One-Step Pyrolysis Preparation of 1.1.1 Oriented Gold Nanoplatelets Supported on Graphene and Six Orders of Magnitude Enhancement of the Resulting Catalytic Activity

Article abstract of DOI:10.1002/anie.201508908

Pyrolysis of chitosan films containing Au³⁺ renders 1.1.1 oriented Au nanoplatelets (20 nm lateral size, 3-4 nm height) on a few layers of N-doped graphene (${overline {{rm{Au}}} }$/fl-G), while the lateral sides were 0.0.1 oriented. Comparison of the catalytic activity of ${overline {{rm{Au}}} }$/fl-G films with powders of unoriented Au NPs supported on graphene showed that ${overline {{rm{Au}}} }$/fl-G films exhibit six orders of magnitude enhancement for three gold-catalyzed reactions, namely, Ullmann-like homocoupling, C-N cross coupling, and the oxidative coupling of benzene to benzoic acid. This enhancement is the result of the defined morphology, facet orientation of Au nanocrystals, and strong gold-graphene interaction.

Full text of DOI:10.1002/anie.201508908

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201508908
German Edition: DOI: 10.1002/ange.201508908
Metal–Graphene Catalysis
One-Step Pyrolysis Preparation of 1.1.1 Oriented Gold Nanoplatelets
Supported on Graphene and Six Orders of Magnitude Enhancement of
the Resulting Catalytic Activity
Ana Primo, Ivan Esteve-Adell, Simona N. Coman, Natalia Candu, Vasile I. Parvulescu,* and
Hermenegildo Garcia*
Abstract: Pyrolysis of chitosan films containing Au3+ renders
1.1.1 oriented Au nanoplatelets (20 nm lateral size, 3–4 nm
height) on a few layers of N-doped graphene (Au/fl-G), while
the lateral sides were 0.0.1 oriented. Comparison of the
catalytic activity of Au/fl-G films with powders of unoriented
Au NPs supported on graphene showed that Au/fl-G films
exhibit six orders of magnitude enhancement for three gold-
exposed facets was recently achieved by using additives that
coordinate strongly to one facet, arresting its growth in
comparison to other crystallographic planes.[14,15] These
oriented Au nanocrystals can be subsequently supported on
a solid, aimed at adsorption of the nanocrystal with some
preferential orientation. Graphenes are among the currently
preferred supports, owing to their large surface area, easy
dispersability, and strong metal–support interactions. There
are now numerous examples showing that the catalytic
activity of MNPs supported on graphene is higher than the
activity of analogous MNPs supported on other carbon forms
or metal oxides.[16–18]
À
catalyzed reactions, namely, Ullmann-like homocoupling, C
N cross coupling, and the oxidative coupling of benzene to
benzoic acid. This enhancement is the result of the defined
morphology, facet orientation of Au nanocrystals, and strong
gold-graphene interaction.
Herein, we report the preparation of 1.1.1 facet-oriented
Au nanoplatelets of average lateral dimension 20 nm and 3–
4 nm height, supported on a few layers of N-doped graphene
films (Au/fl-G, Au: 1.1.1 oriented Au nanoplatelets, fl: few
layers, G: graphene) that exhibit about six orders of
magnitude higher catalytic activity than the analogous
unoriented Au catalyst (Au/fl-G) for three different gold-
catalyzed reactions.
