Obtaining Highly Crystalline Barium Sulphate Nanoparticles via Chemical Precipitation and Quenching in Absence of Polymer Stabilizers

Abstract

Here we report the synthesis of barium sulphate (BaSO4) nanoparticles from Ba(OH)2/BaCl2 solutions by a combined method of precipitation and quenching in absence of polymer stabilizers. Transmission electron microscopy (HRTEM), Fourier transforms infrared spectroscopy (FTIR), and X-ray diffraction (XRD) were employed to characterize the particles. The Scherrer formula was applied to estimate the particle size using the width of the diffraction peaks. The obtained results indicate that the synthesized material is mainly composed of nanocrystalline barite, with nearly spherical morphology, and diameters ranging from 4 to 92 nm. The lattice images of nanoparticles were clearly observed by HRTEM, indicating a high degree of crystallinity and phase purity. In addition, agglomerates with diameters between 20 and 300 nm were observed in both lattice images and dynamic light scattering measurements. The latter allowed obtaining the particle size distribution, the evolution of the aggregate size in time of BaSO4 in aqueous solutions, and the sedimentation rate of these solutions from turbidimetry measurements. A short discussion on the possible medical applications is presented.

Ángela B. Sifontes 1 and Edgar Cañizales 2 and Jhoan Toro-Mendoza 3 and Edward Ávila 1 and Petra Hernandez 1 and Blas A. Delgado 2 and G. Brenda Gutierrez 1 and Yraida Diaz 3 and Eliandreina Cruz-Barrios 3

Academic Editor:Yong Zhu

1, Centro de Quimica, Instituto Venezolano de Investigaciones Cientificas (IVIC), Apartado 20632, Caracas 1020-A, Venezuela
2, Área de Analisis Quimico Inorganico, PDVSA, INTEVEP, Los Teques 1070-A, Venezuela
3, Centro de Estudios Interdisciplinarios de la Fisica, Instituto Venezolano de Investigaciones Cientificas (IVIC), Apartado 20632, Caracas 1020-A, Venezuela

Received 26 November 2014; Revised 9 February 2015; Accepted 10 February 2015; 13 August 2015

This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

1. Introduction

The baryte group consists of baryte, celestine, anglesite, and anhydrite. Baryte is a sulfate of barium with chemical formula BaSO₄ . It is generally white or colorless, chemically inert, insoluble in water, with high density, and the main source of barium. Although baryte contains a "heavy" metal (barium), it is not considered to be a toxic chemical reagent by most governments because of its extreme insolubility [1]. Commonly, baryte is used for the production of barium hydroxide for sugar refining and as a white pigment for textiles, paper, and paint [2]. It is also suitable for other purposes due to its high specific gravity (4.5), opaqueness to X-rays, inertness, and whiteness [3]. Recently, more attention has been paid on the massive production of BaSO₄ nanoparticles for its multiple applications in the oil industry, electronics, TV screen, glass, car filters, paint industry, and ceramics [4] as well as in medical applications. In particular, barium sulfate suspension in water is used as the universal contrast medium for the examination of the upper gastrointestinal tract [5]. Currently, the development of nanoparticle-based X-ray contrast agents is providing an increasing contribution to the field of diagnostic and molecular imaging. For example, the nanoparticles surface can be modified to enhance their specificity by attaching targeting moieties, increase their circulation half-life by adding appropriate coatings (e.g., polymers, silica), and improve their functionality by adding other components, including fluorescent markers and therapeutic agents [6]. However, although significant progress has been made in preparing stable nanoparticles, their industrial production can present serious difficulties that limit their potential applications. For example, BaSO₄ nanoparticles tend to form aggregates due to the high ratio surface area-volume. Expensive costs and complex methodologies also attempt against massive industrial production [3].

Among the methods to obtain nanoscaled materials, chemical synthesis has numerous advantages such as simple technique, low costs, less instrumentation, doping, and high yield [4]. These methods could be classified as (a) direct precipitation [7], (b) microemulsion [8], (c) membrane separation [9], and (d) organic modification [10]. In general, particle sizes between 16 and 50 nm can be obtained. Nonetheless, precipitation seems to be the most suitable method due to its simple procedure and ease of mass production, and less apparatus is required [11]. In spite of this, the difficulties in controlling the size, polydispersity, and strong tendency to aggregate could hinder their potential applications [11]. In this sense, it is important to find more efficient methodologies of synthesis. Accordingly, in this paper we report the synthesis and characterization of barium nanoparticles by a chemical precipitation route, in the absence of polymer stabilizers.

