Dynamics of Photo‐Induced Surface Oxygen Vacancies in Metal‐Oxide Semiconductors Studied Under Ambient Conditions

Abstract

Surface‐enhanced Raman spectroscopy (SERS) is a powerful analytical technique commonly used in the detection of traces of organic molecules. The mechanism of SERS is of a dual nature, with Raman scattering enhancements due to a combination of electromagnetic (EM) and chemical contributions. In conventional SERS, the EM component is largely responsible for the enhancement, with the chemical contribution playing a less significant role. An alternative technique, called photo‐induced enhanced Raman spectroscopy (PIERS) has been recently developed, using a photo‐activated semiconductor substrate to give additional chemical enhancement of Raman bands over traditional SERS. This enhancement is assigned to surface oxygen vacancies (V o) formed upon pre‐irradiation of the substrate. In this work, the exceptional chemical contribution in PIERS allows for the evaluation of atomic V o dynamics in metal oxide surfaces. This technique is applied to study the formation and healing rates of surface‐active V o in archetypical metal‐oxide semiconductors, namely, TiO2, WO3, and ZnO. Contrary to conventional analytical tools, PIERS provides intuitive and valuable information about surface stability of atomic defects at ambient pressure and under operando conditions, which has important implications in a wide range of applications including catalysis and energy storage materials.

Enhancing hybrid metal-semiconductor systems beyond SERS with PIERS (photo-induced enhanced Raman scattering) for trace analyte detection

“…A 3 step-analysis approach was followed to analyze series of Raman spectra without UV, after UV and under constant UV, following the procedure and analysis reported previously. 22 Firstly, the rate of decay of the Raman band intensities due to photo-bleaching was measured for each different experimental set-up under identical conditions, excluding UV exposure. The second step analyzes samples after an initial exposure to UV irradiation, allowing the rate of induced vacancy healing to be determined.…”

“…The primary reason for this relies on the chosen fitting approximations, based on the model presented previously. 22 The model assumes the healing of induced vacancies can effectively be modelled as an exponential decay. Although this is shown to be accurate for the UV lamp data, after 15 minutes the LED data appears to behave more like a linear decrease rather than an exponential decay.…”

“…In addition, due to the size and nature of the UV lamps, real time Raman measurements under UV exposure are challenging practically. Although enhancements under constant UV exposure have been reported previously, 22 the UV source was reported to be switched off during measurements where objects often had to be refocused on the substrate before taking measurements between UV exposures, due to experimental constraintssee Figure 1. In this paper we report an alternative UV source which can overcome all these issues, allowing real time Raman monitoring under UV irradiation, a UV LED.…”

“…In addition, surface defect states can interact with adsorbed molecules, resulting in an additional CE pathway. [20][21][22] Surface oxygen vacancies, Vo, induced through high energy UV irradiation (254 nm) on photocatalytic MOS substrates has come to be known as PIERS (Photo-Induced Enhanced Raman Spectroscopy). The mechanism is thought to rely on photo-induced carrier production, where induced carrier lifetimes are increased due to carrier separation due to the hybrid metal-semiconductor system.…”

“…Under ambient conditions, or exposure to O2 or H2O, the induced vacancies were shown to "heal", decreasing the Raman band intensities back to the original background enhancement level. 22 Previous PIERS studies conducted have primarily used fixed laboratory UV sources, typically UV gas discharge lamps, which are generally larger, have significant power supply requirements and cannot be as easily adjusted and are therefore hard to incorporate into portable Raman systems for field analysis. In addition, due to the size and nature of the UV lamps, real time Raman measurements under UV exposure are challenging practically.…”