Shape and stoichiometry control of bismuth selenide nanocrystals in colloidal synthesis

Abstract

Three kinds of Bi–Se nanocrystals (Bi₂Se₃, Bi₈Se₉ and Bi_1.007Se_0.993) have been synthesized via a hot-injection approach. The colloid morphology, from hexagonal to octahedron, strongly depends on the injection temperature. The growth mechanism of Bi–Se nanocrystals is revealed here. A higher temperature could cause the reduction reaction from Bi³⁺ to Bi, and accelerate the reaction speed of Bi–Se, thus causing a difference of monomer concentration in bulk solution, which impacts the shapes of bismuth selenide nanocrystals as well as the composition. The three Bi–Se nanocrystals also show different optical and photoelectrocatalyst performance.

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Nanocrystal engineering is of great interest over the past few years in catalysis,¹ electrocatalysis,^2,3 and photocatalysis,⁴ due to the size- and shape-controlled properties of nanomaterials. Among various methods to obtain nanocrystals, colloidal synthesis is highlighted due to its flexible processing chemistry⁵ and potential for large-scale production. Through controlling the reaction environment, like temperature, time, capping molecules, precursor ratio and monomer concentration, engineering shapes as well as constituents of nanocrystals is feasible. Guo et al. reported wurtzite ZnSe nanorod couples connected by twinning structures by purification and re-adding monomer approach.⁶ Han et al. found that the colloidal morphology of Bi₂S₃ nanocrystals, such as nanodots and nanorods, strongly depends on the preparation temperature and the ratio of Bi/S precursors, which lead to different optical and electrical properties.⁷ Sang-Min Lee et al. pointed out that the shape of CdS nanocrystals can be adjusted by the temperature-mediated phase control of the initial seeds.⁸ It is widely accepted that colloidal synthesis is an effective way to explore unique nanostructures of relevance for applications.

Bismuth selenide, a V–VI semiconductor, is mostly known as topological insulator material with a simple band structure and a single Dirac cone on the surface. However, with a narrow band gap (∼0.3 eV in bulk) tunable by shape, bismuth selenide is able to utilize near infrared solar energy, which is admirable for applying in photoelectrocatalyst.⁹ Moreover, Bi₂Se₃ is also popular in thermoelectric devices.¹⁰ Consequently, the investigation on shape-controlled synthesis of Bi₂Se₃ is of great interests. Liu et al. reported growth of Bi₂Se₃ nanobelts synthesized through a co-reduction method under ultrasonic irradiation,¹¹ Yan et al. had synthesized various Bi₂Se₃ nanocrystals with various morphologies using CVD method,¹² Kadel et al. expressed nano-flake like Bi₂Se₃ synthesized by hydrothermal method.¹³ Though there are some reports on colloidal synthesis of Bi₂Se₃,^14,15 to date, to the best of author's knowledge, there has been no reports on the synthesis of Bi–Se nanocrystals with tunable shape and composition by a simple temperature-controlled colloidal way.

Herein, Bi₂Se₃, Bi₈Se₉ and Bi_1.007Se_0.993 with hexagonal, disk and octahedron-like shapes respectively were synthesized by simply controlling the reaction temperature. The growth mechanism was discussed in detail. It is found that in higher temperature, formation of shapes with lower chemical potential were promoted. Moreover, optical and photoelectrochemical properties of three Bi–Se nanocrystals had been investigated.

It is known that Se powder is not soluble in oleyl amine until heated to 205 °C,¹⁶ but our previous work shows that in assistance of dimethylamine borane (DMAB), Se powder could firstly be reduced to alkylammonium selenide and then dissolved in oleyl amine at 110 °C.¹⁷ In this work, we prepared Se–DMAB–OLA solution as Se precursor and Bi(NO₃)₃–OLA as Bi precursor. By injecting Se–DMAB–OLA into Bi precursor at 220 °C, 250 °C and 290 °C, three kinds of Bi–Se nanocrystals were obtained (more experiment details in ESI†). In a typical method, 1.5 mmol (0.089 g) DMAB, 1.5 mmol (0.118 g) Se powder and 3 mL oleyl amine were loaded into a two-neck flask (10 mL) connected to a Schlenk line. After purging for several times, the temperature was raised up to 60 °C under vacuum for degassing about 30 min. Then the flask was heated to 100 °C under Ar for 10 min, the turbid solution gradually turned clear and yellow, as the Se powder reduced by DMAB. The Se–DMAB solution was kept at 60 °C for injection process. Meanwhile, 1 mmol (0.485 g) Bi(NO₃)₃·5H₂O with 10 mL oleyl amine were loaded into a three-neck flask (100 mL) connected to a Schlenk line. After purging and degassing process, the flask was heated up to determined temperature (220 °C, 250 °C and 290 °C) under Ar for 30 min to form a homogenous solution. The Se–DMAB solution made before was then injected to Bi-precursors, kept for 1 h for crystal growth. The final precipitates were washed with ethanol for several times, and dispersed in toluene.

