# The Growth of Bismuth Sulfide Nanorods from Spherical-Shaped Amorphous Precursor Particles under Hydrothermal Condition

Authors: Pravas Kumar Panigrahi; Amita Pathak
Journal: Journal of Nanoparticles
 (2013)

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## Abstract

A surfactant/solid-template-free hydrothermal process has been developed for the synthesis of single-crystalline nanorods of bismuth sulfide (Bi2S3) using triethanolamine as a complexing agent for the Bi3+ ions and elemental sulfur, solubilized in monoethanolamine, as the sulfur source. X-ray diffraction and morphological studies of a series of samples synthesized at different reaction conditions suggest that the growth of nanorods occurred at the expense of the low-crystalline spherical precursor particles of aminium compounds of bismuth sulfide or bismuth sulfate formed at room temperature. In the process, the reaction condition is optimized for obtaining crystalline nanorods of pure Bi2S3 with high aspect ratio. From the XRD, XPS, and HRTEM analysis of the samples, the growth of nanorods was assessed to be due to the cooperative effects of solid-solution-solid transformation and controlled oriented attachment. The hydrothermal process parameters and the presence of water in the reaction system have been found to play a crucial role in the formation of high aspect ratio nanorods. The optical band gap of the synthesized sample at optimized conditions is found to be 1.46 eV as calculated from its diffused reflectance spectrum at room temperature.


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## Body

## 1. Introduction

Bismuth sulfide (Bi2S3) is a direct band gap semiconductor with large absorption coefficient, high band gap energy (1.3–1.7 eV) [1], and having energy conversion efficiency close to theoretical maximum attainable value. These qualities make Bi2S3 potentially suitable for the fabrication of optoelectronic and thermoelectric devices as well as applications in photovoltaic thermoelectric transport, photoconductivity, electrical photoresponse, and field emission [2–7]. Nanostructures of Bi2S3, especially the one-dimensional (1D) nanostructures (such as wires, rods, tubes, and ribbons), are in fact considered to be the best contenders for these applications due to quantum confinement effect [8, 9]. Due to quantum confinement of the charge carriers in 1D nanostructure, the thermoelectric efficiency of a material is enhanced through the increase in its figure-of-merit value brought about by the increase in the Seebeck coefficient and the decrease in the coefficient of thermal conductivity through boundary scattering of the heat carriers [10]. Similarly, the electron transport in crystalline nanowires is expected to be several folds faster than through a polycrystalline structure. It is because of these reasons that the optoelectronic properties of single-crystalline 1D nanostructures of Bi2S3 have been widely studied for the design and fabrication of fast, miniaturized optoelectronic devices (such as optical switches and photodiode arrays) [11, 12] or for augmenting the performance of existing ones.Due to the immense technological importance of 1D nanostructured Bi2S3 in nanoscale devices fabrication, there has been a surge of interest in their synthesis. There has been a variety of synthetic approaches reported so far for their synthesis, which include evaporation-condensation route [1], molecular precursor decomposition route [13], solventless synthesis route [14], electrochemical deposition route [15], solvothermal/hydrothermal route [16, 17], and ionic liquid-assisted route [18]. Among these processes, solvothermal/hydrothermal method is one of the most studied solution-based synthesis route for the fabrication of 1D nanostructures of Bi2S3 because this technique allows the opportunities to control the morphology of the final products through the variation of different reaction parameters, which include reaction temperature, reaction time, reaction medium (or solvent), bismuth and sulfur sources, complexing agent, and surfactant [19–21]. Besides, in the context of the possibilities to modify the growth behaviour of Bi2S3 crystal to anisotropic one, the use of soft templates (such as surfactant, complexing agent, and block copolymer) as growth-directing agent has received significant amount of research interest in the recent past. Among various growth-directing agents, the use of a complexing agent to change the growth behaviour (i.e., both nucleation and growth rate) and to confine it in a desired direction remained an interesting aspect. The complexing agents are generally polyfunctional ligands that coordinate to the surface of either the ions or the nanoparticles formed, modify the particle surface energy, and thereby enforce the unidirectional growth of the crystal. These bi- or multidentate ligands in some cases also act as a soft template in the growth process.There are a number of reports on the synthesis of 1D nanostructured Bi2S3 by using organic ligands, such as long chain amines, alcohols, and acids as complexing agents. Yu et al., for example, reported the use of EDTA as a coordinating agent for the preparation of Bi2S3 nanorods [22]. EDTA plays multiple roles as a coordinating agent for Bi3+ as well as a soft template for the growth of nanorods under solvothermal condition [23]. On the other hand, Ota and Srivastava [24] utilized a multifunctional organic molecule (i.e., tartaric acid) as a capping agent for producing nanorods of Bi2S3. Similarly, Chen et al. [25] synthesized highly crystalline Bi2S3 nanorods using bismuth citrate and thiourea as precursor materials in the presence of cetyltrimethylammonium bromide. They claimed that the growth of nanorods is assisted by linear bismuth citrate polymer template. In addition, a number of polyols, for example, ethylene glycol [26], diethylene glycol (DEG) [27], and so forth, have been reported to assist in the direction-arrested one-dimensional growth of Bi2S3. In this process, the multiple –OH groups cap the formed particles of Bi2S3 and make them stable. These polyols could also exist in long chains due to the effective hydrogen bonding between them that serves as a template for growth of Bi2S3 nuclei to form nanorods [27]. Triethanolamine (TEA) is another important tetradentate chelating ligand containing three –OH and one amine group, which, like diethylene glycol, exists as a long chain polymeric framework in the presence of water due to the effective hydrogen bonding [28]. Moreover, TEA is miscible with water in all proportions and can be used in hydrothermal process successfully. Hence, it is expected that the use of this organic ligand can be an alternative route for the synthesis of Bi2S3 nanorods. However, the use of TEA for the synthesis of 1D nanostructure of Bi2S3 powders is limited only to a few reports [29].Therefore, the present work aims at synthesizing nanorods of Bi2S3 by using TEA as a complexing agent under hydrothermal condition. In the developed process, a novel and inexpensive sulfur source, that is, elemental sulfur solubilized in monoethanolamine (MEA) has been used. The essence of this work lies in the formation of amorphous spherical precursors at room temperature and subsequently their transformation into nanorods under hydrothermal condition. A growth mechanism is also proposed.


