There is always mechanical grinding. You can then afterwards filter or centrifuge (if its in solution you can centrifuge and remove the pellet (large particles) or supernatant (small particles)). If you have a large variety of sizes and some are of the desired size you can also do this without the mechanical grinding.

Yes, Ball milling is the way to make bigger particles smaller. This is called the top-down method. i.e. mechanical grinding too. Take your Bi2Te3 micron size particles in the ball mill container, add the balls and run the grinding. depending on rotation speed, time, solvent (some times), etc, one can control the size of the particle. There are many types of ball for this top-down method. Please refer to your requirement.