a–b GIWAXS two-dimensional reciprocal space maps of a control sample, b treated sample at an incidence angle of 0.12°, probing the surface of the material. Low q signals of the new surface species are marked with a green circle. Measurements were done under vacuum conditions. c Peak intensities of different crystal phases as a function of the estimated probing depth, normalized to their respective maximum value for control and treated samples. The intensity of surface species is approximately the same for the probed depth (red). d Relative change in pseudo-cubic unit cell parameters as a function of probing depth. Values were calculated from radial profiles of reciprocal space maps and averaged over two sets of samples. GIWAXS probing depth was varied by changing the angle of incidence of the X-ray beam from between 0.12 and 0.3°. The error in the probing depth (horizontal error bars) was estimated from the angular error due to the surface roughness of the sample. The vertical error bars were determined from the standard deviation from repeated measurements on two sets of samples.