## 2. Experimental Details

### 2.1. Materials

Bismuth nitrate (Bi(NO3)3·5H2O), triethanolamine (HOC2H4)3N), monoethanolamine (HOC2H4NH2), and elemental sulfur were purchased from Merck, India, and used as received without further purification.


### 2.2. Synthesis

In a typical preparation, 1 g of bismuth nitrate was solubilized in a mixture of 20 mL of TEA and 120 mL of distilled water by stirring continuously for 1 h. The clear solution was obtained possibly through the formation of [Bi(TEA)x3+] complex [29, 30]. Elemental sulfur (0.132 g) was separately dissolved in 10 mL of MEA, and it was then poured into the solution of [Bi(TEA)x]3+ complex. A brown precipitate was obtained immediately, which turned gray within few seconds. The entire solution mixture containing the gray colored solid precursor (i.e., the precursor sol) was then transferred to a Teflon-lined stainless steel autoclave and subjected to hydrothermal reaction at 160°C. After cooling the autoclave naturally to room temperature, the precipitates were collected, washed with water for several times and finally with ethanol, and dried. For the investigation of the reaction mechanism, the precursor sol formed at room temperature (prior to hydrothermal treatment) was also collected separately and characterized. This sample was referred to as “precursor sample” in the later part of the paper.


