Polynuclear Clusters: Bridging between Metal Ion and Metal Oxide
2016; Elsevier BV; Volume: 1; Issue: 6 Linguagem: Inglês
10.1016/j.chempr.2016.11.010
ISSN2451-9308
Autores Tópico(s)Inorganic Chemistry and Materials
ResumoDr. Wei Wang is an associate professor at the Fujian Institute of Research on the Structure of Matter (FJIRSM), Chinese Academy of Sciences. Wei received his BSc degree in material science and engineering from the University of Science and Technology of China in 2008. He joined Prof. Douglas Keszler's group at OSU and obtained his PhD in 2013. After completing his PhD study, Wei received a post-doctoral fellowship from the Center for Sustainable Materials Chemistry. In 2014, Wei joined FJIRSM. His current research focuses on exploring aqueous cluster chemistry for developing novel solution precursors to fabricate high-performance electronics and energy-storage devices. Dr. Wei Wang is an associate professor at the Fujian Institute of Research on the Structure of Matter (FJIRSM), Chinese Academy of Sciences. Wei received his BSc degree in material science and engineering from the University of Science and Technology of China in 2008. He joined Prof. Douglas Keszler's group at OSU and obtained his PhD in 2013. After completing his PhD study, Wei received a post-doctoral fellowship from the Center for Sustainable Materials Chemistry. In 2014, Wei joined FJIRSM. His current research focuses on exploring aqueous cluster chemistry for developing novel solution precursors to fabricate high-performance electronics and energy-storage devices. How do you make metal hydroxide from a metal-nitrate solution? This is a question that any chemist could easily answer. Take Cr(OH)3, for example. You could just add some base, e.g., NaOH or NH3·H2O, to the Cr(NO3)3 solution and bring the solution's pH up. Solid Cr(OH)3 would precipitate when the pH approaches ∼7. The reaction is simply as follows:Cr3+(aq) + 3OH−(aq) = Cr(OH)3(s)(Equation 1) Now, let's go further with some more questions. In the reaction above, how does each individual metal ion come together to form the metal-hydroxide solid? How does the reaction pathway that these ions go through affect the properties of the resulting oxide or hydroxide? These questions just turn Equation 1 into Equation 2, where the blank needs to be filled in. The answer might not seem that obvious. In my opinion, the answer to the two questions is the holy grail of the aqueous chemistry of metal ions. Polynuclear metal-oxo clusters, which are the intermediate state between metal ions and metal oxides and hydroxides, certainly hold the key to answering these questions.Cr3+(aq) + xOH−(aq) = (___) + yOH−(aq) = Cr(OH)3(s)(Equation 2) Aqueous polynuclear metal-oxo clusters consist of multiple metal ions interconnected by oxo (–O–) or hydroxo (–OH–) bridging ligands. In the past 20 years, aqueous polynuclear metal-oxo clusters have demonstrated their importance in natural processes, in material synthesis, and in everyday applications. In nature, they govern the dissolution, migration, and deposition process of minerals, and they're often appealing in the study of contaminate transportation.1Nyman M. Polyoxometalates and other metal-oxo clusters in nature.in: White W.M. Encyclopedia of Geochemistry: A Comprehensive Reference Source on the Chemistry of the Earth. Springer International Publishing, 2016: 1-5Crossref Google Scholar The well-established structure of polynuclear metal-oxo clusters provides a great platform for studying the dissolution of minerals computationally.2Ohlin C.A. Villa E.M. Rustad J.R. Casey W.H. Nat. Mater. 2010; 9: 11-19Crossref PubMed Scopus (90) Google Scholar In the lab, metal-oxo clusters containing aqueous solution are used in the deposition of high-quality oxide thin films.3Keszler, D.A., and Wang, W. (2016). Process to form aqueous precursor and aluminum oxide film. US patent US9340678 B2, filed June 14, 2011, and published May 17, 2016.Google Scholar The cluster structures can serve as a starting point in theoretical calculations during spectroscopic analysis, which reveals the condensation pathway of metal ions.4Wang W. Liu W. Chang I.Y. Wills L.A. Zakharov L.N. Boettcher S.W. Cheong P.H.-Y. Fang C. Keszler D.A. Proc. Natl. Acad. Sci. USA. 2013; 110: 18397-18401Crossref PubMed Scopus (54) Google Scholar Meanwhile, the nanometer-sized pre-nucleation cluster has been directly observed by cryogenic transmission electron microscopy during the formation of magnetite.5Baumgartner J. Dey A. Bomans P.H.H. Le Coadou C. Fratzl P. Sommerdijk N.A.J.M. Faivre D. Nat. Mater. 