WO2022211739A1 - Ambient scalable synthesis of surfactant-free nanostructured chalcogenide particles for near room-temperature thermoelectric applications - Google Patents

Ambient scalable synthesis of surfactant-free nanostructured chalcogenide particles for near room-temperature thermoelectric applications Download PDF

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WO2022211739A1
WO2022211739A1 PCT/SG2022/050186 SG2022050186W WO2022211739A1 WO 2022211739 A1 WO2022211739 A1 WO 2022211739A1 SG 2022050186 W SG2022050186 W SG 2022050186W WO 2022211739 A1 WO2022211739 A1 WO 2022211739A1
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ag2se
chalcogenide
mixing
metal
ions
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French (fr)
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Si Yin Tee
Yin Win Khin
Ming-yong Han
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Agency For Science, Technology And Research
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    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10N—ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10N10/00—Thermoelectric devices comprising a junction of dissimilar materials, i.e. devices exhibiting Seebeck or Peltier effects
    • H10N10/80—Constructional details
    • H10N10/85—Thermoelectric active materials
    • H10N10/851—Thermoelectric active materials comprising inorganic compositions
    • H10N10/852—Thermoelectric active materials comprising inorganic compositions comprising tellurium, selenium or sulfur
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B19/00—Selenium; Tellurium; Compounds thereof
    • C01B19/007—Tellurides or selenides of metals
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B82—NANOTECHNOLOGY
    • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
    • B82Y30/00—Nanotechnology for materials or surface science, e.g. nanocomposites
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2002/00—Crystal-structural characteristics
    • C01P2002/70—Crystal-structural characteristics defined by measured X-ray, neutron or electron diffraction data
    • C01P2002/72—Crystal-structural characteristics defined by measured X-ray, neutron or electron diffraction data by d-values or two theta-values, e.g. as X-ray diagram
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2004/00—Particle morphology
    • C01P2004/01—Particle morphology depicted by an image
    • C01P2004/03—Particle morphology depicted by an image obtained by SEM
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2004/00—Particle morphology
    • C01P2004/01—Particle morphology depicted by an image
    • C01P2004/04—Particle morphology depicted by an image obtained by TEM, STEM, STM or AFM
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2006/00—Physical properties of inorganic compounds
    • C01P2006/40—Electric properties
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10N—ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10N10/00—Thermoelectric devices comprising a junction of dissimilar materials, i.e. devices exhibiting Seebeck or Peltier effects
    • H10N10/01—Manufacture or treatment

Definitions

  • the present disclosure relates to a method of forming a metal chalcogen.
  • Chalcogenide semiconductors have been gaining widespread attention in the past decade owing to their remarkable intrinsic properties that may be exploited in diverse areas of applications. Although their utilities in photovoltaics and optoelectronics have been widely carried out, development is still lacking when it comes to their use in thermoelectric application.
  • Various methods may have been developed to manufacture chalcogenide semiconductors, but these methods tend to be undesirable and/or not feasible for large scale production.
  • the traditional melting- annealing-sintering processes are used to grow Ag2Se materials with the disadvantages of high costs of energy and time.
  • organic colloidal method utilizes large amount of organic solvent (and/or surfactant) or high synthesis temperature and some approaches may require complex instrumentation, which are not feasible for large scale production.
  • the solution should at least provide a method for the production of chalcogenide semiconductors in large scale with controlled phases and composition, without conventional organic ligands and surfactants.
  • a method of forming a metal chalcogenide comprising: providing an aqueous suspension comprising a chalcogen; mixing the aqueous suspension with a reducing agent to render chalcogenide ions; and mixing the chalcogenide ions with a metal precursor in the presence of a control agent to have the metal chalcogenide precipitated, wherein the control agent comprises ascorbic acid.
  • FIG. 1A shows the x-ray diffraction (XRD) pattern of as-synthesized Ag2Se powder produced from the method of the present disclosure and the spark plasma sintering (SPS) treated pellet at 200°C.
  • FIG. IB is a scanning electron microscopy (SEM) image of the Ag2Se powder of FIG. 1A.
  • FIG. 1C is a transmission electron microscopy (TEM) image of the Ag2Se powder of FIG. 1A.
  • FIG. ID is a digital image of the as-synthesized Ag2Se powder of FIG. 1A.
  • FIG. IE is a digital image of the hot-pressed Ag2Se pellet of FIG. ID.
  • FIG. IF is a schematic diagram of a pot-synthesis of the present method, which is a one-pot aqueous synthetic approach at room temperature using water soluble precursors (e.g. Se, NaBFU, ascorbic acid, AgNO,).
  • water soluble precursors e.g. Se, NaBFU, ascorbic acid, AgNO,
  • FIG. 2A shows temperature dependence of electrical conductivity of SPS Ag2Se pellets with and without ascorbic acid (AA).
  • FIG. 2B shows temperature dependence of Seebeck conductivity of SPS Ag2Se pellets with and without ascorbic acid (AA).
  • FIG. 2C shows temperature dependence of thermal conductivity of SPS Ag2Se pellets with and without ascorbic acid (AA).
  • FIG. 2D shows temperature dependence of zT values of SPS Ag2Se pellets with and without ascorbic acid (AA).
  • FIG. 3A shows temperature dependence of electrical conductivity of SPS Ag2Se, 1.0%Cu:Ag 2 Se and 1.5%Cu:Ag 2 Se samples.
  • FIG. 3B shows temperature dependence of Seebeck conductivity of SPS Ag2Se, 1.0%Cu:Ag 2 Se and 1.5%Cu:Ag 2 Se samples.
  • FIG. 3C shows temperature dependence of thermal conductivity of SPS Ag2Se, 1.0%Cu:Ag 2 Se and 1.5%Cu:Ag 2 Se samples.
  • FIG. 3D shows temperature dependence of zT values of SPS Ag2Se, 1.0%Cu:Ag 2 Se and 1.5%Cu:Ag 2 Se samples.
  • FIG. 4A shows XRD patterns of commercial Ag2Se from Sigma Aldrich.
  • FIG. 4B compares thermoelectric properties of Cu doped Ag2Se synthesized from the present method against commercial Ag2Se, specifically the temperature dependence of electrical conductivity.
