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Thermoelectric Enhancement of Ternary Copper Chalcogenide Nanocrystals by Magnetic Nickel Doping

Dongsheng Chen , Yan Zhao , Yani Chen , Tingyu Lu , Yuanyuan Wang ,* Jun Zhou ,* and Ziqi Liang *

Dr. D. Chen, Y. Zhao, Y. Chen, Prof. Z. Liang Department of Materials Science Fudan University Shanghai 200433 , China E-mail: zqliang@fudan.edu.cn T. Lu, Prof. J. Zhou Center for Phononics and Thermal Energy Science School of Physics Science and Engineering Tongji University Shanghai 200092 , China E-mail: zhoujunzhou@tongji.edu.cn Prof. Y. Wang School of Environmental and Materials Engineering Shanghai Second Polytechnic University Shanghai 201209 , China E-mail: wangyuanyuan@sspu.edu.cn Y. Zhao Shanghai Advanced Research Institute Chinese Academy of Sciences Shanghai 201210 , China Dr. D. Chen College of Mathematics and Physics Shanghai University of Electric Power Shanghai 200090 , China

DOI: 10.1002/aelm.201500473

Cu 12 Sb 4 S 13 , Cu 3 SbS 3 , and Cu 3 SbS 4 . [ 19 ] By far, only Cu 12 Sb 4 S 13 nanocrystals are intensively studied for TE applications due to their low lattice thermal conductivity (<0.5 W m −1 K −1 ). [ 16–18 ] The best zT of Cu 12 Sb 4 S 13 based TE materials was reported as 1.13 at 302 °C. [ 18 ]

Theoretical studies showed that Cu 3 SbS 4 could exhibit excel-lent TE properties due to its relatively low κ L and large S . [ 20,21 ] The band gap of Cu 3 SbS 4 was reported as narrow as 0.9 eV, which is suitable for mid-temperature TE applications. How-ever, the maximum zT value of Cu 3 SbS 4 only reached less than 0.1 at 300 °C, which is far lower than that of theoretical predic-tion. [ 20 ] The key issue is how to increase carrier density through proper doping.

Recently, magnetic ion doping (e.g., Fe, Co, Ni, and Zn) was applied to ternary Cu 12 Sb 4 S 13 tetrahedrites to improve their TE performance. [ 16,18,22,23 ] For instance, the record zT value was up to 0.95 near 427 °C in synthetic Cu 12−x (Zn,Fe) x Sb 4 S 13 compound. [ 16 ] As another example, Ni-doped Cu 10.5 Ni 1.5 Sb 4 S 13 exhibited the best zT of 0.81 at 427 °C by a direct melting method. [ 22 ] In both cases, Ni-doping all resulted in an increase of electrical conductivity and a decrease of Seebeck coeffi cient or vice versa. However, recent theoretical and experimental studies showed that S and σ can be simultaneously increased in Ni-doped Cu 2 ZnSnS 4 nanocrystals, for instance, due to its spin effect. [ 11,24,25 ] This encourages us to explore the method of magnetic Ni ion doping to enhance the TE performance of ter-nary Cu 3 SbS 4 nanocrystals, which has been barely investigated.

In this work, we investigate TE properties of Ni-doped Cu 3 SbS 4 nanocrystals which were synthesized by using hot-injection method. Two different types of Ni-substituted Cu 3 SbS 4 nanocrystals, Cu 3–x Ni x SbS 4 and Cu 3 Ni x Sb 1–x S 4 , are compared. In Cu 3-x Ni x SbS 4 nanocrystals, enhanced electrical conductivity along with slightly changed Seebeck coeffi cient and reduced lattice thermal conductivity are acquired, which yields a max-imum zT of 0.37 for nominal Cu 2.25 Ni 0.75 SbS 4 nanocrystals at 250 °C. In Cu 3 Ni x Sb 1–x S 4 nanocrystals, by contrast, an increase of electrical conductivity and a decrease of Seebeck coeffi cient are routinely obtained. As a result, a maximum zT of 0.22 is attained for nominal Cu 3 Ni 0.05 Sb 0.95 S 4 nanocrystals at 250 °C mainly due to the reduction of lattice thermal conductivity. Both zT values are signifi cantly larger than those of the state-of-the-art ternary Cu 3 SbS 4 based TE materials.

