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Applied Materials Today xxx (xxxx) xxx

Contents l
ists avail e at ScienceDirect
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Applied Materials Today
j urnal homepage: www.el evier.com/l cate/apmt
o
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o

Anchoring of single-platinum-adatoms on cyanographene: Experiment and
theory
´
Rostislav Langer a,1 , Edvin Fako c,1 , Piotr Błonski b,∗ , Miroslav Vavreˇ ka a,b , Aristides Bakandritsos b ,
c
Michal Otyepka a,b , Núria López c,∗
a
b
c

Department of Physical Chemistry, Faculty of Science, Palack´ University, Olomouc, Czech Republic
y
Regional Centre of Advanced Technologies and Materials, Faculty of Science, Palack´ University, Olomouc, Czech Republic
y
Institute of Chemical Research of Catalonia (ICIQ), The Barcelona Institute of Science and Technology, Tarragona, Spain

a r t i c l e

i n f o

Article history:
Received 18 July 2019
Received in revised form 28 August 2019
Accepted 8 September 2019
Keywords:
Carbon-magnets
Anchoring
Derivatives
Graphene
Single-platinum-adatom
Single-atom-catalyst

a b s t r a c t
Graphene decorated with isolated single atoms (SAs) offers new vista to magnetic and spintronic devices
up to single-atom catalysts. While sp atoms can be efficiently bound to graphene, d-block atoms require
anchoring groups to prevent nanoparticle formation. Identification of suitable binding sites is a challenging task because the interaction among graphene, anchoring groups and adatoms is very complex.
Using density functional theory (DFT) we explored strength and nature of interactions of graphene covalently functionalized by − OH, − CN, − F, and − H groups as anchors for Pt SAs. Both theory and experiment
showed that − CN groups acted as suitable ligand enabling immobilization of 3.7 wt % single Pt adatoms.
The findings imply that CN functionalized graphene, i.e., cyanographene, is a perspective material for
anchoring metal adatoms with potential implications as single-atom-catalysts.
© 2019 Elsevier Ltd. All rights reserved.

1. Introduction
Since the discovery of graphene [1–3], its vast application potential has driven research in many fields [4–6]. Graphene is one of the
most studied materials of recent decades making it a popular model
system for graphitic materials [7 ]. Many of the fascinating properties of pristine graphene can be tuned by functionalization [8 –11]
and put to use in diverse applications, including electrical devices,
sensors, spintronics and other [12–17 ].
High specific surface and affordable price make graphene an
attractive support for heterogeneous catalysis. Atomistic design
of the active phase is vital for unlocking the full potential of heterogeneously catalyzed industrial (and other) chemical reactions
[18 ]. These catalytic reactions take place on (typically) metal surfaces and therefore bulk metal atoms that are not accessible by the
reactants are not involved in the catalysis. There are clear benefits
to be had from reducing the amount of such inactive atoms. The
downsizing of heterogeneous metal catalysts is a prominent way
of improving their catalytic properties, and single atom catalysts
(SACs) are at the border of what is possible in terms of advanc-

∗ Corresponding authors.
´
E-mail addresses: piotr.blonski@upol.cz (P. Błonski), nlopez@iciq.es (N. López).
1
These authors are contributed equally to this work.

ing this trend [19 –21]. The benefit of SACs lies not only in that
they provide the highest possible surface to volume ratio of any
heterogeneous catalyst, but they also give rise to several specific
interactions enhancing selectivity. SACs are at the ultimate frontier
in atom economy with nearly 100% metal loading being utilized to
form accessible active sites. Moreover, the well-defined structure
and strong interaction between the support and SAC suggest easy
reusability and recycling of the catalytic material [22,23].
Particularly, carbon supported SACs have recently been successfully employed for several processes traditionally considered
to be difficult to perform utilizing a heterogeneous catalyst, e.g.
Pd-catalyzed coupling reactions or Pd-SAs enhanced catalytic
reactivity in the semi-hydrogenation of acetylene to ethylene
[24–27 ]. Despite the successful application of SACs for an increasing number of applications, only a few highly stable coordination
environments [28 –32] have yet been experimentally characterized.
Graphene derivatives were identified as promising supports for
single adatoms [33,34], however, stabilizing the individual metal
atoms on the support remains a significant challenge in further
development of efficient and stable SACs. The doping of graphene
with sp elements (particularly sp3 doping) unlocked the means of
adjusting the electronic structure of graphene [16], however, the
relatively low interaction of the pristine graphene layer with the
d-block adatoms [35], implies that a different approach must be
utilized when trying to stabilize these isolated atoms on graphene

