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The ESEM as surface science tool: Observation of 2D film growth and surface reaction dynamics by in situ scanning electron microscopy

  • Marc Willinger (ETH Zürich, Switzerland)
Date
Thursday 2 April 2020
Time
Address
Hotel NH Noordwijk Conference Centre Leeuwenhorst

Marc Willingera, Zhu-Jun Wanga, Cédric Barroob
aScientific Center for Optical and Electron Microscopy (ScopeM), ETH Zürich, Zürich, Switzerland
bChemical Physics of Materials and Catalysis, Université libre de Bruxelles, Brussels, Belgium

Analytical methods that provide direct real-space information about the dynamics of catalyzed surface reactions often require simplified model systems and operate under high-vacuum conditions. There is thus a strong need for the development of methods that enable observation of active catalysts under relevant working conditions. During the last years, we have used in situ scanning electron microscopy to study processes that are induced by the interaction between a catalytically active surface and a surrounding reactive gas-phase.

During CVD growth experiments performed inside the chamber of the scanning electron microscope, we realized that the instrument is in fact very suitable to study the growth of 2D materials. The ability to visualize individual atomic layers of graphene even at growth temperatures of up to 1000 °C enabled studies on the growth dynamics and stacking sequence of single- and few-layer graphene, respectively.[1,2,3] More recently, we have performed studies on the effect of the film-substrate interaction on growth dynamics. We identified conditions for seamless coalescence and were able to reveal how the strength of the film-substrate interaction determines the coalescence behavior.    

The high sensitivity of the secondary electron signal to changes in the work function suggested that the environmental SEM (ESEM) should be capable of detecting reactions between adsorbed species on clean metal surfaces. In order to test that, we performed NO2 hydrogenation on polycrystalline Pt foils. As shown in Figure 1, it is indeed possible to detect surface reactivity in the form of chemical waves that propagate across the surface of individual grains of a polycrystalline platinum catalyst.[4] Real-time imaging in combination with EBSD mapping thus enables a direct assessment of the structure–reactivity correlation on a working catalyst. Due to the versatility of the environmental SEM, such in situ observations are possible across a large range of pressures and not limited to flat samples. In situ SEM can thus be used to extend the capabilities of photoemission electron microcopy to higher pressures and non-ideal surfaces.

Figure 1: A shows an overview image recorded during NO2 hydrogenation on Pt at T = 172 °C; 𝑝NO2:𝑝H2  ≈ 1:10; ptot = 3.6 × 10−2 Pa. Propagating chemical waves give rise to beautiful dissipative structures. B shows the variation in contrast between subsequent light-off events and 3D plots of the secondary electron intensity.

References

  1. Z.-J. Wang et al., ACS Nano, 2015, 9, 1506–1519.
  2. Z.-J. Wang et al., Nature Communications, 2016, 7:13256
  3. Z.-J. Wang et al., Adv. Mater. Interfaces, 2018, 1800255
  4. C. Barroo, Z.-J. Wang, R. Schlögl, M.-G. Willinger, Nature Catalysis, 2019, doi:10.1038/s41929-019-0395-3
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