3.7.1. Rhodamine B Dye

The catalytic properties of *C. pentandra*/Ag-NPs on the reduction of RhB dye with NaBH4 is shown in Figure 16. The decrease in absorbance peak intensity at 554 nm as a function of time reflects the decrease in the concentration of RhB in the system. A controlled experiment was performed on RhB dye in the presence of NaBH4 only. The result indicates that in the absence of *C. pentandra*/Ag-NPs, the peak at 554 nm slightly decreased in absorbance of RhB dye solution (20 mg/L) even after 120 min of reaction time (4%) as shown in Figure 16a. This result also displays the reduction of organic dyes by NaBH4 is possible but not kinetically favorable due to the kinetic barriers differences in the thermodynamic potential of electron donor (NaBH4) and acceptor (RhB dye) [44]. After the addition of *C. pentandra*/Ag-NPs under the same condition, the RhB dye peak–peak at 554 nm decreased quickly and reaches an equilibrium reaction time within 4 min (94%). A new peak was observed around 402 nm corresponding to the peak of Ag as shown in Figure 16b.

This indicates that the ability of *C. pentandra*/Ag-NPs which can act as nanocatalyst and accelerate the reaction. According to Ganguly et al. [45], the reduction of dye molecules generally obeys two stages pathway; initially adsorption of dye molecules onto the Ag-NPs catalyst surface followed by electron transfer phenomenon among catalyst, BH− <sup>4</sup> ions and dye molecules. The peak observed around 400 to 450 nm at all concentrations of RhB tested corresponding to the peak of Ag as evidence of electron relay process between BH− <sup>4</sup> ions and RhB dye during the reduction reaction process. This peak was shifted to the blue shift and less intensity than observed in the case of RhB dye reduction. This was due to the fact that the λmax of RhB is detached from the SPR absorption of Ag-NPs and reduces the possibility of interaction between these two peaks [21]. The Ag peak was increased with decreasing reduction time until the completed reaction shows that more Ag-NPs involved as electron relay transfer process and RhB is detached during the reduction reaction.

**Figure 16.** UV–vis spectra of reduction of RhB dye by (**a**) NaBH4 alone, (**b**) in the presence of *C. pentandra*/Ag-NPs at 20 mg/L RhB and (**c**) the proposed mechanism of *C. pentandra*/Ag-NPs as nanocatalyst for the reduction of RhB dye.

The proposed mechanism of catalytic reaction by *C. pentandra*/Ag-NPs on the RhB dye reduction can be illustrated in Figure 16c. The redox potential of Ag-NPs is in between the RhB dye (−0.48 V) and NaBH4 (−1.33 V) [46]. Thus, can act as electron transfer agents and relay electron from the donor NaBH4 to the acceptor RhB dye. Upon the addition of *C. pentandra*/Ag-NPs, the BH<sup>−</sup> <sup>4</sup> ions dissociate from NaBH4 and donate the electrons as well as transfer it to *C. pentandra*/Ag-NPs. The RhB dye

on the surface of *C. pentandra*/Ag-NPs will be accepted that electrons and reduced dye molecules. The negatively charged of *C. pentandra*/Ag-NPs surface brings the RhB dye molecules closer to the surface of *C. pentandra*/Ag-NPs via electrostatic attraction.
