A Creative “Spark” for High-Entropy Alloys

Abstract

In this issue of Matter, Feng et al. make a breakthrough understanding of an aerosol system for manufacturing unprecedented alloys with excellent catalytic properties. This work will inspire improved 3D nanoprinting techniques for creating new matter forms for broad applications, including additive manufacturing, catalysts, and metallurgy. In this issue of Matter, Feng et al. make a breakthrough understanding of an aerosol system for manufacturing unprecedented alloys with excellent catalytic properties. This work will inspire improved 3D nanoprinting techniques for creating new matter forms for broad applications, including additive manufacturing, catalysts, and metallurgy. Main TextManufacturing of so-called “unconventional” alloys consisting of multiple elements is a complicated process because of the inevitable interplay between thermodynamics and kinetics and a strong dependence on manufacturing conditions. The advent of high-entropy alloys (HEAs),1Yeh J.-W. Chen S.-K. Lin S.-J. Gan J.-Y. Chin T.-S. Shun T.-T. Tsau C.-H. Chang S.-Y. Nanostructured high-entropy alloys with multiple principal elements: Novel alloy design concepts and outcomes.Adv. Eng. Mater. 2004; 6: 299-303Crossref Scopus (6841) Google Scholar which consist of multiple principal elements to enlarge the entropy contribution for issuing stability, represents a paradigmatic shift to the center of materials science. Most HEAs are produced using liquid-phase methods where the increase in the entropy of mixing with additional alloying elements stabilizes the solution and prevents phase separation; however, this strongly depends on the enthalpy of mixing as well as other factors that determine miscibility.The exceptional properties of HEAs were thought to relate to the resultant single-phase solid-solution upon entropy maximization. However, in reality, such entropy, as the only driving force for the mixing, often turns out to be insufficient for alloying: a mixed state from many ingredients is difficult to be kept in the final alloys. The involvement of immiscible elements needs to overcome the encountered positive mixing enthalpy, and the alloy development is hindered by traditional liquid-solid transformations. One solution—avoid the liquid phase altogether. Vapor crystal transformation provides a very kinetic-effective way to form alloys.2Feng J. Chen D. Pikhitsa P.V. Jung Y. Yang J. Choi M. Unconventional Alloys Confined in Nanoparticles: Building Blocks for New Matter.Matter. 2020; 3 (this issue): 1646-1663Abstract Full Text Full Text PDF Scopus (30) Google Scholar Herein, Feng et al. report a novel vapor source technology called “spark mashup,” which is harnessed to create 55 distinct types of alloys with controllable compositions. These alloys are confined in ultra-small nanoparticles (NPs) (<5 nm) and range from binary elements to HEAs2Feng J. Chen D. Pikhitsa P.V. Jung Y. Yang J. Choi M. Unconventional Alloys Confined in Nanoparticles: Building Blocks for New Matter.Matter. 2020; 3 (this issue): 1646-1663Abstract Full Text Full Text PDF Scopus (30) Google Scholar with multiple components. It is worth noting that the research successfully broke the miscibility limit: creating alloys containing immiscible materials and discovering unconventional alloys that have never been reported.Alloying of multiple elements into NPs is a productive approach for research and engineering.3Yao Y. Huang Z. Xie P. Lacey S.D. Jacob R.J. Xie H. Chen F. Nie A. Pu T. Rehwoldt M. et al.Carbothermal shock synthesis of high-entropy-alloy nanoparticles.Science. 2018; 359: 1489-1494Crossref PubMed Scopus (606) Google Scholar Wet-chemistry synthesis allows for remarkable control in terms of NP size and morphology, but extending this approach to multicomponent alloy (MA) NPs involves tight constraints and undue complications.3Yao Y. Huang Z. Xie P. Lacey S.D. Jacob R.J. Xie H. Chen F. Nie A. Pu T. Rehwoldt M. et al.Carbothermal shock synthesis of high-entropy-alloy nanoparticles.Science. 