Final Report Summary - BISNANO (Functionalities of Bismuth based nanostructures)
Executive Summary:
BisNano is a collaborative research project aiming the acquisition of fundamental knowledge on Bismuth-based nanostructures, as a new class of materials towards the development of value-added Bismuth-based products and devices. In the project, we have integrated sixteen complementary academic research groups from Mexico and Europe covering interdisciplinary fields, and three industrial partners.
In the thematic work-packages, research groups working on the synthesis of the nanostructured materials collaborate with others doing the physical-chemical materials characterization and the application development. A large variety of methods have been employed to synthesize over thirty Bi-based materials including thin films, nanoparticles, nano-alloys and nanocomposites.
BisNano is promoting the development of applications in a wide range of domains: development and characterization of Bismuth-based thermocouples and thermopiles, thin film strain sensors, anti-counterfeiting tags, anti-static and fire-retardant thermoplastics and elastomers, electrochemical sensors, thermo-optical devices, self-lubricant coatings. For some of the aimed applications the investigated materials turned out to offer a competitive advantage over commercialized products, in that they are cheaper, better or present a reduced impact on environment.
Prototypes at labscale were developed together with industrial project partners or with third parties, and may be industrially exploited by the creation of spin-offs, technology transfer or direct industrial use. Some industrial demonstrators were shown on international fairs and market studies are under way to estimate the real potential of some applications. A patent concerning a nanofilter system has been submitted.
The BisNano project comprises safety of life as a determinant factor in defining possible Bismuth-based materials for daily life applications. In this spirit, some of the nanomaterials have been evaluated by two partner laboratories: their effect on different cell types as a function of size distribution, concentration and time of exposure was investigated. Novel approaches were developed to identify and visualise metallic NPs in suspensions, but also inside cells, allowing to study their subcellular localisation, and uptake rates. Overall, in vitro and in vivo toxicological studies suggest that Bi-nanoparticles can be safely used in applications and products and do not pose a threat to the environment and human health and safety.
Dissemination of the results was actively performed, with the organization of two symposiums in international conferences, more than 22 publications in peer review journals and a large number of conference communications.
BisNano also promoted the formation of young scientists, with a significant part of PhDs and postdocs involved in the research activities. The project supported, through the collaboration of the partners, several long research stays of young scientists in partner laboratories, helping them to develop social skills by meeting other cultures.
Project Context and Objectives:
BisNano is a medium scale focused research project that started in 2010 following a EU-MEX call upon “adding value at mining at the nanostructure level”. The general objective of BisNano is the acquisition of fundamental knowledge on Bismuth-based nanostructures, as a new class of materials towards the development of value-added Bismuth-based products and devices.
Besides this general aim, specific objectives were formulated:
Development and production of different bismuth-based nanostructures with good control over the size and composition: nanoparticles (NPs), thin films (TFs), nanoceramics and nanocomposites.
Evaluate the safety of the nano-based products at the earliest stage of production in order to predict the potential impact of engineered nanomaterials on the environment (life cycle analysis) and human health.
Determine the size-dependence of the physical properties (optical, mechanical, thermal, magnetic, electric and conductivity) for the assortment of bismuth-based nanomaterials.
Evaluate the functionality of the nanomaterials for a variety of applications; Pb-free piezoelectrics, ferroelectrics, metal detectors, sensors and actuators, ionic conductors for micro solid state fuel cells, self-lubricant coatings and Pb-free lubricants.
Design functional devices based on the properties previously determined and explore their commercialization by the companies involved in the project.
Establish a world leadership concerning the knowledge of bismuth-based materials in Europe and Mexico through the collaborative effort that includes dissemination activities and formation of master’s and PhD students.
The choice of Bismuth was motivated by economical, environmental and scientific issues. Bismuth is a mining material produced in large quantities in Mexico and other Latin American countries. However, the material is usually sold as a raw material and in many cases it is not even extracted due to the small profit margin for the mining companies. One of the project's aim is to promote the use of Bismuth-based materials in a wider range of applications and moreover in high technological applications which may add value to the raw material.
