Mn-doped GaN as photoelectrodes for the photoelectrolysis of water under visible light

Abstract

Hydrogen generation through direct photoelectrolysis of water was studied using photoelectrochemical (PEC) cells made of Mn-doped GaN photoelectrodes. In addition to its absorption of the ultraviolet spectrum, Mn-doped GaN photoelectrodes could absorb photons in the visible spectrum. The photocurrents measured from PEC cells made of Mn-doped GaN were at least one order higher than those measured from PEC cells made of undoped GaN-working electrodes. Under the visible light illumination and a bias voltage below 1.2 V, the Mn-doped GaN photoelectrodes could drive the water splitting reaction for hydrogen generation. However, hydrogen generation could not be achieved under the same condition wherein undoped GaN photoelectrodes were used. According to the results of the spectral responses and transmission spectra obtained from the experimental photoelectrodes, the enhanced photocurrent in the Mn-doped GaN photoelectrodes, compared with the undoped GaN photoelectrodes, was attributable to the Mn-related intermediate band within the band gap of GaN that resulted in further photon absorption.

Photoelectrolysis Using Type-II Semiconductor Heterojunctions

“…Of course, it is difficult to state any conclusion with certainty from these plots, and instead they serve to highlight the complexities in accurately modelling band bending in photoelectrolytic systems. We speculate that, as water splitting has been demonstrated using GaN as an electrode 24 , 25 , χ lies toward the higher of the literature values, but not large enough to push the conduction band edge lower than the hydrogen-production potential; a value of a little less than 4 eV seems reasonable. Here we shall use the average value from the literature of 3.4 ± 0.5 eV.…”

“…However, given only a small overpotential, of 0.050 V, is required for hydrogen production, the system could still plausibly split water. Furthermore, that bulk n-GaN has been shown to successfully split water in a PEC 24 , 25 highlights the likeliness that, at least for x = 0, E ox is sufficiently higher in energy than ε h,0 . Of course, a higher electron affinity would be beneficial for oxygen production, meaning the confined hole states would be at lower energies.…”

“…4a for a pH of 7, which also includes an error of ±0.2 V for the electrode–vacuum potential conversions (again due to disagreement in literature values 20 – 23 ). For smaller electron affinities, ϕ B becomes negative, which seems unlikely given that water splitting has been experimentally demonstrated using GaN 24 , 25 . Indeed, a value of ϕ B ≈ 0.8 eV has been reported for various alkaline solutions 26 .…”

Green Synthetic Fuels: Renewable Routes for the Conversion of Non-Fossil Feedstocks into Gaseous Fuels and Their End Uses

“…The photocatalyst efficiency could be enhanced with the introduction of transition-metal cations (W [340], Cr [341], Zr [342], Mn [343][344][345], Fe [346], Ni [341,347,348], Mo [340,349,350]) or anions (N [351], C [352], S [353,354], B [355]). Doping ions are functional in the modification of electronic and optical properties of semiconductors and improve photoelectrical conductivity and charge-carriers transport and separation [356].…”

Carrier dynamics of Mn-induced states in GaN thin films

“…Recently, significant increasing of experimental efforts 11–14 have been devoted to improving the conversion efficiency of solar cells by using IBs in materials such as quantum dots 15–18 and chalcopyrites 19–21 . Manganese was also doped into GaN to induce extra states for absorption of visible light, and the conversion efficiency of solar cells based on Mn-doped GaN was greatly improved 22–25 . Compared with tandem cell structure, the practical conversion efficiency for single material with IB is still low.…”