Temperature-dependent formation of Ru-based nanocomposites: Structures and properties

Article abstract of DOI:10.1039/c4ra03142c

A new route to produce Ru-based nanocomposites with mixed valence states of ruthenium is reported in this paper via a solid-phase sintering process. Precursor particles were prepared by an intimate mixing of RuCl₃ and native β-cyclodextrin (β-CD) with a molar ratio of 1:1, followed by a sintering process at various temperatures ranging from 573 to 1173 K in ambient atmosphere. The so-obtained composite nanomaterials have been characterized by X-ray diffraction and notably the results show that an adjustment of the temperature enabled us to obtain Ru-based nanoparticles with controllable compositions. The surface-enhanced Raman scattering performances of the obtained nanomaterials have been analyzed using Rhodamine 6G (R6G) as the Raman probe. Their magnetic behaviors have been investigated as a function of the field strengths. The present work provides a significant advance in the development of both transformation in valence states of transition metals and in situ nanocomposites of metal/metal oxide combinations. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4ra03142c

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PAPER
Temperature-dependent formation of Ru-based
nanocomposites: structures and properties†
Cite this: RSC Adv., 2014, 4, 26847
ac
c
Yue Teng,ab Le Xin Song,
Anne Ponchel, Eric Monflier,c Zhi Cheng Shao,a
*
*
Juan Xiaab and Zheng Kun Yanga
A new route to produce Ru-based nanocomposites with mixed valence states of ruthenium is reported in
this paper via a solid-phase sintering process. Precursor particles were prepared by an intimate mixing of
RuCl3 and native b-cyclodextrin (b-CD) with a molar ratio of 1 : 1, followed by a sintering process at
various temperatures ranging from 573 to 1173 K in ambient atmosphere. The so-obtained composite
nanomaterials have been characterized by X-ray diffraction and notably the results show that an
adjustment of the temperature enabled us to obtain Ru-based nanoparticles with controllable
compositions. The surface-enhanced Raman scattering performances of the obtained nanomaterials
have been analyzed using Rhodamine 6G (R6G) as the Raman probe. Their magnetic behaviors have
been investigated as a function of the field strengths. The present work provides a significant advance in
the development of both transformation in valence states of transition metals and in situ
nanocomposites of metal/metal oxide combinations.
Received 8th April 2014
Accepted 9th June 2014
DOI: 10.1039/c4ra03142c
www.rsc.org/advances
Metal/metal oxide nanocomposite materials have attracted
wide interest because they have improved multifunctional
Introduction
properties and potential wide-ranging applications in many
leds.16 For example, composites consisting of nanostructured
ruthenium (Ru) and its oxides (e.g., RuO2) have shown prom-
ising results in experimental studies.17–19 Usually, Ru-based
materials are initially prepared in solution by chemical reduc-
tion of ruthenium salts,20 complexes17a or oxides.21 Thus, the use
of reducing agents [e.g., hydroxylamine, hydrazine, sodium
borohydride or polyols] is the most common method employed.
Recently, Zhang and co-workers reported the preparation of
Ru@RuO2 composites using an electrochemical method.22 In
general, the procedure requires drastic conditions, such as high
reux temperatures and the use of toxic or expensive organic
reagents. Thus, it was found that, aer their direct deposition
onto inorganic oxide supports, 1 wt% Ru-based supported
nanocatalysts were highly active and stable in the catalytic
conversion of ethanol to CO2. Surprisingly, despite these
promising effects, the preparation of Ru-based composites has
not been the subject of further investigations, and in particular,
their synthesis in the presence of molecular templates by solid-
phase sintering in ambient atmosphere has never been
explored. The interest of the procedure is to perform the solid-
phase reaction without adding any strong reducing agents,
neither in solution nor in the gas phase and so to avoid the use
of liquid organic media.
