Synthesis and H2 adsorption on graphitic nanofibres

Article abstract of DOI:10.1016/j.jallcom.2005.02.073

Graphitic nanofibers (GNFs) were successfully synthesised at low temperatures (500 ° C) using NiO powder as the catalyst precursor and a H₂/C₂H₄ gas mixture with a high yield, around 60 g g_catalyst^-1 h^-1. XRD patterns confirm that during the formation of GNFs the NiO was reduced to form metallic Ni. TEM study confirmed the formation of a graphitic phase in the form of GNFs, where the diameter is controllable with synthesis temperature, from 500 to a few nm. Surface treatments have been carried out and these have been used to modify the GNF morphology and surface chemistry. H₂-uptake measurements using a Sievert's apparatus are reported and correlated with the GNF properties.

Full text of DOI:10.1016/j.jallcom.2005.02.073

Journal of Alloys and Compounds 404–406 (2005) 634–636
Synthesis and H adsorption on graphitic nanofibres
2
∗
M. Bououdina, D. Grant, G. Walker
Advanced Materials, School of Mechanical, Materials, Manufacturing, Engineering and Management,
University of Nottingham, Nottingham NN7 2RD, UK
Received 7 June 2004; received in revised form 1 February 2005; accepted 7 February 2005
Available online 11 July 2005
Abstract
◦
Graphitic nanofibers (GNFs) were successfully synthesised at low temperatures (500 C) using NiO powder as the catalyst precursor and
−
1
−1
a H
2
/C
2
H
4
gas mixture with a high yield, around 60 g g
h . XRD patterns confirm that during the formation of GNFs the NiO was
catalyst
reduced to form metallic Ni. TEM study confirmed the formation of a graphitic phase in the form of GNFs, where the diameter is controllable
with synthesis temperature, from 500 to a few nm. Surface treatments have been carried out and these have been used to modify the GNF
morphology and surface chemistry. H
2
-uptake measurements using a Sievert’s apparatus are reported and correlated with the GNF properties.
©
2005 Elsevier B.V. All rights reserved.
Keywords: Graphite nanofibers; TEM; XPS; H2-uptake
1
. Introduction
Many intermetallic materials such as (LaNi , ZrMn2,
different diameters, ranging from 10 to 200 nm. Recently,
H2 adsorption exhibited by GNFs, have attracted attention as
potential candidate as a hydrogen storage materials (HSM).
There are some H2 storage capacities reported for GNFs
from 1–2 wt.% up to 10 wt.% [4–7], but there is a lack of
reproducibility in these measurements. Theoretically, H2 can
be adsorbed on the surface and then incorporated between
graphiticsheetsofGNFs. Thespacebetweenthesheetswhich
5
TiFe, Mg, etc.) have shown a reversible H2 absorp-
tion/desorption reactions [1]. However, most of these
materials are too heavy to meet the DOE H2 capacity target
of 6.5 wt.% for on board transport storage systems. Mg-based
hydrides such as MgH2 and Mg2NiH4 have been rekindled
recently, due to their large H-storage capacities, 7.6 and
˚
is ≥3.35 A, acts like a slit-shaped pore hence enabling GNFs
3
.8 wt.%, respectively, but their technical applications are
to physisorb large amounts of H2, whose kinetic diameter is
˚
limited by their slow hydriding/dehydriding kinetics and
high decomposition temperatures, 275 and 255 C (at 1 atm),
only 2.89 A [4]. H2 capacity of GNFs has been measured by
◦
volumetric [8], gravimetric (TG) [9], and thermal desorption
spectroscopy (TDS) [10]. But, nevertheless, the reported H2
capacities are disparate, and further studies are needed to
better understand the factors that influence the values (as
well as the measurements). The irreproducibility is partially
due to poor characterisation, highly variable GNF samples,
and other experimental parameters concerning the H2-uptake
measurements such as: low amounts of GNFs used (around
50 mg or even less), low H2 purity and humidity of H2 gas.
In this paper, we report the synthesis of graphitic
nanofibers as well the formation of Ni carbide phase at low
temperatures, and the effect of the synthesis temperature on
the type of the graphitic structure formed. The GNF samples
were characterised by XRD, TEM, XPS, BET and H2-uptake
measurements.
respectively [2]. Graphitic nanofibers (GNFs) belong to
carbon nanostructures based on that for graphite. The basic
microstructure consists of stacked graphite layers, which can
be arranged parallel/perpendicular to the fiber axis, or her-
ringbone structure, often with an amorphous component. The
angle between the graphitic planes and the fiber axis which
determine the type of structure is determined by the shape of
the catalyst particle, as reported by Boellard et al. [3]. The
as-prepared GNF morphology and microstructure depend on
catalyst composition, catalyst support, gas mixture, and re-
action temperature. In addition, GNFs can be produced with
∗
Corresponding author. Tel.: +44 115 951 3752; fax: +44 115 951 3800.
E-mail address: gavin.walker@nottingham.ac.uk (G. Walker).
0
925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2005.02.073