2018; 359: 1489-1494Crossref PubMed Scopus (606) Google Scholar In addition, the NP quality seems to be inseparable from the substrate properties.3Yao Y. Huang Z. Xie P. Lacey S.D. Jacob R.J. Xie H. Chen F. Nie A. Pu T. Rehwoldt M. et al.Carbothermal shock synthesis of high-entropy-alloy nanoparticles.Science. 2018; 359: 1489-1494Crossref PubMed Scopus (606) Google Scholar, 4Wong A. Liu Q. Griffin S. Nicholls A. Regalbuto J.R. Synthesis of ultrasmall, homogeneously alloyed, bimetallic nanoparticles on silica supports.Science. 2017; 358: 1427-1430Crossref PubMed Scopus (191) Google Scholar, 5Ding K. Cullen D.A. Zhang L. Cao Z. Roy A.D. Ivanov I.N. Cao D. A general synthesis approach for supported bimetallic nanoparticles via surface inorganometallic chemistry.Science. 2018; 362: 560-564Crossref PubMed Scopus (122) Google Scholar Complete alloying is typically prevented by wide gaps between reduction potentials, although this difference was revealed to be a viable method to hollow out NPs.6Oh M.H. Yu T. Yu S.-H. Lim B. Ko K.-T. Willinger M.-G. Seo D.-H. Kim B.H. Cho M.G. Park J.-H. et al.Galvanic replacement reactions in metal oxide nanocrystals.Science. 2013; 340: 964-968Crossref PubMed Scopus (420) Google Scholar Electrostatic adsorption has been exploited to synthesize bimetallic alloy NPs, as evidenced by the speckling effect.4Wong A. Liu Q. Griffin S. Nicholls A. Regalbuto J.R. Synthesis of ultrasmall, homogeneously alloyed, bimetallic nanoparticles on silica supports.Science. 2017; 358: 1427-1430Crossref PubMed Scopus (191) Google Scholar An analogous method reduces sequentially adsorbed heterometallic precursors to synthesize bimetallic NPs, but pairs of components that are immiscible in the bulk exhibit sub-nanometer intraparticle phase segregation.5Ding K. Cullen D.A. Zhang L. Cao Z. Roy A.D. Ivanov I.N. Cao D. A general synthesis approach for supported bimetallic nanoparticles via surface inorganometallic chemistry.Science. 2018; 362: 560-564Crossref PubMed Scopus (122) Google Scholar The carbothermal shock method succeeded in fabricating MA-NPs (>30 nm), strictly requiring defective carbon nanofibers as supports.3Yao Y. Huang Z. Xie P. Lacey S.D. Jacob R.J. Xie H. Chen F. Nie A. Pu T. Rehwoldt M. et al.Carbothermal shock synthesis of high-entropy-alloy nanoparticles.Science. 2018; 359: 1489-1494Crossref PubMed Scopus (606) Google Scholar Interestingly, in one study, small HEA-NPs (ca. 3 nm) exhibited high stability despite deep annealing,7Liu M. Zhang Z. Okejiri F. Yang S. Zhou S. Dai S. Entropy-maximized synthesis of multimetallic nanoparticle catalysts via a ultrasonication-assisted wet chemistry method under ambient conditions.Adv. Mater. Interfaces. 2019; 6: 1900015Crossref Scopus (60) Google Scholar whereas other works showed that the appropriate thermal treatment of slightly larger NPs (>20 nm) afforded precisely defined interfaces.8Chen P.C. Liu X. Hedrick J.L. Xie Z. Wang S. Lin Q.Y. Hersam M.C. Dravid V.P. Mirkin C.A. Polyelemental nanoparticle libraries.Science. 2016; 352: 1565-1569Crossref PubMed Scopus (256) Google Scholar,9Chen P.C. Liu M. Du J.S. Meckes B. Wang S. Lin H. Dravid V.P. Wolverton C. Mirkin C.A. Interface and heterostructure design in polyelemental nanoparticles.Science. 2019; 363: 959-964Crossref PubMed Scopus (123) Google Scholar Targeted applications require alloy-NPs to have precisely tailored compositions and sizes, but a fundamental understanding to achieve such requirements remains lacking.10Kwon S.G. Krylova G. Phillips P.J. Klie R.F. Chattopadhyay S. Shibata T. Bunel E.E. Liu Y. Prakapenka V.B. Lee B. Shevchenko E.V. Heterogeneous nucleation and shape transformation of multicomponent metallic nanostructures.Nat. Mater. 2015; 14: 215-223Crossref PubMed Scopus (153) Google ScholarThe authors here quench a well-mixed vapor directly to crystal solids with nanosize-stabilized mixing. This discovery contradicts the conventional HEA trends of “smaller is less stable” and “mixing multiple elements in a nanoparticle is more difficult.” The nanosize effect, arising fundamentally from surface confinement, produces an enormous enhancement factor to mixing entropy in HEAs and gives even binary alloys a mixing entropy comparable with that of a bulk HEA containing > 25 elements. Intriguingly, such size effect on entropy reveals a close similarity to the relation between the Hawking temperature and the size of a black hole (known as a perfect mixer). This work, therefore, represents a breakthrough in concept and fundamental understanding for manufacturing unconventional alloys. The key findings of the paper can be summed up as follows:(1)A brief oscillation spark alternatively vaporizes the two electrodes and controls the feed ratio. This oscillating spark also acts like a piston to enforce the vapor mixing. Then, co-nucleation takes place to form mixed clusters, which are kinetically trapped and grow into random alloy NPs according to the feed composition.