A Novel One-Step Green Method to Synthesis of Palladium Nanoparticles ^†

Abstract

Palladium nanoparticles (PdNPs) are one of the most attractive metal nanomaterials because of their excellent physicochemical properties. PdNPs have been studied for many different applications such as Suzuki cross-coupling reactions, hydrogen purification/storage/sensing, CO oxidation, fuel cells, prodrug activation, and antimicrobial therapy. Recently, PdNPs have been explored as photoabsorbers for photothermal therapy and photoacoustic imaging in the treatment of cancer. Herein, we reported a scalable, efficient, green, and one-step method to synthesize PdNPs. The chitosan polymer was used as a stabilizer and vitamin C was used as a reducing agent. Interestingly, the reaction temperature can be adjusted to the size of PdNPs. When the reaction temperature was increased from 25 °C to 95 °C, the morphology of resulted PdNPs changed from a flower shape to a spherical shape and their nanoparticles’ sizes decreased from 64 nm to 29 nm. The characterization revealed that the obtained PdNPs were relatively uniform in size, shape, and stability in an aqueous solution.

1. Introduction

Palladium nanoparticles (PdNPs) are gaining attention due to their good physicochemical properties including excellent catalytic activity, chemical-thermal stability, and the fact that they are cheap to manufacture [1,2]. They have been discovered in different experiments including Suzuki cross-coupling reactions [3], hydrogen purification/storage/sensing [4], CO oxidation [5], and fuel cells [6], prodrug activation [7,8], antimicrobial therapy [7,9,10], photothermal therapy [7,11], and photoacoustic imaging [11,12].

The reported methods for the preparation of PdNPs are complicated [13], produce toxic reductants [13,14,15,16,17], require high-temperature conditions (e.g., 300 °C) [18,19], and are time-consuming processes [20]. An effective green method for synthesis PdNPs is needed.

Here, we reported a new method for the synthesis of spherical PdNPs. This method is scalable, simple, environmentally friendly, and low-temperature. We utilized chitosan (CS) as a stabilizer, vitamin C as a reducing agent, and water as a solvent. The PdNPs were flower-shaped when the reaction temperature was 20 °C. When increasing the temperature to 50 °C, 75 °C, and 95 °C, the shape of PdNPs changed to spherical. Correspondingly, the size of the obtained nanoparticles was decreased from 64 nm to 29 nm.

2. Materials and Methods

2.1. Materials

Palladium chloride (PdCl₂), chitosan (50 to 190 kDa), hydrochloric acid (HCl), and L-ascorbic acid (AA) were ordered from Sigma-Aldrich (St. Louis, MO, USA).

2.2. Synthesis of PdNPs

Firstly, we dissolved 89 mg PdCl₂ in 50 mL DW containing 82 µL HCl 37% to obtain the HPdCl₄ 0.01 M. After that, the AA-CS solution was prepared by mixing 2 mg CS and 50 mg AA in 15 mL DW. Then, AA-CS solution was preheated in the stirring mantle to the setup temperature (20 °C, 50 °C, 75 °C, and 95 °C). Next step, 10 mL HPdCl₄ 0.01 M was added slowly into the preheated solution. The heating condition was kept for 5 min and we then turned off the heating condition. The resulted solution was left to cool down to room temperature. Finally, PdNPs were collected and were dried to obtain the powder.

2.3. Characterization

The morphologies of PdNPs were obtained by Field-emission transmission electron microscopy (FETEM, JEOL JEM-2010 microscope, Japan). The size distribution of PdNPs was analyzed by an electrophoretic light scattering spectrophotometer (ELS-8000, OTSUKA Electronics Co. Ltd., Osaka, Japan). UV-Vis spectra of PdNPs solutions were recorded by a UV-Vis spectroscopy (Thermo Biomate 5 Spectrophotometer).

3. Results and Discussion

As shown in Figure 1, both the AA-CS and HPdCl4 solutions have absorption peaks below 525 nm. There was a presence of broadband absorption after 525 nm in all four temperature conditions.

The morphologies and sizes of PdNPs were given in Figure 2 and Figure 3, respectively. The TEM images showed that PdNPs were flower-shaped at 20 °C experimental conditions; however, the morphologies changed to spherical shape in the experiments at 50 °C, 75 °C, and 95 °C. The sizes of PdNPs were 63.4 nm, 48.5 nm, 42.5 nm, and 29.6 nm from experiments at 20 °C, 50 °C, 75 °C, and 95 °C, respectively.

4. Conclusions

In conclusion, we reported a simple, environmental friendly, and low-temperature strategy for the synthesis of the PdNPs. The reaction temperature plays an important role in controlling the size of the PdNPs. The resulted PdNPs have a size from 64 nm to 29 nm.