Is NiCo₂S₄ Really a Semiconductor?

Abstract

NiCo 2 S 4 is a technologically important electrode material that has recently achieved remarkable performance in pseudocapacitor, catalysis, and dye-synthesized solar cell applications. 1−5 Essentially, all reports on this material have presumed it to be semiconducting, like many of the chalcogenides, with a reported band gap in the range of 1.2−1.7 eV. 6,7 In this report, we have conducted detailed experimental and theoretical studies, most of which done for the first time, which overwhelmingly show that NiCo 2 S 4 is in fact a metal. We have also calculated the Raman spectrum of this material and experimentally verified it for the first time, hence clarifying inconsistent Raman spectra reports. Some of the key results that support our conclusions include: (1) the measured carrier density in NiCo 2 S 4 is 3.18 × 10 22 cm −3 , (2) NiCo 2 S 4 has a room temperature resistivity of around 10 3 μΩ cm which increases with temperature, (3) NiCo 2 S 4 exhibits a quadratic dependence of the magnetoresistance on magnetic field, (4) thermopower measurements show an extremely low Seebeck coefficient of 5 μV K −1 , (5) first-principles calculations confirm that NiCo 2 S 4 is a metal. These results suggest that it is time to rethink the presumed semiconducting nature of this promising material. They also suggest that the metallic conductivity is another reason (besides the known significant redox activity) behind the excellent performance reported for this material.

CoNi2S4‐Graphene‐2D‐MoSe2 as an Advanced Electrode Material for Supercapacitors

“…Typically, A 1g (375 cm −1 ), E g (240 cm −1 ), T 2g,1 (342 cm −1 ), T 2g,2 (300 cm −1 ), and . [23] Compared with CoNi 2 S 4 -G, weak peaks of MoSe 2 (shown as blue circles) can be observed, corresponding to A 1g (240 cm −1 ), E 2g (285 cm −1 ), and B 1g (352 cm −1 ) vibration modes. [24] Since the positions of A 1g peak of MoSe 2 and E g peak of CoNi 2 S 4 are overlapped with each other in the same field, while with little amount of MoSe 2 and the weak intensity of E 2g and B 1g of MoSe 2 (as seen in Figure S4b, Supporting Information), these peaks are hard to be detected.…”

Capacitance Performance of Nanostructured CoNi2S4 with Different Morphology Grown on Carbon Cloth for Supercapacitors

“…More recently, NiCo 2 S 4 has been reported to exhibit excellent performance owing to its metallic conductivity and significant redox activity . Various attempts have been made to further improve the electrochemical performance of NiCo 2 S 4 ‐based electrodes.…”

Low temperature fabrication of hydrangea-like NiCo2S4 as electrode materials for high performance supercapacitors

“…In addition, the nonlinear GCD curves demonstrate their typical Faradic reactions which are in good agreement with the CV curves. The specific capacitance C m (F g −1 ) can be calculated from the following formula [10]: (1) Where I is the applied discharge current (A), Δt is the discharge time (s), m is the mass of the active material (g), ΔV is the potential drop during discharge (V). Thus, the specific capacitance of NiCo 2 S 4 can be calculated to be 1475, 1420, 1356, 1332, 1243 and 1152 F g ) which shows the potential in high rate application.…”

“…Compared with binary metal oxides/sulfides, ternary metal oxides/sulfides exhibit much higher specific capacitance owing to the rich redox reaction sites of their mixed valence and the synergistic effect of different cations [8]. Very recently, NiCo 2 S 4 attracts much interest since its electrical conductivity is nearly 100 times higher than that of NiCo 2 O 4 , which has a significant impact on its excellent electrochemistry performance [9] and [10].…”