Pristine Ta3N5 Nanotubes: Trap-Driven High External Biasing Perspective in Semiconductor/Electrolyte Interfaces

• Published in: Chemistry : a European journal Vol. 22; no. 51; pp. 18501 - 18511
• Main Authors: Khan, Sherdil, Santos, Marcos J. L., Malfatti, Célia F., Dupont, Jairton, Teixeira, Sérgio R.
• Format: Journal Article
• Language: English
• Published: Weinheim Blackwell Publishing Ltd 19-12-2016; Wiley Subscription Services, Inc
• Subjects: applied biasing; Chemistry; Electrolytes; nanotechnology; Oxidation; photoelectrochemistry; thermal nitridation; trapping states
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201603246

Ta3N5 is a promising photoelectrode for solar hydrogen production; however, to date pristine Ta3N5 electrodes without loading co‐catalysts have presented limited photoelectrochemical (PEC) performance. In particular, large external biasing has been required to run water oxidation, the origin of which is investigated herein. Ta3N5 nanotubes (NTs) prepared by nitridation were characterized by a wide range of techniques. The bandgap was confirmed by a novel PEC technique. Nondestructive synchrotron‐excited XPS has shown the presence of reduced Ta species deeper in the Ta3N5 surface. Lower photocurrent and transient spikes that were intense at lower applied biasing were observed under water oxidation; however, spikes were inhibited in the presence of a sacrificial agent and photocurrent was improved even at low biasing. It was observed for the first time that the lower PEC performance under water oxidation can be attributed to the presence of interband trapping states associated with pristine Ta3N5 NTs/electrolyte junction. These states correspond to the structural defects in Ta3N5, devastate PEC performance, and present the necessity to apply higher biasing. The key to circumvent them is to use a sacrificial agent in the electrolyte or to load a suitable co‐catalyst to avoid hole accumulation under water oxidation, thereby improving the phootocurrent. The findings on the interband states could also provide guidance for the investigation of PEC properties of new types of semiconducting devices.
Semiconductor–liquid interfaces: Under water‐oxidation conditions, the photogenerated holes are trapped in the interband states related to the crystalline defects in Ta3N5. However, in the case of a sacrificial reagent, these holes are efficiently scavenged. A key factor to improve water oxidation at lower biasing is the hole scavenging from the surface of pristine Ta3N5 nanotubes achieved by adding a suitable co‐catalyst on its surface (see figure).