### 2.3. Characterization

The phase and structure analysis of the synthesized samples were carried out on Philips PW 1729 X-ray diffractometer using Co Kα radiation (λ = 1.79 Å). X-ray photoelectron spectroscopy (XPS) studies of the samples were carried out on an XPS system made by Omicron NanoTechnology using Mg Kα (1253.6 eV) as the excitation source. The morphologies of the samples were studied by using JEOL JSM-5800 model and Zeiss EVO-60 model scanning electron microscope. The chemical compositions of the samples were confirmed through energy dispersive analysis by X-ray (EDAX) using Oxford INCA electron microprobe attached to scanning electron microscope. JEOL JEM-2100 model high-resolution transmission electron microscope (HRTEM) was used to examine the microstructural details of the prepared samples. Raman spectroscopy was performed using Renishaw RM-1000 model instrument equipped with an Ar ion laser of wavelength 514.4 nm. The diffuse reflectance spectra of the as-synthesized powders were recorded at room temperature using Varian Cary 5000 UV-Vis-NIR spectrophotometer.


## 2.1. Materials

Bismuth nitrate (Bi(NO3)3·5H2O), triethanolamine (HOC2H4)3N), monoethanolamine (HOC2H4NH2), and elemental sulfur were purchased from Merck, India, and used as received without further purification.


## 2.2. Synthesis

In a typical preparation, 1 g of bismuth nitrate was solubilized in a mixture of 20 mL of TEA and 120 mL of distilled water by stirring continuously for 1 h. The clear solution was obtained possibly through the formation of [Bi(TEA)x3+] complex [29, 30]. Elemental sulfur (0.132 g) was separately dissolved in 10 mL of MEA, and it was then poured into the solution of [Bi(TEA)x]3+ complex. A brown precipitate was obtained immediately, which turned gray within few seconds. The entire solution mixture containing the gray colored solid precursor (i.e., the precursor sol) was then transferred to a Teflon-lined stainless steel autoclave and subjected to hydrothermal reaction at 160°C. After cooling the autoclave naturally to room temperature, the precipitates were collected, washed with water for several times and finally with ethanol, and dried. For the investigation of the reaction mechanism, the precursor sol formed at room temperature (prior to hydrothermal treatment) was also collected separately and characterized. This sample was referred to as “precursor sample” in the later part of the paper.


## 2.3. Characterization

The phase and structure analysis of the synthesized samples were carried out on Philips PW 1729 X-ray diffractometer using Co Kα radiation (λ = 1.79 Å). X-ray photoelectron spectroscopy (XPS) studies of the samples were carried out on an XPS system made by Omicron NanoTechnology using Mg Kα (1253.6 eV) as the excitation source. The morphologies of the samples were studied by using JEOL JSM-5800 model and Zeiss EVO-60 model scanning electron microscope. The chemical compositions of the samples were confirmed through energy dispersive analysis by X-ray (EDAX) using Oxford INCA electron microprobe attached to scanning electron microscope. JEOL JEM-2100 model high-resolution transmission electron microscope (HRTEM) was used to examine the microstructural details of the prepared samples. Raman spectroscopy was performed using Renishaw RM-1000 model instrument equipped with an Ar ion laser of wavelength 514.4 nm. The diffuse reflectance spectra of the as-synthesized powders were recorded at room temperature using Varian Cary 5000 UV-Vis-NIR spectrophotometer.