Bismuth has been proposed as a green-metal to substitute lead and mercury. However, no special efforts have been made to investigate, test and confirm this hypothesis. In this project, different aspects of Pb or Hg substitution by Bi will be evaluated such as piezoelectrics devices and metal detectors. This subject is of great interest for the EC where the environmental policies clearly establish the limits for the use of certain toxic materials and the search for substitutes is under way. In Mexico, no such policies have been established yet. However, it will be an obligation for the participants to encourage the government offices to involve and define laws concerning this important issue. Moreover, if the substitute material can be produced from national raw materials and the know-how is available, then such substitution policies should be easy to establish. Therefore one important workpackage of the project was dedicated to toxicity research including the life cycle analysis of the bismuth-based materials.
Bismuth exhibits unusual electronic properties resulting from its highly anisotropic Fermi surface, low carrier concentration, small effective mass and long mean free path of the charge carriers. As a consequence, Bi nanostructures have been considered good candidates for nano-sized materials with novel physical, chemical and thermal properties. Quantum confinement and finite size effects strongly modify the properties of Bi nanostructures at sizes relative large compared to other metals. Academically, the uncommon but advantageous properties of bismuth and bismuth based compounds have been known for many years. However, these properties have not been completely exploited from a technological point of view and there are still many unexplored possibilities, moreover with the application of nanotechnology novel and unique uses can be expected. The objective of the present project is to examine whether these properties improve, degrade or remain essentially unchanged when such bismuth materials are synthesized at the nanometric scale.
In this project, we have integrated different research groups from Mexico and Europe with the purpose of collaboration to develop and optimize novel applications of bismuth. Within the project there is a homogenous mixture of groups working on the synthesis of the nanostructured materials, the physical chemical characterization of materials and the development of applications, as well as companies with the interest and capacity to assess, test and validate promising new technologies and solutions developed within the project.
Careful and detailed chemical and structural characterization is required to be able to correlate the synthesis parameters with the physical properties. Finally, the project specifically involves the physical evaluation of the theme materials focused on the particular properties required (optical, electrical, magnetic, ferroelectric, etc) for the proposed and possible applications. The time scale of the project is sufficient for the preparation of master’s and PhD students whom will be involved in a very academic-rich environment but will be also very close to industry through the collaboration with industries.
Project Results:
The activities in the BisNano project were divided among the 11 workpackages, starting from the synthesis, physico-chemical characterization, optimization and functional properties. The different deliverables; final reports from the specific workpackages include therefore the results corresponding to their particular objectives. In this final report the results are presented in terms of the bismuth based materials synthesized within BisNano and their particular functional properties. Only the most developed system will be included:
1. Bismuth Nanoparticles: Bi NPs were produced by both chemical and physical methods. One of the chemical methods produced a colloidal dispersion of Zero-valent Bismuth using sodium citrate as a surface modifier to avoid or minimize the agglomeration of nanoparticles, allowing a nice control of the particle size (4-5 nm) and purity, since no oxides were observed. Zero-valent Bismuth nanoparticles were also obtained by a novel synthetic route to prepare nanocrystalline materials, solvent-free, low cost, and that can produce large amounts of sample powders in a fairly short time, known as mechano-chemical synthesis. In this case ZV-Bi NPs were synthesized mixing the bismuth precursor salt BiCl3 with a reducing agent such a NaBH4 without the need of using either a solvent or inert atmosphere. Analysis of the HR-TEM images showed an average size of 9.3 (SD 1.3 nm).
For the physical methods; cathodic arc and plasma sources were tried. Cathodic arc synthesis lead to Bi NPs with no controllable size, therefore a filtering system to select different NPs size was implemented and a patent has been applied for such process. On the other hand, a novel plasma method was implemented (Toroidal Hollow cathode system) leading to interesting results. Bismuth NPs of regular and controlled size were obtained attached to a substrate. The size was controlled by the deposition conditions and range between 40-50 nm (See figure 1) to micron size particles.
Despite the successful synthesis of bismuth nanoparticles, not specific functional properties were measured, partially as a consequence of the difficulty on getting enough material. The industrial partner also tried a milling method to obtain nanometric particles in large quantities, but failed in terms of reducing the size, therefore only submicrometric particles (about 100 nm) were obtained.
2. Bismuth oxide nanoparticles: the synthesis of bismuth oxide nanoparticles was done by two routes; a chemical route which lead t stable colloidal solutions with Bi2O3 NPs presenting the alpha phase. The other method was a pyrolysis synthesis aimed to large production of Bismuth oxide nanoparticles. In this case, the minimum size that could be obtained was about 150 nm, but the advantage was the production of 4.5 Kg/h with 80% efficiency during the process. These Bi2O3 nanoparticles were used as fillers in PVC, presenting better antistatic properties than the traditional ones used in the industry with potential applications.
