Nanostructured carbon-metal hybrid aerogels from bacterial cellulose

Article abstract of DOI:10.1039/c7ra07534k

Infiltration of nickel(ii) hydroxide inside a bacterial cellulose gel followed by thermal treatment gives rise to highly crystalline nickel nanoparticles that remain associated with the generated carbon nanofibers. The involved mechanism, ascribed to a carbothermal reduction, takes place at an exceptionally low temperature (390 °C). This study suggests the reduction of the intermediate nickel oxide phase by carbon and CO generated during pyrolysis of bacterial cellulose, while the nascent nickel nanoparticles catalyze the formation of graphitic carbon in the form of carbon aerogels. This synthesis route also generates equivalent carbon-iron hybrids pointing to the extension of this procedure to other transition metals as well, thus producing conducting carbon foams with functional metal nanoparticles. This soft procedure opens way to new carbon-metal aerogels with potential interest in fields as diverse as catalysis, energy storage and optical devices.

Full text of DOI:10.1039/c7ra07534k

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Nanostructured carbon–metal hybrid aerogels
from bacterial cellulose†
Cite this: RSC Adv., 2017, 7, 42203
a
a
b
a
*
Bernd Wicklein,
Yves Huttel
Judith Arranz,
Alvaro Mayoral,
Pilar Aranda,
a
a
and Eduardo Ruiz-Hitzky
Infiltration of nickel(II) hydroxide inside a bacterial cellulose gel followed by thermal treatment gives rise to highly
crystalline nickel nanoparticles that remain associated with the generated carbon nanofibers. The involved
mechanism, ascribed to a carbothermal reduction, takes place at an exceptionally low temperature (390 ꢀC).
This study suggests the reduction of the intermediate nickel oxide phase by carbon and CO generated
during pyrolysis of bacterial cellulose, while the nascent nickel nanoparticles catalyze the formation of
graphitic carbon in the form of carbon aerogels. This synthesis route also generates equivalent carbon–iron
hybrids pointing to the extension of this procedure to other transition metals as well, thus producing
conducting carbon foams with functional metal nanoparticles. This soft procedure opens way to new
carbon–metal aerogels with potential interest in fields as diverse as catalysis, energy storage and optical devices.
Received 8th July 2017
Accepted 22nd August 2017
DOI: 10.1039/c7ra07534k
rsc.li/rsc-advances
nanostructured carbon. This aspect remains challenging,20
evident by competing views on the formation mechanism of
metal–carbon hybrid materials.21,22
Introduction
Hybrid materials composed of nanostructured carbon and
metal nanoparticles (NPs) give rise to interesting scientic
phenomena1 and diverse technological applications.2–5 For
instance, charge transfer behavior at the carbon/metal interface
results in high catalytic activity,1,6,7 while magnetic5 and optical8
properties of the metal NPs can be explored in sensing and
biotechnological applications.2 Specically, carbon–nickel
nanostructured materials, oen prepared by carbothermal
reduction,9,10 nd widespread use as energy materials and
optical materials.4,6,7,11–13 Herein, specially biomass like ligno-
cellulosics, nanocellulose, or bacterial cellulose are multifar-
ious carbon sources due to their structural pre-organization
displaying porosity, brous morphology, or three-dimensional
(3D) networks, respectively, which can be translated into
nanostructured carbon during their carbonization.14–16 Thus,
careful choice of the pyrolysis conditions not only reduces the
metal precursor to metal nanoparticles but concomitantly can
also produce carbon nanobers,17 carbon aerogels,18 or gra-
phene,19 which integrate the functional metal nanoparticles.