Water soluble biomolecular templates such as cyclodextrins
(CDs) have recently emerged as excellent potential tools for the
preparation of metal or metal oxide materials.1–3 CDs are water
soluble cyclic oligosaccharides formed of 6 (a-), 7 (b-) or 8 (g-)
glucopyranose units that exhibit multifunctional properties,
such as the formation of host–guest inclusion complexes,4–9 and
the stabilization of metallic nanoparticles10–12 and molecule-ion
adducts with inorganic metal salts.13,14 In addition, recent
studies of our group have revealed that transformations in
valence states of transition metal ions could occur during sin-
tering intimate mixtures of a metal salt with b-CD. For example,
it has been demonstrated that the valence of copper can be
easily modulated from Cu(II) to Cu(I) by changing the initial
molar ratio of copper chloride dihydrate to b-CD in the sintering
process in ambient atmosphere.15 These results encouraged us
to further investigate the formation mechanism of metal-based
composites.
aDepartment of Chemistry, University of Science and Technology of China, Jin Zhai
Road 96, Hefei 230026, China. E-mail: solexin@ustc.edu.cn; Fax: +86-551-3601592;
Tel: +86-551-3492002
bCAS Key Laboratory of Materials for Energy Conversion, Department of Materials
Science and Engineering, University of Science and Technology of China, Hefei
230026, China
The present work describes for the rst time the effect of
b-CD on the temperature dependence of transformation in
ruthenium valence states, starting from a molecule-ion solution
of RuCl3 and b-CD with an initial molar ratio of 1 : 1. Aer
c
´
´
Universite d’Artois, Unite de Catalyse et Chimie du Solide, UCCS UMR CNRS 8181,
´
Faculte des Sciences Jean Perrin, Rue Jean Souvraz, F-62300 Lens, France. E-mail:
anne.ponchel@univ-artois.fr; Tel: +33 (0)321791754
† Electronic supplementary information (ESI) available: XRD, HR-TEM, SAED,
EDS mapping, TG, and XPS. See DOI: 10.1039/c4ra03142c
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evaporation of the solvent, the sintering experiments of the
resulting solid mixture have been performed under ambient
atmosphere in a muffle furnace at different temperatures,
ranging from 573 to 1173 K. A series of Ru-based composites
(SP-a to SP-g) have been obtained and characterized by X-ray
diffraction (XRD), X-ray photoelectron spectroscopy (XPS),
Raman and electron microscopy. Several control materials
formed by the same solid-state method but from different
carbon sources (a-CD, g-CD and activated carbon) were con-
ducted to have a better insight into the role of the organic
templates during the sintering process. Unexpectedly, we have
found that the ability of the CDs to modulate the valence
transformation of ruthenium [from Ru(III) to Ru(0) and Ru(IV)]
and stabilize the Ru-based nanocomposites depended both on
the temperature of sintering and chemical nature of the oligo-
saccharides. A possible formation mechanism of the Ru-based
composites was proposed. Further, we found that these
composite materials exhibit different surface-enhanced Raman
scattering (SERS) properties. Finally, eld dependences of the
Ru-based materials have been determined to be highly related
to the composite ratios of Ru(0) to RuO2.
SERS measurements
SP-a, c, d and g were dispersed on a silica substrate (1 cm  1
cm) aer suspended in alcohol. Then, the R6G reagent (used as
probe molecule) was dissolved into pure water. Finally, 20 mL of
the probe solution (10À3 M) was dropped onto the silica
substrate.
Instruments and methods
XRD analyses of solid samples were carried out in a Philips
X'Pert Pro X-ray diffractometer. Samples were irradiated with
monochromatized Cu Ka and analyzed in the 2q range from 20
to 60ꢀ. Elemental analysis was done on an Elementar Vario EL
III elemental analyzer. XPS was performed at Photoemission
Station of National Synchrotron Radiation Laboratory of Hefei
with a VG Scienta R3000 electron energy analyzer, using Al Ka
radiation (1486.6 eV) in ultra-high vacuum (2.00 Â 10À9 torr) at
room temperature. The energy resolution of the instrument is
0.16 eV. The C 1s peak (284.8 eV) was used as the internal
standard for binding-energy calibration. Raman spectra were
collected at room temperature with a LABRAM-HR Confocal
Laser MicroRaman spectrometer with a resolution of 0.6 cmÀ1
Experimental section
in the range 100–2000 cmÀ1
.