3. Bismuth ferrite nanoparticles: BiFeO3 nanoparticles were obtained by a chemical route. Two organic species (either tartaric acid or glycine) were tested to complex Bi3+ and Fe3+ cations in aqueous solution. The use of tartaric acid as ligand in the BiFeO3 synthesis produced nanopowders with crystallite size around 26 nm. The main byproduct formed is the metastable β-Bi2O3 phase, which could be efficiently removed with CH3COOH. Also, some traces of tiny Bi2Fe4O9 NPs of about 5 nm were found in the same sample. The thorough characterization of these BiFeO3 NPs led us to the following relevant findings: (i) the presence of both types of electronic transitions (direct and indirect, located in the visible region) and (ii) a remarkable selective enhancement in the intensity of the BiFeO3 A1 mode (216 cm−1), as a consequence of the resonance Raman effect. When glycine was used as ligand, a porous BiFeO3 network, composed of tightly assembled and sintered nanocrystallites of high purity was obtained.
4. Bismuth vanadate nanoparticles and thin films
The mechanochemical synthesis technique (ball milling) was used to obtain BiVO4 particles from high purity Bi2O3 and V2O5 starting materials and a theoretical study was done in order to understand the growth process. The BiVO4 powders were composed of nanoparticles with crystallite sizes less than 50 nm. Such material was used to produce sputtering targets and produce well-organized structures of BiVO4 (sphere - like, autumn leaves – like and beads in a row – like) thin films by RF - sputtering with particles in the size range of 20 – 100 nm. Different substrate temperatures (250 to 600oC at intervals of 50oC) were used for tailoring the properties (thickness, morphology, orientation of grains and band gap) of thin films. The most efficient deposition temperature was found to be 600oC having a thickness of 400 nm. Both the powders and the films were tested to decompose photo catalytically rhodamine using visible light. As shown in Figure 2, a 100% degradation was achieved in about 150 minutes using the powder and 200 minutes with the films. The Ball milled BiVO4 did also showed effectiveness for the removal of bacteria (E. Coli) under light irradiation.
5. BNT nanoceramics and Thin Films
The objective of producing such material is the search for lead-free piezoelectric ceramics. Currently, Pb(Zr,Ti)O3 and lead-based compounds constitute the best family of piezoelectric materials for integration in microelectronic devices. Pure BNT((Bi0.5Na0.5)TiO3) ceramics, as well as BNT with additions of SrTiO3, Mn and BaTiO3 were prepared. The crystal structures obtained were the following: BNT rhombohedral, BNBST tetragonal, and BNT-6BT rhombohedral and tetragonal (relaxor). This came out from the X-ray analysis and is in agreement with a (1-x)BNT-xBT phase diagram. However, the ferroelectric properties obtained were still inferior to those reported for the traditional PZT material. For the BNT - BT ceramic, the highest remnant polarization of 20.7 mC/cm2and a coercive field of 3.39 kV/mm was obtained for the sample sintered at 1200°C. These ceramics were used to produce targets for pulse laser deposition, since well crystalline samples with a preferred (100) orientation could present piezoelectric coefficients d33 around 500 pC.N−1. As commonly reported in the literature, the main drawbacks of (BNT) –based materials lie in: (i) their relatively high conductivity which is problematic for poling process and polarization switching, (ii) high leakage currents, and (iii) high dielectric losses. In order to reduce the conductivity, looking for solutions to those problems, two main approaches were tested
(i) The growth of superlattices combining Bi0.5Na0.5TiO3 (BNT) to BaTiO3 (BT).
(ii) The growth of doped BNT based thin films.
The supperlattice samples showed very low remanent polarization Pr values that could be explained as due to the presence of charged interface between the BNT and BT layers, or the presence of micro-polar domains. The main achievement of the superlattice approach was the high coercive field values (Ec ) which got closer to that of BNT bulk ceramics (i.e. 35 kV/cm). Such phenomenon can be ascribed to a lower oxygen vacancies content that will imply a reduced pinning effect of the domain walls and thus a better polarization reversal.
For the Sr doped BNT films, it has not been so easy to achieve the desired crystallinity. There is always a loss of stoichiometry between the target and the films that requires a careful compensation. The dielectrical and ferroelectric properties of the Mn doped BNT and BNT-BT thin films are under investigation. The objective is clearly to determine if the Mn doping can lead to a reduction of the dielectric losses.
