KBr spectra were recorded on a Nicolet 5700 (Thermo
electron corporation) spectrometer. UV-Visible spectra were
recorded using a Shimadzu UV-Vis 2001 PC spectrometer
equipped with an integrating sphere. The samples were prepared
as thin films on quartz plates following the same procedure as
used to prepare thin films on electrodes.
Cyclic voltammetry experiments were carried out with a
potentiostat Autolab PGSTAT100 connected to a cell (50 ml)
with a three-electrode system, including an Ag/AgCl reference
electrode, a platinum auxiliary electrode and a glassy carbon
disk electrode (GCE) modified with LDH films as working
electrode. All electrochemical experiments were performed in
0.1 M phosphate buffer solution (PBS pH 7 and 4.5) degassed
by bubbling with argon for at least 30 min before starting the
measurements.
Fig. 1 Results of the profile analysis of XRPD patterns for
(a) Zn2Cr-ZnTSPP and (b) Zn2Cr-FeTSPP: experimental X-ray diffrac-
tion (dot), calculated (line), difference (bottom) and ticks giving the
3. Results and discussion
%
position of Bragg reflections in the space group R3m. Profile para-
meters of the TCH function are for (a) Zn2Cr-ZnTSPP: isotropic size
parameter of Gaussian character (GauSiz = 0.056(1) and anisotropic
size broadening described using IsizeModel16 (spherical harmonics
coefficients: Y00 = 15.3(4), Y40 = ꢀ19.8(8), Y60 = 11.4(4),
Y66+ = 1.9(1)) and for (b) Zn2Cr-FeTSPP (GauSiz = 0.530(7);
Y00 = 9.3(4), Y20 = ꢀ1.6(5), Y40 = ꢀ21.1(8), Y43- = ꢀ1.7(4),
Y60 = 20.3(7), Y63 = ꢀ1.1(3), Y66+ = 5.3(3)). Inset: One-dimensional
electron density rc projection along the c-axis for Zn2Cr-ZnTSPP.
3.1. Structural characterization
It has been demonstrated many times that the coprecipitation
method, by involving the in situ formation of LDH layers
around species we wish to intercalate, facilitates the effective
insertion of bulky porphyrin anions.11,18,19,21,23 In the present
case, the composition of LDH host based on Zn2+ and Cr3+
is peculiar for two reasons. First, the synthesis can be achieved
at low coprecipitation pH; second, it occurs for a unique molar
ratio equal to 2 whatever the proportion of di- and trivalent
cations in the starting solution. This unique ratio is in favour
of an ordered cation distribution within Zn2Cr hydroxide
layers as discussed by several authors.46 This interlayer metal
cation ordering is rather in favour to the ordering of the
interlayer anions.
favour of the intercalation of such dimers. In the present work, we
will use notations Fe(H2O)nTSPP or O(FeTSPP)2 for monomer
or dimer species where the ionic charges have been omitted.
LDH-modified electrodes were prepared as films on a glassy
carbon electrodes (A = 0.07 cm2). Before use, the glassy carbon
electrodes were polished with alumina slurry paste (0.05 mM)
and rinsed with ethanol and distilled water. Hybrid LDH
suspensions (4 mg mlꢀ1) were stirred under the darkness over-
night before use. Aliquots (10 ml) of LDH samples were spread
on the electrode surface and dried for 4 h.