Field emission scanning electron microscope (FE-SEM)
images were recorded on a JEOL-JSM-6700F eld-emitting
microscope. Transmission electron microscopy (TEM) and
high-resolution transmission electron microscopy (HR-TEM)
images were carried out with a JEOL-2010 eld-emission
transmission electron microscope using an accelerating voltage
of 200 kV. STEM (scanning transmission electron microscopy)
and EDS (energy dispersive X-ray spectroscopy) mappings were
obtained on a JEM 2010F eld-emission TEM at 200 kV,
equipped with an X-ray energy dispersive spectrometer (EDS).
Magnetic properties of solid samples were investigated using a
quantum design magnetic property measurement system
superconducting quantum interference device (SQUID)
magnetometer.
Thermal gravimetric (TG) analyses were obtained on a Shi-
madzu TGA-50 thermogravimetric analyzer at a heating rate of
10.0 K minÀ1 in ambient atmosphere with a gas ow of 25 mL
minÀ1. All solid samples were ground to ne powder by mortar
and pestle before analysis. The weight of the solid samples
analyzed was in the range of 5 mg to 7 mg.
Materials
b-CD was purchased from Shanghai Chemical Reagent
Company and recrystallized twice from deionized water.
RuCl3$3H2O was from Aladdin Chemistry Co. Ltd. Commercial
activated carbon was from Sinopharm Chemical Reagent Co.
Ltd. (Shanghai, China). a- and g-CD were purchased from
Aldrich. Rhodamine 6G (R6G) was purchased from Sigma-
Aldrich. All other chemicals were of general-purpose reagent
grade, unless otherwise stated.
Preparation of intimate mixtures
The intimate mixture of RuCl3 and b-CD was prepared by the
follow method. Initially, 1 mmol RuCl3 (207 mg) and 1 mmol
b-CD (1134 mg) were mixed into a round-bottom ask with
deionized distilled water of 100 mL and magnetically stirred at
298 K for 4 h. Then, the distilled water was drawn off by rotary
evaporation under vacuum at 323 K. Finally, a brown residue
was obtained and dried in vacuum to a constant weight. Note
that, in addition, intimate mixtures of RuCl3 with other CDs
(a- and g-CD) and material (activated carbon) were prepared by
the same procedure.
Results and discussion
Sintering experiments of intimate mixtures
Formation and composition of a series of Ru-based
nanocomposites
Sintering experiments of the intimate mixture of b-CD with
RuCl3 were carried out in a muffle furnace at 573, 673, 773, 873, Fig. 1 shows the XRD patterns of the seven materials (SP-a–g)
973, 1073 and 1173 K for 4 h. Aer sintering, the products were prepared from RuCl3 and b-CD. Clearly, the ve characteristic
cooled under a vacuum, and weighted at room temperature, diffraction peaks at 2q angles of 28.3, 35.1, 40.3, 54.5 and 58.1ꢀ
hereaer denoted as SP-a to SP-g, respectively. Their color (indicated by red asterisks) are attributable to the (110), (101),
changes were uniform, all from brown to black.
(111), (211) and (220) phases of crystal structure of RuO2
Also, the sintering products of intimate mixtures of RuCl3 (tetragonal system), respectively (JCPDS 88-0322).23 At the same
with activated carbon, a- and g-CD were obtained at 573, 773, time, the characteristic diffraction peaks of metallic Ru at 38.5
873 and 1173 K for 4 h in ambient atmosphere.
