DOI: 10.1021/cg9010949
Shape-Controlled Solventless Syntheses of Nano Bi Disks and Spheres
2010, Vol. 10 1578–1584
Yue Wang,†,‡ Jing Chen,† Ling Chen,† Yu-Biao Chen,† and Li-Ming Wu*,† †
State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, P. R. China, and ‡Graduate School of the Chinese Academy of Sciences, Beijing 100039, P. R. China Received September 8, 2009; Revised Manuscript Received February 4, 2010
ABSTRACT: Novel nano Bi spheres and hexagonal disks have been synthesized via the thermolysis of a bismuth-thiolate precursor in a mild solventless condition with the help of capping reagents that have shown an isotropic or anisotropic absorption tendency on the Bi crystal faces. Various techniques such as X-ray diffraction, electron dispersive spectroscopy, scanning electron microscopy, transmission electron microscopy, selected area electron diffraction, and atomic force microscopy have been used to characterize the purity, composition, and morphology of the nanoproducts. The experiments also reveal size dependencies based on thermolysis temperature and annealing time of both disks and spheres. In addition, the reactant molar ratio also affects the morphology. Finally, a possible growth mechanism has been proposed, and the as-synthesized disks and spheres have exhibited different UV-vis absorption spectra.
Introduction Nanomaterials have shown a large spectrum of important applications such as catalysis, photography, electronics, optics, optoelectronics, and information storage;1-7 therefore, their controlled syntheses have become one of the most important topics of nanoscience and nanotechnology for decades. The controlled syntheses of transition metals and binary alloys, such as Au, Ag, Pt, Pd, Co, and PtFe, have been proven to be successful.8-14 In contrast, the syntheses of maingroup metal nanoparticles are relatively less studied. Bismuth is the heaviest main group element, and bulk Bi exhibits semimetal behavior with a small band overlap and an anisotropic electron effective mass. Previous studies have found that as the size decreases, Bi could transfer from a semimetal to a semiconductor due to quantum confinement.15,16 For example, this transition is predicted to occur at a diameter of 49 nm at 77 K in a Bi nanowire oriented along the direction. As the wire diameter decreases, the band edge of the lowest subband at the L-point increases, while the highest subband edges at the T-point and L-point move downward in energy. Finally, the energy of the lowest L-point conduction subband edge exceeds that of the highest T-point valence subband edge, indicating that the small band overlap has become to the small direct band gap.15 In addition, the thermoelectric property of nano Bi has been intensively investigated .17-19 Many previous works have focused on the syntheses of onedimensional (1D) Bi wires20,21 or two-dimensional (2D) Bi nanofilms22 by template or molecular beam epitaxy (MBE) methods. Just a few reports on solution methods have shown the formations of zero-dimensional (0D) Bi nanoparticles. For example, Bi nanoparticles were made in DMF by reducing Bi3þ with sodium borohydride (NaBH4) in the presence of poly vinylpyrroldone (PVP) at room temperature.23 Bi nanospheres were obtained from a mixture of ethylene glycol and acetone by reducing BiO3þ with ethylene glycol (EG) at *To whom correspondence should be addressed. E-mail: liming_wu@ fjirsm.ac.cn. Tel: (011)86-591-83705401. pubs.acs.org/crystal
Published on Web 03/18/2010
