Solvothermal synthesis and electrochemical properties of S-doped Bi₂Se₃ hierarchical microstructure assembled by stacked nanosheets

Abstract

Hierarchical S-doped Bi₂Se₃ microspheres assembled by stacked nanosheets were successfully synthesized as the anode of a lithium ion battery, which shows an initial discharge capacity of 771.3 mA h g⁻¹ with great potential in energy storage.

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The limited fuel resources and the increasing environmental pollution have driven the exploration of new strategies for the conversion and storage of clean energy. Lithium ion battery (LIB) is one of the most promising devices that can meet the energy demand and relieve the environment deterioration to a certain extent for outstanding energy storage performance. Searching for low-cost and high-performance electrode materials is of great importance in the regime of LIBs. Semiconductors of A₂B₃ (A = Sb, Bi; B = S, Se, Te) have attracted increasing attention for their potential applications in photoelectrochemical solar cells,^1–5 thermoelectrics,^6–10 electrochemical catalysis,^11,12 electrochemical hydrogen storage,^13–16 and lithium/sodium storage due to their unique characteristics.^17–26 One example is Bi₂Se₃, which has a layered structure formed by a periodic arrangement of layers perpendicular to the trigonal c-axis. Each charge neutralized layer is formed by five covalently bonded atomic sheets Se–Bi–Se–Bi–Se and defined as a quintuple layer (QL) with a thickness of ∼1 nm.²⁶ van der Waals gaps exist between the Se atomic planes, which consist of vacant octahedral and tetrahedral among the adjacent atoms. A large amount of polyhedrons and interlayers in this structure provide space for ions shuttle.²¹ Compared to pure Bi₂S₃ and Bi₂Se₃, alloyed BiSSe has a tuneable visible band gap between ∼1.92 eV (for Bi₂S₃) and ∼1.40 eV (for Bi₂Se₃) and has immense potential in lithium storage owing to its characteristic crystal structure.⁵

For bismuth selenide and bismuth sulfide, many morphologies and structures, such as one-dimensional (1D) nanowires/nanorods/nanotubes,^28,29 two-dimensional (2D) nanoflakes/film,^2,30 three-dimensional (3D) nanoflower-/urchin-like structures^31,32 and other architectural structures,^21,33 have been designed and synthesized. Compared to other morphologies, two dimensional nanostructures exhibit distinct advantages for electrochemical Li-storage owing to their large surface area, small weight and sensible distribution.²⁷ For example, the anode fabricated from hierarchical Bi₂Se_3−xS_x flower-like microstructure assembled from ultrathin polycrystalline nanosheets exhibited a capacity of 235.1 mA h g⁻¹ after 30 charge and discharge cycles.³⁴ Stacked Bi₂Se₃ nanosheets synthesized from the microwave-assisted route showed better electrochemical performance than individual nanosheets.²¹ The progress in the application of Bi₂Se₃ nanostructures as anodes of LIB are summarized in Table S1,† which indicates that hierarchical assembled Bi₂Se₃ nanostructures could display excellent electrochemical performance. Herein, we developed a one-step solvothermal method to prepare S-doped Bi₂Se₃ hierarchical microstructures assembled from nanosheets with good lithium ion storage performance.

The S-doped Bi₂Se₃ hierarchical microstructure was synthesized from stable bismuth trichloride and selenium powder in the presence of mercaptoethanol by a solvothermal method. The gray selenium was pre-dissolved in the mercaptoethanol–amine mixture under ambient conditions, in which mercaptoethanol services as a reductant for reducing the selenium to anion, and also acts as a sulphur source for chemical doping.^35–38 Compared to other syntheses, the reduction of Se powder is very quick under mild conditions, and the solvothermal temperature (120 °C) is relatively low. The experimental details can be found in the ESI.†

Fig. 1a and b show the SEM images of S-doped Bi₂Se₃ hierarchical structure prepared from the presence of mercaptoethanol. The microstructure with scraggy surface can be found in the low magnification image (a), which is attributed to the stacked coin-like aggregates in image (b). The microspheres have a size of several micrometers and the stacked coins have an average thickness of one hundred nanometers. Nanosheets were also found in the undoped sample synthesized without mercaptoethanol (Fig. S1(a) in the ESI†), but with no uniform hierarchical structure. From the TEM images in Fig. 1c and d, layered nanostructure is confirmed to make up the coin-like aggregates. The layers are very thin (∼1 nm) and separated from each other with a space of 0.7 nm. Both S-doped and undoped Bi₂Se₃ samples have a similar specific surface areas, which were determined to be 12.8 m² g⁻¹ and 21.1 m² g⁻¹, respectively, from their nitrogen absorption and desorption curves displayed in Fig S2(a) and (b).†

Fig. 1 (a–b) Low and high magnification SEM images, (c–d) TEM images of the as-synthesized sample.

