Abstract:Using WO3 as a model material, we investigate how different oxygen vacancy concentrations affect trapping of photogenerated charges and photocatalytic reactions in metal oxides.
“…This latter assignment stems from two previous observations. Firstly, outside the SCL (or in the absence of one), photogenerated holes rapidly trap into occupied oxygen vacancy states, resulting in a bleached signal over the W 5+ absorption range, 17 consistent with the rapid initial decay at 700 nm in Figure 2a indicative of a loss of electron absorption. Secondly, catalytically active holes that absorb at 500 nm have been shown to oxidise water on slower timescales, thereby indicating that this absorption arises from holes localised within the depletion layer.…”
supporting
confidence: 59%
“…[14][15] These defects manifest as reduced tungsten centres neighbouring sites absent in oxygen, and thus give rise to a characteristic broad W 5+ absorption at red wavelengths, rising into the near IR. [16][17] There is ongoing discussion in the literature regarding the effects of these states on charge transport and catalysis, with proponents claiming improved conductivity and critics proposing increased trap-mediated recombination. [18][19][20][21][22][23] These states lie energetically below the conduction band edge, with a fairly large energetic distribution.…”
mentioning
confidence: 99%
“…[18][19][20][21][22][23] These states lie energetically below the conduction band edge, with a fairly large energetic distribution. 17,24 Recent studies of ultrafast kinetics in WO3 have shown that the presence of large numbers of such occupied vacancy states results in rapid trapping of photogenerated holes (<0.2 ps after excitation), whereas electrons may trap into unoccupied vacancy states that lie above the Fermi level. 17 Modulation of the occupancy of these states through band-bending has been demonstrated to significantly affect electron extraction timescales, 24 but the relationship between the electronic occupancy of these defect states, the degree of band-bending and the ultrafast dynamics of photogenerated charges has yet to be directly probed.…”
In metal oxide-based photoelectrochemical devices, the spatial separation of photogenerated electrons and holes is typically attributed to band-bending at the oxide/electrolyte interface. However, direct evidence of such band-bending impacting upon charge carrier lifetimes has been very limited to date. Herein we use ultrafast spectroscopy to track the rapid relaxation of holes in the space-charge layer and their recombination with trapped electrons in WO 3 photoanodes. We observe that applied bias can significantly increase carrier lifetimes on all time scales from picoseconds to seconds and attribute this to enhanced band-bending correlated with changes in oxygen vacancy state occupancy. We show that analogous enhancements in carrier lifetimes can be obtained by changes in electrolyte composition, even in the absence of applied bias, highlighting routes to improve photoconversion yields/performance, through changes in band-bending. This study thus demonstrates the direct connection between carrier lifetime enhancement, increased band-bending, and oxygen vacancy defect state occupancy.
“…This latter assignment stems from two previous observations. Firstly, outside the SCL (or in the absence of one), photogenerated holes rapidly trap into occupied oxygen vacancy states, resulting in a bleached signal over the W 5+ absorption range, 17 consistent with the rapid initial decay at 700 nm in Figure 2a indicative of a loss of electron absorption. Secondly, catalytically active holes that absorb at 500 nm have been shown to oxidise water on slower timescales, thereby indicating that this absorption arises from holes localised within the depletion layer.…”
supporting
confidence: 59%
“…[14][15] These defects manifest as reduced tungsten centres neighbouring sites absent in oxygen, and thus give rise to a characteristic broad W 5+ absorption at red wavelengths, rising into the near IR. [16][17] There is ongoing discussion in the literature regarding the effects of these states on charge transport and catalysis, with proponents claiming improved conductivity and critics proposing increased trap-mediated recombination. [18][19][20][21][22][23] These states lie energetically below the conduction band edge, with a fairly large energetic distribution.…”
mentioning
confidence: 99%
“…[18][19][20][21][22][23] These states lie energetically below the conduction band edge, with a fairly large energetic distribution. 17,24 Recent studies of ultrafast kinetics in WO3 have shown that the presence of large numbers of such occupied vacancy states results in rapid trapping of photogenerated holes (<0.2 ps after excitation), whereas electrons may trap into unoccupied vacancy states that lie above the Fermi level. 17 Modulation of the occupancy of these states through band-bending has been demonstrated to significantly affect electron extraction timescales, 24 but the relationship between the electronic occupancy of these defect states, the degree of band-bending and the ultrafast dynamics of photogenerated charges has yet to be directly probed.…”
In metal oxide-based photoelectrochemical devices, the spatial separation of photogenerated electrons and holes is typically attributed to band-bending at the oxide/electrolyte interface. However, direct evidence of such band-bending impacting upon charge carrier lifetimes has been very limited to date. Herein we use ultrafast spectroscopy to track the rapid relaxation of holes in the space-charge layer and their recombination with trapped electrons in WO 3 photoanodes. We observe that applied bias can significantly increase carrier lifetimes on all time scales from picoseconds to seconds and attribute this to enhanced band-bending correlated with changes in oxygen vacancy state occupancy. We show that analogous enhancements in carrier lifetimes can be obtained by changes in electrolyte composition, even in the absence of applied bias, highlighting routes to improve photoconversion yields/performance, through changes in band-bending. This study thus demonstrates the direct connection between carrier lifetime enhancement, increased band-bending, and oxygen vacancy defect state occupancy.
