Atomic Distribution and Morphology of Colloidal Particles Precursors

Sols were prepared by mixing solutions of bismuth nitrate and thiourea in N,N-dimetilformamide. The corresponding colloidal particles were characteriz...
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J. Phys. Chem. C 2007, 111, 188-193

Atomic Distribution and Morphology of Colloidal Particles Precursors of Bismuthinite Marina Vega-Gonza´ lez and Xim Bokhimi* Institute of Physics, The National UniVersity of Mexico (UNAM), A. P. 20-364, 01000 Me´ xico D. F., Mexico ReceiVed: August 11, 2006; In Final Form: October 16, 2006

Sols were prepared by mixing solutions of bismuth nitrate and thiourea in N,N-dimetilformamide. The corresponding colloidal particles were characterized by using X-ray powder diffraction, scanning and transmission electron microscopy, and quasielastic light scattering. Two size distributions of colloidal particles were found: one at 5 nm and another at 300 nm. The small colloidal particles were nanocapsules that aggregated to produce larger nanocapsules with diameters between 10 and 40 nm and a shell thickness of 5 nm, which aggregated to produce the large colloidal particles with the diameter of 300 nm. The capsule shells were noncrystalline, made of Bi-S atomic clusters that contained two bismuth and five sulfur atoms; the clusters formed chains and double Bi-S layers linked via metallic Bi-Bi bonds. Increasing the bismuth concentration in the sol induced the crystallization of the sample into the crystalline structure of bismuthinite. Aging the sol at different temperatures caused aggregation of the large nanocapsules into one-dimensional arrays that also interacted with each other, forming broccoli-like objects with dimensions of some micrometers. Aging the sol at 80 °C gave rise to a dendritic crystallization of bismuthinite.

Introduction

Experimental Section

Bismuthinite, Bi2S3, is a compound of the semiconductors group V-VI with a band gap between 1.25 and 1.7 eV that makes it very attractive in photoelectrochemical cells,1 where it is used in the form of amorphous thin films. Since in the amorphous phase the atomic order is restricted to small regions, quantum effects are important.5 In the amorphous phase, however, it is not possible to change the size of these regions, which hinders the control of the quantum effects. If the material crystallizes, the size of the regions of ordered atoms is the crystallite size, which can be varied through the synthesis even in the nanometric dimensions. Bismuthinite can be prepared with a crystalline order.1,3,4 The different methods described for its synthesis in this atomic order6-12 report that the samples are composed of onedimensional particles,13 many of which are crystalline rods,14 with cross sections that have dimensions on the order of a few nanometers. A detailed analysis of the micrographs15,16 of the reported particles shows that some of them could be nanotubes, which is a particle morphology of interest not only for electronic devices but also for catalysis17 and drug encapsulation.18 To analyze more in detail these one-dimensional particles of bismuthinite, including the origin of their morphology, they were synthesized from a colloidal dispersion, a sol, as reported for the synthesis of boehmite.19 This synthesis method allowed us to find a correlation between the formation of bismuthinite crystallites and the properties of the colloidal particles, which had a noncrystalline atomic distribution that was a precursor of the crystalline structure of bismuthinite. The interaction between the colloidal particles caused their aggregation to form crystallites and one-dimensional objects of bismuthinite that are the precursors of the urchin-like morphologies reported in the literature for this system.20-23

