Distinct photocatalytic performance was observed when Ta 3 N 5 was synthesized from commercially available Ta 2 O 5 or from Ta 2 O 5 prepared from TaCl 5 via the sol−gel route. With respect to photocatalytic O 2 evolution with Ag + as a sacrificial reagent, the Ta 3 N 5 produced from commercial Ta 2 O 5 exhibited higher activity than the Ta 3 N 5 produced via the sol−gel route. When the Ta 3 N 5 photocatalysts were decorated with Pt nanoparticles in a similar manner, the Ta 3 N 5 from the sol−gel route exhibited higher photocatalytic hydrogen evolution activity from a 10% aqueous methanol solution than Ta 3 N 5 prepared from commercial Ta 2 O 5 where no hydrogen can be detected. Detailed surface and bulk characterizations were conducted to obtain fundamental insight into the resulting photocatalytic activities. The characterization techniques, including XRD, elemental analysis, Raman spectroscopy, UV−vis spectroscopy, and surface-area measurements, revealed only negligible differences between these two photocatalysts. Our thorough characterization of the surface properties demonstrated that the very thin outermost layer of Ta 3 N 5 , with a thickness of a few nanometers, consists of either the reduced state of tantalum (TaN) or an amorphous phase. The extent of this surface layer was likely dependent on the nature of precursor oxide surfaces. DFT calculations based on partially oxidized Ta 3 N 4.83 O 0.17 and N deficient Ta 3 N 4.83 consisting of reduced Ta species well described the optoelectrochemical properties obtained from the experiments. Electrochemical and Mott−Schottky analyses demonstrated that the surface layer drastically affects the energetic picture at the semiconductor−electrolyte interface, which can consequently affect the photocatalytic performance. Chemical etching of the surface of Ta 3 N 5 particles to remove this surface layer unites the photocatalytic properties with the photocatalytic performance of these two materials. Mott−Schottky plots of these chemically etched Ta 3 N 5 materials exhibited similar characteristics. This result suggests that the surface layer (1−2 nm) determines the electrochemical interface, which explains the different photocatalytic performances of these two materials.

The present work compares the oxygen evolution reaction (OER) in electrocatalysis and photocatalysis in aqueous solutions using nanostructured NiFeO x as catalysts. The impacts of pH and reaction temperature on the electrocatalytic and photocatalytic OER kinetics were investigated. For electrocatalysis, a NiFeO x catalyst was hydrothermally decorated on Ni foam. In 1 M KOH solution, the NiFeO x electrocatalyst achieved 10 mA cm −2 at an overpotential of 260 mV. The same catalyst was decorated on the surface of Ta 3 N 5 photocatalyst powder. The reaction was conducted in the presence of 0.1 M Na 2 S 2 O 8 as a strong electron scavenger, thus likely leading to the OER being kinetically relevant. When compared with the bare Ta 3 N 5 , NiFeO x /Ta 3 N 5 demonstrated a 5-fold improvement in photocatalytic activity in the OER under visible light irradiation, achieving a quantum efficiency of 24% at 480 nm. Under the conditions investigated, a strong correlation between the electrocatalytic and photocatalytic performances was identified: an improvement in electrocatalysis corresponded with an improvement in photocatalysis without altering the identity of the materials. The rate change at different pH was likely associated with electrocatalytic kinetics that accordingly influenced the photocatalytic rates. The sensitivity of the reaction rates with respect to the reaction temperature resulted in an apparent activation energy of 25 kJ mol −1 in electrocatalysis, whereas the apparent activation energy in photocatalysis was 16 kJ mol −1 . The origin of the difference in these activation energy values is likely attributed to the possible effects of temperature on the individual thermodynamic and kinetic parameters of the reaction process. The work described herein demonstrates a method of "transferring the knowledge of electrocatalysis to photocatalysis" as a strong tool to rationally and quantitatively understand the complex reaction schemes involved in photocatalytic reactions.

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.

The physicochemical properties of a tantalum nitride (Ta3N5) photoanode were investigated in detail to understand the fundamental aspects associated with the photoelectrochemical (PEC) water oxidation. The Ta3N5 thin films were synthesized using DC magnetron sputtering followed by annealing in air and nitridation under ammonia (NH3). A polycrystalline structure with a dense morphology of the monoclinic Ta3N5 films was obtained. A relatively low absorption coefficient (10(4) to 10(5) cm(-1)) in the visible light range was measured for Ta3N5, consistent with the nature of the indirect band-gap. Ultra-fast spectroscopic measurements revealed that the Ta3N5 with different thicknesses films possess low transport properties and fast carrier recombination (<10 ps). These critical kinetic properties of Ta3N5 as a photoanode may necessitate high overpotentials to achieve appreciable photocurrents for water oxidation (onset ∼0.6 V vs. RHE).

a b s t r a c tPresented herein is a detailed discussion of the properties of the lattice dynamic and optoelectronic properties of tantalum(V) oxynitride (TaON) and tantalum(V) nitride (Ta 3 N 5 ), from experimental and theoretical standpoint. The active Raman and infra red (IR) frequencies of TaON and Ta 3 N 5 were measured using confocal Raman and Fourier Transform Infrared spectroscopies (FTIR) and calculated using the linear response method within the density functional perturbation theory (DFPT). The detailed study leads to an exhaustive description of the spectra, including the symmetry of the vibrational modes. Electronic structures of these materials were computed using DFT within the range-separated hybrid HSE06 exchange-correlation formalism. Electronic and ionic contributions to the dielectric constant tensors of these materials were obtained from DFPT within the linear response method using the PBE functional. Furthermore, effective mass of photogenerated holes and electrons at the band edges of these compounds were computed from the electronic band structure obtained at the DFT/HSE06 level of theory. The results suggest that anisotropic nature in TaON and Ta 3 N 5 is present in terms of dielectric constant and effective masses.

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Understanding photophysical and electrocatalytic processes during photocatalysis in a powder suspension system is crucial for developing efficient solar energy conversion systems. We report a substantial enhancement by a factor of 3 in photocatalytic efficiency for the oxygen evolution reaction (OER) by adding trace amounts (~0.05 wt%) of noble metals (Rh or Ru) to a 2 wt% cobalt oxide-modified Ta3N5 photocatalyst particulate. The optimized system exhibited high quantum efficiencies (QEs) of up to 28 and 8.4% at 500 and 600 nm in 0.1 M Na2S2O8 at pH 14. By isolating the electrochemical components to generate doped cobalt oxide electrodes, the electrocatalytic activity of cobalt oxide when doped with Ru or Rh was improved compared with cobalt oxide, as evidenced by the onset shift for electrochemical OER. Density functional theory (DFT) calculation shows that the effects of a second metal addition perturbs the electronic structure and redox properties in such a way that both hole transfer kinetics and electrocatalytic rates improve. Time resolved terahertz spectroscopy (TRTS) measurement provides evidence of long-lived electron populations (>1 ns; with mobilities μe ~0.1-3 cm 2 V -1 s -1 ), which are not perturbed by the addition of CoOx-related phases. Furthermore, we find that Ta3N5 phases alone suffer ultrafast hole trapping (within 10 ps); the CoOx and M-CoOx decorations most likely induce a kinetic competition between hole transfer toward the CoOx-related phases and trapping in the Ta3N5 phase, which is consistent with the improved OER rates. The present work not only provides a novel way to improve electrocatalytic and photocatalytic performance but also gives additional tools and insight to understand the characteristics of photocatalysts that can be used in a suspension system.

The cross-substitution effect of tantalum on the photocatalytic water oxidation activity of BaNbO2N crystals grown by a flux method was investigated.