Cu 3 SbS 4 nanocrystals were synthesized by hot-injection method as schematically illustrated in Figure 1 . [ 26 ] The details are described in the Experimental Section of the Supporting Information. First, two batches of precursor solutions of (i) sulfur in oleylamine (OLA) and (ii) a mixture of CuCl, NiCl 2 , and SbCl 3 in OLA were prepared at 80 °C, respectively. Next, the sulfur solution (i) was injected into the mixture solution

Thermoelectric (TE) materials can directly convert heat into electricity and vice versa because of the Seebeck effect and the Peltier effect. [ 1 ] The energy conversion effi ciency of TE materials is characterized by a dimensionless fi gure of merit zT = S 2 σT / κ , where S is the Seebeck coeffi cient, σ is the electrical conduc-tivity, κ is the thermal conductivity which consists of electronic thermal conductivity κ e and lattice thermal conductivity κ L , and T is the absolute temperature, respectively. [ 2 ] The intimate inter-play between transport properties limits the zT. Simultaneous enhancement of power factor ( S 2 σ ) and reduction of thermal conductivity are favorable for the increase of zT. [ 3 ]

High-performance TE materials such as Bi 2 Te 3 , PbTe, and SiGe have been widely studied. [ 4–6 ] However, Bi, Pb, Te, and Ge are expensive or toxic elements. Mass productive, low-cost, envi-ronmentally benign, and nontoxic TE materials are desirable in practical applications. Possible candidates such as copper-based multinary chalcogenide semiconductors (e.g., Cu x S, Cu 12 Sb 4 S 13 , and Cu 2 ZnSnS 4 ) have been thus widely studied. [ 7–13 ] In binary Cu x S system, the maximum zT reached 1.7 for Cu 1.97 S at 727 °C, which is in part ascribed to the liquid-like substructure of Cu ions. [ 7 ] In quaternary Cu 2 ZnSnS 4 system, the best zT of 0.42 was found at 427 °C, which is ascribed to Ni-doped ion spin entropy. [ 11 ] Ternary Cu-Sb-S systems have recently attracted wide attention for their solar cell [ 14,15 ] and TE applications. [ 16–18 ] The Cu-Sb-S system comprises four major phases of CuSbS 2 ,

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(ii) at 190 °C under nitrogen atmosphere. The reaction was main-tained for 30 min and then cooled down to room temperature. Finally, the brown-black powders were obtained via precipitation by adding a mixture of ethanol and hexane and then centrifuga-tion. The resulting Ni-doped Cu 3 SbS 4 solids were subsequently hot-pressed at 400 °C and 75 MPa for 30 min. The relative den-sities of all the fi nal products were measured as ≈90% of their theoretical densities. In our systematic studies, Cu 3–x Ni x SbS 4 (x = 0, 0.5, 0.75, 1) and Cu 3 Ni x Sb 1–x S 4 (x = 0, 0.05, 0.1) samples have been fabricated and investigated. Based on thermoelectric transport results, two optimal compositions—Cu 2.25 Ni 0.75 SbS 4 and Cu 3 Ni 0.05 Sb 0.95 S 4 —that show the best zT values in their respective categories are chosen for the following detailed studies.

Crystalline structures of as-synthesized Cu 3 SbS 4 samples were characterized by X-ray diffraction (XRD) measurement. Figure 2 shows XRD patterns of neat Cu 3 SbS 4 , Ni-doped Cu 2.25 Ni 0.75 SbS 4 , and Cu 3 Ni 0.05 Sb 0.95 S 4 samples, respectively. The main diffraction peaks in all samples are located at 28.7°, 47.8°,

and 56.7°, corresponding to the refl ections from (112), (204), and (312) crystal planes of Cu 3 SbS 4 (JCPD No. 35–0581) nanocrys-tals, respectively. [ 19 ] By substitution of Sb 5+ (0.6 Å) or Cu + (0.62 Å) with larger Ni 2+ (0.69 Å), all of diffraction peaks shift slightly toward lower angle in doped samples, suggesting that Ni goes to either Sb or Cu site. This is in accord with the case of Sn-doped Cu 3 SbSe 4 nanocrystals. [ 21 ] In Cu 2.25 Ni 0.75 SbS 4 sample, different than Cu 3 Ni 0.05 Sb 0.95 S 4 , Ni-doping increases the intensities of all weak (103), (200), (222), and (224) peaks in original Cu 3 SbS 4 while those strong peaks of (112), (204), and (312) become weaker, further indicating that Ni substitutes different sites.