On the other hand, a good epitaxial growth for the Mn doped BNT and BNT-BT thin films were obtained. However it is worth noting that the tetragonality of the films is probably very weak, since the bulk lattice parameter is very similar to the lattice parameter of the STO substrate. Therefore the piezoelectric properties of the films were weak.
6. Multiferroic thin films
Multiferroics, materials which combine in a single phase two or more of the ferroic properties such as ferromagnetism, ferroelectricity and ferroelasticity, have attracted much attention since, in addition to their fundamental interest, they offer a wide range of applications in the area of information storage, sensors and spintronics. Among the rare single phase multiferroic perovskite oxides, BiFeO3 and BiMnO3 are the most promising candidates for their potential technological applications and as a result, is the subject of intense research.
6.1 BiFeO3 (BFO) is probably the most studied multiferroic system because both polar and magnetic orders coexist at room temperature. BFO films were grown using different approaches, all of them based on pulsed laser deposition, which has shown to be the most effective deposition method. One simple approach based on deposition from a well-crystallize target was initially done, in parallel doping of the BFO using Pb 2+ and Ba2+ was tried on order to inhibit the cycloidal spin structure which limit the magnetization of BFO in the bulk phase. Finally, a novel PLD method combining two plasmas (Bi and Fe2O3) was tried looking for the possibility to obtain pure BFO samples without starting from an expensive BFO target.
The BFO films grown by PLD from a crystalline BFO target showed that the monocrystalline films could be grown epitaxially in the (110) crystallographic plane of appropriate substrates. However, piezoforce microscopy images showed that the surface of the films has domains predominantly in the out‐of-plane polarization, but also regions with small in‐plane components. Domain switching was possible using ±21 V and from the ferroelectric hysteresis loops it was found that the piezoelectric constant d33 for the BiFeO3 (110) film was d33 = 58 ± 1 pm/V. Moreover, the magnetic and magneto-transport properties of Si/SRO/BFO thin films were investigated as a function of the BFO layer thickness. At high BFO thickness the system was characterised by an almost antiferromagnetic BFO layer and no significant magnetic and/or magneto-resistance properties were observed at room temperature or low temperature. Conversely, when the BFO thickness was reduced, weak magnetic properties appeared that were attributed to canted spins. Further reduction of the BFO thickness indicated the appearance of a non-uniform structure, characterised by a BFO matrix in which γ-Fe2O3 platelets were dispersed. For the thinner films, the magnetic response was strongly influence by the magnetic properties of the substrate.
6.2 Doped-BFO films
After structural and chemical characterization of the films, the ferroelectric nature of the sample was studied. For this, piezoresponse force microscopy (PFM) was used which is an electrical configuration of the atomic force microscopy using a conductive tip. This mode is based on the detection of the electromecanic vibrations when an alternative voltage is applied between the tip and the bottom electrode. To fully investigate the polarization direction of each domain, the combined PFM detection in the vertical direction (OP) and in lateral direction (IP) was used. For both PFM modes, the two signals were recorded simultaneously: the amplitude and the phase. The PFM measurements were carried out with the cantilever of the microscope aligned to the substrate side along the [100] direction. Figure 3 displays topography, OP- phase and OP-amplitude images recorded on both Ba-BFO and Pb-BFO 50 nm thin films. Both samples exhibited an OP-amplitude different from zero. The combination of these two results indicates that the out-of-plane projection of the polarization has only one component along the [001] direction. Therefore, it can be concluded that Ba-BFO and Pb-BFO thin films are self-polarized in the direction normal to the surface i.e. in the [001] direction. This result implies that the monoclinic and rhombohedral structures, observed for pure BFO thin films were modified by the doping. The ferroelectric feature was also confirmed by local hysteresis loops measurements since the polarization direction was reversed by an external voltage.
6.3 Reactive crossed beam pulsed laser deposition (RCBPLD) of BFO thin film
The RCBPLD configuration eliminates the need for expensive crystalline BFO target but also allows the deposition of Bismuth ferrite oxide films presenting different composition from the stoichiometric BiFeO3. The Bi and Fe2O3 targets were placed perpendicularly to each other inside the vacuum chamber and were ablated simultaneously. The laser beam was divided into two beams of approximately the same intensity, and each beam was directed towards one of the targets. The substrates were placed in the zone where the expanding plasmas cross each other. However, two substrate orientations were tested: lateral and frontal. Lateral experiments correspond to the substrates facing the iron oxide target and frontal when the substrates were placed in front of the bismuth target. The composition of bismuth iron oxide thin films was changed using Bi plasmas with different densities; the higher the Bi plasma density, the higher the Bi content in the thin films.
