Nothingnes in Chinese / Japanese...

無 is the simple way to express “nothing.”

However, this single character leaves a bit of mystery as to what you might really mean if you hang it as a wall scroll. I'm not saying that's a bad thing; as you can decide what it means to you, and you won't be wrong if you stay within the general context.

More info: 無 is usually used as a suffix or prefix for Chinese and Japanese words (also old Korean). It can be compared to “un-” or “-less” in English. It can also mean “not to have,” no, none, not, “to lack,” or nothingness.

虛空 means empty space, empty sky, or void.

In the Buddhist context, it can mean “emptiness of the material world.” This can also be used as an adjective to modify other words with a meaning of unreal or insubstantial.

空無 is “nothingness” in a Buddhist context.

The first character means empty but can also mean air or sky (air and sky have no form).

The second character means have not, no, none, not, or to lack.

Together these characters reinforce each other into a word that means “absolute nothingness.”

I know this is a term used in Buddhism, but I have not yet figured out the context in which it is used. I suppose it can be the fact that Buddhists believe that the world is a non-real illusion, or perhaps it's about visualizing yourself as “nothing” and therefore leaving behind your desire and worldliness.
Buddhist concepts and titles often have this element of ambiguity or, rather, “mystery.” Therefore, such ideas can have different meanings to different people, and that's okay. If you don't get it right in this lifetime, there will be plenty more lifetimes to master it (whatever “it” is, and if “it” really exists at all).

Soothill defines this as “Unreality, or immateriality, of things, which is defined as nothing existing of independent or self-contained nature.”