The XRD patterns of Zn2Cr-ZnTSPP and Zn2Cr-FeTSPP
%
samples were indexed in the rhombohedral space group R3m,
typical for LDH systems (see ESI, Fig. ESI2w). Although
Zn2Cr-FeTSPP appears less crystalline than Zn2Cr-ZnTSPP,
the position of the 003n basal lines is similar in both compounds
indicating similar interlayer distances. Note that the lower
signal-to-noise ratio seen in the case of Zn2Cr-FeTSPP may
also be attributed in part to iron fluorescence. The cell para-
meters given on Fig. 1(a) and (b) were obtained from the peak
profile analysis and it should be noted that the spherical
2.2. Instrumentation
Powder X-ray diffraction patterns were recorded on X’Pert
Pro diffractometer in Bragg Brentano geometry equipped with
a X’-Celerator Scientific-RTMS detector and using Cu-Ka1/
a1 radiation. The samples were placed on a Si low background
c
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harmonics correction for an anisotropic peak broadening due
to size effects was essential to reach a good fit. The values
of the cell parameter a are identical (a = 3.1058(2) A for
Zn2Cr-ZnTSPP and a = 3.1057(2) A for Zn2Cr-FeTSPP) and
in total agreement with the formation of Zn2Cr LDH host.46
The values of the cell parameter c are also quite similar
(c = 69.21(1) A for Zn2Cr-ZnTSPP and c = 69.79(1) A for
Zn2Cr-FeTSPP) leading to an interlayer distance (d003) of
23.07 A for Zn2Cr-ZnTSPP and 23.26 A for Zn2Cr-FeTSPP
very close to the values reported for Zn2Al-ZnTSPP23
(23.00 A) and Zn2Al-FeTSPP11 (22.95 A) and thus consistent
with the intercalation of the porphyrin molecules with the ring
oriented perpendicularly with respect to the hydroxide layers.
The relatively good crystallinity of Zn2Cr-ZnTSPP allowed
us probing the structure of the interlayer space via the Fourier
transform of the 00l X-ray reflection intensities47 and giving
the one-dimensional electron-density distribution along the
c-stacking axis presented in the inset of Fig. 1(a). This 1D
plot is similar to that reported elsewhere for Zn2Al-ZnTSPP23
with the most intense peaks corresponding to the electron
densities of the hydroxide layers, the maxima at the outer parts
of the interlayer space are due to the sulfonate groups together
with water molecules while the central maxima and the two
peaks on each side arise from the porphyrin ring and zinc atom
at the centre.
When considering previous studies on FeTSPP intercalation
into LDH system using the coprecipitation method, it is
remarkable that whatever the MII/MIII molar ratio (2 r R r 4)
used during the synthesis, chemical and structural analyses of
synthesized materials all display a R value close to 2.11,12
Owing to the coprecipitation pH values applied (7.5 for
ZnRAl-LDH), one can reasonably assume in all cases the
intercalation of O(FeTSPP)2 dimers. By considering the
Fe–O–Fe distance reported for O(FeTPP)2, i.e. 3.516 A with
Fe–O–Fe angles close to linearity 1761,48,49 one can estimate
the surface area per unit charge (S0) for a O(FeTSPP)2 dimer,
assuming an approximate parallelepiped shape of about
16.0 ꢂ15.8 ꢂ 4.3 A for FeTSPP macrocycle (as obtained from
steric energy minimization using the MM2 method50) and a
perpendicular orientation of the porphyrin ring with respect to
LDH hydroxide layers. The value thus obtained equals 15.7 A2/eꢀ.
The comparison with S0 values of Zn2Cr (25.3 A2/eꢀ) or
Zn2Al (25.0 A2/eꢀ) LDH hosts indicates a good surface
compatibility.
Regarding Zn2Cr-ZnTSPP, we cannot either exclude the forma-
tion of aggregates, particularly face-to-face dimers (H-aggregates),
although UV-Vis spectra are weakly resolved (videinfra). Such
a self-association is a frequently encountered phenomenon in
porphyrin chemistry owing to intermolecular Van der Waals-like
attractive forces between the molecules. This type of bonding
being much weaker/longer and more flexible than bridging
ligands certainly makes the structural assembly process with
LDH host layers easier whatever the charge density of the
hydroxide layers, as evidenced here by the high crystallinity of
Zn2Cr-ZnTSPP and that of ZnRAl-ZnTSPP samples reported
elsewhere.23
As said before, good profile refinements were obtained by
assuming anisotropic broadening due to size effects. The broad
features displayed by Zn2Cr-FeTSPP suggest the presence of
very small coherent domain sizes. From the refined spherical
harmonics coefficients, it is possible to determine the volume-
weighed domain size Dhkl in the direction parallel to the
scattering vector qhkl. Related to the platelet-like morphology
of LDH particles, we focused on two directions [003n] and
[110] indicative of the stacking thickness of LDH sheets
and the platelet dimension in the plane, respectively. The
lengths thus calculated were found to be 3/2 times larger for
Zn2Cr-ZnTSPP (thickness D003n of 120 A and in-plane dimension
D110 of 135 A) than for Zn2Cr-FeTSPP (D003n = 75 A and
D110 = 105 A). One can also calculate the number of stacked
interlayers D003n/d003 leading to a value close to 5 in the case of
Zn2Cr-ZnTSPP and 3 for Zn2Cr-FeTSPP.
