PROPIEDADES FÍSICO-QUÍMICAS DEL BISMUTO Y OXIDOS DE BISMUTO: SÍNTESIS, CARACTERIZACIÓN Y APLICACIONES

CLAUDIA MILENA BEDOYA HINCAPIÉ
Ing. Física, Universidad Nacional de Colombia - Sede Bogotá, cmbedoyahi@unal.edu.co

MANUEL JONATHAN PINZÓN CÁRDENAS
Ing. Mecatrónico, Universidad Nacional de Colombia - Sede Bogotá, mjpinzonc@unal.edu.co

JOSE EDGAR ALFONSO ORJUELA
PhD. Física, Universidad Nacional de Colombia - Sede Bogotá, jealfonsoo@unal.edu.co

ELISABETH RESTREPO PARRA ]]> PhD. Ingeniería, Universidad Nacional de Colombia - Sede Manizales, erestrepopa@unal.edu.co

JHON JAIRO OLAYA FLOREZ
PhD. Materiales, Universidad Nacional de Colombia - Sede Bogotá, jjolayaf@unal.edu.co

Received for review March 10^th, 2012, accepted June 29^th, 2012, final version July, 9^th, 2012

ABSTRACT: The physical-chemical properties of bismuth and its oxides have been studied over the last two decades. As a result of this research, the growth of these materials with different crystallographic structures, showing micro and nanometric dimensions has been achieved by using several techniques (cathodic pulverization, laser pulsed deposition and hydrothermal method, among others). The dimensions reached have enabled thin films, nanotubes, nanospheres and nanowires to be obtained, allowing the developing of devices such as heavy metal detectors, optical filters and magnetic field sensors. Due to the progress in these materials in recent years, in this work a bibliographical review was carried out of some techniques employed for processing bismuth based materials with nanometric dimension and some technological applications.

KEYWORDS: Bismuth, nanostructures, bismuth oxide, Aurivillius phases.

RESUMEN: Las propiedades físico-químicas del bismuto y sus óxidos han sido objeto de estudio en las dos últimas décadas. Como resultado de estas investigaciones, estos materiales se han logrado crecer en diferentes estructuras cristalográficas y con dimensiones micro y nanométricas, utilizando diferentes técnicas (pulverización catódica, depósito por laser pulsado, método hidrotérmico, etc). Las dimensiones alcanzadas han permitido obtener películas delgadas, nanotubos, nanoesferas y nanohilos, con los cuales se han desarrollado dispositivos entre los que se destacan detectores de metales pesados, filtros ópticos y sensores de campo magnético. Debido al auge que han tomado estos materiales en los últimos años, en este documento se realizó una revisión bibliográfica de algunas técnicas empleadas para procesar materiales basados en bismuto en forma nanométrica, y algunas de sus aplicaciones tecnológicas.

PALABRAS CLAVE: Bismuto, nanoestructuras, óxido de bismuto, fases Aurivillius.

1. INTRODUCTION

Nanomaterials (thin films, nanoparticles, nano-multilayers) have become important in the current development of material science [1-3]. One of the most used elements for these applications is bismuth. Bismuth is a semimetal with interesting electronic properties such as high carrier mobility, low effective mass, low carrier density, long mean free path; moreover, it is highly anisotropic in its Fermi level, presenting a high magnetoresistance [4]. These properties are due to the spatial arrangement of its atoms [5]. Some reports indicate that a transition from semimetal to semiconductor occurs for thin film thickness of approximately 28 nm [6]. This transition has been observed also in bismuth nanowires oriented in a specific direction (trigonal) for thicknesses close to 55 nm [7].

Also, nanostructured bismuth materials have generated a great deal of interest because of the quantum behavior produced by their size [8], making it very useful for applications such as magnetic field sensors, optical devices, thermoelectric coolers and power generators [4,8]. Because of its low toxicity, bismuth films have recently been used as electrodes in the detection of organic nitro compounds [9] and heavy metals [10], replacing mercury electrodes. Bismuth has also been used in cosmetics and pharmaceuticals in disinfectants, bacteriostatic agents and astringents as well as in the nuclear industry where it is used as a refrigerant [11].

