pubs.acs.org/acsaelm Forum Article Engineering of Thermoelectric Composites Based on Silver Selenide in Aqueous Solution and Ambient Temperature Bingfei Nan, Mengyao Li, Yu Zhang,* Ke Xiao, Khak Ho Lim, Cheng Chang, Xu Han, Yong Zuo, Junshan Li, Jordi Arbiol, Jordi Llorca, Maria Ibáñez, and Andreu Cabot* Cite This: https://doi.org/10.1021/acsaelm.3c00055 Read Online ACCESS Metrics & More Article Recommendations * sı Supporting Information See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. ABSTRACT: 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 Downloaded via 139.47.116.106 on October 31, 2023 at 16:18:21 (UTC). 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. KEYWORDS: thermoelectricity, silver selenide, aqueous synthesis, bismuth sulfide, composite ■ INTRODUCTION Thermoelectric (TE) devices allow for direct, solid state, and = e + L Besides, S2σ is defined as the power factor (PF). A good TE (2) reversible conversion between heat and electricity.1−4 TE material is thus characterized by high S and σ, and low κ values. devices can harvest heat from the ambient environment, Silver selenide (Ag2Se) is one of the very few TE materials potentially increasing the efficiency of a plethora of processes. suitable for use at ambient temperature, where it is They also allow precise control of the temperature and characterized by relatively low thermal conductivity and high effective cooling of hot spots. However, their real-world applications are limited to several niche markets due to their electrical conductivity.5 Ag2Se is an n-type semiconductor with relatively low cost-effectiveness. The high cost of TE devices is a narrow band gap (Eg = 0.07 eV at 0 K). It exists in two stable related to the use of scarce elements such as Te, the need for phases, the low-temperature orthorhombic β-phase and the high-temperature or vacuum-based processes for the synthesis high-temperature cubic α-phase, with a transition temperature of TE materials, and the quasimanual manufacturing of TE of around 407 K.6−8 Numerous approaches exist for the modules. Alternative printing technologies are being developed synthesis of Ag2Se. Among them, the commonly reported worldwide, but the use of organic solvents for the synthesis of solid-state preparation strategy is based on reacting the two the materials and/or the ink formulation is still a major elements, Ag and Se, at high temperatures6,9−15 or using high- drawback to the environmentally friendly and low-cost energy ball milling.8,16 As an example, Chen et al. developed a processing of TE devices. On top of the high cost, the energy porous Ag2Se with hierarchical structures via a wet mechanical conversion efficiency of TE devices is relatively low. The alloying process. Using this approach, a low lattice thermal energy conversion efficiency of a TE material is determined by conductivity of ∼0.35 W m−1 K−1 and a zT of ∼0.7 were a dimensionless figure of merit obtained at 300 K.8 Besides, Ag2Se is also produced by zT = S2 T /k (1) Special Issue: Advanced Thermoelectric Materials and where S, σ, κ, and T are the Seebeck coefficient (μV K ), −1 Devices electrical conductivity (S m−1), thermal conductivity (W m−1 Received: January 13, 2023 K−1), and absolute temperature (K), respectively. κ includes Accepted: April 24, 2023 the electronic thermal conductivity (κe) and lattice thermal conductivity (κL): © XXXX The Authors. Published by American Chemical Society https://doi.org/10.1021/acsaelm.3c00055 A ACS Appl. Electron. Mater. XXXX, XXX, XXX−XXX ACS Applied Electronic Materials pubs.acs.org/acsaelm Forum Article Figure 1. (a) Schematic illustration of the synthesis of binary metal chalcogenides (MX; M = Ag, Cu, Pb, Bi; X = S, Se). (b−d) SEM images of Ag2Se produced from AgNO3:Se molar