2.3. Point -to- face (0D-2D) Heterojunction The combination of TiO2 nanoparticles with g-C3N4 nanosheets resulted in the formation of 2D/0D heterojunction in which 2D interface of g-C3N4 assisted the transfer of photogenerated electrons to TiO2 [152,153]. Yang et al. used bioadhesion and biomineralization mechanism to synthesise TGCN (CNNS/TiO2) heterojunction in which TiO2 nanoparticles with ∼5 nm size were uniformly anchored over the surface of g-C3N4 nanosheets (CNNS).The presence of TiO2 nanoparticles prevented the agglomeration of g-C3N4 nanosheets. Both TiO2 nanoparticles and g-C3N4 nanosheets in the heterostructure ensued in larger surface area (87.9 m2 g−1) which was extremely greater than that of TiO2 nanoparticles (8.4 m2 g−1) and CNNS (20.5 m2 g−1) [154]. According to Bonaccorso et al., high surface area provides copious active sites and lengthens light absorption range [155]. Li et al. reported that the formation of tough chemical bonds at the interfaces of TGCN heterojunction also extends the light absorption to longer wave length [156]. The red shift in absorbance empowers the heterostructure a robust visible light driven photocatalyst [157,158]. The photo oxidation and photo reduction processes are facilitated through the production of more number of electron–hole pairs owing to the strong absorption of visible light by the heterojunction comprising of g-C3N4 nanosheets and TiO2 nano particles [159]. UV–vis DRS results of Fig. 5(a) showed that the absorption maximum for g-C3N4 nanosheets (CNNS) and TiO2 nanoparticles were 476 nm and 414 nm respectively, whereas that for TGCN composite (CNNS/TiO2) lies in between these two [154]. Similar observation has also been reported [160]. As per Tong and his co-authors, the absorption intensity of TGCN heterojunction rises along with the increase of g-C3N4 content [161]. The band gap energies can be calculated from Kubelka-Munk equation presented in Eq. (4). By extrapolating the linear portion of the plot (ahv)n vs (hv) to the X-axis as presented in Fig. 5(b), Jiang group obtained the band gap potentials as 3.00, 2.69 and 2.60 eV for TiO2, CNNS/TiO2 heterojunction and CNNS respectively [154]. The decrease in band gap for the heterostructure in comparison to TiO2 may be attributed to the introduction of g-C3N4. These experimental evidences suggest that g-C3N4 component is responsible for the red shift in the optical absorption edge of the TGCN composite. The apparent quantum yield (AQY) calculations can be used to study the visible light responsiveness of the composite. AQY for production of H2 can be calculated as per Eq. (5) [162].(5)AQY%=2×NumberofevolvedH2moleculesNumberofincidentphotons×100%
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Fig. 5. (a) UV–vis DRS spectra for g-C3N4 nanosheets (CNNS), TGCN composite (CNNS/TiO2) and TiO2 nanoparticles, (b) The plots of (αhv)n versus hν for CNNS, CNNS/TiO2 and TiO2, (c) photoluminescence (PL) studies, (d) EIS Nyquist plots and (e) photocurrent responses of g-C3N4 nanosheets (CNNS), TGCN composite (CNNS/TiO2) and TiO2 nanoparticles [Reproduced from Ref. No. [154]].
Isimjan et al. reported that maximum rate of H2 production was obtained in Pd-TiO2/g-C3N4 heterojunction containing 20 % TiO2 with an optimum AQY of 31 % at 420–443 nm. H2 production rate was decreased by increasing the TiO2 content as it is poorly active in visible light excitation [163].
Apart from excellent visible light absorption, combination of TiO2 nanoparticles and g-C3N4 nanosheets endowed an extended charge transfer process, which was confirmed from photoluminescence (PL) studies, EIS Nyquist plots and photocurrent responses. The PL studies reveal the degree of recombination of charge carriers. Lower the degree of PL emission intensity, less is the possibility of recombination and greater is the charge separation probability [164]. The significant decrease in PL intensity for CNNS/TiO2 shown in Fig. 5(c), indicated minimal recombination of electron-hole pairs as compared to CNNS. The smallest arc radius of CNNS/TiO2 in EIS Nyquist plots of Fig. 5(d) suggested minimum resistance and extended transfer of charge carriers through the interface [154,165]. As shown in the Fig. 5(e), the substantially high photocurrent density displayed by CNNS/TiO2 represents greater interfacial separation of charge carriers [154]. In general, construction of 2D-0D TGCN heterojunction demonstrated good photocatalytic performance due to high surface area, extended visible light absorption and enlarged interfacial charge transfer process.
