J. Phys. Chem. C 2009, 113, 16009–16014
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Syntheses, Growth Mechanism, and Optical Properties of [001] Growing Bi2S3 Nanorods Yue Wang,†,‡ Jing Chen,† Peng Wang,†,‡ 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: May 13, 2009; ReVised Manuscript ReceiVed: July 24, 2009
Uniform Bi2S3 nanorods growing along [001] have been synthesized from the precursor of Bi[S2CN(C8H17)2]3 by two different approaches: solventless and solvothermal methods. The structure, morphology, and formation process of the nanorods have been investigated by SEM, TEM, HRTEM, XRD, and EDX analyses. The growth direction is intrinsically governed by the crystal structure motif and the high surface energy of the (001) plane of orthorhombic Bi2S3 according to the CASTEP calculations. Besides, some important factors that influence the growth of Bi2S3 nanorods, such as the length of alkyl group in the precursor and the reaction temperature are investigated. The optical band gap of the as-synthesized Bi2S3 nanorods is measured to be 1.50 eV indicating a slightly blue shift owing to the quantum size effect. Introduction One dimensional semiconductor nanostructures, such as wire, rod, tube, and ribbon have attracted special interests because of their promising application in nanodevices. The main group metal chalcogenides, namely A2VB3VI (A ) As, Sb, Bi; B ) S, Se, Te), are a class of important semiconductors that are applied as photoconduction targets of television cameras, thermoelectric devices, and optoelectronic devices.1 The bulk Bi2S3 is a direct band gap semiconductor with an energy gap of about 1.3 eV,2 which is used to make photodiode arrays and photovoltaic converters and thermoelectric cooling devices.3,4 Different types of 1D Bi2S3, e.g., wire,5-7 tube,8,9 rod,10-13 and ribbon,14-16 have been synthesized by various methods such as hydro/solvothermal procedure,15-18 microwave irradiation,19,20 sonochemical process,21 and template method.5 In this paper, we have utilized two facile ways to synthesize uniform Bi2S3 nanorods: (1) a solventless approach: firing tri(dialkyldithiocarbamato) bismuth (Bi[S2CN(C8H17)2]3) free of solvent at 200-250 °C and (2) a solvothermal approach: heating Bi[S2CN(C8H17)2]3 in ethanol/DMF mixed solvent without any surfactant at 140-180 °C. The diameter and length of Bi2S3 nanorods can be controlled by the heating temperature or the length of the organic ligands. A possible growth mechanism for Bi2S3 nanorods has been proposed; the theoretical calculations have explained why the thus-obtained Bi2S3 nanorods always grow along the [001] direction. Finally, UV-vis diffuse reflection spectra have been used to investigate the optical properties of the Bi2S3 nanorods. Experimental Section The reactants were used without any further purification: (C8H17)2NH (di-n-hexylamine, Alfa Aesar, 96%), (C6H13)2NH (di-n-octylamine, Acros, 99%), (C4H9)2NH (di-n-butylamine, A.R., Tianjin Chemical Co.), (C3H7)2NH (di-n-propylamine, * To whom correspondence should be addressed. E-mail: liming_wu@ fjirsm.ac.cn. Tel: (011)86-591-83705401. † Chinese Academy of Sciences. ‡ Graduate School of the Chinese Academy of Sciences.