ratios of (b) 2:1, (c) 1.9:1, and (d) 1.8:1. (e) XRD patterns of Ag2Se. vacuum-based technologies such as magnetron sputtering.17 1.0 and 1.5) nanocomposites is produced by solution-blending Various chemical synthetic methods have also been reported and hot pressing. Interestingly, the incorporation of Bi2S3 for the production of silver chalcogenides and particularly results in a significant increase in the Seebeck coefficient. Ag2Se particles, including colloidal,18−21 hydrothermal,22,23 Furthermore, the optimized composition shows low thermal and microwave-assisted24 methods. In some cases, aqueous conductivity and a record-high zT of 0.96 at 370 K. solutions have been used.5,25,26 As an example, Xiao et al. synthesized Ag2Se nanocrystals via a colloidal method, reaching a maximum zT value of 0.23 at the phase transition temperature of around 408 K.19 Wang et al. reported a general ■ EXPERIMENTAL SECTION Materials. Silver(I) nitrate (AgNO3, 99.9+%), copper(II) nitrate aqueous synthesis of nano/microscale binary silver chalcoge- trihydrate (Cu(NO3)2·3H2O, 99%), lead(II) nitrate (Pb(NO3)2, 99+ nides (Ag2X, X = S, Se, Te) based on the reaction of Na2S/ %), and hydrazine hydrate (N2H4·H2O, 64%) were supplied by Fisher NaHSe/NaHTe and AgNO3 aqueous solution at the water Scientific. Selenium powder (Se, 200 mesh, ≥ 99.5% trace metals boiling temperature. The molar ratios of Ag+/X2− were basis), bismuth nitrate pentahydrate (Bi(NO3)3·5H2O, ≥ 99.99%), adjusted from 2:1 to 2:1.1 and the maximum zT value of the thioacetamide (TAA, ≥ 99.0%), and nitric acid (HNO3, 68%) were purchased from Sigma-Aldrich. All chemicals were used without resulting Ag2Se pellet was 0.84 at 380 K.25 further purification using standard solution synthesis procedures.36,37 The TE properties of pristine Ag2Se can be improved Synthesis of Silver Selenide. In a typical synthetic method, through extrinsic and intrinsic doping. Li et al. reported a 0.8494 g of AgNO3 was dissolved into 10 mL of deionized water hydrothermal solution route using ethylene glycol and glycerol (DIW). In parallel, a Se precursor solution was prepared by adding as solvents to prepare Ag2Se at 180 °C, reaching a maximum 0.2078 g of Se powder to 5 mL of N2H4·H2O. The AgNO3 aqueous zT of 0.7 at 317 K for Ag2Se, and up to 0.9 at 300 K when solution was then injected at ambient temperature into the Se adding 0.1 wt % Sn doping at Ag sites.27 Variations in the solution, where a black precipitate was immediately formed. The stoichiometric ratio of silver to selenium were also investigated product was collected by centrifugation and washed using DIW and to control the concentration of free carriers, showing an ethanol three times. The final product was dried and stored in an Ar- obvious effect on the TE performance.11,16,17 In this direction, filled glovebox. Jood et al. introduced an anion excess (≤1% of Se or S) into Synthesis of Metal Chalcogenide (MX). For the production of Ag2Se obtaining a notable improvement in carrier mobility and other binary metal chalcogenides (MX, M = Cu, Pb, Bi; X = S, Se) a zT values up to ∼1.0 in the temperature range of 300−375 similar synthesis strategy was adopted. The detailed parameters are K.13,14 Another important approach to improving Ag2Se shown in Table S1. It is worth mentioning that SnS and SnSe can also performance is to combine it with small amounts of other be prepared by the same approach in an alkaline water environment materials into nanocomposites.28−31 As an example, Ballikaya (e.g., sodium hydroxide aqueous solution), but they require a longer et al. added Cu2Se nanoinclusions in Ag2Se to improve the TE reaction time (ca. 1 h) at ambient temperature. performance and thermal stability.32 Lim et al. obtained a high Synthesis of Bismuth Sulfide. Bi2S3 