Hybrid Interfaces and Nanomaterials for Clean Energy Applications
As the global climate change becomes more and more evident with each passing decade, cutting greenhouse gas emissions from energy production, storage, and utilization must be our top priority in order to minimize future climate changes. From the viewpoint of materials science, the research in energy is closely related to the interfacial transfers of electron/hole, ions and/or atoms, to facilitating the wanted transfers and to suppressing the unwanted transfers. Therefore, the study of physical and chemical properties at interfaces becomes exceptionally desirable in order to explore and fundamentally understand the electrical, optical and chemical phenomena occurring at various hybrid interfaces at nanometer scale. Research in our group is focused on the syntheses, characterizations, modifications, and applications of a variety of hybrid interfaces for clean energy applications including gas sensors, hydrogen storage, solar cells, catalysis, Li‐ion battery and molecular electronics.
For gas sensor research, as an example illustrated in Figure 1, we used gas‐induced changes in the electrical transport properties of a percolating nanocluster‐like palladium thin film on a self‐assembled organic monolayer to rapidly detect hydrogen and other gaseous species. Appl. Phys. Lett. 2005, 86, 203104.
Figure 1. Schematic drawing of a nanopalladium thin film/organic monolayer assembly used in the rapid detection of hydrogen. Conductivity is low in the absence of hydrogen, but higher when hydrogen is present.
For hydrogen storage materials, one of our research interests is to fundamentally understand the kinetics and thermodynamics associated with the diffusion of hydrogen adatoms across the nanoscale interfaces between nanocatalysts and storage materials. This phenomenon is often termed as hydrogen spillover and is believed to be an effective approach to suppress the activation energy during hydriding and dehydriding processes. We have used an in situ electrical method to rapidly probe hydrogen spillover from nanocatalyst to amorphous carbon. We also established a piezoelectric nanogravimetric system for measuring hydrogen mass uptake in thin film materials. Different from any existing bulk gravimetric or volumetric system, our thin film nanogravimetric system allows us to conveniently establish a well‐controlled interface between catalyst and storing material. Meanwhile, we are also working on improving the hydriding kinetics of bulk storage materials including metal hydrides and adsorbents with large surface area through hydrogen spillover. For example, we demonstrated the enhanced hydriding kinetics in palladium‐doped lithium nitride as illustrated in Figure 2. J. Phys. Chem. C, 2009, 113, 8513.
Figure 2. Schematic elucidation of hydrogen spillover‐enhanced hydriding in complex metal hydride‐based hydrogen storage materials.
For solar cell research, one of our research interests is in the field of dye‐sensitized solar cells (DSSCs). We explore new nanoarchitectured electrode designs that can bring new basic sciences to enhance the interfacial electron transport in DSSCs. For example, we recently demonstrated that using the fill factor and open circuit voltage of ZnO‐based DSSCs can be significantly improved by using a Zn‐microtip|ZnO‐nanotip core–shell hierarchy nano‐architecture (shown in Figure 3) as the anode in a DSSC. Unlike a planar electron‐collecting anode, the rough Zn microtip anode provides more surface area to accommodate more semiconductors without significantly increasing the electron diffusion length in the semiconductor layer. J. Phys. Chem. C, 2009, 113, 20521.
Figure 3. Zn‐microtip|ZnO‐nanotip core–shell hierarchy nano‐architecture as anode in dye‐sensitized solar cells.
For nanocatalysis, we are currently focusing on the novel synthesis of precious metal‐based nanocatalysts. Precious metal catalysts such as Pt and Pd are essential for achieving energy‐efficient and greener chemical processes that involve breaking or establishing of H‐H, C‐H, or O‐H bonds. Because of their low natural abundance and high cost, precious metal‐based catalysts in most applications use finely divided metal particles of a few nanometers to maximize the surface‐to‐bulk atomic ratio for atom‐economy. In general, nearly all synthetic methods to prepare precious metal nanoparticles follow a bottom‐up protocol, in which the precursory molecules (precatalysts) of precious metals are first reduced into metal atoms, which then grow into polyatomic particles. However, the use of precursory molecules inevitably lead to various complications, including the energy‐costly and tedious syntheses of precursory molecules and pretreatment of support materials, the possible contamination resulting from the hetero atoms in the precursory molecules, which will likely cause the loss of catalytic activity; the removal of stabilizing agents in many precursory‐based solution methods in order to free the catalytic sites etc.