https://doi.org/10.1016/j.apmt.2019 .100462
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Please cite this article in press as: R. Langer, E. Fako, P. Błonski, et al., Anchoring of single-platinum-adatoms on cyanographene:
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[36–38 ]. Binding isolated metal atoms to already functionalized
graphene, provides additional interaction between the metal and
functional group with respect to pristine graphene and may represent an efficient way for synthesis of new branch of graphene-based
derivatives applicable in electronics and single atom catalysis. To
advance this field, it is necessary to identify suitable anchoring
groups for metal single atoms and deeply understand the nature
of metal-linker-graphene interaction and influence of the adatom
on the electronic structure of the graphene-derivative.
Here, we discuss the possibility of attaching single platinum
adatoms to diluted graphene derivatives, because platinum is often
used as the active center in various important catalytic processes
[25,32,39 ,40]. We considered existing graphene derivatives, i.e.,
fluorinated graphene [41], graphene oxide [42], cyanographene
[43] and graphane [44], i.e., derivatives containing functional
groups (X = − F, − OH, − CN, and − H), which can potentially anchor
the platinum single atoms (Fig. S1 and Table S1). Using theoretical calculations we identified the nitrile group as the most efficient
anchor for the Pt adatoms (in Pt0 and PtII oxidation states, Fig. S2)
which otherwise easily diffuse along the pristine graphene surface
(Fig. S3 and Table S2). We also successfully grafted the Pt single
atoms to cyanographene with high content, 3.7 wt %. It should be
noted, that the cyanographene was very recently used for anchoring the Cu atoms and applied as SAC for oxidative amine coupling
and alcohol oxidation reactions [21]. All these results show that
the nitrile groups are well suited for anchoring of metal single
atoms to graphene and identify cyanographene an ideal platform
for synthesis of a wide portfolio of SACs.
2. Materials and methods
2.1. Computational details
The first-principle spin-polarized density functional theory
(DFT) calculations were performed using the Vienna Ab initio Simulation Package (VASP) code [45,46]. The Perdew-Burke-Ernzerhof
functional (PBE) [47 ] was employed with inner electrons represented as projector augmented waves (PAW) [48 ,49 ]. Dispersion
contributions were introduced through the D3 approach [50,51].
The valence monoelectronic states expanded in plane waves with
a cut-off kinetic energy of 500 eV. The Brillouin zone was sampled
with a 6 × 6×1 (structure and cell optimization) and 15 × 15 × 1
(DOS and energy calculations) –centered grid. Graphene was
modelled as a (3 × 3) and (6 × 6) supercell containing 18 and 7 2
carbon atoms. The molecules were placed in a triclinic box with
18 Å sides. We considered diluted systems containing one functional group attached on-top of the graphene lattice and systems
with two functional groups in the ortho/trans position (Fig. S1). In
all cases the ionic and electronic convergence criteria were 10− 3 eV
and 10-6 eV, respectively, in the self-consistent field (SCF) cycle. A
dipole correction was employed along the z-direction [52]. Transition states were located following the climbing image nudged
elastic band procedure (CI-NEB) [53,54]. Analysis of the charges was
performed using the Bader approach [55]. Total magnetic moments
were calculated as the difference between the numbers of electrons
in occupied majority- and minority-spin states.
The stability of g-X structures (X: fluorine –F, hydroxyl − OH,
nitrile –CN, hydrogen –H or platinum Pt) shown in Fig. S1 was
assessed in terms of the binding energy per group
Ebind = (E g− X − Eg − n · EX )/n

(1)

where Eg− X , Eg , and EX denote the total energy of the functionalized
graphene, pristine graphene, and functional groups, n stands for
number of functional groups. The average C C bond distance in
pristine graphene (1.43 Å) agrees well with experimental results

[56]. A functional group has been placed onto three high symmetry
sites of graphene lattice, i.e. top (t), bridge (b) and six-fold hollow
(h) position. The thermodynamic stability of all reported g-X-Pt
configurations was analysed in terms of the binding energy per
platinum adatom
Ebind/Pt = Eg− X− Pt − Eg− X − EPt

