Bimetallic nanomaterials: Connecting two unusual properties

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When a few atoms from two different types of metal are merged together to form a nanoalloy, the interactions between them can create some fascinating, often useful, properties. Based at Peking University in Beijing, Professor Chinping Chen, along with colleagues including Aixian Shan at the University of Science and Technology Beijing, has spent several years synthesising and studying these unique nanomaterials. The team explore properties including their unusual magnetic behaviours, and their performance as catalysts in chemical reactions.

Consisting of just a few atoms, nanomaterials have been proven to display a hugely diverse range of properties. Recently, a surge of research into the microscopic structures has resulted in an ever-more bewildering array of nanostructures, with the aim to engineer and enhance their uses ever further. One particular branch of nanomaterial research is currently exploring ‘bimetallic’ nanomaterials, which are made from molecular-scale alloys of two different metals. These materials have been proven to possess some intriguing chemical and physical properties, including their ability to speed up certain chemical reactions by acting as catalysts.

Fig. 1 Simulations revealing the atomic structures for the NiPt nanoalloy with a truncated-octahedral morphology. It shows 8,000 atoms with the white and red spheres representing Pt and Ni atoms, respectively. Both are for the same particles viewed at different orientations.

Professor Chen is at the forefront of research into bimetallic nanomaterials. He is particularly interested in the properties of nanoalloys hosting metals from two specific parts of the periodic table. “Bimetallic nanomaterials involving transition and noble metals receive intensive attention due to their functionality for applications in magnetism and catalysis,” he explains. The ‘transition’ metals Professor Chen mentions occupy the central block of the periodic table and can lose electrons from or obtain electrons for their 3d orbitals with the ‘noble’ metals, such as Pt, exhibiting similar behaviour on their 5d orbitals.

Fig. 2 Electron is activated by heat from the low energy state in Ni to the high energy one in the nearby Pt, and then going even further to the Pt which is a longer distance from the Ni atom.

Bimetallic nanomaterials involving transition and noble metals receive intensive attention due to their functionality for applications in magnetism and catalysis.

What seemed strange was the fact that the structures’ magnetism, and their performance as catalysts, appeared on the surface to be two entirely unrelated properties. Professor Chen, however, believes that electrons transferring between the metals could be responsible. “One of our focus points is to understand the roles electron transfer might play in tuning the magnetic and catalytic properties,” he continues. “We aimed to reveal the underlying mechanism for these two seemingly unconnected properties.”

Fig. 3 High resolution eperimental image for a single NiPt nanoparticle. The diameter is about seven nanometres. The abundance of Ni and Pt atoms are revealed.

Fig. 4 Experimental image for NiPt hollow spheres with uniform diameter of about 35 nanometres. The shell thickness is about two nanometres.

Synthesising bimetallic nanoalloys in the lab
To do this, Professor Chen’s team first aimed to synthesise nanomaterials made from transition and noble metals themselves. In a 2014 study, the researchers found that nanoalloys composed of nickel (Ni), a transition metal, and platinum (Pt), a noble metal, could be synthesised fairly easily. Their method of choice was a ‘wet chemical’ route; a technique which involved reacting a compound containing nickel with a liquid of molecules containing platinum at room temperature. The team perfected this technique in subsequent studies, giving them high degrees of control over the nanomaterials they could produce.

Through controlled variations in the reactants and the reducing agents used in the wet chemical method, Professor Chen, Shan and their team were able to synthesise bimetallic nanomaterials with varying numbers of atoms of both nickel and platinum. This resulted in structures with the general chemical formula: Ni(x)Pt(y), where x and y represent the relative proportions of nickel and platinum atoms in the structure respectively. The shapes and sizes of their synthesised structures varied drastically, depending on the surfactants and the reducing agents. Where some structures were almost spherical, others displayed intricate flower-shaped forms. The diameters of the nanostructures were also highly varied; representing a range between 13 and 100 nanometres.