In contrast, the counter‐strategy in nanomaterials synthesis, namely a top‐down strategy, has been largely overlooked for preparation of precious metal‐based nanoparticles. We recently demonstrated an innovative precursor‐free synthetic strategy to prepare the supported precious metal nanoparticles, starting directly from the corresponding bulk metals or metal alloys to bypass the complications associated with the synthetic methods using precursory molecules. As illustrated in Figure 4, bulk precious metals such as Pt, Pd, Ru, Rh, Ir, Au, Ag can be ruptured into nanonucleis in liquid Li at relatively low temperatures according to their phase diagrams with Li. The metal–Li liquid alloys can be quenched into a metal–Li solid solution. Next, Li in the Li–Pt solid solution can be converted into LiOH powder via a controlled gas–solid reaction 2Li (s) + 2H2O (g) → 2LiOH (s) + H2 (g) at 100°C, which prevents the aggregation of the as‐formed Pt nanoparticles. Furthermore, the metal nanoparticles in LiOH powder can then be mixed with any water‐insoluble support materials. The LiOH in the mixture can be selectively leached off with copious amounts of water under ambient condition, allowing Pt nanoparticles to be adsorbed on the neighboring support materials. This precursor‐free process avoids the harsh pre‐treatments of the support materials that are often required in precursor‐based syntheses. As a result, many soft materials including polymer materials can be used as support, provided they are not water soluble. In addition, the trapping of nanocatalysts in the deep cracks of solid support materials can be mitigated since the entering of particles into the deep cracks would be much harder than the precursory molecules. J. Am. Chem. Soc., 2010, 132, 2151.
Figure 4. Top‐down synthesis of supported Pt nanoparticles from the Li–Pt solid solution.
Through our close and active collaborations with researchers at Argonne National Laboratory and National Renewable Energy Laboratory, students in our group will use a variety of cutting‐edge techniques, including atomic force microscopy, scanning electron microscopy, UHV thin‐film sputter systems, thermal evaporation, photo/electron‐beam lithography, electrical transport measurement systems, electrochemical measurement system, advanced photon source (APS at Argonne) to explore the fundamental science at nanoscale hybrid interfaces and to synthesize the corresponding materials for clean energy applications.
Postdoctoral Research Associate, Argonne National Laboratory, 2004–2006
Research Associate, Texas A&M University, 2003–2004
Ph.D., University of Alabama, 2003
B.S., East China University of Science and Technology, 1995
Inorganic–organic hybrid interfaces and nanomaterials for applications in sensors, energy storage and conversion, solid‐state lighting, and molecular electronics.
Three‐Dimensional Photonic Crystal Fluorinated Tin Oxide (FTO) Electrodes: Synthesis, Optic and Electrical Properties. Yang, Z.; Gao, S.; Li, W.; Vlasko–Vlasov, V.; Welp, U.; K. W. Wai; Xu, T.(2011) ACS Applied Materials & Interfaces, 3: 1101–1108.
Enhanced Electron Collection in TiO2 Nanoparticle‐Based Dye‐Sensitized Solar Cells by An Array of Metal Micropillars on A Planar Fluorinated Tin Oxide Anode. Yang, Z.; Xu, T.; Gao, S.; Welp, U.; Kwok, K. W. (2010) J. Phys. Chem. C, 114: 19151–19156.
Synthesis of Supported Platinum Nanoparticles from Li–Pt Solid Solution. Xu, T.; Lin, C.; Wang, C.; Brewe, D.; Ito, Y.; Lu, J. (2010) J. Am. Chem. Soc., 132: 2151–2153.
Enhanced Electron Transport in Dye‐Sensitized Solar Cells Using Short ZnO Nanotips on A Rough Metal Anode. Yang, Z.; Xu, T.; Yasuo, I.; Welp, U.; Kwok, W. K. (2009) J. Phys. Chem. C, 113: 20521–20526.
Hydrogen spillover enhanced hydriding kinetics of palladium‐doped lithium nitride to lithium imide. Lin, C.; Xu, T.; Yu, J.; Ge, Q.; Xiao, Z. (2009) J. Phys. Chem. C, 113: 8513–8517.
Direct mass determination of hydrogen uptake using a quartz crystal microbalance. Kulchytskyy, I.; Kocanda, M. G.; Xu, T. (2007) Appl. Phys. Lett., 91: 113507.
Self‐assembled monolayer‐enhanced hydrogen sensing with ultrathin palladium films. Xu, T.; Zach, M. P.; Xiao, Z. L.; Rosenmann, D.; Welp, U.; Kwok, W. K.; Crabtree, G. W. (2005) Appl. Phys. Lett., 86: 203104.
Periodic holes with 10 nm diameter produced by grazing Ar+ milling of the barrier layer in hexagonally ordered nanoporous alumina. Xu, T.; Zangari, G.; Metzger, R. M. (2002) Nano Lett., 2: 37–41.
Rectification by a monolayer of hexadecylquinolinium tricyanoquinodimethanide between gold electrodes. Xu, T.; Peterson, I. R.; Lakshmikantham, M. V.; Metzger, R. M. (2001) Angew. Chem. Int. Ed., 40: 1749–1752.