(2)

where Eg− X− Pt , Eg− X , and EPt and denote the total energy of functionalized graphene doped by Pt adatom, functionalized graphene
and Pt adatom, respectively.
The finite model calculations of g-(CN)2 -Pt (Fig. S2) were
performed using Gaussian software [57 ] employing PBE exchangecorrelation functional [47 ,58 ] and empirical dispersions D3 [51].
All structures can be retrieved from the ioChem-BD database [59 ]
following the link [https://doi.org/10.19 061/iochem-bd-1-101].
2.2. Materials
Cyanographene (g-CN) was synthesized according to the previously reported procedure [43], K2 [PtCl4 ] was purchased from
Sigma-Aldrich, ethanol absolute from Penta, ultrapure water
(18 M cm–1 ) was used in all cases, produced from a Puris Esse-UP
Ultra Pure Water System.
2.3. Synthesis of the cyanographene/Pt(II) hybrid (g-CN/Pt)
g-CN (120 mg in 12 mL H2 O) and K2 [PtCl4 ] (162 mg in 12 mL
H2 O) were mixed and microwave-heated at 60 ◦ C for 15 min.
(8 50 W, 300 Anthon par microwave synthesizer). Then, the mixture was centrifuged and washed three times with water and once
with ethanol.
2.4. Experimental techniques
The samples were dispersed in water, sonicated for 5 min and
one drop put on copper grid with Holey carbon film. After drying on
the air at room temperature, the samples were measured by High
Resolution Transmission Electron Microscope (HR-TEM) Titan G2
(FEI) with Image corrector on accelerating voltage 8 0 kV. Images
were taken with BM UltraScan CCD camera (Gatan). Energy Dispersive Spectrometry (EDS) was performed in Scanning TEM (STEM)
mode by Super-X system with four silicon drift detectors (Bruker).
STEM images were taken with HAADF detector 3000 (Fishione).
The concentration of metals was determined by the atomic
absorption spectroscopy (AAS) technique, using a graphite furnace
(ContrAA 600; Analytik Jena AG, Germany) equipped with a highresolution Echelle double monochromator (spectral band width,
2 pm at 200 nm). A xenon lamp was used as a continuum radiation
source.
High-resolution X-ray photoelectron spectroscopy (HR-XPS)
was performed using a PHI VersaProbe II spectrometer (Physical Electronics) equipped with an Al K␣ source (15 kV, 50 W). All
spectra were acquired at room temperature (22 ◦ C), under vacuum
(1.4 × 10–7 Pa). High-resolution spectra were acquired by setting
the pass energy to 23.500 eV and the step size to 0.200 eV. These
spectra were analyzed using the MultiPak (Ulvac–PHI, Inc.) program. The binding energy values were reported with respect to the
C1s lowest peak energy at 28 4.7 0 eV.
3. Results and discussions
The bond strengths of functional groups to graphene depend
on the hybrid state of the neighboring carbon atoms. Generally,
the bonds are weak if the sp3 carbon bearing the functional group
is attached to three sp2 carbon atoms, however, their strength

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Fig. 1. The most stable structures of g-X-Pt complexes: two-sided fluorinated g-Pt-F2 , hydroxylated g-Pt-(OH)2 , nitrilated g-(CN)2 -Pt and hydrogenated g-Pt-H2 .

Fig. 2. Spin-polarized PDOS plots of two-sided functionalized graphene (a) g-X2 , (b) g-X2 -Pt. The Fermi level is set to zero. Band-gaps are depicted by vertical bars; numbers
correspond to the width of the band-gap (in eV).

increase if the sp3 carbon is attached to another sp3 (bearing an
attached group in trans-position) and just two sp2 carbon atoms
(see Section 2.1 in Supporting Information for details). Therefore,
we have focused on the double-sided functionalized graphene g-X2
with functional groups in ortho/trans positions (see Section 2.2 in
Supporting Information for details) and the results concerning onesided functionalized graphene g-X are shown in Section 2.3 and 2.4
in the Supporting Information.
We considered all unique positions of Pt atoms relative to the
functional groups (Fig. S4). Surprisingly, the Pt adatom cleaved the
bonds of –F, − OH, and –H to graphene and they released fluorine,
hydroxyl, and hydrogen groups attached on-top of the Pt adatom,
forming graphene–Pt–F (− OH, or –H) complexes (Fig. 1). This process was exothermic with binding energies per platinum adatom
from − 2.7 9 to − 4.05 eV (Eq. 2, Table S3, and Fig. S5). An easy diffusion of –Pt–X moiety following a top-bridge path was enabled by

