As you can see my surname contains a ij, so I've always had an interest in it.
Here is a Wikipedia article about IJ,
https://en.wikipedia.org/wiki/IJ_(digraph).
Another thing to note, that if a person name or place name starts with IJ, then both letters are in capitals. e.g. peoples names IJzerman, IJdoo and IJsbranditie IJsbrandsdr. and the name of a lake IJsselmeer
With the Dijkgraaf surname, if they went to the United States, this would always become Dykgraaf, in some other English speaking countries it varies and only some become Dykgraaf. That Wikipedia article also mentions that in the Netherlands, some people use the unofficial spelling with a Y, and even a famous Dutch soccer team changed to using a Y.