1- Introduction2- Discovery of ruthenium and Occurrence.Fro.docx

1- Introduction
2- Discovery of ruthenium and Occurrence.
From( Properties and Applications of Ruthenium ) chapter of
boook
Link to citing information
https://www.intechopen.com/books/noble-and-precious-metals-
properties-nanoscale-effects-and-applications/properties-and-
applications-of-ruthenium
3-Chemical and physical properties
boook
4-Compounds of Ru (refining of the platinum-Group Metals)
(Refining of the platinum-group metals)Chapter of book
Please make this part two pages.
5- Ruthenium complexes
boook
6-Extraction (preparation of Ru)
1-CONCENTRATE COMPOSITION
3- SEPARATION TECHNIQUES USED IN THE REFINING OF
THE PLATINUM-GROUP METALS
From (Refining of the Platinum-Group
Metals) chapter of book.
7- General applications
book.

8-Catalytic activity of ruthenium, general application in
catalysis.
book.
9- Application of Ru nanoparticles in catalysis
Articles (4– 9– 6 – 19-20) that6 articles you have done.
10- Application of Ru in some other different fields.
Articles (12-14-16-17-18- last article).
11- Summary and conclusions
book.
12- References
Journal of
Materials Chemistry A
REVIEW
P
ub
li
sh
ed
o
n
08
O

ct
ob
er
2
01
9.
D
ow
nl
oa
de
d
by
M
is
si
ss
ip
pi
S
ta
te
U
ni

ve
rs
it
y
L
ib
ra
ri
es
o
n
4/
7/
20
20
1
0:
07
:5
6
P
M
.
View Article Online

View Journal | View Issue
Metallic rutheniu
S
d
S
i
s
P
Q
S
H
r
n
e
e
aSchool of Materials Science and Engineerin
Beijing, Beijing 100083, China. E-mail: qipe
bCenter for Programmable Materials, Schoo
Nanyang Technological University, 50 Nanya
Cite this: J. Mater. Chem. A, 2019, 7,
24691
Received 10th June 2019
Accepted 8th October 2019
DOI: 10.1039/c9ta06178a
rsc.li/materials-a
This journal is © The Royal Society of C
m-based nanomaterials for
electrocatalytic and photocatalytic hydrogen

evolution
Sumei Han,†a Qinbai Yun,†b Siyang Tu,a Lijie Zhu,c Wenbin
Cao*a and Qipeng Lu *a
Developing a sustainable technology to produce hydrogen
efficiently is crucial to realize the “hydrogen
economy”, which may address the growing energy crisis and
environmental pollution nowadays.
Electrocatalytic and photocatalytic hydrogen evolution reactions
have received great attention during
the past few decades since they can realize hydrogen production
from the water splitting reaction
directly. Although platinum has been widely used as a catalyst
for the electrocatalytic and photocatalytic
hydrogen evolution reaction (HER), its high cost and limited
supply make it imperative to develop
alternative high-performance catalysts. Ruthenium (Ru), the
cheapest one among platinum-group
metals, has been emerging as a promising candidate recently.
Until now, tremendous efforts have been
devoted to improving the HER performance of metallic Ru-
based catalysts through the rational design
and synthesis of Ru nanomaterials, in which the size,
morphology, chemical composition and crystal
phase could be controlled. In this review, we summarized the

synthesis of various metallic Ru-based
nanomaterials as catalysts for the HER, including pure Ru
nanocrystals, Ru-based bimetallic
nanomaterials and Ru/non-metal nanocomposites. Then, we
covered the recent progress in the
utilization of metallic Ru-based nanomaterials as catalysts for
the electro- and photo-catalytic HER;
meanwhile, the mechanisms and fundamental science behind
morphology/composition/crystal
structure–performance relationships were discussed in detail.
Finally, the challenges and outlook are
provided for guiding the development of metallic Ru-based
electro- and photo-catalysts for further
fundamental research and practical applications in renewable
energy-related areas.
umei Han received her B.E.
egree from the University of
cience and Technology Beijing
n 2018. She is currently a Ph.D.
tudent under the supervision of
rof. Wenbin Cao and Prof.
ipeng Lu at the University of
cience and Technology Beijing.
er research interests are
elated to the development of
oble metal nanomaterials for
lectrocatalytic hydrogen
volution.

Wenbin Cao received his B.E.
degree from the Northeast Insti-
tute of Technology in 1992. He
obtained his M.E. degree from
Northeastern University in 1995
and completed his Ph.D. with
Prof. Changchun Ge at the
University of Science and Tech-
nology Beijing in 1998. He joined
the research group of Shourong
Yun at the Beijing Institute of
Technology as a postdoctoral
fellow. He worked in Osaka
University as a COE researcher from 2000 to 2002. Then he
joined
the University of Science and Technology Beijing. His current
research interests include photocatalysis, electromagnetic
absorbing
materials and phase transition materials.
g, University of Science and Technology
[email protected]; [email protected]
l of Materials Science and Engineering,
ng Avenue, Singapore 639798, Singapore
cSchool of Instrument Science and Opto-Electronics
Engineering, Beijing Information
Science and Technology University, Beijing 100192, China
† S. Han and Q. Yun contributed equally to this work.
hemistry 2019 J. Mater. Chem. A, 2019, 7, 24691–24714 |

24691
Journal of Materials Chemistry A Review
P
ub
li
sh
ed
o
n
08
O
ct
ob
er
2
01
9.
D
ow
nl
oa
de

d
by
M
is
si
ss
ip
pi
S
ta
te
U
ni
ve
rs
it
y
L
ib
ra
ri
es
o
n

4/
7/
20
20
1
0:
07
:5
6
P
M
.
View Article Online
1. Introduction
The world-wide increasing energy demand makes it imperative
to the traditional and
rapidly depleting fossil fuels. Moreover, the consumption of
conventional fossil fuels has led to severe environmental
pollution and climate change.1,2 In order to solve the above
issues, since the 1970s, the concept of “hydrogen economy” has
been emerging to construct a clean and renewable energy
system based on the electrical energy generated by hydrogen.3,4
In this system, hydrogen gas (H2) serves as the energy carrier,
which can react with oxygen (O2) to generate electricity in fuel
cells, leaving water as the only byproduct (2H2 (g) + O2 (g) /
2H2O (l), DH ¼ �286 kJ mol�1).1,5,6 However, H2 does not
exist
abundantly in nature. Steam reforming of natural gas, partial

employed methods to produce H2 in industry. Although these
technologies could produce considerable amounts of H2 with
low cost, their utilization relies on fossil fuels with the
emission
of greenhouse gases, including carbon dioxide (CO2), nitrous
oxide and water vapor.7,8 Thus, it is of great importance to
develop a more sustainable technology to produce H2 and
realize the “hydrogen economy”.
The water splitting reaction is a well-known route to produce
H2 and O2 at the same time. The hydrogen evolution reaction
(HER), a half reaction of water splitting (2H+ + 2e� / H2), can
be driven by solar energy or electricity derived from other types
of renewable energy; thus there will be no CO2 emission during
the H2 production process.
9,10 Electrocatalytic hydrogen evolu-
tion will not generate any harmful by-products. Meanwhile, this
technology does not require large, centralized plants, which
could meet the requirements of different users from a large
scale (local fueling stations and industrial facilities) to a small
scale (individual households).11 But the high energy consump-
tion (180 MJ for 1 kg H2) and the short life time of
electrolyzers
are two main drawbacks of this technology.12 Improving the
electrocatalytic efficiency and prolonging the life time of
Qipeng Lu received his B.E.
degree from the Taiyuan Univer-
sity of Technology in 2008. He
obtained his M.E and Ph.D.
degrees from Beijing Jiaotong
University in 2010 and 2014,
respectively. As a visiting student,
he studied in Prof. Yadong Yin's
group at the University of Cal-

ifornia, Riverside, from 2011 to
2013. He then carried out his
postdoctoral research with Prof.
Hua Zhang at Nanyang Techno-
logical University, Singapore, in 2014. In 2018, he joined the
faculty
of the School of Materials Science and Engineering, University
of
Science and Technology Beijing. His research interests are
related to
the synthesis of nanostructured materials for energy conversion.
24692 | J. Mater. Chem. A, 2019, 7, 24691–24714
electrolyzers could reduce the cost of H2 production. One of the
most effective strategies is developing high-performance elec-
trocatalysts, which could reduce the overpotential during the
HER and thus lower electrical energy consumption.13
Currently,
the most utilized HER electrocatalyst is platinum (Pt) due to its
near-zero Gibbs free energy of adsorbed hydrogen (DGH),
which
means an appropriate hydrogen binding energy.14,15 For the
photocatalytic HER, Pt has also been the most frequently used
co-catalyst.16 The Pt co-catalyst can promote the separation of
electron–hole pairs and lower the activation barrier, thus facil-
itating photocatalytic reactions.17 However, the high cost and
limited world-wide supply of Pt severely hinders its large-scale
a lower
cost
is crucial for the development of clean H2 production by water
splitting.
Ruthenium (Ru), the cheapest one in platinum-group
metals, is a promising alternative HER catalyst since the bond
strength of Ru–H is comparable to that of Pt–H.18–20

Moreover,
Ru also exhibits good corrosion resistance in both acidic and
alkaline electrolytes.21 Although Ru colloids showed promising
catalytic activity for light-induced H2 evolution from water as
early as 1979,22 it is only recently that the utilization of Ru-
based
HER catalysts has received great attention with the advances of
nanotechnology. Preparation of metallic Ru-based nano-
materials is an effective strategy to increase the HER activity of
Ru, since more surface atoms serving as the active sites can be
exposed.23 The shape control of Ru-based nanomaterials is
effective in tuning the HER activity since different facets with
different atomic arrangements usually have diverse hydrogen
adsorption energies.24,25 Alloying Ru with other metals and
synthesizing bimetallic Ru-based core–shell nanomaterials
have also been proven as effective methods to enhance the HER
and
the synergetic effect between different metals.26,27 Moreover,
compositing Ru nanostructures with carbon, carbon nitride and
semiconductors could ensure that the active sites of Ru-based
nanomaterials are fully exposed, meanwhile preventing the
aggregation of the catalysts during the HER process.19,28–30
Normally, bulk Ru crystallizes in a hexagonal close packed
(hcp)
phase. Recently, it has been shown that Ru may crystalize in
face-centered cubic (fcc) or 4H phases under certain synthesis
conditions.28,31–34 As different arrangements of Ru atoms in
different crystal phases will change the electronic and
geometric structures of Ru-based catalysts,35 superior HER
activities are expected to be achieved in Ru nanomaterials with
unconventional crystal phases. From the aforementioned
aspects, the synthesis of metallic Ru-based nanomaterials and
their applications in the electrocatalytic and photocatalytic HER
are becoming a research hotspot; however, there has been no

-
cant progress in this area until now.
In this review, we will
various kinds of metallic Ru-based nanomaterials, including
pure Ru nanocrystals (NCs), Ru-based bimetallic nanomaterials
and Ru/non-metal nanocomposites (Ru-carbon, Ru-carbon
nitride, Ru-semiconductor, etc.). Then the basic principle of the
electrocatalytic HER and the application of these metallic Ru-
This journal is © The Royal Society of Chemistry 2019
Review Journal of Materials Chemistry A
P
ub
li
sh
ed
o
n
08
O
ct
ob
er
2
01

9.
D
ow
nl
oa
de
d
by
M
is
si
ss
ip
pi
S
ta
te
U
ni
ve
rs
it
y
L

ib
ra
ri
es
o
n
4/
7/
20
20
1
0:
07
:5
6
P
M
.
View Article Online
based nanomaterials for the electrocatalytic HER will be dis-
cussed. Next, we will introduce the use of metallic Ru-based
nanomaterials as co-catalysts for the photocatalytic HER.
Finally, we will give some perspectives on the challenges and
promising directions in this research area.
2. Synthesis of metallic Ru-based
nanomaterials