Sample Preparation. The Sol Precursor of Bismuthinite. Bismuth nitrate [Bi(NO3)3·5H2O; Baker, 99.22%] was dissolved in N,N-dimethylformamide (DMF; Aldrich, 99.97%) with molarities of 0.05, 0.2, or 0.4 to produce colorless solutions. Thiourea (H2NCSNH2; Aldrich, 99%) was also dissolved in DMF with the adequate molarity to get a thiourea/bismuth nitrate molar ratio of 1.5; the solution was also colorless. For each bismuth concentration, the corresponding thiourea solution was dropped into the one of bismuth at 20 °C, while the mixture stirred during 20 min, to produce a yellow colloidal dispersion that presented the Tyndall effect. Dispersions were aged at 20, 30, 60, or 80 °C, for 2 months, 14 days, and 22 and 22 h, respectively. Bismuthinite with a Large Crystallite Size. Bismuth nitrate was dissolved in deionized water at a molarity of 0.2, while the thiourea was also dissolved in water at a molarity of 0.3. At room temperature, the thiourea solution was added to the bismuth one and stirred for 20 min; thereafter, it was annealed at 80 °C for 22 h. Experimental Techniques. X-ray Powder Diffraction. The X-ray diffraction patterns of the samples were measured in a θ-θ Bruker D-8 Advance diffractometer having the BraggBrentano geometry, Cu KR radiation, a graphite secondary-beam monochromator, and a scintillation detector. Diffraction intensity as a function of the angle 2θ was measured between 3° and 110°, with a 2θ step of 0.02° and a counting time of 9 s per point. Bismuthinite crystalline structure was refined via the Rietveld method using Fullprof code.24 Crystallite morphology was modeled by using spherical harmonics as base functions,25 while the background was modeled with a polynomial function that, in addition to the constant, linear, quadratic, and cubic terms in 2θ, also contained the terms (1/2θ) and (1/2θ)2. The standard deviations given in parentheses in the text and tables show the last figure variation of a number. When they correspond to Rietveld refined parameters, their values are not estimates of

* Corresponding author. Tel. +52 55 5622 5079. Fax: +52 55 5622 5008. E-mail: [email protected].

10.1021/jp0652087 CCC: $37.00 © 2007 American Chemical Society Published on Web 11/30/2006

Colloidal Particles as Precursors of Bismuthinite the probable error in the analysis as a whole, but only of the minimum possible probable errors based on their normal distribution.26 Electron Microscopy. Samples were analyzed with transmission electron microscopy (TEM) in a Jeol JEM-100CX, and in a Jeol JEM-2010F microscope with a NORAN Advantage microanalyzer system without a window between sample and detector, which allows the detection of low atomic number elements. Samples were prepared by using two different methods: in the first one, the precipitate was dispersed in ethanol before placing it in the copper grid with Formvar support; in the second one, a drop of the sol was deposited directly on the support. The scanning electron microscopy (SEM) analysis of the samples was performed in a Jeol JSM 5600-LV microscope; in this case, samples were prepared by depositing a portion of the precipitate on the aluminum sample holder and covering it with a thin gold film. Quasielastic Light Scattering. The particle size distribution in the sol was determined with a Brookhaven Instrument with a BI200SM goniometer and a BI9000AT digital correlator; the light source was a 35 mW He-Ne laser from Melles-Griot (model 9167EB-1). Since this equipment works with dilute dispersions, only the 0.05 M Bi(NO3)3·5H2O sample was analyzed. The scattering ampules were immersed in an indexmatching fluid at room temperature; scattering was measured at the fixed angle of 90°. Particle size distribution was determined by using the non-negatively constrained least-squares method. Results and Discussion Bismuth nitrate was completely soluble in DMF at 20 °C for all bismuth concentrations; its dissolution produced a colorless transparent solution. Thiourea was also completely soluble in DMF at this temperature for all thiourea concentrations; its dissolution produced a colorless transparent solution. When these two solutions were mixed and stirred for 20 min, the mixture became yellow and transparent and scattered light, indicating that it was a colloidal dispersion. The quasielastic light scattering experiments on the sol with the lowest bismuth concentration detected two particle size distributions: one around 5 nm, produced by particles that will be named as small colloidal particles, and the other one around 300 nm, produced by particles that will be named as large colloidal particles, which were the most abundant. A drop of this yellow colloid was deposited on a copper grid to analyze it with the transmission electron microscope. In the micrograph (Figure 1A), we observed the large colloidal particles, which appeared to be made of connected small rings with a thickness dimension (of the rings) on the same order of magnitude as the small colloidal particles (Figure 1A); similar micrographs were obtained for the precipitate formed after aging the sol in a covered recipient for 2 months at room temperature (Figure 1B). This suggests that the large colloidal particles were formed by aggregation of the small colloidal particles. The EDS analysis of the colloidal particles detected the presence of only bismuth and sulfur; they did not contain nitrogen or oxygen. The colloidal particles observed in these micrographs were similar to those observed in the micrographs of the colloidal particles of the sol precursor of boehmite (Figure 1C,D) prepared by dissolving aluminum tri-sec-butoxide in 2-propanol,19 those of the sol of hydrotalcite,27 and those of the sol prepared with titanium butoxide.27 In the present work, we never found isolated small colloidal particles; they were always aggregated. Therefore, we could not