2. BISMUTH-BASED SYSTEMS

The physicochemical properties of bismuth nanostructures differ from those presented in higher dimensions due to different structures such as wires and wells that show quantized states [12] and surface defects [13]. For instance, Bi in bulk has no thermoelectric properties due to electron and hole contributions canceling each other. However, quantum confinement effects have been demonstrated in nanowires with diameters below 50 nm, presenting thermoelectric properties with high efficiency for diameters close to 10 nm [14]. Some research groups have synthesized bismuth thin films [15], nanowires [16], nanotubes [17], nanoplates [18] and nanospheres [19] using different techniques, which will be presented.

2.1. Nanostructures
Bismuth nanoparticles have been widely exploited in heterogeneous catalysis, magnetic recording media, microelectronics, thermoelectric devices and lubrication [20,21].

Also, these hold outstanding tribological properties in presence of large charges [18]. A range of physicochemical methods recently applied to obtain nanoforms of bismuth have been studied, such as: cluster beam, laser ablation, pulsed laser vaporization, flame spraying, electron beam techniques, electron beam lithography, pulse electroplating techniques, including supercritical fluids and solution dispersion methods. The latter is a novel and simple method which has been used to obtain nanoparticles by using a suitable solvent [21]. Moreover, these structures have been synthesized using microwave irradiation for quality, simplicity, speed, low cost and the possibility to be processed at low temperatures [22]. Although this method is simple enough to use a domestic microwave oven and is most often performed in solution, the lack of control and understanding of microwave reaction variables in the process is a drawback to its widespread application [23]. Fig. 1 shows the transmission electron microscopy (TEM) image of a nanotube synthesized by this method in a vacuum

Figures 2 and 3 present patterns of X-ray Diffraction (XRD) peaks of Bi nanoparticles and Bi₂O₃ nanorods. Bi nanoparticles obtained by thermal plasma change to nanorods when these are treated thermally at 250° C in atmosphere of oxygen by using different time intervals (2 and 3 hours) [24]. Figure 4 shows TEM micrographs of these nanostructures.

The formation of bismuth nanospheres was obtained on silicon wafers by a hydrothermal method [25]. Similarly, by means of an ethanol-thermal method bismuth nanobelts have been obtained from bismuth hydroxide on Si at 180°C for different reaction times (Fig. 5). The electrical properties of these nanobelts were measured from I-V curves, different properties from bulk bismuth materials were found. This study established that Bi nanobelts can be used as sensitive magnetic sensors or electronic nanodevices.

On the other hand, the ultrafast optical response of glassy materials that contain metal nanoparticles has led to the fabrication of these materials doped with Au, Ag, Cu and Bi generally by melt quenching, sol-gel, sputtering, ion exchange, ion implantation and femtosecond laser irradiation Bismuth is very sensitive to preparation conditions. Growing this material on glass substrates is difficult; Geng Lin et al. in 2011 proposed a simple method to precipitate Bi nanoparticles on glass, using heat treatment processes and conventional melting [26].

2.2 Bismuth oxide (Bi₂O₃)
Bismuth oxide (Bi₂O₃), an important metal oxide semiconductor has been receiving considerable attention because it exhibits excellent optical and electrical properties such as wide band-gap, high refractive index, high dielectric permittivity and good photoconductivity [27]. Owing to these unique characteristics, bismuth oxides have been studied for use in various domains such as fuel cells, sensor technology, oxide varistors, ionic conductors [28,29], photoelectric materials, high temperature superconductors and functional ceramics [30]. Also it is an important component in the manufacturing of transparent ceramic glass, optical coatings [31], ceramics and catalysts [32]. Similarly, the industrial field of manufacturing technologies based on Bi₂O₃ optical fiber has advanced significantly as a result of a variety of high quality optical fiber production, such as bismuth oxide fiber doped with erbium (Er) and highly nonlinear bismuth oxide fibers[33].

