Understanding Surface Reactivity of Amorphous Transition-Metal-Incorporated Aluminum Oxide Thin Films

Abstract

The electronic structure of multimetal, amorphous oxides can be varied across a wide range of elemental compositions. Bulk properties such as conductivity, work function, and absorption can thus be tailored to suit a range of applications spanning from carrier-selective contacts to catalysis. Missing, however, is an understanding of how the surface reactivity is impacted in mixed metal-oxide amorphous films. Here we investigate the propensity of Al (1−x) M (x) O y (M = Fe, Mn) amorphous oxide films to dissociate water into hydroxyl groups in a humid environment and find comparable hydroxylation at the low relative humidity (∼0.3% RH) probed by ambient pressure X-ray photoelectron spectroscopy. In contrast, films with both Al and Fe show an increased formation of methoxy groups upon methanol exposure compared to pure Al-and Fe-oxide end members, indicating that the coordination environment of the amorphous oxide network impacts the acidity and redox character of surface metal and oxygen sites. These results provide guidance for the rational design of amorphous oxide layers with tailored chemical reactivity or passivity for a given application.

Understanding methanol dissociative adsorption and oxidation on amorphous oxide films

“…S1† shows example survey spectra collected at 690 eV for 50 mTorr CH 3 OH and 100 mTorr oxygen at 25 °C for the Al8Fe2 and Al4Fe6 films depicting the expected features. The incident photon energy was calibrated at room temperature by cross-referencing O 1s (690 eV, 750 eV) and Al 2p (490 eV, 350 eV) core levels in duplicate films previously calibrated to the Au 4f (84 eV) of an auxiliary reference, 21 where the resultant O 1s and Al 2p binding energy (BE) is dependent on composition as discussed in Section 3.2. Subsequent measurements at elevated temperatures were calibrated to the Al 2p core level as an internal standard.…”

“…The identified features were fit using a line shape with 70% Gaussian and 30% Lorentzian character with average fitted values shown in Tables S1 and S2 †. The OH + OCH 3 feature in the O 1s core level was constrained at 1.65 eV above the bulk O 1s BE based on previous measurements made for duplicate films in water, 21 unless stated otherwise.…”

“…This shoulder at 1.65 eV above the bulk is attributed in part to OH species based on previous measurements of duplicate samples in water. 21 Any oxidized carbon species on the surface will also have similar O 1s BEs, and are primarily attributed to OCH 3 species based on the C 1s spectra (Fig. 1b).…”

“…20 In our previous work, we investigated the influence of these new electronic states on the surface acidity, basicity, and the hydroxylation behavior in a humid environment. 21 In the current work, we study the influence of these new electronic states on the surface reactivity towards CH 3 OH.…”

Predicting Electronic Structure of Realistic Amorphous Surfaces

“…For example, structurally amorphous materials can act as the dielectric layers in transistors, [ 1,2 ] surface passivation layers, [ 3,4 ] and photocatalysts. [ 5,6 ]…”