180-200 C.24 However, in both experiments, the extra reductant, NaBH4 or EG, is necessary; otherwise Bi products cannot be produced. Recently, we found that free-standing Bi nanofilms and 0D Bi nanorhomobuses could be produced by the thermolysis of the single phased Bi[SC12H25]3 precursor by a solventless method under vacuum condition,25 which, to our best knowledge, is the first time Bi nanoparticles have been prepared below 100 C without extra reductant. Li and coworkers have pointed out that the capping molecule in a solution phase approach affects the morphology of the Bi nano wire and belt.24 On the basis of this point, the mild solventless conversion of Bi[SC12H25]3 precursors at low temperature might also be suitable to detect the influence of the capping reagent. Herein, we introduced two organic capping agents, 1-dodecanethiol (C12H25SH, DT) and poly vinylpyrroldone (PVP), into such a solventless reaction system and successfully prepared single-crystalline Bi nanodisks and nanospheres, respectively. The capping agents are found to play an important role in controlling the shape and size of the nanoproducts. Meanwhile, some other reaction factors, such as thermolysis temperature, reactant molar ratio, and annealing time are also studied by a batch of designed parallel reactions. In addition, the single-crystalline uniform Bi nanodisks and nanospheres have shown different UV-vis absorption spectra. Experimental Section Except dodecanethiol (C12H25SH, DT, Lancaster, 98%), Bi(NO3)3 3 5H2O (A.R.), poly vinylpyrroldone (PVP), K-30 (mp 265 C), ethanol (A.R), and CHCl3 (A.R.), CH2Cl2 (A.R.), DMF (A.R.) were purchased from Shanghai Chemical Co. All reactants are used as received. Route 1: Synthesis of Bi Nanodisks. For a typical reaction, 25 mL of CHCl3 and 12.5 mmol (3 mL) of DT were added into an aqueous Bi(NO3)3 3 5H2O solution (0.233 g, 0.48 mmol in 32 mL of H2O). After 1 h of stirring, the aqueous phase was discarded and CHCl3 was subsequently evaporated. A yellow waxy (because the presence of the excessive DT) precursor was then obtained and collected in a Pyrex boat and transferred into a long Pyrex tube (6 cm 1 m) with stopcocks on both ends and purged with N2 for 10 min, and subsequently heated at 120 or 135 C for 10 h to generate a black r 2010 American Chemical Society
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product. Such product was dispersed with CHCl3 and reprecipitated with ethanol to remove the byproducts, and dried in a vacuum. Route 2: Synthesis of Bi Nanospheres. For a typical reaction, to an aqueous Bi(NO3)3 3 5H2O solution (0.233 g, 0.48 mmol in 25 mL DMF) was added 0.4 mL, 1.67 mmol of C12H25SH, and 60 min later, the yellow precipitate was filtered from the yellow suspension and washed several times with DMF and ethanol, and then dried in air. 0.204 g of such a yellow precipitate and 1.667 g of PVP were dissolved in 15 mL of CH2Cl2 with stirring for 30 min; a solid was obtained after the removal of CH2Cl2 by a rotary evaporator. The thus-obtained solid was transferred to a boat, which was put into a long Pyrex tube (6 cm 1 m), capped with stopcocks on both ends, and purged with N2, and then heated to 105 C for 1 h. The final black solid product was washed with CH2Cl2 and ethanol by centrifugation several times and dried in a vacuum. Characterizations. Various techniques such as X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and atomic force microscopy (AFM) were used to characterize the structure, composition, size, and shape of the as-synthesized nanoproducts. The XRD patterns were collected at room temperature with the aid of a D-MAX-2500 diffractometer with Cu KR radiation. The TEM images were obtained using a JEM 2010 transmission electron microscope equipped with a field emission gun operating at 200 kV. Images were acquired digitally using a Gatan multipole scanning CCD camera with an imaging software system. Energy dispersive X-ray spectrometry (EDX) analyses were performed on a carbon-film-coated Cu grid with the aid of a JEM 2010 transmission electron microscope equipped with an Oxford INCA spectrometer. The elemental chemical analyses were performed by Vario EL III (Elementar Co.). Thermogravimetric analyses (TGA) and differential scanning calorimetry (DSC) were carried out with a NETZSCH STA 449F3 unit at a heating rate of 5 C/min under a nitrogen atmosphere. The UV-vis spectra were measured on a Perkin-Elmer Lambda-35 spectrophotometer.
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Figure 1. XRD patterns of the as-synthesized Bi: (a) disks, made at 135 C for 10 h by Route 1; (b) spheres, made at 105 C for 1 h by Route 2.