The functional characterization of films produced under different deposition conditions was done using the atomic scale microscopies to analyze the ferro-piezoelectric domains (CR-PFM) and ferroelectric hysteresis loops. Analysis of structure and ferroelectric properties showed that the BFO films consisted of two crystalline phases, one ferroelectric BiFeO3 and the other piezoelectric Bi25FeO40. Correlation between structural and functional characterization was useful to determine the deposition conditions to obtain BiFeO3 as the dominant phase. The optical properties of these films were also studied by ellipsometric and UV-VIS spectroscopy, as shown in Figure 3, where the variation of the optical gap of the BixFeyOz films is presented for films with increasing Bi content.
6.4 BiMnO3 films
Concerning BiMnO3, this multiferroic compound is certainly the only single phase material which exhibits a true ferromagnetic ordering. However, BiMnO3 is metastable at room pressure and it is, hence, difficult to be synthesized. Thus, extreme conditions such as high pressures of at least 6 GPa and high temperatures around 1100K are required to obtain this compound in bulk form. One way to stabilize such metastable phase is to grow this compound in thin film form since high pressure requirement for the stabilization can be successfully replaced by the epitaxial strains imposed by the substrate. The goal was to grow thin films of pure BiMnO3 to determine if the mechanical and chemical strain imposed by the substrate as well as through the decrease of film thickness could influence the structural and the magnetoelectric properties. However, after trying different deposition conditions and target compositions, it was found that it is very difficult to form the pure BiMnO3 phase. Using a stoichiometric or a Bi-rich target: Bi1.1MnO3target it was evident that all samples were poor in Bi because the volatility of this element or contained parasitic phases such as Bi2Mn4O10.
7. Bismuth Thin Films
Studies about the synthesis and properties of Bismuth thin films is an old subject, however the deposition of high quality films is still a challenge. Bulk Bismuth was a key material for the observation of quantum size effects, since the combination of a low electron density a the Fermi level with the long mean free path, gives extraordinary conditions for confinement effect at relative large sizes (40 nm). Moreover, Bulk Bismuth presents other extraordinary properties such as semimetallic behavior, magnetoresistance, diamagnetism, high Seebeck coefficients, etc….
Bismuth has an A7 rhombohedral crystal structure and two atoms per unit cell. The Brillouin zone show a hole pocket and three electron pockets, which occupy only a few thousandth of the Brillouin zone. The electron pockets are not exactly ellipsoids, but are often approximated as such. They also have two of their principal axes slightly tilted from the crystal axes. Due to the complicated band structure, most of the transport properties and the Seebeck coefficient of bismuth are anisotropic. However, the effect of such anisotropy has usually not being considered to explain the discrepancies in the properties of Bi thin films measured by different groups or under different deposition conditions. This is even more relevant considering that Bi thin films might present a high degree of preferred orientation, as shown in extreme for evaporated Bi thin films, where the basal plane (hcp lattice) is parallel to the film plane and the XRD patterns show only strong (003), (006) and (009) reflections. In the BisNano project different groups worked on the deposition of Bi thin films aiming to understand the conditions that lead to such a strong preferred orientations and therefore to find conditions for the production of high quality Bi thin films. For such purpose, the structural variations of Bi thin films deposited by evaporation, sputtering (dc and rf) and pulse laser deposition methods were compared. The results indicate that basal orientation is obtained under low energetic deposition conditions, while very high energies lead to a pure (012) orientation and intermediate energies to polycrystalline samples, as shown in Figure 4.