was synthesized in an zT of 0.89 at 343 K through the simple blending of Ag2Se with aqueous solution following an alternative procedure inspired by Te nanorods.18 Besides, carbon nanotubes were also used as an previous publications.38,39 Briefly, TAA (0.510 g) was dissolved in 160 mL of DIW with rapid stirring. At the same time, Bi(NO3)3·5H2O effective nanofiller for enhancing the TE performance of (1.584 g) was added to 20 mL of 0.4 M HNO3 aqueous solution, and Ag2Se.5,23 then it was added drop by drop to the TAA solution. The mixture was Bismuth sulfide (Bi2S3) is an n-type semiconductor reacted for 15 h at room temperature with continuous strong stirring. composed of relatively abundant, nontoxic, and low-cost The coarse product solution was washed with DIW and ethanol five elements.33 Bi2S3 has poor zT values at ambient temperature, times. Finally, it was dried and stored in an Ar-filled glovebox. because of moderate electrical conductivity. However, it is Nanopowder Blend and Consolidation. Ag2Se/Bi2S3 compo- characterized by high Seebeck coefficients (ca. −400 μV/K) site powders were produced by blending the proper ratio of particles and low thermal conductivities. Bi2S3 has been used as a of the two materials in solution under ultrasonication for 1 h. Next, doping phase to promote the TE properties of some TE the dried blended powders were placed in a furnace and annealed at materials, including Cu1.8S34 and Bi2Te2.7Se0.3.35 250 °C for 1 h in an Ar/H2 flow. The annealed powders were loaded Herein, we detail a facile, rapid, room temperature, and into a graphite die (Ø 10 mm × 10 mm cylinders) and hot-pressed for aqueous-based general approach to producing highly crystal- 5 min at 50 MPa and 250 °C inside an argon-filled glovebox. The hot- line Ag2Se. Besides, a series of Ag2Se-x wt % Bi2S3 (x = 0, 0.5, pressed pellets were then polished and used for TE characterization. B https://doi.org/10.1021/acsaelm.3c00055 ACS Appl. Electron. Mater. XXXX, XXX, XXX−XXX ACS Applied Electronic Materials pubs.acs.org/acsaelm Forum Article Figure 2. Structural and Chemical Characterization of Ag2Se and Bi2S3. (a) TEM micrograph of Ag2Se. (b) HRTEM micrograph of Ag2Se, detail of the orange squared region, and its corresponding power spectrum. (c) EELS chemical composition maps from the red square area of the STEM micrograph of Ag2Se. Individual Ag M4,5-edges at 367 eV (red), Se M1-edges at 232 eV (green), and composites of Ag−Se. (d) SEM image of Bi2S3. (e) TEM and (f) HRTEM micrograph of Bi2S3, detail of the orange square region, and its corresponding power spectrum. From the crystalline domain, the Bi2S3 lattice fringe distances were measured to be 0.351 0.354, and 0.193 nm, at 66.70° and 96.72°, which could be interpreted as the orthorhombic Bi2S3 phase visualized along its [213] zone axis. (g) EELS chemical composition maps from the red square area of the STEM micrograph of Bi2S3. Individual Bi N4,5-edges at 440 eV (red), S L2,3-edges at 165 eV (green), and composites of Bi−S. (h) XRD pattern of Bi2S3. ■ RESULTS AND DISCUSSION Figure 1a shows a schematic illustration of the aqueous and molar ratio of 1.9:1, energy-dispersive X-ray spectroscopy (EDX) analysis shows the Ag:Se atomic ratio in the final Ag2Se ambient temperature synthesis process used to produce binary particles to be 2.2 (Figure S1). However, for the sake of metal chalcogenides (MX; M = Ag, Cu, Pb, Bi; X = S, Se). The convenience, we denote the silver selenide as Ag2Se. The SEM MX chalcogenide is produced by the reaction of the zerovalent images, To demonstrate the versatility of the synthesis chalcogen (X0) powder with N2H4·H2O to form X2− and the approach here reported, EDX data and XRD patterns of immediate reaction of such anions with the metal cations in other