“…For the NiFe(OH) x modified homojunction, the bleach amplitude at 580 nm decreases dramatically relative to the bare homojunction photoanode, which suggests the large decrease of trapped photoelectrons by passivation of surface electron trap states 54 . On the other hand, the absorption amplitude at 694.3 nm also slightly decreases and the half-lifetime decreases to 0.19 ms due to the reduction of electron trap states, resulting from the increase of direct recombination 55 or the charge transfer from homojunction to NiFe(OH) x 56 . Fig.…”
Hematite has a great potential as a photoanode for photoelectrochemical (PEC) water splitting by converting solar energy into hydrogen fuels, but the solar-to-hydrogen conversion efficiency of state-of-the-art hematite photoelectrodes are still far below the values required for practical hydrogen production. Here, we report a core-shell formation of gradient tantalum-doped hematite homojunction nanorods by combination of hydrothermal regrowth strategy and hybrid microwave annealing, which enhances the photocurrent density and reduces the turn-on voltage simultaneously. The unusual bi-functional effects originate from the passivation of the surface states and intrinsic built-in electric field by the homojunction formation. The additional driving force provided by the field can effectively suppress charge–carrier recombination both in the bulk and on the surface of hematite, especially at lower potentials. Moreover, the synthesized homojunction shows a remarkable synergy with NiFe(OH)x cocatalyst with significant additional improvements of photocurrent density and cathodic shift of turn-on voltage. The work has nicely demonstrated multiple collaborative strategies of gradient doping, homojunction formation, and cocatalyst modification, and the concept could shed light on designing and constructing the efficient nanostructures of semiconductor photoelectrodes in the field of solar energy conversion.
“…It has been reported that for a ratio smaller than 2.00, the surface presents oxygen vacancies 50 . The presence of oxygen vacancies generates defects able to act as a trap for holes 51 . In consequence, the holes and excited electrons recombination is limited, implying an improved charge transfer.…”
There is an increased interest in recycling valuable waste materials for usage in procedures with high added values. Silica microparticles are involved in the processes of catalysis, separation, immobilization of complexants, biologically active compounds, and different nanospecies, responding to restrictive requirements for selectivity of various chemical and biochemical processes. This paper presents the surface modification of accessible and dimensionally controlled recycled silica microfiber with titanium dioxide. Strong base species in organic solvents: methoxide, ethoxide, propoxide, and potassium butoxide in corresponding alcohol, activated the glass microfibres with 12-13 µm diameter. In the photo-oxidation process of a toxic micro-pollutant, cyclophosphamide, the new composite material successfully proved photocatalytic effectiveness. The present work fulfills simultaneously two specific objectives related to the efforts directed towards a sustainable environment and circular economy: recycling of optical glass microfibers resulted as waste from the industry, and their usage for the photooxidation of highly toxic emerging micro-pollutants. Inorganic-inorganic oxide-type composite materials offer the most interesting technical solutions in many physicochemical processes of bio-degradation, coating, separation, or catalysis 1,2. A particular case is presented by the silicon dioxide-titanium dioxide (TiO 2-SiO 2) composites that combine the distinctive characteristics of the two oxides. Thus, silicon dioxide is a technically-economically accessible material and can be made in the form of spherical particles 3 , nanofibers 4 , nanotubes 5 , or films 6 , thus being used as an active material, adsorbent or support for other materials with higher selectivity 7. Titanium dioxide prepared in predetermined forms: spheres 8 , nanotubes 9 , nano-threads 10 , or films 11 could be used as an adsorbent or covering, but especially like catalytic material 12. Of course, the combination of the two oxides' properties has been used for the most advanced applications: photocatalytic degradation of organic/pharmaceutical pollutants 13 , fuel cells 14 , membrane reactors 15 , bioreactors 16 , advanced separations 17 and clinical devices 18. Environmental researches proved that the widely used cytostatic drug, N, N-bis(2-chloroethyl)-1,3,2-oxazaphosphinan-2-amine 2-oxide known as cyclophosphamide, which is a cyclic amide, produces residues with mutagenic action. Although mainly known as an anticancer drug, cyclophosphamide is also administrated frequently as an anti-inflammatory or immunolytic agent for rheumatoid arthritis 19 , or nephrotic syndrome 20 , systemic lupus erythematosus 21 , or systemic sclerosis lung fibrosis 22. This important cytostatic compound administrated for autoimmune diseases, and cancer chemotherapy, as well as its metabolites, are released into effluents that reach surface and ground waters 23. The cyclophosphamide action through nucleophilic compounds' alkylation and metabolic activation result in dra...
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