TEM images show that at lower reaction temperature, the synthesized nanocrystal show a hexagonal shape (Fig. 1a and S1†). Corresponding XRD pattern (Fig. 2a) evidences that all the diffraction peaks can be related to rhombohedral Bi₂Se₃ phase (paraguanajuatite, JCPDS no. 33-0214). The characteristic peak represents the prominent growth orientation of Bi₂Se₃ nanoplates along the [015] direction.¹⁸ The selected area electron diffraction (SAED) ring patterns (Fig. 1b) could be related to the (107), (0015) and (1210) planes of Bi₂Se₃, and the interplanar spacing measured from high-resolution TEM (Fig. 1c) of 2.074 Å can correspond to the (110) plane. The sharp characteristic peak, distinct rings in the SAED pattern as well as clear lattice fringes reveal the high crystalline quality of Bi₂Se₃ nanoplates. The elemental composition of synthesized Bi₂Se₃ was determined by X-ray fluorescence (XRF) spectroscopy (Table 1 in ESI†), the atom ratio of Bi to Se is 4.1 : 6, almost the same as stoichiometric ratio of Bi₂Se₃ phase.

Fig. 1 (a)TEM image, (b) SEAD ring patterns and (c) HRTEM of Bi₂Se₃ nanocrystals; (d) TEM image, (e) SEAD ring patterns and (f) HRTEM of Bi₈Se₉ nanocrystals; (g) TEM image, (h) SEAD ring patterns and (i) HRTEM of Bi_1.007Se_0.993 nanocrystals.

Fig. 2 (a) XRD patterns of Bi₂Se₃, Bi₈Se₉ and Bi_1.007Se_0.993; (b) XPS spectra of Bi 4f for Bi₂Se₃, Bi₈Se₉ and Bi_1.007Se_0.993.

With increasing temperature, nanodisks were found at 250 °C (Fig. 1d and S2†). The XRD patterns (Fig. 2a) could correspond to guanajuatite Bi₈Se₉ phase (JCPDS no. 70-2610). SAED ring patterns (Fig. 1d) reveal (1022), (0048) and (0225) planes of Bi₈Se₉, and the HR-TEM image (Fig. 1f) shows the d-spacing of lattice fringes of 2.738 Å, which is related to the (0123) plane of Bi₈Se₉. In even higher temperature, octahedron nanocrystals were formed (Fig. 1g and S3†). For octahedron Bi–Se, the XRD (Fig. 2a) patterns can be accordingly indexed to hexagonal Bi_1.007Se_0.993 phase (nevskite, JCPDS no. 82-2442). SAED patterns (Fig. 1h) show three rings corresponding to the (105), (018) and (0110) planes of Bi_1.007Se_0.993, and the d-spacing of 3.23 Å (Fig. 1i) can be related to (013) plane. XRF results (Table 1 in ESI†) show the atom ratio of Bi to Se of 5.1 : 6 and 1.01 : 1 for Bi₈Se₉ and, Bi_1.007Se_0.993 respectively, which also correspond to the stoichiometric ratios. It is well known that the Gibb's free energy of the surface of nanoparticles increases with the surface–volume ratio, as a result, Bi₂Se₃ with hexagonal shape obtains the highest surface energy, following with Bi₈Se₉ with disk-like shape and ended with Bi_1.007Se_0.993 with octahedron shape. As the reaction temperature increases, the shape of Bi–Se nanocrystal turns more stable, from hexagonal to octahedron, and the atom ratio of Bi to Se gradually increases too along with the temperature. It can be concluded that higher temperature could promote the formation of stabler and Bi-richer nanocrystals. Moreover, XRD patterns ascribed to other impurity crystal phases were not detected. Average crystal size are shown in Fig. S4.†