In the past few years, Ru-based electrocatalysts and photo-
catalysts have gained intensive research interests because of
their lower cost compared to Pt and high catalytic activity in the
HER.19,34 In order to effectively take advantage of Ru for the
HER, until now, various kinds of metallic Ru-based nano-
materials have been prepared, including pure Ru NCs, Ru-based
bimetallic nanomaterials and Ru/non-metal nanocomposites.
In the following sections, the synthesis methods of metallic Ru-
based nanomaterials will be introduced in detail.
2.1. Synthesis of pure Ru NCs
To achieve excellent catalytic activity, pure Ru NCs with
controllable size, morphology, exposed facet and crystal phase
have been synthesized through various wet chemical methods,
such as chemical reduction, hydro(solvo)thermal method and
template method. It is notable that bulk Ru normally adopts the
hcp crystal structure.35 With the development of crystal phase
engineering, Ru nanomaterials with fcc and 4H structures have
been synthesized because of the nanosize effect and exhibited
superior catalytic activity compared to hcp Ru nano-
materials.28,34,36 In this section, besides the morphology and
exposed facet control, we will focus on the synthetic procedures
of Ru NCs with these novel crystal phases.
Wang and co-workers synthesized Ru nanocluster colloids
via a chemical reduction method without using any protective
agents. The Ru nanoclusters with a size of around 1 to 2 nm are
very stable in solution, and no precipitation could be observed
and co-workers
synthesized fcc and hcp Ru nanoparticles (NPs) with tunable
size from 2.0 to 5.5 nm by simple chemical reduction methods,
respectively.36 They discovered that the crystal phase of Ru
NPs
varied with different metal precursors, and reducing and
stabilizing agents. When using RuCl3 as the metal precursor,
triethylene glycol (TEG) as the solvent and reducing agent, and

poly(N-vinyl-2-pyrrolidone) (PVP) as the capping agent,
metallic
hcp Ru NPs were obtained, while by employing Ru(acac)3 as
the
metal precursor and ethylene glycol (EG) as the reducing agent,
fcc Ru NPs could be prepared. Based on these experimental
results, they pointed out that the metal precursor that dissolved
into the organic solvent as a neutral molecule rather than as an
ion led to the formation of Ru NPs with the unconventional fcc
phase.
In order to realize the efficient utilization of Ru atoms in
catalytic reactions, Ru NCs with ultrathin nanostructures, such
as nanosheets (NSs), nanotubes (NTs),38 nanocages
(NCGs)31,39
and nanoframes (NFs),32 have been synthesized to increase the
proportion of exposed surface atoms. Using Ru(acac)3 as the
metal precursor, Wu and co-workers prepared 2D ultrathin Ru
NSs (Fig. 1a) through a solvothermal method.38 Ru3+ was
reduced via the self-decomposition of the metal precursor and
grew into ultrathin NSs with the aid of isopropanol and urea. Ru
triangular nanoplates (NPLs) were prepared by Yan's group
through a facile hydrothermal method with RuCl3$xH2O as the
precursor.40 The shape of the Ru NPLs would become irregular
when the concentration of RuCl3$xH2O and the reducing rate
were increased. Moreover, Ru-capped columns and nano-
spheres could also be synthesized with the aid of Na2C2O4 and
Na2C3H2O4$H2O as the shape-control agent, respectively.
They
claimed that the shape control of Ru NCs was related to both
the
intrinsic characteristics of Ru crystals and the adsorption of
certain reaction species (i.e. Na2C2O4 and Na2C3H2O4$H2O).
For

the crystal-phase based heterostructure, Huang and co-workers
synthesized Ru nanodendrites (Fig. 1b) composed of ultrathin
fcc/hcp nanoblades (Fig. 1c) via a facile solvothermal reduction
of Ru3+ together with Cu2+ followed by the selective etching
of
metallic Cu.21
Seed-mediated growth followed by chemical etching is an
effective synthetic approach to prepare Ru NCs with highly
open
structures such as NTs, NCGs and NFs. The synthetic process
mainly involves three steps: (i) preparing templates or seeds for
the deposition of Ru to form bimetallic nanostructures; (ii)
depositing Ru by epitaxial growth on the templates or metal
seeds; (iii) chemical etching to remove the templates.34 As
a typical example, Zhang's group reported that the hierarchical
4H/fcc Ru NTs could be synthesized by a hard template-medi-
ated method as shown in Fig. 1d, in which 4H/fcc Au nanowires
epitaxial growth of 4H/fcc Ru nanorods (NRs) (Fig. 1f).34 By
using Cu2+ in dimethylformamide as an effective etchant, the
Au templates were removed and hierarchical 4H/fcc Ru NTs
with ultrathin Ru shells and tiny Ru NRs were obtained (Fig.
1g).
Xia's group reported the successful synthesis of Ru cubic NCGs
with ultrathin walls, in which the Ru atoms were crystalized in
a fcc structure rather than the hcp structure.31 To obtain the Ru
cubic NCGs, Pd nanocubes (NCBs) served as seeds to realize
the
epitaxial growth of Ru and thereby formed the core–shell NCBs.
The Pd core was selectively etched away through the reaction
Pd
+ 2Fe3+ + 4Br� / PdBr4
2� + 2Fe2+ using an etchant based on the
Fe3+/Br� pair and then fcc cubic NCGs were obtained.

Moreover,
they also obtained octahedral41 and icosahedral39 Ru NCGs
with
ultrathin walls in the fcc phase by using a similar method. In
addition, fcc Ru NFs can also be obtained by realizing the
preferential growth of Ru on the corners and edges of Pd
truncated octahedra through kinetic control and then removing
the Pd seeds by chemical etching with the aid of the Fe3+/Br�
pair.32 Kinetic control was achieved by adjusting the injection
rate of the RuCl3$xH2O solution using a syringe pump while
the rates of the deposition and surface diffusion of Ru atoms
-
trodes without using any solvents, surfactants and reducing
agents. Cherevko and co-workers prepared a Ru/Ti/SiO2/Si
electrode for the HER and oxygen evolution reaction (OER) via
J. Mater. Chem. A, 2019, 7, 24691–24714 | 24693
Fig. 1 TEM images of (a) ultrathin Ru NSs. Reproduced with
permission.38 Copyright 2016, American Chemical Society. (b)
Ru nanodendrites.
Inset: the size distribution of Ru nanodendrites. (c) XRD
patterns of Ru and RuCu nanodendrites in comparison with the
standard peaks for hcp Ru
(JCPDS no. 06-0663), fcc Ru (JCPDS no. 88-2333) and fcc Cu
(JCPDS no. 04-0836). Reproduced with permission.21
Copyright 2018, The Royal
Society of Chemistry. (d) Schematic illustration of the
formation process of 4H/fcc Ru NTs. TEM images of (e) 4H/fcc
Au NWs, (f) 4H/fcc Au–Ru
NWs and (g) 4H/fcc Ru NTs. Reproduced with permission.34

Copyright 2018, Wiley-VCH.
P
ub
li
sh
ed
o
n
08
O
ct
ob
er
2
01
9.
D
ow
nl
oa
de
d
by

M
is
si
ss
ip
pi
S
ta
te
U
ni
ve
rs
it
y
L
ib
ra
ri
es
o
n
4/
7/

20
20
1
0:
07
:5
6
P
M
.
View Article Online
sputtering. During the preparation, single-crystal Si wafers with
a Ti adhesion layer, 300 nm of Ru was deposited on the
substrate at 250 W RF and 0.085 nm s�1.42
2.2. Synthesis of Ru-based bimetallic nanomaterials
According to the mixing pattern of Ru and the other metal, Ru-
based bimetallic nanomaterials can be divided into two types:
(i) Ru-based alloys and (ii) Ru-based core–shell structures. For
Ru-based alloys, two kinds of metals are distributed homoge-
neously in the NCs. However, for Ru-based core–shell struc-
tures, one kind of metal is located in the core and the other one
nucleates and grows surrounding the core to form a shell.
2.2.1. Synthesis of Ru-based alloys. Synthesizing Ru-based
alloys is an efficient strategy to combine the advantages of
different metals, generate a synergetic effect and reduce the
cost
of noble metal catalysts. The wet chemical approach has been

commonly used in the preparation of Ru-based bimetallic
alloys.
Li's group reported the synthesis of highly active and stable
Co-substituted Ru NSs for the HER through a solvothermal
method.43 They isolated Co atoms into Ru lattice by co-reduc-
tion of Ru(acac)3 and Co(acac)2 in a mixed solution containing
24694 | J. Mater. Chem. A, 2019, 7, 24691–24714
oleylamine and heptanol. Han and co-workers synthesized
a series of necklace-like hollow NixRuy nanoalloys based on
the
galvanic replacement reaction between Ni nanochains and
RuCl3$3H2O.
44 By adjusting the concentration of Ru precursors,
hollow NixRuy nanoalloys with variable Ni to Ru molar ratios
can be obtained due to the Kirkendall effect. Using Ru(acac)3
and Ni(acac)2 as metal precursors, Huang and co-workers re-
ported a wet chemical approach for the preparation of a three-
dimensional (3D) hierarchical structure composed of an ultra-
thin Ru shell and a Ru–Ni alloy core as a catalyst under
universal
pH conditions.45 By tuning the ratios of Ru/Ni precursors,
assemblies with different Ru/Ni ratios were obtained.
It should be noted that for Ru alloys with a non-hcp metal,
composition and the reduction kinetics of the different metal
precursors. Iversen and co-workers presented a systematic
investigation of the Pt1�xRux phase diagram through the
supercritic -
tional range, using an ethanol–toluene mixture as the solvent at
450 �C and 200 bar.46 The crystal phase, particle size and
morphology of the Pt1�xRux NPs were determined by the molar
ratio (i.e. x in Pt1�xRux). The crystallite and particle size of
the

Pt1�xRux NPs were both found to decrease as the content of Ru
P
ub
li
sh
ed
o
n
08
O
ct
ob
er
2
01
9.
D
ow
nl
oa
de

4/
7/
20
20
1
0:
07
:5
6
P
M
.
View Article Online
increased. The crystal phase of Pt1�xRux NPs was fcc when x #
0.2, while the hcp phase emerged as x approached 1. Besides,
the samples exhibited a spherical morphology as x < 0.3 while
elongated particles together with the dominating spherical
morphology were obtained when x $ 0.3. Although the crystal
phase of Ru-based alloys can be predicted using the phase
diagram, the nanosize effect makes it possible to obtain Ru-
based alloys with a novel structure beyond the phase diagram.
By using a chemical reduction method, Kitagawa's group suc-
ceeded in controlling the crystal structure of Au–Ru alloys with
a certain composition in the nanoscale.47 Normally, hcp Ru and
fcc Au do not easily form alloys in the bulk due to the large
lattice mismatch between these two elements. By precisely
tuning the reduction rate with the aid of cetyl-
trimethylammonium bromide (CTAB) and appropriate precur-
sors, fcc and hcp AuRu3 alloy NPs (Fig. 2a, b, c and d) can be

synthesized under ambient conditions, respectively. The crystal
structure of the AuRu3 alloy was dominated by the nuclei
formed from one metal precursor, which started to be reduced
earlier than the other one during the reduction process (Fig. 2e).
Atomic layer deposition (ALD) is a general method to
synthesize bimetallic nanoparticles. Stair and co-workers
prepared RuPt and RuPd alloy NPs and the size, composition
and structure of the bimetallic NPs could be precisely
Fig. 2 HAADF-STEM images of (a) fcc-AuRu3 NPs and (b)
hcp-AuRu3 N
close-up view of 2q ¼ 12� to 19�. (e) Schematic illustration of
the synthesi
the reduction speed of the Au and Ru precursors, respectively.
Reprodu
controlled. The growth of well-mixed RuPd alloy NPs was ach-
ieved using the ALD sequence Ru(EtCp)2-O2-H2-Pd(hfac)2-H2
at
150 �C, which gave a Ru : Pd mole ratio of about 3 : 5. During
this process, Ru(EtCp)2 and Pd(hfac)2 dissociated and O2
burned off the ligands, forming the Ru/Pd oxide. Then H2
reduced the Ru/Pd oxide and thus the RuPd bimetallic NPs were
deposited on the substrate. Similarly, well-mixed RuPt alloy
NPs
were prepared using the sequence Ru(EtCp)2-O2-H2-
MeCpPtMe3-O2-H2 at 150
�C, which yielded a Ru : Pt mole ratio
of 1 : 1.48
2.2.2. Synthesis of Ru-based core–shell structures. Ru-
based core–shell structures have attracted much attention
since the structural design and construction of Ru-based core–
shell structures could enhance their catalytic activities owing

to the modulation of the geometric, strain and electronic
structures.
Solution
phase epitaxial growth is a versatile and facile
method to prepare Ru-based nanomaterials with core–shell
structures. As a prerequisite for heteroepitaxial growth, the
lattice mismatch between the seed and the secondary metal
should be small enough (<5%). When there is a large mismatch,
epitaxial growth is unfavorable due to high strain energy.27,49
In
this process, the deposited shell metal will follow the same
crystalline orientation as the core metal.26 Thus, it is possible
to
Ps. (c) XRD patterns of Au, fcc-AuRu3, hcp-AuRu3 and Ru
NPs. (d)The
s of AuRu3 alloy NPs with fcc and hcp crystal structures. RAu
and RRu are
ced with permission.47 Copyright 2018, Nature Publishing
Group.
J. Mater. Chem. A, 2019, 7, 24691–24714 | 24695