  • FIG. 4C compares thermoelectric properties of Cu doped Ag2Se synthesized from the present method against commercial Ag2Se, specifically the temperature dependence of Seebeck conductivity.
  • FIG. 4D compares thermoelectric properties of Cu doped Ag2Se synthesized from this work against commercial Ag2Se, specifically the temperature dependence of power factor.
  • FIG. 5 shows a non-limiting example of a schematic setup of the present one- pot method.
  • the present method involves an aqueous solution-based one-pot reaction, mild conditions, no surfactants, and is easily scalable and environmental friendly.
  • a 2 denotes for the reduced chalcogen and M + denotes for the metal ion.
  • Ar denotes argon.
  • the present disclosure relates to a method of forming a metal chalcogenide.
  • the method advantageously renders surfactant-free products because organic ligands on the surfactant may undesirably block the charge transfer, substantially decreasing the electrical conductivity, which should be avoided for thermoelectric applications. Details of various embodiments of the present method and advantages associated with the various embodiments are now described below. Where an embodiment or an advantage has been described in the examples section further hereinbelow, it shall not be iterated for brevity.
  • the method comprises providing an aqueous suspension comprising a chalcogen, mixing the aqueous suspension with a reducing agent to render chalcogenide ions, and mixing the chalcogenide ions with a metal precursor in the presence of a control agent to have the metal chalcogenide precipitated, wherein the control agent comprises ascorbic acid.
  • the metal chalcogenide can be a chalcogenide semiconductor.
  • providing the aqueous suspension includes mixing the chalcogen with water.
  • the chalcogen may include sulfur, selenium, and/or tellurium.
  • mixing the aqueous suspension with the reducing agent comprises dissolving the reducing agent in water prior to mixing with the aqueous suspension.
  • the reducing agent may include sodium borohydride.
  • mixing the chalcogenide ions with the metal precursor includes dissolving the metal precursor in water prior to mixing with the chalcogenide ions.
  • the metal precursor includes silver nitrate.
  • the present method may further include washing the metal chalcogenide that has precipitated with water or an alcohol, and drying the metal chalcogenide after the washing.
  • the present method may be carried out in an inert environment. That is to say, all steps may be carried out in the presence of an inert gas, such as nitrogen or argon.
  • an inert gas such as nitrogen or argon.
  • the present method is absent of organic solvent and surfactant.
  • mixing the chalcogenide ions with a metal precursor may further include mixing the metal precursor with a dopant precursor prior to mixing the chalcogenide ions with the metal precursor.
  • the dopant precursor can include or can be copper sulfate or tin nitrate.
  • the dopant precursor may render the metal chalcogenide doped with a dopant of copper or tin.
  • the word “substantially” does not exclude “completely” e.g. a composition which is “substantially free” from Y may be completely free from Y. Where necessary, the word “substantially” may be omitted from the definition of the present disclosure.
  • the articles “a”, “an” and “the” as used with regard to a feature or element include a reference to one or more of the features or elements.
  • the term “about” or “approximately” as applied to a numeric value encompasses the exact value and a reasonable variance.
  • the term “and/or” includes any and all combinations of one or more of the associated listed items.
  • the present disclosure describes a facile one-pot synthesis method to produce high-yield, large-scale production of, for example, n-type crystalline binary chalcogenide Ag2Se and multinary chalcogenide Cu:Ag2Se with controlled phases and composition.
  • elemental Se powder was used to generate Se 2 after reduction with NaBFE in aqueous solution and react easily with Ag + ions to form Ag2Se at room temperature (e.g. 20 to 40°C).
  • ascorbic acid can be used to control the synthesis reaction, speed up the reaction time and enhance performance in thermoelectric application (and hence the ascorbic acid may be termed herein a “control agent”).
  • the incorporation of AA advantageously assists to accelerate the production rate of Ag2Se particles through inhibiting the reaction of Ag ions with NaBFB to form metallic Ag.
  • Cu can be incorporated as a dopant in Ag2Se to greatly improve the overall thermoelectric properties in reducing the lattice thermal conductivity by enhancing phonon scattering.
  • the maximum zT of Cu doped Ag2Se is 0.9 at 298 K and 1.2 at 393 K, the calculated power factor value (oS 2 ) of Cu doped Ag2Se produced from the present method was determined to be ⁇ 7 times higher than a commercially purchased Ag2Se at 323 K.
  • Chalcogenide Ag2Se compound is a narrow band gap semiconductor with an energy gap of 0.07 eV at room temperature, which exhibits unusual low lattice thermal conductivity coupled with high electrical conductivity and relatively high Seebeck coefficient.
  • Ag2Se is a promising n-type thermoelectric materials at room temperature.
  • the present method is a scalable aqueous synthetic approach at room temperature to produce high-yield (>96%) n-type crystalline chalcogenide semiconductor (e.g. Ag2Se) using water soluble precursors.
  • the present method produces phase-pure chalcogenide nanostructures with controlled phases and composition without involving organic ligands and surfactants. Furthermore, the present approach can significantly save material synthesis time and provide a significant advantage for large-scale production.
  • Example 2 Present Method and Characterization
  • elemental Se powder as a non-limiting example was used as a selenium source for reduction to form Se 2 in aqueous solution since it could easily react with Ag + ions at room temperature without requiring the use of additional energy source.
  • Se 40 mmol
  • NaBfU 80 mmol
  • ascorbic acid 60 mmol
  • pH 11 to 5.
  • the reaction mixture was left stirring at room temperature until clear.
  • the powder obtained was loaded into graphite dies and subsequent hot pressed by spark plasma sintering (SPS) into pellets (0 15 mm x 1.0 mm) under vacuum atmosphere.
  • SPS spark plasma sintering
  • the compaction temperature was maintained at 473 K for 10 mins, and the pressure was set to 40 MPa to obtain high-density bulk materials.
  • the relative density of the obtained pellets was above 95% of their theoretical value.
  • the electrical conductivity and Seebeck coefficient of the Ag2Se pellet were measured simultaneously under helium atmosphere, using nickel contacts and a reversed vertically arranged thermocouples and electrodes.
