3.1. Structural and optical absorption studies
Figure 1A illustrates the FTIR spectra of g-C3N4, BiYO3 and BiYO3/g-C3N4 nanocomposite photocatalysts. From the Fig. 1A, the g-C3N4 shows bands between 689–881 cm− 1 corresponding to tri-S-triazine mode. In addition, the C-N stretching peaks at 1205 and 1317 cm− 1 and C = N stretching at 1388–1520 cm− 1 are also observed. The bands appeared at 1621 and 3264 cm− 1 are owing to O-H bending and stretching respectively. The N-H stretching is found at 3071 cm− 1 [Jia et al. 2019]. For BiYO3, Bi-O band’s peak at 530 cm− 1 and a Y-O band allocated at 881–1033 cm− 1 for Y-O are observed. The peaks at 1590 and 3496 cm− 1 can be ascribed to the characteristic H-O-H bending and O-H stretching vibrations individually [Wongli et al. 2017]. For BiYO3/g-C3N4 composites, the increasing amount of BiYO3 seems to slightly influence the peak intensity of the g-C3N4 as observed in the spectra.
Figure 1B depicts the crystalline pattern of the pristine g-C3N4, pristine BiYO3 and BiYO3/g-C3N4 nanocomposites. From the Fig. 1B, the XRD for bare g-C3N4 shows two diffraction peaks were observed at 2θ = 12.9o, 27.6o for (100) and (002) diffraction planes respectively, which are well consistent with the tetragonal phase of g-C3N4 [JCPDS No.87-1526] [Abdelhafeez et al. 2020]. For BiYO3, the diffraction peaks at 2θ = 29.4o, 31.8o, 45.7o, 52.3o, 55.6o, 58.1o and 66.9o corresponds to (111), (200), (220), (311), (222) and (400) diffraction planes respectively. These peaks seem to be consisted with the cubic phase structure of BiYO3 [JCPDS No.27-1047] [Qin et al. 2020]. Both the peaks of the BiYO3 and g-C3N4 are observed in the BiYO3/g-C3N4 nanocomposites. The peak intensity of the g-C3N4 at 2θ = 27.6o seemed to decrease with the increasing content of BiYO3. At the same time, the BiYO3 peaks at 2θ = 31.8o and 45.7o have shifted from its normal position along with a rise in the peak intensity of 55.6 o with the increasing amount of BiYO3 in the BiYO3/g-C3N4 composites. These results evidently supported the formation of BiYO3/g-C3N4 heterojunction.
The morphological structure and elemental composition of BiYO3, g-C3N4 and optimized CB-1 composite photocatalytic materials were investigated through TEM and EDS analysis. Figure 2a demonstrates pure BiYO3 exhibited nanorod like structure and accumulated 2D- nanosheet like structure for g-C3N4 as shown in Fig. 2(b). Further, the optimized CB-1 composite presents 2D- nanosheets of g-C3N4 wrapped with 1D-BiYO3 nanorods, thus revealing the closer surface interaction between BiYO3 and g-C3N4 (Fig. 2c). Figure 2(d) presents the HRTEM image of the optimized CB-1 composite and the lattice fringes with interplanar distances of 0.295 and 0.336 nm for (002) and (200) planes of pure g-C3N4 and BiYO3 with tetragonal and cubic phase structure. Furthermore, the SAED pattern of the optimized CB-1 composite photocatalyst is also shown in Fig. 2(e-g). Figure 3(a-b) displays the EDX spectra of the pure BiYO3 and g-C3N4 containing Bi, Y and O and C and N respectively. Figure 3(c) depicts the existence of Bi, Y, O, C and N elements observed in the EDX image of the optimised CB-1 composite, thus indicating the presence of no other impurities in the prepared BiYO3/g-C3N4 nanocomposite photocatalysts. In accordance with EDX, the Fig. 4 shows the elemental mapping images of Bi, Y, O, C and N elements.