P
ub
li
sh
ed
o
n
08
O
ct
ob
er
2
01

9.
D
ow
nl
oa
de
d
by
M
is
si
ss
ip
pi
S
ta
te

U
ni
ve
rs
it
y
L
ib
ra
ri
es
o
n
4/
7/
20
20

1
0:
07
:5
6
P
M
.
View Article Online
synthesize fcc Ru by epitaxial growth if a core metal with the
fcc
phase is selected. Li's group reported the synthesis of Pd–
[email protected] core–shell structures through an epitaxial-
growth-
mediated method, in which the crystal phase of the Ru shell can
be tuned from hcp to fcc.50 In the whole processes, a sol-
to prepare Pd–Cu alloy
seeds with a homogeneous truncated octahedral shape and
uniform size (19.6 �
was initially induced by galvanic replacement between Ru and

PdCu3 seeds. In this step, the structure of Pd–[email protected]
trans-
formed from core–shell into yolk–shell. Moreover, the experi-
mental results indicated that the PdCu3 and PdCu2.5 seeds were
PdCu2
or Cu seeds would drive the growth of the hcp Ru shell. As the
lattice parameter of Pd–Cu varied with the composition ratio of
Pd to Cu, the appropriate lattice mismatch between the Pd–Cu
alloy substrate and the Ru overlayer led to the epitaxial growth
of the Ru shell in the unconventional fcc phase. As another
example, by using Ru(acac)3 and Pd(acac)2 as metal precursors,
Yang and co-workers adopted a simple solvothermal method to
prepare [email protected] core–shell NPLs (Fig. 3a–c) with
various thick-
nesses and different crystal structures of the Ru shell by tuning
the amount of the Ru precursor.51 During the reaction, the fcc
Pd NPLs served as seeds for the epitaxial growth of the Ru shell
and the Ru atoms preferred to adopt a fcc structure rather than
a hcp structure owing to the similar atomic radii and the small
lattice mismatch between Pd and Ru. However, further increase
of Ru would result in a crystal phase transition of Ru from fcc
to
hcp since the regulation from Pd seeds for Ru growth became
weak with increasing thickness of the Ru shell.

Besides the fcc phase, Ru could also crystalize in some novel
crystal phases, e.g. the 4H phase, by epitaxial growth if unique
substrates are selected. For instance, using 4H/fcc Au NWs as
the initial seeds, Ru(acac)3 as the metal precursor,
Fig. 3 (a) TEM, (b) HAADF-HTEM image and (c) EDX
mapping of [email protected] N
of Chemistry. (d) Schematic illustration of the synthetic route
of Au–Ru N
enlarged sectional view illustrates the epitaxial growth of a Ru
NR on a Au
mapping of the Au–Ru NW. (g) HAADF-STEM images of Au–
Ru NWs. (g1
squares (areas g1 and g2) in (g). Reproduced with permission.33
Copyrigh
structures. Inset: the high magnification TEM image of the
[email protected] core
Royal Society of Chemistry.
24696 | J. Mater. Chem. A, 2019, 7, 24691–24714
octadecylamine as the solvent and surfactant, and 1,2-hex-
adecanediol as the reductant, 4H/fcc [email protected] NWs
with core–
shell structures could be prepared (Fig. 3d, e, f1 and f2).34

HAADF-STEM images and the corresponding statistical survey
showed that Ru NRs only deposited in the 4H phase and fcc-
twin boundary in the 4H/fcc Au NWs (Fig. 3g, g1 and g2),
indicating that the highly reactive 4H and fcc twin structures
could serve as preferential nucleation sites for the hetero-
epitaxial growth of the second metal. Meanwhile, the length of
Ru NRs could be easily tuned by varying the amount of the Ru
precursor. Moreover, in the synthesized bimetallic NWs, the Ru
NRs with highly active 4H or fcc-twin structures could serve as
nucleation sites for further growth of a third metal, such as Rh
or Pt, thus forming Au–Ru–Rh and Au–Ru–Pt hybrid NWs.34,52
Thermal reduction is also an effective approach for the
synthesis of Ru-based core–shell structures. A one-step
synthetic route was proposed by Joo's group to prepare hexag-
onal nanosandwich-shaped [email protected] core–shell
NPLs.53 The co-
decomposition of Ni and Ru precursors initially generated Ni
particles as cores with a hexagonal plate-
that, the Ru shell layer would deposit in a regioselective manner
on the top and bottom of the Ni NPLs as well as around its
center edges. The selective growth of the Ru shell layer can be
attributed to the distinct surface energies of different Ru facets
in the presence of CO gas, as well as the presence of twin
boundaries in the Ni core. This method can be extended to

synthesize trimetallic [email protected] core–shell NPs with
tunable
chemical compositions. Feng and co-workers realized the
assembly of Ru NPs as a shell on the surface of Te NRs.54 The
Te
protected] core–shell
structures
(Fig. 3h) with different molar ratios of Ru to Te were
synthesized
through solvothermal treatment in ethylene glycol. Besides
depositing Ru shells on other metals, Ru nanostructures can
also act as seeds for the growth of secondary metals. For
PLs. Reproduced with permission.51 Copyright 2018, The Royal
Society
Ws. The black dashed line indicates the fcc-twin boundary. The
partial
NW. (e) STEM image of a typical Au–Ru NW. (f1 and f2)
STEM elemental
and g2) Corresponding FFT images taken from the two green
dashed
t 2018, Nature Publishing Group. (h) TEM images of
[email protected] core–shell
–shell structure. Reproduced with permission.48 Copyright
2019, The

P
ub
li
sh
ed
o
n
08
O
ct
ob
er
2

01
9.
D
ow
nl
oa
de
d
by
M
is
si
ss
ip
pi
S
ta

te
U
ni
ve
rs
it
y
L
ib
ra
ri
es
o
n
4/
7/

20
20
1
0:
07
:5
6
P
M
.
View Article Online
instance, [email protected] NPs were synthesized by the thermal
reduction
of the corresponding metal precursors, i.e. Ru(acac)3 and PtCl2.
followed
by the reduction of PtCl2 to form the Pt shells.
55
2.3. Synthesis of Ru/non-metal nanocomposites

2.3.1. Synthesis of Ru–
NPs are easy to aggregate, their active sites may be blocked,
resulting in decreased HER performance. Loading Ru NPs on
the
provide long-term corrosion protection to enhance the stability
of Ru-based electrocatalysts. Carbon materials have been
demonstrated as excellent matrixes to inhibit the aggregation of
and enhance the conductivity of the catalysts.
To date, various methods have been adopted to prepare Ru NCs
loaded on different carbon supports such as commercial carbon
materials, graphene and nitrogen (N)-doped carbon materials.
As shown in Fig. 4a, by using a mechanochemically assisted
method, Ru NPs could be deposited on graphene nanoplatelets
(GnPs) for the HER in both acidic and alkaline media.29 The
mechanochemical reaction between graphite and dry ice
produced carboxylic-acid-functionalized graphene nanoplatelets
(CGnPs). Owing to the abundant carboxylic acid groups on
CGnPs, the Ru3+ ions can be easily adsorbed on the surface of
CGnPs via the coordination between carboxylic acids and Ru3+
metal
with NaBH4 when the Ru precursor and CGnPs were mixed in

an
reduce CGnPs to GnPs, uniform Ru NPs (�2 nm) deposited on
Fig. 4 (a) Schematic illustration of the synthesis of
[email protected] (i) Physica
reduction of Ru ions on CGnP to [email protected] (b) TEM
images of [email protected]
images of Ru NPs on C supports. Reproduced with
permission.56 Copyrig
hybrids. Reproduced with permission.64 Copyright 2018, The
Royal Socie
NPs.
the GnP matrix ([email protected]) were obtained (Fig. 4b). The
synthesis
process avoided the usage of hazardous reagents or tedious
procedures, providing an opportunity for the low-cost and scal-
able production of stable catalysts for practical applications.
Solid-state synthesis has been considered as a facile and
green approach to prepare highly dispersed metal-based
nanocatalysts on carbon materials, as this method can avoid the
use of organic capping agents which may block the active sites
on the surface of catalysts. Zhang and co-workers reported

a simple solid approach to synthesize Ru NPs deposited on
various carbon supports (Fig. 4c), including XC-72 Vulcan
carbon, 3D graphene, Ketjenblack and Super P via mortar
grinding at room-temperature.56 The in situ reduction of the Ru
precursor took place during grinding a mixture of RuCl3,
sodium hydroxide (NaOH), sodium borohydride (NaBH4) and
carbon support in an agate mortar. This process is favorable for
the scalable production of Ru-carbon composites since it does
not need any organic solvents, capping agents or pretreatment
of carbon supports.
The physical sputtering method is a facile and efficient
technique to directly prepare highly dispersed and uniform Ru
NPs on carbon materials. Yang and co-workers prepared gra-
phene supported Ru NP composites through a sputtering
method for the electrocatalytic HER and hydrolytic dehydroge-
nation of NaBH4. They prepared graphene by liquid reduction
of graphene oxide with hydrazine hydrates. Then the obtained
graphene was used as the support and a metallic Ru plate was
used as the target. During the sputtering process, the support
rotated continuously and vibrated cyclically to ensure the
deposition of Ru NPs uniformly. The size of the as-prepared Ru
l cracking of graphite into CGnPs in the presence of dry ice. (ii)
In situ
. Reproduced with permission.29 Copyright 2018, Wiley-VCH.

(c) TEM
ht 2018, Wiley-VCH. (d) TEM images of Ru NPs over N-doped
carbon
ty of Chemistry. The insets in (b) and (d) show the size
distribution of Ru
J. Mater. Chem. A, 2019, 7, 24691–24714 | 24697
P
ub
li
sh
ed
o
n
08

O
ct
ob
er
2
01
9.
D
ow
nl
oa
de
d
by
M
is
si

ss
ip
pi
S
ta
te
U
ni
ve
rs
it
y
L
ib
ra
ri
es

o
n
4/
7/
20
20
1
0:
07
:5
6
P
M
.
View Article Online
NPs fell in the range of 1–2.5 nm, and the mean particle size
was

around 1.7 nm.57
Pyrolysis is also used in the preparation of composite cata-
lysts, i.e. Ru NPs dispersed on carbon materials. The fabrication
process is facile, economical, environmentally friendly and can
be scaled up easily. The synthesis of Ru-carbon composites by
pyrolysis can be divided into two different strategies. In the
t
followed by pyrolysis. As a typical example, Fan and co-
workers
developed a facile and convenient strategy for synthesizing
-72 carbon through adsorption
and subsequent low temperature pyrolysis of Ru3(CO)12.
58 The
Ru3(CO)12 molecules were encapsulated in the pores of the
carbon matrix during the adsorption procedure. Upon pyrolysis
at different temperatures, the molecules were decomposed to
Ru NPs with different sizes on the surface of carbon. Since the
abundant functional groups on the surface of carbon quantum
dots (CQDs) provide favorable sites for the nucleation and
growth of Ru NPs, Liu and co-workers synthesized
[email protected]
CQD hybrid materials by a facile pyrolysis method.59 The

hybrids were prepared by mixing N-doped CQDs with RuCl3 via
a hydrothermal process to achieve a membranous structure,
followed by one-step pyrolysis under an argon atmosphere.
In the second strategy, Ru precursors are mixed with N-
pyrolysis process, the reduction of Ru precursors occurs with
the carbonization of the carbon precursors. Moreover, N-doping
can be introduced into the carbon materials in this process,
resulting in Ru/N-doped carbon composites.60–62 As a typical
example, Wang and co-workers prepared Ru NPs encapsulated
in 3D N-doped graphite carbon materials via a two-step
process.63 First, carbon foam was impregnated in an aqueous
solution of RuCl3$5H2O to adsorb Ru
3+ followed by freeze
drying. Then the mixture was annealed to realize the reduction
of Ru3+, crystallization of Ru NPs and graphitization of carbon
foam simultaneously. Wang and co-workers constructed highly
dispersed Ru NPs over N-doped carbon hybrids (Fig. 4d)
through the calcination of a solid mixture of D-glucosamine
hydrochloride (GAH), melamine and RuCl3.
64 During the calci-
nation process, layered g-

through the thermal condensation of melamine in the low-
temperature zone (<600 �C). In the meantime, GAH was
condensed to form a carbon skeleton in the interlayer of g-
C3N4.
-C3N4/C sandwich-like
structure
effectively inhibits the aggregation of Ru NPs during the calci-
nation process. Then, a high-temperature pyrolysis process at
800 �C induced the formation of graphene-
complete decomposition of g-C3N4. Zhang and co-workers used
a unique precursor, tris(2,20-bipyridyl)-ruthenium(II) chloride
hexahydrate (TBA), to prepare highly dispersed Ru nanoclusters
on N-doped carbon by the pyrolysis method.65 As TBA contains
Ru, N and C simultaneously, its pyrolysis directly results in the
formation of Ru nanoclusters and N-doped carbon, thus
simplifying the synthesis process. In addition, Qin and co-
workers synthesized Ru NPs coated with a thin layer of N-doped
carbon through thermal annealing of polydopamine-coated Ru
NPs ([email protected]). The in situ formed N-doped carbon
layer
24698 | J. Mater. Chem. A, 2019, 7, 24691–24714
protected the agglomeration of Ru NPs during the annealing
process. Importantly, they found that the crystallinity of Ru NPs
was highly related to the annealing temperature and thus