MX (M = Ag, Cu, Pb, and Bi, X = S and Se) are the metal salt solution. In this way, Ag2S, Ag2Se, CuS, Cu2S, displayed in Figure S2, Table S2, and Figure S3, respectively. Cu2Se, Bi2S3, Bi2Se3, PbS, and PbSe particles, which through Figure 2a shows a general view bright field transmission proper processing can be used in a plethora of different electron microscopy (TEM) image of produced Ag2Se. Figure applications,40−48 were easily and rapidly obtained. Figure 1b− 2b shows a high-resolution HRTEM micrograph from an d displays scanning electron microscopy (SEM) images of Ag2Se particle and its corresponding power spectrum revealing Ag2Se produced from different AgNO3:Se molar ratios; 2:1, an orthorhombic crystal phase (space group P212121) with a = 1.9:1, and 1.8:1. Ag2Se particles are characterized by elongated 4.334 Å, b = 7.070 Å, c = 7.774 Å. The high-angle annular dark shapes, an average size of a few hundred nanometers, and high field (HAADF) scanning TEM (STEM) micrographs and crystallinity, as observed by X-ray diffraction (XRD, Figure 1e). electron energy loss spectroscopy (EELS) composition maps XRD patterns show the obtained Ag2 Se to have an of Ag2Se particles show a homogeneous distribution of both orthorhombic crystallographic phase (PDF 00−024−1041) elements (Figure 2c). with lattice parameters a = 4.333 Å, b = 7.062 Å, and c = 7.764 Bi2S3 particles were also synthesized in an aqueous solution Å. At an AgNO3:Se molar ratio of 2:1, a few impurity peaks at at ambient temperature. Figure 2d,e shows representative SEM 38.2° and 44.3° can be indexed with the cubic Ag phase (PDF and TEM images of the obtained product. Bi2S3 particles were 00−004−0783). At a AgNO3/Se molar ratio of 1.8:1, a new highly polycrystalline and presented a flowerlike morphology. peak at 29.6° is ascribed to the hexagonal Se phase (PDF 01− Crystallites had an average size of ca. 10 nm. EDX analysis 086−2246), indicating that the excess Se was not fully showed the atomic ratio of Bi to S to be consistent with incorporated into the Ag2Se lattice. At a AgNO3:Se molar stoichiometric Bi2S3. HRTEM analysis showed the particle ratio of 1.9:1, XRD patterns show pure-phase Ag2Se, with no crystal structure to agree with the Bi2S3 orthorhombic phase crystalline impurities. Thus, we chose this precursor molar (space group = Pmcn) with a = 3.9810 Å, b = 11.1470 Å, and c ratio to prepare the material to be further characterized and = 11.3050 Å (Figure 2f). EELS compositional maps showed a used to produce Ag2Se/Bi2S3 composites. At this AgNO3:Se homogeneous distribution of Bi and S (Figure 2g). Besides, C https://doi.org/10.1021/acsaelm.3c00055 ACS Appl. Electron. Mater. XXXX, XXX, XXX−XXX ACS Applied Electronic Materials pubs.acs.org/acsaelm Forum Article Figure 3. (a) SEM image of the polished Ag2Se-1.0 wt % Bi2S3 pellet and corresponding EDX compositional maps of Ag, Se, Bi, and S (Se-rich regions marked with white circles). (b) XRD patterns of consolidated Ag2Se-x wt % Bi2S3 pellets. Figure 4. Temperature dependence of (a) Seebeck coefficient and S. (b) electrical conductivity, σ. (c) Hall carrier concentration (nH) and mobility (μH) at room temperature. (d) Power factor PF of Ag2Se-x wt % Bi2S3. XRD data confirmed the orthorhombic phase (PDF 03−065− resulting dried powder at 50 MPa and 250 °C inside an argon- 3884) of the Bi2S3 particles (Figure 2h). filled glovebox (see the Experimental section for details). An Ag2Se/Bi2S3 composites were produced by blending the SEM image of the polished surface of the Ag2Se-1.0 wt % Bi2S3 proper ratio of particles in solution and hot pressing the composite and its corresponding EDX elemental maps are D https://doi.org/10.1021/acsaelm.3c00055 ACS Appl. Electron. Mater. XXXX, XXX, XXX−XXX ACS Applied Electronic Materials