M. Bououdina et al. / Journal of Alloys and Compounds 404–406 (2005) 634–636
635
2
. Experimental
GNFs were prepared using NiO powder as the catalyst
precursor, the NiO powder was synthesised using Ni-nitrate
[
Ni(NO3)2·6H2O] (Alfa Aesar) solution added to 2 M ammo-
◦
nia solution (Fisher Scientific) and dried at 120 C. The cat-
alyst precursor was loaded in an alumina crucible and placed
in a tube furnace. The feedstock gas H2/C2H4 at 80:20 sccm,
◦
was flowed for 2 h at a given temperature (500 and 700 C).
The reflux treatment was carried out using 30 mg of GNF
powder in 100 ml of 2 M nitric acid (HNO3) at 110 C for
◦
Fig. 1. XRD patterns of GNF3 as-prepared and after reflux treatment.
−1
ature, increasingwithlowersynthesistemperature, 51 m g
for the 700 C sample compared to 170 m g for the 500 C
sample. After reflux treatment, the surface area increases
three times higher for GNFN7 and only 13% for GNFN3.
2
h. The samples were washed with deionised water and then
◦
2
dried at 80 C. GNF samples were characterised using X-
◦
2
−1
◦
ray diffraction (Siemens D500 equipped with K␣ Cu λ =
˚
1
2
.5418 A), transmission electron microscopy (JEOL JEM-
000FX II), X-ray photoelectron spectroscopy (VG Scien-
tific ESCA Lab X-ray photoelectron spectrometer fitted with
a twin Mg/Al X-ray) and BET surface area analysis (Quan-
3.4. XPS analysis
tachrome Autosorb-1). The GNF samples were degassed at
◦
2
00 C for 2 h prior to multipoint (7 points) BET measure-
XPS analysis was carried out on selected GNF sample
prepared at 500 C. From the C1s spectra, it is clear that
◦
ments. H2-uptake measurements have been carried out using
a Sievert’s apparatus. The samples were degassed at 200 C
under high vacuum for 1 h.
◦
the spectrum consists of a main binding energy peak with
high intensity attributed to graphitic C C species and an-
other small peak with relatively low intensity attributed to
C O. This is in good agreement with values reported in liter-
ature, 284.7 eV [12]. Moreover, no additional binding energy
peaks were detected, which could be attributed to the pres-
ence of impurities. After reflux treatment the C C graphite
peak keeps same position. However, additional binding en-
ergy peaks were detected, which could be attributed to the
presence of impurities, such as Ni(OH)2.
3
. Results
3
.1. GNFs synthesis
The GNFs growth depended on the experimental condi-
tions. It has been found that: (i) the efficiency of the syn-
thesis procedure depends on the temperature [11]; (ii) the
GNFs were synthesised at relatively low temperatures (350–
3.5. TEM analysis
◦
−1
◦
−1
within
4
00 C) with an optimum yield of 57 g g
h
catalyst
◦
the temperatures range 500 C ≤ T ≤600 C, but decreased
TEM images of some selected as-prepared GNFs are
−
1
−1
◦
drastically below this temperature range, 2.4 g g
h
at
displayed in Fig. 2. For the sample prepared at 500 C
catalyst
◦
3
50 C; (iii) the use of a low H2/C2H4 gas ratio mixture, gave
(GNFN3), the fiber diameters could be grouped into three
ranges (Fig. 2a): large (>100 nm), medium (30–100 nm) and
small (<30 nm). Selected area diffraction (SAD) pattern gave
two arcs in line with the fiber axis consistent with a platelet
structure. The diffuse nature of the pattern indicates some
level of amorphisation within the fiber, in agreement with the
relative intensities area observations stated above.
proportionally higher GNF growth rates, as the ratio of car-
bon carrying gas is increased. Furthermore, it is found that
the presence of Ni3C phase as catalyst favours the formation
of more crystalline fibers at low temperatures, fine diffuse
bands can be observed from SAD patterns.
◦
3
.2. XRD
TEM image of GNFs synthesised at 700 C shows mainly
two fiber diameters (Fig. 2b): large (>100 nm) and small
(<30 nm). It was noticed that the fibers had a smaller length
compared to those synthesised at 500 C. SAD pattern of the
100 nm fiber shows well-defined arcs that indicates a platelet
type microstructure, with a more defined crystallinity struc-
ture.
Fig. 1 displays X-ray diffraction patterns of GNFs pre-
◦
◦
pared at different 500 and 700 C before and after reflux
treatment. A very broad carbon diffraction line (0 0 2) can
be observed with the presence of some additional diffraction
peaks with low intensity, attributed to Ni metal.
Table 1
3
.3. BET analysis
Surface area of selected GNFs samples
Sample
As-prepared GNF
Treated GNF
Table1reportsthevaluesofsurfaceareaoftheas-prepared
GNFN3
GNFN7
170
51
192
132
GNFs and after reflux treatment. It can be noticed that the
GNFs surface areas were influenced by the synthesis temper-