The absorption capability of light and Eg values of the photocatalyst are significant factors to determine the photocatalytic degradation behaviour [27]. The optical absorption behaviour of the pure g-C3N4, pure BiYO3 and optimised CB-1 composite photocatalysts were characterised through UV-vis DRS and the results are exposed in Fig. 5(a). From Fig. 5(a), the pristine g-C3N4, pristine BiYO3 and CB-1 shows the absorption edges at 428, 487 and 465 nm respectively. In addition, the following Kubelka-Munk function is used to calculate the Eg values of photocatalysts [Bavani et al. 2021b],
here, Eg, A, h, v, and α are the band gap energy value, constant, Planck’s constant, light absorption frequency and absorption co-efficient respectively. Figure 5(b) displays the αhν vs. photon energy (hv) plot. Here the intercept on X-axis shows the corresponding Eg values. Figure 5(b), the pure g-C3N4, pure BiYO3 and CB-1 composite has Eg values 2.84, 2.71 and 2.81 eV respectively. The acquired results show that the CB-1 composite has lower Eg value than the pristine g-C3N4, owing to the addition of BiYO3 in the BiYO3/g-C3N4 composite.
Photoluminescence (PL) emission spectra of the prepared photocatalyst exposes the separation and transfer efficiency of the photo-produced e−-h+ pairs. In general, the smaller PL emission intensity reveals the lower recombination of charge carriers. As seen in Fig. 6, comparatively the BiYO3/g-C3N4 composite exhibits lower PL emission intensity than that of pure BiYO3, thus revealing the reduced recombination of e−-h+ pairs. Obviously, the addition of BiYO3 in BiYO3/g-C3N4 composite have considerably influenced the PL emission peak intensity. The separation and migration ability of the pristine g-C3N4, pristine BiYO3 and CB-1 composite photocatalysts were characterized by photoelectrochemical studies as presented in Fig. 7(a) and (b). From the Fig. 7(a), the transient photocurrent study indicates the higher photocurrent intensity of optimized CB-1 composite and has greater separation efficiency than the pure g-C3N4 and BiYO3. In electrochemical impedance spectroscopy (EIS) displayed in Fig. 7(b), the smaller semicircular radius of the Nyquist plot indicates the lower resistance. From Fig. 7(b), the optimized CB-1 composite possesses smaller arc radius than the bare g-C3N4 and BiYO3, which represents the lower charge transfer resistance. These outcomes show the creation of BiYO3/g-C3N4 heterojunction has feasibly enhanced the separation and transfer of e−-h+ pairs on the interface of the heterojunction.
3.2. Photocatalytic degradation studies
Photocatalytic degradation activity of a series of BiYO3/g-C3N4 composite photocatalysts were studied via the RhB degradation under VLI. Figure 8(a) displays the poor photocatalytic decomposition ability of the pristine BiYO3 and g-C3N4 when compared with BiYO3/g-C3N4 composite photocatalysts. Here, the photocatalytic degradation efficiency of the pure g-C3N4, BiYO3, CB-1, CB-2 and CB-3 are 49.3, 16 .2, 87.9, 46.7 and 33.5% respectively (Fig. 8(b)), towards the degradation of RhB in 60 min. Among the different ratio of BiYO3/g-C3N4 composites, the CB-1 shows greater photocatalytic efficiency than the other prepared photocatalysts, it is mainly accredited by the formation of BiYO3/g-C3N4 heterojunction. Here, photocatalytic degradation ability of the BiYO3/g-C3N4 composite seem to increase with the increasing content of BiYO3, but the excessive amount of BiYO3 possibly affected the formation of the heterojunction between the two semiconductors and also impeded the transfer of charges. In addition, the kinetic study of the prepared materials for the decomposition of RhB were analysed by the fitting with first-order kinetics equation [Palanisamy et al. 2021],
