Molecular-Level Design of Metal-Oxyfluoride/Organic Solids for Visible-Light Photocatalysis.
The elements, of which everything is comprised, have been conveniently organized into what’s called the Periodic Table – thank you, Dmitri Ivanovich Mendeleev (b. Feb. 8, 1834; d. Feb. 2, 1907). At about the middle of the Periodic Table there’s a group of elements called the transition metals, which includes a lot of stuff we’re familiar with—zinc, nickel, copper, manganese, chromium. Actually, there are a lot of different metals on the Table, the first column of them are called the alkaline metals, the second column is called the alkaline-earth metals; after that there are 10 more columns—and all of those are called the transition metals. As you stand there looking at the Periodic Table and all of those transition metals, note that on the far left of them, the first few columns are very reactive – that means they’re willing to give up their electrons to other stuff – and as your eyes move all the way over to the right few columns of transition metals—the nines and tens—they grow much less reactive. Then, after that there’s a block of around six more columns that are called the main group elements, but that’s another story. Anyway, remember: left to right—more reactive to less reactive. Here’s another term to keep in mind: “d-orbital.” The d-orbital is a valence electron orbital for nearly all of the transition metals. In chemistry, valence electrons are the outermost electrons of an atom, and they’re important in determining how the atom reacts chemically. All of which brings us to Paul Maggard’s Scialog-funded project: “What we’re exploring,” he says, “is combining transition metals that have higher-energy d-orbitals with transition metals that have lower-energy d-orbitals.” Besides the fact that a lot of transition metals are cheap and readily available, what do they have to do with getting useable energy from sunlight? According to Maggard: “In using light you can excite an electron from a lower energy d-orbital that’s in one of the transition metals on the right-hand side of the Periodic Table; it can be excited to the d-orbitals of a much higher energy in one of the transition metals on the left-hand side.” Which will do what, exactly? “It’s possible, with the right combination of metals, to tune a compound’s ability to absorb sunlight from the ultraviolet to the visible, and even into the infrared,” Maggard says. Currently, the transition metals that work for solar energy conversion do so in the form of metal oxides, which are basically tiny particles consisting of atoms of metals and oxygen linked together. Unfortunately, they absorb only ultraviolet light. Researchers have been working furiously to lower the bar – to get them to absorb photons from the less energetic visible and the infrared portion of the electromagnetic spectrum. That could provide more bang for the buck because a lot more sunlight is located within those energy ranges. One of the ways people have done this is by putting other molecules on the surface of the metal oxides to sensitize them to visible light. Michael Gratzel, for example, won the 2010 Millennium Technology Prize for the invention of a dye-sensitized thin-film solar cell of titanium dioxide that’s relative cheap and easy to make. The dye absorbs visible-light photons and excited electrons go running off into the metal oxide. But that’s not the approach that Maggard is taking. “We’re sensitizing the metal oxide from within,” he says. In other words, if he succeeds – a big if, which is why it’s high-risk research – the entire metal-oxide particle will be sensitized to some form of visible light, not just ultraviolet. The process involves taking a lower-reactive and a higher-reactive transition metal, and meshing them together with something called a “ligand”—a ligand is a molecule that binds to a central metal atom to form a larger compound. All of this – the two different metals plus the ligand—forms what Maggard calls a “hybrid material,” meaning that the ligand bridges the different metals in order to aid the flow of electrons from the more lowly-reactive metal atoms to the more highly-reactive metal atoms, and resulting in more “bang” per photon. He’ll be attempting to make this material through what chemists call a “hydrothermal synthetic route” – meaning he’s using fairly mild temperatures to cook this stuff to make it stick together and form the crystals he’ll be producing, and that’s important because it will allow him to investigate previously- difficult-to-test relationships between the crystals’ atomic structure, their electron structure and their optical properties. It’s exacting work. “You have to look at the material at the atomic level,” he says. “And as you look at it, you have to know what the environment is around each particular atom and how the reaction you put it through changed their orbital energies. Further, how did the two metals combine with the ligand to form the new material, and what effect did this have on the overall electronic structure? You really do have to get down to the fine atomic and electronic structures to make some accurate analyses of why the experiment worked the way it did.” Fortunately, there are tools today for working at such an unimaginably small scale. Maggard and his associates use both powder X-ray diffraction and single-crystal X-ray diffraction. These processes involve passing X-rays through the sample materials he creates. The wavelength of X-rays is on the order of the spacing between the atoms, so that when the X-ray photons go through the material they diffract, and from the diffraction pattern he can back-calculate the atomic structure of the material. “Solar photochemistry is cross-cutting over many fields of expertise,” Maggard says, adding that researchers working in the field must have a commensurate range of technical abilities. Increasingly, collaborative work and communication with other scientists in the field is a necessity. “The annual Scialog meetings will provide a great opportunity to strengthen scientific connections to other researchers,” he says. “For example, I’d like to locate collaborators for further in-depth characterization, an understanding of the elementary processes of photon-driven electron transfer, or potential solar cell designs, using the types of semiconductors we’re able to prepare.” Maggard says that he’s also eager to look at other research approaches to solar-photoconversion, and he’s interested in trying to generate some synergy with other researchers at the conference by combining approaches. “For example, one could do collaborative research at combining an efficient light-harvesting array together with the molecular catalysts for CO2 or H2O reduction.” What really grabs him about chemistry in general, and photoconversion chemistry in particular, is the potential impact someone could make with basic scientific discoveries. “Understanding these subtle changes at the atomic scale can yield results that make a dramatic impact on the quality of life, and entirely new technologies might emerge from it,” Maggard says.