The production of carbon–metal materials usually takes
place at high temperature (>600 ꢀC) through carbothermal
reduction10 but shows still a lack of understanding especially
about how nascent metal NPs may inuence the formation of
Bacterial cellulose is a versatile biomaterial for the formation
of nanostructured hybrid materials.23 It consists of nanometer
thick, highly crystalline cellulose Ia bers produced by bacterial
fermentation of glucose.24 The bers are crosslinked forming
a strong hydrogel, which allows for effective impregnation with
aqueous metal salt solution.25 Subsequent reduction, hydro-
thermal treatment, or precipitation may render metal or metal
oxide/hydroxide NPs supported on the cellulose bers suitable
for a variety of catalytic, sensing, or biomedical applications.24
On the other hand, pyrolysis of bacterial cellulose can result in
graphitic carbon ber networks17 or carbon aerogels,15 which are
interesting as energy materials, absorbers, or articial muscles.
In this paper we combined the carbon aerogel formation
from bacterial cellulose with carbothermal reduction of nickel
hydroxide to produce carbon–nickel hybrid foams in view of
their outstanding technological importance. We precipitated
nickel hydroxide nanoparticles on bacterial cellulose, which
aer a carbothermic reaction at a remarkably low temperature
formed metallic nickel–carbon aerogels. The same methodology
was also applied to ferric hydroxide for the production of iron–
carbon hybrids. Potential applications of the synthesized mate-
rials can be envisaged in catalysis, electromagnetic shielding, as
electrode material in Li ion batteries, and as light absorber.
aInstituto de Ciencia de Materiales de Madrid, Consejo Superior de Investigaciones
´
´
Cientıcas (CSIC), c/Sor Juana Ines de la Cruz 3, 28049 Madrid, Spain. E-mail:
Experimental section
Synthesis of carbon–nickel hybrids
bernd@icmm.csic.es
bLaboratorio de Microscopıas Avanzadas, Instituto de Nanociencia de Aragon,
Universidad de Zaragoza, c/Mariano Esquillor, Edicio I+D, 50018, Zaragoza, Spain
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c7ra07534k
´
´
Nata de coco (Fitrite Inc., Philippines) was used as source of
bacterial cellulose.24 The nata de coco cubes were extensively
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washed with deionized water, stirred for 3 days in 0.1 M NaOH (SDT Q600, TA Instruments) coupled with a mass spectrometer
and for 2 days in deionized water and then stored refrigerated in (Thermostar QMS200 M3) were carried out at the ICMM service
Milli-Q water until usage. Nickel acetate (Ni(Ac)2), FeCl3 and facility. X-ray photoelectron spectroscopy (XPS) was carried out
NaOH were purchased from Sigma-Aldrich. In a typical in an ultrahigh vacuum chamber with a base pressure in the low
synthesis bacterial cellulose cubes were incubated in 50 mM 10ꢁ10 mbar. The angle between the hemispherical analyzer
Ni(Ac)2 for 3 days under agitation. Then the cubes were rinsed (Specs, PHOI BOS 100) and the plane of the surface was kept at
with Milli-Q water and immersed in 5ꢀ0 mM NaOH for 24 h 60ꢀ, and the X-ray radiation was the Mg Ka line (hn ¼ 1253.6 eV).
followed by hydrothermal aging at 100 C for 24 h in an auto- The analysis was performed on the photoemission peaks of Ni
clave. Aer 1 day in Milli-Q water to remove NaOH the cubes 2p, C 1s, and O 1s core levels. In all cases, the Shirley back-
were freeze-dried and are henceforth termed BC–Ni(OH)2.
ground and the X-ray satellites were removed and the quanti-
BC–Ni(OH)2 is then pyrolyzed in a tubular furnace (Hobersal, tative XPS analysis was performed with the CasaXPS soware.