Next, the morphologies of both neat and Ni-doped Cu 3 SbS 4 nanocrystals were intensively examined from fi eld-emission scanning electron microscopy (FE-SEM) and transmission elec-tron microscopy (TEM). The SEM graphs of Cu 3 SbS 4 , Ni-doped Cu 3 Ni 0.05 Sb 0.95 S 4 and Cu 2.25 Ni 0.75 SbS 4 nanocrystals are displayed in Figure 3 a,d,g, respectively. Nanocrystals with circle shape and sizes varying from 20 to 30 nm are observed. Compared to that of neat Cu 3 SbS 4 sample, smaller grain size and more grain boundaries are produced in the Ni-doped nanocrystals, which is in favor of reducing lattice thermal conductivity. To further char-acterize the microstructures of these samples, TEM images were taken as shown in Figure 3 b,e,h. They all show nearly spherical monodisperse nanostructures with an average size of 3–5 nm. Then high-resolution (HR) TEM images were acquired to fur-ther understand the nanocrystal growth of all samples. The HRTEM images of Cu 3 SbS 4 , Ni-doped Cu 3 Ni 0.05 Sb 0.95 S 4 , and Cu 2.25 Ni 0.75 SbS 4 nanocrystals are shown in Figure 3 c,f,i, respec-tively. They exhibit clear lattice fringes with an average inter-planar distance of 0.31, 0.307, and 0.306 nm, respectively, which corresponds to the (112) d -spacing of Cu 3 SbS 4 and also agrees well with the XRD results. In these Ni-doped samples, the crystal lattice constant becomes smaller owing to the change of crystal-line structures, suggesting that Ni goes to either Cu or Sb site, which is also supported by two different XRD patterns obtained as seen in Figure 2 . In Cu 2.25 Ni 0.75 SbS 4 , we therefore believe that Ni would prefer to enter Cu site due to the cohesive strength between atoms, while Ni would favor Sb site in Cu 3 Ni 0.05 Sb 0.95 S 4 .

We then investigated the infl uences of the above different crystalline structures and morphologies caused by Ni-doping on TE properties. Temperature dependences of electrical con-ductivity ( σ ), Seebeck coeffi cient ( S ), thermal conductivity ( κ ), lattice thermal conductivity ( κ L ) and zT are plotted in Figure 4 for neat Cu 3 SbS 4 and two optimal Ni-doped Cu 2.25 Ni 0.75 SbS 4 and Cu 3 Ni 0.05 Sb 0.95 S 4 samples in the range of room tempera-ture to 250 °C.

As shown in Figure 4 a, the electrical conductivities of all samples increase with increasing temperature because more electrons are thermally excited from valence band to con-duction band. It is found that σ increases from 69.3 S cm −1 for neat Cu 3 SbS 4 nanocrystals to 229.5 S cm −1 for Ni-doped Cu 2.25 Ni 0.75 SbS 4 nanocrystals at 250 °C. This could be inter-preted by the fact that the Ni-doping causes spin splitting of valence band because of the hybridization of Ni 3d states with Cu 3d states and S 3p states. Thus, the band gap gets narrower and the thermal excitation of the carriers becomes easier. [ 11 ] For Cu 3 Ni 0.05 Sb 0.95 S 4 nanocrystals, Ni-doping also results in a dramatic increase of σ , which is attributed to the fact that the

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Figure 1. Hot-injection synthesis of Cu 3 SbS 4 (namely CSS) nanocrystals for TE applications. a) Schematic of the experimental procedure, and chemical reactions of b) neat and c) Ni-doped samples, respectively.