Ru/N-doped carbon composites can also be easily obtained
through the chemical reduction of Ru precursors. Zhang and
co-workers prepared various Ru NPs on N-doped porous carbon
substrates by reducing RuCl3 with NaBH4.
67 First, various kinds
of biomass, such as lignin, straw and shaddock peel, were
carbonized at 800 �C under N2, followed by annealing under an
atmosphere of ammonia to realize N doping. Then, the ob-
tained products were oxidized with nitric acid. Finally, these
materials were dispersed in RuCl3 solution followed by the
addition of NaBH4. It has been shown that oxidation and N-
doping can accelerate the charge transfer rate between Ru NPs
and the carbon substrates, thus improving the HER
performance.
Ru-based alloys could also be composited with carbon
materials to further enhance the activity and stability of the
catalysts. Pd–Ru NPs encapsulated in porous carbon NSs were
synthesized through a wet-chemical approach.68 Ru3+ and Pd2+
NSs in a mixed solution of RuCl3 and Na2PdCl4. The composi-
tion and structure of the as-formed catalysts could be tuned by

adjusting the ratio of Pd to Ru. Doping Ru in other metal (e.g.
Ni
and Co)-based metal–organic frameworks (MOFs) followed by
one-step annealing under a N2 or Ar atmosphere is another
simple method for the preparation of bimetallic alloys sup-
ported on carbon or N-doped carbon substrates.69–71 For
example, Su and co-workers synthesized RuCo nanoalloys
encapsulated in N-doped graphene layers via one-step anneal-
ing of a Ru-doped Co3[Co(CN)6]2 MOF.
69 During the annealing
process, Ru and Co atoms in the MOF precursor were reduced to
form bimetallic RuCo nanocrystals; meanwhile some remaining
CN-group linkers would transform into N-doped graphene
layers. Electrodeposition is a useful route to synthesize elec-
trodes with higher stability compared with those synthesized
from chemical reduction. Pt–Ru bimetallic electrocatalysts were
prepared by potentiostatic electrodeposition on poly-
acrylonitrile based carbon paper. The electrodeposition process
was carried out in a 250 mL beaker on a stirring hot plate with
RuCl3 and H2PtCl4 as the precursors at 78
�C. Ru and Pt were
deposited on the substrate with a potential of �0.120 V versus
Ag/AgCl.72 This method was also used for the synthesis of Pt–

Ru–M (M ¼ Cr, Fe, Co, Ni, and Mo) decorated Ti mesh for H2
evolution.73
2.3.2. Synthesis of Ru–carbon nitride composites. Similar
to carbon materials, carbon nitrides are widely used as matrices
for the growth of Ru nanomaterials to improve the dispersibility
of Ru NCs and enhance the conductivity of the catalysts.
Moreover, recent studies have demonstrated that carbon
nitrides can tune the electronic and crystal structure of Ru
nanomaterials, thus improving their HER performance.
Thermal polycondensation of compounds containing C and
N is a facile method to prepare carbon nitrides for further
obtaining Ru–carbon nitride composites.74 For example, Qiao's
group synthesized an anomalously structured Ru–graphitic
P
ub
li

sh
ed
o
n
08
O
ct
ob
er
2
01
9.
D
ow
nl
oa
de

d
by
M
is
si
ss
ip
pi
S
ta
te
U
ni
ve
rs
it

y
L
ib
ra
ri
es
o
n
4/
7/
20
20
1
0:
07
:5
6

P
M
.
View Article Online
carbon nitride complex supported on carbon (Ru/g-C3N4/C)
electrocatalysts by annealing a mixture of RuCl3 and dicyan-
diamide (DCDA) under an argon atmosphere.28 They ascribed
the formation of the homogeneously dispersed Ru NPs with an
average size of 2 nm to the strong interaction between Ru NPs
and g-C3N4. Moreover, g-C3N4 can facilitate the formation of
anomalous fcc Ru NPs on the substrates since the adhesion
energy between fcc Ru and g-C3N4 is higher than that between
hcp Ru and g-C3N4. In another interesting work, the C2N
matrix
between hexaketocyclohexane and hexaaminobenzene trihy-
occurred within the C2N layers via the reduction of RuCl3 with
NaBH4 (Fig. 5a). Small Ru NPs (average diameter �1.6 � 0.5
nm)
were homogeneously dispersed within the nitrogenated holey
two-dimensional carbon structure (R[email protected]) (Fig. 5b–
d).

19 In
order to modulate the electronic structures of Ru to enhance its
catalytic activity, Ma and co-workers prepared Ru electro-
catalysts anchored on multi-walled carbon nanotubes
(MWCNTs) as well as encapsulated in amorphous turbostratic-
phase carbon nitride ([email protected]/MWCNTs).
75 During the
preparation processes, Ru NPs were anchored on the surface of
with glycol. Then the ultrathin amorphous t-CNx layer was
chemically coated on the surface of Ru/MWCNTs via the poly-
merization between CCl4 and C2H8N2 followed by thermal
treatment.
Direct pyrolysis of the mixtures containing Ru precursors
and C and N sources is a simple, convenient and widely used
strategy for the preparation of Ru NCs supported on carbon
nitride substrates. Chu and co-workers successfully prepared
�2 nm) with double protective coating layers
Fig. 5 (a) Schematic illustration of the synthesis and structure
of [email protected]
peak at 25.09� belongs to the {002} plane of C2N. (c) TEM
image of [email protected]

and STEM-EDS elemental mapping of [email protected]
Reproduced with permi
through annealing a mixture of tris(2,20-bipyridine)ruthenium
dichloride, cyanuric acid and graphene.76 Cyanuric acid was
the 2,20-bipyridine ligand was converted to N-doped carbon
during the annealing process. The N-doped carbon and C3N4
jointly prevented the aggregation of the Ru NPs.
2.3.3. Synthesis of Ru–semiconductor composites.
Compared to single-component materials, Ru–semiconductor
composites can exhibit better activity and favorable kinetics
chemical synthesis is a facile method to composite Ru with
semiconductors. For instance, using RuCl3 as the precursor,
Akbayrak and co-workers synthesized Ru/MO2 (M ¼ Ti, Zr, Hf
and Ce) composites by a chemical reduction method.77,78 The
Ru3+ ions impregnated on the surface of metal oxides were
reduced with NaBH4 aqueous solution. Through a wet-impreg-
nation reduction method, Ru/RuO2 dual co-
TiO2 nanobelts were constructed for photocatalytic water split-
ting by using RuCl3 solution as the Ru precursor.

79 The ratio of
Ru and RuO2 could be regulated by adjusting the annealing
temperature, when annealing the samples in air. As reported by
Chen and co-workers, Ru–MoO2 nanocomposites were fabri-
cated by in situ carburization of Ru- -btc (btc ¼
1,3,5-benzene-tricarboxylate) under an inert atmosphere.
mixing RuCl3 aqueous solution and Mo-btc.
80 The Ru-
Mo-btc was pyrolyzed at 700 �C for 3 h under a continuous
co-workers successfully
synthesized Ru NPs on N-doped TiO2 NCs with pits on the
surface through the calcination of pre-synthesized RuO2/TiO2
composites under an NH3 atmosphere.
81 During the calcination
process, RuO2 was reduced to metallic Ru with NH3;
meanwhile,
. NMP: N-methyl-2-pyrrolidone. (b) XRD pattern of
[email protected] The broad
2N. Inset: size distribution of the corresponding Ru NPs. (d)
STEM image
ssion.19 Copyright 2017, Nature Publishing Group.

J. Mater. Chem. A, 2019, 7, 24691–24714 | 24699
P
ub
li
sh
ed
o
n
08
O
ct
ob
er

2
01
9.
D
ow
nl
oa
de
d
by
M
is
si
ss
ip
pi
S

ta
te
U
ni
ve
rs
it
y
L
ib
ra
ri
es
o
n
4/
7/

20
20
1
0:
07
:5
6
P
M
.
View Article Online
N-doping was introduced into anatase TiO2. Ager and co-
workers prepared a photocathode containing Ru, TiO2 and InP
sputtered on the surface of a TiO2 passivation layer. The utili-
zation of Ru increased the carrier separation rate and thus
increased the short-circuit current density of the PECs.82

with Ru. For example, Ru/MoS2/carbon paper composites were
prepared via the hydrothermal reaction. During the prepara-
followed by modifying MoS2 with Ru through impregnation in
RuCl3 solution and reduction with H2 under calcination.
30 Joo's
group reported the preparation of cactus-like hollow Cu2�x-
[email protected] NPLs through the process shown in (Fig.
6a).83 First,
Cu1.94S NPLs were transformed into Cu1.8S during the cation
Ru3+ ions were
reduced to metallic Ru at high temperature followed by the
growth of Ru islands, thus forming the cactus-like nano-
structures (Fig. 6b). The crystal phase of the exterior was hcp
Ru
u -
ually leached out, forming the hollow NPLs (Fig. 6d).
2.3.4. Others. Other Ru-based hybrids, such as Ru/Mo2C,
84
Ru/SiO2,

85 Ru/Y(OH)3,
86 Ru/Ru2P
87 and other Ru-based
composites,58,88–91 were also prepared for the HER.
Compositing
Ru with various materials could take advantage of every
Fig. 6 (a) Schematic illustration of the synthesis of hollow
Cu2�[email protected] Ru NP
vertically standing NPLs. (c) HRTEM image of a porous shell
and the corres
mapping images of the lateral face of the vertically standing
NPLs. Repro
24700 | J. Mater. Chem. A, 2019, 7, 24691–24714
component and make use of the synergetic effect of the hybrids
to enhance the HER activity.
MoC2 has a similar d-band structure to Pt group metals and
has been proven to be a promising electrocatalysts for the HER.
The preparation of Ru/MoC2 hybrids combined the advantages
of MoC2 and Ru and could reduce the use of noble metal
catalysts. Using (NH4)6Mo7O24$4H2O, RuCl3 and popcorn as
Mo, Ru and carbon sources, respectively, Ru/Mo2C embedded

in highly porous N-doped carbon framework was fabricated.84
By annealing the mixture of porous popcorn and Mo/Ru sources
under an inert atmosphere, the carbonization of popcorn, the in
situ growth of Mo2C particles and the reduction of Ru
3+ were
achieved simultaneously. As for Ru–SiO2 hybrids, SiO2 was
used
as a support for the growth of Ru NPs. Ru NPs were loaded on
SiO2 supports by an impregnation method using RuCl3 as the
precursor.85 An et
the
suspension of SiO2. Using a rotary evaporator at room temper-
ature, ethanol was evaporated under reduced pressure and Ru3+
obtained mixture in air, RuO2 was formed on SiO2 supports.
Then RuO2 was reduced with NaBH4 in ethanol and Ru NPs
were prepared. In addition, Ru/amorphous yttrium hydroxide
(Y(OH)3) nanohybrids were obtained through a chemical
reduction method.86 As Y(OH)3 has good corrosion resistance
-
cial for the durability of the electrocatalyst. RuCl3 was used as
the Ru precursor, which was reduced with NaBH4. Moreover,