ST116020,ꢁ1Spain) under N2 following three ramps: (i) at Infrared spectra were recorded on a Bruker IFS 66v/s FTIR
ꢁ1
ꢁ1
ꢀ
ꢀ
10 C min to 170 ꢀC, (ii) at 5 C min to 230 ꢀC followed by spectrophotometer with samples diluted in KBr. The textural
ꢀ
ꢀ
a dwell time of 10 h, (iii) at 1 C min to 400 C with a dwell properties were analyzed by collecting nitrogen adsorption/
time of 2 h followed by natural cooling to room temperature desorption isotherms at 77 K (Micromeritics Flowsorb II 2300)
under N2. The resulting material is termed C–Ni.
with samples degassed at 110 ꢀC. The magnetic properties of
C–Ni were assessed by in a superconducting quantum inter-
ference device (SQUID) from Quantum Design, equipped with
a 5 Tesla coil. The magnetization curves, M(H), were recorded at
300 K and at 4.2 K. The temperature dependent magnetization,
M(T), was measured under zero eld cooling (ZFC) and eld
cooling (FC) conditions, applying a cooling eld of 50 kOe and
a measuring eld of 50 Oe.
Synthesis of carbon–iron hybrids
In case of iron bacterial cellulose cubes were incubated in
50 mM FeCl3 for 3 days followed by immersion in 1 M NaOH
overnight. Aer 1 day in Milli-Q water the cubes were freeze-
dried and are henceforth termed of BC–FeO(OH). The pyrol-
ysis under N2 followed four ramps: (i) at 10 ꢀC minꢁ1 to 170 ꢀC,
(ii) at 5 ꢀC minꢁ1 to 230 ꢀC followed by a dwell time of 10 h, (iꢁii1)
at 1 ꢀC minꢁ1 tꢁo1400 ꢀC with a dwell time of 2 h (iv) at 5 ꢀC min
Results and discussion
Synthesis and characterization of carbon–nickel hybrid
ꢀ
to 650 C min followed by natural cooling to room tempera-
ture under N2. The resulting material is termed C–Fe.
materials
We observed that the inltration of bacterial cellulose (BC) with
nickel(II) acetate solution (50 mM) and the subsequent immer-
Characterization
The microstructure of the materials was characterized using sion in NaOH (50 mM) followed by hydrothermal aging at
ꢀ
eld emission scanning electron microscopes (FE-SEM) (FEI 100 C (ESI, Fig. S1†) results in the precipitation of hexagonal
NOVA NanoSEM 230 and Philips XL30 S-FEG) and with Ni(OH)2 nanoparticles on the cellulose bers (Fig. 1A). The inset
a
spherical aberration corrected (Cs-corrected) Scanning image shows freeze-dried bacterial cellulose foams containing
Transmission Electron Microscope (STEM) using a High Angle Ni(OH)2 NPs (denoted as BC–Ni(OH)2). Subsequently, the foams
Annular Dark Field (HAADF) detector mounted in a FEI TITAN were pyrolyzed under N2 following a dened heating prole
60-300 XFEG electron microscope, which was operated at 300 kV with successive ramps to 170 ꢀC, 230 ꢀC, and 400 ꢀC. The careful
and equipped with a CEOS corrector for the electron probe. The choice of the pyrolysis conditions resulted not only in the
corrector was aligned prior observations using a gold standard conversion of bacterial cellulose into carbonaceous bers
˚
sample yielding to a probe size of 0.8 A. For chemical analysis (denoted as C) under preservation of the original foam-like ber
the microscope was tted with an EDAX detector and a Gatan network (Fig. 1B), but also in the transformation of nickel
Tridiem Energy Filter. Electron diffraction pattern were ob- hydroxide into metallic nickel NPs decorating the ex-cellulose
tained by Fast Fourier Transform of high resolution Cs-STEM carbon bers (denoted as C–Ni, Fig. 1C). Spherical aberration
micrographs. In order to image the carbon phase, Cs-corrected (Cs) corrected Scanning Transmission Electron Microscopy
TEM was used. For this analysis a FEI Titan3 was employed (STEM) analysis of the nanoparticles shows both defect free
operating the microscope at 80 kV. The column was tted with single crystals and twinned polycrystals (Fig. 1D) with a d-