Figure 2. XRD patterns of neat Cu 3 SbS 4 , Ni-doped Cu 2.25 Ni 0.75 SbS 4 and Cu 3 Ni 0.05 Sb 0.95 S 4 nanocrystals.

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Figure 3. Morphologies of neat and Ni-doped Cu 3 SbS 4 nanocrystals: a–c) Cu 3 SbS 4 , d–f) Cu 3 Ni 0.05 Sb 0.95 S 4 , and g–i) Cu 2.25 Ni 0.75 SbS 4 nanocrystals. (a,d,g) SEM graphs, (b,e,h) TEM images, and (c,f,i) HRTEM images.

Figure 4. Temperature dependences of a) electrical conductivity, b) Seebeck coeffi cient, c) lattice thermal conductivity (solid lines) and thermal con-ductivity (dashed lines), and d) zT in neat Cu 3 SbS 4 and Ni-doped Cu 2.25 Ni 0.75 SbS 4 and Cu 3 Ni 0.05 Sb 0.95 S 4 nanocrystals, respectively.

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substitution of Sb 5+ with Ni 2+ introduces additional holes as charge carriers in the system. At 250 °C, σ reaches 285.6 S cm −1 , which is notably higher than that of neat Cu 3 SbS 4 and Ni-doped Cu 2.25 Ni 0.75 SbS 4 nanocrystals.

Figure 4 b presents the temperature dependences of S in neat Cu 3 SbS 4 and Ni-doped Cu 2.25 Ni 0.75 SbS 4 and Cu 3 Ni 0.05 Sb 0.95 S 4 nanocrystals. The positive S values for all samples indicate that charge carriers are holes. S monotoni-cally increases with increasing temperature due to the shift of chemical potential. We fi nd that S in Cu 2.25 Ni 0.75 SbS 4 nanocrystals is very close to that in neat Cu 3 SbS 4 nanocrystals although their electrical conductivities are largely different as shown in Figure 4 a. For instance, at room temperature, S is 145.1 µV K −1 in Cu 2.25 Ni 0.75 SbS 4 nanocrystals, which is larger than 137.6 µV K −1 in neat Cu 3 SbS 4 nanocrystals; at 250 °C, S is 171.6 µV K −1 in Cu 2.25 Ni 0.75 SbS 4 nanocrystals, which is almost the same as 171.1 µV K −1 in neat Cu 3 SbS 4 nanocrystals. These fi ndings are quite different from the conventional nonmagnetic doping effect which results in a notable increase of electrical conductivity and decrease of Seebeck coeffi cient. Such abnormal phenomena are presum-ably assigned to the fact that Ni 2+ ions spin effect compen-sates the reducing of S in Cu 2.25 Ni 0.75 SbS 4 nanocrystals. In contrast, in Cu 3 Ni 0.05 Sb 0.95 S 4 nanocrystals, the measured S is only 122.4 µV K −1 , which is lower than that of neat Cu 3 SbS 4 nanocrystals (171.1 µV K −1 ) at 250 °C. Hence, when Sb 5+ is substituted by Ni 2+ , the spin effect is not obvious and Cu 3 Ni 0.05 Sb 0.95 S 4 nanocrystals behave as conventional non-magnetic doped materials.

In order to understand the spin effect in these two samples, the temperature dependent electron paramagnetic resonance (EPR) experiments were conducted and the spectra are displayed in Figure 5 . For neat Cu 3 SbS 4 nanocrystals, lines with Lande factor g = 2.2 are obtained for all fi ve temperatures as shown in Figure 5 a. Figure 5 b indicates that there are two g factors in Cu 2.25 Ni 0.75 SbS 4 nanocrystals where one is g = 1.75 and the other varies from 4.7 at 300 K to 3.59 at 460 K. The latter one is attributed to the Ni 2+ electron spin. [ 11,27,28 ] Such additional elec-tron spin is however not found in Cu 3 Ni 0.05 Sb 0.95 S 4 nanocrystals as shown in Figure 5 c, meaning that Ni-doping does not create new contribution to S . Therefore, the EPR results are in good agreement with the spin effects on the Seebeck coeffi cient in Figure 4 b.

