TiO₂ has been widely used as a photocatalyst for water splitting due to its high activity, low cost, abundance, safety, and stability. However, further utilization of TiO₂ is limited to the wide band gap and fast electron recombination speed, so TiO₂ can only utilize 4% of the energy in the ultraviolet spectral region of the solar spectrum. Therefore, expanding the light absorption range of TiO₂ is of great importance.

Carbon is a suitable dopant with metallic conductivity and large electron storage capacity, and isolated C 2p states can excite high photocatalytic activity under visible light. 

In a study published in Appl. Surf. Sci., the research group led by Prof. LU Canzhong from Fujian Institute of Research on the Structure of Matter of the Chinese Academy of Sciences reported the visible light absorption of ultra-thin carbon-modified TiO₂ nanotube arrays (UTC-doped TiO₂@TNT).  

The researchers prepared TiO₂ nanotube arrays by electrochemical anodic oxidation method using a trace amount of ethylene glycol in the electrolyte as the carbon source. They prepared the ultra-thin carbon-modified TiO₂ nanotube arrays by an ultra-thin ethylene glycol adsorption and vacuum annealing treatment, which is simple and efficient and requires no additional carbon source.

The linear sweep voltammetry (LSV) curves showed that there is a peak photocurrent density (0.6 mA×cm^-2) with a max photoelectric-conversion efficiency (η=0.4%) for the UTC-doped TiO₂@TNT under visible light irradiation, which is significantly improved compared to ordinary TiO₂ materials in the visible region. 

The researchers also evaluated the light absorption range of UTC-doped TiO₂@TNT using an incident photo-to-current efficiency (IPCE) test. The results showed that UTC-doped TiO₂@TNT still has photocurrent at a wavelength of 600 nm, indicating that its photoabsorption range extends to 600 nm. 

Thanks to the induced doping energy level of ultra-thin carbon, the UTC-doped TiO₂@TNT photocatalyst can absorb both ultraviolet and visible light. Because carbon has metallic conductivity and a large electron storage capacity, carbon atoms can trap photogenerated electrons on the surface while enhancing the separation of photogenerated carriers. 

This study shows that introducing ultra-thin carbon can effectively reduce the band gap of TiO₂ and extend the TiO₂ light absorption to the visible region.