Results and Discussion The Structure and Morphologies of Bi Nanodisks and Nanospheres. The XRD (Figure 1a,b) patterns of the assynthesized Bi disks and spheres are all well indexed as rhombohedral Bi (ICSD 64705), and no other impurity is observed. The EDX study also indicates that the products only contain Bi element (Figure 2a,b), the Cu and C observed are coming from the sample grid. The morphologies were studied by SEM and TEM analyses. Figure 3b shows that the products prepared by Route 1 at 135 C for 10 h are monodispersed nanodisks with edge lengths of 100-260 nm and an average thickness of about 17 nm (σ = (16.1%) (Supporting Information, Figures 1 and 2). The size of the nanodisks has shown a temperature dependence; for example, the sample prepared at 120 C for 10 h has a decreased edge length range of 35-80 nm (TEM: Figure 3a, XRD: Supporting Information, Figure 5a). The single-crystalline character of the disk shown in Figure 3c is indicated by the corresponding SAED pattern as shown in Figure 3d; the distinct diffraction spots indicate that the Bi disk belongs to a rhombohedral phase, which is in agreement with the XRD analysis. The SAED pattern also reveals that the {0001} faces of the disk are parallel to the sample holder. Differently, Route 2 produces uniform rhombohedral Bi nanospheres with a narrow size distribution without aggregation. The spheres have smooth faces and good spherical profile. The SAED pattern in Figure 4f shows the good crystallinity of a single Bi nanosphere. The sizes of the spheres also exhibit a temperature dependence; for example, spheres made at 85 C for 30 min (SEM: Figure 4b; XRD: Supporting Information, Figure 9b) are in the range of
Figure 2. EDX patterns of the as-prepared Bi: (a) disks made at 135 C for 10 h by Route 1; (b) spheres made at 105 C for 1 h by Route 2.
100-174 nm, while at 105 C for 60 min, the sizes are 114-214 nm (Figure 4e). The Influence of the Organic Capping Reagent. The organic capping reagent is well-known to influence the morphology and size of metallic nanoparticles,2,8 which usually serves as a “directing” agent as a hard or soft template with different preferential absorption to different crystal faces, and therefore directs the growth of particles into different shapes.1 Here, our parallel experiments nicely show that a different organic capping reagent in a solventless method leads to a different morphology: DT, nanodisks, while PVP, nanospheres. The morphology analyses suggest that DT maybe mainly binds to the {0001} faces of Bi particles, and thus blocks the growth of the {0001} faces and leads to the formation of nanodisks. Differently, PVP does not show any preferential anisotropic absorption and thus leads to the isotropic growth of a sphere. Detailed description can be
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found in the following section Possible Formation Mechanism of the Bi Nanodisks and Nanospheres. The Influence of the Thermolysis Temperature. The melting point of the bulk Bi is relatively low, ∼271.4 C,26 and for the nano Bi particle, such a value is even lower because of the size
Figure 3. TEM images of the as-synthesized Bi nanodisks obtained by Route 1 at (a) 120 C for 10 h; (b) 135 C for 10 h; (c) a single Bi nanodisk shown in (b); (d) the SAED pattern for the disk in (c).
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effect. As a result, we found that the morphology and size of Bi nanoparticles are sensitive to the thermolysis temperature as listed in Table 1. For both Bi-DT and Bi-PVP reaction systems, the size increases with the increase of temperature. And the optimal temperature range for uniform disk from Bi-DT reactions is at 120-135 C, and for spheres from BiPVP reactions is at 85-105 C. All the reactions listed in Table 1 generate single phased rhombohedral Bi. Both Bi-DT reactions annealed at 120 and 135 C for 10 h generate monodispersed disks, and the edge lengths increase from 35-80 nm (SEM: Figure 2a, XRD: Supporting Information, Figure 5a) to 100-260 nm (SEM: Figure 2b, XRD: Figure 1a) with the increase of the temperature. Further increasing the temperature to 150 C, the disk morphology uniformity is destroyed. For example, a shorter annealing time of 5 h at 150 C already generates dispersed spheres as a major shape and a few disks, but the size is obviously increased; for example, the minimum size (the diameter of a sphere) is about 500 nm and the maximum size (the diagonal length of a disk) is 4 μm (SEM: Figure 5a, XRD: Supporting Information, Figure 5c). For Bi-PVP reactions annealed at temperatures lower than 75 C and shorter than 5 h (not listed in Table 1), the products are still yellow and almost none of the Bi peaks are found in the XRD pattern. Uniform Bi nanospheres can be produced at 85 C. As the time increases, the sizes increase from 100-174 nm for 30 min (Figure 4b) to 110-220 nm for 5 h (SEM: Figure 5b, XRD: Supporting Information, Figure 6a). Similar observations are found at 105 C: as the annealing time increases from 1 to 5 h, the sizes of the nanoproducts increase from 114-214 nm (Figure 4e) to 120-250 nm (SEM: Figure 5c, XRD: Supporting Information, Figure 6b).
