External Field Energy Regulates The Activity Of BiVO₄-based Photoanodes For Solar Water Oxidation

Hydrogen energy is a carbon neutral energy.Based on the catalytic properties of semiconductor materials,solar energy can be converted into hydrogen energy through photoelectrochemical water splitting.For water splitting,the oxidation of H₂O or OH^-intoO₂ on photoanodes involves complicated and slow 4-electron reaction,which is difficult to achieve efficiently both in thermodynamics and kinetics.At present,the search of effective photoanode materials and the scientific understanding their water splitting mechanism are the forefront of water splitting field.BiVO₄ is a typical bimetallic oxide semiconductor that be of suitable band gap（～2.4 eV）and valence band position（～2.4 V vs.RHE）,which can drive water oxidation.However,due to the high carrier recombination rate in the bulk and on the surface of BiVO₄ as well as sluggish kinetics of water oxidation on BiVO₄,the water oxidation activity and stability of pristine BiVO₄ photoanodes are lower than expected.In recent years,several modifying strategies have been developed to improve the water oxidation performance of BiVO₄ photoanodes,such as,composition or morphology-tuning,crystal-facet engineering,heterogeneous structure constructing,co-catalysts coupling.However,the water oxidation performance of BiVO₄-based photoanodes is still limited,it is urgent to explore new strategies to regulate BiVO₄-based photoanodes.In this thesis,to address the issues of high carrier recombination rate and sluggish water oxidation kinetics of BiVO₄ photoanodes,several BiVO₄-based photoanode materials have been designed and prepared,such as W or Mo surface doped BiVO₄,BiVO₄/RFS,CoO_x/C/BiVO₄ and NiO/C/BiVO₄.In the presence of magnetic field,interface electric field,thermal field condition,the water oxidation performance and mechanism of the resultant BiVO₄-based photoanodes were investigated.The main results and findings are as follows:（1）Magnetic field regulates the water oxidation activity and mechanism of W or Mo surface doped BiVO₄ photoanode:Based on the feature that the pristine BiVO₄ photoande is prone to initiate photocorrosion dissolving out VO₄^3-,and VO₄^3-has a propery to exchange with WO₄^2-or MoO₄^2-,a photoelectrochemical treating approach has been developed to prepare W or Mo surface doped BiVO₄ film photoande in saturated WO₄^2-or MoO₄^2-electrolyte,respectively（W-BiVO₄,Mo-BiVO₄）.Since W or Mo surface doping improves the charge separation and injection efficiencies of BiVO₄ photoande,W-BiVO₄ and Mo-BiVO₄ photoandes show better water oxidation activity and stability.At 1.23 V vs.RHE,the water oxidation photocurrent density of W or Mo doped BiVO₄ photoandes be improved about 50%and 40%relative to the pristine BiVO₄ photoande,respectively.After 2 h of reaction,95%of the water oxidation activity remained on the W-BiVO₄ and Mo-BiVO₄photoandes,which was higher than 64%of activity retention on the pristine BiVO₄photoande.For the W-BiVO₄ and Mo-BiVO₄ photoandes,the introduction of a vertical magnetic field can significantly improve their water oxidation activity,and the improvement is is proportional to the intensity of magnetic field.The mechanism investigations found that Lorentz force can be generated when the photogenerated carriers of W-BiVO₄ and Mo-BiVO₄ move in magnetic field,which inhibit the nonradiative recombination rate of carriers,and promote the water oxidation activity of W-BiVO₄ and Mo-BiVO₄ photoanodes efficiently.