Patent Application
20110220166
The following is the non-patent literature documents of some prior art that presently appears relevant:
This invention pertains generally to compositions and a method for making films, nanostructures and nanowires in templates and on substrates, including but not limited to metal-semiconductor nanostructures and semiconductor nanostructures on semiconductor substrates, and a device having the same. Particularly described are methods for making cobalt antimonide nanostructures on gold and Co—Sb substrates.
Thermoelectrics are renewable energy materials that benefit from low dimensionality. Cobalt triantimonide and its derivatives are considered to be the most suitable thermoelectric materials for applications in the range around 600 K. CoSb3 exhibits excellent electrical transport properties, one of the highest values for hole mobility in a semiconductor due to a high degree of covalent bonding.
Cobalt and antimony form three intermediate compounds (CoSb, CoSb2 and CoSb3). CoSb3 has a very narrow range of solid solubility, forms a eutectic with Sb at about 621° C. and melts incongruently at 873° C. It belongs to a group that has the skutterudite (CoAs3) structure indicated by MX3 where M represents a metal atom and X a pnictide atom. The compound crystallizes in a body-centered cubic structure with the space group Im3. The unit cells consists of eight corner-shared MX6 octahedra, which produce a large void at the center of (MX6)8 clusters occupying the body center position in the unit cell. This open site may be further filled with large, “rattling”, rare earth ions (La or Ce) to decreases thermal conductivity. CoSb3 is a semiconductor with a band gap of 0.5 V as determined experimentally and 0.57 V by calculations. Thermoelectric and electrical transport properties of CoSb3 are sensitive to dopant concentrations. Important dopants include Fe and Ni (substituted for Co) and Sn and Te (substituted for Sb). Thin films of CoSb3 have been prepared by different techniques including pulsed laser deposition and dc magnetron sputtering. Thermoelectric materials with filled skutterudite structure for thermoelectric devices. For example, U.S. Pat. No. 6,342,668 Fleurial, et al., Jan. 29, 2002, teaches a method for fabricating a thermoelectric alloy of a filled skutterudite structure from powders using a sintering method.
Although most studies on skutterudites have been focused on the effects of rattler and dopant concentrations on thermoelectric properties, only recent work concentrated on low dimension skutterudites such as thin films and nanowires. Unlike other thermoelectric materials such as Bi2Te3, CoSb3 has a cage structure that offers a unique opportunity for nanowires to combine rattling effect and quantum confinement in obtaining higher figure of merit ZT and more efficient thermoelectric devices.
Since theoretical calculations first predicted a drastic increase of ZT for thermoelectric nanowires, well beyond the stagnant bulk value of 1, significant progress has been made in synthesizing these structures by way of electrodeposition. Most of the electroplating baths were developed first for thin films and then demonstrated for nanowires using nanoporous templates. For example, Behnke et al in “Electrodeposition of CoSb3 nanowires”, Proceedings of the 18th International Conference on Thermoelectrics, 1999: p. 451-453, teaches how to prepare nanowires with Co:Sb≈1:3 using an electrochemical deposition process. However, CoSb3 nanowires were not obtained directly by electrodeposition but after an additional post-deposition annealing that created the conditions for Co and Sb to react on adjacent layers and form CoSb3. After annealing, EDS pointed to the presence of cobalt rich phases in small amounts or noncrystalline due to the antimony loss. Therefore, post-deposition annealing is a challenging method to form crystalline CoSb3 and the right conditions were not achieved.
Chen et al [“Ordered CoSb3 nanowire arrays synthesized by electrodeposition.” Chemistry Letters, 2006. 35(2): p. 170-171; and “Different preferred orientation CoSb3 nanowire arrays synthesized by electrodeposition”. Chinese Journal of Inorganic Chemistry, 2006. 22(5): p. 949-951.] teach on the synthesis of crystalline CoSb3 nanowires using electrochemical deposition. CoSb3 nanowire arrays were deposited on an alumina template from a solution of SbO+, Co2+ and tartaric acid.
To improve efficiency of thermoelectric devices, thermoelectric materials with controlled structure and composition need to be developed. I wide selection of thermoelectric materials is now available. However, the temperature ranges at which these materials operate impose some limitation on this selection. Bulk materials are somehow limited in achieving the required levels in thermoelectric properties, including, for example, thermal and electrical conductivity. In order to overcome these limitations, a combination of controlled structures and composition must be achieved. The availability of new nanostructures with novel properties will significantly expand the utilization of these materials for heat waste recovery.
Described herein are methods of making nanostructures and nanowires that overcome one or more of the drawbacks of the existing methods. Also described are particular nanostructures and nanowires made using a conductor or semi-conductor substrate made inside and over a non-conductive template. Also described are particular pillar nanostructures made in the absence of a template or over a template.