(2)They also developed a thermodynamic approach to distinguish the nanoscale stability of the alloy from the general miscibility tendency. The nanosize-stabilized mixing considerably enhances the role of mixing entropy in HEAs.(3)In terms of great applicability in many facets, the alloy NPs were proven to be high-performance fuel cell catalysts and were treated as building blocks to a new additive manufacturing technique for printing new three-dimensional (3D) nanostructured arrays.In particular, the work breaks miscibility limits to successfully mix bulk-immiscible systems in alloy NPs. The ability to alloy any dissimilar metals in NPs is the most important principle in spark mashup. Such unlimited mixing is possible because the formed metallic bonds are of electronic nature and underlie the quantum physical effects, which can be interpreted via the electron density and work function (Figure 1). Their findings radically depart from conventional theories of alloy development, based on liquid-solid transformation. Considering that NPs can be used as integral building blocks to introduce new materials, the alloy NPs serve as high-performance fuel-cell catalysts, and they also demonstrate their “smart 3D arrangement” in gases via “lines of forces” to form various HEA nanostructure arrays, in which electrical field lines are harnessed to be as 3D drawing tools. Until now, existing nanoscale additive manufacturing techniques have not been able to scale down HEA structures to the sub-microns and materialize them over such a wide range of compositions. Thus, at the intersection of the outwardly disparate fields of NP science and additive manufacturing lies the promise of revolutionary new nanomanufacturing approaches. These novel results, therefore, provide a strong foundation for developing new alloys and their structures. 3D HEA nanostructures can trigger new electronic, optical, structural, and mechanical properties for fabricating devices in a previously inaccessible environment.Compared with traditional alloy development, the work provides a more advanced way to create unconventional alloys of distinct families. In addition, it also provides a foundation and a platform technology for the discovery of new materials, new structures, new theories, and new applications, as well as the development of new frontiers in the fields of HEAs, catalysts, and additive manufacturing. A truly creative “spark.” Main TextManufacturing of so-called “unconventional” alloys consisting of multiple elements is a complicated process because of the inevitable interplay between thermodynamics and kinetics and a strong dependence on manufacturing conditions. The advent of high-entropy alloys (HEAs),1Yeh J.-W. Chen S.-K. Lin S.-J. Gan J.-Y. Chin T.-S. Shun T.-T. Tsau C.-H. Chang S.-Y. Nanostructured high-entropy alloys with multiple principal elements: Novel alloy design concepts and outcomes.Adv. Eng. Mater. 2004; 6: 299-303Crossref Scopus (6841) Google Scholar which consist of multiple principal elements to enlarge the entropy contribution for issuing stability, represents a paradigmatic shift to the center of materials science. Most HEAs are produced using liquid-phase methods where the increase in the entropy of mixing with additional alloying elements stabilizes the solution and prevents phase separation; however, this strongly depends on the enthalpy of mixing as well as other factors that determine miscibility.The exceptional properties of HEAs were thought to relate to the resultant single-phase solid-solution upon entropy maximization. However, in