Enhanced thermoelectric properties of Bi₂S₃ polycrystals through an electroless nickel plating process

Abstract

Bi₂S₃ is an eco-friendly alternative compound for thermoelectric devices. However, the low electrical conductivity of the pristine Bi₂S₃ hinders the improvement of its ZT value, which further restricts its application in the field of thermoelectricity. In this work, we report the first attempt to optimize the thermoelectric properties of Bi₂S₃ by electroless nickel plating. A nickel plated Bi₂S₃ powder sample was synthesized by electroless nickel plating on the precursor Bi₂S₃ powder prepared by mechanical alloying. Then, the powder was sintered to a bulk material by spark plasma sintering. The relationships between the composition, microstructure and thermoelectric properties of the bulk samples were investigated. The XRD results showed a AgBi₃S₅ second phase which was formed by the reaction of Ag residues with the Bi₂S₃ substrate during the sintering process. The nickel element and AgBi₃S₅ second phase introduced in the nickel plating process directly affect the electronic conductivity and Seebeck coefficient of the nickel plating sample, resulting in the relatively high power factor of 244 μW m⁻¹ K⁻² at 628 K. What's more, the thermal conductivity of the sample was also reduced moderately, obtaining a low value of 0.40 μW m⁻¹ K⁻¹ at 628 K. Therefore, a maximum ZT value of 0.38 was obtained at 628 K for the nickel plated sample, which is three times higher than that (0.12 at 628 K) of pristine Bi₂S₃ materials.

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Introduction

In the past decades, thermoelectric materials have drawn world-wide scientific attention due to the fact that they could be capable of directly and reversibly converting heat energy into electrical power and provide an alternative for power generation and refrigeration.^1,2 Generally, the conversion efficiency of thermoelectric materials is dominated by the dimensionless thermoelectric figure of merit ZT = (S²σ/κ)T, where S, σ, κ and T are Seebeck coefficient, electrical conductivity, thermal conductivity and the absolute temperature, respectively. Because of the well-known interrelated individual parameters, it is indeed a challenge to separately tune the S, σ and κ for the substantial improvement of the ZT value.^3–5 Several effective strategies, including band engineering, nanostructuring and nanoscopic approaches,^6–12 or intrinsically low thermal conductivity^13–15 have been successfully introduced and developed to improve the thermoelectric performance.

Despite achieving significant advances in this field, the state-of-the-art thermoelectric materials, including Bi₂Te₃,^10,16–18 PbX (X = S, Se and Te),^12,19,20 skutterudite,²¹ half-Heusler and SiGe,^22–24 contain the rare and expensive elements such as Te and Hf, or toxic elements such as Pb and Sb, which have restricted their applications to niche markets. In order to make further large-scale application, new robust candidates, mainly comprised of non-toxic, inexpensive and earth-abundant elements, are urgent to be discovered and optimized.^25–27

Compared with tellurium element, sulfur is more abundant in the earth, therefore, sulfur-based thermoelectric materials have received extensive attention such as PbS,^28,29 CoSbS,³⁰ Cu₂S,³¹ SnS,³² CuFeS₂,³³ etc. Among them, Bi₂S₃ is a typical V–VI thermoelectric materials with the band gap E_g of 1.3 eV. Bi₂S₃ has a high Seebeck coefficient and a relatively low thermal conductivity but its low electrical conductivity has resulted in a very low ZT value of 0.05 near the room temperature,³⁴ which is much lower than that of Bi₂Te₃. There is no doubt that improving the electrical conductivity of Bi₂S₃ material is the inevitable way for enhancing its thermoelectric property. Mostly recently, some researchers have reported that it is possible to enhance the electrical properties of the pristine Bi₂S₃. For example, Zhao et al. reported that sulfur vacancies can improve the electrical transport of Bi₂S₃.³⁵ Selenium incorporation in Bi₂S_3−xSe_x solid solution triggered a 2.5-fold increase in the electrical conductivity of Bi₂S₃ nanowires.³⁶ The copper as electron donor in the layered Bi₂SeS₂-based alloys optimized the band structure and increased the carrier concentration, simultaneously nano-scale Bi₂S₃ precipitates enhance the phonon scattering.³⁷ Furthermore, the substitutions of silver,^38,39 copper,^40,41 antimony⁴² and lanthanum⁴³ for bismuth sites, and selenium⁴⁴ for sulfur sites have also been investigated to increase the carrier concentration and electrical properties of Bi₂S₃. It is well known that nickel has a good electrical conductivity, and electroless nickel plating is a mature process that can deposit nickel element uniformly on any substrate. At the best knowledge of us, there has no report about plating nickel on Bi₂S₃ materials. Therefore, in this work, the effect of nickel plating on the thermoelectric properties of Bi₂S₃ thermoelectric materials has been preliminarily investigated. For convenience, nickel plated bismuth sulfide material is referred to as BS@Ni. The bulk of BS@Ni materials obtained the maximum value of figure of the merit as 0.38 at 623 K, which was three times higher than that of untreated Bi₂S₃ materials. This shows that electroless nickel plating is a valuable method to optimize the thermoelectric properties, which is worthy of further studying.