a CEOS corrector for the objective system which allows reaching spacing of 0.20 nm as measured for the atomic columns. This
˚
0.8 A point resolution. The phase composition was analyzed by value can be ascribed to (111) planes in face center cubic (fcc) Ni
˚
X-ray powder diffraction (Bruker D8 Advance instrument with a unit cell value of a ¼ 3.52 A (JCPDS 04-0850). Addition-
equipped with a Cu anode and Ni lter) and Raman spectros- ally, the Fast Fourier Transform (FFT) diffractogram, inset,
ꢀ
copy (Micro-Raman using the 488 nm line of an Ar laser coupled shows reections of the (002) and (111) planes of Ni, corre-
to an Olympus microscope). The Raman spectra were smoothed sponding to the [110] orientation of the crystallite. X-ray powder
applying the Savitzky–Golay method with a 2nd polynomial diffraction (XRD) also conrms the transformation of Ni(OH)2
order and 67 points of window. The nickel content was deter- into Ni upon pyrolysis (Fig. 1E). While the diffractogram of
mined by total reection X-ray uorescence (S2 PicoFox TXRF BC–Ni(OH)2 shows reections at 19ꢀ (001), 33ꢀ (100), 38.5ꢀ (011),
spectrometer, Bruker Nano GmbH). Elemental analysis (CHNS 52ꢀ (012), 59ꢀ (110), and 62.5ꢀ (111) which correspond to
PERKIN ELMER 2400) and thermal analysis including DSC-TGA hexagonal b-Ni(OH)2 (JCPDS 74-2075), the diffractogram of
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Fig. 1 Microstructural characterization: SEM micrographs of BC–Ni(OH)2 (A) and C–Ni (B) with inset photographs of the respective foams. Cs-
corrected STEM-HAADF micrographs of Ni NPs on a carbon fiber in sample C–Ni (C) and an atomic resolution observation of a Ni NP in sample
C–Ni with the inset FFT pattern (D). The white lines indicate the twin boundary. XRD pattern of BC–Ni(OH)2 and C–Ni (E).
C–Ni only shows the (111) and (200) reections assigned to fcc (Fig. 2A) of nickel oxide/hydroxide as evidenced from Ni 2p core
Ni. The lattice parameter a calculated from the main reection level X-ray photoemission spectroscopy (XPS) (ESI, Fig. S4A†).
˚
is 3.516 A and demonstrates the highly ordered crystal structure Indeed, Electron Energy Loss Spectroscopy (EELS) mapping
as already observed in the Cs-STEM image in Fig. 1D. The across a nanoparticle indicates enhanced oxygen concentration
Scherrer crystallite size of the precipitated Ni(OH)2 is 7 nm in at the particle surface layer, while the core, as already described
h001i direction and 29 nm in h100i direction, conrming the in Fig. 1, is composed of pure Ni (ESI, Fig. S4B†). The fact that
typical platelet-like morphology of nickel hydroxide (cf. SEM NiO does not appear in the diffractogram of C–Ni is a strong
image in Fig. 1A). Contrarily, the Ni NPs show a spherical shape indication for a disordered crystal structure, which is also
with a crystallite size of 12.5 nm in h200i and 14.3 nm in h111i corroborated by O 1s core level XPS (ESI, Fig. S4C†) and Raman
direction, respectively. Also STEM characterization suggests spectroscopy (Fig. 2B). The spectrum of C–Ni shows a strong
spherical Ni particles with a particle size distribution centered one-phonon band of the longitudinal optical (LO) mode at
at 9 nm (ESI, Fig. S2†). Concerning bacterial cellulose, its crys- 536 cmꢁ1, attributed to crystalline defects in NiO.
talline Ia structure has vanished aer pyrolysis as apparent
Thermal analysis was conducted to elucidate the formation
ꢀ
from the disappearance of the cellulose (110) and (200) reec- of the nickel nanoparticles by carbothermal reduction of
tions in the diffraction pattern of C–Ni (cf. Fig. 1E) suggesting Ni(OH)2 (ESI, Fig. S5†). The thermogravimetric analysis coupled
the formation of amorphous carbon bers (vide infra).