reality, such entropy, as the only driving force for the mixing, often turns out to be insufficient for alloying: a mixed state from many ingredients is difficult to be kept in the final alloys. The involvement of immiscible elements needs to overcome the encountered positive mixing enthalpy, and the alloy development is hindered by traditional liquid-solid transformations. One solution—avoid the liquid phase altogether. Vapor crystal transformation provides a very kinetic-effective way to form alloys.2Feng J. Chen D. Pikhitsa P.V. Jung Y. Yang J. Choi M. Unconventional Alloys Confined in Nanoparticles: Building Blocks for New Matter.Matter. 2020; 3 (this issue): 1646-1663Abstract Full Text Full Text PDF Scopus (30) Google Scholar Herein, Feng et al. report a novel vapor source technology called “spark mashup,” which is harnessed to create 55 distinct types of alloys with controllable compositions. These alloys are confined in ultra-small nanoparticles (NPs) (<5 nm) and range from binary elements to HEAs2Feng J. Chen D. Pikhitsa P.V. Jung Y. Yang J. Choi M. Unconventional Alloys Confined in Nanoparticles: Building Blocks for New Matter.Matter. 2020; 3 (this issue): 1646-1663Abstract Full Text Full Text PDF Scopus (30) Google Scholar with multiple components. It is worth noting that the research successfully broke the miscibility limit: creating alloys containing immiscible materials and discovering unconventional alloys that have never been reported.Alloying of multiple elements into NPs is a productive approach for research and engineering.3Yao Y. Huang Z. Xie P. Lacey S.D. Jacob R.J. Xie H. Chen F. Nie A. Pu T. Rehwoldt M. et al.Carbothermal shock synthesis of high-entropy-alloy nanoparticles.Science. 2018; 359: 1489-1494Crossref PubMed Scopus (606) Google Scholar Wet-chemistry synthesis allows for remarkable control in terms of NP size and morphology, but extending this approach to multicomponent alloy (MA) NPs involves tight constraints and undue complications.3Yao Y. Huang Z. Xie P. Lacey S.D. Jacob R.J. Xie H. Chen F. Nie A. Pu T. Rehwoldt M. et al.Carbothermal shock synthesis of high-entropy-alloy nanoparticles.Science. 2018; 359: 1489-1494Crossref PubMed Scopus (606) Google Scholar In addition, the NP quality seems to be inseparable from the substrate properties.3Yao Y. Huang Z. Xie P. Lacey S.D. Jacob R.J. Xie H. Chen F. Nie A. Pu T. Rehwoldt M. et al.Carbothermal shock synthesis of high-entropy-alloy nanoparticles.Science. 2018; 359: 1489-1494Crossref PubMed Scopus (606) Google Scholar, 4Wong A. Liu Q. Griffin S. Nicholls A. Regalbuto J.R. Synthesis of ultrasmall, homogeneously alloyed, bimetallic nanoparticles on silica supports.Science. 2017; 358: 1427-1430Crossref PubMed Scopus (191) Google Scholar, 5Ding K. Cullen D.A. Zhang L. Cao Z. Roy A.D. Ivanov I.N. Cao D. A general synthesis approach for supported bimetallic nanoparticles via surface inorganometallic chemistry.Science. 2018; 362: 560-564Crossref PubMed Scopus (122) Google Scholar Complete alloying is typically prevented by wide gaps between reduction potentials, although this difference was revealed to be a viable method to hollow out NPs.6Oh M.H. Yu T. Yu S.-H. Lim B. Ko K.-T. Willinger M.-G. Seo D.-H. Kim B.H. Cho M.G. Park J.-H. et al.Galvanic replacement reactions in metal oxide nanocrystals.Science. 2013; 340: 964-968Crossref PubMed Scopus (420) Google Scholar Electrostatic adsorption has been exploited to synthesize bimetallic alloy NPs, as evidenced by the speckling effect.4Wong A. Liu Q. Griffin S. Nicholls A. Regalbuto J.R. Synthesis of ultrasmall, homogeneously alloyed, bimetallic nanoparticles on silica supports.Science. 2017; 358: 1427-1430Crossref PubMed Scopus (191) Google Scholar An analogous method reduces sequentially adsorbed heterometallic precursors to synthesize bimetallic NPs, but pairs of components that are immiscible in the bulk exhibit sub-nanometer intraparticle phase segregation.5Ding K. Cullen D.A. Zhang L. Cao Z. Roy A.D. Ivanov I.N. Cao D. A general synthesis approach for supported bimetallic nanoparticles via surface inorganometallic chemistry.Science. 