Experimental section

In a typical experiment, to synthesis the Bi₂S₃ precursor powders, commercial purity powders of Bi (99.9%), S (99.95%) under the same 200 mesh were used as raw materials. The powders weighed with a chemical formula Bi₂S₃ were milled at 425 rpm for 12 h in a mixed atmosphere of high-purity argon (95%) and hydrogen (5%), using a planetary ball mill (QM-1SP2, Nanjing University, China). Stainless steel vessels and balls were used, and the weight ratio of the ball to powder was kept at 20 : 1. After that, the Bi₂S₃ precursor powder was mixed in a 30 ml aqueous solution of 0.75 g SnCl₂ (98%) and 1.2 ml HCl (36–38%) and stirred for 5 min to sensitize the Bi₂S₃ precursor powder. Then, after removing the sensitized solution, and the powder was placed in a 30 ml aqueous solution with 0.048 g AgNO₃ (99.8%) and stirred for 20 minutes to activate it. And further, allergic and activated Bi₂S₃ powder was placed in nickel plating solution with a PH value at 9 and stirred for 1.5 h in a 60 °C water bath. Finally, we obtained the nickel plated Bi₂S₃ after separating the supernatant and drying the mixture at 80 °C for 6 h. Nickel plating solution was prepared by successively mixing 30 ml aqueous solution with 4 g NiSO₄·6H₂O (98%), 6 g Na₃C₆H₅O₇·2H₂O (98%), 5.4 g NaH₂PO₂·H₂O (99%) and 110 ml aqueous solution with 5 g HBO₃ (≥99.5%). The BS@Ni powder was charged into column graphite dies with 15 mm in internal diameter and then sintered in a vacuum using a SPS system (FDC SPS6321X, Japan) under 50 MPa at 773 K for 5 min. The sintered bulk samples were disks with a dimension of Φ 15 mm.

The phase structures of powder and bulk of BS@Ni were analyzed by X-ray diffraction (XRD, MiniFlex 600, Japan) with a CuKα radiation. The morphologies and elementary composition of powder and bulk of BS@Ni were characterized by the scanning electron microscopy and the energy dispersive spectrometer (SEM, EDS, EVO18, Germany). The TE properties were evaluated along the sample section which is perpendicular to the pressing direction of SPS. The Seebeck coefficient and electrical resistivity were measured form 323 K to 673 K in helium atmosphere using a Seebeck coefficient/electrical resistance measuring system (ZEM-3, Ulvac-Riko, Japan). The density of the sample was measured by the Archimedes method. The thermal conductivity of the sample was calculated by the relationship of κ = DC_Pρ from the thermal diffusivity (D) measured by the laser flash method (NETZSCH, LFA457, Germany), specific heat capacity (C_P) and density (ρ).

