Flow cell for operando X-ray photon-in-photon-out studies on photo-electrochemical thin film devices.

Background: Photo-electro-chemical (PEC) water splitting represents a promising technology towards an artificial photosynthetic device but many fundamental electronic processes, which govern long-term stability and energetics, are not yet fully understood. X-ray absorption spectroscopy (XAS), and particularly its high energy resolution fluorescence-detected (HERFD) mode, emerges as a powerful tool to study photo-excited charge carrier behavior under operating conditions. The established thin film device architecture of PEC cells provides a well-defined measurement geometry, but it puts many constraints on conducting operando XAS experiments. It remains a challenge to establish a standardized thin film exchange procedure and concurrently record high-quality photoelectrochemical and X­ray absorption spectroscopy data that is unperturbed by bubble formation. Here we address and overcome these instrumental limitations for photoelectrochemical operando HERFD-XAS. Methods: We constructed a novel operando photo-electro-chemical cell by computer numerical control milling, guided by the materials' X­ray and visible light absorption properties to optimize signal detection. To test the cell's functionality, semiconducting thin film photoelectrodes have been fabricated via solution deposition and their photoelectrochemical responses under simulated solar light were studied using a commercial potentiostat in a three-electrode configuration during HERFD-XAS experiments at a synchrotron. Results: We demonstrate the cell's capabilities to measure and control potentiostatically and in open­circuit, to detect X­ray signals unperturbed by bubbles and to fluently exchange different thin film samples by collecting high-resolution Fe K-edge spectra of hematite ( α -Fe 2O 3) and ferrite thin film ( MFe 2O 4, M= Zn, Ni) photoelectrodes during water oxidation. Conclusions: Our cell establishes a measurement routine that will provide experimental access of photo-electro-chemical operando HERFD-XAS experiments to a broader scientific community, particularly due to the ease of sample exchange. We believe to enable a broad range of experiments which acquired fundamental insights will spur further photoelectrochemical research and commercialization of water splitting technologies.

Effect of leaving group on the structures of alkylsilane SAMs.

Multiple transmission and reflection (MTR) infrared spectroscopy has been used to study the kinetics of the formation of self-assembled monolayers (SAM) of octadecylsilanes with different leaving groups, viz. trichloro, trimethoxy, and triethoxy. It was observed that the chlorosilanes form much denser and crystalline-like SAMs and ethoxysilanes form thin SAMs, while methoxysilanes form extremely thin SAMs. The high sensitivity of the MTR IR technique allows the molecular conformations of the alkyl chains and appearance/disappearance of the silanol groups to be scrutinized in detail. This enables the formulation of models for the structures of the SAMs that are in many ways different than the classical picture of silanes on oxide surfaces. We observe that the structure of SAMs depends on the rate of hydrolysis of the leaving groups and thus their chemical nature. SAMs of chlorosilanes resemble a structure of snow moguls or densely packed umbrellas. SAMs of ethoxysilanes, on the other hand, look like stacks of fallen trees, while the molecules of the ultrathin methoxysilane SAMs are lying nearly parallel to the surface, resembling creepers.