The preparation of Au/fl-G is based on the formation of
N-doped G by simultaneous pyrolysis of nanometric films on
quartz under inert atmosphere at 9008C of a natural biopo-
lymer (chitosan), and Au nanoplatelets by reduction of
Au(III). The Au particles are morphologically nanoplatelets,
and their dimensions range from 20 nm to larger than
1000 nm, depending on the Au loading on the polymeric
precursor. In a related system, it was reported that Cu2O
nanocrystals supported on G exhibit greatly enhanced
catalytic activity.[19]
Pyrolysis is known to convert chitosan and other filmo-
genic natural polysaccharides into G.[20] Apparently, the fibrils
of chitosan, the carbonaceous residues derived from them,
and the evolving N-doped G arrest in each step the growth of
Au NPs forming simultaneously in the process, as a conse-
quence of the reductive conditions of the G synthesis and the
low solubility of Au on carbon.[21,22] While many metals form
metal carbides upon heating the metal with carbon at high
temperatures, the noble character of Au results in a phase
segregation, leading to the synchronous formation of N-
doped G and Au NPs. Remarkably, despite the high pyrolysis
temperature, the average lateral size of the Au nanoplatelets
can be as small as 20 nm for films with low Au loading
(ngcmÀ2). Besides being a precursor of N-doped G, one of the
key points in the synthesis is the ability of chitosan films to
adsorb metal ions by strong interactions with the positive
C
atalysis by supported metal nanoparticles (MNPs) has
been a very active field of research in the last thirty years.[1]
Remarkable examples of highly active and selective MNP
catalysts for organic transformations include aerobic oxida-
tions, couplings, and rearrangements.[1] Catalysis by gold
constitutes one of the most salient examples of unforeseen
properties for a material that appear exclusively at the
nanoscale.[2–4] A constant target in this area has been the
development of more efficient Au catalysts, and this has been
generally achieved by changing the support and average
particle size. However, according to an existing theory,[5,6]
control of the crystallographic facet exposed by the NP should
be a powerful tool to boost catalytic activity. Experimental
support of this concept has remained elusive owing to the
difficulty in preparing MNPs having preferential facet ori-
entation, and there are only a few examples of highly active,
morphologically defined Au nanocrystals.[6–9] There has been,
therefore, an interest in developing synthetic strategies for the
preparation of oriented Au NPs, although the success has
been limited until recently.[10–13] The preparation of colloidal
solutions of Au nanocrystals with defined morphologies and
[*] Dr. A. Primo, I. Esteve-Adell, Prof. H. Garcia
Instituto Universitario de Tecnología Química CSIC-UPV
Universidad PolitØcnica de Valencia
Avda. de los Naranjos s/n, 46022 Valencia (Spain)
E-mail: hgarcia@qim.upv.es
Dr. S. N. Coman, N. Candu, Prof. V. I. Parvulescu
Department of Organic Chemistry, Biochemistry, and Catalysis
University of Bucharest
Bd. Regina Elisabeta nr. 4–12, 030018 Bucharest (Romania)
E-mail: vasile.parvulescu@chimie.unibuc.ro
Supporting information and ORCID(s) from the author(s) for this
article are available on the WWW under http://dx.doi.org/10.1002/
anie.201508908.
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amino groups of the glucosamine units of the polymer fibrils
peaks that correspond indisputably to c Au crystals (JCPDS
No 01-1174). As seen in Figure 1, the consequence of the
simultaneous formation of N-doped G and Au NPs is the
orientation of the Au NPs crystals.
À
and metal anions, in this case AuCl4
.
The loading of Au NPs on the G film can be controlled by
À
varying the concentration of AuCl4 in the solution during
impregnation of chitosan films (Scheme 1). In this way, Au
Possible explanations to rationalize the preferential
growth of the 1.1.1 facets include the combination of higher
thermodynamic stability of this facet, together with a templa-
tion effect of N-doped G on the growth of Au nanoplatelets.
This explanation is based on the accepted mechanism for the
chemical vapor deposition of G on metal surfaces.[21,25,26] In
these syntheses of G, it is accepted that carbon atoms start to
deposit on the metal surface and the metal atoms template the
hexagonal arrangement of the growing G formed epitaxially
on the metal surface. In our system, the process would be
reversed: as G sheets are formed in the pyrolysis from
chitosan, Au atoms evolving from the polymeric fibrils will
appear on the surface of G, resulting in an epitaxial growth of
the 1.1.1 facets of Au nanoplatelets deposited on G, thereby
maximizing the interaction between the Au atoms and G
(Supplementary Information, Figure S1).
Scheme 1. Preparation procedure for Au/fl-G films.(i) Spin coating of
an aqÀueous chitosan solution on quartz (2”2 cm2); (ii) Adsorption of
AuCl4 on chitosan film before pyrolysis under inert atmosphere at
9008C.
