Newly developed electrochemical synthesis of Co-based layered double hydroxides: Toward noble metal-free electro-catalysis

Article abstract of DOI:10.1039/c8ta11812d

A novel and enhanced electrosynthesis protocol that allows the deposition of thin films of Co/Al and Co/Fe layered double hydroxides (LDHs) on different supports is hereby proposed. The approach is based on potentiodynamic cathodic reduction. All the films have been characterised by cyclic voltammetry, powder X-ray diffraction, scanning electron microscopy, and Raman and atomic emission spectroscopies. Moreover, LDHs electrosynthesised on carbonaceous materials have also been investigated by X-ray absorption spectroscopy to analyse their local metal structure. These substrates are particularly interesting for the plethora of LDH applications ranging from energy storage, sensing, and electrocatalysis, and are also of industrial importance, due to their low cost, ecocompatibility, and easy handling. In particular, the material was exploited for 5-(hydroxymethyl)furfural (HMF) electro-oxidation.

Full text of DOI:10.1039/c8ta11812d

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Gualandi, E. Scavetta, A. RIVALTA, E. VENUTI, M. Christian, V. Morandi, A. Mullaliu, M. Giorgetti and D.
Tonelli, J. Mater. Chem. A, 2019, DOI: 10.1039/C8TA11812D.
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January 2016 Pages 1–330
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DOI: 10.1039/C8TA11812D
Novel and reproducible protocol allows the electro-synthesis of LDHs for electro-catalytic
application.

J Po lue ran sael od fo Mn aot te rai ad l js u Cs ht emm ai rs gt ri yn sA
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DOI: 10.1039/C8TA11812D
Journal Name
ARTICLE
Newly developed electrochemical synthesis of Co-based Layered
Double Hydroxides: toward noble metal-free electro-catalysis
a
a
a
a
a
Elisa Musella , Isacco Gualandi , Erika Scavetta , Arianna Rivalta , Elisabetta Venuti , Meganne
b
b
a
a
a
Received 00th January 20xx,
Accepted 00th January 20xx
Christian , Vittorio Morandi , Angelo Mullaliu , Marco Giorgetti *, Domenica Tonelli *
A novel and enhanced electrosynthesis protocol that allows the deposition of thin films of Co/Al and Co/Fe layered double
hydroxides (LDHs) on different supports is hereby proposed. The approach is based on a potentiodynamic cathodic
reduction. All the films have been characterised by CVs, powder X-ray diffraction, scanning electron microscopy, and Raman
and atomic emission spectroscopies. Moreover, the LDHs electrosynthesised on carbonaceous materials have been also
investigated by X-ray absorption spectroscopy to analyse their local metal structure. These substrates are particularly
interesting for the plethora of LDHs applications ranging from energy storage, sensing, electrocatalysis, also of industrial
importance, due to their low cost, ecocompatibility, and easy handling. In particular, the material was exploited for the 5-
DOI: 10.1039/x0xx00000x
www.rsc.org/
(hydroxymethyl)furfural (HMF) electro-oxidation.
1
5
coatings, even if the signal was not always reproducible. More
1
6
Introduction
recently, Tonelli’s group
has demonstrated that
a
potentiodynamic approach for synthesising Ni-based LDHs is
the most satisfying in terms of reproducibility if compared with
the potentiostatic, and galvanostatic methods.
In this work, a study aimed at proposing a novel and alternative
potentiodynamic electrosynthesis of Co/Al-NO
Layered double hydroxides (LDHs) are lamellar compounds with
chemical formula [M(II)1−xM(III)
x+
x/n-
x
(OH)
2
] (A
n
2
) × mH O, where
n−
M(II) and M(III) are metal cations and A is the interlayer anion.
1
3 3
and Co/Fe-NO
The LDH properties can be adapted by changing their chemical
LDH is reported.
composition: a wide range of metal cations can be used as M(II)
2
+
2+
2+
2+
2+
2+
3+
3+
3+
The electrodeposition of thin films with a Co(II)/M(III) molar
ratio of 3:1, which reflects the electrolytic bath composition, is
described. Moreover, to verify the versatility of the proposed
synthetic protocol, the synthesis has been carried out with two
different supports, platinum and Grafoil (a graphitic-like
material). Platinum supports have been chosen as reference for
the deposition, since Pt was the noble metal previously used to
study the electrodeposition mechanism. Grafoil, instead, was
chosen since it can be considered a more versatile substrate to
be used in a wide range of applications: it is low cost, flexible
and environmentally-friendly; these features make Grafoil an
optimal candidate for catalytic applications. The films obtained
on the electrodes were characterised by cyclic voltammetry
(CV), microwave plasma atomic emission spectroscopy (MP-
AES), Powder X-ray diffraction (PXRD), Raman spectroscopy,
scanning electron microscopy (SEM) and X-ray absorption
spectroscopy (XAS).
(
Mg , Ni , Co , Cu , Mn or Zn ) and M(III) (Al , Fe , Cr or
3
+
Ga ), and different anions can be intercalated in the interlayers
2
to control the distance between the brucite sheets. Recently,
these compounds have been found to be extraordinary
materials for electrochemical technologies, for instance in the
field of energy storage devices , as electrode coatings for
amperometric or potentiometric sensors and as catalysts for
3
-5
6
-8
1
7
9
-11
water splitting reactions.