The crystalline structure of the S-doped Bi₂Se₃ hierarchical microstructure was determined by powder X-ray diffraction (XRD). As shown in Fig. 2a, all the diffraction peaks are matched with those of hexagonal Bi₂Se₃ (JCPDS 89-2008). The undoped Bi₂Se₃ sample has the same crystal structure (Fig. S1(b)†). In addition, some traces of Bi (JCPDS 89-1329) were detected in the undoped Bi₂Se₃ sample when mercaptoethanol is absent, which could be due to the partial reduction of the Bi precursor under the reaction conditions.

Fig. 2 (a) XRD pattern of S-doped Bi₂Se₃ hierarchical microstructure and (b) its Raman spectrum in comparison with that of undoped Bi₂Se₃ sample.

Raman spectroscopy is used to evaluate the local atomic arrangements and vibrations of the bands. Fig. 2b presents the Raman spectra of undoped and S-doped Bi₂Se₃ samples. Two Brillouin zone center Raman active modes at 123 and 171 cm⁻¹ were observed, which were assigned to in-plane mode E²_g and out-of-plane mode A²_1g.^39,40 Compared to the undoped sample, the S-doped Bi₂Se₃ exhibits a drastic enhancement of the peak at 171 cm⁻¹ (i.e. A²_1g) compared to the peak at 123 cm⁻¹ (i.e. E²_g). The enhancement may be due to partial replacement of Se at the Se2 sites with S, because the chemical bonding between Bi–Se2 is pure weak covalent compared to Bi–Se1 bond and Se atoms are partially replaced with the strong electronegative S.³⁴ The intensity ratio of A²_1g to E²_g increases with the thickness of the crystals decreasing from the bulk into a few quintuple layers.⁴¹ The enhanced ratio of A²_1g/E²_g in the S-doped Bi₂Se₃ microspheres demonstrates their layered structure with an uniform thickness (∼1 nm).

The composition of S-doped Bi₂Se₃ hierarchical microstructure was determined by ICP-OES, and the molar ratio of Bi : Se : S was 1 : 1.22 : 0.83. The EDS result (Fig. S3†) reveals the presence of S in the sample. The distribution of Bi, Se, and S in the microstructure was mapped by the electron energy loss spectroscopy, which is shown in Fig. 3b–d, respectively. The results clearly show that S was homogeneously doped into the Bi₂Se₃ structure.

Fig. 3 (a) Low-magnification STEM image of the hierarchical microsphere; (b–d) EELS mapping of Bi (green), Se (red) and S (blue).

The X-ray photoelectron spectroscopy (XPS) was carried out to further investigate the sample compositions. The survey scan spectrum (Fig. 4a) reveals the obvious presence of Bi, Se and S in the sample. A similar elemental ratio is obtained from semi-quantitative analysis. In high-resolution spectra in Fig. 4b and c, Bi 4d_3/2 and Bi 4d_5/2 are located at 465.7 eV and 441.6 eV, and Bi 4f_5/2 and Bi 4f_7/2 are found at 164.4 eV and 159.1 eV, respectively. It should be noted that S 2p_1/2 and S 2p_3/2 are located at a similar position to that of Bi 4f_5/2 and Bi 4f_7/2 (Fig. 4c). The binding energies of Se 3d_3/2 and Se 3d_5/2 are 53 eV and 55.5 eV, as shown in Fig. 4d.^6,42

Fig. 4 XPS spectra of S-doped Bi₂Se₃ hierarchical microspheres: (a) survey scan; (b) Bi 4d; (c) Bi 4f and S 2p; (d) Se 3d.

All the abovementioned characterization results demonstrate the successful doping of S into Bi₂Se₃ microspheres. Mercaptoethanol played important roles in chemical doping and the formation of hierarchical microstructure. It can make the gray selenium powder easily soluble in an ethylenediamine solution and react with the Bi ions under the ambient conditions. The temporal evolution of the morphology and crystal structure and the effects of EDTA on the formation of a hierarchical microstructure were investigated. Fig. 5a shows XRD patterns of the samples prepared under ambient conditions without a solvothermal treatment, through a solvothermal treatment at 120 °C for 1 h, 3 h, 12 h, and 24 h, and prepared without EDTA. The main peaks of (015) and (110) were located at identical positions in these samples. In addition, the peaks of (006) and (101) are emerged and sharpened in the samples with a long solvothermal treatment time (i.e. >3 h). These samples have better resolved XRD patterns because of their good crystallization. It should be noted that the addition of EDTA has no influence on the crystal structure of microspheres.

