A review on TiO2/g-C3N4

2.4. Line -to- face (1D-2D) Heterojunction
Depending on the geometry, 1D nanostructures are classified into four categories such as nanowires (NW), nanorods (NR), nanobelt (NB) and nanotubes (NT) [166]. NR are usually considered as NWs with a small aspect ratio and these are surfaced as straight and immutable [167]. Nanorod, nanobelt and nanotube shaped TiO₂ were coupled with g-C₃N₄ nanosheets to construct 1D-2D TGCN heterojunction [147,168,169]. Fig. 6 shows FESEM images of (a) TiO₂ nanotubes, (b) g-C₃N₄ nanosheets and (c) TiO₂ nanotube- g-C₃N₄ nanosheet composite [168]. Lu et al. synthesised TGCN composite in which TiO₂ nanorods (TNRs) of around 15 nm were anchored on g-C₃N₄ sheets of 200–300 nm length. The transfer of photoinduced charges from g-C₃N₄ sheets to TNRs through the interface restricted electron-hole pairs recombination that enhances the photo activity. Moreover, the improved absorption of RhB on g-C₃N₄ sheets followed by the synergistic effect between reduction of Cr (VI) and degradation of RhB contributed the significant increase in photocatalytic activity of g-C₃N₄/ TiO₂ composite [169]. Shen et al. constructed black TiO₂ nanobelts/g-C₃N₄ nanosheets heterojunction that had degraded about 95.1 % MO and generated 555.8 μ mol h⁻¹ g⁻¹ of H₂ under visible light irradiation in separate experiments. The elevated photoactivity may be attributed to the sufficient interfacial contact between TiO₂ nanobelts and g-C₃N₄ nanosheets that promoted the separation of charge carriers and hastened the electron transfer. The heterostructured photocatalyst also widens its absorption edge and extends the visible light range mainly due to the synergistic effect of Ti³⁺, Ov in TiO₂ and g-C₃N₄ [147]. The introduction of Ti³⁺ and Ov present at the bottom of the TiO₂ CB contracts the band gap of TiO₂ nanobelt due to which the optical absorption is improved [[170], [171], [172]]. Anjum and his co-workers observed that about 96.6 % 2-chlorophenol was degraded under visible light by g-C₃N₄/TiO₂ nanotube composite [173]. Jo et al. reported that isoniazid (an antibiotic) degradation efficiency was quite higher (90.8 %) in 4 h by the Z scheme g-C₃N₄/TiO₂ nanotube photocatalyst as compared to Z scheme g-C₃N₄/TiO₂ nanoparticles (79.5 %) during same time interval [168]. The high performance of former might be attributed to its enlarged surface area and tubular structure that increased the adsorption of pollutants and lengthened the lifespan of the photogenerated charge carriers [174,175].

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Fig. 6. Field emission-scanning electron microscopy (FESEM) images of (a) TiO₂ nanotubes, (b) g-C₃N₄ nanosheets and (c) g-C₃N₄ nanosheet- TiO₂ nanotube composite. [Reproduced from Ref. No. [168]].