low diffusion barriers (0.13-0.44 eV, Table S4) similar to the diffusion barrier of isolated platinum adatom (0.15 eV, Table S2). This
finding agrees with the experimental observation that − OH groups
were released from graphene after the Pt deposition [60,61]. On
the other hand, Pt adatom did not cause the release of –CN from
the graphene surface and is instead coordinated on top of the –CN
group, forming a linear g–CN–Pt complex (Fig. 1). The anchoring of
Pt adatom above or in the vicinity of the –CN group (Fig. S5c1-4)
prevented any potential detrimental processes, e.g. agglomeration,
diffusion or leaching, with a high top-bridge diffusion barrier of
1.34 eV (Table S4). Such high diffusion barriers reveal that thermal
effects do not compromise the long term stability of this material. The g-CN based catalysts remained highly stabile even upon
recycling [21,62]. Additionally, when a Pt adatom is bound on top
of a –CN group, this leaves the coordination sphere of Pt accessible which is desirable for catalytic activity. One shall note that
both the release of –F, − OH, and –H from the graphene lattice to

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Fig. 3. (a) Charge density difference plot of periodic model g-CN2 -Pt (isovalue 5 meÅ− 3 ) and (b) charge density difference plot of finite model g-CN2 -PtII in gas phase (isovalue
5 meÅ− 3 ).

Fig. 4. Representative HR-TEM image of a flake from the g-CN/Pt hybrid and HR-XPS of the Pt 4f region (a), higher magnification HR-TEM image showing high-contrast spots
probably originating from embedded Pt(II) ions (b), HAADF-STEM images showing the heavier (and thus brighter) single metal ions embedded in the g - CN support (c,d) and
EDS elemental mapping performed on g-CN/Pt for carbon (e), nitrogen (f), platinum (g), and combined C, N and Pt (h).

form g–Pt–F(F2 ), g–Pt− OH(OH2 ), and g–Pt–H(H2 ) complexes and
the formation of g–CN(CN2 )–Pt was predicted for one-sided and
two-sided functionalized graphene (Tables S5-S16).
The chemical functionalization of graphene, which is a nonmagnetic zero bandgap semi-metal [2] with its valence and
conductive bands crossing at the Dirac point at the Fermi level (EF ),
significantly changes its electronic structure. The covalent bonding of –H, − OH, and –CN to the graphene lattice in trans-positions
opened the graphene’s zero-band-gap, while fluorinated graphene
is a p-type metal (Fig. 2a and Fig. S6-S9 , Table S3 and Tables S5S8 ) [63–65]. All g–X2 systems are non-magnetic. The deposition
of Pt adatom onto the fluorinated graphene leading to the formation of graphene–Pt–F2 complex, further strengthen the metallic
character of the system (Fig. 2b, Fig. S10). Also, the electronic gap
at EF for the graphene–(OH)2 system was closed by the formation
of the g-Pt-(OH)2 complex (Fig. 2b and Fig. S11) although with a
much lower DOS at EF . Accordingly, the DOS of the latter system
is spin symmetric, whereas the g-Pt-F2 complex is spin-polarized.
The formation of graphene-Pt-H2 complex lower the electronic
gap of the hydrogenated graphene from 0.27 to 0.19 eV, while the
electronic gap of cyanographene (0.26 eV) was hardly modified
(0.25 eV) due to the presence of Pt bound on top of the –CN group

(Fig. 2b and Fig. S12-S13). The conductive character of the material
can be highly advantageous in electrochemical sensing, capacitors,
(photo)redox and (photo)electrocatalytic applications due to the
possibility of storing and shuttling electrons [17 ,21,43,62,66]. Both
g-Pt-H2 and g-(CN)2 -Pt are non-magnetic with spin symmetric
DOSs. Whereas for the metallic g–Pt–F2 and g-Pt-(OH)2 complexes
the charge depletion on Pt was the greatest (respectively 0.47 e and
0.34 e), the charge lost on Pt atom bound on-top of –CN was only
0.06 e, and in the g-Pt-H2 complex there was hardly any chargetransfer from/to Pt observed (Table S3). Fig. 3 and Fig. S14 show the
difference-electron densities calculated for the g–CN–Pt complex
with platinum in both oxidation Pt0 and PtII . The picture demonstrates a polar-covalent bond formation between Pt and N (Ebind/Pt
= − 2.29 eV) associated with the electron transfer to antibonding d *
state of the Pt atom, which is accompanied with the accumulation
of electrons in bonding states between Pt, N, and C atoms, shown
as a charge depletion both on the Pt site within the donut-shaped
ring parallel to the graphene plane, and on the N site.
Both platinum atom Pt0 and PtII ion which represents a common
oxidation state of platinum were considered in the gas phase and
water environment using an implicit solvent model. As expected,
the calculations (Eq. 2) identified that PtII binds to g-CN more

