Amine-borane assisted synthesis of wavy palladium nanorods on graphene as efficient catalysts for formic acid oxidation

Article abstract of DOI:10.1039/c4cc05019c

Wavy palladium (Pd) nanorods were obtained by controlled synthesis by using amine-boranes as the reducing agents. Thanks to the unique structure and strong interaction with graphene, the as-synthesized Pd nanorods supported on graphene exhibit much enhanc

Full text of DOI:10.1039/c4cc05019c

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Amine–borane assisted synthesis of wavy
palladium nanorods on graphene as efficient
catalysts for formic acid oxidation†
Cite this: Chem. Commun., 2014,
50, 12843
Received 1st July 2014,
Accepted 5th September 2014
Cheng Du,a Yuxiang Liao,a Xing Hua,a Wei Luo,*ab Shengli Chen*a and
Gongzhen Chenga
DOI: 10.1039/c4cc05019c
www.rsc.org/chemcomm
Wavy palladium (Pd) nanorods were obtained by controlled synthesis better control of the reducing rate over commonly used reducing
by using amine–boranes as the reducing agents. Thanks to the unique agents like borohydrides. Amine borane complexes are normally
structure and strong interaction with graphene, the as-synthesized used as reducing agents in OAm for controlled synthesis of metal
Pd nanorods supported on graphene exhibit much enhanced electro- nanocrystallites.18–21 For example, Han’s group have reported
catalytic activity towards formic acid oxidation as compared with Pd the shape controlled synthesis of Pd nanocrystals by combination
nanoparticles.
of OAm and alkylammonium alkylcarbamate (AAAC) using the
borane-tert-butylamine complex as the reducing agent.22 However,
Extensive efforts have been devoted to controlling the morphologies the effect of different boranes on the morphology of nanomaterials,
of metal nanocrystals for enhanced performances in various to the best of our knowledge, has been rarely explored. Taking Pd
applications.1 Especially, 1-D noble metal nanocrystals such as nanocrystals for example, this study demonstrates the capability
nanorods have been attracting considerable recent attention of amine–boranes in morphology-controlled synthesis of metal
due to their unique optical, electronic, magnetic, and catalytic nanocrystallites.
properties.2–5 Solution-phase synthesis has been considered as
By using N,N-diethylaniline-borane as the reducing agent in
one of the most promising and convenient route to produce metal OAm, wavy Pd nanorods several tens of nanometers in lengths
nanorods.6–10 The solution-based formation of metal nanorods in and 3.0 Æ 0.5 nm in widths were formed at temperature below
most cases proceeds through the formation of NP seeds followed by 100 1C. In a typical synthesis, 0.1 mmol Pd(acac)2 (acac =
their oriented growth, both of which are crucially impacted by the acetylacetonate) was dissolved in 10 mL OAm, and heated to
solvent, reducing agent, nature and composition of metal precursors 60 1C for about 1 h under a nitrogen flow to remove the oxygen.
and templates used.11,12 Although template-assistant nanocrystal Then 0.1 mL N,N-diethylaniline-borane was quickly injected
synthesis offers better size and morphology control, it suffers from into the solution, a visible color change from colorless to a
the complicated and cumbersome template formation and removal brown-black was observed, indicating the reduction of Pd(II) to
processes. Current template-free synthesis of metal nanorods mostly Pd(0). The reaction temperature was raised to 90 1C under
relies on the use of relatively weak reductive agents such as ascorbic magnetic stirring and kept for about 1 h, the wavy Pd nanorods
acid, polyol, oleylamine (OAm), etc., therefore high temperatures and were obtained after certification.
long reaction time are mostly required.13–17
The formation of wavy Pd nanorods was indicated by trans-
Herein, we show that the introduction of amine–borane with mission electron microscopy (TEM) (Fig. 1a) and high-resolution
appropriate reducibility, namely, N,N-diethylaniline-borane, TEM (HRTEM) (Fig. 1b) images. The HRTEM characterization
could facilitate and optimize the solution-based synthesis of showed lattice spacing of parallel fringes of 0.23 nm, which
Pd nanorods. As the classical Lewis acid–base adducts, the correspond to the (111) planes of face-centered cubic (fcc) Pd.
