Chemical Limits on X-ray Nanobeam Studies in Water

Operando X-ray studies of chemical reactions have gainedincreasinginterest lately, fueled by the emergence of a new generation of powerfulfocused X-ray sources. Although it is well known that ionizing radiationcauses damage to samples via radical chemistry, this effect is oftenoverlooked in studies of working devices or catalysts where intensefocused beams are used as nanoscale probes. Here, we show how an X-raynanobeam directly causes a phase transition in shape-controlled Pdnanoparticles and that a large oxidative potential must be appliedto counteract the effect. In addition, we present a chemical reaction-diffusionmodel that offers a plausible qualitative explanation of the observations,and which also suggests that prohibitive concentrations of reactivespecies will arise under any focused X-ray probe, calling into questionthe validity of these methods as applied to aqueous chemical and catalyticsystems.