Noble metals are commonly used in heterogeneous catalysis due to their active catalytic sites. However, carbon layer deposition, or coking, can block these sites and reduce performance, even at thicknesses below 25 nm. Rapid monitoring of ultrathin coking layers is therefore essential for timely surface renewal and sustained catalytic efficiency. Conventional analysis methods are expensive, destructive, require bulky instruments, and lack in-situ, real-time capabilities. In this work, we present a simple, image-based optical method for non-destructive monitoring of sub-25 nm coking layers. This approach uses direct imaging of dielectric-loaded plasmonic azimuthally chirped gratings (DL-pACGs) on the coked gold surface, where reflection images reveal azimuthal dark bands from plasmon coupling. The position of these bands correlates with carbon thickness, enabling quantitative analysis. Ellipsometry data from thin carbon films were incorporated into simulations, showing strong agreement with experimental results. This method is simple, effective, and non-destructive for quantifying thin coke films.
Noble metal catalysts are widely used in heterogeneous catalysis, including organic synthesis, carbon dioxide reduction, catalytic converters, and fuel cells. However, these catalytic reactions often suffer from issues such as poisoning, aging, coking or fouling on the metal surface, which significantly impair catalytic activity and stability. The presence of coke, for instance, can block active sites and impede heat transfer, leading to decreased catalyst performance and requiring replacement or reactivation. The typical thickness of the coking layer formed on noble metal electrodes during catalytic chemical reactions can vary depending on factors such as reaction conditions, catalyst material, the type of coke formed, and operating time. Carbon deposits on metal catalyst surfaces can range from a few nanometers to several micrometers, depending on the extent of coking and the reaction environment. Studies have shown that even an ultra-thin coke layer with a thickness of less than 25 nm is sufficient to block active sites, thereby reducing catalytic efficiency.
To overcome these issues, it is beneficial to monitor the coke load on the surface of the metal catalysts in real time during deactivation and regeneration processes. This approach enables a comprehensive evaluation of catalyst availability and offers valuable insights into deactivation and regeneration mechanisms. Such knowledge can be used to develop strategies to optimize catalyst performance and extend their lifetime, as well as to recover deactivated noble metal catalysts. Implementing such an approach ensures a more efficient and sustainable utilization of these valuable catalysts, thereby contributing to a greener future.
Techniques for obtaining coke-related information can be broadly categorized into optical and non-optical methods. Conventionally, coking information has been obtained by analyzing used catalyst extracted from reaction chambers through combustion or high-temperature analysis techniques such as temperature programmed oxidation, differential thermal analysis, and thermogravimetric analysis. Vacuum-based techniques, such as X-ray photoelectron spectroscopy and secondary ion mass spectrometry, are also commonly utilized to investigate catalyst coking and other forms of poisoning. However, these techniques typically do not offer spatially resolved information about coke formation. To address this limitation, advanced microscopy techniques, such as hyperspectral confocal fluorescence microscopy combined with tip-enhanced fluorescence microscopy, have been developed to directly investigate the surface coke formation. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM), often coupled with energy-dispersive X-ray spectroscopy, are also widely used for detailed analysis of coke morphology, crystalline structure, and elemental composition.
Optical techniques, such as spectroscopic ellipsometry, can determine film thickness and refractive index, while optical transmission and reflection measurements are commonly used to determine thin film thickness. In addition, optical sensing, which integrates light sources and sensors with optical fibers, enables signal transmission and in-situ data collection. These systems can operate under extreme conditions, such as high temperatures and pressures, making them suitable for coke detection.
However, these optical methods typically require a stable light source, a flat and large sample surface and a bulky spectrometer, making it difficult to monitor the coking layer on microelectrodes. Moreover, a reference beam is necessary to remove the effect of the source intensity fluctuation, rendering the evaluation of the complex refractive index and thickness of extremely thin or non-transparent thin films challenging. For extremely thin films, the interference fringes are not pronounced. As a result, the fitting procedures may yield multiple solutions, which makes quantitative analysis difficult. Most of the methods mentioned above are either invasive to the sample, or require post-data analysis, making them unsuitable for reliable and continuous on-site monitoring of the coking process. Currently, no efficient methods are available for providing non-destructive and on-site quantitative results for coke detection.