In Figure 4 c, the total thermal conductivity ( κ ) and lat-tice thermal conductivity ( κ L ) are plotted as a function of

temperature in neat Cu 3 SbS 4 , Ni-doped Cu 2.25 Ni 0.75 SbS 4 and Cu 3 Ni 0.05 Sb 0.95 S 4 nanocrystals. κ L (= κ−κ e ) was calcu-lated using the Wiedemann–Franz law κ e = LσT where L = 2.45 × 10 −8 W Ω K −2 is the Lorentz number. [ 29 ] For all sam-ples, κ L are below 1 W m −1 K −1 within the entire tempera-ture range. The reason is that phonon scattering is signifi -cantly enhanced by additional phonon-boundary scattering at grain boundaries in nanocrystals. Moreover, κ L in Ni-doped Cu 2.25 Ni 0.75 SbS 4 and Cu 3 Ni 0.05 Sb 0.95 S 4 nanocrystals are lower than that in neat Cu 3 SbS 4 nanocrystals. At 250 °C, the min-imum κ L reaches 0.68 W m −1 K −1 in Ni-doped sample, which corresponds to a 17% decrease than that in neat Cu 3 SbS 4 nanocrystals (0.80 W m −1 K −1 ). This is because the Ni doping induced crystal distortion would strengthen the anharmo-nicity and the phonon-phonon scattering. Comparing to that of Cu 3 SbS 4 nanocrystals, the κ of Cu 2.25 Ni 0.75 SbS 4 nanocrys-tals is smaller when T < 125 °C and larger when T > 125 °C. For Cu 3 Ni 0.05 Sb 0.95 S 4 nanocrystals, the κ is larger than that of Cu 3 SbS 4 nanocrystals in all temperature ranges. Increased κ is presumably originated from the increased σ and κ e in Ni-doped nanocrystals.

As a result, as shown in Figure 4 d, zT values of all Ni-doped samples obviously increases with temperature and are signifi cantly higher than that of Cu 3 SbS 4 sample. For the nominal composition of Cu 2.25 Ni 0.75 SbS 4 sample, the zT reaches the peak value of 0.37 at 250 °C due to remarkable increased σ along with little changed S and the decreased κ L . Meanwhile, the maximum zT reaches 0.22 at 250 °C due to reducing κ L for the nominal composition of Cu 3 Ni 0.05 Sb 0.95 S 4 sample.

In conclusion, we have employed hot-injection synthetic method of producing neat Cu 3 SbS 4 and two types of optimally doped Cu 2.25 Ni 0.75 SbS 4 and Cu 3 Ni 0.05 Sb 0.95 S 4 nanocrystals. In both Ni-doped samples, the electrical conductivity increased and the lattice thermal conductivity decreased with the intro-duction of Ni 2+ . When Cu + was substituted by Ni 2+ , the See-beck coeffi cient remained little changed due to the spin split-ting of valence band. However, when Sb 5+ was replaced by Ni 2+ , the Seebeck coeffi cient signifi cantly decreased as usual. Such distinct spin effects of magnetic Ni 2+ were confi rmed by EPR spectra. We have thus demonstrated that the sub-stitution of Cu + by Ni 2+ can notably improve the TE perfor-mance of ternary Cu 3 SbS 4 system, giving a maximum zT of 0.37 for the nominal composition of Cu 2.25 Ni 0.75 SbS 4 sample at 250 °C.

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Figure 5. Temperature-dependent EPR spectra of a) Cu 3 SbS 4 , b) Cu 2.25 Ni 0.75 SbS 4 , and c) Cu 3 Ni 0.05 Sb 0.95 S 4 nanocrystals, respectively.

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Experimental Section See the Supporting Information for details.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements This work was funded by National Natural Science Foundation of China (NSFC) under Grant No. 51473036 (Z.L.) and 11404244 (J.Z.). J.Z. also acknowledges the support from the program for New Century Excellent Talents in Universities (Grant No. NCET-13-0431) and the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning (Grant No. TP2014012).

Received: December 21, 2015 Revised: March 20, 2016

Published online: April 13, 2016

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