Chemical limits on X-ray nanobeam studies in water

Operando X-ray studies of chemical reactions have gained increasing interest lately, fuelled by the emergence of a new generation of powerful focused X-ray sources. Although it is well known that ionizing radiation causes damage to samples via radical chemistry, this effect is often overlooked in studies of working devices or catalysts where intense focused beams are used as nano-scale probes. Here, we show how an X-ray nano-beam directly causes a phase transition in Pd nanoparticles, and that a large oxidative potential must be applied to prevent the process. We present a chemical reaction-diffusion model which offers a plausible qualitative explanation of the observations, and which also suggests that prohibitive concentrations of reactive species will arise under any focused X-ray probe, calling into question the validity of these methods as applied to aqueous chemical and catalytic systems.