pubs.acs.org/acsaelm Forum Article shown in Figure 3a. Besides, the morphology of a fractured contribution associated with the preferential scattering of Ag2Se-1.0 wt % Bi2S3 sample and its compositional map and electrons over holes at the Ag2Se/Bi2S3 interphase owing to the EDX compositions of all fractured pellets are shown in Figure upward band bending generated at the Ag2Se side. It is S4 and Table S3. An overall homogeneous distribution of the therefore crucial to maintain the Bi2S3 content below 1% to constituent elements, including S and Bi, is observed, denoting achieve an optimal thermoelectric performance in Ag2Se−Bi2S3 atomic doping of the Ag2Se with these two elements. Only composites. Notice that a similar evolution of the Seebeck some small Se-rich inhomogeneities can be found, as marked coefficient with dopant concentration, first increasing and later with white circles. Figure 3b shows the XRD patterns of the decreasing at higher dopant concentrations, has been reported hot-pressed Ag2Se-x wt % Bi2S3 composites, which can be in other systems, and diverse mechanisms have been indexed with the orthorhombic Ag2Se phase (PDF 00−024− reported.54−57 Besides, previous studies have also shown 1041). As the Bi2S3 content increases, the XRD peaks shift increased Seebeck coefficients without affecting the electrical slightly to lower angles, indicating an expansion of the lattice conductivity.58−60 The preserved electrical conductivity of associated with the partial substitution of Ag+ ions (0.67 Å) by Ag2Se after being mixed with Bi2S3 is attributed to the notable Bi3+ ions (1.03 Å) with a larger ionic radius.49 However, when increase in charge carrier mobility, which compensates for the the Bi2S3 content exceeds 1.0 wt %, the XRD peaks no longer moderate decrease in charge carrier concentration. shift due to the limited solubility of Bi3+ in the matrix. No Figure 4d shows the power factor (PF, S2σ) of Ag2Se-x wt % impurity XRD peaks and particularly Bi2S3 peaks were Bi2S3 samples as a function of temperature. For the pure Ag2Se detected, indicating notable alloying of Bi2S3 with Ag2Se.34 sample, the PF slightly increases, from 2.06 to 2.24 mW m−1 The density of the composite slightly decreases with the K−2 over 300−390 K. The PF of the Ag2Se −1.0 wt % Bi2S3 increase of the Bi2S3 fraction due to the lower density of Bi2S3 composite is significantly larger, reaching up to 2.66 mW m−1 (∼6.78 g/cm3) compared with Ag2Se35 but all samples reach K−2 at 360 K. Notice also that the Ag2Se-1.0 wt % Bi2S3 pellet relative densities above 90% (Table S4). exhibits good stability even after multiple tests (Figure S5). The temperature dependence of the Seebeck coefficient (S) The experimental thermal diffusivities, α, are presented in of the different composites is shown in Figure 4a. All samples Table S4. The measured heat capacities, Cp, and the limit Cp show n-type semiconducting behavior with negative S values. calculated by the Dulong−Petit law are shown in Figure S6. The absolute values of S monotonously decrease with Notice that while the measured Cp is very close to the temperature over the entire measured range. The S of the calculated limit for the Ag2Se-x wt % Bi2S3 composite and pure Ag2Se sample reaches up to −158.4 μV/K at 300 K and slightly below this limit for the pure Ag2Se sample, the limit is decreases to −146.7 μV/K at 390 K. With the introduction of approximately within the error range of the measured Bi2S3, the absolute S values increased significantly reaching up values.61,62 The total thermal conductivity (κtot) is determined to −178.5 μV/K at 300 K for the Ag2Se-1.0 wt % Bi2S3 pellet. by the equation As displayed in Figure 4b, the temperature dependence of = Cp tot (3) the electrical conductivity (σ) of the