Fine adjustment of the composition, location engineering (core/shell or heterodimer), morphological control and surface structure of each individual domain.
Fine adjustment of the composition, location engineering (core/shell or heterodimer), morphological control and surface structure of each individual domain.

Combining noble metals and metal oxides in hybrid nanoparticles opens up a range of optical and catalytic properties that could prove useful in a whole host of applications. Designing advanced functional metal-metal oxide nanoparticles depends on controlling the size, shape, crystal structure and conformation accurately. But a simple and cost effective synthesis route to hybrid nanoparticles with controlled physical properties has proved elusive, especially in water.

Now researchers from the Institut Català de Nanociència i Nanotecnologia (ICN2), Institució Catalana de Recerca i Estudis Avançats (ICREA), and Vall d’Hebron Institut de Recerca (VHIR) in Barcelona, Spain believe that they have developed a one-pot approach for synthesizing Au nanoparticles with porous coatings or shells of tiny CeO2 nanoparticles of varying thickness. Different CeO2 shell modes of growth give the final nanoparticles ‘dimensionality’, or the ability to maximize the interfaces between the two materials, which leads to anisotropic electric field distribution ideal for plasmonic detection and catalytic applications.

“The aim of our work was to accomplish the synthetic challenge of producing well-defined, stable colloidal nanostructures composed through the controlled integration of two different immiscible elements, Au and CeO2,” says Victor Puntes, who led the effort. “This challenge was motivated by the unique structural features and synergetic optical and catalytic properties that these complex NCs possess.”

In a quick and straightforward process, chloroauric acid (HAuCl4) is reacted with cerium nitrate (Ce(NO3)3) in a solution of sodium citrate, a well-known non-toxic and biocompatible reagent. It is already used in the synthesis of noble metal nanocrystals, but the researchers have extended its remit to include oxide nanoparticles as well.

“The ability [of sodium citrate] to act as a complexing agent of Ce+3/+4 ions, adjusting their oxidation and hydrolysis rates in water, enables the controlled deposition of CeO2 onto Au nanocrystals,” explains Puntes.

The reaction produces Au@CeO2 nanoparticles with an Au core typically around 5 nm in diameter surrounded by tightly bound CeO2 nanocrystals varying in dimension from 2-3 nm. The novelty and power of the approach lies in its ability to control the nucleation and growth processes of the different components. For example, by raising the Ce3+ ion ratio from 1:0.5 to 1:6, the thickness of the CeO2 shell increases from 2 nm to 12 nm without affecting the Au core. Conversely, Ce3+ ion ratio of 1:1 creates clover-like structures, while heterodimers were initially obtained.

“Remarkably, the absence of any calcination step in our method facilitates control of the overall morphology while circumventing aggregation and sintering problems during post-synthesis thermal treatments,” points out Puntes. “Moreover, because no organic solvents are used and no toxic waste is formed during the reaction, our scalable synthesis method can be defined as sustainable, viable, and low-cost.”

The researchers believe that their approach can be applied to other useful systems, such as Ag/CeO2 or PdAg/CeO2.

Bastús et al., Applied Materials Today 15 (2019) 445-452,