Catalysing oxidation reactions
Once Professor Chen and Shan’s team were able to synthesise a wide range of bimetallic nanomaterials in the lab, they could begin to explore the unique properties they displayed. Previous studies had revealed that the nanostructures are effective at catalysing chemical reactions, although their performance hadn’t yet been quantified in detail. In their 2014 study, Professor Chen and colleagues carried out reactions between two different alcohols and oxygen – a process in which the -OH and -H bonds in the alcohol molecule are replaced with a double-bond to one oxygen atom. To catalyse the reaction, the researchers used their wet chemical technique to synthesise particles of NiPt (composed of equal parts nickel and platinum), which formed hollow, uniformly-sized nanospheres.

Fig. 5 Schematic illustration showing different stages in the synthesis process towards the formation of hollow NiPt nanospheres. (1) nanoparticles of Ni–B compounds show up in the solution. (2) Pt and Ni forms NiPt alloying state surrounding Bi-B nanoparticles. (3) NiPt alloying shell grows further at the expense of the enclosing Ni-B. (4) the redundant atoms or ions would migrate out of the NiPt shell through nano-sized pinholes. Eventually, hollow spheres form.

By acting as surfaces where the alcohol oxidation reaction could efficiently take place, the team observed that their NiPt nanostructures performed remarkably well as catalysts. “We measured the electrocatalytic properties for the oxidation of ethylene glycol and methanol,” Professor Chen explains. “The performances are apparently better than that of current commercially-available catalysts made from platinum and carbon”. The discovery was significant, with potential uses of the nanocatalysts including fuel cells, biotechnology and environmental chemistry. However, Professor Chen, Shan and their team were not yet able to confirm with solid evidence whether electrons transferring between nickel and platinum were responsible for their observations.

Mysterious magnetic properties
Professor Chen’s team explored the issue further in a 2017 study, when they studied the unusual magnetic properties of lab-synthesised NiPt₂, which forms a ‘truncated octahedron’- shaped structure. In the experiment, the researchers paid particular attention to the ‘magnetic moment’ of the atoms within the nanostructure. Magnetic moments of each atom are responsible for the magnetism of the magnet when they align with each other. The magnitude of magnetism increases when the magnetic moments associated with atoms align more strongly with each other; increasing the magnetic pull of the object as a whole.

Fig. 6 Numerical simulations reveal that two nanometres of the shell thickness is the thinnest limit for a NiPt alloying hollow structure. Any shell thickness less than two nanometres will collapse as time passes. For one nanometres of shell thickness starting from (a), the hollow sphere structure will collapse to form the one in (b). On the contrary, two nanometres or larger of the shell thickness, the shape of hollow sphere remains like one evolving from (e) to (f).

Applying common sense to magnetics, for a magnetic material, such as Ni or Co, the magnetism would decrease with an increasing temperature. However, the magnetic moment of each atom is considered as having nothing to do with the temperature or even the applied magnetic field. However, in two of their previous experiments, Prof Chen and colleagues did observe some intriguing results. In 2010 they found an unusual enhancement of magnetic moment associated with the Co ions in the Co₃O₄ compound, which is also dependent on the magnetic field. They also reported in 2012 that the magnetic moment of Co in an Au-Co nanohybrid greatly increases from the value it is supposed to be. Although electron transfer is a favourite candidate to explain these phenomena, more investigations remain necessary.

XANES provides evidence supporting the fact that, indeed, fractions of electrons would transfer from Ni to the neighbouring Pt atoms.

In his experiment, “two unusual magnetic properties show up,” Profesor Chen recalls. “The magnitude of the average magnetic moment for the nickel atoms increases greatly, by a factor of more than three compared with their counterparts in a bulk metal at room temperature. Also, the magnitude of the magnetic moment of each atom increases enormously from low to room temperature. This anomalous behaviour has never been reported in the literature.” This time, however, the researchers were able to confirm whether electron transfers were responsible for their observations for the first time.