(100), 42.3 (002) and 44.0ꢀ (101) (hexagonal system, JCPDS 89-
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is dominant, i.e., the molar ratio of Ru(0) to RuO2 is about 5 : 1),
SP-d (RuO2 and metallic Ru are almost equivalent) and SP-g
(only RuO2) have been further characterized by FE-SEM and
TEM analyses in order to show the differences in their
morphology or surface structure. Fig. 2a–d shows surface
structures of the four Ru-based materials. Obviously, SP-a, SP-c
and SP-d have analogous ordered three-dimensional structures
shaped like doughnuts comprising nanoparticles with a mean
diameter of 50–70 nm. In contrast, a large agglomerate diam-
eter (>200 nm) and a relative large mean diameter (about 120
nm) of particles are observed in the pure RuO2 (SP-g). The HR-
TEM image of SP-g shows that the interplanar spacing is 0.255
nm (Fig. S2, ESI†), corresponding to the (101) plane of tetrag-
onal RuO2. The selected area electron diffraction (SAED) pattern
recorded indicates the single crystallinity of the RuO2 crystal
and can be indexed to the tetragonal RuO2 structure along the
[010] zone axis, corresponding to the nanocrystal grown along
[101] direction (Fig. S2, ESI†).
Fig. 1 (A) XRD patterns of SP-a–g, (B) XPS-(Ru 3p) spectra of SP-a, c, d
and g, and (C) molar percentages (%) of Ru(0) and RuO2. The peaks and
percentages of RuO2 and Ru are marked by red and blue, respectively.
4903)24 (indicated by blue asterisks), were also detected in SP-
a–f.
The change of the molar percentages of Ru(0) and RuO2 in
the series of composites is further illustrated by XPS measure-
ments as shown in Fig. 1. At lower sintering temperatures (573–
773 K), the relative content of metallic Ru becomes higher with
the increase of temperature (indicated by the green arrow).
Contrarily, at higher sintering temperatures (873–1173 K), the
relative content of metallic Ru decreases with the increase of
temperature and nally the metallic Ru phase disappears
completely to give pure RuO2 phase in SP-g (indicated by the
black arrow). This result, which shows that the content of
metallic Ru passes through a maximum at a temperature of
about 773 K, strongly suggests two signicant points regarding
the redox properties of RuCl3 in the presence of b-CD.
First, the reduction of RuCl3 to Ru(0) is likely to be associated
with the generation of graphite particles during the decompo-
sition process of b-CD. In addition, it can be mentioned that
besides the RuO2 diffraction lines, no signal assigned to a Ru(0)
phase was ever detected in the sintering products of free RuCl3
under the same conditions (Fig. S1, ESI†).
Second, the formation and constitution of the composites
consisting of metallic Ru and RuO2 is highly dependent on, and
can be manipulated by, changes in sintering temperatures.
The Ru-based nanocomposites: SP-a (RuO2 is dominant, i.e.,
Fig. 2 FE-SEM and TEM (insets) images of (a) SP-a, (b) SP-c, (c) SP-d
and (d) SP-g. Size distributions of particles of (e) SP-a, (f) SP-c, (g) SP-d
the molar ratio of Ru(0) to RuO2 is about 1 : 9), SP-c (metallic Ru and (h) SP-g based on the TEM images.
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STEM and EDS analyses were performed to further charac- Raman-active modes (Eg, A1g and B2g) of RuO2,27 appear clearly.
terize the detail structure of the nanoparticles in SP-c. Result Additionally, there are two bands at 1026 and 1252 cmÀ1, which
indicates that a low content of oxygen but a high content of are the overtones of 513 and 626 cmÀ1 Raman bands. The
ruthenium appear at the center of some nanoparticles (Fig. S3, results of Raman spectra suggest that the graphite carbon
ESI†). This is a strong indication that the Ru phase within the generated in this carbonization process of b-CD may act as a
nanoparticles is not homogeneously distributed, showing a reduction agent of Ru(III) to Ru(0).
native oxide shell in combination with an underlying Ru core.