J. Phys. Chem. C, Vol. 111, No. 1, 2007 189

Figure 1. TEM micrographs of colloidal particles of a sol precursor of bismuthinite: (A) Z-contrast image of a large colloidal particle for 0.05 M bismuth nitrate and (B) bright-field image of colloidal particles made of a few small colloidal particles (the bismuth nitrate molarity was 0.4). TEM micrographs of colloidal particles of a sol precursor of boehmite: (C) Z-contrast of a large colloidal particle and (D) Z-contrast of an isolated small colloidal particle.

present simple and evident micrographs about their morphology as was done for the boehmite system,19 where one isolated small colloidal particle (Figure 1D) was rotated at different angles, producing similar projections on the plate forming the image, independently of the rotation angle, which demonstrated that its morphology corresponded to a hollow sphere.19 But, since both small colloidal particles (those in the sol precursor of boehmite and in the present work) had similar projections on the micrographs, we adopted the same interpretation of the projection and assumed that the small colloidal particles in the present work were also hollow spheres. This assumption helped us to understand clearly and congruently the information contained in the micrographs of the present system. For example, in Figure 1B, there were zones with a reduced number of small colloidal particles, and their projection produced many nearly round rings. If they were real rings, and they were oriented at random, their projection would generate ellipses with different eccentricities, including some that would look like straight lines. The micrograph, however, shows that all rings produced by the projections of the particles had an eccentricity that approximated to one. The only geometry form of particles oriented at random that could project this image is the one of a hollow sphere, which is in accord with the assumptions that the small colloidal particles in the sol precursor of bismuthinite were hollow spheres. For simplicity of the discussion in the following paragraphs, these small colloidal particles will be named as nanocapsules, because they were hollow spheres and had dimensions in the range of nanometers. From the micrographs (Figure 1B) it was observed that the nanocapsules, the small colloidal particles, aggregated to form larger nanocapsules; the most abundant had diameters between 10 and 40 nm and a shell thickness of 5 nm. By electron microscopy, at low resolution, these were the nanocapsules that were observed in the large colloidal particles, which gives them (the large particles) the appearance of a porous material (Figure 1A-C).

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Vega-Gonza´lez and Bokhimi TABLE 2: Relative Atom Positions of Bismuthinite with an Average Crystallite Size of 2 nma atom

site

x

y

z

B1 B2 S1 S2 S3

4c 4c 4c 4c 4c

-0.0211(7) 0.3756(6) 0.128(5) 0.352(3) 0.189(4)

0.25 0.25 0.25 0.25 0.25

0.668(1) 0.468(2) 0.170(4) -0.169(4) 0.966(6)

a Lattice parameters a ) 1.228(2) nm, b ) 0.3851(2) nm, c ) 0.9139(5) nm.

Figure 2. X-ray powder diffraction. patterns of samples prepared with different bismuth nitrate molarities: (A) 0.05, (B) 0.2, (C) 0.4. Indices and marks correspond to crystalline bismuthinite. The crystalline phase in C had an average crystallite size of 7 nm.

Figure 4. (A) Characteristic atomic cluster of the system. (B) Two neighboring atomic clusters showing their interaction; each cluster shares two edges to form filaments parallel to the b-axis.

Figure 3. Rietveld refinement plot (Rwp ) 0.051) of the sample prepared with the lowest bismuth concentration. Marks correspond to bismuthinite. The points in the upper curve correspond to the experimental data and the continuous line to the calculated ones; the lower curve is the difference between experimental and calculated data.

TABLE 1: Relative Atom Positions of Bismuthinite (Space Group Pnma) with an Average Crystallite Size of 50 nma atom

site

x

y

z

B1 B2 S1 S2 S3

4c 4c 4c 4c 4c

0.0165(1) 0.3400(1) 0.0538(6) 0.3745(5) 0.2184(6)

0.25 0.25 0.25 0.25 0.25

0.6738(1) 0.4662(1) 0.1335(6) 0.0579(6) 0.799(6)

a Lattice parameters a ) 1.12978(2) nm, b ) 0.398087(5) nm, c ) 1.11436(2) nm.