Direct information on the morphology of Au crystals and
their preferential 1.1.1 facet orientation, regardless of the
loading of Au and thickness of G, was obtained by electron
microscopy. TEM images of Au/G (both fl and ml) samples
revealed that the preferential morphology of Au nanocrystals
is nanoplatelets, whose size distribution and average dimen-
sions depend on the concentration of Au on chitosan. Low-
loading Au/fl-G films have a Au content of 3.2 ngcmÀ2
(16.2 pmolcmÀ2). For low-loading Au/fl-G films that do not
exhibit any XRD pattern, the average lateral dimension of the
nanoplatelets determined by TEM was about 20 nm
(Figure 2; Supporting Information, Figures S2 and S3). A
few nanoplatelets detached from the G surface, allowing
estimation of a 3–4 nm for the thickness of these nano-
platelets (see marked nanoplatelets in Figure S4), an estima-
tion that was confirmed more accurately by AFM. It is worth
noting that these relatively small average dimensions are
remarkable considering the high temperature required in the
synthesis of Au/fl-G films and the known tendency of Au NPs
to undergo aggregation at high temperatures. For these films
with low Au loading, as well as for those with larger Au
loading, transmission Kikuchi diffraction[27] of relatively large
film areas (2.5 ” 2.5 mm2) showed the preferential 1.1.1 facet
orientation of Au nanoplatelets (Figure 2; Supporting Infor-
mation, Figure S5), consistent with the XRD of the thicker
Au/mL-G films. Comparison of the FESEM images showing
the presence of Au with that of the 1.1.1 facet based on the
transmission electron diffraction pattern showed a coinci-
dence over 90%. Moreover, these images also reveal the strict
0.0.1 orientation of the planes perpendicular to the larger
1.1.1 surface. Another interesting observation related to the
interplay between Au and G is the presence on G of some
pathways around Au nanoplatelets with significantly
decreased thickness of the G sheet. It seems that formation
of Au nanoplatelets causes depletion on the number of G
layers, possibly owing to the activity of Au nanoplatelets as
catalytic sites improving graphitization by removal of residual
oxygenated functional groups or the random walk of Au
nanocrystals on the G sheets defining some pathways. AFM
loadings from a few ngcmÀ2 up to mgcmÀ2 on the Au/fl-G film
can be obtained in a reproducible way. The preparation
procedure was repeated independently many times with
identical Au nanocrystal orientation and consistent character-
ization data for the Au/fl-G films. This one-step synthesis of
Au/fl-G contrasts with other reported methods for the
preparation of Au NPs supported on G that require prior
preparation of G,[23] and even modification of the G surface.[24]
For the sake of comparison, Au/fl-G powders were also
synthesized by supporting preformed Au NPs on N-doped G
(see the Experimental Section).
XRD of Au/fl-G samples (loading of Au 3.2 ngcmÀ2) did
not exhibit any peaks, owing to the low film thickness and Au
loading. In contrast, thicker samples of multilayer G (ml-G,
ml: multilayer; thickness > 40 nm) containing Au exhibit
a single peak at 398, accompanied by a weaker 2.2.2 peak at
828, characteristic of 1.1.1 oriented crystals. Figure 1 shows
Figure 1. XRD patterns of Au/fl-G (1 wt%; left) and Au/mL-G
(2.4 mg”cmÀ2; right) showing the different indexation of the peaks.
The broad band at about 228 observed in the Au/mL-G corresponds to
the characteristic diffraction of ml-G.
XRD of Au/mL-G (Au loading 2.4 mg ”cmÀ2) compared to
Au/fl-G (Au loading 1 wt%) lacking any preferential orien-
tation. In the latter case, XRD presents more diffraction
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Figure 3. Top view at two different magnifications of the AFM image
taken for Au/fl-G (3.2 ngcmÀ2). The size (height and lateral dimen-
sions) of five Au nanoplatelets circled in the right frame are presented
in the bottom part of the image. Notice that according to the low
magnification image shown in the left frame, Au nanoplatelets are
consistently located on valleys of the G film of height lower than
10 nm.
