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Spin-1/2 Heisenberg antiferromagnetic chains are excellent one-dimensional platforms for exploring quantum magnetic states and quasiparticle fractionalization. Understanding its quantum magnetism and quasiparticle excitation at the atomic scale is crucial for manipulating the quantum spin systems. Here, we report the fabrication of spin-1/2 Heisenberg chains through on-surface synthesis and in-situ reduction. A closed-shell nanographene is employed as a precursor for Ullman coupling to avoid radical fusing, thus obtaining oligomer chains. Following exposure to atomic hydrogen and tip manipulation, closed-shell polymers are transformed into spin-1/2 chains with controlled lengths by reducing the ketone groups and subsequent hydrogen desorption. The spin excitation gaps are found to decrease in power-law as the chain lengths, suggesting its gapless feature. More interestingly, the spinon dispersion is extracted from the inelastic spectroscopic spectra, agreeing well with the calculations. Our results demonstrate the great potential of fabricating desired quantum systems through a combined on-surface synthesis and reduction approach.