The presence of the organic anions within the LDH struc-
ture was confirmed by FTIR (see ESI, Fig. ESI3w) and UV-Vis
spectroscopies. Fig. 2 compares the absorption spectra of
free porphyrins (curves a and c) with those of intercalated
complexes (curves b and d), prepared as thin films on quartz
plates. We have verified that Zn2Cr-LDH film does not absorb
in the analyzed wavelength range (not shown). Spectra of
solid metallo-porphyrins exhibit a strong adsorption band
(Soret band) and only two Q bands. They are situated at
424, 561 and 603 nm for ZnTSPP and at 408, 578 and 621 nm
for FeTSPP, as reported in the literature.11,23 As previously
This lower coherency for Zn2Cr-FeTSPP is likely to result
from structural strains accompanying FeTSPP intercalation.
This idea of strain effect is reinforced by the fact that it
was necessary to refine both Lorentzian and Gaussian size
broadening to accurately model the XRD peak shape. In the
case of Zn2Cr-FeTSPP, the isotropic Gaussian size parameter
was found 10 times larger than for Zn2Cr-ZnTSPP. Such a
mixed Gaussian and Lorentzian character more marked for
Zn2Cr-FeTSPP suggests the presence of both strain and size
effects and we believe these strains are the result of the struc-
tural accommodation of aggregates oxo bridged diiron(III)
complexes within the interlayer space of Zn2Cr LDH. Indeed
as discussed above in the experimental part, the dimerisation
equilibrium of FeTSPP in aqueous homogeneous solution is
expected to occur in the pH range 4–8.40,42 It is interesting to
note the synthesis pH of Zn2Cr-FeTSPP i.e. 6.2 falls within
this pH range. Accordingly, intercalated species in the as-prepared
Zn2Cr-FeTSPP sample are likely to be O(FeTSPP)2 dimers.
Fig. 2 Normalized UV-Vis spectra of thin films of ZnTSPP
(a), Zn2Cr-ZnTSPP (b) FeTSPP (c) and Zn2Cr-FeTSPP (d).
c
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observed in aqueous solution, the peak positions, reported
for thin film prepared from aqueous solution of FeTSPP,
do not correspond to the free monomer Fe(H2O)nTSPP in
aqueous solution (Soret band at 394 nm and Q bands at
503 and 531 nm)39,42
A broadening of Soret bands is observed when porphyrins
are intercalated in LDH interlayer, which is generally attributed
to some degree of aggregation and stacking of the porphyrin
molecules.11 Zn2Cr-ZnTSPP spectrum shows an intensive and
broad Soret band centered at ca. 421 nm and two Q bands
at 557 and 600 nm in agreement with ZnTSPP formation
(Fig. 2, curve b). An additional absorption band less intense is
observed at ca. 499 nm. It is not expected for monomeric
ZnTSPP nor uncomplexed TSPP but more probably to
J-aggregates of H4TSPP.51Moreover for ZnTSPP, the Soret
band broadening was less noticeable with Zn2Al-ZnTSPP,23
suggesting that this porphyrin is more favorable to aggregate
when it is intercalated in Zn2Cr-LDH than in Zn2Al-LDH
host structure. These results may be related to the cationic
order in Zn2Cr hydroxide layers strongly suggested by several
authors. However it is difficult to determine unequivocally the
nature of aggregates. The complex broad Soret band of
Zn2Cr-ZnTSPP is probably related to the presence, in different
ratio, of H-aggregates (414 nm) and monomers (422 nm).42
For Zn2Cr-FeTSPP, the Soret band is centered at ca. 415 nm
and Q bands are situated at 575 and 619 nm (Fig. 2, curve d).
