Graphitic carbon nitride (g-C₃N₄) is an interesting conjugated polymer, being metal-free, stable and active in the lower ranges of visible light with a bandgap of 2.7eV [1]. Titanium dioxide (TiO₂) has been researched for photoelectrolysis of water since the late 1960. It is one of the most researched wide-band gap semiconductors, which is only active in ultra-violet-light range [2]. In this work, we report the fabrication and photoelectrical properties of thin film g-C₃N₄ modified TiO₂ nanotube photoelectrodes.

Graphitic carbon nitride (g-C₃N₄) was produced with a thermal polycondensation of urea in nitrogen atmosphere for 2 hours at 550^oC, 575^oC and 600^oC in two different ovens. The samples were designated F550, F575 and F600 for the samples made in the MTI Corporation GSL-1100X Tube Furnace, and TC at the start for the Lenton WHT6/30 Thermal Chamber. To achieve an even film on the substrate, urea was placed in a quartz crucible for the tube furnace and covered with aluminium foil. Holes were cut in the foil and the substrate was placed on top, not covering the holes. More aluminium foil was used to seal the crucible. The substrate was TiO₂ nanotubes produced by anodization in ethylene glycol at 60V for 3 hours. Preliminary experiments have been carried out and shown enhancement of photocurrent response.

The samples were characterized with UV-vis spectroscopy, Scanning Electron Microscopy (SEM) imaging, Energy Dispersive X-Ray (EDX) and X-ray Diffraction (XRD). For the UV-vis reflectance spectroscopy, the tube furnace results are shown in fig. 1, where F550, F575 and F600 shows significantly increased absorption for wavelengths over 500nm. For F575, the pure TiO₂ and all the samples made in the thermal chamber, a clear absorption edge was present around 400nm, corresponding to the TiO₂ bandgap. F550 and F600 do not show the absorption edge, as they show an increased absorption all the way down to 350nm. It is also clear that the surface has been modified by looking at the samples, especially F600 which has gotten a very dark, almost black, colour, as shown in fig. 2.

The SEM image, fig. 3, shows the TiO₂ nanotubes with thin patches of g-C₃N₄ spread intermittently on top, and the EDX measurements give the variation in g-C₃N₄ density from sample to sample. F550 and F600 show an increased g-C₃N₄ (12-17% of C and N) content compared to all the other samples (around 1% C and none detected of N). XRD was done on the powders to verify that the substance was indeed g-C₃N₄.

In summary, depositing g-C₃N₄ on top of TiO₂ nanotubes can give a significant increase in absorption across the whole visible spectrum, and this probably comes from the increased g-C₃N₄ content seen in F550 and F600. Though even a small amount of g-C₃N₄ will increase the absorption in the 500-800nm range.

[1] W.-J. Ong, L.-L. Tan, Y. H. Ng, S.-T. Yong, and S.-P. Chai, “Graphitic Carbon Nitride (g-C₃N₄)-Based Photocatalysts for Artificial Photosynthesis and Environmental Remediation: Are We a Step Closer To Achieving Sustainability?,” Chem. Rev., vol. 116, no. 12, pp. 7159–7329, Jun. 2016.

[2] K. Hashimoto, H. Irie, and A. Fujishima, “TiO ₂ Photocatalysis: A Historical Overview and Future Prospects,” Jpn. J. Appl. Phys., vol. 44, no. 12, pp. 8269–8285, Dec. 2005.