Fig. 5 (a) XRD patterns; (b–f) SEM images of samples prepared at ambient temperature, and at 120 °C for 1 h, 3 h, 12 h, and 24 h, and prepared in the absence of EDTA.

The SEM images of these samples are shown in Fig. 5b–f. Uniform microspheres (diameter: about 300 nm) were found in the sample formed under ambient conditions before the solvothermal treatment (Fig. 5b). The size of microspheres increases with the solvothermal treatment time increasing from 3 h to 12 h [Fig. 5c–e]. The formation of a hierarchical microstructure in the sample prepared in the absence of EDTA [Fig. 5f] shows that EDTA did not influence the sample morphology. Further characterization with TEM (Fig. S4†) demonstrated the absence of a sheet-stacking structure in the sample formed without a solvothermal treatment, and the presence of a stacking structure in the samples obtained with a solvothermal treatment. This highlights the importance of a solvothermal treatment in the formation of a hierarchical microstructure.

The elemental ratios of Bi : Se : S in these samples were determined to be 1 : 0.83 : 0.02, 1 : 0.89 : 0.02, 1 : 0.88 : 0.03, 1 : 0.70 : 0.24, and 1 : 1.22 : 0.83 by ICP-OES. The results show that a low temperature (e.g. ambient temperature) and short reaction time (i.e. 1 h) lead to a low S content, and an extension of the reaction time from 1 h to 3 h, 12 h, and 24 h could promote S-doping. Furthermore, the addition of EDTA results in a higher S content than the sample prepared in the absence of EDTA, as evidenced by their elemental ratios (1 : 1.22 : 0.83 vs. 1 : 0.66 : 0.40).

The formation reactions of S-doped Bi₂Se₃ are proposed in eqn (1)–(4).^35,43–45 On one hand, mercaptoethanol could be deprotonated during the catalysis of ethanediamine to give hydroxyethylmercaptide (eqn (1)) because the pK_a of thiol (–SH) group in mercaptoethanol is 9.61. The hydroxyethylmercaptide formed is chalcophilic and could react with Se powder to form hydroxyethylthioselenide [eqn (2)]. On the other hand, BiCl₃ is coordinated with mercaptoethanol to form a soluble complex in the mixture of mercaptoethanol and ethanediamine (eqn (3)). When two precursor solutions are mixed at ambient temperature, highly reactive hydroxyethylthioselenide react with bismuth–mercaptoethanol complex to generate Bi₂Se₃ [eqn (4)]. S was doped into Bi₂Se₃ during the high temperature solvothermal treatment.⁴⁶

HOCH₂CH₂SH + NH₂CH₂CH₂NH₂ ⇋ HOCH₂CH₂S⁻ + H₂NCH₂CH₂NH₃⁺ (1)

HOCH₂CH₂S⁻ + Se → HOCH₂CH₂SSe⁻ (2)

3HOCH₂CH₂SH + BiCl₃ ⇋ (HOCH₂CH₂S)₃Bi + 3HCl (3)

3HOCH₂CH₂SSe⁻ + 2(HOCH₂CH₂S)₃Bi → Bi₂Se₃ + 9HOCH₂CH₂S⁻ (4)

The S-doped Bi₂Se₃ hierarchical microspheres were fabricated into the anode of the Li-ion batteries to examine their electrochemical performance. The CV curve is shown in Fig. 6a and two pairs of peaks are found between 0.01 and 3.0 V. During the charging process, the sharp peak at 0.92 V corresponds to the long and gentle charging platform in Fig. 6b and the minor peak at 1.81 V could be expressed as a small slope around 2 V in the curves. In the discharging process, a small peak at 1.55 V was formed first, followed by a sharp peak at 0.53 V. The unique structure of S-doped Bi₂Se₃ hierarchical micro-spheres plays a key role in such electrochemical behavior. The hierarchical layers provide a large space (0.7 nm) and passage for Li-ion intercalation. The larger space than the radius of Li⁺ (0.076 nm) makes the intercalation and de-intercalation easy. At a high potential (i.e. 1.55 V), Bi ions are exchanged with Li ions by breaking the electrovalent bonds through the equation of Bi₂Se₃ + xLi⁺ + xe⁻ → Li_xSe₃ + 2Bi. The weak van der Waals force between the Bi₂Se₃ layers makes it easy to intercalate Li⁺ at a low potential (0.53 V) through an electrochemical reaction of Bi₂Se₃ + xLi⁺ + xe⁻ → Li_x⁺[Bi₂Se₃]^x−.⁴⁷ In the charging process, the inverse procedure occurred.