Figure 4. Low- and high-magnification images of Bi nanospheres prepared by Route 2 under different conditions: (a, b) 85 C for 30 min; (d, e) 105 C for 60 min; (c) TEM images of Bi nanospheres made at 85 C for 30 min; and (f) a single Bi sphere with the SAED pattern in the inset. Table 1. Morphology (Size)-Temperature Dependence of the Single Phased Rhombohedral Bi Nanoparticles no.
precursora:
morphology
size (nm)
temp (C)
time (h)
image
1 2 3 4 5 6 7 8 9 10
Bi-DT Bi-DT Bi-DT Bi-PVP Bi-PVP Bi-PVP Bi-PVP Bi-PVP Bi-PVP Bi-PVP
disks disks spheres þ disks spheres spheres spheres polyhedra spheres þ others flakes þ cubes þ others flakes þ cubes þ others
35-80 100-260 500-4000 100-174 110-220 114-214 120-250 150-600 200-2000 200-3000
120 135 150 85 85 105 105 120 135 150
10 10 5 0.5 5 1 5 5 5 5
Figure 3a Figure 3b Figure 5a Figure 4b Figure 5b Figure 4e Figure 5c Figure 5d Figure 5e Figure 5f
a:
Bi-DT: 0.39 g, 0.48 mmol of Bi[SC12H25]3 mixed with 11.06 mmol of DT; Bi-PVP: 0.204 g, 0.25 mmol of Bi[SC12H25]3 mixed with 1.667 g of PVP.
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Figure 5. SEM images of Bi made by (a) Bi-DT precursors at 150 C, 5 h; (b-f) Bi-PVP precursors at 85, 105, 120, 135, and 150 C for 5 h, respectively. Table 2. Morphology of the Rhombohedral Bi Products That Is Influenced by the Bi: Capping Reagent Molar Ratio no.
precursor
ratioa:
morphology
size (nm)
reaction condition (C/h)
image
1 2 3 4 5 6 7 8 9 10
Bi-DT Bi-DT Bi-DT Bi-DT Bi-DT Bi-PVP Bi-PVP Bi-PVP Bi-PVP Bi-PVP
1: 2.6 1: 8.6 1: 17.3 1: 21.6 1: 26 1: 0 1: 10 1: 30 1: 60 1: 100
varied shapes rhombuses þ cubes rhombuses þ cubes nanodisks nanodisks irregular polyhedra polyhedra þ spheres spheres spheres spheres
60-300 100-300 100-250 30-85 35-80 80-250 80-250 100-300 114-214 100-200
120/10 120/10 120/10 120/10 120/10 105/1 105/1 105/1 105/1 105/1
Supporting Information, Figure 3 Figure 6a Figure 6b Figure 6c Figure 2a Figure 7a Figure 7b Figure 7c Figure 4e Figure 7d
a: For Bi-DT, the ratio means Bi(NO3)3 3 5H2O: DT; for Bi-PVP, the ratio means Bi(SC12H25)3: PVP, and the mole of PVP is calculated by using Mr(C6H9NO) = 111 other than Mr[(C6H9NO)n] = ∼40 000.
