Oxynitrogenography: Controlled Synthesis of Single-Phase Tantalum Oxynitride Photoabsorbers

Abstract

We propose an in-situ UV-vis monitoring technique called 'oxynitrogenography' as an approach towards the controlled and reproducible synthesis of thin films of different Ta-O-N phases, including the elusive β-TaON phase. The optical absorption changes are measured during annealing of the film at increasing NH 3 /H 2 O ratios, and can be directly correlated to the presence of different phases (Ta 2 O 5 , β-TaON, mixed TaON-Ta 3 N 5 , Ta 3 N 5 ) due to the abrupt change in the absorption edge. After additional XRD analysis, the thermodynamic equilibrium conditions to obtain these various phases are determined, and a phase diagram is constructed. We observe that there is a very narrow range of parameters for the thermodynamic stability of the β-TaON phase. We observe that the carrier mobility increases with the nitrogen content in the sample, from 10 -5 cm 2 /Vs in Ta 2 O 5 , to 10 -2 cm 2 /Vs in β-TaON and the mixed TaON-Ta 3 N 5 , until 10 -1 cm 2 /Vs in Ta 3 N 5 . While the carrier mobility of β-TaON and Ta 3 N 5 is comparable to that of BiVO 4 , the lifetime in the order of milliseconds is comparable to that of crystalline silicon. This is much higher than previously reported and compares favorably with the currently most promising metal oxide-based semiconductors (BiVO 4 , Fe 2 O 3 , WO 3 , Cu 2 O) for photo-electrochemical (PEC) water splitting. Although these long lifetimes may be partly caused by (de-)trapping from shallow trap states, these results clearly demonstrate that a high phase purity is an essential prerequisite for efficient (oxy)nitride-based absorber materials.

Role of Oxygen Defects on the Photocatalytic Properties of Mg‐Doped Mesoporous Ta3N5

“…There also appears to be a shoulder at a higher binding energy (534.3 eV), probably owing to the carboxylic groups in the contaminant . For the N 1s state, however, overlapping between the N 1s and Ta 4p 3/2 peaks always occurs, which is frequently seen in the N 1s spectrum of Ta 3 N 5 that is free of surface oxynitride species . Notably, there is a slight shift (≈0.6 eV) of the Ta 4d peaks towards a higher binding energy after doping with Mg (Figure d), indicating a modified local chemical environment for Ta that arises from substitution of N 3− with O 2− .…”

Formation and suppression of defects during heat treatment of BiVO4 photoanodes for solar water splitting

“…High temperature in situ UV-Vis spectroscopy measurements were performed by placing the BiVO 4 lms on a sample holder in a custom quartz tube equipped with a at quartz window, as described previously. 41,42 This tube was placed in an oven and annealed in air up to 700 C with a heating ramp of 7 C min À1 . The transmission spectra were measured with a high sensitivity CCD spectrometer (Maya2000 Pro, Ocean Optics) in a wavelength range from 300 to 850 nm, and a deuterium-halogen lamp (DH-BAL-2000, Ocean Optics) as the light source.…”

Highly Active GaN‐Stabilized Ta3N5 Thin‐Film Photoanode for Solar Water Oxidation

“…Second, the conduction band of GaN is more negative than those of Ta 3 N 5 , BaTaO 2 N, LaTiO 2 N and SrNbO 2 N, which tends to mitigate the migration of photogenerated electrons to the GaN on a thermodynamic basis, thus reducing charge recombination . Third, GaN has a long hole lifetime of 10–500 ns that is superior to the lifetimes of (oxy)nitrides, which are generally below 10 ns . Reduced charge recombination and improved hole mobility in GaN are critical semiconducting properties that allow the development of efficient (oxy)nitride photoanodes.…”