（2）Interfacial electric field regulates the water oxidation activity and mechanism of BiVO₄/resorcinol-formaldehyde resin heterojunction photoanode:On the basis of the match of resorcinol-formaldehyde resin’s（RFS）and BiVO₄’s band positions,as well as the semiconductor heterojunction theory,BiVO₄/RFS heterojunction film was designed and prepared to drive water oxidation.Compared with the BiVO₄ photoanode,BiVO₄/RFS photoanode exhibted better water oxidation activity,stability and kinetics.At 1.23 V vs.RHE,the water oxidation photocurrent of BiVO₄/RFS photoanode is 1.22 times higher than that of BiVO₄ photoanode.After 120 min of reaction,the water oxidation activity of BiVO₄/RFS photoanode can be well maintained at～80%,which is higher than the 50%of activity retention of BiVO₄ photoanode.In addition,the lifetime of holes on BiVO₄/RFS photoanode was 1.93 ms,shorter than that on BiVO₄ photoanode（8.95 ms）,and the holes of BiVO₄/RFS can drive water oxidation reaction quickly.The RFS nanoparticles coupling with BiVO₄ film did not change the light absorption and electrocatalytic water splitting activity of BiVO₄significantly,but modified the hydrophilicity of BiVO₄/RFS film,which is conducive to the achievement of water splitting on BiVO₄/RFS.BiVO₄/RFS photoanode be of better water oxidation performance could be attributed to the following reason.The E_CB（-0.47 V vs.RHE）and E_VB（1.64 V vs.RHE）of RFS match well with the BiVO₄(E_CB=-0.05 V vs.RHE,E_VB=2.45 V vs.RHE).The electric field formed at RFS nanoparticles-BiVO₄ film interface can promote the directional separation and transfer of their carriers,and reduce the carrier recombination rate.（3）Thermal field regulates the water oxidation activity and mechanism of amorphous carbon/nickel or cobalt oxide/BiVO₄ composite photoanode:Based on the advantage of candle flame burning metal ion-solution to form metal oxide/amorphous carbon composites rapidly,a simple candle flame treating method was developed to prepare NiO or CoO_x/C/BiVO₄film photoanodes.Relative to the BiVO₄ photoanode,CoO_x/C/BiVO₄ and NiO/C/BiVO₄ photoanodes showed higher water oxidation activity,stability and kinetics.At1.23 V vs.RHE,the water oxidation photocurrent of CoO_x/C/BiVO₄ and NiO/C/BiVO₄photoanodes was 2.32 times and 2.41 times higher than that of BiVO₄ photoanode,respectively.The lifetime of holes on CoO_x/C/BiVO₄ and NiO/C/BiVO₄ photoanodes was1.63 ms and 3.43 ms,respectively,which is significantly shorter than that on BiVO₄photoanode（8.96 ms）.Thus,the holes on CoO_x/C/BiVO₄ and NiO/C/BiVO₄ photoanodes can initiate water oxidation quickly.After 120 min of reaction,the water oxidation activity of CoO_x/C/BiVO₄ and NiO/C/BiVO₄ photoanodes was maintained in the range of 84～87%,which is higher than the 43%of activity retention on BiVO₄ photoanode.The better water oxidation performance of CoO_x/C/BiVO₄ and NiO/C/BiVO₄ can be attributed to the following reasons:（1）CoO_x and NiO are p-type semiconductors,while BiVO₄ is n-type;The p-n heterojunction effect can promote the directional separation and transfer of BiVO₄’s holes to CoO_x and NiO,and reduce the carrier recombination rate.（2）CoO_x and NiO are typical oxygen evolution electrocatalysts,they can accelerate the water oxidation kinetics of CoO_x/C/BiVO₄ and NiO/C/BiVO₄ photoanodes.（3）Due to the photothermal properties of amorphous carbon,CoO_x and NiO,they can absorb light with lower energy than the band gap of BiVO₄,and promote the CoO_x/C/BiVO₄ and NiO/C/BiVO₄ photoanode temperature rising and driving water oxidative through photothermal synergy.The research of this thesis could provide valuable method references for the design and preparation of similar semiconductor materials.Meanwhile,the findings of this thesis about the regulation of BiVO₄-based photoanodes for water oxidation through external field energy can provide useful concepts and theories references for the construction of efficient photoelectrochemical water splitting systems.