The atoms were distributed in a noncrystalline array (Figure 2A,B) independently if they were in the small or in the large colloidal particles; the atomic local order, however, was similar to the atomic local order in crystalline bismuthinite28 (Figure 2C). This allows, in a first approximation, the fitting of the experimental diffraction pattern produced by the colloidal particles (Figure 3) by simulating their atom distribution with the crystallography of bismuthinite (Table 1), obtained by refining the crystalline structure of a bismuthinite sample having

a large crystallite size (50 nm), starting from the crystallography reported for bismuthinite28 and using the Rietveld refining method. In the model used in the refinement for fitting the experimental diffraction pattern of the nanocapsules, the crystallite size was modeled by using only the spherical harmonic Y00, which is isotropic in the space coordinates and corresponds to an isotropic crystallite form. The obtained average crystallize size was 2 nm, which is similar to the thickness of the small nanocapsules, those associated with the small colloidal particles. Table 2 summarizes the values obtained for the relative atom positions and the lattice parameters, some of which differ considerably from the ones obtained for the bismuthinite sample with the crystallite size of 50 nm (Table 1). It is important to remark that the model used for the diffraction pattern for the colloidal particles was only a first approximation, because the true positions of the atoms in the particles did not have the translational symmetry that is intrinsic to the model used in the refinement. Therefore, the data in Table 2 give only a first idea of the real atom distribution in the colloidal particles. From this analysis, however, it was possible to propose an approximation for the characteristic atomic cluster of the system: It had two bismuth and five sulfur atoms (Figure 4a). Clusters shared two of their edges, forming one-dimensional chains along the b-axis (Figure 4b). These chains interacted with each other via a weak metallic Bi-Bi bond (Figure 5). The shortest atomic bond length between bismuth atoms of two neighboring chains was 0.366 nm, which was similar to the interatomic distance between layers in metallic bismuth (0.36 nm).29 This interaction between chains gave rise to double layers perpendicular to the a-axis and connected via metallic Bi-Bi bonds (Figure 5). The atom distribution outside of the chains was widely open, which easily allowed the reordering of the atomic clusters to form the curved surfaces of the colloidal particles.

Colloidal Particles as Precursors of Bismuthinite

J. Phys. Chem. C, Vol. 111, No. 1, 2007 191

Figure 5. Layers made of filaments of atomic clusters (the filaments are perpendicular to the figure plane). It is evident that the interaction between layers occurs via metallic Bi-Bi bonds.

Figure 7. Micrographs of the sample prepared with a bismuth nitrate molarity of 0.4 and aged at 30 °C for (A) 1 day, TEM bright field image; B) 1 day, SEM image; (C) 3 days, TEM bright field image; (D) 3 days, SEM image. The insets in A and C are the corresponding electron diffraction patterns of the crystallites in the micrographs.

Figure 6. X-ray powder diffraction patterns of the samples prepared with a bismuth nitrate molarity of 0.4 and aged at 30 °C for (A) 1 day, (B) 3 days, (C) 10 days, and (D) 14 days. Indices and marks correspond to crystalline bismuthinite.

The fact that the atomic clusters shared two edges to build chains of clusters parallel to the b-axis indicated that the binding of the clusters along this axis was very stable, and the growing of the particles along it would be favored. This result explains why the crystalline samples of bismuthinite are made of onedimensional objects with their length axis along the crystallographic b-axis.12,15 It is necessary to comment here that, in the JCPDS file 17-0320 for bismuthinite,30 the space group assigned to the crystalline structure is Pbnm, while for our refinement we used the space group Pnma; both are equivalent representations of the same space group, but the c-axis in the first representation is equivalent to the b-axis in the second one.31 Therefore, we are reporting that the length axis of the onedimensional objects is parallel to the b-axis, while in refs 12 and 15 of the present work, they report that this length axis is parallel to the c-axis, because they used the JCPDS file 170320 as reference for the crystalline structure of bismuthinite. The increase of bismuth concentration in the sol favored the crystallization of the sample (Figure 6). This could be caused by the increase of the number of colloidal particles in the sol, which makes more probable the collisions between them to create their interaction. The noncrystalline atomic local order in the nanocapsules shell was transformed into the crystalline order of bismuthinite (Figure 2C). Since the generation of a large precipitate in the sol prepared with the 0.4 M bismuth solution occurred after aging the sample for 2 months at 20 °C, the interaction between the large colloidal particles at this temperature should be relatively weak, because of the low frequency of the collisions between them and their low kinetic energy. This explains why the corresponding aged samples prepared at 20 °C with the bismuth molarities of 0.05 and 0.2 M gave rise to a noncrystalline diffraction pattern