Ls from Cu1.94S NPLs. (b) Top-view and side view HRTEM
images of the
ponding FFT pattern. (d) STEM image and the corresponding
elemental
duced with permission.83 Copyright 2017, Wiley-VCH.
P
ub
li
sh
ed
o
n
08
O

ct
ob
er
2
01
9.
D
ow
nl
oa
de
d
by
M
is
si
ss

ip
pi
S
ta
te
U
ni
ve
rs
it
y
L
ib
ra
ri
es

o
n
4/
7/
20
20
1
0:
07
:5
6
P
M
.
View Article Online
NaBH4 also provided an alkaline environment to facilitate the
hydrolysis of Y(NO3)3$6H2O. The Ru NPs with a small size of
2.9

scaffold could trap Ru to inhibit further growth, leading to the
3. Ru-based electrocatalysts for H2
evolution
3.1. Principle
During the last few decades, the HER, a half reaction of water
splitting, has attracted much attention for the clean production
of H2. Since the water splitting reaction requires a large over-
potential, i.e. excess potential compared to the thermodynamic
potential value the production of H2 from water is difficult. The
adoption of electrocatalysts can reduce the overpotential,
resulting in the high efficiency of the HER.
In general, the electrocatalytic HER occurring on the
surface of the el
the Volmer reaction, during which a proton adsorbed on the
active site of the electrocatalyst reacts with an electron trans-
ferred from the external circuit, forming an adsorbed
hydrogen atom (H*). The second step is H2 generation occur-
ring in two different mechanisms. The formation of H2 is
through the combination of two H* in the Tafel mechanism
when the H* coverage is high enough, while in the Heyrovsky
mechanism one H* prefers to combine with one proton from

the electrolyte and an electron to produce H2. The catalytic
activity varies with the pH of electrolytes. In acidic
electrolytes,
protons are reduced in the H* generation process, and the
intrinsic activity of the electrocatalysts is highly related to the
Gibbs free energy for hydrogen adsorption (DGH).
92 If the bond
strength between the active sites and H* is too weak, H* will be
unstable for further reactions. In contrast, if the bond strength
is too strong, the active sites would be blocked, and the bond is
hard to break, thus preventing the release of H2.
93–95 In alka-
line electrolytes, the Volmer step was proven to be the rate-
determining step for the HER.43 In this step, the adsorbed H2O
� to supply enough protons.
Thus, extra energy is required for catalysts to overcome the
energy barrier of water dissociation (DGB) to break the H–O–H
bonds. Pt has been regarded as the best solid-state electro-
catalyst for the HER due to its near-zero DGH. However, the
scarcity and high cost of Pt as well as its low stability in
alkaline media limit its wide application. Recently, Ru has
been proven as an efficient alternative to Pt owing to its high
theoretical intrinsic activity with a moderate bond strength of

�65 kcal mol�1 with hydrogen,18 which is slightly lower than
that of Pt–H. Besides, Ru exhibits abundant d orbital electrons
for promoting the adsorption and activation of H*. Moreover,
Ru-based materials exhibit strong corrosion resistance in both
acidic and basic media. Additionally, the price of Ru is 1/3 that
of Pt, lowering the cost of electrocatalysts.20 Therefore,
tremendous efforts have been devoted to the preparation of
Ru-based electrocatalysts and the enhancement of their cata-
lytic activities during the past few years.
In order to improve the catalytic performance of Ru-based
electrocatalysts, a lot of studies are focused on increasing the
number and activity of the active sites of electrocatalysts and
promoting the electron transfer efficiency between the electrode
and electrocatalysts. Until now, several strategies have been
proved vital in the improvement of HER performance: (a) defect
engineering. Defects, such as atomic steps, kinks, and phase
boundaries, could serve as the active sites for the HER; mean-
while, these defects could modulate the electronic structure of
Ru, thus enhancing the catalytic activity.33 (b) Crystal phase
engineering. The crystal phase of Ru is highly associated with
the
DGH and DGB. For instance, the calculation results from Qiao
and co-workers have demonstrated that the DGH values of
Ruhcp

and Rufcc were �0.83 and �0.48 eV, respectively.41 From
a thermodynamic point of view, the hydrogen bonding of fcc Ru
is weaker than that of hcp Ru, thus facilitating the H* desorp-
tion process in the Heyrovsky step. Meanwhile, from a kinetic
viewpoint, the DGB values of Ruhcp and Rufcc in the Volmer
step
were 0.51 and 0.41 eV, respectively, resulting in easier H*
generation for the Rufcc catalyst in alkaline electrolytes. There-
fore, the synthesis of Ru nanomaterials with a novel crystal
phase is one of the most promising strategies to develop high-
performance electrocatalysts for the HER. (c) Constructing Ru-
based composites. Alloying Ru with other metals or
constructing
core–shell structures can tune the value of DGB and the elec-
tronic structure (e.g. d-band center) of Ru-based materials.43,75
For example, DFT calculation results have revealed that the
DGB
value of the [email protected] core–shell structure was 0.84 eV,
lower than
that of the pure Ru crystal (0.93 eV), thus facilitating H2 evolu-
tion.96 The DFT calculation results by Huang and co-workers
demonstrated that the d-
dulating the surface electronic envi-
ronment for easier H–H formation.45 In addition, the HER

performance could also be improved via depositing Ru NPs on
highly conductive substrates, which could ensure fast electron
transport and inhibit Ru NPs from aggregation and corrosion.64
Therefore, the rational design and precise preparation of Ru-
based composites can improve the HER activity. In the
following sections, based on the components and structures of
Ru-based nanomaterials, we will mainly discuss the utilization
of three types of metallic Ru-based electrocatalysts for the
HER:
(i) Ru NCs; (ii) Ru-based bimetallic nanomaterials; and (iii) Ru/
non-metal nanocomposites. Meanwhile, the key performance
parameters of these mentioned electrocatalysts are summarized
in Table 1.
3.2. Ru NCs for the electrocatalytic HER
Ru NCs can be directly used as electrocatalysts because of their
high intrinsic catalytic activity and relatively low cost. Wu and
co-workers reported the synthesis of free-standing ultrathin
Ru NSs with high activity toward water splitting.38 The HER
performance of Ru NSs was better than that of Ru powders
owing to their smaller DGH (0.289 eV) and enhanced HER
kinetics. However, the catalytic activity of Ru NSs was still
lower than that of the commercial Pt/C. Recently, Huang and
co-workers synthesized Ru nanodendrites composed of fcc/

J. Mater. Chem. A, 2019, 7, 24691–24714 | 24701
Table 1 Comparison of the parameters of the Ru-based HER
electrocatalystsa
Catalyst Electrode Electrolyte
Mass loading
[mg cm�2]
Scan rate
[mV
s�1]
Overpotential
@ 10 mA
cm�2 [mV]
Exchange
current
density [mA
cm�2]
Tafel

slope
[mV
dec�1]
TOF [H2
s�1] Ref.
Pure Ru crystals Ru GCE 0.5 M
H2SO4
0.102 5 20 — 46 — 36
Ru GCE 0.5 M
H2SO4
0.352 10 83 — 46 0.87 (100
mV)
130
Ru GCE 1.0 M
KOH
0.034 2 23 1.81 29.4 0.22 (30
mV)

34
Ru GCE 1.0 M
H2SO4
0.428 10 20 — 29 17.38 (100
mV)
131
Ru GCE 1.0 M
NaOH
0.428 10 25 — 65 — 131
Ru-based alloys Ni43Ru57 GCE 0.5 M
H2SO4
0.28 5 41 0.62 �31 — 44
Ru3Ni3 GCE 1.0 M
KOH
0.102 5 39 — 26.9 — 45

Ru3Ni3 GCE 0.5 M
H2SO4
0.102 5 39 — 53.9 — 45
Co-substituted Ru GCE 1.0 M
KOH
0.153 5 13 — 29 2.15 (30
mV)
43
Ru-based core–shell
structures
Au–Ru NWs GCE 1.0 M
KOH
0.08 2.0 50 0.35 30.8 0.31 (50
mV)
33

[email protected] GC-RDE 0.1 M
KOH
0.02 10 41 — 36 — 51
[email protected] GCE 1.0 M
KOH
0.05 5 30 — 30 — 96
H2SO4
0.285 5 86 — 36 0.82 (100
mV)
54
Ru–C composites Ru/C RDE 1.0 M
KOH
0.590 — 14 — 32.5 — 58
Ru/C GCE 1.0 M
KOH

0.498 — 14 — 30 — 65
Ru/C GCE 0.5 M
H2SO4
0.86 10 61 — 59 10 (100
mV)
76
Ru/C GCE 1.0 M
KOH
0.86 10 81 — 88 24 (100
mV)
76
Ru/C GC-RDE 1.0 M
KOH
0.035 2 43.4 — 49 — 21
Ru layers/hollow C

sphere
GCE 1.0 M
KOH
0.418 2 18 — 47 0.25 (15
mV)
102
Ru/N-doped
graphene
GCE 1.0 M
KOH
0.857 10 40 — 76 — 60
Ru/N-doped C GCE 1.0 M
KOH
0.247 5 32 — 53 — 64
Ru/N-doped C Carbon
paper

1.0 M
KOH
— 5 26 — 36 10.8 (100
mV)
132
hcp Ru/N-doped C GCE 0.5 M
H2SO4
0.28 — 27.5 — 37 1.6 (25
mV)
66
Ru/N-doped C Graphite
foam
1.0 M
KOH
0.013 1 21 2.43 31 4.55 (100
mV)

62
Ru/N-doped
graphite C
GCE 0.5 M
H2SO4
0.36 2 25 — 31 0.68 (30
mV)
63
Ru/N-doped C GCE 0.1 M
KOH
0.20 — 47 — 14 — 103
Ru–C composites [email protected] RDE 0.5 M
H2SO4
0.75 5 13 — 30 — 29
[email protected] RDE 1.0 M

KOH
0.25 5 22 — 28 — 29
KOH
0.42 2 10 0.8 47 — 59
Ru/porous N-doped
C
GCE 0.1 M
KOH
0.159 5 30 0.089 28.5 — 61
24702 | J. Mater. Chem. A, 2019, 7, 24691–24714 This journal
is © The Royal Society of Chemistry 2019
P
ub

li
sh
ed
o
n
08
O
ct
ob
er
2
01
9.
D
ow
nl
oa

de
d
by
M
is
si
ss
ip
pi
S
ta
te
U
ni
ve
rs
it

y
L
ib
ra
ri
es
o
n
4/
7/
20
20
1
0:
07
:5

6
P
M
.
View Article Online
Table 1 (Contd.)
Catalyst Electrode Electrolyte
Mass loading
[mg cm�2]
Scan rate
[mV
s�1]
Overpotential
@ 10 mA
cm�2 [mV]
Exchange

current
density [mA
cm�2]
Tafel
slope
[mV
dec�1]
TOF [H2
s�1] Ref.
KOH
0.27 — 14 5.8 59 — 67
Ru/3d NPC GCE 1.0 M
KOH
0.498 — 15 — 31 1.45 (40
mV)
123

Pd50Ru50/CNs GCE 0.1 M
KOH
0.354 10 37.3 — 67.9 — 68
Pd50Ru50/CNs GCE 0.5 M
H2SO4
0.354 10 45.1 — 67.6 — 68
NiRu/N-doped C GCE 0.5 M
H2SO4
0.273 5 50 — 36 — 70
RuCo/N-doped C GCE 1.0 M
KOH
0.275 2 28 10�2.48 31 — 71
PtRu/porous C
sphere
GCE 0.5 M
H2SO4

0.354 5 19.7 1.57 27.2 4.03 (100
mV)
133
Ru–carbon nitride
composites
H2SO4
0.285 5 13.5 1.9 30 1.95 (50
mV)
19
KOH
0.285 5 17 — 38 1.66 (50
mV)
19

Ru/C3N4/C GCE 0.1 M
KOH
0.204 — 79 — — 4.2 (100
mV)
28
[email protected]/MWCNTs GCE 1.0 M
KOH
0.28 10 39 — 28 — 75
RuC2N2 GCE 1.0 M
KOH
0.20 — 12 — — — 103
RuC2N2 GCE 0.1 M
KOH
0.20 — 47 — 14 — 103
RuC2N2 GCE 0.5 M
H2SO4

0.20 — 29 — 29 — 103
Ru/semiconductor
composites
Ru/MoO2 GCE 1.0 M
KOH
0.285 2 29 — 31 — 80
Ru/MoS2/CP GCE 1.0 M
KOH
0.408 5 13 — 60 — 30
Cu2�xS/Ru GCE 1.0 M
KOH
0.23 2 82 — 48 — 83
Others Ru/Y(OH)3 GCE 0.1 M
KOH
0.283 5 100 0.7 66 — 86

[email protected]–Ru GCE 0.5 M
H2SO4
0.283 5 51 0.32 35 1.10 (100
mV)
89
Ru/Cu-doped RuO2 GCE 1.0 M
KOH
0.285 2 28 — 35 — 90
NiO/[email protected] Ni
scaffold
— 1.0 M
KOH
— 2 39 — 75 0.36 (100
mV)
88

a GCE: glassy carbon electrode; RDE: rotating ring disk
electrode; GC-RDE: glassy carbon rotating disk electrode.
P
ub
li
sh
ed
o
n
08
O
ct
ob
er
2
01

9.
D
ow
nl
oa
de
d
by
M
is
si
ss
ip
pi
S
ta

te
U
ni
ve
rs
it
y
L
ib
ra
ri
es
o
n
4/
7/
20

20
1
0:
07
:5
6
P
M
.
View Article Online
hcp nanoblades by a solvothermal method.21 The micro/mes-
oporous electrocatalysts exhibited robust efficiency and
stability for the HER in alkaline media, surpassing commercial
Pt/C. The overpotential of Ru nanodendrites/C to achieve
a current density of 10 mA cm�2 was 43.4 mV, and its current
densities were larger than those of Pt/C for an overpotential
above 60 mV. Apart from the abundant active sites provided by
the dendrite structure, the superior HER performance of Ru
nanodendrites/C also resulted from their small charge transfer
resistance.