In order to modify electrode surfaces with LDH films, different
methods have been proposed; the most common one consists
on synthesizing the LDH through a co-precipitation method and
then depositing a fixed amount of a colloidal solution onto the
support by drop casting. A drawback of this method is the poor
1
2
adhesion of the film to the support material. An alternative,
cheap and fast route was proposed by Scavetta et al.
1
3,14
in
2
004 consisting in the electrosynthesis of LDH thin films through
a potentiostatic reduction of nitrates which leads to hydroxide
ions generation. The great advantage of the proposed method
is the short time for the electrode fabrication, the well-adhered
Finally, among all the possible applications above mentioned,
these modified electrodes were tested for the 5-
hydroxymethyl)furfural (HMF) electro-oxidation. Nowadays,
(
HMF molecule is considered a fundamental platform chemical
a. Dipartimento di Chimica Industriale ‘Toso Montanari’, Università di Bologna,
Viale Risorgimento 4, 40136 Bologna, Italy
Istituto per la microelettronica e i microsistemi (IMM), Consiglio nazionale delle
18
,
i.e., it is a key precursor for a great number of chemicals
1
9,20
b.
which find application in fuel and polymer industry.
ricerche, Via Gobetti 101, 40129 Bologna, Italy
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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of   0,827 Å. Data were collected in a flat plate configuration
by keeping fixed the incident angle at two degrees. The XRD
Experimental
View Article Online
DOI: 10.1039/C8TA11812D
pattern was collected consecutively in the range 5° < 2θ < 50°,
with steps of 0.02° and an acquisition time of 0.2 s per step.
The morphology and chemical compositions of the LDH films
were investigated by Field Emission-Scanning Electron
Microscopy (FE-SEM) using a LEO 1530 ZEISS instrument
equipped with Schottky emitter, operated at an acceleration
voltage variable from 5 to 15 kV, and an Everharte-Thornley and
an In-lens detectors for secondary electrons imaging. Energy
Dispersive X-ray Spectroscopy (EDS) measurements were
Chemicals
Cobalt (II) nitrate hexahydrate (98% pure), aluminium nitrate
nonahydrate (>96% pure), 5-hydroxymethyl-2-furaldehyde
(99% pure) and sodium hydroxide (98% pure) were supplied by
Sigma-Aldrich. Iron (III) nitrate nonahydrate (99% pure) was
purchased from Riedel-de Haën. The supporting electrolyte for
all the electrochemical experiments was NaOH at the
concentration of 0.1 mol/L. The salt solutions were prepared
with doubly distilled water. Graphite GTJ (Graphite>99.8%) was
used as support and purchased from VED (https://www.ved.it);
2
performed with an Oxford 30 mm EDS Silicon Drift Detector.
The elemental analysis was performed using an Atomic
Emission Spectrometer MP-AES Agilent 4210 controlled by a
computer via Agilent MP Expert software.
2
the geometrical area was 2cm , the bulk density was 1.12 g/mL,
-4
the resistivity 7.33∙10  cm; carbon content was established
at 99.5%, while ash content at 0.5%. SEM images of this support
are reported in SI1. Platinum foil had a geometrical area of 1
Raman spectra were recorded with the Renishaw System
RM1000. The spectrometer was coupled to a Leica DMLM
microscope equipped with 50x, 20x and 5x objectives which
allowed, with the 50x objective, a nominal spatial resolution < 2
µm in xy and 4 µm in z. The excitation wavelength was from an
argon laser tuned at 514.5 nm. The output power of the laser
was around 25 mW, but when needed the light intensity
impinging on the film was reduced by neutral density filters to
avoid sample damage.
2
cm .
Support modification
Cleaning the electrode is a critical step to achieve a well
adherent coating. For the graphitic electrode (Grafoil), the
surface was rinsed in ethanol for 10 min and then dried to
constant weight. The LDH films were deposited on the electrode
surface by cathodic reduction of a freshly prepared 0.03 M
solution containing M(II) and M(III) at a molar ratio of 3:1. For
Al-containing LDHs, the electrochemical reaction was carried
out by applying a variable potential (2 CVs cycles between -1.3
Samples for the XAS measurements were prepared following
the procedure described above with the unique exception of
-1
the scan rate, which was 10 mV s . XAS experiments were
performed at ELETTRA Synchrotron Radiation Laboratory
-1
and 0.0 V, vs SCE) with a scan rate of 30 mV s , for Fe containing
2
2
(
Basovizza, Trieste), on the XAFS beam line 11.1 , recording the
-1
LDHs the scan rate was 40 mV s and the potential window was
1.1 and 0.0V, vs SCE. After performing the modification, the
data in fluorescence mode using a large area Si drift diode
detector (KETEK). XAS spectra were recorded at the Co and Fe
K-edge (7709 eV and 7112 eV, respectively) using a Si(111)
double crystal monochromator. Extended X-Ray Absorption
Fine Structure (EXAFS) curves were recorded at room
temperature. X-Ray Absorption Spectroscopy spectra were
deglitched, calibrated, and normalized using the Athena
-
electrode was immediately rinsed with water.
For the Pt electrodes, the surface was first polished to a mirror-
like surface by a mechanical cleaning, using sand-paper, and
then the electrode was submitted to an electrochemical
treatment: 250 cycles between -0.25 V and +1.30 V were
performed in 0.1 M H
was deposited following the same procedure described above
-
1 21
2 4
SO at a scan rate of 1 V s . The film
2
3
program.