At temperatures higher than 120 C, miscellaneous shapes, such as sphere, flake, cube, etc. coexist. And the sizes have increased largely, from 600 nm to 3 μm (SEM: Figure 5d-f, XRD: Supporting Information, Figure 6c-e). In a short summary, both Bi-DT and Bi-PVP reactions show that the morphology, size, and the size distribution of the nanoproducts depend on the thermolysis temperature: the higher the temperature, the larger the size, and the wider the size distribution. Influence of the Reactant Molar Ratio. The quantity of the organic capping reagent is another key parameter to control the morphology of the Bi nanoproduct by a solventless method. As listed in Table 2, the capping reagent molar ratio in both Bi-DT and Bi-PVP reactions has influenced the uniformity of the morphology and the aggregation of the nanoproduct. In the Bi-DT system, the molar ratio of the starting reactants Bi(NO3)3 3 5H2O/DT=1:2.6, which is close to the stoichiometry of the neutral salt of [Bi(SC12H25)3], leads to particles with sizes of a large distribution (60-300 nm) and varied shapes (SEM: Supporting Information, Figure 3, XRD: Supporting Information, Figure 7a). When the molar ratio is 1:8.6 and 1:17.3, rhombuses and cubes with sizes of 100-300 and 100-250 nm were obtained, respectively (SEM: Figure 6a,b; XRD: Supporting Information, Figure 7b,c). When the amount of DT is about 7-fold higher than the cation-anion charge balance requirement, that is, molar ratio = 1:21.6, nanodisks are formed with a edge-length range of 30-85 nm (SEM: Figure 6c,d; XRD: Supporting Information, Figure 7d); after further increasing the ratio to 1:26, uniform nanodisks with edge lengths of 35-80 nm were formed (Figure 3a). Note that from 1:21.6 to 1:26, both
Figure 6. TEM images for Bi nanoproducts made from Bi-DT precursors with different molar ratios at 120 C for 10 h: (a) Bi3þ/ DT = 1:8.6; (b) Bi3þ/DT = 1:17.3; (c, d) Bi3þ/DT = 1:21.6.
the shape and size of the product do not change obviously, and this is probably because the amount of DT with a Bi/DT molar ratio of 1:21.6 is already enough to fully cap the newly formed Bi particles to ensure the formation of the nanodisks under the condition, and more DT is not necessary.
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For the Bi-PVP system, we have chosen the optimal temperature, 105 C, to investigate the ratio effect of Bi(SC12H25)3/PVP. When no PVP is used, most of the products are irregular polyhedral particles with sizes of 80-250 nm (SEM: Figure 7a, XRD: Supporting Information, Figure 8a). When the molar ratio of Bi(SC12H25)3/PVP is 1:10 (note that the number of moles of PVP is calculated by using Mr (C6H9NO) = 111 instead of Mr [(C6H9NO)n] = ∼40 000), irregular polyhedral particles and spherical particles are obtained (SEM: Figure 7b, XRD: Supporting Information, Figure 8b). When the ratio is 1:30, spheres with a good spherical shape but a wide size distribution of 100-300 nm are generated (SEM: Figure 7c; XRD: Supporting Information, Figure 8c). When the ratio is 1:60, beautiful spheres with a narrower size distribution of ∼114-214 nm are formed (Figure 4e). Further increasing the ratio to 1:100 does not narrow the size distribution, yet some negative influence becomes distinct, such as difficulties in washing,
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separating the sphere from the capping reagents, etc. (SEM: Figure 7d, XRD: Supporting Information, Figure 8d). The Influence of the Annealing Time. We found that the annealing time is another factor to determine the shape and size of the Bi product, but such influence is less important than that of the thermolysis temperature. The nanodisks made from Bi-DT precursors at 135 C have shown a size increase with annealing time elongation; for example, the edge lengths are 100-260 nm for 10 h (Figure 3b), and 150-500 nm for 20 h (SEM: Figure 8a, XRD: Supporting Information, Figure 5b). In the case of Bi nanospheres made from Bi-PVP precursors at 85 C, the spherical shape and the size distribution have not obviously changed with the increase of the annealing time. For example, for annealing at 85 C for 10 min (SEM: Figure 8b, XRD: Supporting Information, Figure 9a), 60 min (SEM: Figure 8c, XRD: Supporting Information, Figure 9c), and 300 min (Figure 5b), the shapes and the sizes of the thus-synthesized Bi nanoproducts are nearly the same. Differently, a shapetime dependence of the products made by the thermolysis of Bi-PVP precursors at 105 C has been observed. For example, uniform Bi nanospheres were obtained for 30 min (SEM: Figure 8d, XRD: Supporting Information, Figure 9d) or 60 min (Figure 4e). Polyhedra as a minor shape have been found for 120 min (SEM: Figure 8e, XRD: Supporting Information, Figure 9e). Half of the products are polyhedra for 210 min (SEM: Figure 8f, XRD: Supporting Information, Figure 9f). Uniform polyhedra with sizes of 120250 nm were obtained for 300 min (Figure 5c). Such a sphere to polyhedron transformation could be the crystallization process. Possible Formation Mechanism of the Bi Nanodisks and Nanospheres. Our previous work25a has pointed out that in the temperature range of 90-150 C, bismuth thiolate has undergone a redox reaction via a radical process to generate Bi element as eqs I and II represent: BiðSC12 H25 Þ3 f Bi þ 3 3 SC12 H25 ðIÞ 2 3 SC12 H25 f H25 C12 SSC12 H25
Figure 7. SEM images for the nanoproducts made from Bi-PVP precursors with different molar ratios at 105 C for 1 h: (a) Bi(SC12H25)3/PVP = 1:0; (b) Bi(SC12H25)3/PVP = 1:10; (c) Bi(SC12H25)3: PVP = 1:30; (d) Bi(SC12H25)3/PVP = 1:100.