In one method, nanostructures are produced using a template. The method may also optionally include the step of heating during or after the nanostructures are grown. In one method of making nanostructures, the deposition time is increased to grow pillars. In one method of making cobalt antimonide nanostructures, the nanoscale structures are further doped. In one method of making cobalt antimonide nanostructures, the nanoscale structures are further roughened by a selective dissolution of Sb. In one method, thin films and nanowires were both grown on a gold-coated polycarbonate track-etched (PCTE) membrane. Thin films of cobalt antimonide were grown on the nanostructured Au surface. Nanowires were grown inside the pores of the PCTE membrane. The thin film deposition was performed on a nanostructured gold surface and nanowires were grown electrochemically by template synthesis. Any size of PCTE or anodized aluminum oxide (AAO) membrane can be used.
In one aspect, this invention includes a variety of methods of producing a non-stoichiometric material using electrochemical deposition. Stoichiometric CoSb3 is produced under certain deposition conditions including electrolyte composition, stirring rate, temperature and deposition (i.e. constant potential or pulsing).
In another method, electrolytes were prepared by dissolving 0.003 M Sb2O3 and 0.172 M CoSO4.7H2O in aqueous solution containing 0.125 M potassium citrate monobasic and 0.196M citric acid. Sb2O3, CoSO4.7H2O, C6H7KO7 (potassium citrate monobasic), and C6H8O7 (citric acid) were used. According to another method, other electrolyte compositions were used, including:
According to yet another method, deposition has been made at temperatures between 25 and 65° C.
According to yet another method, constant and pulsing electrodeposition was used.
The invention is described with reference to the several FIG.s of the drawing, in which,
The invention is described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. The invention may, however, be embodied in many different forms and should not be construed as limited to the exemplary embodiments set forth herein. Rather these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the drawings, the size and relative sizes of layers and regions may be exaggerated for clarity.
The terminology used herein is for the purposes of describing particular embodiments only and is not intended to be limiting of the invention. Embodiments of the invention are described herein with reference to schematic illustrations of idealized embodiments (and intermediate structures) of the invention. As such, variations from the shapes of the illustrations as a result, for example, of manufacturing techniques and/or tolerances, are to be expected. Thus, embodiments of the invention should not be construed as limited to the particular shapes of regions illustrated herein but are to include deviations in shapes that result, for example, from manufacturing. Also, well-known functions or constructions are not described in detail since they would obscure the invention in unnecessary detail.
An exemplary embodiment of a method for manufacturing Co—Sb nanostructures according to the present invention is characterized in that the shape of a final Co—Sb nanostructure. Cathodic electrodeposition was performed under potentiostatic conditions with a conventional three-electrode setup consisting of a computer-controlled bipotentiostat (Model: AFCBP1). The reference electrode was Ag/AgCl electrode (3 M NaCl). All potentials are given here relative to the Ag/AgCl (0.194 V vs Standard Hydrogen Electrode). The counter electrode was an Au wire. Electrolyte was purged with N2 for at least 15 minutes before each experiment.
Electrochemical characterization of the CoSb3 nanowires including cyclic voltammetry (CV) and deposition were performed using a bi-potentiostat (Model AFCBP1, Pine Instrument Co.). Structural characterization was performed using XL30-SFEG, a high-resolution scanning electron microscope (SEM) with energy dispersive X-ray spectroscopy (EDS) capability.
Co—Sb thin film deposition was performed on the nanostructured gold surface and CoSb3 nanowires were grown electrochemically by template synthesis. The Au coating was characterized by EDS and XRD. Results have shown that the gold film is polycrystalline and has (111) preferred orientation. FIG. 1.c shows the Au coated side of the membrane. Au layer is thin and it doesn't cover the pores. Instead, a conductive ring is formed at the base of the pore (FIG. 1.a), which provides the electrical contact for deposition.
In another aspect, this invention includes a submicron non-stoichiometric Co—Sb material that was produced from a solution containing 0.003 M Sb2O3+0.172 M CoSO4.7H2O+0.125 M potassium citrate+0.196 M citric acid. A small amount of Sb2O can be dissolved in water according to the following equation:
Sb2O3+H2O=2HSbO2 log(HSbO2)=−3.92 (1)
In acidic solution, antimony can also exist in the form of SbO+:
Sb2O3+2H+=2SbO++H2O log(SbO+)=−3.05−pH (2)
Combining Eqns. (1) & (2), equilibrium between HSbO2 and SbO+ can be established at a given pH:
SbO++H2O=HSbO2+H+ log [(HSbO2)/(SbO+)]=−0.87+pH (3)
While the concentration of HSbO2 in a saturated aqueous solution is always 1.20×10−4 M, the concentration of SbO+ varies with pH. For pH<0.87, the amount of SbO+ in the solution exceeds that of HSbO2. Since pH of the as-prepared solution is 2.29, the concentration of HSbO2 in the solution is 26 times greater than that of free SbO+. Noteworthy is that the equilibrium amounts of both HSbO2 and free SbO+ in the solution are small compared to the starting 0.003 M Sb2O3, and the rest of the initial Sb2O3 input stays in complexes between SbO+ and citrates. As the deposition proceeds, depletion of HSbO2 and SbO+ is counterbalanced by the dissociation of citrate complexes, so that new equilibrium is established.
