Highly efficient and stable g‑C3N4 decorated Ta3N5 nanotube on n-Si substrate for solar water oxidation

Abstract

Tantalum nitride (Ta3N5) nanotube (NT) is an up-and-coming visible-light-absorbing photoelectrode material for photoelectrochemical (PEC) solar water oxidation with a small band gap of nearly 2.09 eV and the ability to capture a large fraction of the solar band spectrum up to 600 nm to enhance the PEC performance due to the increased light driving and the total increase of surface region for the high light incorporation and photogenerated charge carrier collection. In here, we deposited Ta metal film onto the n-Si wafer substrate by magnetron sputtering technique and developed a Ta3N5 NT thin-film photoanode with a length of ~1.2 µm by fluorinated-based electrochemical anodization, followed by calcination system in a N2/NH3 mixture gas flow condition. In our experimental results, the film calcined at 700 °C for 7 h showed high visible range of light absorption, well-ordered crystallinity, and a high conductive intermediate layer between the substrate, resulting in an optimized photocurrent density (J) value of ~0.27 mA/cm2 at 1.23 VRHE in photoelectrochemical water oxidation. However, this photoelectrode suffered from a special self-oxidation problem under PEC working conditions. Thus, to improve the photostability and PEC activity of Ta3N5 NT, a thin layer of g-C3N4 with band gap energy of about 2.79 eV was fabricated by an electrophoretic deposition (EPD) method under fixed voltage of (20 V) for 30 sec, 1, 2, 3 and 4 min. The g-C3N4 layer usually possesses several unique properties such as thermal stability, robust deposition process, reasonable band position, and an ability to absorb more visible light region. At an increased the deposition time of 2 min, the g-C3N4 thin layer resolutely coated the surface of Ta3N5 NT and exhibited the maximum J value of ~0.59 mA/cm2 at 1.23 VRHE under solar light irradiation. The modified g-C3N4/Ta3N5NT/Si photoelectrode showed excellent photostability and PEC performance. In the absence of any specific literature report on the preparation and analysis of g-C3N4/Ta3N5NT/Si composites for PEC solar water oxidation, we expect our study to motivate further research to promote the next age generation of g-C3N4/Ta3N5NT-based photoelectrodes with highly efficient PEC cells.