SCHEME 1: Reactions to Generate the Precursor Suspension A
Acros, 99%), Bi(NO3)3 · 5H2O (A.R., Shanghai Chemical Co.), ethanol (A.R., Shanghai Chemical Co.), CS2 (A.R., Shanghai Chemical Co.), DMF (A.R., Shanghai Chemical Co.), and CH2Cl2 (A.R., Shanghai Chemical Co.). Preparation of the Bi(S2CNR2)3 Precursors. Precursors (namely, P1, P2, P3, and P4) were prepared according to the equations shown in Scheme 1. Below 0 °C, CS2 had reacted with amine and KOH to produce dithiocarbamate salt, into which the DMF solution of Bi(NO3)3 · 5H2O was added to generate Bi(S2NR2)3 precipitation. In a typical procedure, 2 mL of di-n-octylamine, 20 mL of ethanol, and 0.364 g of KOH were added into a 50 mL-glass flask and stirred for 30 min. Subsequently, the flask was put into an ice bath, then 0.40 mL of CS2 in 10 mL of ethanol was added into the flask, and the reactants were stirred for another 3 h to ensure a full reaction. Finally, 1.045 g of Bi(NO3)3 · 5H2O in 7.5 mL of DMF solution was added with vigorous stirring. About 60 min later, the reaction was completed, and the thus-obtained yellow suspension is named suspension A. Bi2S3 Nanorods Prepared by a Solventless Method. The yellow suspension A was filtered, and the precipitation was washed with DMF and ethanol, then dried in air at room temperature, and collected as precursor (P). Such precursor was put into a silica boat, then transferred into a long silica tube (6 cm × 1 m), capped with stops with outlets on both ends, purged with N2 for 10 min, and heated at 200-250 °C for 60 min to produce a black solid. This black solid was dispersed in CH2Cl2, then precipitated with ethanol to remove the possible byproducts, and finally washed with water and ethanol. The products were dried at room temperature. Bi2S3 Nanorods Prepared by a Solvothermal Method. The yellow suspension A was transferred into a Teflon-lined stainless steel autoclave which was sealed and heated. The heating ramp
10.1021/jp904448k CCC: $40.75 2009 American Chemical Society Published on Web 08/13/2009
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Figure 1. XRD pattern of the as-synthesized Bi2S3 nanoproduct made by a solventless method at 250 °C for 1 h.
was done in 120 min from room temperature to the target temperature. Three parallel reactions have been carried out at 140, 160, and 180 °C for 5 h, respectively. The black solid product was collected by centrifugation, then washed with CH2Cl2, ethanol, and water several times, and dried in air at room temperature. Characterization. X-ray powder diffraction (XRD), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), and scanning electron microscopy (SEM) were used to characterize the structure, composition, size, and shape of the as-synthesized nanoproducts, respectively. The XRD patterns were collected with the aid of a D-MAX-2500 diffractometer with CuKR radiation at room temperature. The TEM images were obtained using a JEM 2010 TEM 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. The EDX analyses were performed on a carbon-film-coated Cu grid with the aid of a JEM 2010 TEM equipped with an Oxford INCA spectrometer. The chemical elemental analyses were performed by Vario EL III (Elementar Co.). The optical diffuse reflectance spectrum was measured at room temperature using a Perkin-Elmer Lambda 900 UV-vis spectrophotometer equipped with an integrating sphere attachment and BaSO4 as reference. Computational Details. The CASTEP method was used to calculate the crystal energy and the surface energy. The unit cell model of the Bi2S3 crystal was set up according to ICSD 89323, Pbnm, a ) 11.123 Å, b ) 11.282 Å, and c ) 3.971 Å. The surface models of (100), (010), and (001) planes were constructed with about 7-9 layers of atoms cleaved on the planes before a vacuum slab of 10.00 Å in thickness was built on the corresponding crystal surface. The energy of each plane was calculated using the generalized gradient approximation (GGA) with the gradient corrected functional Perdew-BurkeErnzerhof (PBE). The ultrasoft pseudopotentials (USP) were applied with a plane wave energy cutoff of 220 eV. The 2 × 6 × 1, 6 × 2 × 1, and 2 × 2 × 1 k points were used for the Brillouin zone of models of (100), (010), and (001), respectively. The other parameters were set as default. The self-consistent calculations are converged only when the total energy converges to less than 10-6 eV/atom. Results and Discussion Structure and Morphologies of Bi2S3 Nanorods Prepared by the Solventless Method. The X-ray diffraction peaks of the as-synthesized products by the solventless method are well indexed as orthorhombic Bi2S3 (ICSD 89323) (Figure 1). The morphology was studied by SEM and TEM analyses. As shown in Figure 2a,b, the Bi2S3 produced by the thermolysis