As previously discussed, owing the coprecipitation pH value of
Zn2Cr-FeTSPP (pH 6.2), we can assume that the predominant
intercalated species are oxo-bridged dimers O(FeTSPP)2,
corresponding to a red shift Soret band. This hypothesis seems
to be confirmed with the presence of a vibration band at
873 cmꢀ1 corresponding to the Fe–O–Fe antisymmetric
stretching39,49 (see ESI, Fig. ESI3w).
Fig. 3 Cyclic voltammograms recorded in the potential range between
ꢀ1.0 and 1.0 V using Zn2Cr-Cl/GCE (a) and Zn2Cr-ZnTSPP/GCE at
pH 7.0 (b) and pH 4.5 (c), inset shows the plot of the first anodic peak
current vs. Ov (d) (0.1 M PBS under argon, n = 50 mV sꢀ1).
Fig. 4 Cyclic voltammograms of Zn2Cr-FeTSPP/GCE recorded at
pH 4.5 in the potential range between ꢀ1.0 and 1.0 V (a) or in the
cathodic potential range (b), at pH 7.0 (c), inset shows the plot of Fe anodic
peak current vs. Ov (d) (0.1 M PBS pH under argon, n = 50 mV sꢀ1).
3.2. Electrochemical characterization
depending on the nature of axial ligands on metal.36 Indeed,
the equilibrium distribution of porphyrins species depends on
the pH, with a predominant concentration of Zn(H2O)2TSPP,
Zn(H2O)(OH)TSPP and Zn(OH)2TSPP at pH 5.2, 6.2 and 8.5,
respectively.54 Moreover as suggested by Su et al., the radical
cation Zn[TSPP+ꢃ], which can be also formed by a dismutation
reaction between the dication Zn[TSPP2+] and the reduced
macrocycle Zn[TSPP], is particularly stable at pH 4.53 This
fact can explain the enhanced reversibility of the first electron
transfer at pH 4.5. The current intensity of the first anodic
peak is proportional to the square root of the scan rate (Ov) in
the 50–400 mV sꢀ1 range, indicating that the oxidation of
[TSPP] macrocycle inside the LDH host is governed by a
diffusional process, in which electrolyte ions play a key role in
the charge balancing effect52,55,56 (Fig. 3d). The number of
electroactive species (n*) in the film is obtained by integration
of the area under the first anodic peak recorded at slow scan
rate (n = 10 mV sꢀ1, n* E2 ꢂ 10ꢀ10 mole). This value
corresponds to the lowest value found for other LDH containing
redox active anions, such as anthraquinone sulfonates, ferrocene
sulfonates or 2,20-azino-bis(3-ethylbenzthiazoline-6-sulphonate)
(0.25–2 ꢂ 10ꢀ9 mole).52,55–57 Indeed, we have previously
shown that only 2 to 20% of intercalated anions in the
LDH-modified electrodes are electroactive, depending on their
These hybrid materials can be examined by electrochemical
methods such as cyclic voltammetry (CV). Electrochemical
behaviour of porphyrins is known to be pH dependent.
ZnCr-LDH host structure is stable in acid medium and we
may assume that no dissolution of hybrid materials occurs at
pH 4.5.52 Cyclic voltammetry curves of glassy carbon electrodes
(GCE) coated with 40 mg of ZnCr-LDH intercalated with Cl,
ZnTSPP and FeTSPP were recorded in 0.1 M PBS at pH 7 and
4.5 under argon atmosphere, as shown in Fig. 3 and 4. Under
these experimental conditions, Zn2Cr-Cl LDH is not electro-
active and the current base line is very similar to that obtained
with a bare glassy carbon electrode (Fig. 3a).