The pre-edge background was removed by
subtraction of a linear function extrapolated from the pre-edge
region, and the XANES spectra were normalized at the unity by
extrapolation of the atomic background. The EXAFS analysis was
-1
but for Al-containing LDHs the scan rate was 20 mV s , and for
Fe containing LDHs the scan rate was 30 mV s .
-1
2
4
performed by using the GNXAS package that considers
Multiple Scattering (MS) theory. Briefly, the method is based on
the decomposition of the EXAFS signals into a sum of several
Single scattering (SS) and Multiple Scattering (MS)
contributions, i.e.: the n-body terms. It allows the direct
comparison of the raw experimental data with a model
theoretical signal. The procedure avoids any filtering of the data
and allows a statistical analysis of the results. The theoretical
signal is calculated ab-initio following a previously published
Apparatus
All the electrochemical tests were carried out in a single
compartment, three-electrode cell. Electrode potentials were
measured with respect to an aqueous saturated calomel
electrode (SCE). A Pt wire was used as the counter electrode.
The CV curves were recorded using a CHInstruments Mod.
6
60C, controlled by a personal computer via CHInstruments
software.
X-ray diffraction patterns were obtained with CuK radiation in
reflection mode by means of an X'Pert PANalytical
diffractometer equipped with a fast X’ Celerator detector. The
samples were analysed directly inserting the Pt electrode in the
optical centre of the diffractometer. X-Ray Diffraction
measurements of LDHs materials on graphite foil were recorded
at the MCX beamline at Sincrotrone ELETTRA, Basovizza, Trieste
Italy, by using a monochromatic X-ray beam with a wavelength
2
5
structural model for LDHs that has been adapted to this set of
samples and contains the relevant two-body (2), three-
2
6
body (3) and four body (4) MS terms. For the case of the Co-
edge the following n-body terms have been included in the
fitting procedures: the two-atom contributions (2) Co-O with
degeneracy of six and the two-atom contributions (2) Co--Co
with degeneracy of six. This also accounts for two Co--Fe
interactions at the same distance. This approximation holds due
2
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ARTICLE
to the relative identical scattering power of both metals. Also, induce a pH increase necessary for the LDH precipitation, and
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1
7,29,30
DOI: 10.1039/C8TA11812D
the following n-body terms have been included in the fitting they are as follows
procedures for the Fe K-edge case: the two-atom contributions 퐻 +푒 → 퐻푎푑푠
:
+
−
+
−

(2) Fe-O with degeneracy of six and the two-atom 2퐻 +2푒 → 퐻
2
−
퐸°=0.0푉
−
+
−
contributions (2) Fe--Co(Fe) with degeneracy of six. Data 푁푂
analysis was performed by minimising a -like residual 푁푂
3
+2퐻 +2푒 → 푁푂
+10퐻 +8푒 → 푁퐻 +3퐻
푂+2푒 → 퐻 +2푂퐻
2 2
2
+퐻
2
푂
퐸°=0.93푉
퐸°=0.36푉
퐸°=−0.83푉
퐸°=0.01푉
−
+
−
4+
3
2
푂
−
−
function that compares the theoretical signal, mod(E), to the 2퐻
experimental one, exp(E). The phase shifts for the 푁푂
photoabsorber and backscattered atoms were calculated ab-
−
−
−+2푂퐻−
3
+퐻
2
푂+2푒 → 푁푂
2
initio according to the muffin-tin approximation and allowing 5- The mechanism of LDH electrodeposition has been proposed to
5% overlap between the muffin-tin spheres. The Hedin- possibly occur in two different ways: the first one involves the
1
2
7
3
Lundqvist complex potential was used for the exchange- precipitation of Al(OH) followed by the insertion of the bivalent
correlation potential of the excited state. The core hole cation to form the clay structure. The second proposed
lifetime28 was fixed to the tabulated value and included in the mechanism is the direct precipitation of the LDH without
1
4
phase shift calculation. The experimental resolution in the involving the hydroxide of the trivalent metal . It is worth
fitting analysis was about 1 eV, in agreement with the stated mentioning that various chemical synthesis approaches have
value for the beamline used.
been described in the literature and there is not a unique
To analyse the products of the oxidation procedure, the residue explanation for the reactions sequence of the LDH production,
3
1
obtained after evaporation of the solvent was taken from the even for the chemical route. It is also important to notice that
cell and suspended in 2 mL 0.1M NaOH. 50 L of this solution the support characteristics (i.e., morphology, composition, and
were diluted to 5 mL and analysed by means of an Agilent electrical conductivity) determine its activity in terms of
Infinity 1200 liquid chromatograph equipped with an Aminex reduction potential at which the above reported reactions
HPX 87-H 300 mm 7.8 mm column, using a 0.005 M H
2
SO
4
occur, current distribution and mass transfer processes. In fact,
solution as the mobile phase. The identification of the products it is well known that the electrochemical reduction of nitrate is
was achieved using reference commercial compounds and their a complex process, that depends on a lot of factors (such as the
concentrations were determined from the calibration curves support, but also temperature, pH and composition of the bath,
3
2
(
correlation coefficient = 0.999). The HMF conversion (%) and etc.) . Moreover, during the electrodeposition of LDH
the selectivity for the oxidation products were calculated using compounds, two further issues must be considered: the first
the following equations:
one is the growing of the film, which alters the conductivity and
the area available for nitrate adsorption; the second one is
related to the continuous modification of pH and, therefore, of
mol of HMF consumed
mol of initial HMF
HMF conversion(%) =
x 100
2
9
nitrate reduction potential and electrochemical products.
