The mechanism of current-doubling reactions at ZnO photoanodes

Schoenmakers, G. H.; Vanmaekelbergh, D.; Kelly, J. J.

doi:10.1039/a606734d

Cited by 25 publications

(24 citation statements)

References 20 publications

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“… 11 , 23 , 31 This photocurrent doubling mechanism (where one photon generates two conduction band electrons) has been observed previously under low light intensity conditions on TiO 2 and ZnO. 11 , 32 , 33 However, we note that Schoenmakers et al 33 have reported a reduction in the quantum efficiency of methanol oxidation on ZnO from 2 to 1 when increasing the light intensity from ∼0.005 to 5 suns. The results we report here provide an explanation for Schoenmakers et al’s observation and indicate that under ∼1 sun irradiation conditions, where there is significant hole accumulation at the oxide surface, both steps of methanol oxidation are driven by photogenerated valence band holes.…”

Section: Discussionsupporting

confidence: 75%

Kinetics of Photoelectrochemical Oxidation of Methanol on Hematite Photoanodes

et al. 2017

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The kinetics of photoelectrochemical (PEC) oxidation of methanol, as a model organic substrate, on α-Fe2O3 photoanodes are studied using photoinduced absorption spectroscopy and transient photocurrent measurements. Methanol is oxidized on α-Fe2O3 to formaldehyde with near unity Faradaic efficiency. A rate law analysis under quasi-steady-state conditions of PEC methanol oxidation indicates that rate of reaction is second order in the density of surface holes on hematite and independent of the applied potential. Analogous data on anatase TiO2 photoanodes indicate similar second-order kinetics for methanol oxidation with a second-order rate constant 2 orders of magnitude higher than that on α-Fe2O3. Kinetic isotope effect studies determine that the rate constant for methanol oxidation on α-Fe2O3 is retarded ∼20-fold by H/D substitution. Employing these data, we propose a mechanism for methanol oxidation under 1 sun irradiation on these metal oxide surfaces and discuss the implications for the efficient PEC methanol oxidation to formaldehyde and concomitant hydrogen evolution.

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Section: Discussionsupporting

confidence: 75%

Kinetics of Photoelectrochemical Oxidation of Methanol on Hematite Photoanodes

et al. 2017

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“…They considered the most probable explanation was that alcohol adsorbs at the TiO 2 surface and blocks the sites that would otherwise act as recombination. A similar proposal was made by Schoenmakers, Vanmaekelbergh and Kelly [42] for the oxidation of methanol on single-crystal ZnO. In a study of photoelectrochemical oxidation of methanol in Na 2 SO 4 solution John and co-workers [43] conclude that the limiting step was interfacial charge transfer and that the concentration of hydroxyl ions (the precursors of hydroxyl radicals) did not influence photocurrents at pH 9.…”

Section: Small Electrodessupporting

confidence: 57%

Synthesis, characterization and activity of photocatalytic sol–gel TiO2 powders and electrodes

Marugán

Christensen

Egerton

et al. 2009

Applied Catalysis B: Environmental

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“…With pure ZnO as a top layer, it is furthermore assured that the conduction band edge is located energetically higher than the reduction level [43]. However, ZnO dissolves in most electrolytes making an anticorrosion layer indispensable [44]. Among other oxides, TiO 2 is known to be chemically stable, making it a potential candidate for a thin photoanode overlayer preventing corrosion of the underlying…”

Section: Oxides As Pec Electrodesmentioning

confidence: 99%

Deep level related photoluminescence in ZnMgO

Trunk

Venkatachalapathy

Galeckas

et al. 2010

Applied Physics Letters

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Zinc oxide (ZnO) has been used in a wide range of products for many years, including, among others, varistors, surface acoustic wave devices and cosmetics. Besides these established applications, ZnO and its ternary alloys are now also being considered as potential materials for optoelectronic applications, such as light emitting diodes, photovoltaics, sensors, displays, etc. Unlike other materials, which could be used alternatively, ZnO has the advantage of being inexpensive, chemically stable and relatively plentiful. In spite of the long research history, fabrication of defect free ternary alloys and stable p-type ZnO is still challenging. The aim of this work was therefore to provide a better understanding of ZnO ternary alloys, so that -based on the gained knowledge -their optical properties can be further improved and, in a second step, optoelectronic applications based on these materials can soon be commercialized. The work carried out in this thesis was two-fold: the first part aimed at identifying the origin of defect related luminescence phenomena in ZnMgO, and the second part was dedicated to the exploration of a novel ZnCdO-based heterostructure photovoltaic applications.In the case of ZnMgO, luminescence properties of deep level defects were studied by photoluminescence (PL) spectroscopy and a model was proposed to explain the changes in the deep band emission with increasing Mg content. In this model, the observed trends can be understood by considering interaction of native zinc and oxygen defects of the ZnO sublattice with Mg interstitials (Mg i ). In summary, the deep level bands at 3.0 and 2.8 eV, which show a blueshift with increasing Mg content, were assigned to free-to-bound type transitions between zinc interstitials (Zn i ) with the valence band edge and between the conduction band edge with zinc vacancies (V Zn ), respectively. A red band at 2.0 eV, which does not show an apparent shift of the peak energy for increasing Mg content, is associated with the oxygen vacancies (V O ). Two luminescence bands at 2.3 and 2.5 eV, which are redshifted for higher Mg concentrations, were assigned to transitions between zinc and oxygen interstitials and between zinc interstitials and zinc vacancies, respectively. The redshift is interpreted in terms of a competing supply of electrons from slightly deeper Mg i donor states. The ZnMgO band gap diagram, which the model is based on, has contributed to gain valuable information about the nature of the deep defects both in ZnO and ZnMgO and is therefore of fundamental interest.In the second part of this work, focused on ZnCdO, a stacked heterostructure was designed for iii iv Abstract application in a photoelectrochemical cell, which is used for hydrogen production by photoelectrolysis using the semiconductor as an absorber. Optical and photoelectrochemical measurements led to the conclusion that the optical emission band for the ZnCdO heterostructures is broadened compared to a ZnO single layer. The broadened emission could be explained by combined ...

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The mechanism of current-doubling reactions at ZnO photoanodes

Cited by 25 publications

References 20 publications

Kinetics of Photoelectrochemical Oxidation of Methanol on Hematite Photoanodes

Kinetics of Photoelectrochemical Oxidation of Methanol on Hematite Photoanodes

Synthesis, characterization and activity of photocatalytic sol–gel TiO2 powders and electrodes

Deep level related photoluminescence in ZnMgO

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