## 3. Results and Discussion

The XRD analysis of the product, obtained after 18 h of hydrothermal treatment at 160°C, is displayed in Figure1. The positions and relative intensities of all the peaks are in good agreement with those of the orthorhombic crystal structure of Bi2S3 and are indexed according to JCPDS file number 17-0320. The calculated cell parameters (a = 11.133 Å, b = 11.304 Å, and c = 3.977 Å) are also found to be well matched with the standard values (i.e., a = 11.14 Å, b = 11.30 Å, and c = 3.98 Å). Within the detection limit of the XRD method, no unidentified peaks in the diffractogram are noticed, suggesting that the final product is free from impurities. Energy dispersive analysis by X-ray (EDAX) of the sample reveals S-to-Bi atomic ratio as 1.45 against the ideal value of 1.5 for pure Bi2S3.Figure 1
XRD pattern of the Bi2S3 sample obtained after 18 h of hydrothermal treatment at a temperature of 160°C.The composition of this sample was also verified by X-ray photoelectron spectroscopy (XPS) (Figure2). The survey spectrum (Figure 2(a)) reveals the presence of elements, such as Bi, S, O, and C, which indicates the high purity of the resulting product. The peak for O arises, possibly, due to the adsorbed gases and/or oxides of C on the surfaces of the samples [29]. This observation is common in case of ultrafine powder samples when exposed to atmosphere. The high resolution XPS spectra in Bi region (Figure 2(b)) shows two peaks at 157.7 and 163 eV, which can be assignable, respectively, to the binding energies of Bi 4f7/2 and Bi 4f5/2 in Bi2S3. The broad peak at around 224.9 eV (Figure 2(c)) corresponds to the binding energy of S 2s. The observed values are found to be in close agreement with the data reported by Grigas et al. [31]. Raman spectroscopy studies were carried out to further validate the composition of the samples. Room-temperature Raman spectrum (recorded between 200 and 1400 cm−1) for the sample obtained after 18 h of hydrothermal treatment at 160°C is displayed in Figure 3. The spectrum depicts five distinct vibrational peaks at about 260, 306, 422, 606, and 965 cm−1, which corresponds to the characteristic Raman bands of crystalline Bi2S3 samples [24]. These are also in good agreement with the nanoparticles of Bi2S3 reported by Rabin et al. [7, 32].XPS analysis of Bi2S3 synthesized after 18 h of hydrothermal treatment at a temperature of 160°C; (a) survey spectra, (b) Bi region, and (c) S region.
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(c)Figure 3
Raman spectra of the Bi2S3 sample synthesized through hydrothermal process at 160°C and incubation time period of 18 h.The particle morphologies of the synthesized Bi2S3 powders were studied by electron microscopy. Representative SEM and bright field TEM micrographs of the sample obtained after 18 h of hydrothermal treatment at 160°C are shown in Figures 4(a) and 4(b), respectively. The micrographs reveal the formation of nanorods of Bi2S3 with average diameters and lengths ranging between 70 and 170 nm and 0.550–3 μm, respectively. TEM image of a typical nanorod having diameter of 90 nm and length of 2.5 μm and its detailed lattice fringe pattern are illustrated in Figures 4(c) and 4(d), respectively. The calculated fringe width of 0.56 nm (shown in Figure 4(d)) matches with the d-spacing of the (200) planes of the orthorhombic Bi2S3 crystal [12], suggesting that the growth of the nanorods of Bi2S3 occurred along the preferential [001] (c-axis) direction. Furthermore, the spotted pattern of the selected area electron diffraction (SAED) of the selected nanorod (shown as an inset in Figure 4(d)) infers single-crystalline nature of the synthesized sample.Bi2S3 sample obtained through hydrothermal process at 160°C after 18 h of incubation period; (a) SEM image, (b) corresponding TEM image, (c) TEM image of a selected nanorod, and (d) HRTEM image of the selected nanorod with the corresponding SAED pattern (inset).
(a)
(b)