2018; 362: 560-564Crossref PubMed Scopus (122) Google Scholar The carbothermal shock method succeeded in fabricating MA-NPs (>30 nm), strictly requiring defective carbon nanofibers as supports.3Yao Y. Huang Z. Xie P. Lacey S.D. Jacob R.J. Xie H. Chen F. Nie A. Pu T. Rehwoldt M. et al.Carbothermal shock synthesis of high-entropy-alloy nanoparticles.Science. 2018; 359: 1489-1494Crossref PubMed Scopus (606) Google Scholar Interestingly, in one study, small HEA-NPs (ca. 3 nm) exhibited high stability despite deep annealing,7Liu M. Zhang Z. Okejiri F. Yang S. Zhou S. Dai S. Entropy-maximized synthesis of multimetallic nanoparticle catalysts via a ultrasonication-assisted wet chemistry method under ambient conditions.Adv. Mater. Interfaces. 2019; 6: 1900015Crossref Scopus (60) Google Scholar whereas other works showed that the appropriate thermal treatment of slightly larger NPs (>20 nm) afforded precisely defined interfaces.8Chen P.C. Liu X. Hedrick J.L. Xie Z. Wang S. Lin Q.Y. Hersam M.C. Dravid V.P. Mirkin C.A. Polyelemental nanoparticle libraries.Science. 2016; 352: 1565-1569Crossref PubMed Scopus (256) Google Scholar,9Chen P.C. Liu M. Du J.S. Meckes B. Wang S. Lin H. Dravid V.P. Wolverton C. Mirkin C.A. Interface and heterostructure design in polyelemental nanoparticles.Science. 2019; 363: 959-964Crossref PubMed Scopus (123) Google Scholar Targeted applications require alloy-NPs to have precisely tailored compositions and sizes, but a fundamental understanding to achieve such requirements remains lacking.10Kwon S.G. Krylova G. Phillips P.J. Klie R.F. Chattopadhyay S. Shibata T. Bunel E.E. Liu Y. Prakapenka V.B. Lee B. Shevchenko E.V. Heterogeneous nucleation and shape transformation of multicomponent metallic nanostructures.Nat. Mater. 2015; 14: 215-223Crossref PubMed Scopus (153) Google ScholarThe authors here quench a well-mixed vapor directly to crystal solids with nanosize-stabilized mixing. This discovery contradicts the conventional HEA trends of “smaller is less stable” and “mixing multiple elements in a nanoparticle is more difficult.” The nanosize effect, arising fundamentally from surface confinement, produces an enormous enhancement factor to mixing entropy in HEAs and gives even binary alloys a mixing entropy comparable with that of a bulk HEA containing > 25 elements. Intriguingly, such size effect on entropy reveals a close similarity to the relation between the Hawking temperature and the size of a black hole (known as a perfect mixer). This work, therefore, represents a breakthrough in concept and fundamental understanding for manufacturing unconventional alloys. The key findings of the paper can be summed up as follows:(1)A brief oscillation spark alternatively vaporizes the two electrodes and controls the feed ratio. This oscillating spark also acts like a piston to enforce the vapor mixing. Then, co-nucleation takes place to form mixed clusters, which are kinetically trapped and grow into random alloy NPs according to the feed composition.(2)They also developed a thermodynamic approach to distinguish the nanoscale stability of the alloy from the general miscibility tendency. The nanosize-stabilized mixing considerably enhances the role of mixing entropy in HEAs.(3)In terms of great applicability in many facets, the alloy NPs were proven to be high-performance fuel cell catalysts and were treated as building blocks to a new additive manufacturing technique for printing new three-dimensional (3D) nanostructured arrays.In particular, the work breaks miscibility limits to successfully mix bulk-immiscible systems in alloy NPs. The ability to alloy any dissimilar metals in NPs is the most important principle in spark mashup. Such unlimited mixing is possible because the formed metallic bonds are of electronic nature and underlie the quantum physical effects, which can be interpreted via the electron density and work function (Figure 1). Their findings radically depart from conventional theories of alloy development, based on liquid-solid transformation. Considering that NPs can be used as integral building blocks to introduce new materials, the alloy NPs serve as high-performance fuel-cell catalysts, and they also demonstrate their “smart 3D arrangement” in gases via “lines of forces” to form various HEA nanostructure arrays, in which electrical field lines are harnessed to be as 3D drawing tools. Until now, existing nanoscale additive manufacturing techniques have not been able to scale down HEA structures to the sub-microns and materialize them over such a wide range of compositions. Thus, at the intersection of the outwardly disparate fields of NP science and additive manufacturing lies the promise of revolutionary new nanomanufacturing approaches. These novel results, therefore, provide a strong foundation for developing new alloys and their structures. 