  • the specific heat capacity (C p ) was measured by differential scanning calorimeter.
  • FIG. 1A shows the X-ray diffraction (XRD) patterns of the as- synthesized Ag2Se powder and hot pressed Ag2Se pellet.
  • JCPDS 24-1041
  • JCPDS standard orthorhombic Ag2Se phase
  • the morphology of the Ag2Se powder was examined by scanning electron microscope (SEM) and transmission electron microscopy (TEM). As shown in FIG.
  • the Ag2Se particles consist of particles with dimension of -200 nm.
  • the elemental composition and purity of the Ag2Se were analyzed by inductively coupled plasma - atomic emission spectrometry, wherein the chemical components were identified to be Ag and Se with a ratio of 2:1 respectively, indicating the high purity of the product.
  • Ag2Se can undergo a polymorphic phase transition whereby the low-temperature orthorhombic phase at room temperature behaves as a semiconductor, while the high-temperature cubic phase with a phase transition temperature at about 135°C or 408 K exhibits the properties of a metal.
  • AA played a role in controlling the reaction as well as determining the reaction duration. Addition of AA speeds up the reaction mechanism from 3 days to 2 hours. Without AA in the reaction medium, the reaction kinetic promotes the formation of Ag metal instantaneously upon adding AgNCb to Se 2 due to the excess NaBtU. To complete the reaction of 2 Ag + + Se 2 Ag2Se, it takes approximately 3 days to form the product. Whereas, the presence of the ascorbic acid neutralizes the excess NaBtB as indicated by the drop in pH from alkaline to acidic and promotes the formation of Ag2Se, shortening the reaction duration to 2 hours to form Ag2Se product.
  • S the Seebeck coefficient
  • s the electrical conductivity
  • T the absolute temperature
  • K to tai is the total thermal conductivity from the electronic (K e ) and lattice (KI) thermal conductivities.
  • the challenge to produce high-efficiency thermoelectric materials lies in simultaneously optimizing power factor (S 2 G) and K to tai in the same solid, since these two factors are closely correlated. As shown in FIG.
  • FIG. 2A shows the Seebeck coefficient of Ag2Se pellets as a function of temperature.
  • the negative sign of the Seebeck coefficient indicates that Ag2Se are n-type semiconductor. It is seen that Seebeck coefficient is inversely proportional to electrical conductivity evidenced from the drop of Seebeck coefficient as temperature increased.
  • Both Ag2Se pellets formed with AA and without AA display a similar trend in Seebeck coefficient.
  • metals usually display Seebeck coefficients of a few tens of pV/K, which are much lower than the semiconductor Ag2Se demonstrated herein.
  • the semiconductor- superionic conductor phase transition in Ag2Se leads to larger Seebeck coefficient of low temperature phase and lower Seebeck coefficient of high temperature.
  • FIG. 2C reveals the similar trend of temperature dependence thermal conductivity as observed in electrical conductivity. It was observed that a pronounced increase in thermal conductivity with temperature increasing to 353 K particularly to the Ag2Se sample formed without using AA. The thermal conductivity of this sample is significantly higher than the Ag2Se formed using AA. Combining the electrical and thermal transport properties, the calculated zT value of Ag2Se pellets are shown in FIG. 2D. The effect of AA addition during synthesis has clearly shown that zT value is enhanced considerably. The maximum zT value of 0.8 at 298 K and 1.1 at 393 K are obtained for the Ag2Se sample formed using AA, which is significantly higher than sample (formed without using AA) of 0.6 at 298 K and 1.0 at 393 K.
  • the thermal conductivity of the material can be lowered to reduce the phonon mean free path.
  • Successful strategy includes to reduce thermal conductivity by creating point defects that scatter heat carrying phonons in the alloy. The introduction of randomness in the lattice by alloying often results in sufficient disorder or point defects to produce phonon scattering and is of great advantage in the improvement of the performance of thermoelectric materials.
  • Doping can be used to induce extrinsic defects, and it is applied to adjust the carrier concentration by regulating the Fermi level.
  • Cu or Sn can be used as a non-limiting examples, which demonstrated positive effects.
  • Cu doped chalcogenide samples with the use of AA was synthesized and the thermoelectric properties of SPS hot pressed 1.0%Cu:Ag 2 Se andl.5%Cu:Ag 2 Se were investigated.
  • the 1.0% and 1.5% are mole percentages.
  • the dopant (e.g. Cu or Sn) amount in Ag2Se can range from 1 to 1.5% (mol%), e.g. at 1 mol% or 1.5 mol%.
  • Elemental Se powder was used as a selenium source for reduction to form Se 2 in aqueous solution since it could easily react with Ag + ions at room temperature without requiring the use of additional energy source.
  • Se 40 mmol
  • NaBtU 80 mmol
  • ascorbic acid 60 mmol
  • the temperature-dependent electrical property of Cu doped Ag2Se samples is presented in LIG. 3A.
  • the electrical conductivity of the Ag2- x Cu x Se is observed to increase after Cu doping and the increment is quite similar for the two Cu doped Ag2Se samples.
  • the samples of Cu doped Ag2Se have an initial s value of -1.2 x 10 3 S/cm at 298 K, which rapidly increase to -1.7 x 10 3 S/m at 393 K and decreases after phase transition.
  • the Seebeck coefficients of Cu doped Ag2Se samples in LIG. 3B decrease accordingly owing to the increase in carrier concentration.
  • Thermal conductivity is the sum of lattice and electronic contribution.
  • Ag2Se is a semiconductor with relatively low electrical conductivity, and electrical contribution to the total thermal conductivity may be neglected, while the lattice thermal conductivity is also very low due to the reduction of grain size.
  • reduction to nanometer grain size serves to introduce a large density of interfaces in which phonons can be effectively scattered resulting in the reduction of lattice thermal conductivity without having to compromise carrier mobility values.
  • temperature increased to 393 K, the orthorhombic to cubic phase transition appeared.
  • Ag2Se began the change from semiconductor to superionic conductor and the electrical conductivity sharply increased.
  • the lattice thermal conductivity dropped contrarily but acutely.