different composites shows a typical nondegenerate semiconductor characteristic where ρ is density. Figure 5a displays the obtained thermal with σ monotonously increasing with temperature. Relatively conductivity of Ag2Se and Ag2Se-x wt % Bi2S3 samples over the similar σ values were obtained for the different doping composites. The charge carrier concentration (nH) and mobility (μH) as a function of the Bi2S3 amount were measured by Hall and are displayed in Figure 4c. As the concentration of Bi2S3 increases, the nH for Ag2Se-x wt % Bi2S3 exhibits a moderate decrease (6.4 × 1018 cm−3 for pure Ag2Se and 4.9 × 1018 cm−3 for Ag2Se-1.5 wt % Bi2S3). In contrast, the μH for Ag2Se-x wt % Bi2S3 samples first rises and then decreases gradually with rising Bi2S3 concentration. In detail, the μH is 958.9 cm2 V−1 s−1 for the pure Ag2Se sample, and the largest value of 1310.4 cm2 V−1 s−1 is found as Bi2S3 amount increases to 1.0 wt %. A further increase of Bi2S3 doping concentration to 1.5 wt % Figure 5. Thermal conductivity of (a) total thermal conductivity, κtot. reduces the μH of Ag2Se-x wt % Bi2S3 samples. (b) Lattice thermal conductivity κL. The increase in the absolute value of the Seebeck coefficient with the introduction of Bi2S3 is in part associated with the whole temperature range. The pure Ag 2 Se pellet is decrease in the charge carrier concentration. Previous studies characterized by a moderate κtot of 0.99 W m−1 K−1 at 300 also demonstrate an increase in the absolute value of the K and 1.15 W m−1 K−1 at 390 K. These values are consistent Seebeck coefficient and also the charge carrier mobility with with previous reports on Ag2Se.25 With the introduction of the partial replacement of Se with S.13,50 Besides, the Bi doping Bi2S3, κtot significantly decreases. The Ag2Se-1.5 wt % Bi2S3 within the Ag2Se lattice expands the lattice, as observed by sample displayed the lowest κtot, 0.76 W m−1 K−1 at 300 K and XRD, which according to previous publications could increase 0.90 W m−1 K−1 at 390 K. Figure 5b displays the lattice the density of states near the Fermi level,51−53 thereby further thermal conductivity (κL) obtained by subtracting the enhancing the Seebeck coefficient of the Ag2Se-based electronic contribution to the thermal conductivity calculated materials. At too high, Bi2S3 precipitates are found as a using a single parabolic band (SPB) model according to secondary phase inside the Ag2Se matrix, which reduces the Wiedemann−Franz (κe = LσT, where L is the Lorentz charge carrier mobility and the absolute value of the Seebeck number) from the total thermal conductivity (κL= κtot − κe). coefficient. This reduction may be related to a higher bipolar The Lorenz number L is calculated by E https://doi.org/10.1021/acsaelm.3c00055 ACS Appl. Electron. Mater. XXXX, XXX, XXX−XXX ACS Applied Electronic Materials pubs.acs.org/acsaelm Forum Article ÅÄÅ |S| ÑÉÑ L = 1.5 + expÅÅÅÅ ÑÑ resulted in an abrupt drop of zT. Thus, the material application Ñ ÅÅÇ 116 ÑÑÑÖ (4) is limited to a temperature range extending up to about 390 K. zT values remain constant throughout the whole temper- The plot is displayed in Figure S7a. Figure S7b shows the ature range tested, providing a high average zT (zTave) temperature dependence of κe. While similar κe values were calculated as obtained for the different materials, composites displayed Th lower κL, down to 0.34−0.17 W m−1 K−1 for Ag2Se-1.5 wt % zTave = 1 zT dT Bi2S3. The lower κtot measured for the composites is associated Th Tc Tc (5) with a more effective scattering of phonons at point defects created by Bi3+ and extensive interphases between Ag2Se and where Th is the hot-side temperature, Tc is the cold-side Bi2S3 in the case of the highest doped samples. Numerous temperature, and zTave is thus