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(d)In order to investigate the possible growth mechanism and the chemistry involved in the synthesis of Bi2S3 nanorods, a series of samples were synthesized at different reaction conditions and were analyzed by XRD and electron microscopy. Figure 5 represents the XRD patterns of the room-temperature precipitate that was separated prior to hydrothermal treatment and the samples obtained after 1, 3, and 12 h of hydrothermal treatment at a fixed temperature of 160°C. XRD pattern in Figure 5(a) showed broad and diffused peaks centered at around 2θ = 30.88°, 35.23°, and 51.73° indicating the precursor sample to be predominantly amorphous. The positions of these diffused peaks are slightly shifted from the standard values of major peaks reported for the orthorhombic Bi2S3 phase (JCPDS file no. 17-0320) but could not be assigned to any compound reported in the available literature. These unidentified broad peaks were however observed to diminish in intensity and eventually disappeared to give way to diffraction lines characteristic of the pure Bi2S3 phase. This could be achieved through hydrothermal treatment of the precursors at temperatures ≥160°C and incubation periods ≥6 h (see Figure S1 in Supplementary Material available online at http://dx.doi.org/10.1155/2013/367812). It was interesting to note that though the orthorhombic phase of Bi2S3 was realized through hydrothermal processing of the precursors at temperatures as low as 120°C and incubation period of just 3 h, but it was accompanied by the highest intense peak (at 2θ≈30.28°) of the unidentified phase. It was also realized that this peak persisted even though it prolonged the incubation period to 24 h at a temperature of 120°C. The peak however diminished in intensity with an increase in hydrothermal temperatures beyond 120°C, and in effect, the pure orthorhombic phase of Bi2S3 was accomplished on hydrothermal processing of the precursors at 160°C for 12 h (Figure 5(d)), although the products after 1 and 3 h of heating periods contain some amount of unidentified phase (as indicated by * in Figures 5(b) and 5(c)). On the basis of optimization studies, however, the pure and crystalline phases of Bi2S3 with relatively high aspect ratio were obtained on hydrothermal processing of the precursors at 160°C for 18 h, as depicted in Figure 1.Figure 5
XRD patterns of the samples synthesized at different conditions; (a) at room temperature, after hydrothermal treatment at 160°C for (b) 1 h, (c) 3 h, and (d) 12 h.It may thus be inferred that the broad peaks of the precursors (at 2θ = 30.88°, 35.23°, and 51.73°) correspond to an intermediate compound that are poorly crystalline and decompose to Bi2S3 at hydrothermal processing temperatures ≥160°C and incubation period ≥6 h. Liu et al. [16] reported the formation of similar intermediates, which decomposed to generate Bi2S3 nanoribbons. They assigned the broad peaks located at 2θ≈30.57°, 29.62°, and 52.75° to cubic NaBiS2, which got formed through hydrothermal reaction between Bi3+-glycerol complexes and elemental sulfur/Na2S2O3 in aqueous solution of NaOH. Ota and Srivastava [33] also predicted the formation of Bi2S3 nanotubes via decomposition of NH4BiS2 intermediates that were claimed to be produced from the reaction of [Bi(TEA)x]3+ complex with CS2 and NH3 in presence of Triton X-100 under hydrothermal condition. It was noted that the observed 2θ values of our precursors were slightly shifted from those reported for the cubic NaBiS2 phase. Therefore, based on these observations and drawing analogies with the reported literature in conjunction with the knowledge of the reagents involved in the present preparation, we can rationally predict the intermediates (precursors) to be aminium compounds of bismuth sulfide [(BiS2)−] or thiosulfate [[Bi(S2O3)]3−]. Hydrothermal processing of these precursors probably generates nanorods of Bi2S3. It however needs to be mentioned that though Na3Bi(S2O3)3·nH2O and NaBiS2 are reported in the literature [16, 34], the existence of aminium/ammonium intermediates is difficult to establish through XRD studies because of their poor crystalline nature and nonavailability of the standard literature claiming their formation in the crystalline form.In order to substantiate the prediction from XRD analysis, the composition of the precursor sample (collected prior to hydrothermal treatment) was further studied by XPS. The survey spectrum, shown in Figure6(a), reveals the presence of Bi, S, O, N, and C. Here, C 1s with standard binding energy (BE) value of 284.6 eV is taken as a reference. As C 1s peak in Figure 6(a) lies at 285.5 eV, instead of its ideal value of 284.6 eV, all the observed BEs in the XPS analysis are corrected accordingly [35, 36]. The small peak around 400 eV in survey spectrum may be attributed to N 1s of amine analogue [37, 38] present in the