The cyclic voltammetric curves recorded during the synthesis of
the Co/Al-NO and Co/Fe-NO LDHs using the newly proposed
mol of product
Selectivity(%) =
푥100
mol of HMF converted
3
3
The Faradaic efficiency (FE) of FDCA product was calculated by
synthetic path are displayed in Figure 1.
mol of FDCA formed
FE (%) =
x 100
mol of total electrons passed/6
1
Moreover, H-NMR spectra were recorded at 400 MHz, on a
Bruker Avance 400 spectrometer.
Results and discussion
Electrochemical deposition on Platinum and Grafoil
In order to find the best experimental conditions to obtain an
LDH film, the electro-synthesis was carried out both on Pt foil
and on carbonaceous materials, investigating some parameters
such as the scan rate and the potential window. Recently, our
1
6
group has proposed a potentiodynamic approach for the
electrochemical reduction of a nitrates solution of nickel and
aluminium which consists of several reactions that contribute
to the precipitation, close and on the electrode surface, of the
LDH. However, the precipitation of single metal hydroxides or
oxides could be a side reaction. The reactions, causing the
Figure 1. CV curves recorded at Pt (a and c) and Grafoil (b and
d) substrates soaked in solutions of Co and Al (a and b) and Co
and Fe nitrates (c and d)
From the CVs, it is possible to note a small difference between
the two supports as to the first voltammetric cycle (displayed in
+
-
disappearance of H ions and/or the generation of OH ions,
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black): in the case of platinum two peaks are detectable (the Morphological characterisation of the synthesised LDHs
View Article Online
DOI: 10.1039/C8TA11812D
less cathodic one can be ascribable to hydrogen evolution, the
other to nitrate reduction), whereas for Grafoil just one peak The morphological characterisation of the LDHs electro-
can be seen.
For Co/Al-NO
synthesised on Grafoil and Pt was performed using FE-SEM. All
the two peaks are located for Pt at -0.6 and -1 V, the supports appeared homogeneously covered and no
3
3
whereas for Grafoil the peak is at -1.1V. For Co/Fe-NO a shift detachment of the film was observed in any case. Investigating
toward less cathodic potentials is registered: the two peaks on more thoroughly the LDHs films, different compactness and
Pt are at -0.4 and -0.9 V, respectively, whereas on Grafoil the thicknesses of the coatings were discerned. Figure 3 illustrates
peak is at -0.97 V.
the morphologies of the materials on Pt and Grafoil.
Another difference can be observed in the total charge passed The images show great differences among LDHs and supports.
during the deposition, which is practically the same for both When Grafoil (Fig. 3 c, d, g, h) was used for the
electrodes. Considering that the geometrical area of Pt foil is electrodeposition, all the LDH coatings appeared very compact
half that of the Grafoil, it means that when Pt is used as a and consisted of basically round particles with a uniform size
support the charge is double. This can be explained by a higher distribution. In particular, the smallest particles (10-20 nm)
efficiency of LDHs formation on Pt compared to Grafoil.
were observed for the Fe-containing LDH. Furthermore, the film
is so compact that the particles seem almost fused to each
other. This observation can be further confirmed from Figure
Physical characterisation of the synthesised LDHs
With the aim to verify the structure of the LDHs, PXRD analysis 3d, which shows a cross-section: the material seems to display
was carried out directly on the graphitic and Pt electrodes.
a flower-like morphology on the layer in contact with the
substrate whereas in the upper part it looks more compact. In
the case of Pt, the morphology of the LDH coatings shows
surprising differences (Fig. 3 a, b, e, f). As far as the Co/Al LDH is
concerned the particles appear in spherical groups on which
flower-like architectures are grown, indicating the formation of
3
D hierarchical nanostructures. In case of the Co/Fe LDH the
morphology consists of well-defined highly interconnected
tubular particles which are the largest (about 1 m long and 150
Figure 2. PXRD patterns of the LDHs electro-synthesised on a) nm wide) among the electro-synthesised LDHs. At the best of
Pt and b) Grafoil. The clipped reflections are due to Pt or Grafoil. our knowledge, this kind of structure has never been reported
for an electrochemical synthesis route of Co/Fe and Co/Al-LDHs.
As can be seen from Figure 2, the patterns of all the samples In both cases, underneath the larger particles there was a more
show the main diffraction maxima typical of the LDH phase, compact film similar to that observed on Grafoil.
indexed as 003 and 006 reflections.