3D HEA nanostructures can trigger new electronic, optical, structural, and mechanical properties for fabricating devices in a previously inaccessible environment.Compared with traditional alloy development, the work provides a more advanced way to create unconventional alloys of distinct families. In addition, it also provides a foundation and a platform technology for the discovery of new materials, new structures, new theories, and new applications, as well as the development of new frontiers in the fields of HEAs, catalysts, and additive manufacturing. A truly creative “spark.” Manufacturing of so-called “unconventional” alloys consisting of multiple elements is a complicated process because of the inevitable interplay between thermodynamics and kinetics and a strong dependence on manufacturing conditions. The advent of high-entropy alloys (HEAs),1Yeh J.-W. Chen S.-K. Lin S.-J. Gan J.-Y. Chin T.-S. Shun T.-T. Tsau C.-H. Chang S.-Y. Nanostructured high-entropy alloys with multiple principal elements: Novel alloy design concepts and outcomes.Adv. Eng. Mater. 2004; 6: 299-303Crossref Scopus (6841) Google Scholar which consist of multiple principal elements to enlarge the entropy contribution for issuing stability, represents a paradigmatic shift to the center of materials science. Most HEAs are produced using liquid-phase methods where the increase in the entropy of mixing with additional alloying elements stabilizes the solution and prevents phase separation; however, this strongly depends on the enthalpy of mixing as well as other factors that determine miscibility. The exceptional properties of HEAs were thought to relate to the resultant single-phase solid-solution upon entropy maximization. However, in reality, such entropy, as the only driving force for the mixing, often turns out to be insufficient for alloying: a mixed state from many ingredients is difficult to be kept in the final alloys. The involvement of immiscible elements needs to overcome the encountered positive mixing enthalpy, and the alloy development is hindered by traditional liquid-solid transformations. One solution—avoid the liquid phase altogether. Vapor crystal transformation provides a very kinetic-effective way to form alloys.2Feng J. Chen D. Pikhitsa P.V. Jung Y. Yang J. Choi M. Unconventional Alloys Confined in Nanoparticles: Building Blocks for New Matter.Matter. 2020; 3 (this issue): 1646-1663Abstract Full Text Full Text PDF Scopus (30) Google Scholar Herein, Feng et al. report a novel vapor source technology called “spark mashup,” which is harnessed to create 55 distinct types of alloys with controllable compositions. These alloys are confined in ultra-small nanoparticles (NPs) (<5 nm) and range from binary elements to HEAs2Feng J. Chen D. Pikhitsa P.V. Jung Y. Yang J. Choi M. Unconventional Alloys Confined in Nanoparticles: Building Blocks for New Matter.Matter. 2020; 3 (this issue): 1646-1663Abstract Full Text Full Text PDF Scopus (30) Google Scholar with multiple components. It is worth noting that the research successfully broke the miscibility limit: creating alloys containing immiscible materials and discovering unconventional alloys that have never been reported. Alloying of multiple elements into NPs is a productive approach for research and engineering.3Yao Y. Huang Z. Xie P. Lacey S.D. Jacob R.J. Xie H. Chen F. Nie A. Pu T. Rehwoldt M. et al.Carbothermal shock synthesis of high-entropy-alloy nanoparticles.Science. 