Different studies were done concerning the properties of the Bi thin films, such as the optical, mechanical, magnetoresistance and functional properties. For the optical properties, the dielectric functions were estimated as a function of the film thickness and their response compared to other metals such Au and Ag, showing a completely different behavior at low energies. This non-Drude behavior occurred together with high 2 values especially in the near infrared, likely due to the excitation of interband transitions in the visible and near infrared ranges. The hardness was measured for thick films (2 micros) deposited on Si containing very low values similar to other soft metals, such as silver. It was also found that film adhesion to Si or stainless steel substrates was very difficult to attain. The magnetoresistance response showed dependence with the film thickness similar to other metals, not any extraordinary response. Nevertheless, three interesting functional properties were studied: Bi films as electrochemical sensors to detect heavy metals in water, Bi-metal thermopiles that could be used as thermoelectric sensors due to the very high sensitivity to small variations in the temperature and Bi films as a strain sensor. The optical properties could also be of interest to produce metamaterials according to theoretical calculations and first trials.
8. Bismuth Oxide Thin Films
Bismuth oxide (Bi2O3) is an interesting semiconductor, whose optical gap ranges between 2 – 3.96 eV depending on the crystallographic phase. It has been proposed as a candidate for a wide range of applications, such as gas sensor, optical coating, catalysis, electrochromic and photocatalysis. Nevertheless, their real use is less extensive, maybe due to the difficulty on forming a pure phase, which could be stable for technological applications. At least six polymorphs have been identified for the Bi2O3, the stable phases are: α (monoclinic) from RT to 730°C, and δ (face centered cubic) above 730°C up to melting temperature. Two intermediate metaestable polymorphs that appear during cooling down from the δ-phase: β (tetragonal) and γ (body centered cubic); the transformation into these phases occurs around 650 and 640°C, respectively, and it depends on the purity and texture of the samples. In the BisNano project, the growth of Bismuth oxide thin films was extensively done with a couple of purposes. Firstly, find the conditions where specific phases could be obtained, secondly to obtain the cubic-delta phase and study its thermal stability and finally to study the photocatalytical properties of the different Bi2O3 phases. For such films, different characterization techniques (XRD, Raman, XRR, FTIR, Ellipsometry) were used to study their properties (optical and electrical) and functional applications (Ionic conductor and photocatalysis).
8.1 Bismuth Oxide Growth: The bismuth oxide thin films were deposited by rf magnetron sputtering using a pure alpha-Bi2O3 target, by pulsed laser deposition and using spray pyrolisis. Under the two PVD methods, it was clear that without substrate heating, the stoichiometric oxide could not be produced, even though a Bi2O3 target was used. Without proper substrate heating, the films presented a mixture of Bi and Bismuth oxide substoichiometric phases. However, when the substrate temperature was increased at temperatures as low as 150 oC stoichiometric oxide films were obtained indicating that reduction of the oxide precursor occurred. The major diversity of phases could be obtained using RF sputtering, whilst by PLD or Spray Pyrolisis, the alpha or beta phases were obtained. The electrical and optical properties of the films presenting the different phases were studied as a function of the temperature. The main result was that the electrical properties of the films were those of an intrinsic semiconductor, highly insulating at low temperatures and increasing with the temperature following an Arrhenius behavior. The optical gap ranged between 1.7 to 2.2 eV depending on the predominat phase of the as-deposited films. However, annealing of the films, lead to an increase in the optical gap, clearly observed as an increased in the visible transparency of the films.
8.2 Delta Phase: Among the several electrolyte candidates, face-centered cubic (fcc) delta-Bi2O3 is recognized as one of the most important oxide ionic conductors, with oxide ionic conductivity of 1-1.5 S/cm at 730-830 oC, compared to 0.01 S/cm for optimally doped YSZ at 760 oC. The delta-Bi2O3 phase is one of the six polymorphs of Bi2O3 and exists only in temperatures ranging from 730 to 824 oC (Bi2O3 melting point). Therefore, a lot of effort has been done on the stabilization of the delta-Bi2O3 phase to lower temperatures, where it has been shown that by the addition of appropriate dopants, such as rare-earth oxides or transition metal oxides, the delta-phase can be stabilized and maintained even at room temperature. The deposition of delta-Bi2O3 thin films was done using MS but starting from a Bi2O3 target (alpha-phase) in a reactive O2:Ar (20:80) atmosphere. Similarly to previous reports, the conditions to obtain the delta-phase are highly constrained in terms of substrate temperature (120-150 oC) and power (100-150 W). However, when these films were heated after deposition, the delta phase was quickly changed. Therefore, it was necessary to improve the thermal stability of the films. This was achieved by borrowing the concepts of doping to stabilize the phase from the solid state solution methods. For this, Ta atoms were added during the deposition by attaching a pure Ta wire to the Bi2O3 target and making a co-deposition. The Ta-doped films presented the same structural delta-phase, but the structure could be maintained from RT to 500 oC after annealing in air. Unfourtanely, the impedance spectroscopy measurements indicated that the addition of Ta induced a decrease in the ionic conductivity of the films.
