CRYSTAL GROWTH & DESIGN
Bioinspired Bi2S3 Thin Films on 4-Mercaptobenzoic Acid Functionalized Self-Assembled Monolayers Sheng-Cong Liufu, Li-Dong Chen,* Qun Wang, and Qin Yao State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, P. R. China
2007 VOL. 7, NO. 4 639-643
ReceiVed May 31, 2006; ReVised Manuscript ReceiVed January 8, 2007
ABSTRACT: Self-assembled monolayers (SAMs) of 4-mercaptobenzoic acid (4-MBA) were used to induce the nucleation and growth of bismuth sulfide thin films from aqueous solution at low temperature. The effects of 4-MBA SAMs on the shape, structure, and crystallographic orientation of Bi2S3 crystals were investigated using scanning electron microscopy, atomic force microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, high-resolution transmission microscopy, and selected area electron diffraction. It was found that Bi2S3 crystals on the 4-MBA SAMs showed a rectangular parallelepiped shape and exhibited completely oriented crystalline domains with [001] directional preferred growth. However, the precipitated Bi2S3 films on the unfunctionalized surface consisted of quasispherical Bi2S3 polycrystalline particles. The mechanism of differences in the crystal properties can be explained by taking into account the liquid growth medium and the effect of 4-MBA SAMs. Introduction Bismuth sulfide is a direct band gap (Eg ) 1.3 eV) semiconductor that crystallizes in the orthorhombic system (pbnm space group).1 Because of its good photoconductivity and useful photovoltaic properties, it can be applied in photodiode arrays and photovoltaic converters.2 Bismuth sulfide is also a well-known material with a lamellar structure3 and thus may exhibit excellent electrochemical hydrogen storage properties.4 Extensive investigations on the synthesis of Bi2S3 thin films have been carried out by various methods including chemical bath deposition,5 electrodeposition,6 spray pyrolysis,7 and vacuum evaporation techniques.8 But typically, the products are poorly crystallized or contain some impurities. In some cases, high temperature or vacuum equipment may be required, and even the metallorganic precursor compound must be prepared first. It is therefore highly desired to develop a simple, mild, and cost-effective method for fabricating crystalline Bi2S3 thin films with high purity. New synthetic approaches are often inspired by biomineralization, in which crystals with finely tuned sizes, crystallographic orientation, and morphology can be obtained through sitedirected nucleation and self-controlled growth.9 There has been recent interest in the use of two-dimensional organic layers with specific functional groups to initiate the nucleation of crystals from solution.10 By simply mimicking the role of interfaces in nature, self-assembled monolayers (SAMs) on various substrates are often employed to generate appropriate interfaces at which the synthesis of inorganic crystals11 can be realized. In biomineralization, SAMs play an active role as structural and/or chemical templates for the control of nucleation and growth of inorganic crystals at near room temperature in aqueous solutions. Recently, this technique has been mainly used to prepare oxide thin films, such as TiO2 and ZnO films,12 by hydrolysis reaction. There have been a few studies about the nucleation and growth of sulfide crystals on SAMs. For instance, PbS crystals/films were deposited on the SAMs with various functional groups on Au substrate.13 Nakanishi and co-workers fabricated CdS and ZnS nanoparticles mono- or multilayers on the thiolanchored SAMs on gold.14 But only a few works pay attention * Corresponding author. E-mail:
[email protected]. Tel: 86 21 52412522. Fax: 86 21 52413122.