with mass spectroscopy (TG-MS) of BC–Ni(OH)2 in nitrogen
The textural analysis of the C–Ni foams reveals a BET specic shows the evolution of CO, CO2, and larger cellulose decom-
surface area of 61 m2 gꢁ1 (ESI, Fig. S3†) and an apparent density position products, respectively, (260 ꢀC), the evolution of water
of 11 kg mꢁ3 at a porosity of 99%. Before pyrolysis BC–Ni(OH)2 (315 ꢀC), and a further evolution of CO and CO2 (390 ꢀC)
has an apparent density of 8 kg mꢁ3, which suggests that the (Fig. 3A). The differential scanning calorimetry (DSC) curve of
foam only slightly collapsed during pyrolysis maintaining its BC–Ni(OH)2 shows a strong endothermic peak at 319 ꢀC, an
ꢀ
original foam-like structure. Elemental analysis of C–Ni indi- exothermic peak at 338 C, and a second, weaker endothermic
cates a carbon content of 9–16 wt% and a Ni content of up to peak at 388 ꢀC (Fig. 3B). The thermal transition at 319 ꢀC can be
89 wt% as determined by TXRF.
attributed to the decomposition of nickel hydroxide to NiO
Detailed High-Resolution (HR) STEM investigation of C–Ni under release of water in agreement with the TG-MS data. This
revealed a core–shell structure of the nanoparticles. The Ni core transformation to the fcc NiO phase (JCPDS 78-0643) is also
is covered by a 1.2 ꢂ 0.4 nm thick, partially disordered shell conrmed by XRD of a BC–Ni(OH)2 sample that was heated
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Fig. 2 Cs-corrected STEM-HAADF micrograph of a Ni@NiO core@shell nanoparticle (A). Raman spectrum of C–Ni showing NiO bands and the D
(sp3) and G (sp2) bands of carbon (B).
until 320 ꢀC under N2 (Fig. 3C). The DSC peak at 388 ꢀC is prompt the conjecture that Ni acts as catalyst in the graphiti-
related to the transformation of NiO to metallic nickel as sug- zation of cellulose. In fact, it has been shown that Ni particles
ꢀ
gested by the diffractogram of the sample annealed at 380 C, can catalyze the conversion of amorphous to graphitic carbon at
ꢀ
where the (111) and (200) reections of fcc Ni appear (Fig. 3C). temperatures of 550 C resulting in disordered graphite layers
The formation of metallic nickel at such low temperatures is on the particle surface.29,30 Indeed, in the present study, bacte-
ꢀ
indeed extraordinary as will be discussed below.
rial cellulose in presence of Ni(OH)2 decomposed about 70 C
In parallel to the nickel transformations the bacterial cellu- below the decomposition temperature of pure bacterial cellu-
lose undergoes a carbonization process. The Raman spectrum lose, which is a strong indication of the catalytic activity of the in
in Fig. 2B shows the carbon D (1367 cmꢁ1) and G (1586 cmꢁ1
)
situ formed nickel species, i.e. NiO and Ni.22
bands attributed to pyrolyzed cellulose in C–Ni. The G band
corresponds to in-plane lattice vibrations of sp2 hybridized
carbon atoms in graphene, while the D band originates from
structural defects and sp3 hybridized carbon typically observed
in graphene oxide.26 The band intensity ratio ID/IG together with
the width of the G band give an estimate of the structural quality
of the graphitic carbon layers.27 In C–Ni the ID/IG value is 0.78
and the FWHM (full width at half maximum) is 100 cmꢁ1 for the
G band and 185 cmꢁ1 for the D band. Both ndings suggest
a signicant amount of structurally ordered graphitic carbon
similar to observations in graphene from pyrolyzed saccha-
rides.19,28 Indeed, Cs-corrected TEM bright eld (BF) investiga-
tions in Fig. 4A show the presence of graphene-like layers
Carbothermal reduction mechanism of BC–Ni(OH)2
A formation mechanism of the carbon–nickel hybrid based on
the DSC, TG-MS and XRD results is proposed in the schematic
