Engineering of Thermoelectric Composites Based on Silver Selenide in Aqueous Solution and Ambient Temperature

The direct, solid state, and reversible conversion between heat and electricity using thermoelectric devices finds numerous potential uses, especially around room temperature. However, the relatively high material processing cost limits their real applications. Silver selenide (Ag2Se) is one of the very few n-type thermoelectric (TE) materials for room-temperature applications. Herein, we report a room temperature, fast, and aqueous-phase synthesis approach to produce Ag2Se, which can be extended to other metal chalcogenides. These materials reach TE figures of merit (zT) of up to 0.76 at 380 K. To improve these values, bismuth sulfide (Bi2S3) particles also prepared in an aqueous solution are incorporated into the Ag2Se matrix. In this way, a series of Ag2Se/Bi2S3 composites with Bi2S3 wt % of 0.5, 1.0, and 1.5 are prepared by solution blending and hot-press sintering. The presence of Bi2S3 significantly improves the Seebeck coefficient and power factor while at the same time decreasing the thermal conductivity with no apparent drop in electrical conductivity. Thus, a maximum zT value of 0.96 is achieved in the composites with 1.0 wt % Bi2S3 at 370 K. Furthermore, a high average zT value (zTave) of 0.93 in the 300–390 K range is demonstrated.


Supporting experimental section 1.Structural and chemical characterizations
The NP morphology was characterized using a field emission scanning electron microscope (SEM, Zeiss Auriga) operated at 5.0 kV.The elemental composition ratios were analyzed via an energy dispersive X-ray spectrometer (EDX) inside the SEM at 20.0 kV.X-ray diffraction analyses (XRD) were carried out on a Bruker AXS D8 Advance X-ray diffractometer with Cu-Kα radiation (λ=1.5406Å).Transmission electron microscopy (TEM), high resolution TEM (HRTEM), and scanning TEM (STEM) images were obtained on a FEI Tecnai F20 field emission gun microscope operating at 200 kV and equipped with high angle annular dark field (HAADF) and Gatan quantum electron energy loss spectroscopy (EELS) detectors.

Thermoelectric property measurements
The electrical resistivity and Seebeck coefficient of hot-pressed samples were measured automatically and simultaneously in a Linseis LSR-3 equipment under a helium atmosphere.All samples were measured at least three times under heating and cooling cycles processes.
Considering the system-measurement accuracy, the error in the measurement of resistivity and Seebeck coefficient measurements was estimated to be ca.5%.Thermal conductivity values of hotpressed pellets were calculated according to κ= α Cp ρ, where κ is total thermal conductivity, α is thermal diffusivity (mm 2 s -1 ), Cp is specific heat capacity (J g -1 K -1 ), and ρ is the density of pellets (g cm -1 ).λ was directly measured on a Linseis XFA 600 Xenon Flash apparatus with an estimated error of ca.5%.Cp values were calculated by Dulong-Petit approximation (3R law), and Cp curves were also obtained in a Netzsch differential scanning calorimetry (DSC) at a heating rate of 5 ℃/min in N2 atmosphere.Relative ρ values of the samples were estimated by the Archimedes' method.Hall carrier concentrations (nH) and mobilities (μH) at room temperature were measured using the Van der Pauw method and Hall Bar measurements (ezHEMS, NanoMagnetics) using a magnetic field of 1 T.      Round-robin studies indicate that the error in the measurement of Cp can be as high as between 5 and 10%.Thus we included 10% error bars in our experimental data in Figure S6.Besides, notice that Dulong-Petit's law states that the molar-specific heat capacity of a solid element or compound at constant volume is approximately equal to 3R, where R is the molar gas constant.This law is valid for elements and compounds at high temperatures, where the vibrational degrees of freedom of the atoms are excited.Dulong-Petit's may fail at low temperatures due to higher-energy vibrational modes not being populated. 1,2 n the case of Ag2Se-1.0 wt% Bi2S3, the measured specific heat becomes slightly higher than Dulong-Petit's limit at temperatures higher than 330 K.This can be attributed to the presence of defects and impurities, particularly Bi 3+ ions, in the Ag2Se crystal structure that can act as additional vibrational modes and contribute to the specific heat.Furthermore, the lattice vibrations of the crystal may be affected by the presence of defects, resulting in higher specific heat.4][5] Notice in addition, that the phase transition temperature decreases slightly after the introduction of 1.0 wt% Bi2S3, which is consistent with the phenomenon observed in Cu2Se alloyed with Ag2Se. 6

Calculation of Lorenz number
Here, the Lorenz number L is calculated based on the measured Seebeck coefficient.So we used the equation 7  As shown in Figure S8, above 400 K, when the phase transition of Ag2Se takes place, the Seebeck coefficient (S) of the pure Ag2Se significantly decreases and then slightly rises.Above 400 K, the electrical conductivity (σ) values continue to decrease to 5.96×10 4 S m -1 .Therefore, the zT curve shows a sharp decrease at the phase transition temperature, consistent with previous reports. 8,9 his result highlights the significant deterioration of the TE performance of Ag2Se at higher temperatures after the phase transition.Therefore, careful selection of the operating temperature range is crucial when using Ag2Se for thermoelectric applications.
Figure S1.EDX spectra of the products obtained from the reaction between Ag and Se powders taken in the ratio (a) 2:1, (b) 1.9:1, (c) 1.8:1.

Figure S2 .
Figure S2.SEM images of MX NPs (M=Ag, Cu, Pb and Bi, X=S and Se).

Figure S4 .
Figure S4.(a-b) Cross-section SEM micrographs of Ag2Se-1.0 wt% Bi2S3 pellet and corresponding compositional maps of Ag, Se, Bi and S from the spectrum 1 in the Figure b (Se-rich regions marked with white circles).

Figure S6 .
Figure S6.(a) Dulong-Petit specific heat capacity (Cp) of Ag2Se pellet and experimental temperature dependence of Cp of Ag2Se and Ag2Se -1.0 wt% Bi2S3 pellets.(b) The calculated Cp from the Dulong-Petit approximation (blue line) is 0.254 J g -1 K.

Table S1 .
Synthesis of binary metal chalcogenides MX nanoparticles (M=Ag, Cu, Pb and Bi, Sn, X=S and Se).

Table S5 .
Comparison of TE properties of Ag2Se-based materials.