2. Materials and Methods

2.1. Chemicals and Instruments

The starting reagents barium hydroxide, barium chloride, and sulfuric acid were used as received. Double distilled water was used in all experiments. The prepared barium sulfate was characterized by transmission electron microscopy using a JEOL JEM-2100 microscope with LaB₆ filament (accelerating voltage of 200 kV). The samples were prepared by suspending the powders in an ethanol-based liquid and pipetting the suspension onto a carbon/collodion coating. Fourier transform infrared (FT-IR) spectra were measured with a Perkin Elmer 100 spectrometer (in the range of 2000-500 cm^-1 ) by incorporating the samples in KBr (1 : 99 mg) disks to confirm the characteristic vibrational bands. X-ray diffraction (XRD) patterns of BaSO₄ were recorded on a panalytical diffractometer, model X'Pert Pro, and CuK-α radiation in the [figure omitted; refer to PDF] range between 5 and 80°, operating at 40 kV and 20 mA. Phases were identified using the MATCH! 2 program (version 2.2.3, Build 361) Crystal Impact, coupled to the Release 2011 PDF-2 database [12]. PDF card number 024-1035 (or ICSD card number 33730) was used to carry out the Rietveld refinement to all the peaks [13], which were in accordance with the reported values in the literature. The structure of BaSO₄ was refined with the Rietveld (1969) program GSAS [14] using the graphical interface EXPGUI [15]. The peak shapes were modeled using the pseudo-Voigt peak shape function 3, which included the axial divergence correction at low angle [16]. Background was initially determined manually and then modeled using the Chebyshev polynomial function. The isotropic atomic displacement parameters were refined as one overall [figure omitted; refer to PDF] for the nonoxygen atoms starting from a value of 0.05 Ų . Nevertheless, one constraint was applied to [figure omitted; refer to PDF] for modeling the isotropic atomic displacement parameters of oxygen atom following riding-model. The mean grain size ( [figure omitted; refer to PDF] ) of the prepared BaSO₄ nanoparticles was estimated from X-ray line broadening of the listed reflections in Table 1, using the Scherrer equation [17] ( [figure omitted; refer to PDF] where [figure omitted; refer to PDF] is the wavelength of the X-ray radiation, [figure omitted; refer to PDF] is a constant taken as 0.89, [figure omitted; refer to PDF] is the diffraction angle, and [figure omitted; refer to PDF] (rad) is the full width at half-maximum (FWHM)).

Table 1: Selected peaks employed to calculate the particles size with Scherrer's equation. These are the nine first peaks that appearance powder pattern of BaSO₄ synthesized.

[figure omitted; refer to PDF]	[figure omitted; refer to PDF] /°	[figure omitted; refer to PDF]	Int./U.A.	Rel. Int./%	FWHM/ [figure omitted; refer to PDF]
200	19.9969	4.43666	211.47	13.68	0.0895
011	20.4650	4.33622	513.75	33.24	0.0892
111	22.8076	3.89586	795.40	51.46	0.0882
210	23.5789	3.77013	155.81	10.08	0.0879
002	24.8785	3.57606	448.89	29.04	0.0875
210	25.8694	3.44128	1521.90	98.46	0.0872
120	26.8582	3.31679	1090.44	70.55	0.0870
211	28.7665	3.10096	1545.73	100.00	0.0868
112	31.6281	2.83364	535.66	34.65	0.0868

Both initial particle size distribution and particles size variation were characterized by dynamic light scattering measurements (DLS) on a Brookhaven BI 9000AT goniometer. Measurements were carried out at a fixed angle of 90° using a 633 nm laser. The sedimentation rate was obtained from Turbiscan LAB software in scanning mode. The light source is an electro luminescent diode in near infrared (880 nm). Two synchronous optical sensors receive the transmitted light through the sample (180°) and the backscattered light by the sample (45°). The sample was dispersed in a deionized water solution at a phase fraction of 0.1% w/v. The suspension was sonicated during 15 min before measurement.