of Fig. 5, where the interdependence of nickel reduction and
cellulose transformation is deꢀpicted. The thermal decomposi-
tion of BC occurs around 260 C, while nickel hydroxide dehy-
drates at 315 ꢀC to NiO. At 340 ꢀC decomposed cellulose is
further transformed into carbon-rich char31,32 (cf. DSC, Fig. 3B)
possibly activated by the presence of NiO as in pure bacterial
ꢀ
cellulose this process does not occur (ESI, Fig. S9†). At 390 C
a further evolution of CO and CO2 occurs in BC–Ni(OH)2 but not
in BC. This can be explained by a carbothermal reduction of NiO
with carbon10 produced during charring of cellulose. Integra-
tion of the TG-MS peak at 390 ꢀC gives a CO/CO2 ratio of
0.24 (ESI, Fig. S10†), which suggests that the solid state reaction
˚
around the nanoparticles with a d(002) spacing of 3.6 A similar
to graphite. Interestingly, the graphitic layers only appear on the
NP surface, while further away they are more disordered (ESI,
Fig. S6†) and form a carbon matrix as can be observed in EELS
maps (Fig. 4B). This observation is also supported by dif-
fractograms of C–Ni that do not show any reections of graphite
(cf. Fig. 1E), which suggests that the graphitic carbon is disor-
dered and the domain size is below the coherence length of the
employed X-rays source. A lack of two-dimensional graphitic
order is also suggested by the Raman spectrum of C–Ni, which
shows the absence of the second-order 2D band attributed to
the electron band structure of the graphene lattice (ESI,
Fig. S7†).27 The composition of the carbon phase was further
investigated by FTIR (ESI, Fig. S8†) and XPS revealing that sp3
C–C or C–H bonds represent about 32% of the carbon bonds,
while approximately 53% of the carbon atoms are bonded to
oxygen (Fig. 4C). Additionally, it has been found that approxi-
mately 15% of the carbon is sp2. The fact that graphitic carbon
layers only appear in the immediate vicinity of the NPs may
2NiO + C / CO2 + 2Ni
(1)
is operating at this temperature. This is also in agreement with
the Ellingham diagrams that represent the Gibbs free energies
of formation of oxides (DGꢀ) versus temperature. Below z715 ꢀC
the formation of CO2 (C + O2 / CO2) is preferred over that of
CO (2C + O2 / 2CO). Indeed, reaction (1) is thermodynamically
feasible already at a temperature above z210 ꢀC, at which point
the DGꢀ(T) curves of NiO and CO2 intersect.33 However, in
reality, carbothermal reduction of nickel oxide with graphite34
or sucrose35 typically occurs at temperatures above 800 ꢀC,
reecting slow reaction kinetics. Contrarily, in the present case
this process occurs at considerably lower temperatures (viz.,
390 ꢀC). Therefore, additional reactions are considered that may
simultaneously take place and involve the gas phase.34
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Fig. 4 Carbon layer characterization of C–Ni: TEM micrograph of
a NiO@Ni NP covered by a carbon layer (A). EELS mapping of NiO@Ni
NPs surrounded by carbon (B). Color code: red: Ni; green: O; blue: C.
C 1s core level XPS spectrum of C–Ni including fitting curves of
components located at 284.6 eV (C]C), 285.7 eV (C–C and C–H), and
287 eV (C–O and C]O) (C).
Fig. 3 Study of carbothermal reduction of nickel: TG-MS curves of
H2O, CO, and CO2 produced from BC (dashed lines) and BC–Ni (solid
lines) under N2 atmosphere. (A) The existence and formation regimes
of Ni(OH)2, NiO, and Ni are indicated. DSC curve of BC–Ni(OH)2 in
nitrogen (B). XRD pattern (C) of BC–Ni(OH)2 annealed under nitrogen
at 320 ꢀC and 380 ꢀC. The inset photograph in (C) shows the corre-
sponding samples, where charring is manifested by the color change
from brown to black with increased annealing temperature.