  • the thermal conductivity was then reduced due to the intensive scattering of phonons by the extra point defects introduced by Cu doping. Due to the enhanced electrical-transport properties and reduced lattice thermal conductivity, the Cu doped Ag2Se samples show enhanced zT values compared to the Cu free sample over the entire measured temperature range, as shown in FIG. 3D.
  • a peak zT of 0.9 at 298 K and 1.2 at 393 K are obtained for the 1.0% Cu:Ag2Se, which are higher than those of most of the state- of-the-art n-type Ag2Se materials.
  • the present method is a green synthetic method operating at room temperature aqueous solution-based to produce large-scale nanostmctured metal chalcogenides (nano structured materials and easy for pelletization).
  • the current method controls product’s phases and composition without the need for organic ligands and surfactants.
  • the present method uses AA to control the synthesis reaction, speed up the reaction and achieve enhanced performance in thermoelectric application.
  • Cu or Sn can be doped in Ag2Se to significantly improve the overall thermoelectric properties in reducing the lattice thermal conductivity by enhancing phonon scattering.
  • the present method differs in that Ag2Se nanostructures are synthesized from aqueous synthetic approach at room temperature, particularly for thermoelectric application.
  • Ag2Se nanostructures are synthesized from aqueous synthetic approach at room temperature, particularly for thermoelectric application.
  • thermoelectric application particularly for thermoelectric application.
  • phase-pure high crystallinity binary chalcogenide nanostructures with Cu doping in high-yield (>96%) and high zT (0.9 at 298 K and 1.2 at 393 K) for practical scale-up.
  • the present method is demonstrated to prepare phase-pure binary chalcogenide nanostructures with high crystallinity towards thermoelectric application. Specifically, features of the present method include:
  • thermoelectric application Use of AA to control the synthesis reaction, speed up the reaction and achieve enhanced performance in thermoelectric application
  • thermoelectric (TE) power generators have emerged as a promising alternative green technology that permit a direct conversion of waste-heat directly into electrical energy.
  • Chalcogenide semiconductors are promising TE material, amongst which Ag2Se is a low temperature TE material that can work at ambient temperature, and is a good alternative to the highly toxic telluride based materials.
  • the current method taught herein to prepare chalcogenide semiconductors e.g. Ag2Se
  • the present method being a facile one-pot synthesis method, advantageously produces high-yield, large-scale production of n- type crystalline Ag2Se with controlled phases and composition without conventional organic ligands and surfactants under room temperature.

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Abstract

Herein disclosed is a method of forming a metal chalcogenide, the method comprising: providing an aqueous suspension comprising a chalcogen; mixing the aqueous suspension with a reducing agent to render chalcogenide ions; and mixing the chalcogenide ions with a metal precursor in the presence of a control agent to have the metal chalcogenide precipitated, wherein the control agent comprises ascorbic acid.

Description

AMBIENT SCALABLE SYNTHESIS OF SURFACTANT-FREE NANOSTRUCTURED CHALCOGENIDE PARTICLES FOR NEAR ROOM- TEMPERATURE THERMOELECTRIC APPLICATIONS
Cross-Reference to Related Application
[0001] This application claims the benefit of priority of Singapore Patent Application No. 10202103339V, filed 31 March 2021, the content of it being hereby incorporated by reference in its entirety for all purposes.
Technical Field
[0002] The present disclosure relates to a method of forming a metal chalcogen.
Background
[0003] Chalcogenide semiconductors have been gaining widespread attention in the past decade owing to their remarkable intrinsic properties that may be exploited in diverse areas of applications. Although their utilities in photovoltaics and optoelectronics have been widely carried out, development is still lacking when it comes to their use in thermoelectric application. Various methods may have been developed to manufacture chalcogenide semiconductors, but these methods tend to be undesirable and/or not feasible for large scale production. The traditional melting- annealing-sintering processes are used to grow Ag2Se materials with the disadvantages of high costs of energy and time. In another example, organic colloidal method utilizes large amount of organic solvent (and/or surfactant) or high synthesis temperature and some approaches may require complex instrumentation, which are not feasible for large scale production.
[0004] There is thus a need to provide for a solution that addresses one or more of the limitations mentioned above. The solution should at least provide a method for the production of chalcogenide semiconductors in large scale with controlled phases and composition, without conventional organic ligands and surfactants. Summary
[0005] In a first aspect, there is provided for a method of forming a metal chalcogenide, the method comprising: providing an aqueous suspension comprising a chalcogen; mixing the aqueous suspension with a reducing agent to render chalcogenide ions; and mixing the chalcogenide ions with a metal precursor in the presence of a control agent to have the metal chalcogenide precipitated, wherein the control agent comprises ascorbic acid.
Brief Description of the Drawings [0006] The drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the present disclosure. In the following description, various embodiments of the present disclosure are described with reference to the following drawings, in which:
[0007] FIG. 1A shows the x-ray diffraction (XRD) pattern of as-synthesized Ag2Se powder produced from the method of the present disclosure and the spark plasma sintering (SPS) treated pellet at 200°C. XRD diffraction pattern was indexed to the standard orthorhombic Ag2Se phase (JCPDS = 24 1041), no peak of impurities was observed, indicating the formation of pure Ag2Se.
[0008] FIG. IB is a scanning electron microscopy (SEM) image of the Ag2Se powder of FIG. 1A.
[0009] FIG. 1C is a transmission electron microscopy (TEM) image of the Ag2Se powder of FIG. 1A.
[0010] FIG. ID is a digital image of the as-synthesized Ag2Se powder of FIG. 1A. [0011] FIG. IE is a digital image of the hot-pressed Ag2Se pellet of FIG. ID. [0012] FIG. IF is a schematic diagram of a pot-synthesis of the present method, which is a one-pot aqueous synthetic approach at room temperature using water soluble precursors (e.g. Se, NaBFU, ascorbic acid, AgNO,).
[0013] FIG. 2A shows temperature dependence of electrical conductivity of SPS Ag2Se pellets with and without ascorbic acid (AA). [0014] FIG. 2B shows temperature dependence of Seebeck conductivity of SPS Ag2Se pellets with and without ascorbic acid (AA).