2.2. Preparation of the Samples

The direct precipitation (DP) procedure followed consists in adding a Ba(OH)₂ ·8H₂ O solution to BaCl₂ ·2H₂ O in presence of water. The molar ratio of Ba(OH)₂ to BaCl₂ was 3 : 1. The resulting solution was stirred at room temperature while a sulfuric acid solution (50% (v/v)) was added (step 1, precipitation). The precipitated solid material was washed and filtered four times with hot distilled water (step 2). The sample was frozen at -26°C for 24 hours. Subsequently, the supernatant liquid was carefully decanted and then a volume of distilled water was added again. This procedure was repeated 5 times. The final step involved a freeze-dry cycle and was oven-dried at 105°C for 12 hours (step 3, purification) [18]. All the experiments were performed three times in order to observe reproducibility.

3. Results and Discussion

Figure 1 shows the XRD pattern and final Rietveld plot of the synthesized sample. It is shown that the solid is composed of baryte. The BaSO₄ crystalized in orthorhombic crystals, space group Pnma , with cell parameters [figure omitted; refer to PDF] Å, [figure omitted; refer to PDF] Å, and [figure omitted; refer to PDF] Å. The final Rietveld refinement with the reported model gave figures of merit: [figure omitted; refer to PDF] , [figure omitted; refer to PDF] , [figure omitted; refer to PDF] , [figure omitted; refer to PDF] (249 reflections), and [figure omitted; refer to PDF] . Finally, the Scherrer equation was used to estimate the particle size by averaging the value of FWHM for the first nine lines of the powder pattern of BaSO₄ (see Table 1). Thus, the average particle size estimated for the BaSO₄ synthesized in this work was 91.72 nm.

Figure 1: XRD pattern of synthesized sample (a) and final Rietveld plot for BaSO₄ (b). The continuous green line represents the calculated pattern and the red discontinuous line is the observed pattern.

(a) [figure omitted; refer to PDF]

(b) [figure omitted; refer to PDF]

TEM and HRTEM images (Figures 2-6) show that the sample was composed of both isolated and aggregated particles. The morphology of BaSO₄ particles is nearly spherical. The nanoparticles size determined from micrographies is between 2.6 and 20 nm, with an average size of 14.5 nm. Besides, large (>20 nm) fully crystallized structures are observed. Typical sizes of aggregates were found to be about 11-300 nm (Figure 3).

Figure 2: TEM image of BaSO₄ nanoparticles (a); HRTEM image showing lattice fringes of BaSO₄ nanoparticles (b).

[figure omitted; refer to PDF]

Figure 3: TEM image of BaSO₄ nanoparticles (a); typical aggregates (b).

(a) [figure omitted; refer to PDF]

(b) [figure omitted; refer to PDF]

Figure 4: HRTEM image of BaSO₄ nanoparticle (a); FFT power spectrum (b); masked power spectrum FFT (c); image after performing inverse FFT on the masked power spectrum (d).

[figure omitted; refer to PDF]

Figure 5: HRTEM image of BaSO₄ nanoparticles (a); FFT image (b) (particle 1), (c) (particle 2); masked power spectrum FFT (c) (particle 1), (d) (particle 2); image after performing inverse FFT on the masked power spectrum (c) and (d).

[figure omitted; refer to PDF]

Figure 6: HRTEM image of BaSO₄ nanoparticle (a); HRTEM image of BaSO₄ nanoparticle (b); a magnification of the section marked with a white square in (a); FFT image (c); masked power spectrum FFT (d); image after performing inverse FFT on the masked power spectrum (e). A crystallographic representation of BaSO₄ crystal (f).

[figure omitted; refer to PDF]

In Figure 4 HRTEM analysis shows that the lattice fringes with an in interlayer distance of 0.327 nm is close to the 0.331 nm lattice spacing of the [figure omitted; refer to PDF] crystal planes of barite phase. This was confirmed by fast Fourier transformation (FFT) patterns, which is also in accordance with XRD results. For this study, noise reduction was performed on the square-labeled area of Figure 4(a). Masking was applied on the intense spots of the FFT power spectrum. The resulting image after performing inverse FFT on the masked power spectrum is showed in Figure 4(d).