Fig. 6 Electrochemical performance of the anode fabricated from the typical hierarchical microstructure: (a) CV curve of S-doped Bi₂Se₃ in the potential range of 0.01–3.0 V at a scanning rate of 0.1 mV s⁻¹; (b) charge/discharge voltage curves of the anode in the first three consecutive cycles measured with a current density of 50 mA g⁻¹. (c) Cycling performance of S-doped Bi₂Se₃ anode compared to the pure Bi₂Se₃ anode under a current density of 50 mA g⁻¹; (d) rate stability of S-doped Bi₂Se₃ and pure Bi₂Se₃ anodes measured with a current density of 50, 100, 250, 500 mA g⁻¹ for 10 cycles.

Fig. 6b shows the specific capacities of the first three charge–discharge cycles. A high initial discharge capacity of 771.3 mA h g⁻¹ was obtained. From the first three charge curves, the reversible charge specific capacities were determined to be 578.6, 561.1 and 524.5 mA h g⁻¹. Small irreversible capacity loss in the first three cycles (192.7, 62.0 and 41.9 mA h g⁻¹) demonstrates their good performance.

The long term cycling stability of the S-doped Bi₂Se₃ anode is compared with the pure Bi₂Se₃ anode in Fig. 6c. For the pure Bi₂Se₃ anode, when a current density of 50 mA g⁻¹ is applied, the capacity decreases drastically from the initial 1027.5 mA h g⁻¹ to 110.2 mA h g⁻¹ after 20 cycles, and to 77.9 mA h g⁻¹ after 30 cycles of charge and discharge. In addition, when the current density is varied from 50 mA g⁻¹, 100 mA g⁻¹, 250 mA g⁻¹ to 500 mA g⁻¹, the specific discharge capacity also drastically decreases from ∼1035.8 mA h g⁻¹, ∼276.7 mA h g⁻¹, ∼97.2 mA h g⁻¹ to ∼39.3 mA h g⁻¹ [Fig. 6d]. It could only recover to ∼119.6 mA g⁻¹ after the current density is changed back to 50 mA g⁻¹.

In contrast to the pure Bi₂Se₃ sample, the S-doped Bi₂Se₃ sample exhibits a slower decrease in capacity with increasing number of charge and discharge cycles and is remained at 109.4 mA h g⁻¹ at the 100^th cycle. Moreover, when the current density is varied from 50 mA g⁻¹, 100 mA g⁻¹, 250 mA g⁻¹ to 500 mA g⁻¹, the capacity is changed from ∼669.1 mA h g⁻¹, ∼446.3 mA h g⁻¹, ∼210.1 mA h g⁻¹ to 59.2 mA h g⁻¹, which is higher than that of pure Bi₂Se₃ sample except for the initial capacity. The recovered specific discharge capacity (∼180 mA g⁻¹ at 50 mA g⁻¹) is also higher than that of pure Bi₂Se₃. These results demonstrate that the S-doped Bi₂Se₃ sample exhibits better electrochemical performance than the undoped counterpart, which is attributed to the certain retention of a hierarchical structure after long term cycling, as observed in the SEM image (shown in Fig. S5†).

Conclusions

Hierarchical S-doped Bi₂Se₃ microspheres assembled from nanosheets were synthesized by a one-step solvothermal method, in which mercaptoethanol was used as both the sulphur source and reductant. The S-doped content could be controlled by simply changing the solvothermal treatment time. The hierarchical S-doped Bi₂Se₃ microspheres show an excellent initial discharge capacity of 771.3 mA h g⁻¹, indicating a potential application to energy storage. Their cycling stability and rate performance are better than that of pure Bi₂Se₃ materials due to sulfur doping. This study highlights the feasibility of tuning the electrochemical performance of bismuth chalcogenides by chemical doping.

Acknowledgements

This study was supported by the National Natural Science Foundation of China (Grant No. 51302079 and 81471657), Jiangsu Provincial Key Laboratory of Radiation Medicine and Protection, and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). Z. Li acknowledges the support from the program of Jiangsu Specially Appointed Professorship.

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Footnotes

† Electronic supplementary information (ESI) available: Details of sample preparation, characterization, XRD, BET, EDX, and SEM images. See DOI: 10.1039/c6ra01301e

‡ F. X. Mao and J. Guo make equal contributions to the manuscript.

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