Figure 2. a and b) TEM images of Au/fl-G (3.2 ngcmÀ2) at two differ-
ent magnifications. Insets: corresponding particle size distribution. c
and e) EDS analysis of Au of two different films (Au loa-
ding=3.2 ng”cmÀ2) showing the presence of Au in red. d and f) Facet
orientation of the nanoplatelets presented the 1.1.1 facet (blue) and
the 0.0.1 planes (pink) obtained by transmission Kikuchi diffraction.[27]
Insets: color codes.
Figure 4. a) Raman spectrum of Au/fl-G (3.2 ngcmÀ2). The presence of
a sharp 2D band on top of a broad 2D background can be clearly
observed at 2850 cmÀ1. b) SEM image of Au/fl-G (3.2 ngcmÀ2). Inset:
particle size distribution.
measurements of Au/fl-G also confirm the nanoplatelet
morphology of Au crystals and their uniform height of
about 3–4 nm. Figure 3 and Figure S6 (Supporting Informa-
tion) show top views of Au/fl-G, as well as measurements of
the height of Au nanoplatelets distributed on a relatively large
area of the Au/fl-G film. Cuts on the film indicate that the
thickness of the G is about 20 nm, although there are certain
areas around Au crystals with G thickness about 8 nm. The
existence of certain pathways around Au nanoplatelets
observed in FESEM is also seen in the top AFM images of
Au/fl-G, which show that most Au nanoplatelets are located
in valleys of the G film.
sharp 2D, not observed in analogous G samples, arises as
consequence of the influence of Au nanoplatelets on G,
making the G film in the surroundings of the Au nanoplatelets
thinner and increasing the intensity of the band by plasmon
Raman enhancement. It is noteworthy that the position of the
2D band appears generally at values below 2750 cmÀ1.[24]
Other groups have observed this shift and attributed it to
charge transfer from G to the adsorbate.[29] Accordingly, the
abnormal shift of the 2D band toward higher values could be
attributed to a charge transfer from G to Au nanoplatelets,
owing to the different work function of the two components.
X-ray photoelectron spectroscopy (XPS) established the
composition and distribution of each element in the material
into different coordination environments. As expected, the
Au/fl-G films contain C (89 at%), N (1 at%), O (5 at%), and
Au (4 at%). Figure 5 and Figure S7 (Supporting Information)
also show the high-resolution C 1s and the Au 4f peaks
recorded for Au/fl-G. The experimental C 1s peak can be
deconvoluted into three main contributions of 66, 28, and 6%,
The Raman spectrum of Au/fl-G (Figure 4) shows the
characteristic G and D bands appearing at 1600 and
1350 cmÀ1, accompanied by
a broad 2D band around
2700 cmÀ1, expected for G obtained by pyrolysis of chitosan.
The IG/ID ratio was 1.13, which is a common value for this G
with N-doping (from glucosamine units of chitosan) and
residual O functionalities.[28] An abnormal feature of the
Raman spectrum was, however, the appearance of a sharp
peak at about 2850 cmÀ1 on top of the broad 2D band. The
appearance of a narrow 2D peak is generally associated to the
presence of single or double G layers. It is likely that this
=
À
corresponding to graphenic sp2 (284.5 eV), C O/C N
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Table 1: Comparative activity data determined for Au/fl-G film and Au/fl-
G.
Catalyst
Conversion [%] Selectivity [%] Turnover Number
Au/fl-G film[a]
Au/fl-G[a,b]
0.3
0.2
83 to 2
16 to 3
100
3.7”105
79
Figure 5. XPS C 1s (a) and Au 4f (b) peaks, as well as their best fitting
to individual components recorded for the Au/fl-G sample
(3.2 ngcmÀ2).
(285.5 eV), and CO2H (288.5 eV) C atoms, respectively. The
Au 4f core level peak presents the expected two contributions
owing to Au 4f5/2 and Au 4f7/2 separated by 3.73 eV, arising
from spin-orbit splitting at 88.18 and 84.45 eV, respectively.