[0015] FIG. 2C shows temperature dependence of thermal conductivity of SPS Ag2Se pellets with and without ascorbic acid (AA).
[0016] FIG. 2D shows temperature dependence of zT values of SPS Ag2Se pellets with and without ascorbic acid (AA).
[0017] FIG. 3A shows temperature dependence of electrical conductivity of SPS Ag2Se, 1.0%Cu:Ag2Se and 1.5%Cu:Ag2Se samples.
[0018] FIG. 3B shows temperature dependence of Seebeck conductivity of SPS Ag2Se, 1.0%Cu:Ag2Se and 1.5%Cu:Ag2Se samples.
[0019] FIG. 3C shows temperature dependence of thermal conductivity of SPS Ag2Se, 1.0%Cu:Ag2Se and 1.5%Cu:Ag2Se samples.
[0020] FIG. 3D shows temperature dependence of zT values of SPS Ag2Se, 1.0%Cu:Ag2Se and 1.5%Cu:Ag2Se samples.
[0021] FIG. 4A shows XRD patterns of commercial Ag2Se from Sigma Aldrich. [0022] FIG. 4B compares thermoelectric properties of Cu doped Ag2Se synthesized from the present method against commercial Ag2Se, specifically the temperature dependence of electrical conductivity.
[0023] FIG. 4C compares thermoelectric properties of Cu doped Ag2Se synthesized from the present method against commercial Ag2Se, specifically the temperature dependence of Seebeck conductivity.
[0024] FIG. 4D compares thermoelectric properties of Cu doped Ag2Se synthesized from this work against commercial Ag2Se, specifically the temperature dependence of power factor.
[0025] FIG. 5 shows a non-limiting example of a schematic setup of the present one- pot method. The present method involves an aqueous solution-based one-pot reaction, mild conditions, no surfactants, and is easily scalable and environmental friendly. A2 denotes for the reduced chalcogen and M+ denotes for the metal ion. Ar denotes argon. Detailed Description
[0026] The following detailed description refers to the accompanying drawings that show, by way of illustration, specific details and embodiments in which the present disclosure may be practised.
[0027] Features that are described in the context of an embodiment may correspondingly be applicable to the same or similar features in the other embodiments. Features that are described in the context of an embodiment may correspondingly be applicable to the other embodiments, even if not explicitly described in these other embodiments. Furthermore, additions and/or combinations and/or alternatives as described for a feature in the context of an embodiment may correspondingly be applicable to the same or similar feature in the other embodiments.
[0028] The present disclosure relates to a method of forming a metal chalcogenide. The method advantageously renders surfactant-free products because organic ligands on the surfactant may undesirably block the charge transfer, substantially decreasing the electrical conductivity, which should be avoided for thermoelectric applications. Details of various embodiments of the present method and advantages associated with the various embodiments are now described below. Where an embodiment or an advantage has been described in the examples section further hereinbelow, it shall not be iterated for brevity.
[0029] The method comprises providing an aqueous suspension comprising a chalcogen, mixing the aqueous suspension with a reducing agent to render chalcogenide ions, and mixing the chalcogenide ions with a metal precursor in the presence of a control agent to have the metal chalcogenide precipitated, wherein the control agent comprises ascorbic acid. The metal chalcogenide can be a chalcogenide semiconductor. [0030] In various embodiments, providing the aqueous suspension includes mixing the chalcogen with water. In various embodiments, the chalcogen may include sulfur, selenium, and/or tellurium.
[0031] In various embodiments, mixing the aqueous suspension with the reducing agent comprises dissolving the reducing agent in water prior to mixing with the aqueous suspension. In various embodiments, the reducing agent may include sodium borohydride. [0032] In various embodiments, mixing the chalcogenide ions with the metal precursor includes dissolving the metal precursor in water prior to mixing with the chalcogenide ions. In various embodiments, the metal precursor includes silver nitrate.
[0033] The present method may further include washing the metal chalcogenide that has precipitated with water or an alcohol, and drying the metal chalcogenide after the washing.
[0034] The present method may be carried out in an inert environment. That is to say, all steps may be carried out in the presence of an inert gas, such as nitrogen or argon. [0035] The present method is absent of organic solvent and surfactant.
[0036] In various embodiments, mixing the chalcogenide ions with a metal precursor may further include mixing the metal precursor with a dopant precursor prior to mixing the chalcogenide ions with the metal precursor. The dopant precursor can include or can be copper sulfate or tin nitrate. The dopant precursor may render the metal chalcogenide doped with a dopant of copper or tin.
[0037] The word “substantially” does not exclude “completely” e.g. a composition which is “substantially free” from Y may be completely free from Y. Where necessary, the word “substantially” may be omitted from the definition of the present disclosure. [0038] In the context of various embodiments, the articles “a”, “an” and “the” as used with regard to a feature or element include a reference to one or more of the features or elements.
[0039] In the context of various embodiments, the term “about” or “approximately” as applied to a numeric value encompasses the exact value and a reasonable variance. [0040] As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items.
[0041] Unless specified otherwise, the terms "comprising" and "comprise", and grammatical variants thereof, are intended to represent "open" or "inclusive" language such that they include recited elements but also permit inclusion of additional, unrecited elements.
Examples
[0042] The present disclosure describes a facile one-pot synthesis method to produce high-yield, large-scale production of, for example, n-type crystalline binary chalcogenide Ag2Se and multinary chalcogenide Cu:Ag2Se with controlled phases and composition. In the present approach, as a non-limiting example, elemental Se powder was used to generate Se2 after reduction with NaBFE in aqueous solution and react easily with Ag+ ions to form Ag2Se at room temperature (e.g. 20 to 40°C). In the present method, there are no organic ligands and surfactants used, instead, ascorbic acid (AA) can be used to control the synthesis reaction, speed up the reaction time and enhance performance in thermoelectric application (and hence the ascorbic acid may be termed herein a “control agent”). Considering the lengthy reaction time to synthesize Ag2Se in traditional methods, the incorporation of AA advantageously assists to accelerate the production rate of Ag2Se particles through inhibiting the reaction of Ag ions with NaBFB to form metallic Ag. Moreover, Cu can be incorporated as a dopant in Ag2Se to greatly improve the overall thermoelectric properties in reducing the lattice thermal conductivity by enhancing phonon scattering. The maximum zT of Cu doped Ag2Se is 0.9 at 298 K and 1.2 at 393 K, the calculated power factor value (oS2) of Cu doped Ag2Se produced from the present method was determined to be ~7 times higher than a commercially purchased Ag2Se at 323 K.
