How to remove the border behind the button?

There is a grey border behind my app button, how can I remove this?

I defined the ImageButton like this:

<ImageButton
    android:id="@+id/btn_photo_lib"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:onClick="startPhotoLibAction"
    android:src="@drawable/library_blau_2" />