In Figure 5(a) (nanoparticles 1, 2) HRTEM analysis shows that the lattice fringes with an interlayer distance of 0.338 nm correspond to lattice spacing of the (210) crystal planes of BaSO₄ , which is in accordance with XRD results. Figures 6(b) and 6(c) show a FFT of Figure 5(a), and the diffraction points can be indexed to the (210) plane. Masking was also applied on Figures 5(b) and 5(c), specifically on the intense spots of the FFT power spectrum. The resulting image (Figures 5(d) and 5(e)) was obtained after performing inverse FFT on the masked power spectrum (Figures 5(f) and 5(g)).

In the HRTEM image (Figure 6), the planar spacing obtained is 0.355 nm, corresponding to spacing for (200) planes of orthorhombic structure BaSO₄ . All the planar spacing determined from HRTEM images are in accordance with the database of Joint Committee on Powder Diffraction Standards (JCPDS) card 024-1035.

Figure 7 shows the FT-IR spectra of the synthesized BaSO₄ nanoparticles. It was previously mentioned that the sulfate group has four fundamental vibrational modes attributed to one nondegenerate mode ( [figure omitted; refer to PDF] ), one doubly degenerate mode ( [figure omitted; refer to PDF] ), and two triply degenerate modes ( [figure omitted; refer to PDF] and [figure omitted; refer to PDF] ). In general, the IR spectrum of baryte exhibits several significant bands [19]. Two intense bands correspond to asymmetric stretching and bending ( [figure omitted; refer to PDF] and [figure omitted; refer to PDF] ), and two weak ones correspond to symmetric stretching and bending ( [figure omitted; refer to PDF] and [figure omitted; refer to PDF] ) [19]. In the FT-IR spectra the bands centered at 1073-1185 cm^-1 and the shoulder at 982 cm^-1 were assigned to symmetric stretching vibration of [figure omitted; refer to PDF] group. At 1636 cm^-1 the stretching vibration [figure omitted; refer to PDF] of [figure omitted; refer to PDF] group was identified. The peaks obtained at 609 and 637 cm^-1 were attributed to the out-of-plane bending vibration of the [figure omitted; refer to PDF] [10, 19]. The absorption peaks appeared at 3420 cm^-1 are due to antisymmetric stretching vibrations of H₂ O molecules localized on vacant sites of Ba. The peaks around 2000 cm^-1 are overtones and a combination of stretching and bending vibrations bands of the sulphur-oxygen [20].

Figure 7: FT-IR spectra of BaSO₄ nanoparticles.

[figure omitted; refer to PDF]

3.1. Particle Size Distribution, Aggregates Size Evolution, and Sedimentation Rate

Figure 8 shows the particle size distribution from dynamic light scattering (DLS) measures of an aqueous suspension of BaSO₄ nanoparticles. As can be observed, the main peak appears around 800 nm with an average size of [figure omitted; refer to PDF] μ m. Even though the micrographies indicate a particle size less than 100 nm, the distribution shows particles sizes above 5 microns, an indicative that DLS yields an average radius of aggregates formed by several particles. The strong tendency to form aggregates was corroborated and the results are shown in Figure 9, where the time evolution of both radius and percentage of polydispersity is plotted. The average size evolves linearly with time from 1200 μ m to 4200 μ m in 1 h. However, the percentage of polydispersity fluctuates around 30%.

Figure 8: Particle size distribution of a BaSO₄ /W suspension (0.1% w/v) determined by dynamic light scattering.

[figure omitted; refer to PDF]

Figure 9: Particle size variation of a BaSO₄ /W suspension (0.1% w/v) determined by dynamic light scattering.

[figure omitted; refer to PDF]

Due to the large size of aggregates, it is expected to observe sedimentation and the results from turbidimetry can be observed in Figure 10. The photon mean free path ( [figure omitted; refer to PDF] ) represents the mean distance travelled by photons before undergoing a diffusion phenomenon. Thus, the measurement is performed by the Turbiscan by sending a light beam though the cell and detecting the photons crossing the dispersion, without being diffused. Therefore as more photons cross the cell, more important is the [figure omitted; refer to PDF] value and the sample is more translucent, an indicative that less particles are in the beam path. The initial slope of Figure 10, corresponding to the sedimentation rate, was 412 μ m/min, with a correlation coefficient of 0.99. The total average sedimentation rate in a 60 min measurement was 171 μ m/min.

