Cyclic voltammogram (CV) studies were employed to characterize the capacitive performances of the NiS 2 nanocubes. Fig. 4a shows the CVs of the NiS 2 nanocubes electrodes (a mass loading of 5 mg) in 3.0 M KOH electrolyte at different scan rates in the range 5–50 mV s−1. As seen in Fig. 4a, the CVs are almost symmetric, indicating good reversibility of the oxidation and reduction processes. The CVs show a broad redox peak due to faradic reactions of NiS 2 nanocubes and it indicates that the Faradaic pseudocapacitive property of NiS 2 is based on the surface redox mechanism of Ni2+ to Ni3+ at the surface. Fig. 5 shows a simple possible route for diffusion of ions and electrons during the electrochemical process. The single nanocrystal, NiS 2 nanocube, shows different diffusion rates of electrons along different crystal planes. And due to the fewer inner crystal interfaces of NiS 2 nanocubes, the diffusion rate of electrons in NiS 2 nanocubes is much quicker than that of polycrystal NiS 2 nanospheres. Moreover, a great number of inner crystal interfaces of NiS 2 nanospheres may cause large inner resistance and contact resistance, which stops electrons from diffusing quickly.

Figure 4 (a) Cyclic voltammetry experiments within a 0.0–0.60 V range at a scan rate 5, 10, 20, 30 and 50 mV s−1 were performed on the NiS 2 nanocube electrodes in 3.0 M KOH electrolytes at room temperature; (b) The galvanostatic charge–discharge curves of NiS 2 nanocube electrodes during current densities were 1.25–12.5 A g−1 in 3.0 M KOH electrolytes; (c) Specific capacitances of NiS 2 nanocube, nanosphere, nanoparticle electrodes derived from the discharging curves at the current density of 1.25–12.5 A g−1 in 3.0 M KOH electrolytes; (d) Ragone plot of the estimated specific energy and specific power at various charge/discharge rates in 3.0 M KOH electrolytes; (e) Charge/discharge cycling test at the current density of 1.25 A g−1 in 3.0 M KOH electrolytes. Full size image

Figure 5 A simple route for diffusion of ions and electrons during the electrochemical process. Full size image

Chronopotentiometry (CP) curves at different current densities are shown in Fig. 4b and Fig. S5. The symmetrical characteristic of charging/discharging curves is good, which means that the NiS 2 nanocube electrodes with excellent electrochemical capability and redox process are reversible. The relationships between the specific capacitances calculated by CP curves and current densities are given in Fig. 4c. Based on the CP curves, NiS 2 nanocube electrodes have the large specific capacitance and reach up to 695 F g−1 at a current density of 1.25 A g−1 and remain 158 F g−1 even 12.5 A g−1, while that of NiS 2 nanoparticle electrodes is 34 F g−1 at 12.5 A g−1. The specific capacitance of NiS 2 nanocube is significantly better than some nickel based nanomaterials, such as NiO nanowires (0.5 A g−1, 180 F g−1)31, NiO nanotubes (0.28 A g−1, 47 F g−1)32, Mesoporous NiO (165 F g−1)33, NiO with ordered mesoporous structure (120 F g−1)34, but lower than NiO flowers35 (1 A g−1, 710 F g−1) and other supercapacitor materials36,37.

Specific energy and specific power are the two key factors for evaluating the power applications of electrochemical supercapacitors. A good electrochemical supercapacitor is expected to provide both high energy density and specific capacitance. Fig. 4d shows the Ragone plot for NiS 2 nanostructured electrodes in 3.0 M KOH aqueous solution. For NiS 2 nanocubes electrodes, it also has well specific energy and specific power. The specific energy of NiS 2 nanocube electrodes decreases from 15.7 to 3.6 Wh kg−1, while the specific power increases from 254 to 2537 W kg−1 as the galvanostatic charge/discharge current density increases from 1.25 to 12.5 A g−1. As a comparison, NiS 2 nanoparticle electrodes have very small specific energy (changing from 4.3 to 0.8 Wh kg−1).

It is important for electrode materials to have good specific capacitance retention. Supercapacitors should work steadily and safely, which requires the specific capacitance of electrode materials to change as little as possible. Relationships of the specific capacitance against the cycling number of NiS 2 nanostructured materials are shown in Fig. 4e. It shows its excellent specific capacitance retention under 1.25 A g−1. After 200 continuous charge–discharge cycles, NiS 2 nanocubes electrodes almost retain the same specific capacitance as its initial value. More importantly, NiS 2 nanocubes electrodes still retain more than 93.4% of its specific capacitance after 3000 continuous charge–discharge cycles, while the specific capacitance of NiS 2 nanoparticles has decreased nearly to zero.