(Figure 2A,B): The interaction between the corresponding colloidal particles was negligible; therefore, to promote the interaction between them, samples were annealed at temperatures higher than 20 °C. Since the evolution of the system with time occurred relatively slowly at 30 °C, the samples for the analysis at this temperature were prepared with a high bismuth molarity (0.4 M), to increase the probability of the collisions between particles, and aged between 1 and 14 days. Again, as observed in the samples analyzed at 20 °C, the ordering of the atoms in a crystalline array increased with aging time (Figure 6) because it increased the number of collisions between the colloidal particles. Aging changed sample texture as well: after aging the sample for 1 day at 30 °C, it was made of nanocapsules that began to interact with each other, forming one-dimensional objects, fibers, with nearly cylindrical geometry, an average diameter of 20 nm, and lengths of several hundred nanometers (Figure 7A). These fibers initially aggregated parallel to each other along their length axis, forming objects about 100 nm in diameter and 200 nm length. When the aggregation reached a given length, in one of its extremes the aggregated linear objects began to separate, generating a broccoli-like structure with a stem that had a diameter of 100 nm and a flower-like region with a diameter that depended on the length of the one-dimensional aggregated objects (Figure 7A). This flower-like region appeared like an urchin by scanning electron microscopy when it was observed with the electron beam parallel to its growing dimension (Figure 7B); this kind of morphology has been reported previously for the Bi2S3 system.6,22 After 1 day of aging, the diameter of the flower-like region of the broccolilike objects reached about 2 µm. When the sol was aged for 3 days at 30 °C, the interacting capsules aligned along one dimension and coalesced to produce one-dimensional objects 1 µm length (rods or tubes) that had a smooth surface. These objects continue bundling together along their length axis, forming the broccoli-like aggregates with a stem of about 100 nm in diameter (Figure 7C) and an average diameter of the flower-like region of 5 µm (Figure 7D). These

192 J. Phys. Chem. C, Vol. 111, No. 1, 2007

Figure 8. SEM micrographs of the sample prepared with a bismuth nitrate molarity of 0.4 and aged at 30 °C for (A) 7 days, (B) 10 days, (C and D) 14 days.

results are useful to understand the origin of the “acicular crystallites” reported when bismuthinite is synthesized via thermal decomposition of tris(benzylthiolato)bismuth in a sealed tube22 or via an hydrothermal treatment.32 The one-dimensional objects formed after aging the sol for 3 days did not show any more that they were formed by the arrangement of nanocapsules, as in the one-dimensional objects obtained after aging the sample for only 1 day. It is interesting to mention that the cross section of the one-dimensional objects was similar to the diameter of the large nanocapsules from which they were formed, which suggests that the one-dimensional objects would be eventually tubes and not rods. If they would be rods, their diameters would be smaller than that of the capsules, because only the capsules’ shells are made of atoms; and if they collapsed into a compact volumetric object, the diameter of the one-dimensional object would be smaller than that of the capsules. When aging time was increased, the broccoli-like aggregates came into contact to each other (Figure 8A-C), sintering in their contact regions, until eventually they formed a porous film after 14 days of aging (Figure 8C,D). Aging effects were also analyzed when samples were annealed at 60 °C. Since at this temperature the time evolution was faster, the sample was prepared from a 0.2 M bismuth solution to reduce the interaction between the colloidal particles through their collisions. In this case, the time intervals between the different analyses were of only a few hours. The time evolution of the sol was also analyzed when it was annealed at 60 °C, at which time its yellow color became brown due to the aggregation of the colloidal particles. Just after reaching this temperature, a drop of the colloidal dispersion was deposited onto a copper grid for its analysis in the transmission electron microscope. The corresponding micrograph showed that the colloidal particles aggregated, forming a continuous medium with holes of about 200 nm in diameter (Figure 9A); only a few of the objects formed through this aggregation were onedimensional. This morphology remained unchanged, even after aging the sample at this temperature for 30 min. But after aging the sol for 1 h the morphology generated by the colloidal particles was completely different: The one-

Vega-Gonza´lez and Bokhimi

Figure 9. TEM bright field micrographs of the sample aged at 60 °C for: (A) a few minutes and (B, C, and D) 1 h.