Result and discussion

Fig. 1 show XRD patterns and the main peak enlargement patterns of Bi₂S₃ and BS@Ni powders and bulks. The standard diffraction patterns of Bi₂S₃ (PDF# 17-0320) and AgBi₃S₃ (PDF# 50-1645) cited from the database are also plotted in Fig. 1(a) for comparison. Most of diffraction peaks can be indexed to the standard diffraction patterns of Bi₂S₃, but a second phase of AgBi₃S₅ was detected in the XRD patterns of BS@Ni bulk sample. The second phase may be produced by the reaction between the residual Ag element from activation process and Bi₂S₃ materials during the sintering process. The effect of the second phase on the thermoelectric performance of Bi₂S₃ bulk sample will be discussed later. In the Fig. 1(b), the XRD patterns of the bulk part are obviously shifted to the low angle compared with that of the powder parts, and the full widths of half maximum are reduced, indicating that crystallization process occurred in the process of SPS sintering, which led the increase of grain size. Furthermore, the BS@Ni sample produce a greater shift of the (130) diffraction peaks to lower angle, indicating that the nickel elements entered into the interstitial site of the Bi₂S₃ matrix and increased the crystal plane distance. Meanwhile, the XRD pattern of the powder of BS@Ni sample is also obviously shifted to the low angle compared with that of pure Bi₂S₃ phase because of the addition of Ni and Ag elements, but no significant second phase is detected.

Fig. 1 XRD patterns and the main peak enlargement patterns of Bi₂S₃ and BS@Ni powders and bulks. (a) XRD patterns of Bi₂S₃ and BS@Ni powders and bulks. (b) Main peak enlargement patterns of Bi₂S₃ and BS@Ni powders and bulks.

The morphologies of Bi₂S₃ and BS@Ni powders are shown in Fig. 2(a) and (b). The morphologies of the powders before and after nickel plating are almost the same, which are consisted of different particles whose sizes ranging from 0.3 μm to 2 μm. Furthermore, Fig. 2(c) and (d) show the face element scanning results from the area of Fig. 2(a) and (b). As shown in the figures, the ratio of Bi content to S content in the pristine Bi₂S₃ powder is 39.60 to 60.40, which is close to the stoichiometry ratio of 2 : 3. On the other hand, the ratio of Bi to S of the BS@Ni powder is 38.68 to 58.79, which is the same as the ratio of pristine Bi₂S₃ powder. It indicates that the composition of Bi₂S₃ did not change during the nickel plating process. In addition, the content of Ag and Ni is only about 1.29% and 1.23% respectively. Combined with nickel plating process, we believe that due to the small size of micron-scale of powders, it is difficult to conduct coarsening or hydrophilic treatment, resulting in a difficult formation of Ag catalytic nucleation center in the activation process. Meanwhile, part of the catalytic nucleation center of Ag element failed to reduce Ni ions, so that the final powder contained only a small amount of Ni element and a small amount of residual Ag element. It is precisely because of the low contents of these two elements that the diffraction peaks cannot be observed in the XRD patterns.

Fig. 2 SEM images and face scanning results of element contents of Bi₂S₃ and BS@Ni powders. (a) SEM image of Bi₂S₃ powder, (b) SEM image of BS@Ni powder, (c) face element scanning result of Bi₂S₃ powder from (a), (d) face element scanning result of BS@Ni powder from (b).

After sintering, the morphologies and mapping results of elements distribution of Bi₂S₃ and BS@Ni bulks are shown in Fig. 3. The morphology of Bi₂S₃ sintered block is relatively dense, which is consistent with the experimental result that the relative density of Bi₂S₃ bulk is 97.5%. In contrast, the relative density of BS@Ni is only 85%. As shown in Fig. 3(b), more pores in the BS@Ni bulk lead to its lower relative density. Combined with the XRD results of nickel plated block in Fig. 1(a), more pores in the morphology may be due to the fact that the silver particles which are on the surface of the powder reacted with the Bi₂S₃ substrate during the sintering process and generating AgBi₃S₅. It leads to the instability of the structure. The unstable structure leads to the loss of sulfur and generating more pores further. In addition, we also observe obvious layered structure in Fig. 3(b). What's more, the face scanning results of elements distribution are shown in the Fig. 3(c)–(f) respectively. As shown in Fig. 3(d), More Ni elements are distributed at the edge of Bi₂S₃ substrate but the distribution of Ag elements is relatively uniform, and there are partial distributions within the bulk in Fig. 3(f). The reason for this phenomenon may be that, in the recrystallization process of sintering, the theory of crystal growth kinetics shows that the Bi₂S₃ with layered structure is more likely to grow along the surface steps. The steps are also easy places to deposit Ni or Ag. Therefore, the Ni elements deposited on the surface of the substrate are continuously shifted to the boundary position as the crystal grows but the Ag elements reacted with the matrix and therefore obtaining a relatively uniform distribution. Finally, the result of element distribution is shown in Fig. 3(d) and (f). In order to facilitate the understanding, the process of nickel plating on bismuth sulfide powders and the element distribution in the sintered BS@Ni bulk are made into a schematic diagram, which is drawn in the Fig. 4.