Indeed, thermodynamic calculations show that CO can easily
reduce NiO at temperatures below 400 ꢀC according to reaction (2).36
In fact, TG-MS revealed a CO/CO2 ratio (2.4 ꢃ 10ꢁ1) that is a 100-fold
higher than what is required to reduce NiO at 390 ꢀC according to
the Ellingham diagrams, viz. 2 ꢃ 10ꢁ3. In other words, BC–Ni(OH)2
ꢀ
generates sufficient CO to facilitate reaction (3) at 390 C. On the
other hand, reaction (3) is known to be catalyzed by nascent nickel
and thus, may contribute by auto-catalyzing the reduction of NiO.
The role of bacterial cellulose in the carbothermal reduction
of NiO appears to be important as inferred by comparing the
process with glucose, the monomer in cellulose. Precipitated
NiO + CO / CO2 + Ni
CO2 + C / 2CO
(2)
(3)
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other transition metal oxides with DGꢀ > DGꢀ(CO2), that is,
above the CO2 (2CO + O2 / 2CO2) line in the Ellingham
diagram. In this regard, oxides of Co, Fe, Sn, Cu, or Mo should
be reduced. Exemplarily we corroborated this possibility by
successfully extending the method to Fe(III)-hydroxide.
b-FeO(OH) (JCPDS 34-1266) was precipitated on BC (ESI,
Fig. S12†) and was reduced to Fe/g-Fe2O3 according to XRD
(Fig. 6A). Quantitative analysis based on XRD peak integration
revealed a high Fe/g-Fe2O3 ratio of 27/73ꢀ, while the temperature
of the metal reduction is as low as 640 C (ESI, Fig. S13†). The
Scherrer size of the Fe phase is around 35 nm and consistent
with nanoparticles observed in Fig. 6B.
Fig. 5 Formation mechanism of C–Ni: schematic showing the trans-
formation of BC–Ni(OH)2 into C–Ni through various steps; the dehy-
dration of Ni(OH)2, the decomposition of BC (BC-d), the carbothermal
reduction of NiO, and the carbonization of BC (not drawn to scale).
Ni(OH)2 was mechanically mixed with glucose at a 1/1 wt ratio
and subjected to the same pyrolysis conditions as BC–Ni(OH)2.
The X-ray diffractogram (ESI, Fig. S11†) of the product clearly
showed the presence of NiO, amounting to approx. 15% with
respect to Ni as per peak integration, while in case of BC all NiO
was reduced to Ni. This nding suggests that the carbothermal
reduction with BC was more efficient as with glucose, which is
possibly attributed to the intimate contact of the Ni(OH)2
nanoparticles precipitated directly on the BC bers (cf. Fig. 1C).
Concerning the graphitic carbon layer on the surface of the
nanoparticles it is suggested that the amorphous carbon
produced from charred cellulose is graphitized by the in situ
formed Ni0 (vide supra).20,22 But it has also been shown that re-
deposition of volatile aromatic and aliphatic products from
saccharide or biomass pyrolysis can form graphene-like struc-
tures upon adsorption on solids.37 A similar mechanism may
also simultaneously operate in the present case and thus,
contribute to the formation of the carbonaceous layers on the
nickel NPs. However, the disordered and presumably porous
microstructure of this carbon layer is likely to allow diffusion of
oxygen to the Ni surface in ambient atmosphere and thus, lead
to the observed NiO shell between the metallic Ni core and the
external carbon layer.
