Just a gentle hint: the 'thing in itself' is rooted in Kantian metaphysics and I cannot see how it can support your thinking about the 'concept' of 'morality'. Here's why.
It is true that in the Groundwork of the Metaphysics of Morals Kant refers to things-in-themselves but not in any way that will help you. He writes:
... one must concede and assume behind the appearances [of objects] something else that is not an appearance, namely the things in themselves; even if - since they can never become known to us, but only ever how they affect us - we of ourselves rest content with being unable to get any closer to them or ever know what they are in themselves' (G: 4:451: Kant, Groundwork of the Metaphysics of Morals, rev. ed., tr. M. Gregor & Jens Timmermann, Cambridge: CUP, 2012: 60.)
Or more crisply :
... we can achieve only cognition of appearances, never of things in themselves (ibid.)
So if a concept or morality were a thing in itself, you could ex hypothesi not know what it was. This seems an unpromising start for an analysis or characterisation of the nature of either concepts or morality. A more fruitful exercise would be, if you want to approach your essay from a Kantian angle, to consider what Kant understood morality to be - i.e. what was his concept of morality. He identified 'The autonomy of the will as the supreme principle of morality' (4: 440: Gregor & Timmermanns: 51) and connected morality with reason in such a way that on his account the moral law is one of the principles of (practical) reason. Morality binds all rational beings as such (4: 408: Gregor & Timmermanns: 23). Your essay could elaborate this bare indication. The Stanford philosophy website should help here - or any of the standard commentaries - Onora O'Neill, Christine Korsgaard, Scruton. Roger Scruton's succinct but excellent, Kant: A Very Short Introduction, is probably the clearest and most straightforward place to start.
C. Korsgaard, Creating the Kingdom of Ends, Cambridge: CUP, 1996: Part 1.
ISBN 10: 0521499623 / ISBN 13: 9780521499620
O. O'Neill, Constructions of Reason, Cambridge: CUP, ch. 4 & 5.
ISBN 10: 0521388163 / ISBN 13: 9780521388160
R. Scruton, Kant: A Very Short Introduction, Oxford: OUP, 2001.
ISBN 10: 0192801996 / ISBN 13: 9780192801999