X-ray diffractometer was used to identify phases in alloys and to study the crystal structure of minerals. Is this still a useful tool when studying new materials like nanoparticles on surfaces?
Yes if you put the diffractometer on a synchrotron beam then you can study nanoparticles on surfaces. You may even try to find structure of a single nonocrystal.
Do not underestimate X-ray diffractometer even if it is placed in a laboratory with just an X-ray tube. It can still do a lot of sample characterization and can even solve a new structure of enough complexity. On top of it now-a-days you can have powerful laboratory sources of X-rays including table-top synchrotron beam.