Constructing Ru electrocatalysts with a porous and hierar-
chical structure is an effective strategy to increase the active
sites and enhance mass transport during the HER. Recently,
Zhang's group demonstrated that hierarchical 4H/fcc Ru NTs
(Fig. 7a) exhibited a lower overpotential and Tafel slope (Fig.
7b)
in comparison with Ru/C and even Pt/C in alkaline media.34
The
HER performance of the hierarchical 4H/fcc Ru NTs was still
Two main reasons accounted for the excellent HER perfor-
mance of the hierarchical 4H/fcc Ru NTs. On one hand, the
unique hierarchical and porous structure provided a large
J. Mater. Chem. A, 2019, 7, 24691–24714 | 24703
Fig. 7 (a) TEM image of hierarchical 4H/fcc Ru NTs. (b) The
polarization curves in 1.0 M KOH of various electrocatalysts.
(c) The stability test of
4H/fcc Ru NTs. The polarization curves are recorded before and
after 10 000 potential cycles from 0.03 to �0.04 V (vs. RHE).
Reproduced with
permission.34 Copyright 2018, Wiley-VCH. (d and e) The

STEM images of Ru3Ni3 nanosheet assemblies. (f) Surface
valence band photoemission
spectra and (g) the polarization curves of the as-prepared
samples in 1 M KOH. Reproduced with permission.45 Copyright
2019, Elsevier. (h) STEM
image of Co-substituted Ru NSs. (i) The polarization curves of
Ru/C, Pt/C, RuCo alloy and Co-substituted Ru. (j) Free energy
diagrams of the
Volmer steps of the HER on various metal surfaces with
different amounts of Co substitution including atomic
configurations of reactant initial
states, intermediate state, final states and additional transition
states. Reproduced with permission.43 Copyright 2018, Nature
Publishing Group.
P
ub
li
sh
ed
o

n
08
O
ct
ob
er
2
01
9.
D
ow
nl
oa
de
d
by

M
is
si
ss
ip
pi
S
ta
te
U
ni
ve
rs
it
y
L
ib

ra
ri
es
o
n
4/
7/
20
20
1
0:
07
:5
6
P
M
.

View Article Online
surface area and large number of active sites. On the other
hand, the Ru NTs were rich in atomic steps, kinks and phase
boundaries, which could modulate the electronic structure and
increase the catalytic activity.
3.3. Ru-based bimetallic nanomaterials for the
electrocatalytic HER
3.3.1. Ru-based alloys. Alloying Ru with other metals to
form bimetallic alloys is one of the effective approaches to
prepare catalysts with high efficiency and robust stability. The
introduction of another metal can not only generate a certain
Ru induced by the hetero metal–metal bond. Moreover, the
synergistic effect of different metals favors the activation of the
catalyst during the HER process.97–100
For example, 3D hierarchical Ru–Ni NS assemblies (Fig. 7d
and e) composed of an ultrathin Ru shell and a Ru–Ni alloy core
exhibited superior catalytic performance and stability for the
HER in alkaline solution compared with the commercial Pt/C
catalyst.45 With the increase of Ni content, the d-band center of
Ru–
24704 | J. Mater. Chem. A, 2019, 7, 24691–24714

bond strength with H for easier H–H formation. Thus, Ru–Ni
alloys with a higher Ni content exhibited better HER perfor-
mance. The Ru–Ni alloys with different component ratios
(Ru3Ni3, Ru3Ni2, and Ru3Ni1) exhibited smaller overpotentials
than commercial Pt/C at a current density of 10 mA cm�2 and
Ru3Ni3 showed the smallest overpotential (Fig. 7g). As shown
in
the Tafel plots, the Tafel slopes of Ru3Ni3, Ru3Ni2, and
Ru3Ni1
were calculated to be 26.9, 29.9, and 30.5 mV dec�1,
respectively,
lower than those of Ru NS assemblies (58.3 mV dec�1) and
Pt/C
(46.8 mV dec� -band center, the large
surface area of the hierarchical structure provided a large
number of active sites which also contributed to the enhanced
catalytic activity. Han and co-workers prepared necklace-like
hollow NixRuy nanoalloys, which exhibited enhanced electro-
catalytic HER activity and stability in acidic media.44
Especially,
the Ni43Ru57 nanoalloy exhibited an overpotential of 41 mV at
a current density of 10 mA cm�2 and a Tafel slope of �31 mV
dec�1, close to the performance of commercial Pt/C. The
excellent catalytic performance of Ni43Ru57 can be ascribed to
the appropriate component ratio and the effective electronic

coupling of Ni and Ru, which increase the interfacial electron
transfer efficiency and active sites on the surface. Very
recently,
P
ub
li
sh
ed
o
n
08
O
ct
ob

er
2
01
9.
D
ow
nl
oa
de
d
by
M
is
si
ss
ip
pi

S
ta
te
U
ni
ve
rs
it
y
L
ib
ra
ri
es
o
n

4/
7/
20
20
1
0:
07
:5
6
P
M
.
View Article Online
Li's group prepared Co-substituted Ru NSs with a single Co
atom dispersed in the Ru lattice (Fig. 7h), which exhibited
excellent HER catalytic activity in 1 M KOH (Fig. 7i).43 Since
the
water dissociation kinetics of the Volmer step is crucial to the
rate of the HER, the energy barrier of O–H bond cleavage is of

importance. Single Co atom substitution can reduce the energy
barrier of water dissociation and boost the electrocatalytic
activity and durability, while the energy barriers increased when
increasing the number of substituted Co atoms to two and three
per unit cell (Fig. 7j). The presence of the Co–Co bond in RuCo
and RuCo2 alloys would lead to a decrease of the catalytic
activity.
3.3.2. Ru-based core–shell structures. Constructing Ru-
based core–shell structures is an effective approach to tune the
crystal structure of Ru and boost the electrocatalytic activity for
the HER due to the strain effect. In the core–shell structure, the
lattice strain resulting from the lattice mismatch between the
core and the shell could alter the electronic structure and the
interaction between H and OH, leading to enhanced HER
activity.55 Moreover, the use of Ru can be reduced in the core–
shell structures, thus decreasing the cost of the electrocatalysts.
For example, Feng and co-workers prepared core–shell struc-
tures with Ru NPs assembled into a shell over the surface of Te
NRs ([email protected]).54 The HER performance of
[email protected] was better
than that of Te and Ru in acidic solution. [email protected] NRs
with a Ru/
Te ratio of 0.6 ([email protected]) exhibited the best
performance

among the composite NRs with different Ru/Te ratios. The
overpotential to reach a current density of 10 mA cm�2 was 86
mV for [email protected], less than that of Ru alone. Moreover,
the
Tafel slope of [email protected] was 36 mV dec�1, close to the
typical
Fig. 8 (a) TEM image of Au–Ru nanowires with the core–shell
structu
solution. (c) TOF values of Au–Ru NWs in 1.0 M KOH
compared with tho
Copyright 2018, Nature Publishing Group. (d and e) HAADF-
STEM image
of various electrocatalysts. (g) DG diagram for water activation
in the HER
Reproduced with permission.96 Copyright 2018, American
Chemical Soc
value of the Pt/C catalyst (30 mV dec�1). The enhanced HER
activity of [email protected] NRs was attributed to the
interaction between
the semimetal Te core and the active metallic Ru layer as well
as
the large surface area of [email protected] core–shell structures.

Qiao's group revealed that the large compressive strain in the
core–shell [email protected] nanostructure resulted in the
y
enhanced HER activity compared to the strain-free RuPt alloy
under alkaline conditions.55 The Pt/Ru interfacial interactions
contributed to the formation of the unconventional fcc struc-
tured Ru core and introduced compressive strain into the Pt
shell to accommodate the interfacial lattice mismatch between
Pt and Ru. The compressive strain could optimize the adsorp-
tion–desorption energetics toward H intermediates and OH
spectator species during the catalytic reaction, thus resulting in
superior HER activity. The 4H/fcc Au–Ru NWs with a core–
shell
structure (Fig. 8a) were used as electrocatalysts for the HER in
alkaline solution and exhibited excellent electrocatalytic
performance.34 The Au–Ru NWs showed a much smaller over-
potential (50 mV at 10 mA cm�2) (Fig. 8b) and Tafel slope
(30.8
mV dec�1) than those of Pt/C and Ru/C. The exchange current
density and turnover frequency (TOF) (0.31 H2 s�1 at 50 mV)
(Fig. 8c) were also larger than those of other reported HER
catalysts and even Pt/C. Several reasons could account for the
superior HER performance of the 4H/fcc Au–Ru NWs. First, the
Au–Ru NWs with a one-dimensional structure led to smaller
charge transfer resistance than Pt/C and Ru/C during the HER

process. Second, the hierarchical structures and the atomic
concave and convex surfaces provided abundant active sites for
the HER. Third, the electronic band structure could be altered
re. (b) Polarization curves of different electrocatalysts in 1.0 M
KOH
se of some other HER electrocatalysts. Reproduced with
permission.33
of a mesoporous [email protected] nanorod. (f) Polarization
curves in 1.0 M KOH
on different surfaces. Inset: the Volmer reaction at the
Ru/Pd(111) site.
iety.
J. Mater. Chem. A, 2019, 7, 24691–24714 | 24705
P
ub
li
sh

ed
o
n
08
O
ct
ob
er
2
01
9.
D
ow
nl
oa
de

d
by
M
is
si
ss
ip
pi
S
ta
te
U
ni
ve
rs
it
y

L
ib
ra
ri
es
o
n
4/
7/
20
20
1
0:
07
:5
6
P

M
.
View Article Online
by the lattice strain and electronic charge transfer between Au
and Ru, leading to improved activity.
Yang and co-workers synthesized a series of two-dimensional
[email protected] core–shell NPLs for the HER.51 They
reported that the
rational design and synthesis of Ru-based core–shell nano-
structures can tune the crystal structure of Ru shells, thus
tuning the HER performance of Ru. The different crystal
structures of Ru shells lead to different reaction mechanisms on
when the thickness of the Ru shell increased owing to the
increased Ru content and then sharply increased as the crystal
phase of the Ru shell changed from fcc to hcp. When the
thicknesses of the [email protected] NPLs and fcc Ru shell
reached ca. 2.3
and 0.6 nm, respectively, the NPLs exhibited the best catalytic
properties and good stability for the HER in alkaline media. The
small Tafel slope (36 mV dec�1) indicated a Tafel–Volmer
mechanism with electrochemical desorption of H2 as the rate-

determining step in the HER. However, the [email protected]
NPLs
(thickness �2.6 nm) with the hcp Ru shell followed the
Volmer–
Heyrovsky mechanism with the Volmer step as the rate-limiting
step. As another example, the mesoporous [email protected]
core–shell
NRs (Fig. 8d and e) prepared by Li's group exhibited superior
HER catalytic activity to Pt/C and solid [email protected] NRs,
with an
overpotential of 30 mV at 10 mA cm�2 (Fig. 8f) in 1.0 M KOH
solution and a high mass activity of 722.9 A g�1 at �0.06 V vs.
the reversible hydrogen electrode (RHE).101 With a monolayer
of
Ru deposited on a Pd(111) substrate as the model (Ru/Pd(111)),
Fig. 9 (a) TEM and HAADF-STEM images (top) and the
corresponding e
different catalysts (b) in N2-saturated 0.5 M aq. H2SO4 solution
and (c) in
Copyright 2018, Wiley-VCH. (d) TEM image, (e) HRTEM
image and (f) atom
(d) shows the corresponding particle size distribution of the Ru
NPs. (g) Po
[email protected] annealed at different temperatures and the
Pt/C catalyst. Rep

24706 | J. Mater. Chem. A, 2019, 7, 24691–24714
density functional theory (DFT) calculation results revealed that
the Ru/Pd(111) site was favorable for the dissociation barrier
with a Gibbs free-energy of 0.84 eV, lower than that of Ru
(0001),
Pd (111) and Pt (111) (Fig. 8g). Since [email protected] NRs
exposed a large
amount of Ru/Pd(111) on the surface and possessed superior
charge-transfer capability due to their mesoporous structure,
they can exhibit better HER performance in alkaline media than
Ru/C, Pd/C and Pt/C.
3.4. Ru/non-metal nanocomposites for the electrocatalytic
HER
3.4.1. Ru–carbon composites. The conductivity of the
electrocatalysts is important for achieving good HER perfor-
mance since poor electrical conductivity will lead to a voltage
drop across the electrode, producing an extra overpotential and
lowering the catalytic activity. In addition, more energy will be
consumed during the electrocatalytic process if the conductivity
of the electrocatalysts is poor. The good conductivity of carbon
materials makes them ideal candidates to composite with Ru-
based nanomaterials.57,101 Moreover, loading Ru-based nano-

materials on carbon materials can also prevent the aggregation
of the catalysts and thus ensure full exposure of the active sites
of Ru-based nanomaterials during the HER process.
As Fan and co-workers reported, Ru NPs deposited on carbon
substrates exhibited excellent catalytic properties for the HER
in
alkaline solution.58 Among the samples synthesized at various
calcination temperatures, the Ru/C composites prepared at 300
lemental mapping images of [email protected] (bottom).
Polarization curves of
N2-saturated 1.0 M aq. KOH solution. Reproduced with
permission.
29
ic-resolution TEM image of the as-synthesized [email protected]
The inset in
larization curves obtained from various catalysts. (h) Nyquist
curves for
roduced with permission.59 Copyright 2018, Wiley-VCH.

