4.1. RhB photodegradation
The photocatalytic performance was evaluated by studying RhB photodegradation as shown in Fig. 8a. The removal efficiencies of SC1, SC2, SC3, SC4, CN, and CS samples were 97%, 58%, 28%, 31%, 88%, and 72% respectively in 90 minutes of light illumination. The 0.1% loaded sample showed maximum photodegradation among all the samples. However, the gradual increase in carbon soot content drastically decreased the performance. This was attributed to the limited exposure of light on the photocatalyst due to the presence of excess soot particles in the nanocomposite material [34]. Besides, the CS sample showed notable efficiency which was basically due to the high SSA value that resulted in the adsorption of dye molecules on the soot particles. The rate of reaction of all the photocatalyst samples was obtained by applying pseudo – 1st order kinetics equation [ ln C/C0 = -kt, where k (min− 1) is the rate constant] as shown in Fig. 8b. It was noticed that the SC1 sample degradation rate was 1.44 times faster than the pristine CN sample. This was due to the addition of carbon soot nanoparticles. The present work has been compared with some of the previously reported works using different pollutants as given in Table − 3.
Table – 3 Comparative study of present work with some of the published literature.
S. No. | Spherical CNMs Loading (%) | Target Pollutant | Target Conc. (mg/L) | Time (Minutes) | Degradation (%) | Ref. No. |
1 | - | Cr (IV) | 75 | 240 | ~ 100 | [13] |
2 | 0.3 | RhB | 10 | 60 | ~ 100 | [21] |
3 | 2 | RhB | 5 | 40 | 91 | [34] |
4 | 1 | RhB | 5 | 60 | 97 | [37] |
5 | 0.03 | MB | 3 | 300 | - | [38] |
6 | 0.1 | RhB | 20 | 90 | 97 | This Work |
The degradation mechanism was further explored by conducting radical quenching experiments (Fig. 8c). here, 10 mM each of Ammonium Oxalate, (AO) p-Benzoquinone (pBQ), and Isopropanol (IPA) were added to the dye solution to quench the photogenerated holes (h+), superoxide anion (•\({\text{O}}_{2}^{-}\)), and hydroxyl radical (•OH) respectively. It was discovered that with the addition of a h+ scavenger, the removal efficiency dropped to 2% whereas when •\({\text{O}}_{2}^{-}\) and •OH scavenging agents were added, and the removal percentage dropped to 46% and 44% respectively. On this note, it can be said that h+ majorly influenced the photodegradation process followed by superoxide and hydroxyl radicals. The image of the final colour of CN and SC1 sample solutions after the photodegradation experiment is shown in Fig. 8d. Clearly, the solution treated with SC1 becomes colourless whereas CN treated solution was slightly coloured.
The consistency and stability of the material were studied by doing repeatability and reusability test as displayed in Fig. 9(a, b). For four times consecutively, the degradation efficiency remained constant and during the reusability study, up to three cycles, there was a minute drop in the overall efficiency however, in the fourth cycle the efficiency dropped to 30%. This could be majorly due to the loss of material during the collection and drying process. Furthermore, the XRD pattern of the used SC1 sample showed an identical pattern to that of the fresh SC1 sample (Fig. 9c) which implies that the structural characteristics were essentially unperturbed post-experiment.
4.2. Proposed mechanism
Based on the above results, a plausible degradation mechanism is proposed as shown in Fig. 10. In general, when the light of required energy is incident on the photocatalyst material, electron-hole pairs (e− – h+) are generated and the e− gets excited to conduction band (CB) from the valence band (VB) leaving behind a hole void in the VB which eventually participate in photo redox reactions. Simultaneously, a large portion of charge carriers are recombined resulting in low performance. To overcome this problem, nanocomposites are developed that contain a small fraction of electron-accepting species like carbon nanomaterials. Due to the incorporation of carbon soot nanoparticles in this case, the photoelectrons are easily delocalised in the honeycomb structure of the soot nanoparticles facilitating a synergistic charge separation and transportation [37]. The transferred electrons will presumably migrate towards the surface and capture the surface-bound oxygen molecules to produce superoxide radicals and thus participate in photocatalytic reactions [38].