The thermal decomposition of b-CD has already been the
However, we also notice that not all the nanoparticles display subject of several investigations in the literatures.28,29 Thus
such a core–shell structure. We speculate that this phenomenon authors have reported that the process occurred in three stages:
may be related to two factors (i) incomplete dispersion of the (i) the release of physically absorbed and crystallized water below
RuCl3 in its mixture of b-CD, and (ii) the growth of the nano- 110 K, (ii) the dissociation of cyclodextrin rings to form saturated
composites with more kinetically controlled than thermody- or unsaturated organic compounds below 573 K, and (iii) the
namically controlled process, e.g., the effect of the size of the formation and oxidation of graphite carbon at high temperatures
nanoparticles on the oxidation rate of Ru(0).
(>573 K). A comparison of the TG curves between free b-CD and
In conclusion, the temperature dependence of trans- the mixture (1 : 1, molar ratio) of b-CD and RuCl3 at the entire
formation in valence states of Ru gives a strong impression that temperature range reveals signicant decomposition differences
b-CD seems to play an important role as the composite particle (see Fig. S4, ESI†). The b-CD in the mixture completely decom-
precursor.
poses much earlier (710 K) when compared to free b-CD (850 K),
implying that the presence of RuCl3 favors the cleavage of
glycosidic bonds. TG data is in accordance with the results of
XRD and XPS. For example, when the decomposition of b-CD
ends, a solid product with a residual mass of 7.8% at 773 K was
collected. Notably, this experimental value is in well-agreement
with the ratio of Ru(0) to RuO2 previously estimated by XPS
analysis (i.e. 5 : 1 corresponding to 78.8 wt% metallic Ru).
Several control experiments using different oligosaccharides
were performed in parallel for comparison. The XRD patterns of
sintering products of the mixtures of RuCl3 with a- and g-CD at
the same sintering temperatures as those used in the case of b-
CD indicate that although the Ru-based composites can be
produced in the presence of each of the CDs, there are two
signicant characteristics that need to be pointed out (Fig. S5,
ESI†).
First, the relative contents of metallic Ru in the composites
are quite different even at the same sintering temperatures. For
example, at 573 K, while the treatment with a-CD leads to the
highest content of Ru, by treatment with b-CD, RuO2 is clearly
the dominant phase. Interestingly, at this temperature, the use
of g-CD results in almost equivalent amounts of Ru(0) and
RuO2. Second, in the presence of a- or g-CD, with the increase of
temperature from 573 to 873 K, the relative content of Ru(0)
decreases while the content of RuO2 increases gradually.
These observations reveal that the generation of the Ru-
based nanocomposites by means of in situ chemical processes is
highly dependent on the chemical nature of the oligosaccha-
rides. On the one hand, this nding suggests that there are
different ways, in which RuCl3 can interact with the oligosac-
charides in aqueous solution, causing each molecule-ion
system to generate a characteristic compositing process of the
Ru-based crystallite nanoparticles formed during the thermal
process. On the other hand, different thermal decomposition
processes (Fig. S6, ESI†) of the oligosaccharides may be an
important reason for different formation rates of graphite
carbon, leading to a difference in compositions of the
composites.30
Formation process of the Ru-based nanocomposites
Raman spectra of the Ru-based materials in Fig. 3 provide
further information about the structural nature of the
composites, allowing us to speculate the role of b-CD. As shown
in Fig. 3, in the case of SP-a, the occurrence of a band at 1589
cmÀ1 (G band of the C–C vibration of graphite layers25) indicates
the existence of graphite (mass percentage: 0.36%, based on
elemental analysis) in this material. Undoubtedly, the graphite
particles were originated from the decomposition process of b-
CD. The very low value of the relative content of graphite should
be a reason for the absence of the X-ray diffraction lines of
graphite.26 Nevertheless, in SP-c, SP-d and SP-g, the G band
disappears completely. One reason is the presence of an inter-
action between graphite and RuCl3, and the other is the
oxidation of graphite particles with the increase of temperature.
