Tuning the properties of visible-light-responsive tantalum (oxy)nitride photocatalysts by non-stoichiometric compositions: a first-principles viewpoint

Abstract

Finding an ideal photocatalyst for achieving efficient overall water splitting still remains a great challenge. By applying accurate first-principles quantum calculations based on DFT with the screened non-local hybrid HSE06 functional, we bring rational insights at the atomic level into the influence of non-stoichiometric compositions on essential properties of tantalum (oxy)nitride compounds as visible-light-responsive photocatalysts for water splitting. Indeed, recent experiments show that such non-stoichiometry is inherent to the nitridation methods of tantalum oxide with unavoidable oxygen impurities. We considered here O-enriched Ta3N5 and N-enriched TaON materials. Although their structural parameters are found to be very similar to those of pure compounds and in good agreement with available experimental studies, their photocatalytic features for visible-light-driven overall water splitting reactions show different behaviors. Further partial nitration of TaON leads to a narrowed band gap, but partially oxidizing Ta3N5 causes only subtle changes in the gap. The main influence, however, is on the band edge positions relative to water redox potentials. The pure Ta3N5 is predicted to be a good candidate only for H(+) reduction and H2 evolution, while the pure TaON is predicted to be a good candidate for water oxidation and O2 evolution. Non-stoichiometry has here a positive influence, since partially oxidized tantalum nitride, Ta(3-x)N(5-5x)O5x (for x≥ 0.16) i.e. with a composition in between TaON and Ta3N5, reveals suitable band edge positions that correctly bracket the water redox potentials for visible-light-driven overall water splitting reactions. Among the various explored Ta(3-x)N(5-5x)O5x structures, a strong stabilization is obtained for the configuration displaying a strong interaction between the O-impurities and the created Ta-vacancies. In the lowest-energy structure, each created Ta-vacancy is surrounded by five O-impurity species substituting the five N sites characterizing one octahedral environment.

Enhancing Charge Carrier Lifetime in Metal Oxide Photoelectrodes through Mild Hydrogen Treatment

“…This method has been successfully applied to investigate the thermodynamic stability of various defective (photo-)catalytic materials, and was shown to be well in-line with experimental observations. [50][51][52][53][54][55] found to have relatively low formation energies (less than 100 meV). Upon insertion of hydrogen, the formation energies of both point defects were found to increase, indicating a smaller likelihood of their formation.…”

“…[ [50][51][52][74][75][76][77][78][79][80] However, our main focus on this study was to show the trend rather (relative difference) rather than giving absolute numbers. A systematic computational investigation of the impact of relevant self-intrinsic defects on the optoelectronic properties of BiVO4 at the HSE06 level of theory is indeed under progress and will be reported soon.…”

Establishing Efficient Cobalt-Based Catalytic Sites for Oxygen Evolution on a Ta3N5 Photocatalyst

“…24,44 In a typical experiment, a total of 0.5 g Ta 2 O 5 was wrapped with quartz wool and placed into a tube furnace (ID 26 mm) to provide homogeneous nitirided product. The tube furnace was initially purged with N 2 before the introduction of NH 3 flow at room temperature.…”

Synthesis and characterization of metastable transition metal oxides and oxide nitrides

“…Respecting the experimental accuracy of 2 % this is in an acceptable accordance with the expected ideal composition TaON. It should be mentioned that investigations regarding non-stoichiometric phases of β-TaON and Ta 3 N 5 have been published recently [55]. From these results a deviation from the ideal anion composition should not be completely ruled out.…”