In recent years, several successful strategies including precipitation method, microwave-assisted synthesis, electrodeposition, and chemical vapor deposition (CVD), have been reported to prepare Bi₂O₃ with various morphologies, such as quantum dots, nanoparticles, nanobelts, nanorods, nanotubes nanofibers and thin films [28].

2.2.1. Bi₂O₃ phases
Bi₂O₃ is a complex system with four main polymorphs: a (monoclinic), b (tetragonal), g (body-centered cubic) and d (face-centered cubic), each one with unique physical properties [28] (electrical, optical, photoelectrical, etc.). For example at 300 K, the band gap E_g of a -Bi₂O₃ is equal to 2.85 eV, while for b phase is 2.58 eV [36]. Properties of bismuth oxide, Bi₂O₃, have been studied in recent years, since its monoclinic structure is one of the most important materials for synthesizing a number of high-temperature superconductors and Bi-containing ferroelectric compounds [37]. Particularly, the high applicability of d phase is well known because of its high oxygen ionic conductivity [38]. This property is due to the quarter of the oxygen sites that are vacant in the fluorite-type lattice, according to Mairesse [39]; the electronic structure of Bi³⁺ is characterized by the presence of 6s2 pair electrons, leading to high polarisability of the cation lattice, oxide ion mobility and the ability of the Bi³⁺ to accommodate highly disordered surroundings.

2.2.2. Bi₂O₃ nanowires
Nanostructured materials, especially, semiconductor nanowires have attracted great attention recently becoming one of the most active research projects in the nanoscience area [40]. One of the most interesting structures is the nanowire of Bi₂O₃, because of its physical properties, such as, high refractive index (n_dBi2O3=2.9) [41], dielectric permittivity (e_r=190) and oxygen ionic conductivity (1.0 S/cm), as well as a remarkable photoconductivity and photoluminescence response [42]. Although some reports about synthesis and applications of Bi₂O₃ nanowires can be found, transition phases within polymorphisms is not studied enough. These studies are important for improving their applications in optical elements and storage devices, among others [43].

The phase transitions of a- and b- to d and d - to liquid phases for Bi₂O₃ nanowires were obtained recently by using the oxidative metal vapor transport deposition technique which exploits spatial-temporal separation of Bi evaporation, vapor transport, droplet formation, and oxidative nucleation and growth [44]. Results showed the size effect only for the d to liquid phase, decreasing its transition temperature. In Fig. 6 SEM images of a- and b synthesized polymorphisms are presented, showing that the a- Bi₂O₃ phase (Fig. 6 (a)) consists of uniform nanowires close to 100 nm in diameter and several hundreds of micrometers in length. On the other hand, as is shown in Fig. 5(b), b-Bi₂O₃ contains exclusively ultrathin nanowires about 10 nm in diameter and several mm in length [43].

Until now, bismuth oxide nanowires have been successfully synthesized by different techniques including metalorganic chemical vapor deposition-MOCVD, chemical methods, and oxidative metal vapor transport deposition. These techniques employ high temperatures and chemical reaction routes in solid state, which are energetically intensive and their integration into silicon-based microelectronic devices is difficult. Therefore, bismuth nanowires with a high crystalline quality have been prepared using a novel method of induced stress at low temperatures on silicon substrates and subsequently annealed at low temperature in an oxygen environment [28].

Also, oblique prim-like a-Bi₂O₃ phase was prepared by a simple one-step aqueous method at low temperature (50 °C) and ambient pressure. This method is suitable for industrial production because complex systems or conditions are not required [30]. Other geometries as Bi₂O₃ flower-shape has been successfully synthesized via a citric acid assisted hydrothermal process consisting of several nanosheets and nanoparticles [28]. These structures are illustrated in fig. 7.