2018; 359: 1489-1494Crossref PubMed Scopus (606) Google Scholar Wet-chemistry synthesis allows for remarkable control in terms of NP size and morphology, but extending this approach to multicomponent alloy (MA) NPs involves tight constraints and undue complications.3Yao Y. Huang Z. Xie P. Lacey S.D. Jacob R.J. Xie H. Chen F. Nie A. Pu T. Rehwoldt M. et al.Carbothermal shock synthesis of high-entropy-alloy nanoparticles.Science. 2018; 359: 1489-1494Crossref PubMed Scopus (606) Google Scholar In addition, the NP quality seems to be inseparable from the substrate properties.3Yao Y. Huang Z. Xie P. Lacey S.D. Jacob R.J. Xie H. Chen F. Nie A. Pu T. Rehwoldt M. et al.Carbothermal shock synthesis of high-entropy-alloy nanoparticles.Science. 2018; 359: 1489-1494Crossref PubMed Scopus (606) Google Scholar, 4Wong A. Liu Q. Griffin S. Nicholls A. Regalbuto J.R. Synthesis of ultrasmall, homogeneously alloyed, bimetallic nanoparticles on silica supports.Science. 2017; 358: 1427-1430Crossref PubMed Scopus (191) Google Scholar, 5Ding K. Cullen D.A. Zhang L. Cao Z. Roy A.D. Ivanov I.N. Cao D. A general synthesis approach for supported bimetallic nanoparticles via surface inorganometallic chemistry.Science. 2018; 362: 560-564Crossref PubMed Scopus (122) Google Scholar Complete alloying is typically prevented by wide gaps between reduction potentials, although this difference was revealed to be a viable method to hollow out NPs.6Oh M.H. Yu T. Yu S.-H. Lim B. Ko K.-T. Willinger M.-G. Seo D.-H. Kim B.H. Cho M.G. Park J.-H. et al.Galvanic replacement reactions in metal oxide nanocrystals.Science. 2013; 340: 964-968Crossref PubMed Scopus (420) Google Scholar Electrostatic adsorption has been exploited to synthesize bimetallic alloy NPs, as evidenced by the speckling effect.4Wong A. Liu Q. Griffin S. Nicholls A. Regalbuto J.R. Synthesis of ultrasmall, homogeneously alloyed, bimetallic nanoparticles on silica supports.Science. 2017; 358: 1427-1430Crossref PubMed Scopus (191) Google Scholar An analogous method reduces sequentially adsorbed heterometallic precursors to synthesize bimetallic NPs, but pairs of components that are immiscible in the bulk exhibit sub-nanometer intraparticle phase segregation.5Ding K. Cullen D.A. Zhang L. Cao Z. Roy A.D. Ivanov I.N. Cao D. A general synthesis approach for supported bimetallic nanoparticles via surface inorganometallic chemistry.Science. 2018; 362: 560-564Crossref PubMed Scopus (122) Google Scholar The carbothermal shock method succeeded in fabricating MA-NPs (>30 nm), strictly requiring defective carbon nanofibers as supports.3Yao Y. Huang Z. Xie P. Lacey S.D. Jacob R.J. Xie H. Chen F. Nie A. Pu T. Rehwoldt M. et al.Carbothermal shock synthesis of high-entropy-alloy nanoparticles.Science. 2018; 359: 1489-1494Crossref PubMed Scopus (606) Google Scholar Interestingly, in one study, small HEA-NPs (ca. 3 nm) exhibited high stability despite deep annealing,7Liu M. Zhang Z. Okejiri F. Yang S. Zhou S. Dai S. Entropy-maximized synthesis of multimetallic nanoparticle catalysts via a ultrasonication-assisted wet chemistry method under ambient conditions.Adv. Mater. Interfaces. 2019; 6: 1900015Crossref Scopus (60) Google Scholar whereas other works showed that the appropriate thermal treatment of slightly larger NPs (>20 nm) afforded precisely defined interfaces.8Chen P.C. Liu X. Hedrick J.L. Xie Z. Wang S. Lin Q.Y. Hersam M.C. Dravid V.P. Mirkin C.A. Polyelemental nanoparticle libraries.Science. 2016; 352: 1565-1569Crossref PubMed Scopus (256) Google Scholar,9Chen P.C. Liu M. Du J.S. Meckes B. Wang S. Lin H. Dravid V.P. Wolverton C. Mirkin C.A. Interface and heterostructure design in polyelemental nanoparticles.Science. 2019; 363: 959-964Crossref PubMed Scopus (123) Google Scholar Targeted applications require alloy-NPs to have precisely tailored compositions and sizes, but a fundamental understanding to achieve such requirements remains lacking.10Kwon S.G. Krylova G. Phillips P.J. Klie R.F. Chattopadhyay S. Shibata T. Bunel E.E. Liu Y. Prakapenka V.B. Lee B. Shevchenko E.V. Heterogeneous nucleation and shape transformation of multicomponent metallic nanostructures.Nat. Mater. 