Wang et al. of the tri(dialkyldithio-carbamato) bismuth precursor at 250 °C for 1 h is uniform nanorods, with diameters of 59-84 nm and lengths of 2-3 µm. The image of a single Bi2S3 nanorod has been shown in Figure 2c, of which the selected area electron diffraction (SAED) pattern (Figure 2d) shows good crystallinity. The HRTEM images (Figure 2e,f) reveal that the perpendicular interlayer spacing is about 0.398 nm, corresponding to the interlayer distance of (001) plane of the Pbnm Bi2S3 phase, which indicates the [001] growth direction of the nanorod. The elemental analysis of the nanorods was examined by EDX (Supporting Information (SI), Figure 1a) and only Bi and S were found except C and Cu that come from the copper grid and carbon film of the sample holder. Thus the EDX analysis confirmed the binary component of the product. Structure and Morphologies of Bi2S3 Nanorods Prepared by the Solvothermal Method. In order to realize the reaction at lower temperature, we attempt to prepare Bi2S3 nanorods with solvothermal method in a relatively low temperature (140-180 °C) by heating suspension A as described above. The XRD patterns of the thus-made products match well with that of the orthorhombic Bi2S3 (Figure 3). All of the products have a rodlike morphology, and the lengths and diameters increase with the increase of temperature. For example, rods produced at 140 °C have lengths of 30-100 nm and diameters of 7-18 nm (Figure 4a), at 160 °C, lengths of 220-350 nm and diameters of 15-40 nm (Figure 4b), and at 180 °C, lengths of 220-560 nm and diameters of 18-60 nm (Figure 4c). Further investigation on the morphology and crystal growth orientation of Bi2S3 nanorods was preceded by HRTEM and SAED. A highly crystalline Bi2S3 nanorod (Figure 5a,b) obtained at 160 °C, shows a lattice spacing of 0.373 nm corresponding to the distance between (101) planes as well as 0.398 nm corresponding to that of (001) planes. As indicated by the long arrow in Figure 5c, the nanorod grows along the [001] direction, which recalls what was reported previously. Another example obtained at a different temperature (180 °C for 5 h) has been shown in Figure 5d, which also exhibits (101) (610) diffractions (Figure 5e) indicating the [001] growth direction. We have shown that two different approaches, either the solventless method from the solid Bi[S2CN(C8H17)2]3 precursor or the solvothermal method from the suspension of Bi[S2CN(C8H17)2]3 precursor, can both generate Bi2S3 nanorods. The comparisons between these two approaches are listed in Table 1. The optimal reaction temperature for the solventless method (200-250 °C) is higher than that for the solvothermal method (160-180 °C). As to the reaction time, although the heating time in the solventless method is only 1 h, 12 h is required for drying. Thus the solventless approach is more timeconsuming than the solvothermal method. Furthermore, compared to the solvothermal method, the solventless method shows less controllability of the product size. Perhaps the size of the product is sensitive to the temperature and the precursor is heated more evenly in liquid than in solid. However, the solvothermal method holds some disadvantages, for example, an expensive steel autoclave must be used, and the whole reaction proceeds under high pressure, which has a potential of explosion. On the contrary, the solventless method needs no expensive device and the whole process is under normal pressure, much safer than that of the solvothermal approach. We have also noticed that the growth direction of all of the as-prepared Bi2S3 nanorods is along [001] direction no matter which method is utilized, solventless or solvothermal. Does it imply that the reaction condition does not affect the growth orientation? Extended literature studies revealed that all the
[001] Growing Bi2S3 Nanorods
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Figure 2. (a and b) SEM images for Bi2S3 nanorods prepared at 250 °C for 1 h by the solventless method, (c) TEM image of a single Bi2S3 nanorod and its SEAD pattern (d), HRTEM image (e), and (f) the enlargement of the selected area in panel e.