8.3 Photocatalysis
Photocatalysis experiments were performed using Methyl Orange (MO) dye in a concentration of 10-6 M. The films were immersed in 10 mL of the dye solution and were exposed to UV light (centered at 380 nm). The absorbance spectra of the dye solution were recorded each 30 minutes. A strong dependence on the acidity of the dye solution was observed, since the degradation at pH=6 was about 7% in 150 min of UV exposure; while the maximum degradation achieved was at pH=2, giving 96% of dye degradation. However, for the films deposited by spray pyrolisis, the acidity of the solution affected the film, causing detachment of the film from the substrate. At a pH=3 the films were still adhered to the substrate and the maximum degradation achieved was around 80% after 120 minutes of UV exposure, while for the sputtered films higher degradation was achieved. The difference might be related to the different crystallographic phase observed. Sputtered films having different phases were compared, and in such case it was clear that the maximum degradation was obtained for the films presenting the delta phase, confirming the previous assumption. The delta phase has a more disorder structure and therefore the absorption is larger.
9. Nanocomposite Thin Films: Different types of composite films were prepared where Bi or a bismuth based nanostructure was immersed in a matrix.
9.1 For mechanical-tribological applications, the hypothesis was that the addition of a soft metal, such as Bi into a hard coating could help the reduction of the coefficient of friction without affecting in large scale the hardness of the coating, leading to materials with low wear. A lot of efforts were done in this direction, but even from the synthesis, the results indicated that the addition of Bi was detrimental for the coating stability due to the formation of bismuth oxide phases. In the best cases, it was possible to produce hard coatings (CrN or NbN) containing very low Bi percentages and a 50-70% decrease in the coefficient of friction was observed. However, the coatings became rougher and the hardness was reduced. These results were interesting, but a lot of control during the film growth is required to assure a positive response. For the measurements of the coefficient of friction two Pin-on-Disk systems were designed and built. One of the Pin-on-Disk equipment was aimed to work under vacuum and the other at large loads.
9.2 Bismuth nanoparticles were also added into polymers by a combination of techniques, were the Bi NPs were produced by a physical method and the polymer film was deposited by plasma enhanced chemical vapor deposition on top of a Bi-NPs layer dispersed on a substrate. In this case, the aim was to use the Bi NPs to enhance the electrical conductivity of the polymer, looking for a transparent conductive coating. However, both conductivity and light absorption were increased. Co-deposition of Bi and a carbon-film was also tried, but film-substrate adhesion could not be improved.
9.4 A very interesting approach was the incorporation of Bi NPs into Al2O3 or SiO2 matrices by a layer by layer deposition using pulsed laser ablation. The light transmission of such systems showed a sharp increase upon melting of the Bi NPs and a very large hysteresis cycle was observed upon cooling. This makes this system very attractive for thermally-driven optical switching devices, taking advantage of the relatively low Bi melting point (544 K). Thin films with embedded NPs of different sizes were prepared, both multilayer and single layer films. All the prepared films showed the typical hysteresis cycle, however the temperature of the changes and therefore the width of the cycle, and the contrast depend on the average nanoparticles size.
9.5 The addition of BiOCl and Bi2O3 NPs (as discussed above) into polymers was also investigated to produce antistatic polymers with applications in sole shoes or plastic containers for the food or cosmetic industry. Similarly, some test were done for the BiOCl Nps added into industrial polymers as a fire retardant, the results were interesting since only a 5 at% induced an improvement, without observing a detriment in the mechanical properties of the polymers.
9.6 Bi NPs were also added into lubricants oils. The Bi NPs were produced by lase ablation directly into the oil, in such a way the concentration and size could be varied but only small quantities could be produced. In order to be able to measure the tribological properties of these Bi-loaded oils, a Four-Ball system was designed and built, which is an interesting Technological Development from BisNano, since the system fulfills the technological characteristics of an industrial equipment but a lab-scale size.