Figure 10: Sedimentation rate obtained from the initial slope of the photon mean free path ( [figure omitted; refer to PDF] ) as a function of time.

[figure omitted; refer to PDF]

The combined effect of forming aggregates and sedimentation could limit possible medical applications of BaSO₄ nanoparticles. Nevertheless, this study was processed without any additives (including stabilizers, antioxidants, and lubricants). In this sense, in general, barium sulfate suspension oral for gastric and colonic radiological work, containing a physiologically inert stabilizer, consists of a solution of a water-soluble cellulose ether and carboxymethylcellulose and also contains a suitable penetrant of cationic origin as lauryl sulfoacetate [21]. Besides, in order to improve the possibilities of application, further studies in particle-particle interactions and the main mechanisms responsible for coarsening are necessary to implement new methodologies to obtain more kinetically stable systems. In this sense, our procedure could be optimized in order to low the polydispersity index around a particle size below 10 nm. It is also noticeable that the starting reagents have low impact toxicity, compared to others which, in some cases, make use of benzene [22]. This latter is an additional advantage of our procedure in order to use the synthesized nanoparticles in medical applications.

3.2. Formation Mechanism of BaSO₄ Nanoparticles

The synthesis of barium sulfate was carried out using a mix of Ba(OH)₂ /BaCl₂ , where the barium chloride solution was added to increase the concentration of Ba²⁺ ions (in excess). Subsequently, with the addition of [figure omitted; refer to PDF] (H₂ SO₄ ), BaSO₄ crystals were gradually formed.

The increase in pH could result in an increase in the negative charge of BaSO₄ . According to Zhang et al., this could be attributed to the adsorption of OH^- ions on the positive charge centers of BaSO₄ particles [23]. The reaction of barium (Ba²⁺ ) and sulphate ( [figure omitted; refer to PDF] ) ions into barium sulphate (BaSO₄ ) is [20, 24, 25] [figure omitted; refer to PDF]

In general, a precipitation process is described by processes that include the creation of a supersaturation level followed by the generation of nuclei (nucleation) and the subsequent growth [8]. A number of secondary processes like aging, ripening, agglomeration, and breakage might occur depending on the nature of the process itself and the chemical compound [8]. Moreover, in a highly supersaturated solution the instability of the system will result in a fast local concentration fluctuation [26]. Basically, there are three variables that govern the rate of nucleation: the temperature [figure omitted; refer to PDF] , the degree of supersaturation [figure omitted; refer to PDF] , and the surface energy [figure omitted; refer to PDF] . In this sense, if the supersaturation ratio is increased and the surface energy is decreased, the result is a higher nucleation rate, which finally leads to the formation of particles of smaller size [27, 28].

The synthesis process involves as second step a quenching. Quenching is simply cooling rapidly to a lower temperature, -26°C. The final step in the precipitation process was an artificial aging at -26°C (aging above room temperature). Temperature has a significant influence on solubility and crystal growth of barium sulphate [29]. It is then expected that the growth of particle size is stopped by quenching with cold water.

The results presented here prove that barium sulfate nanoparticles can be prepared in the absence of polymers stabilizers polymeric employing a simple methodology. Moreover, the nanomaterial synthesized has applications as a potential contrast agent for X-ray examination.

4. Conclusions

Barium sulphate nanoparticles were synthesized using a precipitation method in absence of polymer stabilizers or solvents. The XRD study shows that the synthesized nanoparticles crystallize in orthorhombic system with space group Pnma . This method allowed obtaining high purity, spherical, and morphologically homogeneous nanoparticles. X-ray diffraction, transmission electron microscope (TEM), and high resolution transmission electron microscope (HRTEM) measurements confirmed an average particle size in the range from 4 to 92 nm and agglomerates with diameters between 20 and 300 nm. FTIR study shows the presence of the sulphate group in the synthesized nanoparticles. Dynamic light scattering and turbidimetry measurement indicates a strong tendency of nanoparticles to form aggregates and precipitate in aqueous solution. Finally, this methodology can be employed to synthesize BaSO₄ nanoparticles for possible applications as a potential contrast agent for X-ray examination.

Acknowledgment

The authors acknowledge financial support by IVIC through project 1077 and technical assistance of Lic. Liz Cubillan (FTIR).

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

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