[0043] The present method is described in further details, by way of non-limiting examples, as set forth below.
[0044] Example 1: General Introduction
[0045] Chalcogenide Ag2Se compound is a narrow band gap semiconductor with an energy gap of 0.07 eV at room temperature, which exhibits unusual low lattice thermal conductivity coupled with high electrical conductivity and relatively high Seebeck coefficient. Ag2Se is a promising n-type thermoelectric materials at room temperature. The present method is a scalable aqueous synthetic approach at room temperature to produce high-yield (>96%) n-type crystalline chalcogenide semiconductor (e.g. Ag2Se) using water soluble precursors. The present method produces phase-pure chalcogenide nanostructures with controlled phases and composition without involving organic ligands and surfactants. Furthermore, the present approach can significantly save material synthesis time and provide a significant advantage for large-scale production. [0046] Example 2: Present Method and Characterization
[0047] In the present approach, elemental Se powder as a non-limiting example was used as a selenium source for reduction to form Se2 in aqueous solution since it could easily react with Ag+ ions at room temperature without requiring the use of additional energy source. As a non-limiting example, Se (40 mmol) was added in water (100 mL) under mild magnetic stirring and continuous argon gas purging. NaBfU (80 mmol) in water (50 mL) was introduced into the Se suspension under the protection of argon, followed by ascorbic acid (60 mmol) in water (50 mL) to adjust pH from 11 to 5. The reaction mixture was left stirring at room temperature until clear. Subsequently AgNCL (80 mmol) in 50 mL of water was added slowly to the flask. The reaction (i.e. the formation of black Ag2Se particles) was deemed completed when no trace of orange Se in the supernatant was observed. The precipitate was purified by three rounds of centrifugation/washing treatment with water/ethanol. The powder was then dried in oven at 60 C overnight.
[0048] The powder obtained was loaded into graphite dies and subsequent hot pressed by spark plasma sintering (SPS) into pellets (0 15 mm x 1.0 mm) under vacuum atmosphere. The compaction temperature was maintained at 473 K for 10 mins, and the pressure was set to 40 MPa to obtain high-density bulk materials. The relative density of the obtained pellets was above 95% of their theoretical value. The electrical conductivity and Seebeck coefficient of the Ag2Se pellet were measured simultaneously under helium atmosphere, using nickel contacts and a reversed vertically arranged thermocouples and electrodes. Thermal conductivity (K) was calculated using the equation k = apCp, in which (p) is the density and thermal diffusivity (a) is obtained by a flash diffusivity method. The specific heat capacity (Cp) was measured by differential scanning calorimeter.
[0049] In the present example, high purity Ag2Se powder was prepared by the aqueous synthetic approach and hot pressed in 15 mm diameter pellet. FIG. 1A shows the X-ray diffraction (XRD) patterns of the as- synthesized Ag2Se powder and hot pressed Ag2Se pellet. The diffraction pattern was indexed to the standard orthorhombic Ag2Se phase (JCPDS = 24-1041), also known under the mineral name naumannite. Upon SPS treatment at 200°C, crystallinity increased while no peak of impurities was observed, indicating formation of pure Ag2Se. The morphology of the Ag2Se powder was examined by scanning electron microscope (SEM) and transmission electron microscopy (TEM). As shown in FIG. IB and 1C, the Ag2Se particles consist of particles with dimension of -200 nm. The elemental composition and purity of the Ag2Se were analyzed by inductively coupled plasma - atomic emission spectrometry, wherein the chemical components were identified to be Ag and Se with a ratio of 2:1 respectively, indicating the high purity of the product. Ag2Se can undergo a polymorphic phase transition whereby the low-temperature orthorhombic phase at room temperature behaves as a semiconductor, while the high-temperature cubic phase with a phase transition temperature at about 135°C or 408 K exhibits the properties of a metal.
[0050] In the synthesis of Ag2Se powder, AA played a role in controlling the reaction as well as determining the reaction duration. Addition of AA speeds up the reaction mechanism from 3 days to 2 hours. Without AA in the reaction medium, the reaction kinetic promotes the formation of Ag metal instantaneously upon adding AgNCb to Se2 due to the excess NaBtU. To complete the reaction of 2 Ag+ + Se2 Ag2Se, it takes approximately 3 days to form the product. Whereas, the presence of the ascorbic acid neutralizes the excess NaBtB as indicated by the drop in pH from alkaline to acidic and promotes the formation of Ag2Se, shortening the reaction duration to 2 hours to form Ag2Se product.
[0051] The influence of AA on the thermoelectric properties of the Ag2Se pellets was studied in this example. The thermoelectric performance of a material is evaluated by conversion efficiency which depends on the dimensionless TE figure-of-merit, zT = S2G77ktotai (K total = Ke+ Ki), where S is the Seebeck coefficient, s is the electrical conductivity, T is the absolute temperature and Ktotai is the total thermal conductivity from the electronic (Ke) and lattice (KI) thermal conductivities. The challenge to produce high-efficiency thermoelectric materials lies in simultaneously optimizing power factor (S2G) and Ktotai in the same solid, since these two factors are closely correlated. As shown in FIG. 2A, a trend of drastic increase in electrical conductivity with temperature increasing from room temperature to 393 K was observed in both Ag2Se samples and this clearly shows the electrical transition originated from the atomic rearrangement of Ag+ disordering during the structural transition. FIG. 2B shows the Seebeck coefficient of Ag2Se pellets as a function of temperature. The negative sign of the Seebeck coefficient indicates that Ag2Se are n-type semiconductor. It is seen that Seebeck coefficient is inversely proportional to electrical conductivity evidenced from the drop of Seebeck coefficient as temperature increased. Both Ag2Se pellets formed with AA and without AA display a similar trend in Seebeck coefficient. In comparison, metals usually display Seebeck coefficients of a few tens of pV/K, which are much lower than the semiconductor Ag2Se demonstrated herein. In this example, the semiconductor- superionic conductor phase transition in Ag2Se leads to larger Seebeck coefficient of low temperature phase and lower Seebeck coefficient of high temperature.