reducing ability of amine–boranes can be flexibly tuned by The selected area electron diffraction (SEAD) pattern (Fig. 1a,
varying the alkyl substitution on nitrogen, thus offering a much inset) indicates the polycrystalline nature of Pd nanorods. The
energy-dispersive X-ray (EDX) spectra further confirm their Pd
nature (Fig. S1, ESI†). The powder X-ray diffraction pattern of the
Pd nanorods shows a broad Pd(111) peak, indicating that the
nanorod has small dimensions of crystal domains, which agrees
with HRTEM results. X-ray photoelectron spectroscopy (XPS)
measurement was carried out to ascertain the surface composi-
tions and chemical states of the as-prepared Pd nanorods, which
a College of Chemistry and Molecular Sciences, Wuhan University, Wuhan,
Hubei 430072, P. R. China. E-mail: wluo@whu.edu.cn, slchen@whu.edu.cn;
Tel: +86 2768752366
b Suzhou Institute of Wuhan University, Suzhou, Jiangsu, 215123, P. R. China
† Electronic supplementary information (ESI) available: Experimental details,
Fig. S1–S9. See DOI: 10.1039/c4cc05019c
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Fig. 1 Morphology and structure analyses of Pd nanorods. Representative
(a) low-magnification and (b) high-magnification TEM images of Pd nanorods.
(a inset) selected area electron diffraction pattern of Pd nanorods. (c) XRD
pattern of Pd nanorods. (d) XPS pattern of Pd nanorods.
Fig. 2 (a)–(d) TEM images of the Pd nanorods obtained from the reaction
in oleylamine at 90 1C for 1, 5, 20, and 60 min, respectively.
gave 3d5/2 and 3d3/2 peaks of Pd(0) at ca. 335.5 and 340.7 eV in the previous study,13 which results in a quick depletion of
respectively (Fig. 1d).23,24
metal precursor and excludes NP growth through atomic
We found that the use of N,N-diethylaniline-borane is essential deposition. The thermodynamically unstable small nanoparticles
for the formation of Pd nanorods. When N,N-diethylaniline-borane then coalesce into 1-D wavy structures through a long time attach-
was replaced by triethylamine–borane, monodisperse Pd NPs ment processes. It is interestingly seen that the widths of the
with particle sizes of 3.7 Æ 0.3 nm were obtained in the same nanorods were generally smaller than the diameters of the Pd NP
synthesis process (Fig. S2a and b, ESI†), similar to a recent report precursors, indicating the fusion and reconstruction of Pd surface
on preparation of Pd NPs by Sun and coworkers.20,21 However, atoms during the attachment. The less efficient formation nanorods
a mixture of Pd NPs and nanorods was observed when by using other amine–boranes should be due to their relatively weak
2-metylpyridine-borane was used (Fig. S2c and d, ESI†). This or strong reducing ability. It has been reported that reducing agents
indicated that the reducing agents play an important role in the have a great impact on the size control of metal NPs due to the
kinetic control of nucleation and growth of Pd nanocrystals different reducing kinetics.20 Current efforts are geared toward
during the synthesis. As far as we know, this is the first example understanding the kinetic-control of the reducing ability of amine–
of morphology-controlled synthesis of Pd nanocrystallites by boranes.
using amine–boranes.
As well as the reducing agents, we found that the final
To gain further insights into the formation of Pd nanorods, morphology of Pd nanocrystals also depended on the nature
time-dependent morphology evolution of Pd nanocrystals in of metal precursors. Pd(acac)2 has to be used for the successful
N,N-diethylaniline-borane synthesis was monitored by TEM synthesis of Pd nanorods. Other Pd precursors such as PdCl2
imaging. As shown in Fig. 2a, small Pd NPs with diameters of and Pd(OAc)2 did not yield high quality Pd nanorods (Fig. S3,
3–4 nm were observed 1 min after injecting the N,N-diethylaniline- ESI†). When Pd(acac)2 was replaced by PdCl2 as the metal
borane. As the reaction time was prolonged to 5 min, some of the precursor, the products were mainly NPs (Fig. S3a and b, ESI†).
