AVS2010 Session SS-WeA: Chemisorption and Surface Reactions

Previous results from Gabor Somorjai’s group demonstrated the production of chemicurrent during catalytic reactions on Pt and Pd surfaces using a Schottky diode structure described as a “catalytic nanodiode” [1-2]. During the exothermic oxidation of CO, some fraction of the chemical energy may be dissipated by formation of hot electrons in the catalytic metal via electronic excitation. If the catalytic metal film is thin enough (nanometer scale) some of these hot electrons may be collected on the semiconductor side of the Schottky barrier in the form of a “chemicurrent”. We have fabricated several versions of catalytic nanodiodes, and during CO oxidation we also detect a current that is unambiguously a result of the chemical reaction. We typically measure current densities up to 100 nA/mm² and reaction conversion efficiencies in the range of 10^-5-10^-3 electrons per CO₂ produced; results which are quantitatively similar to reports in more recent publications [3-4].

However, details of the electronic nature this chemical signal indicates that it is derived from a voltage source; not from a current source. In fact, the chemical signal is primarily, if not entirely, due to the thermoelectric voltage generated by changes in the lateral temperature gradient between the two electrical contacts in the nanodiode. We have used a 3D heat transfer model to simulate the time dependent temperature profile in the nanodiode during CO oxidation on Pt/GaN devices. This information, along with independent experimental measurements of the Seebeck coefficient allows us to quantitatively simulate the thermoelectric voltage signal generated during a typical experiment involving time-dependent CO oxidation. Our results indicate that the nanodiode is simply operating as a thermal detector via a thermoelectric voltage. Unfortunately we have not yet found evidence supporting true “chemicurrent” formation during CO oxidation on Pt/GaN nanodiodes.

References:

[1] Z.J. Xiao and G.A. Somorjai, J. Phys. Chem. B 109 (2005) 22530.

[2] J. Xiaozhong, A. Zuppero, J.M. Gidwani, and G.A. Somorjai, J. Amer. Chem. Soc. 127 (2005) 5792.

[3] J.Y. Park, J. R. Renzas, B.B. Hsu, and G.A. Somorjai, J. Phys. Chem. C, 111 (2007) 15331

[4] J.Y. Park, J. R. Renzas, A.M. Contreras, and G.A. Somorjai, Topics in Catalysis, 46 (2007) 217

The grafting of organic molecules on semiconductor surfaces initiated by UV light has become an efficient means to tailor the chemical and physical properties of surfaces of materials, enabling their integration with various applications of the devices. The mechanism of photochemical grafting of alkenes to group IV semiconductors (diamond, silicon, germanium, etc) has remained poorly understood. We have demonstrated that a previously unrecognized process—photoelectron emission from semiconductors to reactant liquid—is a nearly universal mechanism for initiating grafting of alkenes to surfaces and is broadly applicable to a wide range of semiconductors.

The charge transfer processes that occur during the photochemical grafting to diamond surfaces were investigated by spectrally resolved photoelectron yield experiments. X-ray and ultraviolet photoelectron spectroscopy measurements (XPS, UPS) establish a clear correlation between the photoelectron yield, the grafting efficiency at different wavelengths, and the valence electronic structure of the substrate and of the reactant molecule.

While our initial work focused on detailed studies on diamond, more recently we have shown that this mechanism is also responsible for initiating UV-induced grafting onto other semiconductors, most notably both silicon and germanium. By intentionally reducing the bulk carrier lifetime in Si (by doping with Au) and comparing the grafting efficiency, we showed that the rate of UV-induced grafting is independent of the bulk carrier lifetime. This observation is important as it allows us to immediately rule out the bulk exciton mechanism as the primary pathway. Our results also showed that the rate of grafting was directly connected to the electron affinity of the reactant molecules. These results are important because they show that photoemission can also dominate as an initiation process with smaller bandgap semiconductors, such as silicon and germanium, where photoemission and exciton processes can both take place. We have hypothesized that the reason why the photoemission can be dominant even on silicon is that photoemission is an irreversible process, while in an exciton process the concentration of holes is reduced by recombination processes. Our studies provide new insights into the nature of photochemical functionalization on the surfaces of semiconductors and a fundamental understanding of the mechanism will facilitate the design and synthesis of well defined functional interfaces.