P
ub
li
sh
ed
o
n
08
O
ct
ob
er
2
01
9.

D
ow
nl
oa
de
d
by
M
is
si
ss
ip
pi
S
ta
te
U

ni
ve
rs
it
y
L
ib
ra
ri
es
o
n
4/
7/
20
20

1
0:
07
:5
6
P
M
.
View Article Online
�C showed the best HER performance. Only a small over-
potential of 14 mV was required at 10 mA cm�2, smaller than
that of commercial Pt/C. The excellent HER performance of Ru/
-
ture of Ru NPs as well as the high intrinsic activity of Ru. Baek
and co-workers reported an efficient and stable HER electro-
catalyst with Ru NPs uniformly dispersed on GnP substrates
(Fig. 9a), which exhibited superior HER performance to Pt/C in
both acidic and alkaline media (Fig. 9b and c).19 The
[email protected]
electrocatalyst exhibited high turnover frequencies at 25 mV
(0.67 H2 s

�1 in 0.5 M H2SO4 solution; 0.75 H2 s
�1 in 1.0 M KOH
solution) and small overpotentials at 10 mA cm�2 (13.5 mV in
0.5 M H2SO4 solution; 17.0 mV in 1.0 M KOH solution). It was
urface area and narrow
particle size distribution contributed to the large number of
active sites exposed on the surface of the electrocatalyst, which
substrates with high conductivity facilitated the charge transfer
efficiency between the active sites and electrode. Zou and co-
workers embedded Ru into a hierarchically porous carbon
network (Ru-HPC) for the HER in alkaline solution through the
thermal treatment of CuRu-MOF followed by the removal of Cu
atoms with FeCl3.
29 Ru-HPC achieved a current density of 25 mA
cm�2 at an overpotential of 22.7 mV and showed an ultrahigh
TOF of 1.79 H2 s
�1 at 25 mV. Moreover, the HER performance of
Ru-HPC with a low Ru content of only 5.55% was better than
that of 20% Pt/C, decreasing the cost for practical application.
The superior HER performance of Ru-HPC resulted from the

highly exposed Ru active sites and the high conductivity of
HPC.
Lu and co-workers synthesized hollow carbon sphere-
s) and hollow carbon sphere-
Ru layers (HCRLs) as electrocatalysts for the HER in alkaline
media.102 The HCRNs (Ru content: 4.8 wt%) and HCRLs (Ru
content: 23.5 wt%) displayed high TOFs of 0.77 s�1 and 0.25
s�1
at 15 mV and small overpotentials of 33 mV and 18 mV at 10
mA
cm�2, respectively. Besides, the Tafel slopes of HCRNs and
HCRLs were smaller than those of Pt/C, which indicated that
the reaction followed the Volmer–Heyrovsky mechanism. These
experimental results, as well as the DFT calculations, revealed
that the superior HER performance could be attributed to the
lowered DGH for the HER and enhanced electron transfer from
the carbon shell to the encapsulated Ru.
Compared to pure carbon materials, N-doped carbon
materials would result in better HER performance when
composited with Ru-based nanomaterials since the doped N
atoms could modulate the electronic properties of carbon
atoms by intramolecular charge transfer, which is helpful to

promote the HER performance. For example, the
[email protected]
doped CQD hybrids (Fig. 9d–f) exhibited extremely high
catalytic activity and durability under alkaline conditions.59
The composites only required an overpotential of 10 mV to
achieve a current density of 10 mA cm�2, lower than that of the
Pt/C catalyst (Fig. 9g). The DFT calculation results indicated
that the charge density of the hybrids redistributed with
electrons transferred from Ru to CQDs, leading to the elec-
tron-enrichment of the CQDs and hole-enrichment of the Ru
cluster. It should be noted that a moderate N doping content is
crucial for achieving the optimized electronic properties of
carbon materials, since excessive N doping would destroy the
structure of the carbon skeleton and thus impair the electrical
conductivity between the catalysts and the electrode (Fig. 9h).
Chen and co-workers prepared Ru, N-codoped carbon NWs, in
which the atomically dispersed Ru coordinated to N and C
(RuCxNy) and the carbon atoms adjacent to the Ru center
served as active sites for the HER.103 The Ru, N-codoped
carbon NWs prepared at 700 and 800 �C exhibited better HER
performance than even Pt/C in alkaline media.
3.4.2. Ru–carbon nitride composites. Carbon nitrides have

been intensively investigated as effective supports for the
synthesis of highly efficient Ru-based electrocatalysts for the
HER. Carbon nitride can modulate the binding energy between
Ru and H, thus tuning the HER activity of Ru-based electro-
catalysts. Ma's group found that the carbon nitride layer could
the d-band center and the protective layer to avoid the aggre-
activity and stability of Ru.75 Baek and co-workers found that
when Ru NPs were stabilized in the holes of two-dimensional
holey C2N substrates, the binding energy between Ru and H
was
similar to that between Pt and H, leading to rapid proton
adsorption, reduction and H2 release.
19 Besides, the high H2O
capture rate for the increased Ru–H2O binding energy and the
much easier dissociation of H2O, which offered faster proton
supply, also contributed to the high electrocatalytic activity of
[email protected] in both acidic and alkaline solutions. The
[email protected]
electrocatalysts displayed small overpotentials (13.5 mV in 0.5
M H2SO4 solution; 17.0 mV in 1.0 M KOH solution) at 10 mA
cm�2 (Fig. 10a and b) and a high TOF (0.67 H2 s

�1 in 0.5 M
H2SO4 solution; 0.75 H2 s
�1 in 1.0 M KOH solution) at 25 mV
(Fig. 10c and d), as well as excellent stability in both acidic and
alkaline media, comparable to or even better than those of the
commercial Pt/C catalyst for the HER.
It was reported that the existence of g-C3N4 as the support
could facilitate the growth of anomalous fcc Ru (Fig. 10e),
while
only hcp Ru NCs formed when loaded on a C substrate.28 The
electrocatalytic HER activity of the as-prepared Ru/g-C3N4/C
was
excellent with a smaller overpotential (Fig. 10f) and higher TOF
value (Fig. 10g) than Ru/C in both acidic and alkaline media.
However, the water dissociation issue must be considered
under alkaline conditions. Since the energy barrier of water
dissociation of fcc Ru and hcp Ru was lower than that of Pt, the
activity of Ru/g-C3N4/C surpassed that of Pt/C in alkaline
media
even though the DGH of Pt is near 0 (Fig. 10h). The enhance-
ment of the catalytic activity and stability was attributed to the
formation of fcc Ru NPs and the strong interaction between Ru
and g-C3N4.

3.4.3. Ru–semiconductor composites. The combination of
Ru and semiconductors is able to take the advantage of each
component and generate a synergistic effect among them, thus
enhancing the HER performance.
The heterointerfaces between Ru and semiconductors can
promote the dissociation of water, providing Hads intermediates
to produce H2. For example, cactus-like hollow
Cu2�[email protected]
NPLs exhibited robust electrocatalytic activity for the HER in
J. Mater. Chem. A, 2019, 7, 24691–24714 | 24707
Fig. 10 Polarization curves of various electrocatalysts in (a) 0.5
M H2SO4 solution and in (b) 1.0 M KOH solution. TOF values
of [email protected]
compared with those of other HER electrocatalysts in (c) 0.5 M
H2SO4 and (d) 1.0 M KOH solutions. Reproduced with
permission.
19 Copyright
2017, Nature Publishing Group. (e) HAADF-STEM image and
the corresponding FFT image (inset) of Ru NPs showing a fcc

structure. (f) Polar-
ization curves of different electrocatalysts recorded in N2-
saturated 0.1 M KOH solutions. (g) The relationship between
the TOF and measured
potentials for Ru/C3N4/C and commercial Pt/C electrocatalysts
in 0.1 M KOH solution. The benchmark according to the
metallurgically prepared
commercial Ni–Mo alloys. (h) Gibbs free energy diagram of the
HER on different surfaces including the reactant initial state,
intermediate state,
final state, and an additional transition state representing water
dissociation. DGH* indicates hydrogen adsorption free energy
and DGB indicates
the water dissociation free energy barrier. Reproduced with
permission.28 Copyright 2016, American Chemical Society.
P
ub
li
sh
ed

o
n
08
O
ct
ob
er
2
01
9.
D
ow
nl
oa
de
d
by

M
is
si
ss
ip
pi
S
ta
te
U
ni
ve
rs
it
y
L

ib
ra
ri
es
o
n
4/
7/
20
20
1
0:
07
:5
6
P
M

.
View Article Online
alkaline media owing to the facile dissociation of water in the
Volmer step and the highly exposed active sites.83 Ru–MoO2
NPs
exhibited excellent electrocatalytic activity in both acidic and
alkaline solutions.80 The composites exhibited a very low over-
potential to achieve 10 mA cm�2 under both acidic and alkaline
conditions (55 mV in 0.5 M H2SO4 and 29 mV in 1.0 M KOH)
and
superior stability. Particularly, their performance in alkaline
solution was better than that of commercial Ru powders and
even Pt/C. The Tafel slope of Ru–MoO2 was 31 mV dec
�1 in
alkaline media, indicating a typical Tafel–Volmer mechanism
for the HER. Both experimental and computational results
demonstrated that the enhanced HER activity resulted from the
synergistic effect between Ru and MoO2 as well as the
enhanced
conductivity of the hybrid. The interface electronic structure
was tuned by the electron transfer between MoO2 and Ru, thus
improving the HER activity. Besides, the Ru/MoS2/CP hybrids
showed outstanding catalytic performance (a small over-

potential of �13 mV at �10 mA cm�2) in alkaline media,
surpassing Ru and MoS2 electrocatalysts and even commercial
20 wt% Pt/C.30 The excellent HER performance could be
mainly
ascribed to the interfacial synergy between Ru and MoS2 since
Ru could promote water dissociation and the nearby unsatu-
rated Mo and S atoms facilitated the hydrogen adsorption
process. Meanwhile, the transfer efficiency of electrons was
promoted by the CP, oxygen incorporated into MoS2 and Ru-
decoration. Moreover, the vertically aligned MoS2 NSs exposed
abundant edge sites as active centers and their basal planes
thus leading to the enhanced HER performance.
24708 | J. Mater. Chem. A, 2019, 7, 24691–24714
4. Ru-based photocatalysts for H2
evolution
4.1. Principle
As a promising solar energy utilization method, photocatalytic
H2 evolution has been widely studied during the past few
decades.104 Photocatalytic H2 evolution by semiconductors can
absorb photon energy to generate electron–hole pairs. Second,
electrons and holes transfer to the semiconductor surface.
Finally, electrons react with protons to generate H2. The overall

procedure converts solar energy into chemical energy. However,
there are still several problems which seriously limit the effi-
ciency of photocatalytic H2 production. First of all, the band
structure of the semiconductors should meet the requirements.
In order to achieve photocatalytic water splitting, the bandgap
energy (Eg) of the photocatalyst should be larger than 1.23 eV
to
meet the redox potentials of the H+/H2 and O2/H2O pairs. To
enable smooth proceeding of electron transfer and the
following H2 evolution steps, a larger band gap (>2.0 eV) is
required for the overpotential associated with these steps.
Moreover, a large fraction (ca. 46%) of solar energy lies in the
visible light region, and thus the bandgap of the photocatalysts
should be smaller than 3.0 eV in response to visible light.
Therefore, the ideal bandgap of the semiconductors for pho-
tocatalytic H2 evolution is 2.0 eV < Eg < 3.0 eV.
105 Except the
inherent band structure of semiconductors, there are two
external problems that seriously affect the photocatalytic
activity, which are charge recombination and surface