6
36
M. Bououdina et al. / Journal of Alloys and Compounds 404–406 (2005) 634–636
◦
◦
Fig. 2. TEM images of as-prepared GNFs: (a) at T = 500 C and (b) at T = 700 C.
3
.6. H2-uptake measurements
cursor and H2/C2H4 mixture gas. XRD, XPS and TEM
analysis confirmed the formation of GNFs. The surface
area of as-prepared GNFs increased after reflux treatment,
about three times higher than as-prepared sample. H2-
uptake measurements showed no significant uptake for these
samples.
After degassing, 100 bar of H2 pressure were admitted
into the sample chamber. After 20 h, no significant H2-uptake
was measured for either the as-prepared or the reflux treated
samples (<0.2 wt.%).
4
. Discussion
Acknowledgements
GNFs were synthesised in high yield using NiO as the cat-
alyst precursor and achieved a yield of 60 g g
This project is supported by EPSRC and DSTL. The au-
thors like to thank K. Dinsdale and M. Roe for their help with
TEM and XPS analysis.
−
1
−1
h . Both
catalyst
XRD and TEM analyses confirm the formation of GNF. NiO
was used as the catalyst precursor. NiO diffraction peaks were
notdetectedonXRDpatternsofas-preparedGNFs. However,
new peaks were observed and indexed as Ni metal. TEM im-
ages show that the diameter as well as the length vary with
the preparation temperature, ranging from a few nanometres
to 500 nm, and can be microns in length. BET measurements
carried out on prepared GNFs showed an increase in surface
area with lowering synthesis temperatures as well after ni-
tric acid reflux treatment. No H2-uptake has been observed
for both as-prepared and treated samples. This may be due
to the amorphous carbon film which remain on the surface
of the fibers even after reflux treatment or to the remaining
residual impurities that formed during the growth or surface
treatment. No significant H2-uptake has been detected so far,
further surface treatments are underway in an attempt to im-
prove the H2 adsorption. Any adsorbed H2 was merely due
to physisorption to the external surface of the GNFs.
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[
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[
GNFs were successfully synthesised by thermal CVD
method using unsupported NiO powder as a catalyst pre-