*2.6. Degradation Products of Mn2+:ZnS Assisted Photodegradation of SDB*

The mechanism for the Mn2+:ZnS assisted photodegradation of SDB was established using LC–MS. The results obtained for entirely degraded SDB are given in Figure S3. The SDB solution before photodegradation is shown in the inset. The main possible products from the SDB transformation are shown in Figure S4. The structure of those probable transformation products was proposed based only on LC-MS fragmentation. The peaks of SDB were observed at 461 m/z. Only the parent dye was present before irradiation, as expected [126]. Afterwards, quite a few fragments were obtained at m/z 124, 182, 240, 307.8 and 558, indicating subsequent photodegradation of SDB. Possible structures with those m/z values are depicted in Figure S4.

### **3. Experimental**

### *3.1. Materials*

Analytical grade chemicals, which did not require further purification, were used for the syntheses. Manganese carbonate, zinc acetate, sodium sulfide and nicotinic acid were purchased from Merck India Ltd., and used to prepare Mn2+:ZnS Qds. The SDB dye (laboratory reagent grade) was also supplied by Merck India Ltd. Table S2 shows the formula and other data of this dye. In order to adjust the pH values of the suspensions, NaOH and HCl solutions were used (Merck India Ltd., Mumbai). Using double distilled water, varying concentrations of SDB solutions were made by diluting the prepared stock solution.

### *3.2. Apparatus*

The surface morphology of the nanoparticles was studied using a scanning electron microscope JEOL JSM—6390LV, Tokyo, Japan. A small piece of extrudate of 10 mm diameter was mounted on specimen stubs using carbon tape and was over coated with gold using the JFC 1600. This ion sputtering device performs rapid and efficient gold coating on microscopic specimens, allowing surface visualization. The SEM measurements were performed at 15 kV accelerating voltage. Different voltages and magnifications were used as indicated in the SEM images.

The transmission electron microscopy was performed using a JEOL Model JM 2100 TEM device. An extremely small amount of material was suspended in water/ethanol (just enough to obtain slightly turbid solution). The solution was ultrasonicated to disperse the particles. A drop of the solution was then pipetted out and the drop was placed on a carbon-coated grid of 200 mesh. The measurements of particles observed in the TEM images were carried out using ImageJ software.

The absorption spectrum was used to find the optical properties of the as-prepared material, adopting a Cary Win UV spectrophotometer. The Belsorp mini II (BEL Japan Inc., Osaka, Japan) was used for N2 gas adsorption–desorption analysis at −196 ◦C (with liquid nitrogen as coolant). The Brunauer–Emmett–Teller (BET) method was used to measure the surface areas, whereas the pore size distribution and volume were determined by the Barrett–Joyner–Halenda (BJH) model. The thermo gravimetric analysis (TGA) was conducted with a Perkin Elmer STA 6000 TG/DT model at 10 ◦C/min, from 30 to 650 ◦C, in N2 atmosphere. For tuning the pH, a Systronic pH-meter was used. The ultrasound assistance in the dye degradation experiment was accomplished in an Aczet ultrasonic bath reactor with operating frequency of 40 kHz, a power rating of 120 W, and a 2.5 L capacity with dimensions of 235 × 135 × 100 mm. The UV irradiations were provided using a 40 W mercury lamp (Osram), at 254 nm emission, with incident light intensity of 221.23 W/m2. Finally, product analysis was done by mass spectrometry by the Agilent 1290 Infinity UHPLC system (Agilent Technologies, Santa Clara, CA, USA).