A qualitative evaluation of the adhesion between thin coatings
As far as the Co/Al-LDH is concerned it is evident that this and substrates can be made using a scratch testing technique.
material displays the narrowest reflections when synthesised In this work, the test was performed applying a constant load
on Pt, its crystal domains must be larger and better formed (0.1N) on the LDH coating by a Rockwell C diamond indenter
compared to the other LDHs.
while the specimen moved horizontally. The SEM images
Generally, it is possible to confirm that the substrate plays a obtained for the LDH films on Pt and Grafoil surfaces with
central role in the formation of the crystal domains: in fact, on scratch tracks are reported in SI2 (a, b, c, d), respectively. In the
Pt it was possible to collect the pattern with a conventional X- first case, the failure is only confined into the scratch track
ray tube using the Cu K radiation (  1.5406 Å). On the other (transverse cracking) and the coating resulted well-adherent, as
hand, with Grafoil as a support, the same instrument does not already reported for the films obtained from potentiostatic
allow us to gather a clear signal for LDH phase. The XRD pattern procedure.1 The Grafoil support, due to its intrinsic
was successfully recorded using a synchrotron radiation source morphology, is particularly fragile and, using the indenter, the
5
(
  0.827 Å). Comparing the results obtained for the two flakes of Grafoil tend to detach one to another and fold up
supports, it should be noticed that on Pt the reflections are (SI2b). Consequently, when thin LDH films are electro-deposited
generally narrower than on Grafoil, suggesting a better on Grafoil and the adhesion is good, they follow the shape of
formation of the crystalline phase.
the support track produced by the indenter, as expected and
Eventually, for both Pt and Grafoil, it is remarkable to notice pointed out from Figures SI2c and d (images after the scratch
that the new approach for the electrosynthesis permits a direct test at 2 magnifications).
access to the analytical characterisation of LDHs. Unlike, by Moreover, the films deposited on Pt are thicker than those
7
using the potentiostatic method previously reported , a good obtained on the graphitic substrate (≈3 m compared to 800
signal was recorded only after 10 repetitions of the synthetic nm). The LDH films were also characterised by semi-
protocol.
quantitative SEM/EDS analysis to determine the M(II)/M(III)
atomic ratio (see SI3). All the values were around 3, practically
identical to the molar ratio of the cations present in the
electrolytic solutions used for the synthesis. Moreover, in order
4
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to validate the SEM/EDS values, MP-AES was also performed. shoulder at lower wavenumbers, and the sharper bands at 528
View Article Online
DOI: 10.1039/C8TA11812 D- 1
-1
The average results of three independent measurements for and 1050 cm . Much weaker features appear below 300 cm
-1
Co/Al and Co/Fe-LDHs was 3.3 and 3.2, respectively. These (which should be ascribed to lattice modes) and at 711 cm . A
values confirm the SEM/EDS analysis and are compatible with thorough discussion of the Raman spectrum is not of interest
-1
an LDH stoichiometry.
here, and the analysis of the bands at 472 and 528 cm , typical
of these materials, must rely on previous studies on the
vibrational properties of cobalt hydroxides and cobalt–based
1
0,33-35
LDHs.
In the literature there is no general agreement about the
assignment, but it is reasonable to assume that the band at 528
-1
-1
cm is correlated to the strong band observed at 522 cm in
Co(OH) , and, therefore, is assigned to hydroxyl groups bound
2
to the Co cation, despite the small mismatch with the data for
chemically synthesised Co/Al-LDHs. The broader feature at 472
-
1
-1
cm appears to correspond to the 471 cm band observed in
3
3
the IR spectrum of a Co/Al hydrotalcite-like compound , and
assigned to hydroxyl groups bound to Al. The occurrence of a
band at 454 cm in Co/Fe-LDH Raman spectrum (vide infra) has
-1
1
0
suggested that the shoulder of this band again arises from the
hydroxyl bound to Co cation. Finally, also the strong band at
1
1
050 cm- lies at lower wavenumbers and with much larger
1
0
intensity than reported in Ref. , where the weak feature
-1
observed at 1057 cm in Co/Al-LDH is virtually absent in Co/Fe-
LDH. In this work, the band is assigned to OH deformation
modes, based on previous observations, but an alternative
assignment, relying on the high intensity, could be the
symmetric stretching of the nitrate anion.
Similar to the Co/Al-LDHs, the Co/Fe samples show bands at
-1
shifts of 454 and 521 cm (Figure 4 right). It should be noted
-1
that the band at 1049 cm is barely detected in any of the Co/Fe
LDH samples, in agreement with the literature data.
Generally, the Raman spectra of the Co/Al-LDHs display a
stronger intensity with respect to those of Co/Fe-LDHs,
regardless of the support. Although a quantitative estimate is
not possible, such a behaviour is unlikely to be related to an
intrinsic property of the compound, but it is usually related to a
higher degree of crystallinity of the Co/Al system.
Most importantly, and independently of the details of the
spectral analysis, the Raman measurements have been
employed to probe the chemical identity of the materials
obtained on the two different electrodes used for the growth.
The spectra of LDHs sharing the same cations on Pt and Grafoil
are virtually identical and, therefore, we can assume that the
structure and the composition of the LDH do not depend on the
nature of the support.
Figure 3. SEM images of thin films of Co/Al-LDH deposited on a)
and b) Pt, c) and d) Grafoil; and of Co/Fe-LDH deposited on e)
and f) Pt, g) and h) Grafoil.