Pd NPs started to attach with each other to generate short nanorods However, when the Pd(OAc)2 was used, both nanorods and NPs
with lengths of about 7 nm (Fig. 2b). By further increasing the were produced (Fig. S3c and d, ESI†). It should be the relatively
reaction time to 20 min, the nanorod grew up to about 15 nm weak metal–ligand binding in Pd(acac)2 which enabled the
(Fig. 2c). Upon reacting for 60 min, Pd nanorods nearly evolved to instantaneous generation of a large number of small Pd NPs
their final length, and no changes were observed upon further at the very beginning of the reaction.
increasing the reaction time (Fig. 2d).
Pd is known to be a good electrocatalyst for the formic acid
A similar NP-attachment mechanism was also found in the oxidation reaction (FAOR) by promoting the oxidation via the
formation of ultrathin Pd nanowires in a very recent report by ‘‘direct pathway’’, HCOOH - CO2 + 2H+ + 2eÀ.25 To explore the
Wang et al.13 This type of 1-D nanocrystal formation vitally application of the synthesized Pd nanorods as FAOR electro-
relies on the instantaneous generation of a large number of catalysts, we loaded them on different carbon materials (graphene,
small Pd NPs (within 1 min) due to the use of a moderate EC 300J, and XC-72) and investigated the electrocatalytic properties
reducing agent herein or relatively high reaction temperatures of the obtained samples. Probably due to the long time ultrasonic
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treatments in the loading processes, the supported Pd nano- demonstrate that the 1-D nanorods play an important role in the
rods were shorter than the original ones. A similar phenomenon was improvement of electrocatalytic activity and stability toward
seen in the recent work on Pd nanowires by Wang et al.13 Prior to FAOR.29,30
electrocatalytic measurements, the carbon-supported samples were
We believe that the enhanced electrocatalytic activity and
washed with acetic acid at 70 1C overnight to remove the surface stability of the nanorods should be related to the unique
organics.19 The electrochemical behaviors of various prepared Pd surface structure of this morphology. Since the nanorod
catalysts were investigated by cyclic voltammetry (CV) measurements catalyst exhibited CV charges which are only slightly higher
in HCOOH free/contained 0.1 M HClO4 respectively.
than the other two catalysts (Fig. 3a), the electrochemical active
In HCOOH free solution, all samples exhibited characteristic surface areas (ECSA) should play a minor role in the significantly
CV peaks associated with hydrogen adsorption–desorption enhanced activity. In other words, the nanorods should have much
below 0.1 V (vs. SCE), while those above 0.4 V are assigned to higher area specific activity for the FAOR reaction. It is also seen
the oxidation of surface metal and the reduction of thus formed from Fig. 3a that the three catalysts exhibited similar onset
oxides (Fig. S5a, ESI†).26 The sample with graphene as the potentials for the hydrogen adsorption and the surface oxidation.
support exhibits the highest double layer charging current due We also investigated the CO stripping behaviours of these Pd
to the high specific surface areas of graphene. In HCOOH- nanocatalysts and no substantial difference was observed (Fig. S9,
containing solution, the catalyst with graphene as the support ESI†) in either the stripping potentials or the stripping charges.