[0052] A good thermoelectric material should possess low thermal conductivity other than having favorable electrical transport properties. FIG. 2C reveals the similar trend of temperature dependence thermal conductivity as observed in electrical conductivity. It was observed that a pronounced increase in thermal conductivity with temperature increasing to 353 K particularly to the Ag2Se sample formed without using AA. The thermal conductivity of this sample is significantly higher than the Ag2Se formed using AA. Combining the electrical and thermal transport properties, the calculated zT value of Ag2Se pellets are shown in FIG. 2D. The effect of AA addition during synthesis has clearly shown that zT value is enhanced considerably. The maximum zT value of 0.8 at 298 K and 1.1 at 393 K are obtained for the Ag2Se sample formed using AA, which is significantly higher than sample (formed without using AA) of 0.6 at 298 K and 1.0 at 393 K.
[0053] Example 3: Effect of Dopant
[0054] To further improve on the thermoelectric efficiency, the thermal conductivity of the material can be lowered to reduce the phonon mean free path. Successful strategy includes to reduce thermal conductivity by creating point defects that scatter heat carrying phonons in the alloy. The introduction of randomness in the lattice by alloying often results in sufficient disorder or point defects to produce phonon scattering and is of great advantage in the improvement of the performance of thermoelectric materials. Doping can be used to induce extrinsic defects, and it is applied to adjust the carrier concentration by regulating the Fermi level. Among the dopant elements, Cu or Sn can be used as a non-limiting examples, which demonstrated positive effects. In this context, Cu doped chalcogenide samples with the use of AA was synthesized and the thermoelectric properties of SPS hot pressed 1.0%Cu:Ag2Se andl.5%Cu:Ag2Se were investigated. The 1.0% and 1.5% are mole percentages. In fact, the dopant (e.g. Cu or Sn) amount in Ag2Se can range from 1 to 1.5% (mol%), e.g. at 1 mol% or 1.5 mol%. [0055] To fabricate the Cu doped Ag2Se, the following steps were carried out. Elemental Se powder was used as a selenium source for reduction to form Se2 in aqueous solution since it could easily react with Ag+ ions at room temperature without requiring the use of additional energy source. As a non-limiting example, Se (40 mmol) was added in water (100 mL) under mild magnetic stirring and continuous argon gas purging to form a Se suspension. NaBtU (80 mmol) in water (50 mL) was introduced into the Se suspension under the protection of argon, followed by ascorbic acid (60 mmol) in water (50 mL) to adjust pH from 11 to 5. The reaction mixture was left stirring at room temperature until clear. Subsequently, a portion of the AgNCL (80 mmol) in water (38 mL) was replaced with 1 mmol and 1.2 mmol CuSCL that were, respectively, pretreated in ascorbic acid (4 mmol and 4.8 mmol) solution (12 mL), and further gradually added into the Se suspension containing AA, followed by stirring at room temperature for 2 hrs. In other words, the AgNCL precursor solution added contained CuSCL. The reaction (i.e. the formation of black Ag2Se particles doped with Cu) was deemed completed when no trace of orange Se in the supernatant was observed. The precipitate was purified by three rounds of centrifugation/washing treatment with water/ethanol. The powder was then dried in oven at 60°C overnight. Other than from CuS04, tin nitrate can be used as a dopant precursor to render a Ag2Se particles doped with Sn.
[0056] The temperature-dependent electrical property of Cu doped Ag2Se samples is presented in LIG. 3A. The electrical conductivity of the Ag2-xCuxSe is observed to increase after Cu doping and the increment is quite similar for the two Cu doped Ag2Se samples. The samples of Cu doped Ag2Se have an initial s value of -1.2 x 103 S/cm at 298 K, which rapidly increase to -1.7 x 103 S/m at 393 K and decreases after phase transition. Concurrently, the Seebeck coefficients of Cu doped Ag2Se samples in LIG. 3B decrease accordingly owing to the increase in carrier concentration. Also, all the Cu doped Ag2Se samples exhibit negative Seebeck coefficient over the entire temperature range, indicating n-type conduction. In the Cu doped Ag2Se samples, part of Ag elements of Ag2Se is substituted by Cu to form Ag/Cu cations, which distribute randomly in the Se framework, thus intensifying the crystal disorder, which may give rise to a lower lattice thermal conductivity than that of Ag2Se without Cu dopant. LIG. 3C depicts the variation of the thermal conductivity of Ag2Se and Cu doped Ag2Se samples with temperature. In comparison with Cu free Ag2Se sample, the findings show that there is no significant change in thermal conductivity with Cu doped samples from 298 to 393 K. After the phase transition, the thermal conductivity of the Cu doped samples decreased significantly, much lower than the Cu free Ag2Se from temperature range 393 K to 473 K.
[0057] Thermal conductivity is the sum of lattice and electronic contribution. At low temperature, Ag2Se is a semiconductor with relatively low electrical conductivity, and electrical contribution to the total thermal conductivity may be neglected, while the lattice thermal conductivity is also very low due to the reduction of grain size. Theoretically, reduction to nanometer grain size serves to introduce a large density of interfaces in which phonons can be effectively scattered resulting in the reduction of lattice thermal conductivity without having to compromise carrier mobility values. As temperature increased to 393 K, the orthorhombic to cubic phase transition appeared. Along with the phase transition, Ag2Se began the change from semiconductor to superionic conductor and the electrical conductivity sharply increased. Although the electrical contribution to total thermal conductivity could not be neglected during the phase transition, the lattice thermal conductivity dropped contrarily but acutely. The thermal conductivity was then reduced due to the intensive scattering of phonons by the extra point defects introduced by Cu doping. Due to the enhanced electrical-transport properties and reduced lattice thermal conductivity, the Cu doped Ag2Se samples show enhanced zT values compared to the Cu free sample over the entire measured temperature range, as shown in FIG. 3D. A peak zT of 0.9 at 298 K and 1.2 at 393 K are obtained for the 1.0% Cu:Ag2Se, which are higher than those of most of the state- of-the-art n-type Ag2Se materials.