Fig. 7 Experimental image for Ni_0.20Pt_0.80 nanoflowers, about 30 nm in size.

Explanations in electron transfer
In the same 2017 study, Professor Chen, Shan and their colleagues analysed their synthesised nickel and platinum nanoparticles using the technique of X-ray near-edge spectroscopy (XANES). By firing X-rays at molecular structures, XANES operates by detecting how much of the radiation is absorbed by the electrons within the molecule’s constituent atoms, giving researchers an idea of how the electrons are distributed in the allowed orbitals with different energy. “XANES provides evidence supporting the fact that, indeed, fractions of electrons would transfer from nickel to the neighbouring platinum atoms,” says Professor Chen.

Through numerical calculations, the researchers discovered that this effect could indeed increase the magnitude of magnetic moments in nickel and platinum nanoalloys. In addition, they showed that more electrons will transfer from nickel to platinum at higher temperatures, due to the electrons being activated by heat. Finally, the team’s calculations revealed that electron transfer could be responsible for the enhanced catalytic performance in their nanomaterials – a result which, if confirmed, would finally explain how magnetic and catalytic properties are connected.

Through these discoveries, Professor Chen, Shan and their team have now afforded us the best explanation yet of the properties of bimetallic nanomaterials. In the future, their research could inform developments of new, more intricate nanostructures, with even more specialised properties than current nanomaterials.

What do you think is the most exciting aspect of your work?

First, to understand, by providing solid evidence, the important role the electron transfer effect plays on the mysterious magnetic properties and to dig out the underlying connection between the magnetic and analytical properties of Ni-Pt nanoalloys. This could also be true for the other types of multicomponent metallic alloys or even compounds.

Second, to explore the techniques and phase formation mechanisms for the synthesis of Ni-Pt nanoalloys with variations in structure, morphology, relative composition ratio, and size.

References

Chen, W. Chen, C.P. Guo, L. (2010). Field-dependent low-field enhancement in paramagnetic moment with nanoscaled Co3O4. Journal of Applied Physics, 108(7), 073907.
Zhang, D-F. Zhang, Q. Huang, W.-F. Guo, L.Chen, W.-M. Chu, W.-H. Chen, CP. Wu, Z.Y. (2012). Low temperature fabrication of Au-Co cluster mixed nanohybrids with high magnetic moment of Co. ACS Applied Materials and Interfaces, 4, 5643-5649.
Shan, A. Cheng, M. Fan, H. Chen, Z. Wang, R. Chen, C.P. (2014). NiPt hollow nanocatalyst: Green synthesis, size control and electrocatalysis. Progress in Natural Science: Materials International, 24(2), 175-178.
Shan, A. Chen, C.P. Zhang, Wei. Cheng, D. Shen, X. Yu, R. Wang, R. (2017). Giant enhancement and anomalous temperature dependence of magnetism in monodispersed NiPt₂ nanoparticles. Nano Research, 10(9), 3238-3247.
Shan, A. Chen, Z. Li, B. Chen, C. Wang, R. (2015). Monodispersed, ultrathin NiPt hollow nanospheres with tunable diameter and composition via a green chemical synthesis. J. Mater. Chem. A, 3, 1031–1036.

Research Objectives
Prof Chinping aims to systematically synthesise, by wet chemical routes, the bimetallic nanoalloys of Ni(x)Pt(y), with variations in morphology, size, and composition.

DOI
10.26904/RF-125-1013

Funding
National Natural Science Foundation of China (11674008 and 11174023)

Collaborators
Aixian Shan, Center for Green Innovation, School of Mathematics and Physics, University of Science and Technology Beijing, Beijing 100083, People’s Republic of China sax2005@163.com

Bio
Chinping Chen received his PhD in 1995 from SUNY Buffalo, USA. He became Associate Professor at Peking University, Beijing, China in 2002. Prof Chen’s current research interest lies in nano-materials with magnetic and catalytic properties.