Critical Role of the Semiconductor–Electrolyte Interface in Photocatalytic Performance for Water-Splitting Reactions Using Ta₃N₅ Particles

Abstract

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.

Enhanced Kinetics of Hole Transfer and Electrocatalysis during Photocatalytic Oxygen Evolution by Cocatalyst Tuning

“…After this long-term test, the action spectra were taken to calculate the QE; the results are shown in Figure 2 (and summarized in Table S1). The QE estimated between 400 and 680 nm correlates well with the estimated absorbance of the Ta3N5 samples (which agrees with previous reports (Figure S1)), [7][8][9][10][11][12][13][14][15][16][17][18] which is consistent with the bandgap excitation of Ta3N5. The QE was observed >10% for wavelengths ranging between 440 and 520 nm, with a maximum value of 30% at 480 nm.…”

“…[1][2][3][4][5][6] The main challenge lies in identifying materials suitable for efficient photocatalysis, i.e., with an optical response in the visible light region, and energy levels compatible with chemical reduction and/or oxidation. These properties converge in Ta3N5, [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] which can theoretically achieve a solar to hydrogen efficiency of approximately 17% using a single semiconductor; nevertheless, non-biased, overall efficient water splitting using this material has not been achieved. For efficient photocatalysis with the powder semiconductor, a solid-electrolyte interface is effectively utilized for charge separation, therein directly inducing surface chemical redox reactions.…”

Dendritic Tip-on Polytriazine-Based Carbon Nitride Photocatalyst with High Hydrogen Evolution Activity

“…[23][24][25][26][27][28][29][30] Density functional theory (DFT) is a useful and powerful technique to obtain the relevant photophysical and optoelectronic parameters of a given photocatalyst. 31,32 In addition to structure determination, DFT calculations can lead to the simulation of several semiconductor properties such as the bandgap, the dielectric constants and band positions, with good accuracy thanks to recently developed functionals such as HSE06. 33 Furthermore, the excitonbinding energy and effective masses of electrons and holes are very important parameters, accessible from calculations, in 2 order to qualify a semiconductor as good photocatalytic material.…”

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

“…24 A thin TaN layer on the surface (~ 2 nm), which formed depending on the synthesis method, was observed to change the energetic profile on the Ta 3 N 5 -electrolyte interface, thus changing the photocatalytic activity. 24 Hence, despite the kinetic hindrance governed by the intrinsic properties, research design involving perturbing the surface properties of Ta 3 N 5 is believed to further improve its photocatalytic activity. 24 Decreasing the bandgap of a photocatalyst will increase the number of potentially absorbable photons but, in turn, reduce the driving force for charge separation and the rate of subsequent redox reactions on the surface.…”

“…24 Hence, despite the kinetic hindrance governed by the intrinsic properties, research design involving perturbing the surface properties of Ta 3 N 5 is believed to further improve its photocatalytic activity. 24 Decreasing the bandgap of a photocatalyst will increase the number of potentially absorbable photons but, in turn, reduce the driving force for charge separation and the rate of subsequent redox reactions on the surface. [25][26] An upward band bending (in the case of an n-type semiconductor) promotes easier hole transfer to the surface and subsequently initiates the OER on the surface.…”