The shape of the experimental Au 4f peak indicates that there
is a single component, Au0, without significant contribution of
AuI and AuIII. The binding energy values of the Au 4f peak in
Au/fl-G are, however, shifted by 0.4 eV to higher values
relative to bulk Au. This binding energy shift is again
compatible with the occurrence of a strong gold–support
interaction.
Au/fl-G film[c]
Au/fl-G[b,c]
14.7
5.1
95.2 to 5
4.8 to 6
100 to 5
9.2”106
10
Au/fl-G film[d]
Au/fl-G[b,d]
23.9
35.2
100
100
1.4”107
21.8
Supported Au NPs exhibit remarkable catalytic activity
for a series of reactions, including selective oxidations,
reductions, and couplings.[2,30,31] To evaluate the influence of
preferential 1.1.1 facet orientation and the strong metal–
support interaction on the catalytic performance, the activity
of the Au/fl-G film (quartz plates, 3.2 ngcmÀ2) was compared
with that of an analogous Au/fl-G sample in which Au NPs
obtained by the polyol method were deposited on fl-G at
0.1 wt% Au content. The polyol reduction is known to render
Au NPs of about 5 nm average size.[32,33] The fl-G was
obtained by pyrolysis of chitosan and subsequent exfoliation
of the turbostratic carbon residue by sonication.[28] Preformed
Au NPs were subsequently adsorbed on fl-G. Au/fl-G
exhibited the XRD shown in Figure 1, devoid of any
preferential facet orientation. The comparison of the influ-
ence of facet orientation and strong metal–support interac-
tion was evaluated for two representative coupling reactions,
namely, the Ullmann-like homocoupling of iodobenzene to
[a] Reaction conditions: Iodobenzene 2.00 mmol, 4 mL 1,4-dioxane,
base=2 mmol (KOCH3), reaction temperature: 1608C, time 24 h,
catalyst: Au/fl-G films 1”1 cm2 or Aufl-G powder 10 mg. [b] When
exactly the same total amount of Au present in Au/fl-G is used for Au/fl-
G, no reaction was observed (see text for comments). [c] Reaction
conditions: Bromobenzene 1,2 mmol, aniline 1 mmol, KOCH3
2.1 mmol, 1,4-dioxane 4 mL, reaction temperature: 2008C, time 24 h.
[d] Reaction conditions: Benzene 10 mmol, PhI(OAc)2 1 mmol, solvent
17 mmol HOAc, reaction temperature: 958C, time=24 h, catalyst: Au/fl-
G films 1”1 cm2 or Au/fl-G powder 10 mg.
The Ullmann-like homocoupling of iodobenzene affords
biphenyl as major product, accompanied by minor amounts of
ortho and para isomers of iodobiphenyl. Similarly, the
reaction of aniline and bromobenzene under basic conditions
affords diphenylamine accompanied (Au/fl-G) or not (Au/fl-
À
biphenyl [Eq. (1), Table 1], the C N cross coupling of anilines
[Eq. (2)], and the degradative oxidation of benzene to
benzoic acid [Eq. (3)].
À
Control experiments showed that fl-G does not exhibit
any noticeable catalytic activity in comparison to that of the
samples containing Au. In contrast, Au/fl-G films exhibited
remarkable catalytic activity. Hot filtration tests removing the
Au/fl-G plates after 8 h reaction time showed that the
conversion stops in all cases. Furthermore, Au was not
detected in the liquid phase after removal of Au/fl-G at
final reaction time. When the catalytic activity of unoriented
Au/fl-G sample was tested at the same Au/substrate mol ratio
as Au/fl-G films (about 10À9), no catalytic activity was
observed. However, at higher Au/substrate mol ratio (about
10À3), Au/fl-G was also significantly active. No leaching was
observed for Au/fl-G powders. The results are collected in
Table 1.
G) by triphenylamine, arising from the double C N coupling.
