Reports: ND552424-ND5: In-situ Monitoring of Heterogeneous Catalytic Reactions Under Realistic Conditions

Peng Jiang, University of Florida

Research Activities and Important Findings:

1.   We have systematically investigated a number of plasmonic nanostructures enabled by the spin-coating-based templating nanomanufacturing platform. These include periodic arrays of nanodimples, nanoholes, nanodots, and nanorings. The nanodimple arrays show promising surface plasmon (SP) properties. Their nanostructures and SP properties can be easily tuned by adjusting the templating conditions (e.g., oxygen plasma etching durations). Simultaneous achievement of very high surface plasmon resonance (SPR) sensitivity (up to ~800 nm per refractive index unit which is significantly higher than most of the available SPR substrates) and surface-enhanced Raman scattering (SERS) enhancement factors (> 108) has been demonstrated for gold nanodimples with both monolayer and double-layer structures. Numerical simulations based on a finite-difference time-domain (FDTD) model have been employed to complement the experimental measurements. Good agreement between experiments and theoretical predictions has been achieved.

2.  A simple colloidal transfer technology that enables scalable fabrication of monolayer non-close-packed silica colloidal crystals on a large variety of substrates has been developed.  The lattice spacing of the transferred monolayer colloidal crystal can be easily adjusted by a simple thermal treatment.  The resulting non-close-packed colloidal monolayers with tunable lattice spacing are of great importance in fabricating plasmonic nanostructures for sensitive chemical detection. We have also demonstrated the templating fabrication of periodic arrays of gold nanodots by using a transferred monolayer non-close-packed colloidal crystal as structural template. The resultant plasmonic array exhibits high SERS enhancement factor (~3.8 ´ 107) for adsorbed benzenethiol molecules.

3.  We have systematically investigated chemical sensing using digital versatile discs (DVDs) which show both high SPR sensitivity (> 850 nm/RIU) and large figure of merits (> 100). To improve the substrate stability (e.g., for high-temperature chemical sensing), we have successfully developed a templating technology for fabricating gold-coated titania SPR gratings which are stable at temperatures higher than 500°C. We have also conducted theoretical investigations on a number of structural parameters of DVD-templated gratings. We found that the slope of the periodic gratings plays a critical role in determining the final SPR sensitivity.

4.  We have also conducted an in-situ SPR study on electrochemical heterogeneous catalysis in order to evaluate the possible use of the above DVD-templated gratings in catalytic mechanism studies with detection of reaction intermediates. Anatase titania or other metal oxide gratings can be easily replicated by using sol-gel processes. A 50 nm thick gold is finally sputtered on the templated grating. The values of the SPR sensitivity and the figure of merit is determined to be 737 nm/RIU and 73.1, respectively. For further electrochemical methanol oxidation study, platinum nanoparticles are electrochemically deposited on gold surface. Constant step voltage is applied from 0.00 to 1.00 V with 0.05 V of incremental voltage in nitrogen purged methanol containing sulfuric acid electrolyte solution (0.05 M H2SO4 + 2.00 M CH3OH) with chronoamperometry (CA) while optical probe is equipped to investigate the resonance peak position along the voltage change. Significant elevated peak shift (about 8 nm) is observed in the low voltage region (from 0.00 to 0.55 V).

Educational Activities:

1.  Three UF graduate students and two undergraduates have worked on this multidisciplinary project.

2.   A 11th-grade student has been recruited through the UF Student Science Training Program (SSTP). He worked on the project in summer 2013.

3.  These students have co-authored in eight peer-reviewed papers (4 have been published, 2 have been accepted, and 2 are under review).

4.   Three oral presentations related to the project will be given by the students at the AIChE annual meetings in San Francisco, CA in November 2013.

Contributions within Discipline:

1.   The templated plasmonic nanostructures with very high SPR sensitivities and SERS enhancement factors are of great interest in developing ultrasensitive chemical and biological sensors for real-time, label-free, specific, and quantitative detection of small molecules, biomarkers, viruses, and bacteria, as well as reaction intermediates in heterogeneous catalytic reactions. The fundamental experimental measurements and numerical simulations help us to better understand the structure-property relationships of the templated subwavelength plasmonic nanostructures.

2.   The FDTD optical simulations developed from this project facilitate the fundamental understanding of optical diffraction, reflection, scattering, and transmission from various subwavelength-structured gratings.

Contributions to Other Disciplines:

1.   The templated plasmonic nanostructures (e.g., periodic nanohole arrays), which show enhanced optical transmission, can be used as transparent conducting electrodes for flexible displays.

2.   The templated titania plasmonic gratings with very high SPR sensitivity and large SPR figure of merits can be used as in-situ optical probes for exploring heterogeneous catalytic reactions under realistic conditions (e.g., high temperatures and high pressures).

Impact on Student Education and Training:

The students involved in the project have gained experience in conducting innovative research in chemical synthesis (e.g., preparation of colloidal nanoparticles), standard microfabrication (e.g., clean-room operation, metal deposition, dry etching), material self-assembly, optical characterization and modeling, image analysis, and chemical/biological sensing. They have also learned how to write technical papers and how to effectively communicate their results to colleagues at international meetings. The graduate students gained valuable mentoring experience in coaching the undergraduate and high school students.