Figure 3. XRD patterns of the as-synthesized Bi2S3 nanoproducts by the solvothermal method under different temperatures for 5 h: (a) 140, (b) 160, and (c) 180 °C.
orthorhombic Bi2S3 nanorods reported constantly grow along [001] direction. For example, Lou et al. prepared uniform Bi2S3 nanorods with Bi[S2P(OC8H17)2]3 precursor in oleylamine;12 Wei et al. obtained Bi2S3 nanorods with Bi particles and Na2S2O3 as starting materials from mixed solvent (EtOH/H2NCH2CH2NH2/ H2O);13 Wang et al. prepared Bi2S3 nanorods using a polyol method through reaction between bismuth nitrate and thiourea.15 Why does this happen? We consider the intrinsic crystal structural feature that is independent of the reaction condition probably determines the growth orientation of Bi2S3 nanorods. Why the 1D Bi2S3 Nanoproduct Favors Growth along the [001] Direction. To verify our thoughts, the “CASTEP Calculation” program is used to calculate the surface energies of (100), (010), and (001) planes of the Bi2S3 crystal. The thermodynamic energies of three models are listed in Table 2, and the surface energy is achieved through formula (I)
Esur ) (Etol - nEcell)/S
(I)
Esur is the surface energy of the Bi2S3 crystal, Etol is the total energy of the surface model, Ecell is the energy of one unit cell of the bulk Bi2S3, “n” refers to the number of unit cell in the established model, and S is the surface area of the calculation model.
The results listed in Table 2 clearly show that the (001) plane has the highest surface energy. Hence, to decrease the free energy effectively, growth along the c axis is preferred. Why is [001] the energy favorable growth direction of Bi2S3 16 nanorods? The orthorhombic Bi2S3 (D2h , Pbnm)22 has cell parameters of a ) 11.123 Å, b ) 11.283 Å, and c ) 3.971 Å with 20 atoms per unit cell. The crystal structure features a pseudolayer of [Bi2S3]∞ that is weakly connected via Bi2-S1 ) 3.31 Å interactions along the b axis (Figure 6a). Each pseudolayer contains [Bi4S6]∞ ribbons (indicated by a circle in Figure 6a) that are linked by relatively weak Bi1-S1) 3.03 Å bonds along the a direction. Such a ribbon (Figure 6b) is constructed by strong Bi-S bonds in the range of 2.58-2.74 Å in the format of a one-dimensional chain which is linked by relatively weak Bi-S bonds of 2.96 and 3.05 Å (indicated by thinner bonds in Figure 6a). There are two crystallographic independent Bi atoms, Bi1 is 6-fold coordinated with three strong Bi-S bonds and three weak ones (Figure 6c) and Bi2 has a 5-fold coordinated square pyramid sphere (three strong ones and two weak ones). The two long Bi2-S distances (3.31 Å, marked as dashed line in Figure 6d) connecting two neighboring layers are out of the Bi-S bonding range. In short, the single crystal structure of the orthorhombic Bi2S3 exhibits 1D chain motif extending along [001]. As the calculation indicated, (001) planes have the highest surface energy, and they should have the fastest growth velocity; therefore, during the ripening process, big Bi2S3 particles preferred to grow into rods along [001] direction by consuming small particles. On the other hand, most of the Bi-S bonds along the c axis have the shortest bond length and strongest bond energy, so once the chemical bond between Bi3+ and S2is formed along [001], it is very hard to be cleaved. This means that the Bi2S3 nanorods grown along the c axis are rather stable thermodynamically or kinetically. Furthermore, the cleavage between neighboring layers along the b axis is facile; thus, (010) planes have a lower surface energy. Comparatively, to break Bi-S bonds about 3.03 Å along the a axis is a little difficult; thus, the surface energy of (100) planes is slightly higher than that of (010) planes. Therefore, the Bi2S3 nanocrystal tends to grow along the [001] direction and cleave in the (010) plane and eventually develop into 1D rod. This explains well why
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Figure 4. TEM images for Bi2S3 nanorods obtained at different temperature by the solvothermal method: (a) 140, (b) 160, and (c) 180 °C.
Figure 5. Images of Bi2S3 nanorods made by solvothermal method at different temperatures for 5 h: (a and b) TEM and HRTEM, 160 °C, (c) the enlarged HRTEM image of rectangle part in picture b, which indicates that the nanorods grow along the [001] direction, and (d and e) TEM and SAED, 180 °C.
TABLE 1: Comparison between the Solventless and Solvothermal Methodsa method
P
ORT (°C)
TPT (h)
SCA
SP
devices
PD
solventless solvothermal
P1 (C8) P1 (C8)
200-250 160-180
∼24 ∼12
poor good
facile facile
cheap expensive
almost none some degree
a P (precursor); ORT (optimal reaction temperature); TPT (total preparing time); SCA (size control ability); SP (the simplicity of preparation); devices; PD (potential danger).
