Hydrosulphuric acid (H2S) hydrothermal treatment is employed to enhance the catalytic activity of the nickel sulfide (NiS) counter electrode (CE) for the Pt-free dye-sensitized solar cell (DSSC). It can be found that a small amount of residual nickel hydroxide (Ni(OH)(2)) and nickel (Ni) are still within the as-prepared NiS film electrodeposited by a cyclic voltammetry (CV) approach, which would have impact on the electrocatalytic activity and chemical stability of the NiS CE. After the H2S hydrothermal treatment, not only the Ni(OH)(2) and Ni are removed, but also NiS is recrystallized and reorganized, thus resulting in a surface evolution from granular to nanoporous morphology. CV, electrochemical impedance spectroscopy (EIS), and Tafel polarization curve are conducted to compare the electrocatalytic activity and long-term stability of CEs for I-3 reduction. The results demonstrate that the DSSC with the optimal treated NiS CE obtains an enhanced photovoltaic conversion efficiency of 7.37% compared to that using the as-deposited NiS CE without H2S treatment (4.84%) under full sunlight illumination (100mWcm(-2), AM1.5 G) due to its superior electrocatalytic activity of the high purity of NiS, and increased specific surface area from the nanoporous structure. Therefore, the H2S hydrothermal treatment can be considered as a simple and effective method for use in the preparation of pure metal sulfide films on conductive substrates. (C) 2015 Elsevier Ltd. All rights reserved.