1
0:
07
:5
6
P
M
.
View Article Online
backreaction (SBR). Semiconductors absorb photon energy to
form electron–hole pairs. Some of the electron–hole pairs can
transfer to the photocatalyst surface. Holes will oxidize H2O to
produce O2 and electrons will reduce H
+ to produce H2. The
combination of these two half reactions contribute to the
overall water splitting reaction. However, the recombination of
the electron–hole pairs may occur immediately on the surface or
bulk of the semiconductors during the transfer process,
reducing the number of the electrons and holes participating in
the water splitting reaction. Additionally, SBR is another issue

that lowers the photocatalytic efficiency, which means that the
photogenerated H2 and O2 will react to form H2O on the
surface
of the photocatalyst.106
Loading metals, especially noble metals, on semiconductors
is an effective way to solve the charge recombination and SBR
issues. Pt, Au, Ag, and Rh are widely used as co-catalysts to
deposit on semiconductors or construct metal–semiconductor
hybrid nanostructures.107 Considering the high work functions
of metals, they usually have much lower Fermi levels than
semiconductors. When a metal comes into contact with a n-type
conduction band of the semiconductor to metal until the
equilibration of Fermi levels from the both sides. The defor-
mation of the band structures between the metal and the
semiconductor leads to the formation of a Schottky barrier at
the metal–semiconductor interface. The Schottky barrier can
serve as an effective electron trap due to which electrons are
u -
bination of the photogenerated electron–hole pairs can be
inhibited. Meanwhile, the metal can act as reaction sites for the
reduction of H+ to H2 by electrons, while O2 generation
remains
Fig. 11 (a) TEM image of Ru nanoparticles. (b) Photocatalytic

H2 evolution
v) TEOA aqueous solution (pH ¼ 7) under visible light
irradiation (l $ 4
HRTEM image and schematic illustration (inset) of Ru–N-
PTNs. (d) Photo
irradiation. Reproduced with permission.81 The Royal Society
of Che
SrTiO3:La,Rh/Au/BiVO4:Mo photocatalyst. (f) Time courses of
the water
photocatalyst under simulated sunlight (AM 1.5 G) at 288 K and
5 kPa (o
dependence of the photocatalytic water splitting activity of
Cr2O3/Ru-m
under AM 1.5 G simulated sunlight. Reproduced with
permission.120 Cop
on the surface of the host photocatalyst. As the generation of
H2
and O2 occurs at different reaction sites of the photocatalyst,
SBR can be effectively prevented.
4.2. Ru–semiconductor composites for the photocatalytic
HER
Although Ru has a relatively low cost and abundant supply

compared to other noble metals, there have been few reports
related to the utilization of Ru as a highly efficient co-catalyst
for
a long time.109 Because the work function of Ru (4.71 eV) is
lower
than that of most noble metals (Pt: 5.65 eV, Ir: 5.27 eV, Au: 5.1
eV, and Rh: 4.98 eV),110 the efficiency of electron transfer in
Ru–
semiconductor may be lower than that in other noble metal–
semiconductor photocatalysts.111 However, some researchers
have revealed that Ru-based photocatalysts could exhibit equal
or even higher photocatalytic activity compared to Pt-based
photocatalysts under certain conditions. In 2003, Hara and co-
workers reported Ru loaded TaON with superior photocatalytic
H2 generation activity. TaON with 0.05 wt% Ru loading (0.05
wt% Ru–TaON) exhibited a H2 evolution rate of ca. 120 mmol h
�1
under visible light (420–500 nm). In contrast, Pt, Rh and Ir
loaded TaON delivered H2 generation rates as low as 2–8 mmol
h�1. The quantum efficiency in 0.05 wt% Ru–TaON in aqueous
ethanol solution was 2.1%.112 The authors ascribed the good
photocatalytic performance of 0.05 wt% Ru–TaON to the inter-
face electronic structure between Ru NPs and TaON, which

more attention has been paid to utilizing Ru as co-catalysts for
photocatalytic H2 evolution. Kudo and co-workers found that
the introduction of the Ru co-catal
the photocatalytic H2 evolution activity of ZnS–CuInS2–
from EY-sensitized systems catalyzed by Ru and Pt in 80 mL of
10% (v/
20 nm). Reproduced with permission.116 Copyright 2015,
Elsevier. (c)
catalytic H2 evolution rates of various photocatalysts under
solar light
mistry. (e) Schematic of overall water splitting on the Ru-
modified
splitting reaction on a Cr2O3/Ru-modified
SrTiO3:La,Rh/Au/BiVO4:Mo
pen symbols) and 331 K and 10 kPa (closed symbols). (g)
Temperature
odified SrTiO3:La,Rh/Au/BiVO4:Mo at a background pressure
of 5 kPa
yright 2016, Nature Publishing Group.
J. Mater. Chem. A, 2019, 7, 24691–24714 | 24709

P
ub
li
sh
ed
o
n
08
O
ct
ob
er
2
01
9.
D

ow
nl
oa
de
d
by
M
is
si
ss
ip
pi
S
ta
te
U

ni
ve
rs
it
y
L
ib
ra
ri
es
o
n
4/
7/
20
20
1

0:
07
:5
6
P
M
.
View Article Online
AgInS2.
113,114 Notably, the Ru co-catalyst had higher and steadier
catalytic activity than other noble metal co-catalysts (Pt, Rh,
and
Ir). Moreover, the Ru loaded ZnS–CuInS2–AgInS2
photocatalyst
showed higher activity than the state-of-the-art Pt-loaded CdS
photocatalyst under the same reaction conditions. Fukuzumi
and co-workers reported that the employment of the Ru co-
catalyst achieved efficient H2 production under basic conditions
(pH ¼ 10) in a system composed of 2-phenyl-4-(1-naphthyl)-
quinolinium perchlorate (QuPh+-NA) and dihydronicotinamide

adenine dinucleotide (NADH) as the photocatalyst and electron
donor, respectively.85,115 The activity of the Ru co-catalyst
was
comparable to that of commercially available Pt under such
basic conditions. Moreover, the concentration change of the
photogenerated radical species (QuPhc-NA) was determined by
UV-vis spectroscopy to investigate the electron injection rate
from QuPhc-NA to Ru NPs. It was shown that the electron
transfer rate from QuPhc-NA to Ru was much faster than the H2
evolution rate on the Ru NP surface; thus the rate determining
step was the H2 evolution step. Lu and co-workers reported that
eosin Y (EY)-sensitized metal Ru (Fig. 11a) showed 4.9 times
higher H2 generation activity (Fig. 11b) than EY sensitized
metal
Pt. And an apparent quantum efficiency (AQE) of 46.3% at 520
nm was achieved.116 This performance was because of the
stronger interaction between Ru and EY than Pt and EY. Wang
and co-workers prepared Ru loaded and N-doped pit-rich TiO2
nanocrystals (Ru–N-PTNs) by calcining RuO2-PTNs under
a reducing NH3 atmosphere as shown in Fig. 11c.
81 Ru–N-PTNs
exhibited higher H2 generation activity (33.6 mmol(H2) g
�1 h�1)

than RuO2-PTNs (17.6 mmol(H2) g
�1 h�1) and RuO2-P25 (14.5
mmol(H2) g
�1 h�1) (Fig. 11d). In most of the above photocatalytic
HER systems, Ru can make intimate contact with the host
catalyst, thus facilitating electron transfer and inhibiting elec-
tron–hole recombination.
The introduction of metallic Ru with RuO2 together as dual
co-catalysts into semiconductors could realize full water split-
ting. Xu and co-workers prepared Ru/RuO2 deposited TiO2
nanobelts (NBs) as photocatalysts for H2/O2 evolution simulta-
neously.79 To increase the crystallinity and improve the contact
between the Ru co-catalysts and TiO2 NBs, the as-prepared Ru/
TiO2 NBs were annealed at different temperatures in air. During
this process, metallic Ru was partially oxidized to RuO2. The
sample annealed at 400 �C exhibited the best catalytic activity
towards photocatalytic water splitting with gas production rates
of 25.34 mmol h�1 g�1 and 1.21 mmol h�1 g�1 for H2 and
O2
evolution, respectively. The good photocatalytic activity can be
attributed to the Schottky barrier of Ru/TiO2 and the hetero-
junction of RuO2–TiO2, which improved the transfer of the

photogenerated electrons and holes, respectively. Thus,
enhanced overall water splitting could be achieved.
Compared to the widely used Pt co-catalyst, Ru can effectively
suppress the SBR between H2 and O2, thus enhancing the
photocatalytic activity. Kudo and co-workers constructed a Z-
scheme system (i.e. (Ru/SrTiO3:Rh)-(BiVO4)-(Fe
3+/Fe2+)) by using
Ru as the co-catalyst for overall water splitting under visible
light irradiation.117 They found that the photocatalysis system
using the Ru co-catalyst showed quite stable H2 and O2 gener-
ation rates and proceeded steadily for a long time (>70 h) even
24710 | J. Mater. Chem. A, 2019, 7, 24691–24714
under the relatively high pressures of H2 and O2. However, the
activity of the photocatalysis system using the Pt co-catalyst
decreased gradually due to the back-reactions accompanied
explore the utilization of the Ru co-catalysts in the Z-scheme
system.118,119 Domen's group designed a Z-scheme system
(Fig. 11e) based on La- and Rh-co-doped SrTiO3
(SrTiO3:La,Rh)
and Mo-doped BiVO4 (BiVO4:Mo) powders embedded into a
Au
layer. In order to maximize the photocatalytic HER perfor-

mance, Ru and RuOx species were employed as the H2 and O2
evolution co-catalysts, respectively, and Cr2O3 shells capping
noble metal nanoparticles could suppress the backward reac-
tions whilst maintaining the function of the noble metal as a H2
evolution catalyst. Due to the synergistic effect of the co-cata-
lysts and Cr2O3 shells, the obtained photocatalyst exhibited
a high water splitting activity (Fig. 11f and g) in pure water
without any supporting electrolytes, buffering reagents, pH
adjustment, or applied voltage.120 The solar-to-hydrogen
energy
conversion efficiency reached 1.1% and the apparent quantum
yield reached 33% at 419 nm.
In order to modify the electronic structure of Ru and
generate a synergetic effect between different metals, Ru-based
bimetallic co-catalysts have been prepared for enhancing the
photocatalytic HER performance. Domen's group found that
bimetallic Ru/Pt deposited Y2Ta2O5N2 exhibited much higher
photocatalytic H2 evolution activity than Pt or Ru single metal
deposited photocatalysts. The H2 evolution activity of the
Ru/Pt–
Y2Ta2O5N2 catalyst under visible light (833 mmol h
�1 g�1) was 22
times greater than that of Pt–Y2Ta2O5N2 catalyst (37 mmol h

�1
g�1).121 Wei Chen and co-workers prepared Pt–
CdS
for H2 generation under visible light.
122 The H2 evolution rate of
Pt–Ru/CdS (18.35 mmol h�1 g�1) was ca. 1.7 times that of
Pt/CdS
(10.58 mmol h�1 g�1) and 2.9 times that of Ru/CdS (6.43
mmol
h�1 g�1). The synergetic effect between Pt and Ru facilitated
electron migration from the conduction band of the host cata-
lyst to the co-catalyst, weakened SBR and improved the charge
separation efficiency.
5. Summary and outlook
In this review, we have summarized the research progress in the
past few years on metallic Ru-based nanomaterials for the HER,
with focus on the synthetic strategies, electrocatalytic and
photocatalytic HER performances and the related mechanisms
of the HER. Several types of Ru-based catalysts such as pure Ru
NCs, Ru-based bimetallic nanomaterials and Ru/non-metal

1- Introduction2- Discovery of ruthenium and Occurrence.Fro.docx

1- Introduction2- Discovery of ruthenium and Occurrence.Fro.docx

Recommended

Recommended

More Related Content

Similar to 1- Introduction2- Discovery of ruthenium and Occurrence.Fro.docx

Similar to 1- Introduction2- Discovery of ruthenium and Occurrence.Fro.docx (20)

More from jeremylockett77

More from jeremylockett77 (20)

Recently uploaded

Recently uploaded (20)

1- Introduction2- Discovery of ruthenium and Occurrence.Fro.docx