In the case of the oxidative degradation of benzene by
phenyiodoso diacetate, biphenyl is the primary product that
undergoes efficient degradation to form benzoic acid. As seen
in [Eq. (4), Table 1] the carbon of the carboxylic acid group of
benzoic acid derives from the oxidation of biphenyl inter-
mediate. The most remarkable fact from Table 1 is that the
turnover number values obtained for Au/fl-G film are all over
six orders of magnitude higher than those of Au/fl-G for the
same reaction. This higher catalytic activity is due to the high
activity of minute amounts of Au when deposited as oriented
nanoplatelets on fl-G, the preferential orientation of the Au
nanoplatelets, and the occurrence of strong Au-G interac-
tions, as evidenced by the particle size, the 2D band shift in
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General procedure for the Ullmann-like homocoupling: All
Raman, and the Au 4f binding energy in XPS. When a Au/fl-
G film with higher Au loading (2.4 mg ”cmÀ2) was used as the
catalyst, similar catalytic activities at the final reaction times
were observed, but turnover numbers of these unoptimized
loadings were about two orders of magnitude lower, probably
owing to the larger Au nanocrystal size (up to 1000 nm). At
the end of the reaction, the Au/fl-G film becomes detached
from the quartz plate, but could be recovered from the
reaction mixture. A second use of the detached Au/fl-G
catalyst for the oxidative degradation showed similar catalytic
activity (22%) as that of the fresh catalyst. Furthermore, SEM
images of the used Au/fl-G catalyst did not show significant
changes in the morphology and size of the Au nanoplatelets
(Figure S8).
In conclusion, we have shown a reliable procedure for the
preparation of 1.1.1 facet-oriented Au nanoplatelets sup-
ported on fl-G by pyrolysis at 9008C of a chitosan precursor
containing AuCl4À. The resulting Au/fl-G films exhibit an
extremely high catalytic activity for coupling and oxidation
reactions compared to unoriented Au/fl-G analogues.
Although further work is necessary to fully clarify the origin
of the remarkable catalytic activity, it seems that it derives
from the combination of strong metal–support interaction
and preferential facet orientation. Another issue is to show
the general applicability of this procedure for other metals
that do not dissolve on carbon, and assessment of any
preferential facet orientation.
reagents were purchased from Sigma–Aldrich and used as received
without any purification. To a solution of iodobenzene (2.00 mmol) in
4 mL of 1,4-dioxane, KOCH3 (2 mmol) and catalyst (1 ” 1 cm2 plate of
Au/fl-G film or 10 mg of Au/fl-G powder, 0.1 Au loading) were added.
The resulting mixture was stirred in an autoclave for 24 h at 1608C.
After the reaction, the catalyst was collected by filtration and the
reaction products were analyzed and identified by GC-MS
(THERMO Electron Corporation instrument, Trace GC Ultra and
DSQ, TraceGOLD): TG-5SilMS column with the following specifi-
cations: 30 m ” 0.25 mm ” 0.25 mm, working with a temperature pro-
gram that starts at 508C maintained for 2 min and afterwards
increasing the temperature at a rate of 108CminÀ1 up to 2508C that
was maintained for 10 min, resulting in a total run time of 32 min. The
pressure of He used as the carrier gas was 0.38 Torr. Mass spectra of
the products were acquired at 70000 resolutions. Biphenyl (2): MS
(EI) m/z (rel.int): 154 (M+, 100%), 128 (4), 115 (4), 76 (12), 63 (3), 51
(3); o-Iododiphenyl (3a): MS (EI) m/z 23(rel.int): 280 (M+, 66.8%),
152 (100), 140 (8), 127 (8), 76 (14), 63 (4); p-iododiphenyl (3b): MS
(EI) m/z (rel. int): 280 (M+, 100%), 152(78), 140 (6), 127 (7), 76 (12),
63 (3).
General procedure for the C-N reaction: To a solution of
bromobenzene (1.2 mmol) and aniline (1 mmol) in 4 mL of 1,4-
dioxane, KOCH3 (2.1 mmol) and catalyst were added. The resulting
mixture was stirred in an autoclave for 24 h at 2008C. After the
reaction, the catalyst was collected by filtration and the reaction
products were analyzed and identified by GC-MS (THERMO
Electron Corporation instrument).
