Trapping of different stages of BaTiO3 reduction with LiH.

Hua Guo,Aleksander Jaworski,Reji Nedumkandathil,Zoltan Bacsik,Adam Slabon,Ulrich Häussermann,Zili Ma

doi:10.1039/d0ra07276a

Abstract

We investigated the hydride reduction of tetragonal BaTiO3 using LiH. The reactions employed molar H : BaTiO3 ratios of 1.2, 3, and 10 and variable temperatures up to 700 °C. The air-stable reduced products were characterized by powder X-ray diffraction (PXRD), scanning electron microscopy, thermogravimetric analysis (TGA), X-ray fluorescence (XRF), and 1H magic-angle spinning (MAS) NMR spectroscopy. Effective reduction, as indicated by the formation of dark blue to black colored, cubic-phased, products was observed at temperatures as low as 300 °C. The product obtained at 300 °C corresponded to oxyhydride BaTiO∼2.9H∼0.1, whereas reduction at higher temperatures resulted in simultaneous O defect formation, BaTiO2.9−xH0.1□x, and eventually – at temperatures above 450 °C – to samples void of hydridic H. Concomitantly, the particles of samples reduced at high temperatures (500–600 °C) display substantial surface alteration, which is interpreted as the formation of a TiOx(OH)y shell, and sintering. Diffuse reflectance UV-VIS spectroscopy shows broad absorption in the VIS-NIR region, which is indicative of the presence of n-type free charge carriers. The size of the intrinsic band gap (∼3.2 eV) appears only slightly altered. Mott–Schottky measurements confirm the n-type conductivity and reveal shifts of the conduction band edge in the LiH reduced samples. Thus LiH appears as a versatile reagent to produce various distinct forms of reduced BaTiO3 with tailored electronic properties.

Highlights

The discovery that the archetypical perovskite BaTiO3 can be converted to BaTiO3ÀxHx by hydride reduction sparked enormous interest.[1,2,3] The oxyhydride BaTiO3ÀxHx attains a cubic structure in which O2À and HÀ ions commonly – and as in a solid-solution – form an octahedral environment around Ti that is in a mixed IV/III oxidation state (Fig. 1).[1]
These observations led to the conclusion that the hydride species in BaTiO3ÀxHx is labile and that the material represents a versatile precursor toward new mixed-anion compounds.[6]
What mechanism could apply for BaTiO3 reduction? We recently showed that the reduction of BaTiO3 can be achieved with a larger range of metal hydrides.[13]

Summary

Introduction

The discovery that the archetypical perovskite BaTiO3 can be converted to BaTiO3ÀxHx (with H-contents up to x z 0.6) by hydride reduction sparked enormous interest.[1,2,3] The oxyhydride BaTiO3ÀxHx attains a cubic structure in which O2À and HÀ ions commonly – and as in a solid-solution – form an octahedral environment around Ti that is in a mixed IV/III oxidation state (Fig. 1).[1]. In inert gas atmospheres containing D2 a hydride exchange H/D occurs at hydrogen release temperatures.[4] oxynitrides BaTiO3ÀxNy may be prepared by heating BaTiO3ÀxHx under N2 ow at 400– 600 C.5. These observations led to the conclusion that the hydride species in BaTiO3ÀxHx is labile and that the material represents a versatile precursor toward new mixed-anion compounds.[6] Lately it has been shown that BaTiO3ÀxHx shows a remarkable activity as heterogeneous catalyst for ammonia synthesis, possibly following the Mars–van Krevelen mechanism.[7]

Methods

Results

Conclusion