*2.2. UV-Visible Analysis*

Figure 2a shows the UV-visible spectra of the Ni-Mo sulfide/Al2O3 catalysts with different Ni/Mo molar ratios, compared with NiS2 and MoS2. From the UV-visible spectra, the band edge of MoS2 was located around 600−800 nm, which belonged to the absorption of visible-light. Compared with MoS2, with increasing the Ni/Mo molar ratio, the absorption boundaries of the Ni-Mo sulfide/Al2O3 catalysts were gradually red shifted. Continuous shift of the absorption boundaries suggests that the band gaps of the Ni-Mo sulfide/Al2O3 catalysts can be controllably adjusted through changing the Ni/Mo molar ratio. The relationship between the incident photon energy and the absorption coefficient of a semiconductor can be determined by the Kubelka–Munk equation [16,17]:

$$\alpha(\text{hv}) = \text{C}(\text{hv} - \text{Eg})^{\text{n/2}},\tag{1}$$

where α is the absorption coefficient and its value can be achieved by the equation: α = (1 − R)<sup>2</sup>/2R; R is the diffuse reflectance and its relationship with absorbance can be defined by R = <sup>10</sup>−A; A is absorbance. ν is frequency, h is Planck's constant, and C is a constant. For a direct transition semiconductor, n = 1; for an indirect transition semiconductor, n = 4. The nature of transition is possible to be determined through plotting the graph of (αhν)<sup>2</sup> versus hν; therefore, the band gap energies can be deduced by extrapolating the straight-linear portions of the plot to intersect the photon energy axis. As shown in Figure 2b and Table 1, the band gaps obtained in such a way were 2.17, 2.00, 1.81, 1.56, and 1.30 eV for the Ni-Mo sulfide/Al2O3 catalysts, of which Ni/Mo molar ratios were 2/6, 3/5, 4/4, 5/3, and 6/2, respectively. For all the catalysts, the influence of the chemical compositions of the Ni-Mo sulfide/Al2O3 catalysts on the band gap can be observed. When the Ni/Mo molar ratio increased, the band gap decreased gradually. This indicates that changing the Ni/Mo molar ratio can significantly adjust the band gaps of the Ni-Mo sulfide/Al2O3 catalysts. Meanwhile, the changes in band gaps also illustrate that the relative redox abilities of the Ni-Mo sulfide/Al2O3 catalysts were effectively changed.

**Figure 2.** UV-vis diffuse reflection spectra for the Ni-Mo sulfide/Al2O3 catalysts with different Ni/Mo molar ratios (**a**) relationship of absorbance and wavelength; (**b**) relationship of absorption coefficient and incident photon energy.