TABLE 2: Surface Energy of Different Bi2S3 Crystal Surfaces unit cell model of (100) plane model of (010) plane model of (001) plane
Etol (eV)
Esur (×10-4 kJ/m2)
-4591.0808 -4589.0716 -4589.1504 -16062.1464
3.5878 3.4964 4.2307
the Bi2S3 nanorods prefer to grow along the c axis instead of the b or a axes. Possible Formation Process of Bi2S3 Nanorods. In order to investigate the morphologhy-temperature relationship, a batch of parallel reactions have been carried out, and some representative TEM images are displayed in Figure 7. The detailed experimental conditions have been listed in Table 3. When precursor P1 (C8) was heated at 160 °C for 1 h, black Bi2S3 was obtained as dispersed nanoparticles and a small amount of rods (TEM: Figure 7a, XRD: SI, Figure 2a, EDX: SI, Figure 2e). Black Bi2S3 was also obtained at 180 °C (XRD: SI, Figure 2b, EDX: SI, Figure 2f) but as bundle of nanorods (TEM: Figure 7b). Such bundle morphology is interesting; considering 180 °C is a relatively low firing temperature for a solventless method, the not-yet-reacted precursor at 180 °C may absorb on the
surface of the then-already-formed nanorod so as to lead to the bundle aggregation. At a high temperature of 200 °C, uniform Bi2S3 nanorods with diameters of 43-86 nm and lengths of 517-849 nm are obtained (TEM: Figure 7c, XRD: SI, Figure 2c). This observation may suggest that the bundles of Bi2S3 began to break from the cores at 200 °C, and develop into larger rods. At higher 250 °C, as shown in Figure 2, uniform nanorods with a diameter of 55-91 nm and lengths up to 2-3 µm are formed. The temperature range of 200-250 °C has been approved to be the optimal temperature. Beyond such temperature, for example at 280 °C, the Bi2S3 nanorods are still formed, but the size and shape are relatively highly dispersed (TEM: Figure 7d, XRD: SI, Figure 2d). All of these experimental observations suggest that the formation of Bi2S3 nanorods may involve several distinctive stages as depicted in Scheme 2: (i) at 160 °C the nucleation of Bi2S3, and the formation of dispersed small Bi2S3 particles; (ii) around 180 °C, a ripening process of those small particles in a format of bundles of 1D rod-like Bi2S3; (iii) at 200-250 °C, [001] growth of nanorods that are intrinsically governed by the high surface energy of (001) planes.
[001] Growing Bi2S3 Nanorods
J. Phys. Chem. C, Vol. 113, No. 36, 2009 16013 TABLE 3: Description of the Nanoproducts Made from P1 (C8) ) Bi[S2CN(C8H17)2]3 with Solventless Method at Different Temperature T (°C)
time (h)
phase
shape
size (dia.; l.)a
160 180 200 250 280
1 1 1 1 1
Bi2S3 Bi2S3 Bi2S3 Bi2S3 Bi2S3
particle bundle rod rod Rod not uniform
N/A N/A 43-86 nm; 517-849 nm 55-91 nm; 2-3 µm 58-274 nm; >4 µm
a
The ‘dia.; l’ means the diameter and length of nanorod.
SCHEME 2: Proposed Formation Process of Bi2S3 Nanorods Synthesized by Heating Bi[SCN(C8H17)2]3 Precursor with a Solventless Method
are made according to Scheme 1 with different amines. For instance, P2 (C6) means Bi[S2CN(C6H13)2]3 and P3 (C4) represents Bi[S2CN(C4H9)2]3. Similar to P1 (C8) described above, the solventless treatment of P2 (C6) and P3 (C4) at 250 °C for 1 h also produced pure orthorhombic Bi2S3 phase (ICSD 89323) (SI, Figure 3). However, the rods made from P2 (C6) and P3 (C4) are obviously larger (Figure 8) than that from P1 (C8) (Figure 2). The detailed description of these products is listed in Table 4. Clearly, the diameter of the rod shows a nice dependence on the length of the alkyl group, which shows that the alkyl ligand with short length seems to make the aggregation of the nanoproduct easier so as to generate bigger rods. Figure 6. Single crystal structure of Bi2S3: (a) viewed appropriately along the c axis, the pseudolayer [Bi2S3]∞ is weakly connected via Bi2-S1 ) 3.31 Å interactions (dashed line) along the b axis; (b) a 1D [Bi4S6]∞ ribbon circled in Figure 6a; (c, d) the local coordination environment of Bi1 and Bi2 atoms. Bond distances are in Å.