We also deduced X-ray photoelectron spectroscopy (XPS) to probe the surface valence state of three kinds of Bi–Se nanocrystals. Fig. 2b shows the high-resolution spectra of Bi 4f of Bi₂Se₃, Bi₈Se₉ and Bi_1.007Se_0.993. In the XPS spectrum of Bi 4f in Bi₂Se₃, the two symmetrical peaks appear at the binding energies of 158.2 eV (Bi 4f_7/2) and 163.6 eV (Bi 4f_5/2) with a splitting of 5.3 eV is in consistence with the literature.¹⁹ However, in case of Bi₈Se₉, Bi 4f spin–orbit (SO) doublet become broader compared to that of Bi₂Se₃ with an asymmetrical characteristic. Since the metallic Bi–Bi bond are located at 157.0 eV (Bi 4f_5/2) and 162.3 eV (Bi 4f_7/2),¹⁹ we consider that each asymmetrical peak of Bi 4f in Bi₈Se₉ is a convolution of two peaks, as further shown in Fig. S5,† due to different bonding environment of Bi atoms (Bi–Se bond and metallic Bi–Bi bond). The phenomenon is also observed in Bi-rich surface of Bi₂Se₃ by Hewitt et al.²⁰ As for Bi_1.007Se_0.993, the peaks of Bi 4f SO doublet become even broader. Interestingly, there are less Bi–Bi bonds and more Bi–Se bonds in Bi_1.007Se_0.993 compared with those of Bi₈Se₉ (Fig. S6†). This is because theoretically, searching from Findit software we can find that Bi₈Se₉ is composed of Bi and Se atoms without any oxidation and reduction of the two elements (ICSD-42665 ²¹) and Bi_1.007Se_0.993 of Bi, Bi^2.979+ and Se²⁻ ions (ICSD-79019 ²²). So theoretically, there should be more Bi–Bi bonds in Bi₈Se₉ than those in Bi_1.007Se_0.993, which is in consistence with our result, though there are also a little Bi–Se bonds in Bi₈Se₉ in our experiment. It can be concluded that in three Bi–Se nanocrystals, Bi atoms have different bonding environments, as illustrated in Fig. S7† using CrystalMaker software.

XRD, TEM and XPS results show that in oleyl amine system, by controlling the reaction temperature, shape and composition control of Bi–Se nanocrystal is available. There have been several theories investigating how temperature infects the shapes of nanocrystals. Lee reported that the shapes might be adjusted by temperature-mediated phase control of initial seeds during the nucleation processes. Under different temperature condition, the crystalline phase of nanocrystal seeds might be different.²³ However in Peng's theory, they considered that temperature-dependence of shapes might be another form of concentration dependence.²⁴ Herein, before the nucleation process and formation stage, we concern more about the Bi-precursors heated to different temperature, since changes of color in high-temperature Bi-precursors were observed. As shown in Fig. S8,† when kept at 220 °C for half an hour, the color of Bi-precursor stays white, however when increasing the temperature to 250 °C or even higher (290 °C), the color of Bi-precursor suddenly turns black. The three Bi-precursors, kept under 220 °C, 250 °C and 290 °C, were centrifuged and washed several times before deposited on soda-lime glasses for XRD tests. XRD patterns of three Bi-precursors (Fig. 3) show that under 220 °C, no sharp peak is observed, implicating the absence of any well-formed crystals. As temperature increases, sharp peaks at around 26° appear, which are in consistence with bismuth (JCPDS no. 44-1246). This is resulted to the reducibility of oleyl amine under higher temperature which could reduce Bi³⁺ to Bi. It can be concluded that in preparation of Bi₂Se₃, only Bi³⁺ and Se²⁻ took part in the reaction, resulted in paraguanajuatite Bi₂Se₃. For the formation of Bi₈Se₉ and Bi_1.007Se_0.993, besides Bi³⁺, Bi also took part in the reaction and resulted in Bi-rich products. Chemical reaction equations for the formation of three kinds of Bi–Se composite are proposed here:

2Bi³⁺ + 3Se²⁻ → Bi₂Se₃

2Bi + 6Bi³⁺ + 9Se²⁻ → Bi₈Se₉

0.345Bi + 0.662Bi³⁺ + 0.993Se²⁻ → Bi_1.007Se_0.993

Fig. 3 XRD patterns of Bi-precursors heated under 220 °C, 250 °C and 290 °C for formation of Bi₂Se₃, Bi₈Se₉ and Bi_1.007Se_0.993 respectively.

Moreover, in higher temperature, as Peng mentioned in their theory,²² the consumption of Se²⁻, Bi and Bi³⁺ monomers in bulk solution for the nucleation process as well as the number of nuclei formed in nuclei stage should both be higher than those under low temperature. Consequently, the monomers concentration is lower in high temperature solution, which means the relative chemical potential of the remaining monomers in the growth stage is also lower. Thermodynamically, to achieve the balance, octahedron nanocrystals are more stable in bulk solution of lower monomer concentration than disk-like ones, let alone hexagonal ones, since the Gibb's free energy of the surface of nanoparticles increases with the surface–volume ratio as mentioned above. In this way, as reaction temperature grows, the shape of Bi–Se nanocrystal turns from hexagonal to octahedron like. Reaction process is illustrated in Scheme 1.