Figure 10. X-ray powder diffraction. Patterns of the sample aged for 22 h at (A) 60 °C and (B) 80 °C. Indices and marks correspond to crystalline bismuthinite.

dimensional objects formed by the aggregation of the nanocapsules abounded (Figure 9B) and coexisted with nonaggregated colloidal particles (Figure 9C) and with nets of nanocapsules aligned linearly (Figure 9D). These different morphologies coexisted even after aging the sample for 22 h. The formation of the one-dimensional objects was correlated with an increase in the crystallinity of the sample, as was evident from the corresponding X-ray diffraction pattern (Figure 10A). The noncrystalline contribution of the weakly interacting colloidal particles, including those forming the nets, produced the very broad peaks of the diffraction pattern. These results show how an increase of the sample annealing temperature favored not only the crystallization of bismuthinite through an increase of the interaction between the colloidal particles through their collisions but also the formation of onedimensional objects through their aggregation. When the sample annealing temperature was 80 °C, during the first minutes the colloidal particles aggregated, forming nets (Figure 11A). After 2 h, this arrangement was transformed into

Colloidal Particles as Precursors of Bismuthinite

J. Phys. Chem. C, Vol. 111, No. 1, 2007 193 Acknowledgment. We thank M. Aguilar, A. Morales, L. Rendo´n, R. Herna´ndez, and C. Magan˜a, for technical support; the Laboratorio Central de Microscopı´a, of the Instituto de Fı´sica of The National University of Mexico (UNAM), for use of the electron microscopy facilities; and Dr. Miriam Estevez Gonza´lez, from the Centro de Fı´sica Aplicada y Tecnologı´a Avanzada of The National University of Mexico, for use of the quasielastic light scattering measurement facilities. This work was financially supported by the “Proyecto Universitario de Nanotecnologı´a” of The National University of Mexico. M.V. Thanks the CONACyT for the financial support. References and Notes

Figure 11. TEM bright field micrographs of the sample aged at 80 °C for (A) a few minutes, (B) 2 h, and (C and D) 22 h.

large three-dimensional aggregates (Figure 11B) that eventually transformed into crystalline Bi2S3 (Figure 10B) with a dendritic growing (Figure 11C,D), when the annealing time was 22 h. The above results show clearly that increasing aging temperature favored the reordering of the atoms into a crystalline distribution, because the increase in temperature increased the particles kinetic energy, augmenting the probability and strength of interaction between them. Conclusions The mixing of dissolutions of bismuth nitrate and thiourea in N,N-dimethylformamide produced transparent yellow sols with colloidal particles with two different size distributions. The small colloidal particles had the form of nanocapsules with a diameter of 5 nm which aggregated to form nanocapsules with diameters between 10 and 40 nm with a shell thickness of 5 nm; the large colloidal particles had diameters of 300 nm and were formed by aggregation of the large nanocapsules. The shell of the nanocapsules were formed of Bi-S clusters, made of two bismuth and five sulfur atoms that shared two of their edges along the b-axis of crystalline bismuthinite and formed a structure made of double Bi-S layers, perpendicular to a-axis, linked via metallic Bi-Bi bonds. When the large colloidal particles interacted each other, the atoms reordered into the crystalline structure of bismuthinite, and the capsules aggregated, forming one-dimensional objects that ordered in bundles along their length dimension to form broccoli-like objects that looked like an urchin when they were observed with the electron beam parallel to their largest dimension. The evolution of the morphologies of the particles’ aggregates was analyzed when the sol was aged at 20, 30, 60, and 80 °C. This morphology varied from almost isolated noncrystalline colloidal particles to dendritic crystallites of bismuthinite.

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