New Materials under Extreme Environments
Synthesizing novel materials in extreme environments may seem like an exotic approach, but nature again provides us with an outstanding example: diamond, whose stability field is at high pressure and temperature, but whose metastable field extends far beyond equilibrium back down to ambient conditions. Knowledge about this high pressure phase of carbon led to the development of techniques to mimic the high pressure and temperature conditions under which it is created and the billion dollar diamond abrasives industry. Ingeniously, knowledge about the carbon bonding of diamond has led to an alternative synthesis pathway which uses sp3 bonded carbon in a methane plasma as the source for chemical vapor deposition growth of diamond in a near vacuum environment; high pressure is no longer required. Extreme environments provide a much broader arena in which to search for materials with desirable properties. This is an emerging field which holds great promise for the discovery of unique materials. My group has been focusing on hydrogen-rich systems under high pressures for two reasons: they are relevant to outer solar system bodies like the gas giants and icy satellites, and they may have potential for materials applications. Moderate pressure and/or variable temperature can stabilize structures to hold additional weakly bound molecular hydrogen that can then be easily released for convenient hydrogen storage. We can also use high-energy x-rays to create highly excited states – thus gaining access to a vast energy landscape beyond equilibrium – and find novel x-ray inducted photochemistry in simple molecular systems.
In a mixture of hydrogen and water, we discovered a new H2(H2O)2 solid with sII clathrate structure which can be synthesized at moderate pressures and temperatures (<0.2 GPa and 240 K) and is quencheable to ambient pressure at 110 K (Mao et al., Science 2002; Lokshin et al., PRL 2004, U.S. Patent 6,735,960). This finding provides a new mechanism for hydrogen retention in the outer solar system. The H2(H2O)2 sII clathrate contains a reasonable amount of hydrogen and is environmentally clean since the only byproduct after fuel consumption is H2O. In another study of hydrogen and methane, the most abundant organic compound on Earth, we found a vastly expanded pressure stability at low-temperature in the novel van der Waals compound (H2)4CH4 (Mao et al, CPL 2005) which could be a major constituent in icy bodies. The (H2)4CH4 compound contains the highest amount of hydrogen among all known compounds (Mao et al, PNAS 2004a). We wrote review articles (Mao et al, Physics Today, 2007; Struzhkin et al, Chem. Rev, 2007) on these novel hydrogen storage materials, which have proven to be just the tip of an enormously rich iceberg.
My hydrogen storage program at Stanford has made significant contributions to the search for novel compounds with favorable bonding conditions for energy applications. Over the past three years, we have discovered a number of new phases with promising hydrogen storage potential. Ammonia borane, NH3BH3, has attracted considerable attention due to its remarkably high H2 content (19.6 wt% H2) which exceeds the 2015 US Department of Energy target (9 wt% H2) for on-board hydrogen storage systems. We investigated changes in NH3BH3 bonding with pressure and found two new phases (Lin et al., JCP 2008). We also investigated whether ammonia borane could store additional molecular hydrogen at high pressure and found a new phase in the NH3BH3 + H2 system above 6 GPa (Lin et al., PNAS 2009). This new NH3BH3-H2 compound can hold an estimated 8–12 wt % molecular H2 in addition to the chemically bonded H in NH3BH3. Our results were reported in the New York Times http://www.nytimes.com/2009/11/06/science/06sfbriefs.html and in a Stanford press releasehttp://news.stanford.edu/news/2009/may13/ammon-051309.html.
We also investigated changes in the bonding of decaborane (B10H14) in a hydrogen-rich environment at high pressure using Raman spectroscopy, finding a transition at 3 GPa and a significant amount of molecular H2 dissolved in the decaborane (Wang et al. JCP 2009). My group investigated the pressure-composition phase diagram for the H2-SiH4 system using Raman spectroscopy to 7 GPa, and found eutectic behavior with hydrogen-rich and silane rich solids at high pressure (Wang et al., PNAS 2009). These results were reported in SLAC Today http://today.slac.stanford.edu/feature/2009/silane.asp. We then collaborated with theorists to examine potential charge transfer from the silane to the molecular hydrogen in a higher pressure crystalline SiH4(H2)2 compound (Chen et al, PRB 2010). Moving forward, we are continuing to study promising hydrogen-rich systems and have begun expanding to other energy-related materials. We are collaborating with Yi Cui in Materials Science and Engineering to analyze the effect of pressure on nanostructured Li battery materials, specifically LiMn2O4 nanorods.
New x-ray induced photochemistry
The combination of pressure and high-energy x-rays helps to create photon-induced metastable matter. This novel approach has only very recently been investigated on a handful of simple molecular compounds, namely N2, O2, H2O, and NH3. In these simple molecules, high energy x-rays were found to induce dramatic and often unexpected effects. In high-pressure H2O and NH3, we found the x-rays induced dissociation of the molecular species and the formation of a new H2-O2 compound (Mao et al, Science 2006), and N2-H2 respectively. X-ray photons excite the system into highly metastable states, and pressure prevents the system from reaching equilibrium. Although the pathways are drastically different, the final product of high-pressure, x-ray generated H2-O2 is the same as splitting H2O using the alternative method of ultraviolet photon in electrochemical photolysis, which has become a promising direction in hydrogen generation.
This observation has generated many unanswered questions. Why does the splitting of H2O occur selectively in ice VII, but has not been observed in ice VI and compressed liquid H2O? What are the intermediate reaction steps and paths? In-situ probing of high-pressure chemical kinetics under high-energy x-ray irradiation is needed to answer these questions. To this end, I have begun collaborating with Aaron Lindenberg in Materials Science and Engineering and David Reis in Applied Physics who are experts in probing ultrafast dynamics.