Abstract
Photoelectrochemical (PEC) systems represent powerful tools to convert electromagnetic radiation into chemical fuels and electricity. In this context, two-dimensional (2D) materials are attracting enormous interest as potential advanced photo(electro)catalysts and, recently, 2D group-IVA metal monochalcogenides have been theoretically predicted to be water splitting photocatalysts. In this work, we use density functional theory calculations to theoretically investigate the photocatalytic activity of single-/few-layer GeSe nanoflakes for both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) in pH conditions ranging from 0 to 14. Our simulations show that GeSe nanoflakes with different thickness can be mixed in the form of nanoporous films to act as nanoscale tandem systems, in which the flakes, depending on their thickness, can operate as HER- and/or OER photocatalysts. On the basis of theoretical predictions, we report the first experimental characterization of the photo(electro)catalytic activity of single-/few-layer GeSe flakes in different aqueous media, ranging from acidic to alkaline solutions: 0.5 M H2SO4 (pH 0.3), 1 M KCl (pH 6.5), and 1 M KOH (pH 14). The films of the GeSe nanoflakes are fabricated by spray coating GeSe nanoflakes dispersion in 2-propanol obtained through liquid-phase exfoliation of synthesized orthorhombic (Pnma) GeSe bulk crystals. The PEC properties of the GeSe nanoflakes are used to design PEC-type photodetectors, reaching a responsivity of up to 0.32 AW–1 (external quantum efficiency of 86.3%) under 455 nm excitation wavelength in acidic electrolyte. The obtained performances are superior to those of several self-powered and low-voltage solution-processed photodetectors, approaching that of self-powered commercial UV–Vis photodetectors. The obtained results inspire the use of 2D GeSe in proof-of-concept water photoelectrolysis cells.
Highlights
We reveal that germanium selenide (GeSe) nanoflakes with different thicknesses in nanoporous electrodes can act as different light absorbers in nanoscale tandem systems, mimicking photosynthetic systems,[154,155] by creating monolithic “allsolid-state Z-scheme water splitting pathways”.156−159 These expectations are experimentally proven on photoelectrodes fabricated through the spray-coating of single- and few-layer GeSe flakes, which are produced through the liquid-phase exfoliation (LPE) of a synthetized “black-phase” GeSe crystal in an environmentally friendly solvent
These attributes are further supported by the plots of Understanding of Structural, Optoelectronic, and Catalytic Properties of the GeSe Nanoflakes
The work function (WF) evolution of xL-GeSe with the number of layers was elucidated through density functional theory (DFT) calculations, showing WF values from 2.2 eV for 1L-GeSe to 5.3 eV for 6L-GeSe, and water splitting
Summary
The conversion of light energy into chemical fuels and electricity through photoelectrochemical (PEC) cells represents a powerful strategy for sustainable fuel and chemical generation,[1−4] environmental remediation (i.e., pollutant degradation),[5−7] advanced analytical systems (i.e., chemical sensors) for environmental[8,9] and biological monitoring,[9−11] as well as innovative self-powered photodetectors.[12,13] In particular, PEC water splitting is envisioned to produce molecular hydrogen (H2),[14,15] seen as an ideal energy carrier for the storage and distribution of solar energy in the so-called “Hydrogen economy”.16,17 In addition, aqueous PEC cells, including water splitting ones, are emerging for the development of inexpensive, fabricated, environmentally friendly self-powered photodetectors with high spectral responsivity (>tens of mA W−1 in UV−visible spectral region),[12,18−20] fast response (in the order of tens of ms)[12,19,21] and satisfactory sensitivity (typically in the order of 10).[12,19,22] To achieve efficient PEC systems, it is necessary to develop photocatalytic materials that efficiently absorb light in the desired spectral range (UV−visible for solar energy conversion systems),[23] creating free charge carriers with suitable energies to accomplish the targeted oxidation−reduction (redox) reactions before they recombine.[23−25]. Aqueous PEC cells, including water splitting ones, are emerging for the development of inexpensive, fabricated, environmentally friendly self-powered photodetectors with high spectral responsivity (>tens of mA W−1 in UV−visible spectral region),[12,18−20] fast response (in the order of tens of ms)[12,19,21] and satisfactory sensitivity (typically in the order of 10).[12,19,22] To achieve efficient PEC systems, it is necessary to develop photocatalytic materials that efficiently absorb light in the desired spectral range (UV−visible for solar energy conversion systems),[23] creating free charge carriers with suitable energies to accomplish the targeted oxidation−reduction (redox) reactions before they recombine.[23−25] In this context, two-dimensional (2D) materials, including either single- or few-layer flake forms, are attracting ultimate interest as potential advanced photo(electro)catalysts.[26−29] Such attention mainly relies on their large surface-to-volume ratio, which guarantees that the charge carriers, i.e., electrons and holes, are directly photogenerated at the interface with the electrolyte, in which redox reactions take place before charge recombination.[26−29] Both theoretical and experimental works.
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