Where, k, t, C0 and C are the apparent rate constant (min− 1), irradiation time (min), concentration of dye at initial and different time intervals correspondingly. Figure 8(c) and 8(d) represents the k values of pure BiYO3, pure g-C3N4 and CB-1, CB-2 and CB-3 are 0.0021, 0.0125, 0.0319, 0.0096 and 0.00658 min− 1 respectively. The optimised CB-1 heterostructure shows 15 and 2 times greater rate constant value than pure BiYO3 and g-C3N4. The obtained outcomes reveal the introduction of the BiYO3 in the BiYO3/g-C3N4 composite has effectively increased the photocatalytic degradation efficiency. The effect of catalyst amount of the optimised CB-1 composites on the RhB degradation under VLI is shown in Fig. 9. The Fig. 9 depicts the effect of optimised CB-1 catalyst amount from 0.5-2.0 g/L on the degradation of RhB over composite photocatalyst under identical reaction conditions. The obtained results reveal the increasing trend of photocatalytic degradation while increasing the amount of catalyst from 0.5-1.0 g/L, but the later additions of catalyst exhibited reduced photocatalytic activity, attributing to the colloidal formation that tends to scatter the light on the surface of catalyst. Thus, the excessive amount of catalyst is found not to be suitable for the photocatalytic RhB degradation. At the same time, recyclability and stability of the photocatalysts are also the vital factors for practical applications. The recycling experiment was conducted by collecting the photocatalyst at the end of every cycle and several times washing it with water then dried at 80 0C for 6 h. Figure 10(a) displays the optimised CB-1 composite resuming its initial stability with negligible changes after three cycles. Further, to identify the major reactive species in the RhB degradation over CB-1 composite, radical scavenging experiments were preferred. Variety of trapping agents like AO, IPA and BQ were used to trap h+, •OH and O2•− radicals individually [Malathi et al. 2018; Priya et al. 2018; Priya et al. 2020b]. Figure 10(b) represents the BQ and IPA apparently displays the inhibitory effect in the RhB degradation under VLI. But the addition of AO showed no obvious effects on the rate of RhB degradation. These experimental results proved that the O2. •− and •OH are playing the vital roles in RhB degradation by the optimised CB-1 composite under VLI.
3.3. Photocatalytic degradation mechanism
Figure 11 represents the Z-scheme mechanism for the RhB degradation over BiYO3/g-C3N4 in the presence of VLI. Here, the values of conduction band (CB) and valence band (VB) potentials are calculated by using Butler-Gingley equations (3) and (4) respectively [Senthil et al. 2019b; Bavani et al. 2022],
ECB = EVB - ECB (4)
here, Eg, Ee, X, ECB and EVB are the band gap energy, energy of a free electron on the hydrogen scale (~ 4.5 eV), absolute electronegativity and conduction and valence band potentials of a semiconductor correspondingly. In the presence of VLI, both the BiYO3 and g-C3N4 are excited to produce the photo-generated e−/h+ pairs. Simultaneously, the electrons on the CB of BiYO3 are transferred to the VB of g-C3N4 and holes on the VB of the g-C3N4 are transferred into the CB of BiYO3. At the time the O2 reduces the e− in the g-C3N4 to generates O2.•−, due the CB potential of g-C3N4 is more negative than the standard reduction potential O2/O2.• − E˚ (0.33 eV vs. NHE). Similarly, the •OH is created on the oxidation of H2O, due to the more positive VB potential of BiYO3 than H2O/•OH (+ 2.40 eV vs. NHE). Therefore, both the photo reduction and oxidation results in the degradation of organic pollutant into harmless and biodegradable products. Successfully, the generated BiYO3/g-C3N4 Z-scheme heterojunction greatly improved the photocatalytic efficiency by effectively enhancing the charge separation and transfer efficiency and limits the reconnection of e−/h+ pairs. The following equations depicts the photocatalytic degradation of BiYO3/g-C3N4 Z-scheme heterojunction towards the RhB degradation under VLI.
BiYO3 + hv → BiYO3 (e) + BiYO3 (h+) (5)
g-C3N4 + hv → g-C3N4 (e-) + g-C3N4 (h+) (6)
O2 + g-C3N4 (e-) → g-C3N4 + O2•− (7)
H2O + BiYO3 (h+)→ BiYO3+•OH (8)
•OH and O2•− + RhB → degradation products (9)