2015; 14: 215-223Crossref PubMed Scopus (153) Google Scholar The authors here quench a well-mixed vapor directly to crystal solids with nanosize-stabilized mixing. This discovery contradicts the conventional HEA trends of “smaller is less stable” and “mixing multiple elements in a nanoparticle is more difficult.” The nanosize effect, arising fundamentally from surface confinement, produces an enormous enhancement factor to mixing entropy in HEAs and gives even binary alloys a mixing entropy comparable with that of a bulk HEA containing > 25 elements. Intriguingly, such size effect on entropy reveals a close similarity to the relation between the Hawking temperature and the size of a black hole (known as a perfect mixer). This work, therefore, represents a breakthrough in concept and fundamental understanding for manufacturing unconventional alloys. The key findings of the paper can be summed up as follows:(1)A brief oscillation spark alternatively vaporizes the two electrodes and controls the feed ratio. This oscillating spark also acts like a piston to enforce the vapor mixing. Then, co-nucleation takes place to form mixed clusters, which are kinetically trapped and grow into random alloy NPs according to the feed composition.(2)They also developed a thermodynamic approach to distinguish the nanoscale stability of the alloy from the general miscibility tendency. The nanosize-stabilized mixing considerably enhances the role of mixing entropy in HEAs.(3)In terms of great applicability in many facets, the alloy NPs were proven to be high-performance fuel cell catalysts and were treated as building blocks to a new additive manufacturing technique for printing new three-dimensional (3D) nanostructured arrays. In particular, the work breaks miscibility limits to successfully mix bulk-immiscible systems in alloy NPs. The ability to alloy any dissimilar metals in NPs is the most important principle in spark mashup. Such unlimited mixing is possible because the formed metallic bonds are of electronic nature and underlie the quantum physical effects, which can be interpreted via the electron density and work function (Figure 1). Their findings radically depart from conventional theories of alloy development, based on liquid-solid transformation. Considering that NPs can be used as integral building blocks to introduce new materials, the alloy NPs serve as high-performance fuel-cell catalysts, and they also demonstrate their “smart 3D arrangement” in gases via “lines of forces” to form various HEA nanostructure arrays, in which electrical field lines are harnessed to be as 3D drawing tools. Until now, existing nanoscale additive manufacturing techniques have not been able to scale down HEA structures to the sub-microns and materialize them over such a wide range of compositions. Thus, at the intersection of the outwardly disparate fields of NP science and additive manufacturing lies the promise of revolutionary new nanomanufacturing approaches. These novel results, therefore, provide a strong foundation for developing new alloys and their structures. 3D HEA nanostructures can trigger new electronic, optical, structural, and mechanical properties for fabricating devices in a previously inaccessible environment. Compared with traditional alloy development, the work provides a more advanced way to create unconventional alloys of distinct families. In addition, it also provides a foundation and a platform technology for the discovery of new materials, new structures, new theories, and new applications, as well as the development of new frontiers in the fields of HEAs, catalysts, and additive manufacturing. A truly creative “spark.” Unconventional Alloys Confined in Nanoparticles: Building Blocks for New MatterFeng et al.MatterAugust 17, 2020In BriefBrief oscillatory sparks control the feed composition of multicomponent vapors, which are mixed and quenched to directly transform into alloys of distinct families, some of which are unprecedented. This vapor-crystal transformation enables virtually unlimited mixing possibilities for manufacturing alloys with nanosize-stabilized mixing. This fundamental understanding provides new practical capabilities in additive manufacturing and catalysis. The results presented here thus expand the landscape of high-entropy alloys, catalysts, and nanoscale additive manufacturing. Full-Text PDF Open Archive