precursor sample, that is, aminium compounds of (BiS2)− and [Bi(S2O3)3]3−. The standard 2s binding energy for sulfur in their elemental state is reported to be 229 eV [39], which is lowered by 3 eV for S2− state in Bi2S3 [31]. So, the peak at 228 eV in Figure 6(b) may be assigned to 2s BE for S2− state in (BiS2)− species. On the other hand, the peak at 231.5 eV, which is higher than 2s BE for S0 but lower than S6+ state in sulfate ion [38], may be assigned to the presence of S2O32- species (S2+ state) in the precursor sample. In addition, the strong peak for O 1s in the survey spectrum (Figure 6(a)) could be deconvoluted into two peaks (531.1 and 533.6 eV). These two deconvoluted peaks could respectively be assigned to S2O32- species and to adsorbed CO2 in the sample [38]. The peak pair at 159.5 and 164.5 eV (in Figure 6(c)) is close to binding energies of Bi 4f7/2 and Bi 4f5/2 in Bi2S3 [13] and can be assigned to Bi3+ in (BiS2)− or [Bi(S2O3)3]3− species. This is because Bi3+ is not very sensitive to chemical environment; therefore, (BiS2)− or [Bi(S2O3)3]3− species are expected to show ionization potential values comparable to Bi2S3.XPS analysis of the precursor sample; (a) survey spectra, (b) S region, and (c) Bi region.
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(c)(a) TEM image of precursor sample and the corresponding SAED pattern (inset) and SEM images of the sample obtained through hydrothermal treatment at 160°C, (b) after 1 h, (c) after 12 h, and (d) after 24 h.
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(d)Based on the above experimental results, the possible chemistry involved in the preparation of Bi2S3 can be proposed. As it is already discussed in our previous paper [40] that the disproportionation reaction of solution of elemental sulfur in MEA generates a number of ions including thiosulfate (S2O32-) and HS− ions in the presence of OH− ions in the reaction mixture. The TEA chelated complexes of bismuth ions react then with the S2− and S2O32- ions at room temperature to produce aminium compounds of bismuth sulfide and thiosulfate (i.e., (BiS2)− and [Bi(S2O3)]3−) as per reactions (1) and (2):
(1)Bi3++2S2-⇌[BiS2]-(2)Bi3++3S2O32-⇌[Bi(S2O3)3]3-
Under hydrothermal processing, the aminium compounds of [Bi(S2O3)]3− are first converted into the corresponding (BiS2)−, which subsequently undergoes decomposition to generate Bi2S3 nanocrystals. These nanocrystals probably grow into 1D nanostructures of Bi2S3 with increasing the incubation period. The whole process thus can be summarized through the following set of chemical reactions:
(3)(4)(5)
To further study the possible growth mechanism of the formation of one-dimensional structures of Bi2S3, the effect of reaction temperature and reaction time on the morphologies of the samples were studied exhaustively by electron microscopy. Figure 7(a) depicts TEM image of the precursor sample obtained prior to hydrothermal treatment, which suggests the presence of mostly aggregation of nearly spherical nanoparticles of diameters in the range of 20–40 nm. These aggregates are amorphous in nature as indicated from its diffused SAED patterns (inset of Figure 7(a)). This finding is also supported by XRD analysis (Figure 5(a)). Figure 7(b) reveals that the nanorod-shaped morphologies get formed even after 1 h of hydrothermal processing at 160°C, though some agglomerated spherical particles are still present. With the increase of the incubation to 12 h, the number of rod-shaped particles of Bi2S3 increased, while the spherical aggregates reduced drastically (Figure 7(c)). Pure crystalline nanorods of Bi2S3 were eventually realized on increasing the incubation period to 18 h at the hydrothermal reaction temperature of 160°C (as shown in Figure 4(a)). It is interesting to note that on increasing the heating period to 24 h, a few nanowires of Bi2S3 (indicated by arrows in Figure 7(d)) are also formed amidst a pool of nanorods.The effect of reaction temperature on the morphology of the synthesized Bi2S3 sample was also investigated by varying the reaction temperature in the range of 120–180°C for a fixed incubation period (i.e., 6 h). The SEM images of the synthesized samples at hydrothermal temperatures 120, 140, 160, and 180°C (Figure S2, Supporting Information) suggest that rod-like morphologies with diameters in nanometer range are formed after 6 h of incubation period irrespective of the hydrothermal reaction