[0058] In order to compare the thermoelectric properties of 1.0% Cu:Ag2Se prepared from the present method, commercial Ag2Se was purchased and characterized. FIG. 4A shows the XRD diffraction pattern of commercially purchased Ag2Se which correspond to the Ag2Se phase (JCPDS = 24-1041). The powder was then hot pressed by SPS at 200°C. The comparison of electrical conductivity as a function of temperature in FIG. 4B clearly showed the conductivity of the Cu doped Ag2Se of this example is several order of magnitude higher than the commercially purchased Ag2Se across the temperature range from 298 to 480 K. FIG. 4C shows that the Seebeck coefficient of both Cu doped Ag2Se and commercial sample are most desirable at 298 K, the present example demonstrated a Seebeck coefficient of -140 compared to -87 pVK 1 from the commercial product. These values decrease rapidly when temperature increases, showing opposing trend to the electrical conductivity. In agreement with these results, the calculated power factor value (oS2) of Cu doped Ag2Se of the present example was determined to be ~7 times higher than the commercially purchased Ag2Se. At 323 K, the power factor value of 2.4 and 0.33 mWm^K2 were obtained from Ag2Se of the present example and commercial product, respectively (FIG. 4D).
[0059] Example 4: Summary
[0060] The present method is a green synthetic method operating at room temperature aqueous solution-based to produce large-scale nanostmctured metal chalcogenides (nano structured materials and easy for pelletization). The current method controls product’s phases and composition without the need for organic ligands and surfactants. Instead, the present method uses AA to control the synthesis reaction, speed up the reaction and achieve enhanced performance in thermoelectric application. In addition, Cu or Sn can be doped in Ag2Se to significantly improve the overall thermoelectric properties in reducing the lattice thermal conductivity by enhancing phonon scattering. [0061] As compared to traditional methods, the present method differs in that Ag2Se nanostructures are synthesized from aqueous synthetic approach at room temperature, particularly for thermoelectric application. Herein it has been demonstrated the preparation of phase-pure high crystallinity binary chalcogenide nanostructures with Cu doping in high-yield (>96%) and high zT (0.9 at 298 K and 1.2 at 393 K) for practical scale-up.
[0062] The present method is demonstrated to prepare phase-pure binary chalcogenide nanostructures with high crystallinity towards thermoelectric application. Specifically, features of the present method include:
[0063] - Use of highly water soluble precursors for aqueous synthesis. Residues and side product can be easily removed.
[0064] - Use of AA to control the synthesis reaction, speed up the reaction and achieve enhanced performance in thermoelectric application [0065] - Use of a Cu-based precursor or Sn-based precursor to render a dopant in Ag2Se to significantly improve the overall thermoelectric properties in reducing the lattice thermal conductivity by enhancing phonon scattering.
[0066] - Traditional method may focus on the preparation of Cu2Se as thermoelectric material. However, such traditional methods involving high temperature are not applicable and/or desirable for Ag2Se due to the instability of Ag+ ions at high temperature.
[0067] - Traditional organic synthesis involves the large usage of organic capping agent or surfactant which is undesirable for TE application. [0068] Example 5: Commercial and Potential Applications
[0069] Due to concerns about environmental crises, thermoelectric (TE) power generators have emerged as a promising alternative green technology that permit a direct conversion of waste-heat directly into electrical energy.
[0070] Chalcogenide semiconductors are promising TE material, amongst which Ag2Se is a low temperature TE material that can work at ambient temperature, and is a good alternative to the highly toxic telluride based materials. The current method taught herein to prepare chalcogenide semiconductors (e.g. Ag2Se) are economically advantageous for large scale production. The present method, being a facile one-pot synthesis method, advantageously produces high-yield, large-scale production of n- type crystalline Ag2Se with controlled phases and composition without conventional organic ligands and surfactants under room temperature.
[0071] While the present disclosure has been particularly shown and described with reference to specific embodiments, it should be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the present disclosure as defined by the appended claims. The scope of the present disclosure is thus indicated by the appended claims and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced.

Claims

1. A method of forming a metal chalcogenide, the method comprising: providing an aqueous suspension comprising a chalcogen; mixing the aqueous suspension with a reducing agent to render chalcogenide ions; and mixing the chalcogenide ions with a metal precursor in the presence of a control agent to have the metal chalcogenide precipitated, wherein the control agent comprises ascorbic acid.
2. The method of claim 1, wherein providing the aqueous suspension comprises mixing the chalcogen with water.
3. The method of claim 1 or 2, wherein the chalcogen comprises sulfur, selenium, or tellurium.
4. The method of any one of claims 1 to 3, wherein mixing the aqueous suspension with the reducing agent comprises dissolving the reducing agent in water prior to mixing with the aqueous suspension.
5. The method of any one of claims 1 to 4, wherein the reducing agent comprises sodium borohydride.
6. The method of any one of claims 1 to 5, wherein mixing the chalcogenide ions with the metal precursor comprises dissolving the metal precursor in water prior to mixing with the chalcogenide ions.
7. The method of any one of claims 1 to 6, wherein the metal precursor comprises silver nitrate.
8. The method of any one of claims 1 to 7, further comprising: washing the metal chalcogenide precipitated with water or an alcohol; and drying the metal chalcogenide after the washing.
9. The method of any one of claims 1 to 8, wherein the method is carried out in an inert environment.
10. The method of any one of claims 1 to 9, wherein the method is absent of an organic solvent and a surfactant.
11. The method of any one of claims 1 to 10, wherein mixing the chalcogenide ions with a metal precursor further comprises mixing the metal precursor with a dopant precursor prior to mixing the chalcogenide ions with the metal precursor.
12. The method of claim 11, wherein the dopant precursor comprises copper sulfate or tin nitrate.
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