Electrodeposition of CoSb3 involves first the reduction of the absorbed Co2+ and HSbO2 on the electrode to elemental Co and Sb:
Co2++2e−=Co(s) E0=−0.277+0.0295 log(Co2+) (4)
HSbO2+3H++3e−=Sb+2H2O, E0=0.230−0.0591 pH+0.0197 log(HSbO2) (5)
SbO++2H++3e−=Sb+H2O, E0=0.212−0.0394 pH+0.0197 log(SbO+) (6)
Then, the reduced Co and Sb atoms react with each other to form CoSb3. The overall reaction can be expressed as:
Co2++3HSbO2+9H++11e−=CoSb3+6H2O (7)
In aqueous solutions, there is a large separation between the reduction potential of Co(II) and Sb(III). Under acidic conditions, the standard reduction potentials of the redox couples: Co2+/Co0 and HSbO2/Sb0(or SbO+/Sb0) are −0.277 and 0.230V (or 0.212 V) vs. SHE, respectively, which is 0.507 V (or 0.489 V) apart. In these conditions, it is difficult to achieve controlled deposition rates for Co and Sb. However, the difference between the reduction potentials of Co2+ and HSbO2/SbO+ can be reduced by controlling the composition of the electrolyte. Theoretically the concentrations of HSbO2, SbO+ and Co2+ ions in the solution affect the potential at which Sb and Co deposit on the cathode. The concentration of HSbO2 and SbO+ are 1.20×10−4 M and 4.57×10-6 respectively as determined from Eqns. (1) & (2). The equilibrium concentration of free Co2+ is calculated from the MINEQL+ software for the as-prepared solution and equals to 5.56×10−2 M. Of the initial 0.172 M CoSO4, about 0.116 M (or 67.4%) of Co2+ exists in the form of citrate complexes. The concentration of H+ is 5.13×10−3 M. The standard potentials ESb/HSbO2, ESb/SbO+, ECo/Co2+ and EH/H+ are calculated as follow:
ECo/Co2+=−0.277+0.0295 log(Co2+)=−0.314 V vs. S.H.E. or −0.508 vs. Ag/AgCl
EH/H+=0.0592 log(H+)=−0.136 vs. S.H.E. or −0.330 vs. Ag/AgCl
ESb/HSbO2=0.230−0.0591 pH+0.0197 log(HSbO2)=0.017 V vs. S.H.E. or −0.177 V vs. Ag/AgCl
ESb/SbO+=0.212−0.0394 pH+0.0197 log(SbO+)=0.016 V vs. S.H.E. or −0.178 V vs. Ag/AgCl
The gap between ESb/HSbO2 (or ESb/SbO+) and ECo/Co2+ is shortened from 0.507 V (or 0.489 V) under standard conditions to 0.331 V (or 0.330 V) in this solution. Although the gap between the standard potentials are still relatively large and hydrogen evolution is unavoidable, the main purpose of using citrate and citric acid is to form complexes with SbO+ and to increase the solubility of Sb2O3. Without the formation of complexes, at pH=2.29 the maximum amount of Sb2O3 can be dissolved in an aqueous solution is about 6.00×10-5M (i.e., 1.20×10−4 M HSbO2). In this example, when citrate/citric acid is used and complexes are formed at the same pH, the total amount of Sb2O3 dissolved in the solution is increased to 3.0×10−3 M or 50 times greater than the normal condition. Complexes also affect codepositions and formation of compound.
Nanostructured thermoelectric materials combine structural and compositional design. An exemplary embodiment of the principle of the manufacturing method of the present invention can be explained with reference to
This invention teaches a method for engineering unusual nanostructures with multi-layer stoichiometric/nonstoichiometric compositions. In general, in the process of nanowires electrodeposition, the deposit fills the pores of a template from the bottom. For extended deposition time, the deposit grows isotropically over the template, resulting in semispherical mushroom caps that quickly form a film over the template. This type of isotropic growth has been generally observed and is expected to occur in the template synthesis process. However, the SEM micrographs in
An exemplary embodiment of the principle of the manufacturing method of the present invention can be explained with reference to
Although the exemplary embodiments of the present invention have been described, it will be understood by those skilled in the art that the present invention should not be limited to the described exemplary embodiments, but various changes and modifications can be made within the spirit and scope of the present invention as defined by the appended claims.
This application claims priority to Provisional Patent Application No. 61/340,000 filed on Mar. 12, 2010 by the present inventors, and all the benefits accruing therefrom under 35 U.S.C. 119, the contents of which in its entirety are herein incorporated by reference.
| Number | Date | Country | |
|---|---|---|---|
| 61340000 | Mar 2010 | US |