the area under the zT curve previous works have reported a decrease of thermal divided by the value of Th − Tc. As shown in Figure 6c, a zTave conductivity with a minor effect on electrical conductivity = 0.93 is obtained in the temperature range of 300 to 390 K for and have associated this phenomenon with different the Ag2Se-1.0 wt % Bi2S3 sample, significantly above previously explanations, including a strong scattering on phonons created reported values. by precipitates without strongly affecting electrical conductiv- ity,63 hierarchical architecture with multiscale defects differ- ently affecting phonons and electrons,60 phonon scattering by ■ CONCLUSIONS In conclusion, a facile, rapid, high-yield, and component- introduced electrically dopant atoms,64 and preferential controllable room-temperature aqueous synthesis method was phonon scattering by introduced nanoparticles.65 adopted to prepare a plethora of metal chalcogenide MX The temperature dependence of the TE figure or merit, zT, nanoparticles (M = Ag, Cu, Pb, Bi; X = S, Se). Using this is displayed in Figure 6a. For the pristine Ag2Se pellet, the zT procedure, a series of Ag2Se-x wt % Bi2S3 composites was obtained by blending the materials in solution and hot press sintering the obtained dried powder. A maximum zT value of 0.76 for pure Ag2Se was obtained at 380 K. Further investigation illustrates that moderate Bi2S3 doping can effectively increase the absolute S value and reduce κL without significant harm to σ, which contributes to a remarkable PF of 2.66 mW m−1 K−2 and a maximum zT of 0.96 at 370 K. Besides, a remarkable zTave of 0.93 was obtained for Ag2Se-1.0 wt % Bi2S3 nanocomposites, above the values obtained in most previous silver selenide-based thermoelectric materials fab- ricated via wet chemical approaches. ■ * ASSOCIATED CONTENT sı Supporting Information The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsaelm.3c00055. Experimental characterization details, additional SEM, XRD, EDX data, reproducibility results, heat capacities, calculation of the Lorenz number, and comparison with previous literature (PDF) Figure 6. Temperature dependence of (a) zT values of Ag2Se-x wt % Bi2S3, (b) a comparison with reported silver selenide-based thermo- electric materials,5,10,19,25,66,69,70 and (c) a comparison of zTave with ■ AUTHOR INFORMATION Corresponding Authors reported data of silver selenide-based thermoelectric materi- Yu Zhang − Department of Materials Science and Engineering, als.5,11,13,15,18,68 Pennsylvania State University, State College, Pennsylvania 16802, United Sates; Email: yvz5897@psu.edu Andreu Cabot − Catalonia Institute for Energy value increases from 0.62 at 300 K to 0.76 at 380 K. The zT Research−IREC, Sant Adrià del Besòs 08930, Spain; values of the Ag2Se-x wt % Bi2S3 composites increase with the ICREA, Barcelona 08010 Catalonia, Spain; orcid.org/ introduction of 0.5−1.0 wt % Bi2S3. A maximum zT value of 0000-0002-7533-3251; Email: acabot@irec.cat 0.96 was obtained for the Ag2Se-1.0 wt % Bi2S3 sample at 370 K, which is ascribed to the highest PF value and slightly Authors decreased κtot. These zT values are above those previously Bingfei Nan − Catalonia Institute for Energy Research−IREC, reported n-type Ag2Se -based TE materials prepared by wet Sant Adrià del Besòs 08930, Spain; Departament chemistry18,19,22,23,25,27,66 and other methods15,32,67,68 (Figure d′Enginyeria Electrònica i Biomèdica, Universitat de 5b and Table S5). We further determined the thermoelectric Barcelona, Barcelona 08028 Catalonia, Spain properties of a pure Ag2Se sample up to 480 K (Figure S8). We Mengyao Li − Catalonia Institute for Energy Research−IREC, noticed that above 400 K, coinciding with the Ag2Se phase Sant Adrià del Besòs 08930, Spain; School of Physics and transition, a large decrease in the absolute value of the Seebeck Microelectronics, Zhengzhou University, Zhengzhou 450052, coefficient and electrical conductivity was obtained, which China; orcid.org/0000-0002-9082-7938 F https://doi.org/10.1021/acsaelm.3c00055 ACS Appl. Electron. Mater. XXXX, XXX, XXX−XXX ACS Applied Electronic Materials pubs.acs.org/acsaelm Forum Article Ke Xiao − Catalonia Institute for Energy Research−IREC, Sant Adrià del Besòs 08930, Spain; Departament ■ REFERENCES (1) Liu, W.