To identify the exact electrical conductivity of electrodes, we measured EIS spectrum of NiS 2 nanostructure electrodes at room temperature in the frequency range from 0.01 to 105 Hz under open-circuit conditions, which is shown in Fig. 6a. An equivalent circuit used to fit the impedance curve is given in Fig. 6b, which is similar to the circuit employed for the working electrode of supercapacitors. The EIS data can be fitted by a bulk solution resistance R s , a charge-transfer R ct and a pseudocapacitive element C p from the redox process of electrode materials and a CPE to account for the double-layer capacitance. The charge-transfer resistance R ct of all the samples was calculated by ZSimpWin software. And from the calculated results, we found that NiS 2 nanocube electrodes have a lowest value 2.7 Ω. And the detailed R ct values of nanosphere and nanoparticle electrodes are 5.9 Ω and 17.2 Ω respectively. This clearly demonstrates the reduced charge-transfer resistance of the NiS 2 nanocubes electrodes. In addition, the charge-transfer resistance R ct , also called Faraday resistance, is a limiting factor for the specific power of the supercapacitor. It is the low Faraday resistance that results in the high specific power of NiS 2 nanocube electrodes. This cubic structure surface-interface character might also decrease the polarization of the electrode and thus might increase the capacity.

Figure 6 (a) Electrochemical impedance spectra (EIS) for NiS 2 nanostructured electrodes under room temperature in 3.0 M KOH solutions; (b) An equivalent circuit an consisting of a bulk solution resistance R s , a charge-transfer R ct , a pseudocapacitive element C p from redox process of NiS 2 nanomaterials and a constant phase element (CPE) to account for the double-layer capacitance. Full size image

The as-prepared NiS 2 nanomaterials (nanocubes, nanospheres and nanoparticles) are thus evaluated as a cocatalytic enhancing photocatalyst for H 2 production. Fig. 7 illustrates a possible mechanism of H 2 production on the as-prepared NiS 2 nanomaterials. Photons are captured by the ErY for activation of the electrons from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). The activated election is subsequently adopted by the NiS 2 catalyst with suitable Ni 3d and S 3p hybrid orbital. Nickel with a reduced valence state facilitates the proton reduction for the formation of H 2 gas. At the same time, the oxidized ErY is reduced to its ground state by TEOA as the electron donor. Fig. 8 shows the time course of H 2 evolution for different NiS 2 nanomaterials. In our system, in the first 8 hours, 3210 μmol of H 2 gas is generated under the irradiation of visible light (λ > 420 nm) for the NiS 2 nanospheres, while that of nanocubes and nanoparticles is 1980 and 2220 μmol. More importantly, after 12 hours, the total H 2 gas of NiS 2 nanospheres is up to 3400 μmol with an average H 2 production rate of 283 μmol h−1. A high H 2 production rate of 5.66 mmol h−1 g−1 and such H 2 production performance is superior to those reported previously12,38,39. What is more, an apparent quantum efficiency of 13.6% at 420 nm is measured for NiS 2 nanospheres, while that of NiS 2 nanocubes and nanoparticles is 10.8% and 9.6%. On the contrary, only 1980 μmol (NiS 2 nancubes) and 2220 μmol (NiS 2 nanoparticles) of H 2 gas is collected in 12 hours with an average H 2 production rate of 3.30 (NiS 2 nanocubes) and 3.70 mmol h−1 g−1 (NiS 2 nanoparticles). This clearly indicates the significantly improved photocatalytic activity of the NiS 2 nanospheres in cocatalytic enhancing photocatalytic H 2 production. The cocatalytic enhancing photocatalytic performance may be related with the active surface of the defect of NiS 2 crystals. Some perfect NiS 2 crystals can improve the transformation of electrons and the protons are reduced to generate H 2 on some defect NiS 2 crystals which enhances photocatalytic activities.

