Films and crystalline powder of BiI₃ intercalated with ammonia

Abstract

Intercalation, i.e. the insertion of guest species in a crystalline layered structure, is an efficient route for generating new materials with novel properties. Thin films and crystalline powder of BiI₃ layered semiconductor were intercalated by exposure to ammonia vapors at room temperature. The intercalated compound was studied by thermo-gravimetric analysis, differential scanning calorimetry, X-ray diffraction, UV–vis optical absorption, FTIR spectroscopy and Raman scattering. After exposure of BiI₃ to ammonia the formation of a new phase, BiI₃(NH₃)_3.83, was evidenced by thermal analysis. The intercalation process leads to a blue shift of the BiI₃ optical absorption edge by 0.5 eV. The appearance of new Raman lines at 135 and 353 cm⁻¹ in the Raman spectrum of intercalated BiI₃ is considered as an evidence of the chemical interaction between the ammonia molecules and BiI₃ lattice.

Introduction

During the last years, the intercalation of organic/inorganic molecules in various layered materials such as graphite, clay minerals, metal dichalcogenides, layered double hydroxides, etc. has drawn the attention of the researchers at both fundamental and practical viewpoints. The intercalation process can modify the optical and electronic properties of the two components, the host layered material and the guest molecules; moreover, the properties of the intercalated material can be tuned by the proper choice of host–guest system. These features make possible to discuss the relationships between their structure, properties and applications. In this framework, intercalation compounds have been investigated for wider applications in areas such as: photoelectrochemistry, photodetection, photocatalysis, energy storage, nonlinear optics [1].

Due to their sandwiched layer structure, in which a plane of metal ions is surrounded by two planes of halogens ions, the crystalline layered semiconductors, as PbI₂, BiI₃ and CdI₂ are particularly interesting for intercalation with different molecules. The bonds within halogen–metal–halogen layer are strong while those between adjacent layers are weak, related to van der Waals type forces. As a result the insertion of guest molecules into the interlayer spaces is easy to achieve, leading to a change in many physical properties (optical, electrical, morphological, etc.) of the semiconductor layered crystal. In most cases, the works regarding the intercalation of layered metal iodides (PbI₂, BiI₃ and CdI₂) with different guest species were focused on PbI₂ as matrix [2], [3], [4], [5].

BiI₃, a layered semiconductor with the band gap of ∼2 eV, was found interesting especially as a material for nonlinear optics. The crystal structure of bismuth tri-iodide is rhombohedral with the c-axis perpendicular to the basal plane. Each bismuth ion is octahedrally coordinated with six iodine ions, and each structural layer consists in three I–Bi–I sheets. From a planar perspective along the c-axis, only two-thirds of the possible metal sites are occupied. Thus each anion is bound at two cations. According to the band structure calculations [6], the top of the valence band in bismuth tri-iodide is formed by an admixture of the 6s² electrons in Bi³⁺ with 5p⁶ electrons of I⁻. The lowest energy of the conduction band originates from the 6p state of Bi³⁺. Thus the optical properties in BiI₃ bulk crystal are described by the cationic exciton model based on the intra-cation transitions from 6s to 6p states in Bi³⁺ ion [7]. It has to be mentioned that in the case of BiI₃ intercalated with hydrazine a shift of the cationic absorption band to higher energies was observed [8]. The process of intercalation may be accompanied by charge transfer, from the intercalate species to the host lattice. The most papers taken into account the formation of the coordination complexes as result of the chemical interaction between BiI₃ and different organic molecules (tetramethylthiourea, pyridine, thiourea or triazole) [9], [10], [11], [12].

Therefore, the aim of this paper is to supply further information about the intercalation of BiI₃, using ammonia, an inorganic nitrogen-containing molecule, as guest molecules. For this purpose, different techniques as thermo-gravimetric analysis, differential scanning calorimetry, X-ray diffraction, UV–vis optical absorption, FTIR spectroscopy and Raman scattering were used. Another goal of this work was to find an answer to a question which frequently arises in the context of any intercalation process, i.e.: what is the adsorption type of the guest molecules into the host matrix, a physical or a chemical one?