Functional properties of C–Ni
The functional properties of the carbon–metal hybrids were
exemplarily shown for C–Ni, which displayed an electrical
conductivity of 0.3 S mꢁ1 and magnetic properties as a conse-
quence of the formation of the metal nanoparticles. For
instance, the magnetization cycles, M(H), of C–Ni revealed
hysteresis loops typical of a ferromagnetic (FM) material (Fig. 7),
while the temperature dependent magnetization, M(T), (ESI,
Fig. 14†) suggests magnetic anisotropy of the FM phase. The
saturation magnetization (Ms) at 300 K is 40 emu gNiꢁ1. This
value is lower than the Ms of bulk nickel (55 emu gꢁ1), however
Synthesis and characterization of carbon–iron hybrids
Fig. 7 Magnetization cycles M(H) of C–Ni at 4 K and 300 K. The
coercivity Hc of C–Ni is 60 Oe at 300 K and 600 Oe at 4 K, which is
The here described carbothermic synthesis method for the
production of carbon–nickel hybrids could also be applied to consistent with the presence of ferromagnetic Ni.
Fig. 6 XRD pattern (A) of BC–FeO(OH) and C–Fe, which is obtained from pyrolysis of BC–FeO(OH) at 650 ꢀC. The C–Fe pattern shows
reflections attributed to g-Fe2O3 and metallic Fe. SEM micrograph of C–Fe (B).
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this reduction in Ms is commonly observed for nanosized
magnetic particles.38 This combination of electric conductivity
and magnetic permeability can be useful for the production of
electromagnetic interference shields.13 Moreover, preliminary
tests have shown that the generation of the porous, graphitic
ber network can result in very low total reectance in the
visible light range, which is interesting for efficient light
absorption in solar energy applications.
3 Y. Chang, M. Antonietti and T.-P. Fellinger, Angew. Chem.,
Int. Ed., 2015, 54, 5507–5512.
4 Z.-Y. Yu, Y. Duan, M.-R. Gao, C.-C. Lang, Y.-R. Zheng and
S.-H. Yu, Chem. Sci., 2017, 8, 968–973.
ˇ
ˇ
´
´
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Conclusions
In summary, we have shown that highly crystalline nickel
nanoparticles can be synthesized at a remarkably low temper-
ature of 390 ꢀC with the concomitant generation of graphitic
carbon nanober foams produced from bacterial cellulose
networks. These Ni0 nanoparticles are homogeneously distrib-
uted within the carbon aerogel materials. A reaction path was
identied that leads from dehydration of the nickel hydroxide
to nickel oxide and nally to metallic nickel aer carbothermic
reaction with the charred cellulose. Our study suggests the
combination of a solid state reaction of carbon and NiO with
a gas phase reaction involving CO as reductant, which is
generated during pyrolysis of bacterial cellulose. On the other
hand, the nascent nickel nanoparticles catalyze the formation of
graphitic carbon surrounding the nanoparticles. The low
temperature process indicates a graphitization mechanism
different from the dissolution/crystallization mechanism, for
which more work is required for the elucidation of the exact
mechanism. Nevertheless, the here proposed synthesis route is
versatile and was easily extended in a rst attempt to iron,
hinting the possible applicability to other related transition
metals. The nanostructured carbon–nickel foams simulta-
neously showed electrical conductivity and ferromagnetic
behavior, which could be exploited in the future as sustainable
material in diverse advanced applications.
17 E. Jazaeri and T. Tsuzuki, Cellulose, 2013, 20, 707–716.
¨
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Conflicts of interest
There are no conicts to declare.
22 J. Hoekstra, A. M. Beale, F. Soulimani, M. Versluijs-Helder,
J. W. Geus and L. W. Jenneskens, J. Phys. Chem. C, 2015,
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235–246.
Acknowledgements
The authors thank A. Varela and I. Poveda (UAM-Sidi) (SEM), the
ICMM thermal analysis service, Prof. A. de Andres (Raman
measurements), R. Barrio (BET measurements), Prof. M. Garcıa
(magnetic measurements), as well as Prof. M. Camblor, Dr A.
´
´
´
´
Blanco and Dr R. Jimenez for fruitful discussions. Financial
support from CICYT (projects MAT2012-31759, MAT2015-
71117-R). BW acknowledges an IJCI contract.
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