Diphenylamine (5): MS (EI) m/z (rel.int): 169 (M+, 100%), 141
(5), 115 (4), 84 (12), 77 (5), 51 (3); Triphenylamine (6): MS (EI) m/z
(rel.int): 245 (M+, 100%), 167 (22), 141 (12), 115 (9), 77 (10), 51 (5).
General procedure for the oxidative coupling reaction: To
a solution of benzene (10 mmol) and 1 mmol of oxidant [PhI(OAc)2],
acetic acid (17 mmol) and catalyst were added. The mixture was
stirred for 24 h at 958C in an autoclave, and then quenched with water
(10 mL). The reaction mixture was extracted with EtOAc (3 ” 10 mL)
and the combined organic layer was washed with saturated NaHCO3
(2 ” 20 mL), brine (10 mL), dried over Na2SO4, filtered, and concen-
trated. The products were analyzed and identified by using GC-MS
Experimental Section
Synthesis of few-layers graphene (fl-G): Alginic acid sodium salt from
brown algae (Sigma) was pyrolyzed under argon atmosphere using
the following oven program: annealing at 2008C for 2 h, followed by
heating at 108CminÀ1 up to 9008C for 6 h. The resulting graphitic
powder was sonicated at 700 W for 1 h in water and the residue
removed by centrifugation to obtain fl-G dispersed in water.
Au NPs deposition on fl-G (0.1%wt): fl-G from alginate pyrolysis
(100 mg) was added to ethylene glycol (40 mL) and the mixture was
sonicated at 700 W for 1 h to obtain dispersed fl-G. HAuCl4 (0.2 mg)
was added to the reaction mixture and Au metal reduction was then
performed at 1208C for 24 h with continuous stirring. The Au/fl-G
were finally separated by filtration and washed exhaustively with
water and acetone. The resulting material was dried in a vacuum
desiccator at 1108C to remove the remaining water. The amount of
gold present on the films was determined by ICP-OES by immersing
the plates into aqua regia (HNO3/HCl 1:3) at room temperature for
3 h and analyzing the Au content of the resulting solution.
(THERMO Electron Corporation instrument) and
a Bruker
Advance III UltraShield 500 MHz spectrometer, operating at
500.13 MHz for 1H NMR, 125.77 MHz for 13C NMR. Turnover
numbers were calculated by dividing the moles of product formed
by the moles of Au present in the catalyst at final reaction time.
Acknowledgements
Financial support by the Spanish Ministry of Economy and
Competitiveness (Severo Ochoa and CTQ2012-32315) and by
the Generalidad Valenciana (Promoteo 2013-019) is grate-
fully acknowledged. I.E-A. thanks the Spanish Ministry for
a postgraduate scholarship. SMC, NC, and VIP thank PNCDI
II, project 275/2011, for the financial support. We are thankful
to Mrs. Amparo Forneli for her assistance in sample
preparation.
Synthesis of oriented Au NPs over few-layers graphene films
(Au/fl-G): 0.5 g of chitosan from Aldrich (low molecular weight) was
dissolved in water with a small quantity of acetic acid (0.23 g), which is
necessary for complete dissolution of chitosan. The solution was
filtered through a syringe of 0.45 mm diameter pore to remove
impurities present in commercial chitosan. The films were supported
on a quartz plate (2 ” 2 cm2) by casting 500 mL of filtered solution at
4000 rpm during 1 min. In the second synthesis step, the obtained
chitosan films were immersed in a HAuCl4 solution (1 mm or
0.01 mm) during 1 min. The pyrolysis was performed under argon
atmosphere using the following oven program: heating rate at
58CminÀ1 up to 9008C for 2 h. The amount of gold present on the
films was determined by ICP-OES by immersing the plates into aqua
regia at room temperature for 3 h and analyzing the Au content of the
resulting solution.
Keywords: coupling catalysts · graphene support ·
heterogeneous catalysis · oriented gold nanoparticles
How to cite: Angew. Chem. Int. Ed. 2016, 55, 607–612
Angew. Chem. 2016, 128, 617–622
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Received: September 23, 2015
Revised: October 26, 2015
Published online: November 24, 2015
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Angew. Chem. Int. Ed. 2016, 55, 607 –612