to the differences in morphology and crystal structure in the presence or absence of SAMs. The aim of the present work is to fabricate highly crystalline bismuth sulfide thin films at low temperature by using 4-mercaptobenzoic acid (4-MBA) SAMs. The mechanisms of nucleation and growth of bismuth sulfide crystals are discussed in detail. Experimental Section Materials and Synthesis. Analytical grade bismuth nitrate (Bi(NO3)3‚5H2O) and thiourea (Tu, NH2CSNH2) were used as precursors. 4-Mercaptobenzoic acid (4-MBA, 97%, 4-HSC6H4COOH, also known as thiosalicylic acid) was received from Aldrich Chemical Co. The pH value was adjusted by the addition of a few drops of ammonium hydroxide. To prevent the hydrolysis of Bi3+ ions, bismuth nitrate was initially dissolved in a solution of nitric acid. All the chemicals were used without further purification. Deionized water was used throughout. Silicon (100) wafers (p-type, Pt(111)/Ti/SiO2/Si) were used as substrates to prepare Bi2S3 thin films. Before use, the substrate was cleaned ultrasonically in acetone, absolute ethanol, and deionized water. After drying, the substrate was immersed into a 10 mM ethanol solution of 4-MBA to allow self-assembly proceeding for 24 h. Upon removal from the solution, the substrate was rinsed three times by ethanol and dried under vacuum, and the 4-MBA SAMs covered substrate was then obtained. Bi2S3 thin films were prepared from an acidic solution (pH 0.87), which was prepared by mixing 5 mL of bismuth nitrate (0.1 M) and 60 mL of thiourea (0.1 M) in a sealed vessel. The nucleation and growth of Bi2S3 crystals were conducted by immersing the 4-MBA SAMs covered substrate into the solution at 50 °C for 24 h. The temperature was controlled by thermostatic bath. The substrate was horizontally dipped into the solution with the SAMs side upside-down to prevent particles formed in the solution from accumulating on the substrate surface. Bi2S3 crystals may grow over the SAMs as a result of Bi3+ and S2- ions released from the precursor aqueous solution under acidic conditions. The samples were then slightly washed with deionized water and dried naturally. For comparison, the directly precipitated Bi2S3 films as a result of sedimentation were fabricated by putting the unfunctionalized substrate on the bottom of vessel. Characterizations. Morphologies of Bi2S3 thin films were characterized by a field-emission scanning electron microscopy (FESEM; JEOL JSM-6700F). The surface roughness of the prepared thin films was examined on an atomic force microscopy (AFM; Seiko, SPA-300HV) operated in the tapping mode. Scans were taken in air at room temperature. Crystalline structures of the bismuth sulfide were analyzed by an X-ray diffractometer (XRD; Rigaku RINT 2000) operating with Cu KR radiation at 40 kV/40 mA. High-resolution transmission electron
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Figure 1. EDX spectrum taken from a single crystal giving the composition of Bi2S3; the inset shows the sample used in EDX experiments. microscopy (HRTEM) and selected area electron diffraction (SAED) analysis were carried out using a JEOL JEM-2100F microscope operated at 200 kV. TEM samples onto carbon-film-coated Cu grids were obtained by carefully touching the film surface with stainless steel stubs. Energy-dispersive X-ray spectroscopy (EDS) was obtained from an attached Oxford Link ISIS energy-dispersive spectrometer fixed on a JEM-2100F electron microscopy.
Results and Discussion Composition and Morphology. EDS experiments were performed to determine the composition of the obtained thin films on 4-MBA SAMs. Figure 1 displays the energy-dispersed X-ray (EDX) spectrum of the sample as shown in the inset. Only Bi and S peaks are observed in this spectrum in addition to C and Cu peaks, which were generated by the carbon film on the copper grid, suggesting that the products are composed of Bi and S elements. Quantitative EDX analysis shows that the atomic ratio of Bi/S is approximately close to 2:3, giving the composition of Bi2S3. The typical SEM image of Bi2S3 thin films on the 4-MBA SAMs covered substrate is shown in Figure 2a. The specimens on the functionalized surface show densely nucleated arrays of Bi2S3 crystals with uniform shape and size. The close examination of the micrograph (Figure 2b) reveals that the Bi2S3 crystals exhibit rectangular parallelepiped shape with a typical edge width of about 120 nm, and extend to more than 500 nm in length. The root-mean-square (rms) roughness of the film surface is estimated to be 98 nm for the entire measured area (50 × 50 µm2). However, similar morphology is not observed for the precipitated Bi2S3 films on the unfunctionalized substrate, as shown in Figure 2c, in which one can see that the Bi2S3 thin films generally consist of randomly oriented quasispherical particles 3-5 µm in diameter with low density. The closer inspection demonstrates that these spheroids have innumerable pits on their surfaces, which seem to be etched spheroids (Figure 2d). The rms roughness of the precipitated Bi2S3 film surfaces is 924 nm (50 × 50 µm2). Hence, the 4-MBA SAMs are more active than bare metal substrate in producing a higher density of Bi2S3 crystals. We can infer that, in the presence of 4-MBA SAMs, the Bi2S3 nucleation occurs against the carboxylateterminated surface and the crystals grow in perfect crystallographic alignment. Crystallographic Structure and Orientation. Figure 3a shows the XRD pattern of the directly precipitated Bi2S3 thin
Liufu et al.
