Synthesis of Nanostructured BaTaO₂N Thin Films as Photoanodes for Solar Water Splitting

Abstract

Synthesis of nanostructured BaTaO 2 N thin films on metallic Ta substrates, and their application as photoanodes for solardriven photoelectrochemical water oxidation were studied. Ba 5 Ta 4 O 15 nanosheets vertically grown on Ta substrates by a hydrothermal process were converted into perovskite BaTaO 2 N with a branching nanostructure by thermal nitridation under an ammonia gas flow. The crystal quality and photoelectrochemical properties of the BaTaO 2 N thin films were found to improve with increasing nitridation temperature up to 1000 °C. A Ta 4 N 5 interfacial layer was formed between the BaTaO 2 N thin film and the Ta substrate. Under simulated AM 1.5G light, the BaTaO 2 N electrode generated a photoanodic current, although it rapidly decreased due to photooxidative corrosion. The degradation of the BaTaO 2 N electrode could be alleviated by the deposition of a cobalt phosphate layer on its surface. The modified electrode maintained a photoanodic current of 0.75 mA cm −2 at 1.23 V versus the reversible hydrogen electrode with a Faradaic efficiency of almost unity.

Fabrication of BaTaO2N Thin Films by Interfacial Reactions of BaCO3/Ta3N5 Layers on a Ta Substrate and Resulting High Photoanode Efficiencies During Water Splitting

“…The synthesis of the Ta 2 N phase in the work reported herein is different from previous results in which nanostructured BaTaO 2 N thin films prepared by the thermal nitridation of Ba 5 Ta 4 O 15 nanosheets grown on Ta substrates produced Ta 4 N 5 as an interfacial phase between the BaTaO 2 N layer and the substrate . Specifically, Ta 2 N is a more N‐deficient tantalum nitride than Ta 4 N 5 .…”

“…Thus, the presence of this compound between the BaTaO 2 N and Ta layers did not obstruct electron transfer. Using CoO x as a cocatalyst gave a maximum photocurrent density of 4.6 mA cm −2 at 1.23 V (vs RHE), which was much higher than the value of 0.75 mA cm −2 previously obtained from nanostructured BaTaO 2 N thin films . This value is also slightly larger than the highest value of 4.2 mA cm −2 reported for photoanodes prepared by the particle transfer method using BaTaO 2 N powder .…”

“…As shown in Figure S3, Supporting Information, the smooth, uniform thin films were preserved without the generation of voids following heat treatment of the Ta 3 N 5 /TaN/Ta (without a BaCO 3 layer) in a N 2 atmosphere at 1223 K. These findings suggest that the appearance of an uneven texture and voids in the BaTaO 2 N layer was the result of the interfacial reaction between the BaCO 3 and Ta 3 N 5 . Void formation in the resulting layer was also reported in conjunction with the conversion of Ta to Ta 3 N 5 via a Ta 2 O 5 phase using NH 3 nitridation and in the preparation of nanostructured BaTaO 2 N thin films on Ta metal substrates . These voids are likely produced when the reactions evolve gases such as CO, CO 2 , and H 2 O while producing the (oxy)nitride thin films.…”

“…The direct growth of a BaTaO 2 N thin film on a metal substrate was achieved by preparing nanostructured BaTaO 2 N via NH 3 ‐based nitridation of Ba 5 Ta 4 O 15 nanosheets hydrothermally grown on a Ta plate. The photoanodic current generated by the CoO x ‐modified BaTaO 2 N/Ta was 0.75 mA cm −2 at 1.23 V (vs RHE), and the incident photon‐to‐current conversion efficiency (IPCE) was 8% at 400 nm under AM1.5G radiation . The efficiency of this specimen was approximately 82% lower than that obtained from BaTaO 2 N photoanodes prepared from powder by the particle transfer method.…”

Triggering efficient photocatalytic water oxidation reactions over BaNbO2N by incorporating Ca at B site

“…In particular, perovskite oxynitrides such as AM(O, N) 3 (A = La, Y, Ca, Sr, Ba; M = Ti, Nb, Ta) have shown to be visible light active photocatalysts for water splitting reactions . Nevertheless, previous investigations suggest that these compounds generally exhibit poor photocatalytic activities under ordinary conditions, being incommensurate to their strong visible light absorption . Further studies have pointed out that various types of defects generated during sample synthetic procedures such as (Ti 3+ , Nb 4+ and Ta 4+ etc.)…”