To address this issue, we exploit our previous design of plasmonic azimuthally chirped gratings (pACGs), also called plasmonic Doppler gratings, and introduce the so-called dielectric-loaded plasmonic azimuthally chirped gratings (DL-pACGs) for coke detection, as illustrated in Fig. 1a. Dielectric-loaded plasmonic devices do not require the plasmonic metallic substrate to be structured. The morphology of the devices is purely determined by the loaded dielectric structure, while the plasmonic metal remains flat and continuous. This flat metal layer mimics realistic conditions, such as those found on catalytic metal surfaces. This configuration enables non-destructive, on-site monitoring of coke thickness on the metal catalyst surface. A further advantage of the DL-pACGs for coke detection lies in its image-based analysis, which allows for spectrometer-free optical detection of coking, independent of fluctuations in source power or detector sensitivity. This innovative approach provides a reliable and straightforward method for coke detection and holds strong potential for in-situ, real-time optical monitoring of coking phenomena on heterogeneous catalysts. Furthermore, our DL-pACGs can also be adapted to incorporate actively tunable materials in place of PMMA, allowing for dynamic control of the working window in response to external stimuli such as temperature, humidity, pH, or ionic strength. As mentioned, this approach enables spectroscopic analysis without the need for additional far-field dispersive optical elements, like prisms. All these advantages make our pACGs platform particularly well-suited for microfluidic devices and lab-on-a-chip systems. This adaptability positions it as an excellent option for portable, on-site analytical tools.
The DL-pACGs used in this work is a PMMA grating structure with azimuthally varying periodicities, placed on top of the coked gold (Au) surface, as shown in Fig. 1a. The thin carbon layer, with a thickness ranging from 0 to 25 nm, placed between the structured PMMA layer and the Au substrate, significantly affects the reflection intensity profile as a function of the azimuthal angle. Briefly, this design features a series of eccentrically configured circular rings that create a continuous azimuthally varying lattice momentum. The eccentric spatial configuration of rings is designed in such a way that the trajectory of the n ring can be described as,
In Eq. (1), represents the increment in ring radius and represents the shift in the center of the rings. The continuum of azimuthal angle-dependent periodicity ranges from to , corresponding to azimuthal angles of 180° and 0°, respectively. The mathematical relationship between the grating periodicity () and the azimuthal angle () is given by,
This design allows for a range of momentum-matching possibilities between the incident light and surface plasmon polaritons (SPPs) through the periodicity-dependent grating momentum. The condition for momentum matching can be expressed as:
where represents the wavelength of the incident light, is the angle of incidence, is the effective local refractive index of the surroundings, is the permittivity of the metal, m is the resonant order, and is the periodicity. The term corresponds to the momentum of the incident light, represents the azimuthal angle-dependent grating momentum, and indicates the momentum of the surface plasmon polaritons propagating on the metal surface. SPPs propagate along the Au/carbon interface, with evanescent fields extending tens to hundreds of nanometers into the surrounding layers, including carbon, PMMA and air. Therefore, the is determined by the local dielectric environment defined by these adjacent layers, which is a combination of the refractive indices of the PMMA (n = 1.49), carbon (n = 2.2-3.0), and air (n = 1). The refractive index of carbon ranges from 2.2 to 3.0 in the visible region, depending on the thickness and internal morphology of the amorphous carbon layer.
Regarding Eq. (3), we recognize that the dynamic range of the index sensor can be tuned by selecting an appropriate range of grating periodicities. However, the momentum-matching condition does not consider structural parameters such as the grating's width, length, and height. Additionally, because the dielectric properties of thin carbon films depend nonlinearly on thickness, deriving a simple closed-form analytical expression that includes thickness as a variable is not straightforward. Therefore, we use refractive index data for carbon obtained from ellipsometry measurements and apply finite-difference time-domain (FDTD) simulations to model and validate how carbon thickness influences the angular reflection profile. The optimized design was fabricated on the surface of multiple monocrystalline gold flakes coated with carbon layers ranging from 0 to 25 nm in thickness to simulate coking conditions. The parameter scan used for structural optimization is shown in Fig. S1.
According to Eqs. (2) and (3), the optical response of the DL-pACGs, including the central wavelength and the span of the operational spectral window, can be freely designed by selecting appropriate values of and . For a given grating structure, a change in the refractive index of the surroundings () would lead to a change in the momentum matching condition and thus the periodicity for light-surface plasmon coupling. Experimentally, the observed angular profile of the reflection intensity, specifically the dark band in the reflection image, varies with the surrounding refractive index, making the system an effective, spectrometer-free index sensor.
In this work, thin amorphous carbon films with various thicknesses ranging from 0 to 25 nm were prepared and examined using an ellipsometer to obtain thickness-dependent values of complex permittivity for the simulations, which provides the expected intensity angle profile for a specific carbon layer thickness to be compared with experimental results. The carbon layer was deposited via ion-beam deposition, and the DL-pACGs structure was patterned using electron-beam lithography. Additional fabrication details are provided in the Experimental Section. Figure 1b shows the schematic of the fabrication process and the dark-field image of the well-fabricated DL-pACGs. It is noteworthy that the color distribution observed on the DL-pACGs reflects the dispersive properties of our design.