Fig. 3 SEM images and face scanning results of element distribution of Bi₂S₃ and BS@Ni bulks. (a) SEM image of Bi₂S₃ bulk, (b) SEM image of BS@Ni bulk, (c) the Bi element distribution result of BS@Ni bulk, (d) the Ni element distribution result of BS@Ni bulk, (e) the S element distribution result of BS@Ni bulk, (f) the Ag element distribution result of BS@Ni bulk.

Fig. 4 Schematic diagram of nickel plating process and element distribution of BS@Ni bulk.

Further, the thermoelectric properties of Bi₂S₃ and Bi₂S₃@Ni bulks are shown in the Fig. 5. Among them, the errors of electrical properties are 3%, the errors of thermal properties are 5%, and the error of ZT value is 20%. Fig. 5(a) show the temperature-dependent electrical conductivities of Bi₂S₃ and BS@Ni bulks. The curves of electrical conductivity as a function of temperature exhibit distinct difference. The electrical conductivity of pristine Bi₂S₃ increases gradually with temperature rising, showing typical semiconductor behavior. At 628 K, the value of electrical conductivity of pristine Bi₂S₃ is only about 7.153 S cm⁻¹. For the BS@Ni sample, the electrical conductivity monotonously decreases with the temperature increasing, showing typical strong degenerate semiconductor behavior. In numerical aspect, the value of electrical conductivity of BS@Ni sample decrease form 38.4 S cm⁻¹ at 334 K to 28.9 S cm⁻¹ at 628 K. The lowest value is four times that of the conductivity of the original Bi₂S₃ at the same temperature. This is because the second phase of AgBi₃S₅ with high conductivity (which is about 42 S cm⁻¹) was produced during the preparation of nickel plated samples. The incorporation of the second phase and Ni elements resulted in the increase of S vacancies in the lattice and the enhancement of carrier concentration. At the same time, the Ni element distributing at the boundary position also improved the carrier mobility, so that the conductivity of the BS@Ni sample did not drop significantly with the increase of temperature as reported by Tan et al.,⁴⁵ and the value remained four times that of the pristine bismuth sulfide.

Fig. 5 Thermoelectrical properties of Bi₂S₃ sample and BS@Ni sample. (a) Electrical conductivity, (b) Seebeck coefficient, (c) power factor, (d) thermal conductivity, (e) lattice thermal conductivity, (f) ZT.

The temperature-dependent Seebeck coefficients of pristine Bi₂S₃ and BS@Ni sample are plotted in Fig. 5(b). All the samples show negative coefficient value in the whole measuring temperature range, indicating n-type semiconductors. For the pristine bismuth sulfide sample, the absolute value of Seebeck coefficient decreased with the increase of temperature. According to equation , it is clear that the Seebeck coefficient is positively proportionate to the electron effective mass (m*) at Fermi level and negatively to the carrier concentration (n). Therefore, the absolute value of Seebeck coefficient of the pristine Bi₂S₃ sample decreases gradually with the increase of carrier concentration caused by intrinsic excitation with the temperature increasing. Also affected by the intrinsic excitation of BS@Ni sample, the absolute value of Seebeck coefficient of BS@Ni sample increases gradually with the temperature rising. For BS@Ni sample, the hole carriers induced by intrinsic excitation have a more obvious cancellation effect on most electron carriers, which makes the carrier concentration decrease with the increase of temperature, resulting in the increase of absolute value of Seebeck coefficient. Also limited by the high carrier mobility, the absolute value of Seebeck coefficient of BS@Ni sample did not reach the level of the pristine Bi₂S₃ sample, and the maximum value was 291 μV K⁻¹ at 628 K.