2013; 12: 310-314Crossref PubMed Scopus (494) Google Scholar The knowledge could largely benefit the future design of the aqueous synthesis of crystalline materials. In everyday life, coagulating agents containing polynuclear metal-oxo clusters are widely used in water-treatment processes.6Stewart T.A. Trudell D.E. Alam T.M. Ohlin C.A. Lawler C. Casey W.H. Jett S. Nyman M. Environ. Sci. Technol. 2009; 43: 5416-5422Crossref PubMed Scopus (52) Google Scholar The cluster composition and structure could significantly alter the efficiency during the removal of coagulating contaminants. On the basis of the overall charge, aqueous polynuclear metal-oxo clusters can be divided into two categories. For elements exhibiting an oxidation state lower than 3—such as Al(III), Ga(III), Cr(III), and Sc(III)—the clusters are typically positively charged and are often referred to as polyoxocations. For metal ions with an oxidation state of 4 or above—such as Ta(V), Nb(V), Mo(VI), and W(VI)—the clusters are usually negatively charged and are well known as polyoxometalates (POMs). These two types of clusters depict reaction pathways on opposite sides of the pH spectrum: the acidic pH region for polyoxocations and the more basic region for POMs. In our article in this issue of Chem,7Wang W. Fullmer L.B. Bandeira N.A.G. Goberna-Ferrón S. Zakharov L.N. Bo C. Keszler D.A. Nyman M. Chem. 2016; 1: 887-901Abstract Full Text Full Text PDF Scopus (22) Google Scholar we describe the aqueous synthesis of a chromium-containing δ-Keggin polyoxocation (Figure 1). If we examine the structure of zincochromite (a spinel chromium-containing mineral) closely, we can actually carve out an ɛ-Keggin Zn-Cr cluster from zincochromite. It is true that we still cannot answer how the δ-Keggin Zn-Cr cluster transforms into its ɛ isomer or whether it is the building unit for the spinel structure during condensation. However, isolating the δ-Keggin Zn-Cr structure definitely brings us one step closer to the answer. A question I often get is, "why don't you use organic ligands?" It is correct that organic ligands offer great flexibility during cluster synthesis and can add functionality to the resulting solid (e.g., metal-oxide frameworks). However, most of these organic ligands are not naturally abundant. Because our lab focuses on the aqueous condensation reaction pathway, the strong chelation between organic ligands and metal ions could thermodynamically alter the reaction process and deviate it from the natural process. Therefore, the unique cluster-forming processes created in our study open the door to understanding the natural processes and controlling aqueous synthesis. As far as my career goes, the challenging questions I mentioned above should keep me busy for a while. Ultimately, by gaining enough knowledge of how individual ions transform into solids, we will be able to precisely design, predict, and control the material synthesis from aqueous solution. This is a huge project requiring intensive collaboration. During my PhD program, my advisor, Prof. Keszler, created a wonderful cultural environment for us to collaborate with others and explore our research interests. This culture has led to the founding of the Center for Sustainable Materials Chemistry (CSMC), a phase II Center for Chemical Innovation founded by the National Science Foundation. I met Prof. May Nyman, Prof. Carles Bo, and lots of other people through the center's collaboration, and together we've made lots of progress. Currently, I am still collaborating with the CSMC on several research topics. As an individual, I might not be able to do much, but I believe that as a cluster (team), we will lead to lots of exciting solid outcomes. Crystallizing Elusive Chromium PolycationsWang et al.ChemDecember 08, 2016In BriefNyman and colleagues elucidate a mechanism for metal-oxo cluster growth and crystallization that avoids the predominance of fully formed clusters in solution. Rather, only monomer and dimer precursors to the 14-metal-center Keggin ion are observed spectroscopically. Cluster self-assembly and crystallization occur simultaneously at the solution surface with evaporation of HNO3-H2O azeotrope. This pathway allows the isolation of hydrolytically unstable clusters, including the Zn2+-Cr3+Al3+ polyoxocation structurally described here. Full-Text PDF Open Archive
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