temperature. However, some aggregated masses were visible, when the reaction was carried out at a lower temperature, which gradually disappeared as the temperature was increased. It can thus be inferred that the growth of nanorods occurs probably at the expense of amorphous spherical particles.Based on the morphological findings and other experimental observations, it is thus possible to propose a growth mechanism for the preparation of Bi2S3. As it was already mentioned earlier, a large numbers of Bi2S3 nuclei are generated as a result of the decomposition of the precursors at the initial stage of the hydrothermal reaction. These freshly formed nanocrystals or nuclei in the solution are unstable due to the presence of a large number of dangling bonds, defects, or traps on the nuclei surfaces. Hence, under hydrothermal condition, the smaller Bi2S3 nanocrystals precipitate onto comparatively larger particles, leading to growth of the crystals along the energetically favorable direction (i.e., c-direction) to form the nanorods of Bi2S3. The proposed growth mechanism agrees with the morphological findings, where it is evident that with the increase in the incubation period, the Bi2S3 particles grew more rapidly along the length compared to the diameters. This oriented crystal growth can be attributed to the typical anisotropic layered structure of Bi2S3. The process is also thermodynamically driven by the decrease in the total interfacial energy of the particles as a result of merging. An analogous mechanism was suggested by Zhu et al. [41] for the growth of nanorods from the initially formed Bi2S3 particles through a dynamic equilibrium: 2Bi3+ + 3S2−   ↔   Bi2S3. They proposed that the Bi3+ and S2− ions, which were in equilibrium in solution, precipitated onto larger particles, thereby decreasing the total interfacial energy between the particles and the solution. Liu et al. [16] recognized a similar growth sequence for the fabrication of Bi2S3 nanoribbons from NaBiS2 precursors through solvothermal process, but they choose to call the mechanism as “solid-solution-solid (SSS) transformation”. Gates et al. [42] also proposed the synthesis of single-crystal nanowires of selenium from spherical colloidal particles of amorphous selenium in aqueous solution through similar mechanism and called it to be SSS transformation. Based on the discussion, the formation and the overall growth process of the Bi2S3 nanorods through hydrothermal process can be schematically represented by a sketch as shown in Figure 8.Figure 8
Schematic representation of the growth of Bi2S3 nanorods from spherical nanoparticles of precursors through hydrothermal process.On the other hand, the formation of longer rods or wires of Bi2S3 (as shown in Figure 7(d)) probably occurred at the expense of small rod-shaped particles, which merged to reduce the net interfacial energy of the particles. This prediction is supported by bright field TEM images obtained for the sample synthesized at a hydrothermal temperature of 160°C for a period of 18 h (Figure 9). The contrast variation of TEM images (shown in Figures 9(a) and 9(b)) clearly depicts the joining of a pair of Bi2S3 nanorods. It can be seen from the images that rods are continuous across the connected part barring at some portions. For further insight into the microstructure at the junction, the encircled portion of the TEM image was magnified (Figure 9(c)), which shows the lattice planes at the joint. Lattice spacing, calculated from the fringe pattern of the HRTEM image, is found to be 0.476 nm corresponding to (120) plane of orthorhombic phase of Bi2S3. It can be seen from images that fringes are almost continuous across the joint with disrupted continuity only at selected area. The disrupted portion is indicated by an arrow in Figure 9(c).TEM images showing the joining of a pair of Bi2S3 rods (marked by circles); (a) at low resolution, (b) high resolution of image of the encircled portion in (b), and (c) high resolution of image showing the fringe patterns at the joint.
(a)