When oxidation of Zn2Cr-ZnTSPP was carried out at
pH 7, two oxidation peaks appear on the voltammogram at
+0.65 and +0.90 V (Fig. 3b). In acid medium (pH 4.5), the
electrochemical signal increases and the first peak, situated at
E1/2 = +0.60 V, becomes more reversible (Ipa/Ipc = 1.24,
DEp = 80 mV) (Fig. 3c). Since Zn cation is not electroactive,
these oxidation processes occur on the porphyrin ring, leading
to the successive formation of the radical cation Zn[TSPP+
and dication Zn[TSPP2+].53 The attenuation of cyclic voltam-
metric curves at neutral pH can be interpreted in terms of
slowing of the kinetics for heterogeneous charge transfer,
ꢃ
]
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in this dimer configuration is known to be electrochemically
silent,40,59,63 the electrochemical measurements were done at a
pH value of 4.5. Under this acidic condition, one can expect
that Fe(H2O)nTSPP species should be present in the LDH
host.42 Hence, the cyclic voltammogram displays a reversible
peak system at E1/2 = ꢀ0.24 V, which corresponds to the
FeIII(H2O)nTSPP/FeII(H2O)nTSPP redox couple as reported
for FeTSPP in aqueous acid solution (ꢀ0.23 V).40,60,61
A
further oxidation occurs at +0.90 V which corresponds to
the porphyrin ring based oxidation (Fig. 4).40,60 This oxidation
process is irreversible, probably due to the formation of
iron(III)isoporphyrin by a nucleophilic attack on the porphyrin
radical cation generated in the oxidation step.58,60 This
by-product can be reduced in the reverse scan that can explain
the higher reduction current observed in the cyclic voltam-
metry curve recorded in the large potential range (Fig. 4, curve b).
Hence, the reversibility of the FeIII/FeII electrochemical signal
increases (Ipa/Ipc = 1.2, DEp = 76 mV) when scanning only in
the cathodic potential range (Fig. 4, curve b). At pH 7, these
reversible peaks disappear but the oxidation wave remains
with a slightly lower intensity (Fig. 4, curve c). As previously
observed with Zn2Cr-ZnTSPP, this pH effect is reversible, the
same CV was recovered after cycling successively the same
modified electrode in both PBS (see Fig. ESI4B). The intensity
of oxidation peak of Fe(II) recorded at pH 4.5 increases with
Ov, the electron transfer is under diffusion control (Fig. 4d).
We have calculated the amount of electroactive species present
within the film at pH 4.5 by integration of the oxidation peak
of FeII at low scan rate (n = 5 mV sꢀ1). This amount, which
should correspond to monomer species, was found to be also
very low (n* E 1 ꢂ 10ꢀ10 mole).
Iron(III) porphyrins are known as catalysts for the electro-
chemical reduction of molecular oxygen, hydrogen peroxide
and nitrite.40,60,62,64,65 The cyclic voltammograms shown in
Fig. 5A have been recorded at Zn2Cr-FeTSPP modified
electrode in electrolyte solution (pH 4.5) saturated with argon
(curve a) or oxygen (curve b). As expected, the reversible
redox signal observed in deaerated solution was masked by
an intense catalytic redox wave under oxygen saturated
atmosphere. The intensity of this electrocatalytic wave can
be compared to the current peak corresponding to the direct
reduction of oxygen at a bare glassy carbon electrode (curve c).
Similarly when H2O2 is added in the deaerated PBS, an
increase in the reduction peak is observed (Fig. 5B).An ampero-
metric response to successive additions of H2O2 was recorded
at ꢀ0.4 V using a Zn2Cr-FeTSPP/GC rotating disk electrode
(A = 0.196 cm2). A linear variation of the reduction current
as a function of H2O2 concentration was obtained between
1 ꢂ 10ꢀ5 to 1 ꢂ 10ꢀ2 M. This shows that Zn2Cr-FeTSPP has
also a peroxidase-like activity for the reduction of H2O2.60 As
previously reported in the literature for FeTSPP dissolved in
buffer solution or immobilized in polypyrrole,40,62 the presence
of nitrite caused a drastic modification of the voltammograms
(Fig. 5C). In 0.2 mM nitrite solution the cyclic voltammogram
shows a wave for the reduction of FeIII at ꢀ0.26 V but the
return wave becomes quasi-unobservable, except at low scan
rate (n = 5 mV sꢀ1). A second reduction wave occurs at
ꢀ0.68 V and in the reverse anodic scan a new wave appears at
+0.34 V. It should be noted that there is no disproportion of
Fig. 5 (A) Cyclic voltammograms of Zn2Cr-FeTSPP/GCE recorded
under argon atmosphere (a), under saturated O2 atmosphere (b) and
bare GCE under saturated O2 atmosphere (c). (B) Cyclic voltammo-
grams of Zn2Cr-FeTSPP/GCE in the presence of 0 (a), 0.4 (b) and
0.8 mM H2O2 (c), H2O2 calibration curve recorded at ꢀ0.4 V (d).
