Sodium sulfide

Nickel acetate (Ni(Ac)₂, 99%), cobalt acetate (Co(Ac)₂, 99%) and sodium sulfide (Na₂S, 98%) were purchased from Aldrich. The process for the alternate-dip-coating method and fabrication of the nanostructured NiS electrodes are described elsewhere [17 ]. The nanostructured Co₃S₄ electrode was fabricated using the same process. Briefly, a fluorine-doped tin oxide (FTO)-coated glass substrate was subjected to UV-O₃ cleaning for 5 min. A 200 nm thick porous TiO₂ (p-TiO₂) layer was formed on the cleaned substrate by spin-coating of commercial TiO₂ nanoparticle (NP) paste (90-T, Dyesol) diluted in ethanol (1 : 6 weight ratio) at 2500 r.p.m., followed by annealing at 300°C for 1 h. The TiO₂-deposited FTO substrate was placed in a 0.15 M Co(Ac)₂ methanol/water solution and a 0.15 M Na₂S methanol/water solution for 3 min each, washed with distilled water and blown with N₂ gas. This alternate-dip-coating cycle was repeated seven times. For the fabrication of nanostructured NiCo₂S₄ electrodes, a 0.0375 M Ni(Ac)₂ and 0.075 M Co(Ac)₂ mixed solution was used as a metal precursor solution, and the other process was carried out in the same manner. Finally, the nanostructured metal sulfide electrodes were annealed at 200–350°C for 1 h. The deposit weights of the metal sulfides were determined by a quartz crystal microbalance (QCM, Stanford Research System QCM 2000).

Cyanuric chloride (99%), p-hydroxybenzaldehyde (98%), sodium hydroxide (NaOH) (99%), cyclohexanone (99.8%), 4-nitrobenzaldehyde (98%), potassium hydroxide (KOH) (≥ 85%), sodium sulfide (Na₂S) (≥ 98%), sodium bicarbonate (NaHCO₃) (99%), methyl red (MR) (≥ 99%), chlorhydric acid (HCl) (37%), acetone (99%), ethanol (99%), ethyl acetate (99.5%), n-hexane (99%), methanol (99%), n-butanol (99.5%), o-dichlorobenzene (≥ 99%), acetic acid (100%), N,N-dimethylformamide (DMF) (≥ 99.8%), dichloromethane (≥ 99.5%), and tetrahydrofuran (THF) (≥ 99.8%) were prepared from Aldrich and Merck companies and used without further purification.

For the synthesis of GO graphite powder (crystalline, −300 mesh, 99%) was purchased from Alfa Aesar, whereas sodium nitrate (NaNO₃), sulfuric acid (H₂SO₄), potassium permanganate (KMnO₄) and hydrogen peroxide (H₂O₂) were purchased from Merck. Zinc chloride (ZnCl₂), sodium hydroxide (NaOH), cadmium acetate dihydrate (Cd(OOCCH₃)₂·2H₂O), sodium sulfide (Na₂S), ammonia solution and methyl orange were also supplied by Merck. Polyvinyl pyrrolidone (PVP) used in synthesis was purchased from Sigma-Aldrich. All chemicals were used as received without further purification. Deionized water (18.2 MΩ·cm) used in synthesis was obtained from a double-stage water purifier (ELGA PURELAB Option-R7).