2.2.3. Bismuth oxide Layers
Bismuth oxides have been considered as an interesting material because of their dielectric properties and have been used for applications such as optical coatings, metal-insulator-semiconductor capacitors and microwave-integrated circuits. There is also a general motivation to study bismuth oxides deposited as thin films, since bismuth is a common constituent of high-Tc superconductors such as Bi₂Sr₂CaCu₂O_8+x and ferroelectrics such as SrBi₂Ta₂O₉ and Bi₄Ti₃O₁₂ [45]. In addition, Aurivillius phases are known for exhibiting ferroelectric behavior, most of the current ferroelectric materials show bismuth layered structures useful for non volatile random access memories. Other Aurivillius phases also present several properties and applications in photoluminescence devices [46] and oxygen ionic conduction [47]. Recently, it was found that these structures are efficient photo catalysts for separating water and degrading organic compounds. For instance, Kim et al. discovered that PbBi₂Nb₂O₉ is a suitable photocatalyst for separating water into O₂ and H₂. Moreover, it is useful for degrading isotropic alcohol to CO₂ [48] under visible light and recently they have been used in applications like humidity sensors using compounds like Bi₂MO₆ (M=W, Mo) because of high sensitivity, fast response and good reproducibility properties [49]. These Aurivillius structures are well described by superposition of (B2O₂)²⁺ layers and (A_n+1BnO_3n+1)^2- blocks of perovskites [50], where A corresponds to mono- di- or trivalent elements with coordination number 12 (Na+, Sr²⁺, Ba²⁺, Pb²⁺, Ca²⁺, Bi³⁺); B is the transition cation suitable for an octahedron with lower size (Ti⁴⁺, V⁵⁺, Ta⁵⁺, Nb⁵⁺, W6+, etc.) and n is an integer number representing the number of BO₆ octahedron in the perovskite layer along the packing direction. Fig. 8 presents representative structures for BiWO₆ (n=1), SrBi₂Ta₂O₉ (SBT, n=2) and Bi₄Ti₃O₁₂ (BIT, n=3). The perovskite layer is considered to be responsible for the ferroelectric behavior due to the presence of transition metal cations g[51]. These compounds are also known as BLSF (Bismuth layered-structure ferroelectrics) and they are characterized by having a high Curie temperature [49] and a strong anisotropy in their dielectric and ferroelectric properties as dielectric constant, remanent polarization and coercive field related to the crystalline structure [52]. Their spontaneous polarization (PS) can be determined by n; if n is an even number, the material only exhibits Ps along the a axis by the symmetry and sliding planes perpendicular to the b and c axes. On the other hand, if n is odd, a lower Ps is produced along the c axis and a greater Ps occurs along a axis when the sliding planes are perpendicular [53]. The high transition temperature from a high symmetry phase (paraelectric state) to a low symmetry phase (ferroelectric state) encourages the use of these materials in the fabrication of capacitors, sensors, memory storage devices, optical displays and other electro-optical devices using condensers, transductors [54,55].

Several Aurivillius ceramic compounds have been synthesized and most of them have been examined for the substitution of Pb by Bi [56]. The study of these structures has been focused on the properties of n (n=2, 3, 4) oxides, with the compounds with n≥5 the least studied [52]. For applications in piezoelectricity and information storage, the effect of rare earth isovalent ions substitution as La³⁺, Nd³⁺, Sm³⁺, Pr³⁺ in Bi³⁺ perovskite block sites improves the remanent polarization Pr and the fatigue resistance. The inclusion of ions like Na+ or K+ produces charge compensation effects: oxygen vacancy formations, valence state modification or interstitial site formations that influence the dielectric constant, polarisability and electric conductivity. These studies have been recently applied to the influence Na in the Bi₄Ti₃O₁₂ structure obtained by MOCVD, presenting a decrease in the lattice parameter in comparison with Bi₄Ti₃O₁₂ and a charge compensation caused by changes in Bi valence state, from Bi³⁺ to Bi⁵⁺ around Na [57].

