*2.1. Catalyst Characterization*

The pure and doped Qds were characterized by different techniques. To study the sample morphology, detailed imaging investigations were carried out. Figure 1a–c show TEM images of Mn2+:ZnS Qds. Figure 1c shows some particle sizes of the Qds sample. The micrographs reveal monodispersion of particles, with an average size of 3–4 nm (Figure 1d). Larger aggregates of small particles could also be identified.

**Figure 1.** (**a**–**c**) TEM images, (**d**) distribution of particle sizes Mn2+:ZnS Qds.

The morphology of the Qds was also studied by SEM, as shown in Figure 2. The images illustrate the formation of agglomerated nanoparticles with a smooth surface. The morphology shows that aggregates of Qds are formed by primary units of varied orientations. Arbitrary aggregations among the small particles occur, directing the development of disordered crystallites [58]. The micrographs (Figure 2a) at lower magnification illustrate inhomogeneities about the size distribution of the crystallites. However, an image at higher magnification (Figure 2b) shows that the smaller sized particles agglomerate, originating larger ones.

**Figure 2.** (**a**–**c**) SEM images of Mn2+:ZnS Qds; (**d**) EDX spectra of the marked area (spectrum 1).

The elemental composition was studied by energy dispersive X-ray analysis (EDX). Figure 2c shows the synthesized Qds and Figure 2d displays the respective EDX image of the marked area (spectrum 1). The percentages of various elements present in the material are 91.19 wt.% Zn, 8.05 wt.% S and 0.77 wt.% Mn; molar: 70.78% Zn, 28.37% S and 0.85% Mn.

A small amount of S was used for the synthesis. It is known that the presence of large amounts of S during synthesis causes accumulation of S atoms at the Qds surface. These S atoms are easily oxidized and may cause quenching of luminescence intensity [29,59]. Nevertheless, the expected molar ratio Zn:S in the catalyst should be 1:1, but the amount of Zn found is larger. It is possible that, along with ZnS, Zn(OH)2 is also formed, in alkaline conditions, as also shown by other authors [60]. We also used a small amount of Mn2+, as we found that for higher concentrations a reduction in the luminescence intensity was observed. This phenomenon is ascribed to the fact that a larger Mn2+ concentration causes a larger number of Mn2+ emission centers per nanocrystal, and the interaction between the Mn2+-Mn2+ pairs intensifies the non-radiative decay of the Mn2+ excited state, thus causing a decrease in Mn2+ emission intensity [59]. The EDX image (Figure 2d) shows the Mn2+ doped ZnS crystals. A theoretical value of 1% Mn was expected, but a smaller amount was found by EDX. It is possible that the prepared sample was not totally homogeneous and the small part analyzed contained a little less Mn than the bulk. Additionally, the prepared sample was washed with distilled water, which could also lead to some loss of Mn ions.

Ultraviolet-visible (UV-Vis) spectroscopy was used to evaluate the usefulness of the capping agent in stabilizing the growth of the Qds in aqueous solution. The UV-Vis spectra and curve of the band gap of ZnS and Mn2+:ZnS Qds are shown in Figure 3a,b. As found in Figure 3a, the absorption shoulders of the Qds are placed between 290 to 310 nm, while for bulk ZnS, a band is noted near 350 nm, having a band gap energy (*Eg*) value of 3.6 eV [61–63]. The blue shift in the absorption edge with doping is due to the decrease in the particle size. The absorption spectra are steadily blue shifted with increasing Mn content; this shift in the absorption edge can be attributed to the reduction of particle size through the doping process, which is due to the quantum confinement of the excitons, ensuing a more discrete energy spectrum of the discrete nanoparticles. Figure 3b shows the band gap energy curve of pure ZnS and Mn2+:ZnS QDs calculated from the Tauc's relation [63], as: αhυ = α<sup>o</sup> (hυ-*Eg*) 1/2; where hυ is photon energy, *Eg* is optical bandgap of the nanoparticles, and αo, a constant. The *Eg* values for pure and Mn2+:ZnS Qds samples were found between 3.87 and 3.96 eV. The increment in the *Eg* value along with the shifting of the curve may be due to the decrease in the nanoparticle size, attributed to the quantum size confinement effect [61,64]. Thus, transformations in the optical and electronic properties of nanoparticles occur when their dimensions are reduced below threshold values.

The shift in the band gap can be explained by the effective mass approximation model with the particle in a box approach. Based on the first order approximation of Brus equation, the relationship between the particle radius (*r*) and band gap (*Eg*) in ZnS nanocrystal [59,65–67] is given by:

$$r(E\_{\mathcal{S}}) = \frac{0.32 - 2.9\sqrt{E\_{\mathcal{S}} - 3.49}}{2\left(3.50 - E\_{\mathcal{S}}\right)}\tag{1}$$

Using Equation (1), the size of the pure and doped ZnS nanocrystal (considering *Eg* = 3.87–3.96 eV) was calculated to be 3.62–3.96 nm, which is in agreement with the size obtained from TEM measurements.

