Aqueous Synthesis, Doping, and Processing of n-Type Ag₂Se for High Thermoelectric Performance at Near-Room-Temperature

Abstract

Herein, we have successfully synthesized binary Ag2Se, composite Ag0:Ag2Se, and ternary Cu+:Ag2Se through an ambient aqueous-solution-based approach in a one-pot reaction at room temperature and atmospheric pressure without involving high-temperature heating, multiple-processes treatment, and organic solvents/surfactants. Effective controllability over phases and compositions/components are demonstrated with feasibility for large-scale production through an exquisite alteration in reaction parameters especially pH for enhancing and understanding thermoelectric properties. Thermoelectric ZT reaches 0.8–1.1 at near-room-temperature for n-type Ag2Se and Cu+ doping further improves to 0.9–1.2 over a temperature range of 300–393 K, which is the largest compared to that reported by wet chemistry methods. This improvement is related to the enhanced electrical conductivity and the suppressed thermal conductivity due to the incorporation of Cu+ into the lattice of Ag2Se at very low concentrations (x%Cu+:Ag2Se, x = 1.0, 1.5, and 2.0).

Thermoelectric Silver‐Based Chalcogenides

“…Providing enhanced control of stoichiometry, structure, and phase purity, wet‐chemical syntheses in the liquid phase have become an increasingly important way to prepare a variety of metal chalcogenides, [ 41 ] and their nanostructured derivatives. [ 42 ] These syntheses present an energy‐efficient strategy to synthesize a wide variety of binary (Ag 2 Se, [ 43 ] Ag 2 Te [ 44 ] ) and ternary (Ag 4 SeS, [ 45 ] Ag 3 AuSe 2 [ 46 ] ) silver‐based chalcogenides and their hybrids or composites. [ 47 ] In organic solvents, the colloidal synthesis of organic‐coated chalcogenides often involves the thermal decomposition of precursors in the presence of surfactants, [ 45 , 48 , 49 , 50 ] which is unfavorable due to reduced electrical conductivity and usually requires a prior ligand displacement procedure.…”

Advances in Ag2Se-based thermoelectrics from materials to applications

“…1(a) summarizes the temperature-dependent ZT values of Ag 2 Se-based bulk materials reported so far. Typical strategies for improving the ZT values of Ag 2 Se-based bulk materials include the introduction of a secondary phase (Ag 2 Se–0.5 wt% carbon nanotube (CNT), 112 Ag 2 Se–CuAgSe, 113 Ag 2 Se–5 mol% Te, 114 Ag 2 Se–AgSbSe 2 , 115 and Ag 2 Se–AgAI 116 ), alloying (Ag 2 Se 0.9 S 0.1 117 and Ag 2 Se 0.9 Te 0.1 118 ), doping (Ag 2 Se doped by 1% Cu + 119 and Ag 1.9 Sn 0.1 Se 120 ), defect engineering (Ag 2 Se-hierarchical structure (HS) 121 and Ag 2 Se-liquid phase manipulation (LSM) 122 ), and stoichiometry manipulation (Ag 2 Se 1.015 , 123 Ag:Se = 2 ± 0.01, 124 and Ag 2 Se 1.01 125 ). Table 1 also summarizes the thermoelectric properties of Ag 2 Se-based bulk materials.…”

“…Table 1 also summarizes the thermoelectric properties of Ag 2 Se-based bulk materials. Considerable achievements have been made in Ag 2 Se-based bulk materials with the maximum room temperature ZT exceeding 0.9, which is comparable with that of n-type Bi 2 (Te/Se) 3 , and the reported maximum ZT value can exceed 1.2 at 390 K. 119,122,124 These excellent thermoelectric performances of Ag 2 Se make it a promising candidate to be used as a flexible thermoelectric film. Fig.…”

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

“…Various chemical synthetic methods have also been reported for the production of silver chalcogenides and particularly Ag 2 Se particles, including colloidal, − hydrothermal, , and microwave-assisted methods. In some cases, aqueous solutions have been used. ,, As an example, Xiao et al synthesized Ag 2 Se nanocrystals via a colloidal method, reaching a maximum zT value of 0.23 at the phase transition temperature of around 408 K . Wang et al reported a general aqueous synthesis of nano/microscale binary silver chalcogenides (Ag 2 X, X = S, Se, Te) based on the reaction of Na 2 S/NaHSe/NaHTe and AgNO 3 aqueous solution at the water boiling temperature.…”

“…In some cases, aqueous solutions have been used. 5,25,26 As an example, Xiao et al synthesized Ag 2 Se 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 aqueous synthesis of nano/microscale binary silver chalcogenides (Ag 2 X, X = S, Se, Te) based on the reaction of Na 2 S/ NaHSe/NaHTe and AgNO 3 aqueous solution at the water boiling temperature. The molar ratios of Ag + /X 2− were adjusted from 2:1 to 2:1.1 and the maximum zT value of the resulting Ag 2 Se pellet was 0.84 at 380 K. 25 The TE properties of pristine Ag 2 Se can be improved through extrinsic and intrinsic doping.…”