Figure 7 Schematic illustration of cocatalytic enhancing photocatalytic H 2 production over NiS 2 nanomaterials. Full size image

Figure 8 Time course of cocatalytic enhancing photocatalytic H 2 evolution for as-prepared NiS 2 nanomaterials. Full size image

However, the activity decreases as the irradiation is prolonged, which may be caused by the degradation of the ErY and triethanolamine (TEOA) for this activity loss40,41. More importantly, the variation of the reaction mixture solution (ErY in aqueous triethanolamine (TEOA) solution) with/without NiS 2 catalyst has been monitored upon irradiation (Figure S6) for proving the above conclusion. The maximum absorption of the reaction mixture solution (ErY in aqueous TEOA solution, pH = 8.5) is at around 520 nm. In Figure S6, it is seen that the reaction mixture solution in the presence of NiS 2 is much more stable (Figure S6a) compared to that without the catalyst (Figure S6b). The reaction mixture solution goes through rapid decomposition in the absence of electron acceptor when there is light irradiation. However, the decomposition of the reaction mixture solution with NiS 2 nanostructures exists and their decomposition rates after the 12 h irradiation are about 38%-NiS 2 nanocubes, 34%-NiS 2 nanospheres and 37%-NiS 2 nanoparticles, respectively.

The inorganic solid catalyst hybrid H 2 production system lies in the recycle ability of the solid catalyst. After the test for 12 h, the NiS 2 nanostructures was collected via a simple centrifugation and re-dispersed in the same fresh TEOA solution with ErY dye for the second round of testing. As shown in Fig. S7, 2500 μmol of H 2 gas for NiS 2 nanospheres was collected in 12 h, which is over 73.5% of the amount obtained in their first round. That the activity deceased is possibly due to the damage of NiS 2 nanospheres and the damage of their active surface under long time irradiation. Therefore, SEM images of the 2th circle run as indicated in Figure S7 are shown in Figure S8. NiS 2 nanostructures are still nanocubes, nanospheres and nanoparticles. However, NiS 2 nanocubes are not uniform as the before, the surface of NiS 2 nanospheres becomes smoothly and NiS 2 nanoparticles form some aggregations after prolonged light irradiation. The dropping of activity may be also caused due to the change of NiS 2 morphologies.

This excellent H 2 production performance of the NiS 2 nanospheres may be caused by many factors. NiS 2 nanospheres have rough surfaces with high surface area, which largely facilitates the efficient transfer of photogenerated electrons to improve the photocatalytic efficiency. Ion diffusion and electron transport both play an important role in the performance of supercapacitors and enhancing-cocatalytic photocatalysis. Usually, photocatalytic reaction generally occurs on the surface of nanomaterials. Under such a condition, the electron does not need to diffuse into the inner of these materials. These materials with rough surfaces have large surface area structures and always show better in cocatalytic enhancing photocatalysis due to effective surface photocatalytic reaction. This might be the reason for the highest cocatalytic enhancing photocatalytic activities of NiS 2 nanospheres with highest surface area, while the cocatalytic enhancing photocatalytic activities of nanoparticles are better than those of nanocubes.

But the resistance, arising from the inherent low electronic conductivity of active materials and the boundary interfaces among active material particles, would much limit the high power performance of supercapacitors. Moreover, the surface reaction of electrode materials largely limits its electrochemical activities. The electronic transport in a nanosized material electrode is shown schematically in Fig. 9, where some carbon additive is employed to improve the conductivity of total electrode. In some cases, small sized nanocrystal materials cannot effectively shorten the path length for electronic transport. Nanoparticles with a very high specific surface area and high surface energy (such as the as-prepared NiS 2 nanoparticles, NiS 2 nanospheres) are difficult to disperse and mix with a carbon additive. Accordingly, the electronic transport length is still very long because only a small number of nanoparticles can directly contact the carbon additive and obtain electrons (such as NiS 2 nanoparticle, nanosphere). Furthermore, the large interface resistance of NiS 2 nanoparticles and nanospheres still exists, especially when the unit size of the particle is within a typical nano-scale, which is the reason that NiS 2 nanoparticle electrodes and NiS 2 nanosphere electrodes have large R ct and low specific capacitance.

Figure 9 Schematic representation showing the electronic transport length in nanoparticles based electrode. Full size image

However, different supercapabilities of NiS 2 nanostructured electrodes should be attributed to many different reasons. The most important reason must be associated with easier paths for ions, electrons and electrolytes. The as-prepared single crystal NiS 2 nanocubic structure may have novel surface-interface characters and good electrical conductivity for electrochemical charge-discharge process. These novel chemical-physical characters bring the novel surface-interface and diffusion paths, leading to high electrochemical activities.