Figure 8. TEM images of the Bi2S3 products obtained at 250 °C for 1 h using the solventless method by heating: (a and b) precursor P2 and (c and d) precursor P3.
TABLE 4: Ligand Influence on the Size of the Nanorod Figure 7. TEM images for Bi2S3 nanoproducts obtained at different temperature for 1 h by the solventless method: (a) 160, (b) 180, (c) 200, and (d) 280 °C.
By another set of designed experiments, we also found that the length of the alkyl ligand markedly influence the size of the Bi2S3 rod. In the parallel experiments, different precursors
solventless condition (250 °C, 1 h)
solvothermal condition (160 °C, 5 h)
precursor
rod diameter (nm)
phase
rod diameter (nm)
phase
P1 (C8) P2 (C6) P3 (C4) P4 (C3)
55-91 153-256 438-1184 N/A
Bi2S3 Bi2S3 Bi2S3 N/A
15-40 22-60 N/A 120-146
Bi2S3 Bi2S3 N/A Bi2S3 + unknown
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Wang et al. Acknowledgment. Financial support is by the National Natural Science Foundation of China under Projects (20773130, 20733003, 20803080, 20821061), 973 Program (2009CB939801), the “Key Project from Chinese Academe of Sciences, CAS” (KJCX2-YW-H01), the Knowledge Innovation Program of CAS, and the “Key Project from Fujian Institute” (SZD07004). This mansucript is dedicated to Prof. Xin-Tao Wu on the occasion of his 70th birthday. Supporting Information Available: Additional experimental details. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes
Figure 9. Optical absorption spectrum of Bi2S3 nanorods prepared by the solvothermal method at 160 °C for 5 h. The intercept 1.50 eV at F(R)2 ) 0 is the bandgap.
Similar results are found in solvothermal method as listed in Table 4, right column. For example, P1 (C8) produces a rod with a diameter of 15-40 nm (Figure 4); P2 (C6), diameter 22-60 nm (SI, Figure 4a, c); and P4 (C3), diameter 120-146 nm (SI, Figure 4b). Note that when P4 (C3) is utilized, the nano product is a mixture of major Bi2S3 phase and an unknown minor phase (SI, Figure 4d). UV-Vis Diffuse Reflectance Spectroscopy. Bi2S3 is one of the candidate materials for a solar photovoltaic converter with an appropriate band gap (bulk, ∼1.3 eV)4 for the absorption of sun light.23 The room temperature absorption spectrum of the Bi2S3 rods made from P1 (C8) with solvothermal method was investigated and shown in Figure 9. The band gap, calculated from the reflection spectra via the Kubelka-Munk function,24 F(R) ) (1-R)2/2R, is 1.50 eV, which is slightly wider than that of the bulk Bi2S3. This is probably because of the quantum size effect. Conclusion Uniform orthorhombic Bi2S3 nanorods growing along the [001] direction have been synthesized by both solventless and solvothermal methods. The theoretical study has revealed that such growth direction is intrinsically governed by the crystal structure motif and the high surface energy of the (001) plane. The product morphology dependence on the reaction temperature by both methods has been investigated, and a possible solventless formation process for Bi2S3 nanorods from small particle to bundle to rod is proposed. Besides, the influence of the length of the alkyl group on the size of the nano product is significant that precursor with shorter alkyl group yields larger rods. Finally, the UV-vis diffuse reflectance spectrum shows that the band gap of as-prepared Bi2S3 nanorods is slightly wider than that of the bulk Bi2S3, which may be caused by the quantum size effect.
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