films on the unfunctionalized substrate. All the peaks could be indexed as the orthorhombic lattice of Bi2S3 with cell constants a ) 11.10 Å, b ) 11.23 Å, and c ) 3.98 Å, which are close to the reported data (JCPDS 17-0320). It is also shown that there are no significant deviation in relative peak intensities from those of bulk bismuthinite. The XRD pattern of the Bi2S3 thin films on the 4-MBA SAMs is illustrated in Figure 3b. No peak of other phases is observed, indicating that a single phase of bismuthinite structure was also obtained on the SAMs. Despite low-temperature synthesis conditions, the Bi2S3 thin films exhibit good crystallinity, which differs from other Bi2S3 films prepared by traditional chemical solution methods.5 The (002) and (101) reflections exhibit a higher intensity compared to those for a random orientation of bismuthinite. The relatively large differences in peak intensities indicate that the Bi2S3 thin films on the SAMs are composed of aspherical crystalline domains that are elongated in a preferred crystallographic orientation. To determine the detailed crystalline structure, we employed TEM and SAED to investigate the Bi2S3 crystals on the SAMs. The inset of Figure 4a is the ED pattern with an electron beam incident along the [11h0] zone axis. The {001} and {110} reflections appear in the SAED pattern perpendicular and parallel, respectively, to the [001] direction (the long axis of the crystal). A HRTEM image looking down onto the surface of Bi2S3 crystal is given in Figure 4b. The distances between lattice fringes of 3.984 and 7.921 Å correspond to the (001) and (110) planes of the orthorhombic Bi2S3, respectively. Both the HRTEM and SAED pattern indicate that the Bi2S3 crystals growing on the 4-MBA SAMs have [001] directional preferred growth, whereas the directly precipitated Bi2S3 particles on the unfunctionalized substrate exhibit completely different crystalline behavior. Figure 4c shows the low-magnification TEM image and the corresponding SAED pattern of the precipitated Bi2S3 particles on the substrate without 4-MBA SAMs. TEM results demonstrate that these particles are quasispheroids containing nanoscale granular crystals. All of the concentric diffraction rings could be assigned to the polycrystalline Bi2S3 phase. HRTEM image (Figure 4d) reveals that the lattice spacing is the same as that of orthorhombic phase Bi2S3, verifying that these precipitates are indeed Bi2S3 particles. Nucleation and Growth Mechanism. On the basis of the above results, it can be imagined that the differences in morphologies and crystallographic orientations between the Bi2S3 crystals on the SAMs and the directly precipitated Bi2S3 particles are due to the liquid growth medium and the effect of 4-MBA SAMs. The nucleation and growth of Bi2S3 thin films are based on Bi3+ and S2- ions released slowly from the precursor solution. Thiourea is here used as both sulfur source and ligand, which can form complexes (Bi(Tu)n) with Bi3+. Upon heating, the following reactions may occur in the solution
H2NCSNH2 + 2H2O f 2NH3 + CO2 + H2S
(1)
H2S T HS- + H+ with equilibrium constant K1 ) 1 × 10-7 15 (2) HS- T S2- + H+ with equilibrium constant K2 ) 1 × 10-17 15 (3) 2[Bi(Tu)n]3+ + 3S2- T Bi2S3 + nTu Ksp,Bi2S3 ) 5.01 × 10-62 16 (4) The SAMs of 4-MBA can decrease the activation energy required for nucleation of the solid phase and favor the
Bioinspired Bi2S3 Thin Films
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Figure 2. SEM images of (a, b) Bi2S3 thin films on the 4-MBA SAMs and (c, d) the directly precipitated Bi2S3 films on the unfunctionalized substrate.