displayed much higher current density than that of Pd nanorods These results seemed to suggest that the surface of the Pd nanorods
on other two supports, which indicated that the graphene would has similar binding ability to these small adsorbates. In addition, it
be the best support for Pd nanorods to act as FAOR electro- is also implied that the CO poisoning effect is not the origin of the
catalysts, probably owing to its large specific surface area and activity difference between these Pd nanocrystal catalysts. We
excellent conductivity.27,28 In addition, charge transfer across the speculate that the nanorod morphology of Pd may promote the
metal–graphene interface suggested by recent DFT calculations direct pathway of FAOR. This may be related to the surface defects
might also have contributed to the enhanced electrocatalytic formed during the particle fusion.22 Recently, Zou et al.31 have
activity.20
pointed out that the improved catalytic activity of Pd nanorods may
Using graphene as the support, we compared the electro- be caused by the presence of twin planes in these materials, which
catalytic properties of Pd nanocrystals obtained in this study by could promote the HCOO formation pathway. According to the
using different reducing agents and that of the commercial Pd/C charges in the CO striping peaks, we have estimated the ECSAs
of the same metal loading (20 wt%). As manifested by the CV of the prepared catalysts by assuming that the oxidation of a
currents in the hydrogen adsorption–desorption region (Fig. 3a), monolayer of CO on a Pd surface corresponds to a charge of
the Pd nanorods reduced by N,N-diethylaniline-borane gave a 420 mC cmÀ2. The obtained ECSAs are 118, 103 and 101 m2 gÀ1
,
slightly larger electrochemical active surface area than the other respectively, for the catalysts of nanorods, mixture of nanorods
nanocrystals. In the meantime, the Pd nanorods reduced by and nanoparticles, and nanoparticles. Correspondingly, the
N,N-diethylaniline-borane exhibited the highest Pd mass-specific maximum values for the ECSA-normalized currents (specific activity)
currents for HCOOH oxidation, which was followed by the mixture for the three catalysts are 0.048, 0.039 and 0.025 mA cmÀ2
of Pd NPs and nanorods obtained by using 2-metylpyridine-borane respectively. This indicates that the Pd nanorods have much
as the reducing agent (Fig. S6, ESI†). The Pd NPs obtained by using higher specific activity than the Pd nanoparticles for the FAOR.
triethylamine–borane as the reducing agent (Fig. S7, ESI†) exhibited Further work is being undertaken to obtain high-resolution
similar electrocatalytic activity for FAOR to that of the commercial images of these catalysts and their voltammetric behaviors in
Pd/C, which is much inferior as compared with that exhibited by different media, aiming at gaining a better understanding on the
the nanorods and the nanorods-containing mixture. In addition, relation between the electrocatalytic activity and the morphologies
the as-synthesized Pd nanorods exhibit much higher stability of these materials.
than the other two Pd nanocrystals (Fig. S8, ESI†). The results
In summary, a new approach of making Pd nanorods based
on amine–boranes has been described. The morphology of the
as-synthesized Pd nanostructures could be controlled by using
different amine–boranes as reducing agents. Especially, wavy
Pd nanorods are obtained by appropriate combination of
amine–borane and the metal precursor, for example, the moderately
reductive N,N-diethylaniline-borane and Pd(acac)2. These Pd
nanorods formed through a novel NP attachment mechanism.
Thanks to the unique wavy nanorod motif and strong inter-
action between Pd and graphene, the as-synthesized Pd nano-
rods supported on graphene exhibit excellent electrocatalytic
activity towards FAOR. It is expected that the utilization of
amine–boranes as reducing agents may provide a route to the
morphology-controlled synthesis of other 1-D metal motifs for
more applications.
Fig. 3 (a) CVs of graphene-supported Pd nanocrystallites prepared using
different reducing agents, recorded at 25 1C in N2-saturated 0.1 M HClO4
(potential sweeping rate: 50 mV sÀ1). (b) Pd-mass normalized current
densities vs. applied potential in solution of 0.1 M HClO4 and 2 M HCOOH
at 25 1C (potential sweeping rate: 10 mV sÀ1).
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12 D. V. Leff, P. C. Ohara, J. R. Heath and W. M. Gelbar, J. Phys. Chem.,
1995, 99, 7036.
13 Y. Wang, S.-II. Choi, X. Zhao, S. F. Xie, H. C. Peng, M. F. Chi,
C. Z. Huang and Y. N. Xia, Adv. Funct. Mater., 2014, 24, 131.
14 X. Huang and N. Zheng, J. Am. Chem. Soc., 2009, 131, 4602.
15 Y. Imura, K. Tsujimoto, C. Morita and T. Kawai, Langmuir, 2014,
30, 5026.
This work was financially supported by the National Natural
Science Foundation of China (21201134), the Ministry of Science
and Technology of China (2012CB932800 & 2013AA110201), the
Natural Science Foundation of Jiangsu Province (BK20130370), and
the Natural Science Foundation of Hubei Province (2013CFB288).
16 C. W. Xiao, H. Ding, C. M. Shen, T. Z. Yang, C. Hui and H. J. Gao,
J. Phys. Chem. C, 2009, 113, 13466.
17 Y. Sun, L. H. Zhang, H. W. Zhou, Y. M. Zhu, E. Sutter, Y. Ji, M. H.
Rafailovich and J. C. Sokolov, Chem. Mater., 2007, 19, 2065.