Only a few BLSF compounds exhibit relaxation ferroelectric properties as BaBi₂Nb₂O₉ (BBN), BaBi₂Ta₂O₉ (BBT) and K_0.5La_0.5Bi₂Ta₂O₉. This behavior is characterized by a wide dielectric peak which shows a strong dependence on the frequency, possibly due to the microscopic distortion in the macroscopic tetragonal lattice and the disorder in the cations position [59]. The growth of these layered compounds has been carried out by several techniques such as metalorganic chemical vapor deposition (MOCVD), solid state synthesis and chemical etching methods [53, 56-66]. New phases of these materials have been successfully explored and produced [67-71]. A simple method for Bi_3.64Mo_0.36O_6.55 (BMO) nanoparticles fabrication with microwave irradiation and a hybridization with carbon through the combination of hydrothermal and calcination processes have been proposed by Fang Duan et al. in 2011 [72].

The flexibility in the cations accommodation or a combination of them is a special characteristic of Aurivillius oxides. In this way, in these research works one or more perovskite blocks have been inserted to obtain highly complex structures synthesizing by means of the solid state reaction route [73]. Such a method has several drawbacks, mainly poor homogeneity and hence required additional heat treatment and repeated grinding to obtain a nearly pure phase. Generally, the solid state reactions are diffusion controlled, and can be speeded up by prior usage of very fine particles and homogenized ingredients [74].

Future applications in photoluminescence have been investigated in BLSFs compounds such as Bi₄Ti₃O₁₂, Bi₃TiNbO₉, SrBi₂Nb₂O₉, SrBi₂Ta₂O₉, CaBi₄Ti₄O₁₅ and Bi₂WO₆, doped with rare earth (Eu, Er, Tm, etc). Deng Peng et al. in 2012 synthesized CaBi₂Ta₂O₉ doped with several concentrations of Pr by means of the solid state reaction method. The photoluminescence investigated with a blue light of 450 nm was improved with the substitution of Ca by Sr [75]. On the other hand, the introduction of magnetic transition metallic cations into the perovskite layer in the Aurivillius phases has been received with great interest since a new material named multiferroionics including ferroelectric and ferromagnetic effects [51], which involve local spin ordering and distorted structures with respect to the charge center [75]. Multiferroionic research has been mainly focused on modeling systems, such as perovskites with chemical formula ABO₃ (ferrites-bismuth, magnetite-bismuth) and in Aurivillius structures [49]. Multiferroionic materials as BiFeO₃, Bi₅Ti₃FeO₁₅, Bi₆Ti₃Fe₂O₁₈, BiCrO₃ and BiMnO₃ can be employed in a wide range of applications, as in the spintronics for multiple state memories elements [76], magnetically modulated piezoelectric transductors and resonance devices [77].

Experimentally, the production of these compounds by the conventional solid state reaction method has been widely used, taking into account important aspects as the possible formation of other phases because of the presence of ions with different valences. The conventional solid state reaction method also presents other disadvantages such as low reactant mixtures of homogeneity and a low ionic diffusion. Moreover, bismuth oxides are volatile and the reactivity between Ti⁴⁺, or Nb⁵⁺ and Mn³⁺ in the solid state is different, producing compositional changes or defects in the Bi₂O₂ layers. Nevertheless, some Aurivillius phases containing Ti⁴⁺ or Nb⁵⁺ and Mn³⁺ produced by this method have been reported [78]. Compounds with highly distorted structures with meta-stable phases as BiMnO₃ have been synthesized by high pressure methods [79]. The n=4 Aurivillius phase, Bi₅Ti₃CrO₁₅, prepared through solid state reaction by A.T. Giddings et al. (2011) show that the ferroelectricity and paramagnetism with short range antiferromagnetic coupling of the magnetic moments, inhibit it the long range magnetic ordering, therefore, this multiferroic material, is not suitable to be used in new technological applications [80].

In the last years, non conventional methods such as molten salt flux technique have been successfully reported for producing Aurivillius phases reaching high homogeneity in the precursor mixture. The molten salt can act as an environment for facilitating the reactive dissolution in order to obtain a homogenous mixture due to the incorporation of the salt medium So products generally possess a good crystallinity and crystalline morphology [51].

3. CONCLUSIONS

In this review work, properties and production techniques of nanostructure materials based on Bi and Bi oxides have been presented and described. This review is carried out in order to give to the readers a wide outlook of these materials, identifying their properties and potential technological applications.

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