Figure 3. XRD patterns of Bi2S3 thin films (a) on the unfunctionalized substrate (b) on the 4-MBA SAMs.
heterogeneous nucleation on the SAMs surface. COOH terminal groups act as nucleation sites because of their capacity to bind Bi3+ ions. But once the initial nuclei have formed, the effect of SAMs on crystal growth would not be expected. Rather, the growth medium will affect the crystal growth by controlling the supersaturation degree of solution. According to the usual theory of crystal growth, no nucleation would occur in a solution with low supersaturation, whereas homogeneous nucleation occurs at high supersaturation, and crystal growth by heterogeneous nucleation is possible in the intermediate supersaturation region.17 In this work, the supersaturation degree is mainly controlled by the pH value. According to eqs 1-4. the low pH value (pH 0.87) could hinder the forward reaction, which prevents the occurrence of high supersaturation and promotes ion-by-ion growth of Bi2S3 crystals. The experiments showing
species depletion in the precursor solution (solution color from yellow to colorless) suggest that Bi2S3 crystal growth occurs by consumption of Bi3+ and S2- ions. Figure 5a shows the cross sectional SEM image of Bi2S3 films on the 4-MBA SAMs after 24 h growth. It can be seen that Bi2S3 crystals continuously grow from the bottom to the film surface. There is also nucleation in the bulk of the solution in the presence of SAMs. When the product of ion concentrations in the solution exceeds the solubility product (Ksp,Bi2S3), they could group together to create a colloidal Bi2S3 nucleus possessing electrical charge. This nucleus grows as a single grain. Because of grain aggregation, Bi2S3 particles are then produced in the bulk solution. As a result of sedimentation, these black particles finally deposit on the substrate to form Bi2S3 films. Therefore, the precipitated films consist of polycrystalline Bi2S3 particles, whereas the films on the SAMs are composed of single crystals. The formation of Bi2S3 crystals with c-axis orientation on the SAMs is related to the inherent crystal structure of Bi2S3 (shown in the inset of Figure 5a). The crystal structure is constructed by a stacking of continuous chains of strong bonds along the [001] direction. These chains are linked by weak electrostatic interactions forming layers parallel to (010) plane, with such layers being linked by still weaker van der Waals bonds.18 Therefore, the growth rate of the (001) face is high, whereas (110), (11h0) and (010) faces, with a low surface energy, grow slowly. Consequently, one-dimensional Bi2S3 nanorods tend to be formed, as found in early reports.19 We have examined more than 10 crystals with HRTEM and really found that all the crystals grow along the [001] direction. As the Bi2S3 crystals on the SAMs are not always nucleating from the (001) plane, they are thus randomly tilted away from the perpendicular direction of the substrate. The marked planes of Bi2S3 crystals growing on the 4-MBA SAMs are shown in the inset of Figure 5b.
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Figure 4. Typical TEM and HRTEM images of the samples obtained from the Bi2S3 thin films (a,b) on the 4-MBA SAMs and (c,d) on the unfunctionalized substrate. The insets are corresponding SAED patterns.
Figure 5. (a) Cross-sectional SEM image of Bi2S3 films on 4-MBA SAMs, the inset is the three-dimensional structure of Bi2S3 crystal; (b) scheme of Bi2S3 crystal growing on 4-MBA SAMs, the inset shows marked planes of SEM image of Bi2S3 crystals.
Conclusions In summary, we have succeeded in synthesizing highly crystalline Bi2S3 thin films on 4-MBA SAMs covered substrate in an aqueous precursor solution at low temperature. The terminated carboxylate groups could induce the Bi2S3 crystals nucleation on the SAMs, and the single crystal with a rectangular parallelepiped shape was realized through ion-by-ion growth. The crystalline structure was determined to be the orthorhombic phase, and the growth was along the [001] direction. However, bismuth sulfide with a diameter in the micrometer range was precipitated as discrete particles on the unfunctionalized substrate and exhibited polycrystalline phase. The preparation method of the Bi2S3 films growing on SAMs is an environ-
mentally friendly and low-temperature technique and may be an attractive alternative for the preparation of Bi2S3 thin films. Acknowledgment. Financial support from the National Natural Science Foundation of China (Grant 50325208), the China Postdoctoral Science Foundation, the Chinese Academy of Sciences K. C. Wong Postdoctoral Fellowship, and Shanghai Postdoctoral Science Foundation are gratefully acknowledged. References (1) (a) Boudjouk, P.; Remington, M.; Grier, P. D., Jr.; Jarabek, G. B. R.; McCarthy, G. J. Inorg. Chem. 1998, 37, 3538. (b) Sigman, M. B.; Korgel, J. B. A. Chem. Mater. 2005, 17, 1655.
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