You seem close.
I would try to set the URI like this :
String folderPath = Environment.getExternalStorageDirectory()+"/pathTo/folder";
Intent intent = new Intent();
intent.setAction(Intent.ACTION_GET_CONTENT);
Uri myUri = Uri.parse(folderPath);
intent.setDataAndType(myUri , "file/*");
startActivity(intent);
But it's not so different from what you have tried. Tell us if it changes anything ?
Also make sure the targeted folder exists, and have a look on the resulting Uri object before to send it to the intent, it may not be what we are expected.