-D.; Shi, X.-L.; Moshwan, R.; Yang, L.; Chen, Z.-G.; Zou, d′Enginyeria Electrònica i Biomèdica, Universitat de J. Solvothermal synthesis of high-purity porous Cu1.7Se approaching Barcelona, Barcelona 08028 Catalonia, Spain low lattice thermal conductivity. Chem. Eng. J. 2019, 375, 121996. 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Thermoelectric transport of Se-rich Ag2Se in normal phases and phase 0001-5013-2843 transitions. Appl. Phys. Lett. 2014, 104, 133903. Complete contact information is available at: (12) Ahmad, S.; Singh, A.; Bhattacharya, S.; Basu, R.; Bhatt, R.; https://pubs.acs.org/10.1021/acsaelm.3c00055 Muthe, K. P. Near room temperature thermoelectrics: Ag2Se. J. Appl. Phys. 2020, 2265, No. 030429. Funding (13) Jood, P.; Chetty, R.; Ohta, M. Structural stability enables high thermoelectric performance in room temperature Ag2Se. J. Mater. Open Access is funded by the Austrian Science Fund (FWF). Chem. A 2020, 8, 13024−13037. Notes (14) Jood, P.; Ohta, M. Temperature-Dependent Structural The authors declare no competing financial interest. Variation and Cu Substitution in Thermoelectric Silver Selenide. ACS Appl. Energy Mater. 2020, 3, 2160−2167. ■ ACKNOWLEDGMENTS B.N., M.L., Y.Z., K.X., and X.H. thank the China Scholarship (15) Huang, S.; Wei, T.-R.; Chen, H.; Xiao, J.; Zhu, M.; Zhao, K.; Shi, X. Thermoelectric Ag2Se: Imperfection, Homogeneity, and Reproducibility. ACS Appl. Mater. Interfaces 2021, 13, 60192−60199. (16) Lee, C.; Park, Y.-H.; Hashimoto, H. Effect of nonstoichiometry Council (CSC) for the scholarship support. C.C. received on the thermoelectric properties of a Ag2Se alloy prepared by a funding from the FWF “Lise Meitner Fellowship” grant mechanical alloying process. J. Appl. Phys. 2007, 101, No. 024920. agreement M 2889-N. M.I. acknowledges the financial support (17) Perez-Taborda, J. A.; Caballero-Calero, O.; Vera-Londono, L.; from ISTA and the Werner Siemens Foundation. ICN2 Briones, F.; Martin-Gonzalez, M. High Thermoelectric zT in n-Type acknowledges funding from Generalitat de Catalunya Silver Selenide films at Room Temperature. Adv. Energy Mater. 2018, 8, 1702024. 2021SGR00457 and project NANOGEN (PID2020- (18) Lim, K. H.; Wong, K. W.; Liu, Y.; Zhang, Y.; Cadavid, D.; 116093RB-C43) funded by MCIN/AEI/10.13039/ Cabot, A.; Ng, K. M. Critical role of nanoinclusions in silver selenide 501100011033/. ICN2 was supported by the Severo Ochoa nanocomposites as a promising room temperature thermoelectric program from Spanish MCIN/AEI (Grant No.: CEX2021- material. J. Mater. Chem. C 2019, 7, 2646−2652. 001214-S) and was funded by the CERCA Programme/ (19) Xiao, C.; Xu, J.; Li, K.; Feng, J.; Yang, J.; Xie, Y. Superionic Phase Transition in Silver Chalcogenide Nanocrystals Realizing Generalitat de Catalunya. J.L. is a Serra Húnter Fellow and is Optimized Thermoelectric Performance. J. Am. Chem. Soc. 2012, grateful to the ICREA Academia program and projects 134, 4287−4293. MICINN/FEDER PID2021-124572OB-C31 and 2021 SGR (20) Liu, Y.; Cadavid, D.; Ibáñez, M.; De Roo, J.; Ortega, S.; 01061. K.H.L. acknowledges support from the National Dobrozhan, O.; Kovalenko, M. V.; Cabot, A. Colloidal AgSbSe2 Natural Science Foundation of China (22208293). 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