1. Efficient photocatalytic H₂ evolution was achieved on NiMoO₄/T-MCS in NaOH solution.

2. NaOH can facilitate the water oxidation half-reaction to generate ·OH for MB degradation.

3. Surface hydroxylation can enhance the activity and stability of sulfide-based photocatalysts.

4. Dual S-scheme NiMoO₄/T-MCS maximizes redox capacity and enhances charge utilization.

Employing photocatalysis for water splitting to generate hydrogen (H₂) is regarded as a highly viable strategy for attaining green and efficient H₂ production. However, photocatalysis faces significant challenges in charge separation and instability. Notably, sulfide-based photocatalysts are often severely limited by photocorrosion issues. In this work, we employed a NiMoO₄/twinned Mn_0.5Cd_0.5S (NiMoO₄/T-MCS) heterojunction as a model catalyst. By modulating the alkalinity of the aqueous solution to promote rapid consumption of photogenerated holes (h⁺), the surface reaction kinetics was greatly enhanced, achieving an exceptional H₂ evolution rate (r_H2) of 28.22 mmol·g⁻¹·h⁻¹ in 4 M NaOH solution. The external NaOH facilitates the water oxidation half-reaction, preferentially consuming photogenerated h⁺ to generate ·OH radicals. This process triggers surface hydroxylation, significantly boosting both catalytic activity and stability. Furthermore, the generated ·OH radicals effectively degrade methylene blue (MB). Critically, the substantially enhanced surface reaction efficiency—particularly the rapid consumption of photogenerated h⁺—dramatically improves the system’s operational stability. This study demonstrates concurrent H₂ production and pollutant purification solely through solution modulation, alongside a significant enhancement in system efficiency.