  Hydrogen, the smallest and most abundant element in nature, can be
efficiently incorporated within a solid and drastically modify its electronic
state - it has been known to induce novel magnetoelectric effects in complex
perovskites and modulate insulator-to-metal transition in a correlated Mott
oxide. Here we demonstrate that hydrogenation resolves an outstanding challenge
in chalcogenide classes of three-dimensional (3D) topological insulators and
magnets - the control of intrinsic bulk conduction that denies access to
quantum surface transport. With electrons donated by a reversible binding of H+
ions to Te(Se) chalcogens, carrier densities are easily changed by over 10^20
cm^-3, allowing tuning the Fermi level into the bulk bandgap to enter
surface/edge current channels. The hydrogen-tuned topological materials are
stable at room temperature and tunable disregarding bulk size, opening a
breadth of platforms for harnessing emergent topological states.
