New class of photocatalytic materials and a novel principle for efficient water splitting under infrared and visible light: MgB_2 as unexpected example

Abstract

Water splitting is unanimously recognized as environment friendly, potentially low cost and renewable energy solution based on the future hydrogen economy. Especially appealing is photocatalytic water splitting whereby a suitably chosen catalyst dramatically improves efficiency of the hydrogen production driven by direct sunlight and allows it to happen even at zero driving potential. Here, we suggest a new class of stable photocatalysts and the corresponding principle for catalytic water splitting in which infrared and visible light play the main role in producing the photocurrent and hydrogen. The new class of catalystsionic or covalent binary metals with layered graphite-like structures -effectively absorb visible and infrared light facilitating the reaction of water splitting, suppress the inverse reaction of ion recombination by separating ions due to internal electric fields existing near alternating layers, provide the sites for ion trapping of both polarities, and finally deliver the electrons and holes required to generate hydrogen and oxygen gases. As an example, we demonstrate conversion efficiency of ~27% at bias voltage V bias =0.5V for magnesium diboride working as a catalyst for photoinduced water splitting. We discuss its advantages over some existing materials and propose the underlying mechanism of photocatalytic water splitting by binary layered metals.

Water and Seawater Splitting with MgB2 Plasmonic Metal-Based Photocatalyst

“…The experimental studies and theoretical modelling demonstrate that Mg-and B-terminated states in the multi-layered MgB 2 nanosheets are responsible for the excitation of LSPRs and tunable band gaps in the surface states. 33,34,48 Our modelling of A p , R p and T p (Fig. S3) showed that assemblies of MgB 2 nanosheets exhibit evidence of a direct plasmonic absorption, which is distinctly different from previously reported metallic-like interband transitions.…”

“…A schematic model of a plasmonic photocatalytic mechanism of water splitting with non-noble metals is presented in Figure 5. As was shown in our previous works, 33,34 electron-hole pairs in MgB 2 nanostructures are created by interband transitions between π and σ bands due to the light absorption. This transition originated from a van-Hove singularity in the electronic structure of MgB 2 and is separated by the energy of  2.0-2.6 eV.…”

“…In our previous works we have shown that at the surface properties of MgB 2 nanostructures can be actively involve into the process of water splitting in photoelectochemical cells (PECs). 33,34 In this study, we demonstrate that interband damping of LSPRs is also an important channel for the formation of e --h + pairs. These e --h+ pairs are separated over two bands (namely, between the 𝜋 and  bands) have a longer lifetime and are thereby expected to harvest sun energy more efficiently and contribute to the generation of H 2 together with direct electron injection mechanism of hot electrons generated via LSPRs.…”

“…The conversion efficiency has been calculated using bubble microscopy. 38 These measurements confirmed 0.95% Faraday efficiency (each two electrons generated 0.95 molecules H 2 33 ). Note that the decreasing of currents on longer timescales can be also caused by heating of solution during solar illumination.…”

Plasmon-induced nanoscale quantised conductance filaments

“…Recently, plasmon-induced phenomena 6, 7 , particularly in hybrid plasmonic systems 8 , have attracted considerable attention. Plasmon-induced hot carriers could ensure progress in photovoltaics 6 , fast electronics and photo-chemistry 7 , as well as catalytic reactions 9 including those required for green energy production 10 . At the same time, relatively little research has been conducted on plasmon-induced electrochemistry which can be very useful for creation of resistive random access memory (RRAM) devices.…”