Raman spectroscopy
X-ray Absorption Spectroscopy (XAS)
Due to its strong sensitivity for the metal site, local structural
and electronic information can be gained by looking at the X-
Ray Absorption spectra of the electrodeposited materials at
both Fe and Co sites in the LDHs. Since we are interested in bulk
characterisation of the electrodeposited LDHs, XAS
measurements were recorded in transmission mode and,
therefore, only the graphitic substrate has been considered for
this set of data (the Pt substrate would absorb a large fraction
of the X-ray beam). In addition, Grafoil can be considered a
Figure 4. Raman spectra of left) Co/Al-LDH on Pt and Grafoil and
right) Co/Fe-LDH on Pt and Grafoil
The Raman spectra of the Co/Al and Co/Fe-LDHs in the
-1
wavenumber region 200-1200 cm , which is diagnostic for
these LDHs, are reported in Figure 4. The main spectral features
-1
of the Co/Al LDHs are the broad Raman shift at 472 cm , with a
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more versatile substrate to be used in a wide range of Co and Fe K-edges, respectively. These plots report various
View Article Online
DOI: 10.1039/C8TA11812D
applications, and it is cheap and eco-friendly.
theoretical contributions to the total EXAFS signal. The Co-O
and Fe-O first shells largely contribute to the total signal in their
respective panels but the metal-metal signal in the second shell
is rather important for the interpretation of the total
experimental one. It is also interesting to notice that the two-
body (2) Fe -- Co signal has a smaller intensity when compared
to that of the (2) Co -- Co one. This can be ascribed to a
structural disorder at the Fe site.
Table
1
reports the inter-atomic distances and the
2
corresponding EXAFS Debye-Waller factors ( , best-fit) of the
investigated electrodeposited LDHs. The statistical errors
associated with the parameters are indicated in parentheses.
They were determined by correlation maps (contour plots) for
each pair of parameters. These plots were selected among the
parameters having a strong correlation to reflect the highest
error. The inner elliptical contour corresponds to the 95%
confidence level. Panels (e) and (f) report two examples for the
Figure 5. Comparison of the XANES spectra recorded for Co/Fe highly correlated Co-O and Fe-O variables.
and Co/Al-LDH at Co K-edge (a) and comparison of Co/Fe-LDH The trivalent metal (Fe or Al) influences the local structure of
with FeOOH at the Fe K edge (b).
the electrodeposited LDHs, even at the Co site. The Co-O
interaction significantly enhances from 2.053 to 2.082 Å by
3
+
3+
Fig. 5a shows the comparison of the XANES spectra at Co K-edge substituting Fe with Al , while the interaction at the second
recorded for both Co/Fe and Co/Al LDHs. Both traces are almost shell holds at a value of about 3.11-3.13 Å for both Co--Co and
superimposable, and, therefore, the local structure and charge Co--Al (only in the Co/Al-LDH case). Lower EXAFS bond
2
associated to the Co site appear identical in the two variances are observed in Al-based LDHs as seen from both 
2
electrodeposited LDHs. How much that charge is? This can be Co-O and  Co--Co comparison. This suggests a more regular
evaluated by looking at both pre-edge peak position and overall and crystalline structure of the Co/Al with respect to the Co/Fe-
3
6
shape of previously published Co(II) XANES data, which LDHs, probably because less strain is induced by the smaller Al
indicated a Co(II) formal oxidation state of an atom in an atomic size, in good agreement with the better crystallinity
octahedral environment.
pointed out by XRD data.
Fig. 5b displays the XANES at the Fe K-edge of the Co/Fe LDH. A comparison of the EXAFS bond variance of the Co/Fe-LDH
The curve is characterised by a pre-edge peak at about 7114 eV second shell reveals a large contribution from the structural
which is related to the 1s-3d electronic transition and can be disorder at the Fe site with respect to the Co one, as previously
used as a fingerprint for the local charge around Fe. The position suggested by the Figure 6 panel (a) and (b). This fact can be
and the intensity of such a peak indicate mainly the occurrence extended to other LDHs (e.g. Ni/Fe LDH) where the trivalent site
3
7
of the Fe(III) oxidation state in an octahedral environment . is found to be more structurally disordered than the divalent
2
Confirmation of such an assignment comes from a comparison one.
with a Fe(III)OOH reference sample.
To explore more deeply the local structure around the metals
of the deposited LDHs an EXAFS analysis has been performed at
both Co and Fe K-edges. Figure 6 displays details of the best fit
procedure, for the Co/Fe LDH. Due to the presence of two
metals sites, the structure of this electrodeposited material has
been analysed from the two photoabsorbers, i.e., Co and Fe.
The Fourier Transforms (FTs) of the corresponding EXAFS signals
are depicted in the panels (c) and (d). This plot displays the
pseudo-radial distribution of the atoms surrounding the central
one and suggests that two main peaks contribute majorly to the
experimental photo-absorption process at both metal sites.
They are related to the typical LDH structure where the first
Figure 6. Best fit of Co/Fe LDH at both Fe and Co K-edges. Panels
peak is due to the first atomic shell surrounding the Co and the
(
a) and (b) show the details of the EXAFS analysis for the Co and
second peak, at about 2.6 Å, is associated with the second
coordination shell, formed by 6 metal atoms (4 Co and 2 Fe).
The same coordination is observed at the Fe site. Therefore,
only these signals have been used in the fitting procedure. This
is clearly shown in the panels (a) and (b) of the Figure for both
Fe edge, respectively, in terms of individual EXAFS contributions
to the total theoretical signal. The comparison of the total
theoretical signal (black curve) with the experimental one (red
curve) is also shown. The residual signal is reported at the
6
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ARTICLE
bottom. Panels (c) and (d) display the fit of the corresponding
FTs of EXAFS. Panels (e) and (f) display the contour plot for the
error determination of the Co-O and Fe-O first shell distances.