Instead, several bands at 513, 626 and 704 cmÀ1, belonging to
In addition, we performed control experiments by using
activated carbon instead of b-CD. Our data (Fig. S7, ESI†)
Fig. 3 Raman spectra of SP-a, c, d and g. Relative signal intensity was
normalized to the intensity of the peak at 1026 cmÀ1 in SP-c.
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indicate that the introduction of activated carbon causes the
generation of metallic Ru at 573 K, and subsequently the Ru
disappears with the increase of sintering temperature. The
result is due to the well-known rapid oxidation of activated
carbons by oxygen in air and suggests indirectly that the inter-
actions between RuCl3 and the activated carbon are not of the
same type as those occurring the metal precursor and b-CD.
Starting from this observation, a process for the valence
transformation of Ru can be therefore proposed, as illustrated
in Fig. 4. Initially, an intimate mixture of b-CD and RuCl3 was
prepared as a precursor. Subsequently, several basic reactions
(eqn (1)–(5)) occurred during the sintering process of the
mixture. The oxidation of RuCl3 by oxygen in air to form RuO2
(eqn (1)) is simultaneous with the reduction of the RuO2 to
Ru(0) (eqn (4)) by graphite carbon derived from the slow release
in the decomposition process of b-CD (eqn (2)). At the same
time, the oxidation of the Ru(0) to RuO2 occurs in this process at
higher temperatures (eqn (5)). Our data indicate that the release
rate of the graphite carbon from b-CD is relatively low particu-
larly at lower temperatures such as 573 K, which results in that
the oxidation of RuCl3 is dominant at this temperature. As
temperature increases from 573 to 773 K, the release of carbon
accelerates, causing the content increase of Ru in the compos-
ites. For example, aer sintering for 2 h at 773 K, a higher
intensity of the Ru(0) XRD peaks was obtained because of the
quick release of carbon (Fig. S8, ESI†) and the as-obtained Ru
could be maintained aer another 2 h. Moreover, when
temperature is higher than 773 K, the Ru is gradually oxidized to
RuO2, and nally completely disappears at 1173 K when carbon
disappears (eqn (3)). Therefore, this result should be a reection
of an earlier and easier oxidation of activated Ru species since in
general metallic Ru does not oxidize in air at temperatures
below 1073 K.31
Fig.
5
XRD patterns of the sintering products of the intimated
mixtures of b-CD and RuCl3 with molar ratios of 2 : 1 (A) and 1 : 2 (B) at
573 (a), 773 (b), 873 (c), 973 K (d).
when the initial molar ratio of RuCl3:b-CD is not xed at 1 : 1,
like more RuCl3 or more b-CD. It can be explained by the
formation process of the Ru-based nanocomposites as
described in Fig. 4. On the one hand, when the amount of b-CD
is relatively large, extra water molecules were produced in the
sintering process (see eqn (2)). The presence of the water
molecules will promote the formation reactions (eg, eqn (1)) of
RuO2. On the other hand, when the amount of b-CD is relatively
small, the formation reaction (eqn (4)) of Ru will be difficult to
occur because of the lack of released carbon from the decom-
position of b-CD. This observation not only suggests that the
composition of the nal products of the mixtures of b-CD and
RuCl3 is strongly associated with the mixed ratios, but also
further supports the proposal of the formation process of the
Ru-based nanocomposites.
We performed additional sintering experiments using inti-
mate mixtures of b-CD and RuCl3 with molar ratios of 1 : 2 and
2 : 1. XRD analysis (Fig. 5) provides very interesting ndings
with regard to the composition of the nal products by
changing the sintering temperature. We noticed that all the
products obtained by changing the sintering temperature are
proved to be RuO2. Therefore, it does not follow the same trend
The remarkable difference in compositions of the Ru-based
nanocomposites allows us to characterize the relationship
between the compositions and physical properties. Two inde-
pendent experimental techniques, i.e. SERS and magnetic
enrichment effects, were employed to evaluate this question.
