*3.5. Synthesis of TiO<sup>2</sup> and GQD Composite*

Different as-prepared samples of GQDs (2 mg/mL) were dispersed in DI water to obtain a uniform suspension using the ultrasonication method. A total of 200 mg of TiO<sup>2</sup> was added to 50 mL of GQD suspension and stirred at 60 ◦C for 10–12 h, until it dries completely and turned into powder. The GQDs-TiO<sup>2</sup> nanocomposite was obtained in powder form. The nanocomposite samples were named CNT-GQDs-TiO2, RCNT-GQDs-TiO2, and CNF-GQDs-TiO2. The synthesis scheme for respective GQDs is shown in Figure 7.

**Figure 7.** Schematic for the synthesis of N, S co-doped GQDs named as (**a**) CNT-GQDs, (**b**) RCNT-GQDs, (**c**) CNF-GQDs.

#### *3.6. Characterization Techniques*

The structure, crystal facets, and phase of the as-prepared samples were examined by powder X-ray diffraction (XRD, Bruker D2 PHASER, Gyeonggi-do, Korea). The morphological features, crystallinity, and chemical composition were studied by field-emission scanning electron microscopy (FE-SEM, Hitachi High-Tech Korea Co., Ltd., S-4300, Seoul, Korea), field-emission transmission electron microscopy (FE-TEM JEM-2100F/JEOL, Seoul, Korea), and energy-dispersive X-ray spectroscopy (EDX) composition analysis. The X-ray photoelectron spectroscopy data were also obtained (XPS, VG Scientific Co., ESCA LAB MK-II, Korea) using a monochromatic Al Kα X-ray beam with a current of 20 mA. Quantitative analysis of the solid organic elements present in the samples was performed with an elemental analyzer (Thermo Fisher Scientific, Inc. EA1112, Korea). The specific surface area measurements were performed using an adsorption analyzer (BEL BELSORP, Inc. Japan), and thermogravimetric analysis (TGA, NETZSCH, TG 209 F3, Germany) was carried out for the as-prepared samples at a heating rate of 10 ◦C/min and with N<sup>2</sup> gas as a purge gas. Photoluminescence (PL) emission spectra were acquired under excitation at 325 nm (RAM Boss, Maple Dongwoo Optron). The UV-vis absorption was performed using a commercial UV-vis spectrophotometer (Scinco, S-3100, Seoul, Korea). Electrochemical analysis was carried out using an Ivium electrochemical analyzer.

#### *3.7. Photodegradation Test*

Photocatalytic studies of the GQDs-TiO<sup>2</sup> nanocomposites were conducted using a solar simulator (SUN 2000, Abet Technologies, Inc. Gyeonggi-do, Korea) fitted with a 440 nm cutoff filter. The photocatalytic experiments were carried out in the presence of MB as an organic dye pollutant. A total of 20 mg of catalyst were dispersed in 50 mL of MB aqueous solution with an initial concentration of 10 mg/L. Prior to irradiation, the as-prepared solution was stirred continuously in the dark for 1 h at room temperature to achieve adsorption-desorption equilibrium. At 15 min irradiation intervals, a 5 mL aliquot was withdrawn and analyzed via UV-vis spectrophotometry with a maximum absorption peak at 665 nm.

#### *3.8. Free-Radical Scavenger Test*

A free-radical scavenger experiment was conducted to study the involvement of reactive species in the photodegradation of MB. Isopropyl alcohol (IPA) solution, BQ, and EDTA were used as scavengers to analyze the influence of hydroxyl radicals (•OH), superoxide radicals (O<sup>2</sup> •−), and holes (h<sup>+</sup> ), respectively. In the scavenging experiments, 10 mg of a scavenger was mixed with the dye–catalyst solution (20 mg, 10 mg/L). Afterward, the same procedure mentioned in previous section was conducted under identical conditions.

#### **4. Conclusions**

A novel approach for the synthesis of N, S doped GQDs from 1-D carbon nanomaterial with high photocatalytic and photoelectrochemical response was suggested. The mechanism of interfacial charge transfer in GQDs-TiO<sup>2</sup> heterojunction is elucidated through the possible C–O–Ti bonds formation. TGA and XPS analyses revealed that N and S doping with in-plane functional groups of GQDs were responsible for stable interface formation between GQDs and TiO2. Photoelectrochemical measurements provide insights into photocurrent and impedance characteristics of as-prepared GQDs-TiO<sup>2</sup> nanocomposites. Remarkably high degradation percentages of MB under visible-light illumination were observed for CNT-GQDs-TiO<sup>2</sup> (99.78%), RCNT-GQDs-TiO<sup>2</sup> (99.39%), and CNF-GQDs-TiO<sup>2</sup> (99.23%) nanocomposites. Rapid interfacial charge transfer from the GQDs to TiO<sup>2</sup> is the key phenomenon for the enhanced photodegradation of MB. Consecutive degradation cyclic runs to test the highest efficiency and stability of nanocomposites result that the CNF-GQDs-TiO<sup>2</sup> nanocomposites have the fastest photodegradation rate and proved to be the most stable among all. CNT-GQDs-TiO2, RCNT-GQDs-TiO2, and CNF-GQDs-TiO<sup>2</sup> nanocomposites show different characteristic radicals •OH, h+, and O<sup>2</sup> •−, respectively, as

revealed by radical quenching experiments. This study is considered to be a platform for nonmetallic GQDs and metal oxide semiconductor nanocomposites for green-environment, sustainable energy, and optoelectronic applications.

**Author Contributions:** J.R.: Conceptualization, Investigation, Methodology, Validation, Data curation, Writing—original draft, Writing—review and editing. U.K.: Conceptualization, Visualization, Formal analysis, Writing—review and editing. M.P.: Writing—review and editing. B.P.: Writing—review and editing. S.-J.P.: Supervision, Funding acquisition, Writing—review and editing. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was supported by Korea Electric Power Corporation (Grant number: R21XO01- 5). This work was also supported by Nano-Convergence Foundation (www.nanotech2020.org (accessed on 1 May 2021)) funded by the Ministry of Science and ICT (MSIT, Korea) & the Ministry of Trade, Industry and Energy (MOTIE, Korea). [Project Name: Development of high-efficiency activated carbon filter for removing indoor harmful elements (VOCs, radon, bacteria, etc.)].

**Data Availability Statement:** The data is contained within the article.

**Conflicts of Interest:** The authors declare no conflict of interest.