View Article Online
DOI: 10.1039/C8TA11812D
Table 1. EXAFS Analysis. Atomic first and second shell distances
and corresponding EXAFS Debye-Waller factors of the pristine
and oxidized LDHs. Errors (in parentheses) were determined by
Contour plots (see Fig. 6 e and f).
Paramete Co-Al/LDH
Co-Fe/LDH
r
Co K-edge
Fe K-edge
Co K-edge
2.082(8)
0.006(1)
6
Co-O /Å
2.053(4)
0.008(1)
6
2
Co-O /Å2

CNCo-O
Co-Co /Å

CNCo-Co
3.135(6)
3.11(1)
0.0064
4
2
Co-Co /Å2 0.010(1)
6
Figure 7. CVs of Co/Al-LDH on a and c) Pt, b and d) Grafoil.
Figures c) and d) show the reproducibility of the
electrodeposition since they refer to three independent
syntheses.
Co-Al /Å

CNCo-Al
3.11(2)
0.005(2)
2
2
Co-Al /Å2
Fe-O /Å

CNFe-O
1.981(5)
0.010(1)
6
2Fe-O /Å2
In Figure 8 CVs recorded for the Co/Fe-LDH are shown. In this
case only one peaks system is evident on both supports, but its
position is dependent on the material used as a substrate. Epa is
located at 0.15 V for Pt and 0.20 V for Grafoil.
In this case the reproducibility of the electrosynthesis on Grafoil
seems slightly better that on Pt (Figure 8 c and d).
Eventually, the optimal reproducibility observed for the
potentiodynamic electrosynthesis of Co/Al and Co/Fe-LDHs on
both substrates and, especially, on Grafoil is a striking result for
the real LDHs applications. In fact, the graphitic support is cheap
and can be used in several fields such as energy storage,
sensing, industrial electrocatalysis, etc.
Fe-
3
.116(8)
.017(2)
Co(Fe)/Å
2

/
Fe-Co(Fe)
0
6
Å2
CNFe-Co(Fe)
E
0 / eV
7719.4(5)
0.70(4)
7125.1(5)
0.71(5)
7722.1(8)
0.70(3)
2
S
0
Electrochemical LDHs characterisation
The electrochemical characterisations of the LDHs were
performed. A preliminary check of the film stability was studied
by recording several CV curves for the electro-synthesised LDHs.
The signals were superimposable for hundreds of cycles at both
supports, and this suggested a good adhesion of the clays.
Therefore, the mechanical and electrochemical stability can be
considered very good.
The first CV scan recorded for both LDHs was remarkably
different from the following ones (Figure 7): as already
reported,13 during this cycle an irreversible reaction occurs
involving Co(II) sites.
After stabilization two peak couples can be observed for the
Co/Al-LDH on both supports with Epa= 0.14 V and Epa=0.42 V.
The feature at around 0.14 V is hardly detectable for Pt,
whereas it displays a good intensity in the case of Grafoil.
These peaks are related to Co redox system, according to
equation 1 and 2:13
LDH−Co (II) + OHsol ⇆ LDH(OH) – Co (III) + e-
-
Figure 8. CVs of Co/Fe-LDH on a and c) Pt and b and d) Grafoil.
Figures c) and d) show the reproducibility of the
electrodeposition since they refer to three independent
1
2
)
)
-
-
LDH−Co (III) + OHsol ⇆ LDH(OH) – Co (IV) + e
As can be observed from figures 7 c) and 7 d) the signals of three synthesis.
LDHs coming from independent depositions are
superimposable both for Grafoil and Pt supports.
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acid (FDCA) to be employed as a monomer in industrial
View Article Online
DOI: 10.1039/C8TA11812D
synthesis. In this scenario, the proposed system results in a
Electrochemical HMF oxidation
complete noble metal-free catalyst. Moreover, the support
must also be considered in a process development, since a
catalyst prepared with the same metal active phase might
display different catalytic performances depending on the
support used. Thus, in this wide range of studies on innovative
processes aiming at the preparation of metal-supported
catalysts, it is here proposed Grafoil as support since it is low
cost, environmentally friendly, and easy to handle, as already
mentioned.
The solution coming from the exhaustive electrolysis was
analysed by HPLC equipped with diode array detector (see SI 4)
1
and H-NMR (see SI 5-6) in order to identify the oxidation
Figure 9. a) CVs recorded at a Co/Al-LDH coated Grafoil in 0.1 M
products.
-
NaOH, containing HMF at different concentrations,  = 10 mVs
1
Neither HPLC nor H-NMR shows presence of HMF, confirming
1
;
b) I vs time plot obtained for the same Co/Al-LDH at + 0.5 V in
a
virtual conversion of 100%. 5-Hydroxymethyl-2-
0
.1 M NaOH under magnetic stirring, while the HMF
furancarboxylic acid (HMFCA) and FDCA are formed as major
products, and an unidentified aliphatic product can be also
detected. This product was investigated also with ESI-MS
spectrometry (see SI 7), giving a molecular weight of 242.