SERS properties of the Ru-based nanocomposites
SERS is a very useful technique causing strongly increased
Raman signals from compounds, which have been attached to
nanolms, nano-sized structures, architectures, assemblies.32
A
large number of studies have reported on applications of
metallic nanomaterials, such as gold and silver, in SERS.33
However, not enough concern was given to the application
potential of metallic oxide nanomaterials in SERS. Therefore,
we performed a series of SERS experiments with the use of the
Ru-based composites.
Fig. 6 depicts the Raman spectra of the probe molecule R6G
on the Ru-based composites deposited onto a silica glass
substrate. No Raman signal is observed from the R6G on the
silica substrate. However in the presence of the composites,
there are several clear signals at 612, 771, 1361 and 1648 cmÀ1
,
Fig. 4 Formation process of the Ru-based nanocomposites.
due to the carbon–oxygen stretch, ethylamine group wag and
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Fig. 6 SERS spectra of R6G on a silica substrate, and SP-a, c, d and g.
Fig. 7 Field dependences of magnetizations of SP-a, c, d and g at 298
K. The inset is the magnified version of the hysteresis loop in the field
range of 300 Oe.
xanthene ring stretch of R6G, revealing the SERS properties of
the Ru-based nanocomposites. This property was reported
previously, but only for pure metallic Ru phase.34 In particular,
the two composites: SP-a and SP-c, have stronger SERS activity
relative to the other two: SP-d and SP-g. It suggests that the ratio
of RuO2 to Ru(0) in the nanocomposites has an important effect
on the Raman scattering. This is a very interesting phenom-
enon: when the content of metallic Ru is much higher or much
lower than RuO2, the composites show better SERS activity.
Taken this together with the structural features of the
composites, it can be suggested that such a Raman enhance-
ment effect may be attributed to the enhanced localized elec-
tromagnetic eld, which is affected by two factors: the
interaction between Ru(0) and RuO2, and the small size and
large surface area per unit mass of the nanocomposites.35
Although the SERS effect in the Ru/RuO2 composites is relatively
small compared to that generally reported for noble metals,
understanding the relationship between the compositions of
the nanocomposites and their SERS effects is signicant
because it could lead to a strategy for improvement. Our result
may provide a clue to further modify the SERS properties of
metal/metal oxide nanocomposites.
interaction between metallic Ru and RuO2 has induced a
change in the distribution of electronic density in the crystals.
Therefore, it seems tempting to speculate on the possibility
whether inner electronic structures of Ru core levels vary with
the formation of composites.
In line with this, the Ru 3p XPS spectra for SP-a, c, d and g,
which are shown in Fig. 1, exhibit signicant shis in the
binding energy of Ru(IV) and Ru(0). The Ru 3p region of SP-a was
separated into two components: one having higher binding
energies of about 462.2 (3p5/2) and 484.3 (3p3/2) eV due to Ru(IV)
in RuO2,36 and the other having lower binding energies of about
461.1 (3p5/2) and 482.7 (3p3/2) eV due to Ru (0).37
However, we nd that a shi in binding energies for Ru 3p
core level has occurred from one composite to another and
especially from one composite to the pure RuO2. For example,
in the SP-d, the binding energies of Ru(IV) 3p5/2 and 3p3/2
increase largely to 463.3 and 485.7 eV, respectively, when
compared to those of SP-a. We speculate that it is due to
substantially more electron transfer from Ru atoms to oxygen
atoms while the materials were synthesized at higher temper-
atures. This electron transfer effect is also in line with the
decrease in the O 1s binding energy that is measured in the
samples prepared at 573 and 873 K (Fig. S9, ESI†). It should be
Magnetic properties and electronic structures of the Ru-based
nanocomposites
Magnetic properties of SP-a, c, d and g were measured by a noted that the O 1s line shape is asymmetric in both SP-a and
SQUID. Fig. 7 depicts the eld dependences of magnetizations SP-d, probably as a result of the compositional heterogeneity of