Interest in organic functionalization of semiconductors has increased in recent years, as it offers the ability to mate existing knowledge of microelectronics fabrication with the tailorability of organic molecules to precisely control interfacial properties. Such control is necessary for today’s microelectronics with continually decreasing feature sizes. In particular, this study focuses on organic functionalization of the germanium surface; germanium is a group IV semiconductor, like silicon, which may be used in devices for its favorable electronic properties. Here, we study the adsorption of two diisocyanate molecules on the Ge surface: 1,3-phenylene diisocyanate, in which the isocyanate functional groups are connected by a relatively stiff phenylene ring, and 1,4-diisocyanatobutane, in which they are connected by a more flexible alkyl chain. Using multiple internal reflection Fourier transform infrared spectroscopy in ultra high vacuum, we show that both molecules bind to the Ge(100)-2×1 surface primarily by a [2+2] cycloaddition reaction across the C=N bond of one isocyanate functional group. This result is similar to previous results for other isocyanate-containing molecules reacted with the Ge(100)-2×1 surface. X-ray photoelectron spectroscopy results agree with the [2+2] cycloaddition assignment and provide evidence that some isocyanate functional groups interact with the surface via a dative bond through an oxygen lone pair. We propose that this is likely the result of some adsorbates forming an additional interaction with the surface through the second isocyanate functional group. Density functional theory calculations demonstrate the feasibility of such products. The relatively weak binding of a second functional group by dative bonding may make these molecules ideal candidates for studying displacement by subsequent exposure to a second precursor. It may be possible to displace the dative-bonded isocyanate functional group, thereby creating additional free isocyanate groups on the surface.

X-ray absorption fine structure (XAFS) and x-ray diffraction (XRD) techniques give complementary information about the structure of catalytic materials. XRD is effective in crystalline materials that possess medium to long range order (bulk catalysts, substrates and templates) while XAFS provides short range structural details in disordered, amorphous and/or low-dimensional materials. In addition, XAFS gives information about the electronic properties of the catalysts. These two methods have been developed and advanced independently from each other at synchrotron sources in the US and abroad. To analyze catalysts in situ, in particularly in operando (under their operating conditions), a new approach is needed, namely, the simultaneous collection of the XRD and XAFS data in real time as the reaction progresses, together with the online product analysis. Diffraction Anomalous Fine Structure (DAFS) is a structure-sensitive technique that allows to deconvolute multiple phases of the same element (e.g., metal nanoparticles and metal oxide) that can coexist in the sample. Application of quick scanning monochromator mode to XAFS measurement (called QEXAFS) allows to study kinetics of structural transformations within the reactants, catalysts and the reaction products. Such combinations allow to measure the time-dependent changes in the actual structure (in the short, medium and long range order), electronic properties and chemical activity of catalysts synchronously.

The first in US dedicated instrument for such combined measurements was built at the beamlines X18A and X18B of the National Synchrotron Light Source at Brookhaven National Laboratory. The current setup includes transmission and fluorescence XAFS detectors, QEXAFS monochromator enabling 10ms time EXAFS scan time, 2D area detector for XRD, residual gas analyzer and the automated gas mixing system. The upgrades currently under way include the addition of Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) instruments developed jointly by members of Synchrotron Catalysis Consortium at BNL and Harrick Scientific.

I will present several applications of the combined use of these instruments for catalysis research. including the studies of knietics of reduction and oxidation of Cu-ceria catalysts, the investigation of the mechanism of reduction of CuFe₂O₄ with CO, and others.

The formation of O_ad species and their reactivities in CO oxidation on BaO/Pt(111) were studied with temperature programmed desorption (TPD), infrared reflection absorption (IRA) and X-ray photoelectron (XP) spectroscopies. Two BaO/Pt(111) model systems with different BaO coverages were prepared and studied. The Pt(111) surface in both of these systems was not completely covered with BaO. On the system with lower BaO coverage (~50 % of the Pt(111) surface was free of BaO), two different O_ad species form following the adsorption of O₂ at 300 K; O adsorbed on clean Pt(111) sites and at the Pt-BaO interface. On the system with higher BaO coverage (~70 % of the Pt(111) surface is covered by BaO) two types of O_ad are seen at the Pt/BaO interface. The desorption of oxygen from the BaO-free portion of the Pt(111) surface gives an O₂ desorption peak with a maximum desorption rate at ~ 690 K. Recombinative desorption of interfacial O_ad gives two explosive-desorption features at ~ 760 and ~ 790 K in the TPD spectrum. The reactivities of these adsorbed O species with CO to from CO₂ follow their order of desorption; i.e., the O_ad associated with the clean Pt(111) surface and desorbs at 690 K reacts first with CO, followed by the O_ad species at the BaO/Pt(111) interface (first the one that desorbs at ~ 760 K and finally the one that is bound the most strongly to the interface, and desorbs at ~ 790 K).