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(c)It can thus be predicted that under hydrothermal conditions, fusion of the particles at the interface takes place only when joining particles are suitably positioned so as to share a common crystallographic orientation. A similar morphological sequence has also been observed by Yu et al. [43] for the synthesis of nanowires of Bi2S3 from spherical crystals. They suggested that the formation of nanowires of Bi2S3 occurred via merging of short nanorods under hydrothermal reaction at 180°C (for 3 days) of the solution containing spherical crystals of Bi2S3. Deng et al. [23] explained the fabrication of sheet rods of Bi2Te3 on the basis of EDTA-assisted controlling oriented attachment of individual sheets. They too proposed that the joining process was thermodynamically driven by the reduction in the net surface energy of the particles, and the joining of particles was possible, only when they had common crystallographic orientation. On the basis of the present discussion, it can thus be inferred that both controlled oriented attachment and SSS mechanism cooperate in the formation of nanorods/wires of Bi2S3 from smaller rods/spherical particles in the hydrothermal process.In the developed process, TEA, which serves as a stabilizing agent for retaining the bismuth ions in solution through complex formation as well as an efficient capping agent [44, 45], also plays another important role in the formation of shape-controlled 1D structures of Bi2S3. TEA is a tetradentate ligand with three –OH groups and one amine group that effectively coordinates to the ions or particles, and thereby modifies the surface energy states of the particles, making them to grow in one dimension. In the aqueous reaction mixture, the unchelated, free TEA probably forms long chain polymeric framework [28] due to hydrogen bonding. These structures may help in assembling the initially formed nearly spherical particles to grow in one dimension under hydrothermal condition leading to the generation of uniformly sized pure and crystalline 1D structures of Bi2S3. To investigate the role of water in this mechanism, further experiments were carried out by heating a reaction mixture containing only TEA as a solvent (i.e., without water) in an autoclave at 160°C for 6 h and studying the morphology of the synthesized sample. In this case, Bi2S3 nanorods with diameters and lengths in the range of 77–575 nm and 1200–3700 nm, respectively (Figure S3, Supporting Information), were obtained in contrast to the nanorods (diameters and lengths in the range of 44–140 nm and 945–2150 nm, resp.) produced under similar reaction conditions with TEA and water as reaction medium. The results validated the utility of water in the reaction medium for the generation of uniformly sized nanorods with high aspect ratio.Bi2S3 has moderately high band gap energy (Eg) compared to the other sulfides of the same group, making them important materials for various optoelectronic applications. So, the diffuse reflectance spectrum of the Bi2S3 sample prepared at optimized hydrothermal reaction conditions (i.e., at 160°C for 18 h) has been carried out at room temperature, and the result is depicted in Figure 10. The optical band gap was calculated from the point of intersection of the tangent of the absorption edge and the extrapolated line of the diffuse reflectance at lower wavelength, and the value was found to be 1.46 eV. This calculated value lies in the band gap range reported for bulk Bi2S3, which may be attributed to their larger average particle diameters compared to the exciton Bohr radius of Bi2S3 (29.8 nm) [46].Figure 10
Diffuse reflectance spectra of the Bi2S3 samples, synthesized through hydrothermal process at a temperature of 160°C and incubation period of 18 h.


## 4. Conclusions

Uniformly sized single-crystalline Bi2S3 nanorods with average diameters and lengths in the range of 70–170 nm and 0.55–3 μm, respectively, have been synthesized through hydrothermal reaction at a temperature as low as 160°C. These nanorods have been found to be of pure orthorhombic phase, as evident from XRD, XPS, and Raman spectroscopy. Nanorods of Bi2S3 have been generated at the expense of the amorphous precursor precipitates, formed at room temperature, which has been found to consist of aminium compounds of [Bi(S2O3)]3− and (BiS2)−. The precursors undergo decomposition and subsequent growth into nanorods/wires through SSS as well as oriented-aggregation-growth process under hydrothermal conditions. It is found that Bi2S3 nanorods with relatively good aspect ratio and smooth surfaces are produced, only when all the reaction conditions, such as hydrothermal temperature, incubation period, and solvents, are tuned.



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