of the four nanomaterials at 298 K. Two interesting phenomena the oxide near-surface region.38 In the pure RuO2, the binding
are observed from this gure. First, all the four Ru-based energies of Ru(IV) 3p5/2 and 3p3/2 increase dramatically to 464.5
nanocomposites exhibit superparamagnetism. Second, the and 486.4 eV, respectively. It is apparent that such a large shi is
crystallites have very low, but different, magnetizations in elds a reection of signicant differences in the electronic structure
up to the maximum applied magnetic eld (10 kOe). For of Ru(IV) among the composites and especially between the pure
example, the saturation magnetizations (Ms) of SP-a, c, d and g RuO2 phase and its composite materials. This provides evidence
at 298 K are 0.85 Â 10À2, 2.8 Â 10À2, 0.56 Â 10À2 and 1.4 Â 10À2 for our conclusion that the composite interaction between
emu gÀ1, respectively. It is known that the magnetic suscepti- metallic Ru and RuO2 leads to a change in inner electronic
bility of RuO2 is much higher than that of metallic Ru. However, structures of Ru core levels.
in the present case, this relation is reversed and is abnormal.
These ndings provide a basis for understanding the
For example, SP-c with a large amount of Ru(0) exhibits a much magnetic properties of Ru-based composite materials. The Ru-
higher Ms, when compared to pure RuO2 (SP-g). We believe that based materials show magnetic behaviour even though neither
the reason for this difference may be that the in situ composite ruthenium nor ruthenium dioxide is ferro-, ferri-, or
26852 | RSC Adv., 2014, 4, 26847–26854
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antiferromagnetic. We consider that this is a very rare case of
the composite materials between a metal and its oxides. In
particular, the fact that the SP-c with a large amount of metallic
Ru exhibits a much higher Ms, when compared to pure RuO2
(SP-g), is especially interesting because this allows us to illus-
trate some important properties of the composite model. On the
one hand, the composite interaction between Ru and RuO2 has
led to a change of electronic structure of Ru cores based on XPS
analysis, suggesting that there might be a possibility of elec-
tronic transfer between the Ru(0) and the Ru(IV). This electronic
transfer could be one reason for the change in the Ru local
magnetic anisotropy, leading to a distribution of local Ru
magnetic-moment orientations. On the other hand, the
obtained composite powders had a crystallite size in the range
of nanoscale (diameter, <50 nm). This is the other important
reason why these composite materials show magnetic behav-
iour, because at the nanometer scale, each particle may contain
only a single magnetic domain.
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Conclusion
This work demonstrated for the rst time the valence trans-
formation of Ru(III) to Ru(IV) and Ru(0) via a modied
combustion method based on ruthenium chloride and b-CD.
The special interest of the method lies in the fact that it yields a
series of Ru/RuO2 nanocomposites with temperature-depen-
dent composite ratios, but without adding any extra compo-
nents such as inorganic reducing agents, organic solvents and
surfactants. We believe that this method could be used for many
different systems. Particularly, our results rstly revealed that
the composite ratios have a crucial effect on the physical
properties of the nanocomposites, including magnetizations
and SERS effects. In addition, the size of the domains could also
play a role in the physical properties. We consider that the
description of the composition–property relationship provides a
framework for designing and developing metal–metal oxide
nanocomposite materials.
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Acknowledgements
This project was supported by NSFC (no. 21071139) and
National Synchrotron Radiation Laboratory at University of
Science and Technology of China. We thank the French
Ministry of Foreign and European Affairs (MAEE) for their
nancial support (PHC XI GUANGQI 2011, 26157TJ).
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X. Q. Guo, L. X. Song, Z. Dang and F. Y. Du, Bull. Chem.
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