The selectivity toward FDCA, calculated from HPLC, results 49%,
while toward HMFCA is 37%. The Faradaic efficiency was
calculated to be 40.8%. When analysing the FE, it must be
noticed that the oxygen evolution reaction competes with the
electrocatalytic oxidation of HMF to FDCA in alkaline media,
probably causing a decrease of the selectivity. Summarising, the
above reported results suggest that the proposed chemically
modified electrodes can be considered an interesting approach
to produce FDCA from HMF catalytic oxidation.
concentration was increased by progressive additions of a
concentrated solution
A preliminary study of the oxidation of HMF was carried out by
CV. Firstly the suitable experimental working conditions were
explored in terms of applied potential. As an example, Fig. 9a
shows the CVs recorded at Grafoil coated with Co/Al-LDH
following the addition of HMF different amounts to a 0.1 M
NaOH solution. An increase of the anodic current is recorded
which is proportional to HMF concentration. The
chronoamperometric response was obtained by setting the
potential at +0.5 V (Fig. 9b). The same experiments were carried
out using Pt as the support and all of them were repeated also
for the Co/Al-LDH. The sensitivities and the LOD values of the
four calibration lines are reported in Table 2. The Co/Al-LDH
displays a lightly better performance as to the sensitivity in
respect to the Co/Fe-LDH, independently on the support. As
expected, the same material displays the same LOD in despite
of the support and the values are similar for both LDHs.
Moreover, the reproducibility of the sensitivity values for the
HMF determination, as expressed by the slopes of the
calibration lines, is satisfactory as expected by the good
reproducibility of the newly proposed electrodeposition.
Conclusions
Films of Co/Al and Co/Fe-LDHs were prepared by a novel and
better performing electrodeposition route. Both Pt and carbon-
based materials were investigated as supports. All the
characterisations confirm the LDH structure of the electro-
synthesised materials.
PXRD suggests better-formed crystal domains with respect to
the previously proposed potentiostatic approach. Another
remarkable result is related to the morphology of the Co-
containing LDHs on Pt, which display
nanostructure.
The best films in terms of stability and reproducibility have been
obtained on carbon-based materials, whereas a greater charge
has been recorded on Pt, which also means a larger amount of
deposited material.
Finally, the modified electrodes were employed for the electro-
oxidation of HMF, displaying good performances, so suggesting
the importance of a further investigation for applications in the
field of industrial catalysis.
In conclusion, we propose a robust and highly reproducible
electrosynthesis procedure that can be applied to different
supports and this represents on outstanding opportunity in
view of the large number of LDHs applications.
a well-defined
Table 2.LOD values and sensitivity obtained from the calibration
lines for HMF determination by chronoamperometry
Sensitivity
LOD (M)
-1
-2
(
AM cm )
Co/Al-LDH on Pt
Co/Al-LDH
Grafoil
0.417 ± 0.008
on 0.394 ± 0.006
1.69*10-4
1
.73*10-4
2.23*10-4
.12*10-4
Co/Fe-LDH on Pt
0.070 ± 0.002
Co/Fe-LDH
Grafoil
on 0.069 ± 0.004
2
As a proof of concept, Co/Al-LDH on Grafoil has also been tested
for an exhaustive electrolysis (E = +0.5 V, t = 72 h, nitrogen
atmosphere). Nowadays, research efforts are devoted in
developing processes for production of 2,5-furandicarboxylic
8
| J. Name., 2012, 00, 1-3
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1
1
5
6
E. Scavetta, A. Casagrande, I. Gualandi and D. Tonelli, J.
View Article Online
Electroanal. Chem., 2014, 722–723, 15–22.
I. Gualandi, Y. Vlamidis, L. Mazzei, E. Musella, M. Giorgetti,
M. Christian, V. Morandi, E. Scavetta and D. Tonelli, ACS
Appl. Nano Mater., 2019, 2, 143–155.
DOI: 10.1039/C8TA11812D
Conflicts of interest
There are no conflicts to declare.
1
1
1
7
8
9
E. Scavetta, I. Gualandi, D. Tonelli, C. Mousty, V. Prevot and
M. Monti, Electrochim. Acta, 2014, 152, 75–83.
C. Megías-Sayago, A. Lolli, S. Ivanova, S. Albonetti, F.
Cavani and J. A. Odriozola, Catal. Today, 2018, 0–1.
J. N. Chheda, Y. Román-Leshkov and J. A. Dumesic, Green
Chem., 2007, 9, 342–350.
A. Gandini, Green Chem., 2011, 13, 1061–1083.
I. Gualandi, A. G. Solito, E. Scavetta and D. Tonelli,
Electroanalysis, 2012, 24, 857–864.
Acknowledgements
Authors are grateful to the University of Bologna, Italy, for
providing financial support. XAS experiments at ELETTRA
Sincrotrone Trieste were partially supported by the proposal
Ceric 20172042 (MG as PI). Thanks are also due to Dr. M.
Gazzano for performing PXRD measurements. Moreover,
thanks need to be done to Dr. Jasper R. Plaisier for performing
XRD and Giuliana Aquilanti for XAS at ELETTRA. Authors are also
grateful to Daniel Pecorari and Michele Mancinelli for
performing 1H-NMR experiment and to Danilo Bonincontro for
performing HPLC separation. The authors acknowledge also
professor Casagrande for performing ed evaluating scratch
tests. The European Unionʼs Horizon 2020 Research and
Innovation Programme (Grant Agreement no. 696656
Graphene Flagship) for funding. This research was also funded
by the Italian Ministry of Research (MIUR) within PRIN-2015
Project No. NEWLI2015CL3APH.
2
2
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N. Novello and L. Olivi, J. Phys. D. Appl. Phys., 2017, 50, 1–
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