*2.3. Optical Study*

Figure 4a shows the UV-Vis diffuse reflectance spectra of g-C3N4, TiO2, and g-C3N4/TiO2 composites. The exfoliated g-C3N4 nanosheets show an absorption edge at 470 nm with a band gap of 2.93 eV, which was in agreement with previous reports [53]. The absorption spectra of the TiO2 shows an absorption edge at around 400 nm with a band gap of 3.20 eV. The presence of g-C3N4 resulted in the red shift of the absorption edge in all composites, revealing that the composites can be applied to visible-light photocatalysis. In addition, the presence of Ti3+-TiO2 can narrow the wide band gap of TiO2 for harvesting visible light and can provide an increase in electronic conductivity [54].

**Figure 4.** (**a**,**b**) UV-Vis DRS spectra and Tauc plot of g-C3N4, TiO2, and g-C3N4/TiO2 photocatalysts, (**c**) photoluminescence spectra of the as-synthesized g-C3N4, TiO2, and g-C3N4/TiO2.

The band gap energy was calculated using the Tauc plot in Equation (1) and is shown in Figure 4b [55].

$$
\alpha \mathbf{h} \mathbf{v} = \mathbf{A} (\mathbf{h} \mathbf{v} - \mathbf{E} \mathbf{g})^{1/2} \tag{1}
$$

where α is the optical absorption coefficient, h is Planck's constant, ν is photon frequency, A is constant, and Eg is band gap.

The band gaps of pure g-C3N4, TiO2, and 0.5CNS/TiO2 were calculated to be 2.93, 3.20, and 3.17 eV, respectively.

Photoluminescence analysis was performed in order to determine the electron–hole recombination which is shown in Figure 4c. Under excitation at 320 nm, the emission peak of g-C3N4 appears at around 457 nm. The bulk-CN and CNS showed high PL intensity because of the fast recombination of electron–hole pairs, whereas TiO2 showed a broad emission peak at 410 nm and a lower maximum peak than that of the g-C3N4 system. After the hybridization of g-C3N4 and TiO2, the composite showed a much weaker emission peak, implying that the recombination of charge carriers may be effectively inhibited.