Notes and references
1 (a) Z. S. Li, L. Zhang, X. M. Huang, L. T. Ye and S. Lin,
Electrochim. Acta, 2014, 121, 215; (b) C. Kim and H. Lee, Fuel Cells, 18 S. B. Kalidindi, U. Sanyal and B. R. Jagirdar, ChemSusChem, 2011,
2013, 13, 889; (c) F. Ye, H. Liu, Y. Feng, J. L. Li, X. D. Wang and 4, 317.
J. Yang, Electrochim. Acta, 2014, 117, 480; (d) S. S. John, P. Boolchand 19 V. Mazumder and S. H. Sun, J. Am. Chem. Soc., 2009, 131, 4588.
and A. P. Angelopoulos, Langmuir, 2013, 29, 16150. 20 T. Jin, S. J. Guo, J. L. Zuo and S. H. Sun, Nanoscale, 2013, 5, 160.
2 L. Kesavan, R. Tiruvalam, M. H. A. Rahim, M. I. Saiman, D. I. Enache, 21 N. F. Zheng, J. Fan and G. D. Stucky, J. Am. Chem. Soc., 2006,
R. L. Jenkins, N. Dimitratos, J. A. Lopez-Sanchez, S. H. Taylor, 128, 6550.
D. W. Knight, C. J. Kiely and G. J. Hutchings, Science, 2011, 331, 195. 22 B. J. Hu, K. L. Ding, T. B. Wu, X. S. Zhou, H. L. Fan, T. Jiang, Q. Wang
3 R. G. Chaudhuri and S. Paria, Chem. Rev., 2012, 112, 2373.
4 R. Ferrando, J. Jellinek and R. L. Johnston, Chem. Rev., 2008, 845.
5 R. C. Sekol, X. K. Li, P. Cohen, G. Doubek, M. Carmo and
A. D. Taylor, Appl. Catal., B, 2013, 138–139, 285.
and B. X. Han, Chem. Commun., 2010, 46, 8552.
23 I. Yuranov, L. K. Minsker, P. Buffat and A. Renken, Chem. Mater.,
2004, 16, 760.
24 W. J. Zhou and J. Y. Lee, J. Phys. Chem. C, 2008, 112, 3789.
6 L. Q. Liu, S. X. Ouyang and J. H. Ye, Angew. Chem., Int. Ed., 2013, 25 S. Uhm, H. J. Lee and J. Lee, Phys. Chem. Chem. Phys., 2009, 11, 9326.
¨
52, 6689.
26 V. Mazumder, M. F. Chi, M. N. Mankin, Y. Liu, O. Metin, D. H. Sun,
7 J. Zhang, M. R. Langille and C. A. Mirkin, Nano Lett., 2011, 11, 2495.
K. L. More and S. H. Sun, Nano Lett., 2012, 12, 1102.
8 B. P. Khanal and E. R. Zubarev, Angew. Chem., Int. Ed., 2009, 27 X. L. Li, X. R. Wang, L. Zhang, S. Lee and H. J. Dai, Science, 2008,
48, 6888. 319, 1229.
9 C. B. Gao, Q. Zhang, Z. D. Lu and Y. D. Yin, J. Am. Chem. Soc., 2011, 28 X. Huang, X. Y. Qi, F. Boey and H. Zhang, Chem. Soc. Rev., 2012,
133, 19706. 41, 666.
10 B. Lim, M. J. Jiang, J. Tao, P. H. C. Camargo, Y. M. Zhu and Y. N. Xia, 29 W. P. Zhou, A. Lewera, R. Larsen, R. I. Masel, P. S. Bagus and
Adv. Funct. Mater., 2009, 19, 189. A. Wieckowski, J. Phys. Chem. B, 2006, 110, 13393.
11 P. A. Brugger, P. Cuendet and M. Gratzel, J. Am. Chem. Soc., 1981, 30 W. J. Zhou and J. Y. Lee, J. Phys. Chem. C, 2008, 112, 3789.
103, 2923.
31 Y. G. Tang, R. E. Edelmann and S. Z. Zou, Nanoscale, 2014, 6, 5630.
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