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Please cite this article in press as: R. Langer, E. Fako, P. Błonski, et al., Anchoring of single-platinum-adatoms on cyanographene:
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strongly (13.9 2 eV) than Pt0 (2.35 eV) in vacuum. In water, the
interaction of PtII to g-CN significantly dropped to 3.9 8 eV, while
the interaction of Pt0 remained less affected amounting to 2.63 eV.
These calculations suggest that the nitrile group is well suited for
anchoring platinum in both Pt0 and PtII states (Table S17 and S18 ).
The ability of g-CN to bind Pt atoms was evaluated experimentally by mixing of aqueous dispersions of the cyanographene with
the PtII salt. The obtained g-CN/Pt hybrid was analyzed with HRTEM, revealing the absence of any inorganic crystalline phases,
i.e. nanoparticles (Fig. 4a, Fig. S29 ), but unveiled atomic sized
high-contrast spots, suggesting the presence of heavy metal atoms
(Fig. 4b). This was confirmed by the abundant bright spots from
metal atoms, revealed via the sub-Ångstrom resolution aberration
corrected high-angle annular dark-field (HAADF) STEM in Fig. 4c,d.
Furthermore, Pt atoms appear to populate densely and homogeneously the nitrogen rich graphene sheets, as verified with EDS
mapping (Fig. 4b-e). HR-XPS further confirmed the presence and
oxidation state of the PtII ions, with the two spin-orbit split 4f
components appearing at electron binding energies (EBE) of 7 3.1
and 7 6.5 eV. According to AAS analysis, the Pt content on g-CN/Pt
was particularly high (3.7 wt %), surpassing the recently reported
SAC where 3 wt % Pt loading on ceria was achieved [67 ]. No Pt
was detectable in a control analysis of the as synthesized g-CN.
Therefore, g-CN acted as a very effective 2D-ligand for trapping
and sustaining single Pt ions over the out-of-plane functionalized
graphene.
4. Conclusions
In conclusion, we addressed the anchoring of single-platinumadatom on functionalized graphene. Since Pt adatoms easily diffuse
on pristine graphene, which may lead to undesirable aggregation
and nanoparticles formation, we explored graphene derivatives
bearing –F, − OH, –CN and –H functional groups as possible means of
anchoring single Pt atoms and preventing their diffusion. Whereas
the Pt adatom cleaved the bonds of –F, − OH, and –H to graphene
leading to the formation of graphene–Pt–F (− OH, or –H) complexes, the –CN group remained bound to graphene lattice and
anchored the single-platinum-adatom with high energy barrier
against –CN–Pt migration. The anchoring of Pt adatoms on g–CN
provide a scaffold for SACs which are considered as one of the holy
grails in catalysis as they enable the best use of rare and rather
expensive noble metals. As cyanographene was experimentally
prepared in 2017 , the g–CN material seems to be very promising
as an anchor for metals, as recently experimentally demonstrated
with the synthesis of SACs based on Cu and Fe ions [21], but also
in the present work with the effective immobilization of Pt ions
enabling plethora of catalytic reactions which could be examined
in future.
Declaration of Competing Interest
The authors declare no competing financial interests.
Acknowledgements
The authors gratefully acknowledge the support of the Ministry of Education, Youth and Sports of the Czech Republic under
Project No. LO1305, the Operational Programme for Research,
Development and Education of the European Regional Development Fund (Project No. CZ.02.1.01/0.0/0.0/16 019 /00007 54), the
´
support from the Internal Student Grant Agency of the Palacky
University in Olomouc, Czech Republic (IGA PrF 2019 031) and the
Spanish MCIU (RTI2018 – 10139 4 - BI00/MCIU/AEI/FEDER, UE). E.
F. thanks MINECO La Caixa Severo Ochoa for a predoctoral grant

5

through Severo Ochoa Excellence Accreditation 2014 2018 (SEV
2013 0319 ). M. O. acknowledges the ERC grant (68 3024) from the
EU Horizon 2020 Research and Innovation Programme. The authors
gratefully acknowledge BSC-RES for providing generous computaˇ
tional resources. Jana Stráská and Dr. Klára Cépe for the electron
microscopy analysis, Jan Kolaˇik for AAS measurements, Ondˇej
r
r
´
s
Tomanec for HR-TEM, and TomᡠStekly for synthesis of g-CN are
gratefully acknowledged.
Appendix A. Supplementary data
Supplementary material related to this article can be found,
in the online version, at doi:https://doi.org/10.1016/j.apmt.2019 .
100462.
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