Amino-terminated diamond surfaces: Photoelectron emission and photocatalytic properties

Abstract

We report a new approach to making stable negative electron-affinity diamond surfaces by terminating diamond with amino groups (also known as amine groups,-NH 2). Previous studies have shown that negative electron affinity can be induced by terminating diamond surfaces with hydrogen, creating a surface dipole favorable toward electron emission. Here, we demonstrate that covalent tethering of positive charges in the form of protonated amino groups,-NH 3 + , also leads to negative electron affinity (NEA) and facile electron emission into vacuum and into water. Amino-terminated diamond was prepared using a very mild plasma discharge. Valence-band photoemission studies of the amino-terminated diamond samples show a characteristic "NEA" peak, demonstrating that the amino-terminated surface has NEA. Diamond's ability to emit electrons into water was evaluated using photochemical conversion of N 2 to NH 3. Time-resolved surface photovoltage studies were used to characterize charge separation at the diamond interface, and Mott-Schottky measurements were performed to characterize band-bending at the diamond-water interface. XPS studies show that the amino-terminated surfaces provide increased chemical resistance to oxidation compared with H-terminated diamond when illuminated with ultraviolet light.

Atmospheric‐pressure photoelectron emission from H‐terminated and amino‐terminated diamond

“…The mobility of the much heavier SF 6 À anion in pure SF 6 at 1 atm pressure is even smaller, 0.57 cm 2 V À1 s À1 [24]. Based on these results, we conclude that the much higher currents observed with Ar are a consequence of the fact that electrons emitted into Ar remain as un-bound electrons, in contrast to their fate in air or SF 6 . At À10 V applied potential, the ratio of emission currents in Air (0.46 nA) and SF 6 (0.078 nA) is 4.6, close the ratio of ion mobilities (2.25/0.57 ¼ 3.9).…”

“…These changes in gas composition were performed within a time space of several minutes to avoid any significant change in surface composition during the time of the experiment and were shown to be fully reversible over the time scale of the experiment. Notably, a much higher current was observed when the chamber was filled with Ar, and a much lower current observed when the chamber was filled with SF 6 .…”

“…Argon does not form a stable anion, and it is well established that electrons emitted into argon gas remain as free electrons (albeit with some elastic collisions) [18]. In contrast, both O 2 and SF 6 form stable anions: O 2 has an electron affinity of 0.9 V [19], while SF 6 has an electron affinity of 1.03 V [20]. Thus, electrons emitted into argon remain as free electrons, while electrons emitted into air rapidly form O 2 À , and electrons emitted into SF 6 form SF 6 À .…”

“…In contrast, both O 2 and SF 6 form stable anions: O 2 has an electron affinity of 0.9 V [19], while SF 6 has an electron affinity of 1.03 V [20]. Thus, electrons emitted into argon remain as free electrons, while electrons emitted into air rapidly form O 2 À , and electrons emitted into SF 6 form SF 6 À . Previous studies have shown that the mobility of low-energy electrons in Ar is approximately 400 cm 2 V À1 s À1 at 760 Torr and an applied electric field strength comparable to that used in Refs.…”

“…Recently, we showed that diamond can also be used as a solid-state source of electrons able to emit electrons directly into aqueous media [4,5]. This remarkable ability enables novel reactions such as room-temperature reduction of N 2 to NH 3 [4][5][6], and the direct one-electron reduction of CO 2 to CO 2 À , which in turn can form CO þ O [7]. Electron sources typically operate on the basis of thermionic emission [8] or field emission [9,10] and are usually required to operate under high-or ultrahigh-vacuum conditions.…”