*2.1. Materials*

The hexafluorosilicic acid was collected from Lam Thao Fertilizers and Chemicals JSC (Phu Tho, Vietnam) and used as a raw resource for the production of nanosilica. The concentration of the hexafluorosilicic acid solution was 13 wt.% along with a very low concentration of Fe2O3 (0.01–0.02 wt.%) and P2O5 (0.001–0.01 wt.%). A 25 wt.% ammonia solution was purchased from Xilong Scientific Co. (Shantou, China). The NR (NR-Standard Vietnamese Rubber-3L) was purchased from Phuoc Hoa Rubber Co. (Binh Duong, Vietnam). We adopted 2, 2, 4-trimetyl-1, 2-dihydroquinolin (RD), N-cyclohexyl-2-benzothiazole sulfenamide (CBS), and 2, 2'-dibenzothizole disulfide (DM), (RongCheng K&S Chem., Rongcheng, China) as vulcanizing accelerators [42]. Stearic acid, zinc oxide, and sulfur were purchased from Sigma-Aldrich (St. Louis, MO, USA).

#### 2.1.1. Recovery of Nanosilica

In the first step, pristine solutions of both hexafluorosilicic acid and ammonia were diluted with distilled water to 10 wt.% and 20 wt.%, respectively. In the second step, 200 g of 10 wt.% hexafluorosilicic acid and 110 g of 20 wt.% ammonia solution were mixed in a glass reactor vessel with a mechanical stirrer at 200 rpm for 12 h at 25 ◦C to obtain a slurry of nanosilica. Subsequently, the silica nanoparticles were collected with a vacuum filter under atmosphere 200 mg Hg and cleaned a couple of times with distilled water until they reached a neutral pH value. The resulting products were dried using a vacuum oven at 800 ◦C for 3 h and cooled to 25 ◦C before obtaining the final product. Figure 1 provides details of this processing.

**Figure 1.** The processing of fabrication of nanosilica from hexafluorosilicic acid.

The material characteristics of the obtained nanosilica, such as chemical structure, crystallinity, and particle size were scrutinized by Fourier transform-infrared spectroscopy (FT-IR), X-ray diffraction (XRD), and transmission electron microscopy (TEM), respectively.