(C) Cyclic voltammograms recorded at Zn2Cr-FeTSPP/GCE (a) and
bare GCE (b) in the presence of 0.2 mM NaNO2. (Experimental
conditions: 0.1 M PBS pH 4.5 under argon, n = 50 mV sꢀ1.)
electrochemical accessibility.52,55–57 In the present case, the
theoretical amount of intercalated porphyrin tetrasulfonate
anions is at least two or four times lower than disulfonate or
monosulfonate anions. Moreover, these macrocyclic molecules
must be in a specific coordination to be electroactive.
Finally in order to study the reversibility of this pH effect and
to confirm the good stability of the ZnCr-LDH coating in acidic
medium, the same modified electrode was tested successively in
acidic, neutral and acidic media. The Zn2Cr-ZnTSPP/GCE was
stabilized in each medium during 20 cycles (see Fig. ESI4Aw).
The first CV curve is restored by soaking back the electrode in
PBS at pH 4.5. Moreover, this signal remains unchanged after a
further 2 h soaking in PBS pH 4.5.
The electrochemical behaviour of free or immobilized FeTSPP
is known to be pH dependent depending on the nature axial
ligands H2O, OHꢀ, O2ꢀ on the metal.40,58–63 From DRX and
UV-Vis measurements, we can assume that oxo-bridged dimers
O(FeTSPP)2 are present within the interlayer space of
Zn2Cr-LDH host matrix (vide supra). Since iron complexed
c
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nitrite at pH 4 4 and cyclic voltammogram recorded at a bare
glassy carbon electrode shows a negligible reduction current in
nitrite solution (Fig. 5C, curve b). This suggests that nitrite
may interact with intercalated FeTSPP. This phenomenon is
reversible, since when the same electrode was cycled in buffer
solution, the reversible FeIII/FeII signal appears again. As
reported in the literature, interaction between nitrite and iron
porphyrins exists through the reduction of metal centre followed
by a multi-electron catalytic process toꢀform iron-nitrosyl inter-
mediates (FeII[NOꢃTSPP] and FeII[NO TSPP]), the anodic peak
at 0.34 V corresponding to FeII[NOꢃTSPP]/ FeII[NO+TSPP]
oxidation40,62,65
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Metallo-porphyrin anionic complexes ZnTSPP and FeTSPP
were successfully intercalated between Zn2Cr-LDH layers
using coprecipitation method at constant pH 6.2. Careful
structural and electrochemical characterizations of hybrid
materials were conducted, showing the intercalation of guest
molecules as a H aggregates in the case of Zn2Cr-ZnTSPP
and an oxo-bridged diron(III) complexes in the case of
Zn2Cr-FeTSPP. While a small contribution from monomers
(1%) can be taken into account in the electrochemical responses
at pH 4.5. In both cases, we observed a perpendicular arrange-
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Finally, iron species in Zn2Cr-FeTSPP exhibit an electrocatalytic
activity for the reduction of oxygen, hydrogen peroxide and
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at low pH, will find applications as sensors or photocatalysts.
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programs. We thank N. Caperaa for her technical assistance,
S. Therias (LPMM UMR 6505, Clermont-Ferrand) for experi-
mental facilities and K. Lang, Institute of Inorganic Chemistry
of the Academy of Sciences of the Czech Republic, for fruitful
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