Thermally Activated Reactions of Nitrobenzene at the Ge(100)-2 × 1 Surface

Abstract

The adsorption of nitro compounds on the germanium surface serves as a model system for understanding the formation of interfacial germanium oxynitride. We have studied the adsorption and thermal reactions of nitrobenzene on the Ge(100)-2 × 1 surface under ultrahigh vacuum conditions. A combination of infrared spectroscopy, X-ray photoelectron spectroscopy, and temperature-programmed desorption experiments together with density functional theory calculations was used to elucidate possible product structures and their associated reaction mechanisms. Our study suggests that nitrobenzene initially forms a 1,3-dipolar cycloaddition product with a Ge(100) surface dimer as the major species. Minor side products are also formed at 300 K. Upon heating, some of the adsorbates molecularly desorb, while a fraction of the adsorbates undergo oxygen atom insertion into the surface, transforming them into triplet nitrene products. The energetics and pathways of nitrobenzene on the Ge surface are discussed in comparison with those seen on the closely related Si(100)-2 × 1 surface, with some differences arising from the smaller adsorption energy of organic adsorbates on Ge.

Adsorption of heterobifunctional 4-nitrophenol on the Ge(100)-2 × 1 surface

“…However, our results suggest that the oxygen-insertion product does not form in significant quantity for 4-nitrophenol adsorbates. Compared to the previous result of nitrobenzene adsorbates, the binding energy of the N(1s) peak of 4-nitrophenol adsorbates centered at 398.5 eV is significantly higher than that of the oxygen-inserted structure of nitrobenzene adsorbates at 397.3 eV, in which the nitrogen is bonded directly to Ge [38], although whether a mixture of products is formed is not clearly resolved for 4-nitrophenol adsorbates. Also, the major peak of O(1s) at 530.7 eV is closer to that of surface-bound oxygen (Ge-O-N, 531.1 eV) than surface-inserted oxygen (Ge-O-Ge, 530.0 eV).…”

“…Consequently, only this configuration was considered for D-ad. For both pathways, the energetics for the reaction of the first functionality with the Ge dimer is similar to those previously reported for phenol [36] and nitrobenzene [38], whereas reaction of the second functionality faces a larger kinetic barrier and involves smaller thermodynamic stabilization compared to the reaction of the initial tethering groups. However, the effect is much more significant for the pathway that proceeds via initial NO 2 cycloaddition.…”

“…While a peak belonging to the free nitro group at 405.1 eV remains, a new and larger N(1s) peak also appears at 398.5 eV. The large shift toward lower binding energy can be explained by cycloaddition of the nitro group with the Ge dimer, losing the formal charge on the N atom; nitrobenzene adsorbates on Ge(100) that react via the nitro group show a similar N(1s) peak near 398 eV [38]. Therefore we assign the N(1s) at 398.5 eV to surface-bound nitrogen in either N-ad or adsorbates dually bound with both OH and NO 2 termini (D-ad).…”

“…Notably, in a recent study of nitrobenzene adsorption on Ge(100), a fraction of adsorbates had oxygen atoms spontaneously migrate into the Ge surface at 300 K [38]. Since the reactivity of the nitro groups in nitrobenzene and 4-nitrophenol are similar, we may expect an oxygen insertion reaction to also occur for 4-nitrophenol adsorbates bonded to the surface through the nitro group.…”

“…Moreover, the monofunctional analogues of 4nitrophenol, namely phenol and nitrobenzene, have been shown to readily react with group 14 (100)-2 × 1 surfaces. Phenol undergoes O-H dissociative adsorption to form an H adatom and phenoxy adsorbate [35,36], and nitrobenzene reacts via 1,3-dipolar cycloaddition [37,38].…”