ðIIÞ
The Bi3þ is reduced by the thiolate anion (SR-) through electron transfer. And the two produced radicals ( 3 SR) yield the disulfide according to Oae.25b Similar redox reaction has been found in the firing of Ag(SC12H25) at 180 C.27 However, no such redox is found in the reaction of Cu(SC12H25)2 at 155-220 C, in which only the decomposition of
Figure 8. (a) A SEM image of Bi nanoproducts made from Bi-DT, at 135 C for 20 h. The SEM images of Bi nanoproducts made from Bi-PVP at 85 C for (b) 10 min, (c) 60 min; SEM images of Bi nanoproducts made from Bi-PVP at 105 C for (d) 30 min, (e) 120 min, (f) 210 min.
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Scheme 1. The Possible Formation Mechanism of Nano Bi Disks and Spheres by a Solventless Method with a Capping Reagent
Cu(SC12H25)2 yielding Cu2S and C12H24 occurs.28 Therefore, the occurrence of the redox reaction depends on the tendency of a metal being reduced. Usually, the higher electronegativity a metal has, the easier it can be reduced. The redox reaction temperature of M(SR)n (M = Bi, Ag, Cu) increases as the decrease of the electronegativity, Bi (2.02) > Ag (1.93)>Cu (1.90), note that because the relative difficulty of the reduction ability of Cu, the redox of Cu(SR)2 cannot happen as described above. As the increase of the annealing temperature of Bi(SC12H25)3 to 250 C under a vacuum25a or higher under N2 flow, a decomposition happens as indicated by eq III. BiðSC12 H25 Þ3 f Bi2 S3 þ C12 H24
ðIIIÞ
The XRD pattern of a mixture of Bi and Bi2S3 produced by the themolysis at 360 C under N2 flow has been shown in Supporting Information, Figure 10. This agrees with the report on heating bismuth thiolate at 225 C in air by Korgel.29 Similar decomposition is also found for Ag(SC12H25) at 225 and 250 C.27 Therefore, in the thermolysis of Bi(SC12H25)3 and Ag(SC12H25), the redox happens at a relatively low temperature, while at high temperature, redox and decomposition both occur, and the latter become predominate. The DSC-TG curve of Bi(SC12H25)3 from 20 to 400 C under N2 flow has been provided in Supporting Information, Figure 11. After the early formation of Bi, which is regarded as the nucleation process, the following ripen-growing process is fully influenced by the capping reagent. For example, the SAED pattern of a single Bi disk made by a Bi-DT precursor (Figure 3d) has shown six clear diffraction spots that corresponds to the three faces of rhombohedral Bi, (1010), (0110), and (1100), so the height direction of such a disk is along the Æ0001æ direction. Therefore, the excessive DT is highly likely to bind the {0001} faces of Bi particles, and thus blocks the growth along such a direction. In this case, the growth velocities of {1010}, {0110}, and {1100} faces are similar during the ripening process, which are much faster than that of the blocked {0001} faces. As a result, the hexagonal nanodisks are formed. Different observations are found in Bi-PVP reactions, in which the initial nucleation process of Bi should take place at low temperature (