10. The objectives of Workpackage (WP) 4 in BisNano were to assess the potential hazards posed by bismuth-derived nanoparticles (Bi-NPs), developed by WP 3, to the environment and human health. This is a complex task that requires numerous step by step investigations, starting with the physico-chemical properties in relevant environmental and biological fluids. A thorough investigation of how Bi-NPs behave in biological fluids that are relevant to human exposure and for exposure to cells cultured in vitro was performed, but also in environmental fluids that are relevant for exposure to aquatic species. Novel approaches were developed to identify and visualise metallic NPs in suspensions, but also inside cells, allowing studying their subcellular localisation, and uptake rates. These techniques opened the way to fast and affordable visualisation and quantification of non-labelled NPs in complex organisms. By using a combination of several techniques, including fluorescence and electron microscopy we were able to correlate the subcellular localisation on Bi-NPs with observed biological effects; NPs were observed in the lysosomes that led to lysosomal acidification; BNT-BT NPs in particular were observed in the nucleus, which supported the observed genotoxicity. These effects were observed only at high doses of exposed NPs to cells cultured in vitro, which are potentially not representative of a real exposure scenario. Exposure of Bi-NPs to more complex organisms was also investigated, following OECD recommendations. Zebrafish was selected as a model of exposure to aquatic species while rodents were chosen as a mammalian complex model. Exposure to these model organisms showed that there was not acute toxicity, no increase in mortality or developmental defects were observed in zebrafish, and through histopathology examination of exposed rodents reported absence of tissue damage; NPs mainly accumulated in the kidneys of rodents and were expelled in the urine. Overall, although it is not a trivial task to compare the outcomes of in vitro and in vivo toxicological studies, all the data acquired in this project are supportive that Bi-NPs can be safely used in applications and products and do not pose a threat to the environment and human health and safety.
Potential Impact:
The larger impact of the BisNano Project was the establishment of an international group of researchers and PhD - MSc students who became experts on the synthesis and potential applications of Bismuth-based materials. The fact that new researchers (11 MSc and 1 BSc thesis) have been instructed in the subject gives us the confidence that the research will continue. One example of this follow-up is the Phocscleen project; three institutions from BisNano applied for an IRSES FP7 project to continue the research about the photocatalytic properties of the Bi2O3 thin films. There are PhD students who are still working on the subject (26 PhD students from the UNAM, UNAL, CINVESTAV, IZFP, CIO, ININ, UAM and THUU) therefore the academic dissemination activities of BisNano are ensured for the following years.
Since the BisNano objective was the acquisition of knowledge about the Bismuth nanostructures, then most of the exploitation activities are related to the publication of papers. There are 14 Peer reviewed papers already published, plus 6 peer reviewed papers published in the Online MRS Proceedings, 3 other papers in reviewed papers but of local distribution (Latin-American) and one book chapter. The MRS proceedings were edited by BisNano members after the organization of a symposium (Low Dimensional Bismuth-based materials) at the International Materials research Congress (IMRC 2012). The symposium was very successful with most of the contributions from BisNano members, but also invited speakers with expertise on Bismuth. In 2013, another symposium was organized “Adding Value to Mining Products through nanotechnology” in the International Conference on Surfaces, Materials and Vacuum held in Merida, México. This symposium was an opportunity to share experiences with the other three approved projects in the Mexico-Europe call. The total number of congress presentations concerning the BisNano results is about 120. At the moment there are 11 submitted papers and one book chapter, some of them have already been accepted but not published. The journals are ACS Nano, Advanced Materials research, Thin Solid Films, Ceramics International, Physica B, Applied Physics A, International Journal of Quantum Chemistry and the Journal of Physical Chemistry). Moreover, some partners have also expressed that they have papers in preparation which should be submitted by 2014.
One important impact of the project was related to the environmental impacts since bismuth has been proposed as a green-metal to substitute lead and mercury. The toxicity studies performed for five different bismuth compounds clearly indicated that such bismuth materials presented a lower toxicity compared to other materials of technological importance.
As results of the BisNano research there is already one application in the market. One of the partners negotiated a technological transfer agreement for the production of anti-counterfeiting tags based on a bismuth containing nano-ceramic. Other two industrial partners in BisNano have established interest in continuing the investigation to produce fire-retardant polymers. Since one of the companies just entered BisNano after midterm, the results were obtained at the end of the project, but they are confident of the potentiality of using Bismuth-based nanostructures to improve the fire resistant properties of specific polymers, meaning that halogen-free polymers could be produced. Similarly antistatic properties for shoe’s soles have been investigated and tested at industrial scale.
List of Websites:
www.bisnano.eu