Fig. 5(c) shows the temperature-dependent power factors of pristine Bi₂S₃ and BS@Ni samples, which are calculated from the formula PF = α|S|², where α is electrical conductivity and S is Seebeck coefficient, respectively. As shown in the Fig. 5(c), PF of the BS@Ni samples is much higher than that of the pristine Bi₂S₃ sample at all temperature range due to the enhanced electrical transport properties by Ni plating. The peak value of 244 μW m⁻¹ K⁻² at 628 K for BS@Ni samples was two times higher than that of pristine Bi₂S₃ sample at the same temperature.

In usual, we will write the total thermal conductivity κ_total of solids as follows:

κ_total = κ_lat + κ_el, (1)

where κ_lat is the lattice contribution and κ_el is the electronic contribution. κ_el can be calculated using the Wiedemann–Franz–Lorenz relation:

κ_el = LσT, (2)

where L is the Lorenz number, σ is the electrical conductivity, and T is the absolute temperature. Reduced Fermi energy was used to calculate Lorenz number (L) (eqn (3)) varies as Seebeck value changes (eqn (4)) with temperature or composition. The L calculation was estimated in a traditional single parabolic band model (resulting in an L with a deviation of less than 10% as compared with a more rigorous single non-parabolic band and multiple band models calculation), where the reduced Fermi energy was implicitly determined by the Seebeck values (eqn (4)). When the κ_el is calculated, the κ_lat is obtained by subtracting it from the κ_total.

Variations of the κ_total and κ_lat of Bi₂S₃ sample and BS@Ni sample with temperature are shown in Fig. 5(d) and (e). As shown in Fig. 5(d), the κ_total of BS@Ni sample is obviously lower than that of the pristine Bi₂S₃ sample. With the increase of temperature, the κ_total of BS@Ni samples gradually decreased from 0.48 W m⁻¹ K⁻¹ at room temperature to 0.40 W m⁻¹ K⁻¹ at 628 K, this was significantly lower than the 0.54 W m⁻¹ K⁻¹ of the pristine Bi₂S₃ sample at 628 K. This indicates that nickel plating is an effective method to reduce the κ_total of Bi₂S₃ materials. By comparing the values of κ_total and κ_lat in Fig. 5(d) and (e), it is not difficult to find that the κ_lat is dominant in the κ_total composition of the two samples. Compared with the pristine Bi₂S₃ sample, the κ_lat of BS@Ni sample decreased obviously. This indicates that the introduction of the second phase and the Ni element by the nickel plating process increased the scattering of phonons, resulting in the reduction of the κ_lat of the BS@Ni sample, and then led to a significant decrease in the κ_total of the BS@Ni sample.

The curves of ZT value of pristine Bi₂S₃ sample and BS@Ni sample are plotted in the Fig. 5(f). The ZT value of the BS@Ni sample is significantly higher than that of the pristine Bi₂S₃ sample due to the increased conductivity and decreased conductivity by nickel plating process. Its ZT value reaches the maximum value of 0.38 at 628 K, which is three times higher than that of pristine Bi₂S₃ materials. This proves that nickel plating is an effective method to optimize the thermoelectric properties of Bi₂S₃ materials.

Conclusion

In this work, BS@Ni samples was synthesized by electroless nickel plating on the mechanical alloying precursor-Bi₂S₃ powder and further spark plasma sintering. Its composition, microstructure, electrical and thermal transport properties have been investigated. Due to the influence of matrix particle size, the content of coating Ni elements are less, and there are a small amount of residual Ag elements. In the sintering process, residual Ag elements reacted with the base Bi₂S₃ material and formed AgBi₃S₅ second phase. The nickel element and AgBi₃S₅ second phase which introduced in the nickel plating process led to the increase of carrier concentration and mobility, resulting in significantly improvement of the electrical conductivity of BS@Ni sample. Meanwhile the Seebeck coefficient of BS@Ni sample still remained relatively high values of over 180 μV K⁻¹ in the whole measuring temperature range. Therefore, the BS@Ni sample obtained a relatively high power factor of 244 μW m⁻¹ K⁻² at 628 K. Combined with a low thermal conductivity value of 0.40 W m⁻¹ K⁻¹ at 628 K, a maximum ZT value of 0.38 was obtained, which is three times higher than that of pristine Bi₂S₃ materials (ZT = 0.12). Therefore, nickel plating is an effective method to optimize the thermoelectric properties of Bi₂S₃ materials.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 11764025).

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