Gated Hall effect of nanoplate devices reveals surface-state-induced surface inversion in iron pyrite semiconductor.

Understanding semiconductor surface states is critical for their applications, but fully characterizing surface electrical properties is challenging. Such a challenge is especially crippling for semiconducting iron pyrite (FeS2), whose potential for solar energy conversion has been suggested to be held back by rich surface states. Here, by taking advantage of the high surface-to-bulk ratio in nanostructures and effective electrolyte gating, we develop a general method to fully characterize both the surface inversion and bulk electrical transport properties for the first time through electrolyte-gated Hall measurements of pyrite nanoplate devices. Our study shows that pyrite is n-type in the bulk and p-type near the surface due to strong inversion and yields the concentrations and mobilities of both bulk electrons and surface holes. Further, solutions of the Poisson equation reveal a high-density of surface holes accumulated in a 1.3 nm thick strong inversion layer and an upward band bending of 0.9-1.0 eV. This work presents a general methodology for using transport measurements of nanostructures to study both bulk and surface transport properties of semiconductors. It also suggests that high-density of surface states are present on surface of pyrite, which partially explains the universal p-type conductivity and lack of photovoltage in polycrystalline pyrite.

High-performance electrocatalysis using metallic cobalt pyrite (CoS2) micro- and nanostructures.

The development of efficient and robust earth-abundant electrocatalysts for the hydrogen evolution reaction (HER) is an ongoing challenge. We report metallic cobalt pyrite (cobalt disulfide, CoS2) as one such high-activity candidate material and demonstrate that its specific morphology--film, microwire, or nanowire, made available through controlled synthesis--plays a crucial role in determining its overall catalytic efficacy. The increase in effective electrode surface area that accompanies CoS2 micro- and nanostructuring substantially boosts its HER catalytic performance, with CoS2 nanowire electrodes achieving geometric current densities of -10 mA cm(-2) at overpotentials as low as -145 mV vs the reversible hydrogen electrode. Moreover, micro- and nanostructuring of the CoS2 material has the synergistic effect of increasing its operational stability, cyclability, and maximum achievable rate of hydrogen generation by promoting the release of evolved gas bubbles from the electrode surface. The benefits of catalyst micro- and nanostructuring are further demonstrated by the increased electrocatalytic activity of CoS2 nanowire electrodes over planar film electrodes toward polysulfide and triiodide reduction, which suggests a straightforward way to improve the performance of quantum dot- and dye-sensitized solar cells, respectively. Extension of this micro- and nanostructuring strategy to other earth-abundant materials could similarly enable inexpensive electrocatalysts that lack the high intrinsic activity of the noble metals.

Ionization of high-density deep donor defect states explains the low photovoltage of iron pyrite single crystals.

Iron pyrite (FeS2) is considered a promising earth-abundant semiconductor for solar energy conversion with the potential to achieve terawatt-scale deployment. However, despite extensive efforts and progress, the solar conversion efficiency of iron pyrite remains below 3%, primarily due to a low open circuit voltage (VOC). Here we report a comprehensive investigation on {100}-faceted n-type iron pyrite single crystals to understand its puzzling low VOC. We utilized electrical transport, optical spectroscopy, surface photovoltage, photoelectrochemical measurements in aqueous and acetonitrile electrolytes, UV and X-ray photoelectron spectroscopy, and Kelvin force microscopy to characterize the bulk and surface defect states and their influence on the semiconducting properties and solar conversion efficiency of iron pyrite single crystals. These insights were used to develop a circuit model analysis for the electrochemical impedance spectroscopy that allowed a complete characterization of the bulk and surface defect states and the construction of a detailed energy band diagram for iron pyrite crystals. A holistic evaluation revealed that the high-density of intrinsic surface states cannot satisfactorily explain the low photovoltage; instead, the ionization of high-density bulk deep donor states, likely resulting from bulk sulfur vacancies, creates a nonconstant charge distribution and a very narrow surface space charge region that limits the total barrier height, thus satisfactorily explaining the limited photovoltage and poor photoconversion efficiency of iron pyrite single crystals. These findings lead to suggestions to improve single crystal pyrite and nanocrystalline or polycrystalline pyrite films for successful solar applications.

Earth-Abundant Metal Pyrites (FeS2, CoS2, NiS2, and Their Alloys) for Highly Efficient Hydrogen Evolution and Polysulfide Reduction Electrocatalysis.

Many materials have been explored as potential hydrogen evolution reaction (HER) electrocatalysts to generate clean hydrogen fuel via water electrolysis, but none so far compete with the highly efficient and stable (but cost prohibitive) noble metals. Similarly, noble metals often excel as electrocatalytic counter electrode materials in regenerative liquid-junction photoelectrochemical solar cells, such as quantum dot-sensitized solar cells (QDSSCs) that employ the sulfide/polysulfide redox electrolyte as the hole mediator. Here, we systematically investigate thin films of the earth-abundant pyrite-phase transition metal disulfides (FeS2, CoS2, NiS2, and their alloys) as promising alternative electrocatalysts for both the HER and polysulfide reduction. Their electrocatalytic activity toward the HER is correlated to their composition and morphology. The emergent trends in their performance suggest that cobalt plays an important role in facilitating the HER, with CoS2 exhibiting highest overall performance. Additionally, we demonstrate the high activity of the transition metal pyrites toward polysulfide reduction and highlight the particularly high intrinsic activity of NiS2, which could enable improved QDSSC performance. Furthermore, structural disorder introduced by alloying different transition metal pyrites could increase their areal density of active sites for catalysis, leading to enhanced performance.

Synthesis, characterization, and variable range hopping transport of pyrite (FeS2) nanorods, nanobelts, and nanoplates.

We report the growth, structural, and electrical characterization of single-crystalline iron pyrite (FeS2) nanorods, nanobelts, and nanoplates synthesized via sulfidation reaction with iron dichloride (FeCl2) and iron dibromide (FeBr2). The as-synthesized products were confirmed to be single-crystal phase pure cubic iron pyrite using powder X-ray diffraction, Raman spectroscopy, and transmission electron microscopy. An intermediate reaction temperature of 425 °C or a high sulfur vapor pressure under high temperatures was found to be critical for the formation of phase pure pyrite. Field effect transport measurements showed that these pyrite nanostructures appear to behave as a moderately p-doped semiconductor with an average resistivity of 2.19 ± 1.21 Ω·cm, an improved hole mobility of 0.2 cm² V⁻¹ s⁻¹, and a lower carrier concentration on the order of 10¹8-10¹9 cm⁻³ compared with previous reported pyrite nanowires. Temperature-dependent electrical transport measurements reveal Mott variable range hopping transport in the temperature range 40-220 K and transport via thermal activation of carriers with an activation energy of 100 meV above room temperature (300-400 K). Most importantly, the transport properties of the pyrite nanodevices do not change if highly pure (99.999%) precursors are utilized, suggesting that the electrical transport is dominated by intrinsic defects in pyrite. These single-crystal pyrite nanostructures are nice platforms to further study the carrier conduction mechanisms, semiconductor defect physics, and surface properties in depth, toward improving the physical properties of pyrite for efficient solar energy conversion.