**Figure 3.** (**a**) UV-Vis spectra and (**b**) optical band gap curve of Mn2+:ZnS Qds.

The crystal structure and phase composition of the pure ZnS and Mn2+:ZnS Qds were also studied by XRD, as shown in Figure 4. Crystalline Qds were obtained. The peaks at 29.7◦, 48.4◦ and 56.16◦ for Mn2+:ZnS Qds correspond to the (111), (220), and (311) crystallographic planes of cubic crystalline ZnS, respectively [27,68–70]. The dopants are well inserted in the ZnS structure and Mn2+ ions did not alter the phase [27,70,71]. A slight shift for the peak corresponding to the 220 plane is observed, which indicates the inclusion of Mn in the crystal lattice. The average size was calculated by the Debye Scherrer equation using the full width at half maximum of the XRD peaks. The average crystallite size for pure ZnS and Mn2+:ZnS Qds were found to be 1.32 and 1.13 nm, respectively.

**Figure 4.** XRD patterns of (**a**) pure ZnS Qds and (**b**) Mn2+:ZnS Qds.

Figure 5 shows the FTIR scan of Mn2+:ZnS Qds. Several features can be found in the range of 500–4000 cm<sup>−</sup>1, characteristic of various functional groups [5,71]. The absorption peaks above 3000 cm−<sup>1</sup> are due to the Ar-H or =C-H stretching. The bands between 1600–1200 cm−<sup>1</sup> are ascribed to the ring stretching vibrations [5,72]. The broad band at 3335.4 cm−<sup>1</sup> is attributed to the O-H stretching, whereas the band at 930.4 cm−<sup>1</sup> is due to the O-H out of plane bending. The peaks between 1630–1540 cm−<sup>1</sup> and around 1410 cm−<sup>1</sup> are due to the C=C stretching, while the peaks between 1330–1240 cm−<sup>1</sup> are ascribed to C=N stretching. This may be attributed to the coordinate bond formed between Zn2+ ions and the N atoms of the pyridine moiety in nicotinic acid [73,74]. Thus, the FTIR study strongly supports the formation of nicotinic acid capped Mn2+:ZnS Qds. The band at 1036 cm−<sup>1</sup> is attributed to the Zn-OH vibrations [60]. As explained above, the formation of Zn(OH)2 is possible under alkaline conditions, as reported by other authors [60].

**Figure 5.** FTIR of Mn2+:ZnS Qds.

For determining the specific surface area and pore size distribution of solid and porous materials, gas adsorption is a prevailing analysis technique. BJH calculation is a pore size distribution determination method, typically applied to N2 adsorption data. Evaluation of the adsorption and desorption isotherm branches reveals information about the pore volume and pores size distribution. BET is mainly used for the surface area analysis of the prepared nanomaterials. The surface area can be calculated from the quantity of gas required to form a monolayer. In order to confirm the porous structure of Qds, N2 adsorption–desorption experiments were performed at −196 ◦C. Figure S1a shows a type IV adsorption–desorption isotherm with a hysteresis loop, while Figure S1b displays distribution of the pore size. The pores are likely to show a very narrow slit or bottle shaped configuration or a distribution of randomly-shaped micro and mesopores. The Qds surface area value was 10 m2/g, determined using the BET equation. The pores have an average diameter of 1.5 nm (determined by the BJH adsorption procedure), whereas the total pore volume was 0.01 cm3/g.

Thermal analysis was carried out for studying the disintegration, strength and temperatures of phase development of the nanoparticles. To determine the thermal activity of the prepared materials, differential thermal analysis (DTA) and thermogravimetric analysis (TGA) were performed under N2 atmosphere, as shown in Figure 6a,b. The data were recorded with a heating rate of 10 ◦C/min, up to 650 ◦C. Owing to the loss of absorbed water, the endothermic peak occurred near 185 ◦C. Different steps of weight loss are noted in the TGA plot. The initial weight loss shown by the sample up to 230 ◦C (12.5%) is connected to the water molecules physically adsorbed at the surface. The next peak, located at 240–370 ◦C (10.35%), is correlated to the disintegration of organic moieties from the precursors. The next loss of weight (11.43%) is due to the discharge of Mn ions from the sample. Further decrease (5.72%) is attributed to the loss of the S ions, as also reported by other authors [75–77]. Figure 6a indicates that the thermal study started at about 2.8 mg sample weight, which after heating to 600 ◦C remained at a weight of 1.96 mg. Thus, the sample loses 40% of its weight up to 700 ◦C. Figure 6b shows an exothermic progression up to about 460 ◦C, which may possibly be attributed to a shift in phase or crystallinity of the sample.

**Figure 6.** (**a**) Thermogravimetric analysis (TGA) and (**b**) differential thermal analysis (DTA) of Mn2+:ZnS Qds.

The fluorescence spectra of pure ZnS QDs and Mn:ZnS QDs (doped with various concentrations) were measured, using 320 nm as the excitation wavelength optimal for the ZnS QDs [59]. The integrated fluorescence intensities of the emission peak versus absorbances are plotted in Figure 7. With the increase in Mn2+ concentration from 0 to 3%, the florescence intensity of Mn2+ ( 4T1- 6A1) is found to increase steadily.

**Figure 7.** Fluorescence spectra of Mn2+:ZnS QDs with various percentages of Mn2+ ions (λex = 320 nm).