Scheme 1 Reaction process of Bi₂Se₃, Bi₈Se₉ and Bi_1.007Se_0.993.

Shapes impact the properties of Bi–Se nanocrystals. We have investigated the optical properties of three Bi–Se samples by dissolving them in toluene. Fig. S9† shows the room temperature optical absorption spectra of Bi₂Se₃, Bi₈Se₉ and Bi_1.007Se_0.993. Bi₈Se₉ shows the highest absorbance. All three Bi–Se nanocrystals show near-infrared absorbance, which is desirable for photocatalysis and photoelectrocatalysis. The optical absorption coefficient is calculated by using the equation:²³

α = −1/c ln A

where c is the concentration of Bi–Se sample dispersed per 1 mL of toluene (around 10⁻⁵ g mL⁻¹), A is the absorbance. The value of α is found to be higher than 10⁴ cm⁻¹ for three nanocrystals, showing that Bi–Se nanocrystals are direct bandgap semiconductors.²⁵ Bandgap values are calculated by Tauc equation:²⁶

(αhν)ⁿ = A(hν − E_g)

where A is a constant, hν presents the light energy, E_g is the optical band gap energy, α is the measured absorption coefficient, n is 0.5 for indirect band gap and 2 for direct band materials. Here we choose n = 2 since Bi–Se nanocrystals are all direct band semiconductors. The plot of (αhν)² with respect to hν is shown in Fig. S10.† Extending the linear part of the curve on the hν axis, the bandgap values of 0.85 eV, 1.2 eV and 1.25 eV is obtained for Bi₂Se₃, Bi₈Se₉ and Bi_1.007Se_0.993, respectively.

We also investigated the photo-response abilities of Bi₂Se₃, Bi₈Se₉ and Bi_1.007Se_0.993 nanocrystals. A conventional three-electrode PEC cell was constructed. Three Bi–Se samples dissolved in toluene were deposited on ITO glasses as working electrodes, a saturated calomel electrode (SCE) was adopted as counter electrode and a graphite electrode worked as counter electrode. 0.1 M Na₂SO₄ solution was applied as electrolyte, in which the illumination intensity was 40 W cm⁻² from xenon lamp. The linear sweep voltammogram (LSV) of Bi₂Se₃ (Fig. S11a†) under on/off chopping illumination shows a decrease of photocurrent density with negative potential sweeping, implicating an n-type semiconductor character of Bi₂Se₃. In the PEC cell, holes transferred from the valence band of Bi₂Se₃ to electrolyte. Bi₂Se₃ film shows photo response from 0.2 V to 1.3 V vs. SCE, with an photocurrent density of 6 μA cm⁻² at a bias of 1.2 V. This is at the same level of other reported Bi₂Se₃ materials.²⁷ For Bi₈Se₉ films (Fig. S11b†), it is hard to distinguish p/n type from LSV curves, and the photo response range is also narrower compared with Bi₂Se₃. However under 0.3 V, the photocurrent density achieves 3 μA cm⁻², which still have the potential of applying in photocatalysis and photoelectrocatalysis. For Bi_1.007Se_0.993, the photo response is very weak (Fig. S11c†). It can be concluded that both Bi₂Se₃ and Bi₈Se₉ can be used in photocatalyst and photoelectrocatalyst.

Conclusion

In summary, we have successfully synthesized three kinds of Bi–Se nanocrystals, Bi₂Se₃, Bi₈Se₉ and Bi_1.007Se_0.993 with hexagonal-, disk- and octahedron-like shapes respectively in a facile hot-injection way, by changing the reaction temperature. XRD analysis confirmed the formation of three Bi–Se nanocrystals, and the simulated crystal structures were given. Comprehensive analysis from TEM, XRF and XPS confirms the purity of Bi–Se nanocrystals. We also deduced a growth mechanism for three kinds of nanocrystals. From XRD analysis of Bi-precursors kept at different temperature, it is found that beyond 250 °C, Bi³⁺ was reduced to Bi, so at higher temperature, beside Bi³⁺, Bi also took part in the reaction. Moreover, the temperature also infects reaction speed, thus causing differences in shapes of nanocrystals. With different shapes and composition, Bi₂Se₃, Bi₈Se₉ and Bi_1.007Se_0.993 show different optical absorption and photoelectrocatalysis performances. Bi₂Se₃ and Bi₈Se₉ nanocrystals show potential of applying in photoelectrocatalyst.

Acknowledgements

This work was supported by Hunan Provincial Natural Science Foundation of China (13JJ1003 and 2015JJ2175).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra07570c

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