Photoelectrochemical Water Oxidation Research Articles

Graphene Quantum Dots as Hole Extraction and Transfer Layer Empowering Solar Water Splitting of Catalyst-Coupled Zinc Ferrite Nanorods.

Despite the narrow band gap energy, the performance of zinc ferrite (ZnFe2O4) as a photoharvester for solar-driven water splitting is significantly hindered due to its sluggish charge transfer and severe charge recombination. This work reports the fabrication of a hybrid nanostructured hydrogenated ZnFe2O4 (ZFO) photoanode with enhanced photoelectrochemical water-oxidation activity through coupling N-doped graphene quantum dots (GQDs) as a hole transfer layer and Co-Pi as a catalyst. The GQDs not only reduce the surface-mediated nonradiative electron-hole pair recombination but also induce a built-in interfacial electric field leading to a favorable band alignment at the ZFO/GQDs interface, helping rapid photogenerated hole separation and serving as a conducting hole transfer highway, improve the hole transportation into the Co-Pi catalyst for enhanced water oxidation reaction kinetics. The optimized ZFO/GQD/Co-Pi hybrid photoanode delivers a 23-fold photocurrent enhancement at 1.23 V versus the reversible hydrogen electrode (RHE) and a significant 360 mV reduction in the onset potential, reaching 0.65 VRHE compared with the ZFO photoanode under 1 sun illumination in a neutral electrolytic environment. This investigation underscores the mechanism of synergistic interplay between the hole transport layer and cocatalyst in boosting the solar-illuminated water-splitting activity of the ZFO photoanode.

Insights into the role of Co3O4/Ti3C2 MXene hybrid material in photoelectrochemical water oxidation over BiVO4 photoanode

Developing efficient hole transport layers and water oxidation electrocatalysts is vital for improving the performance of photoelectrochemical (PEC) devices for water-splitting. In this work, we synthesized Co3O4 nanoparticles/Ti3C2-MXene hybrid material and utilized it as a dual-function material to transport the photogenerated holes and catalyze the water oxidation reaction. It was found that the modification of BiVO4 with Co3O4/Ti3C2-MXene prolonged the lifetime of the photogenerated holes by 13.7-fold (i.e. from 0.26 s for BiVO4 to 3.56 s for Co3O4/Ti3C2/BiVO4) and significantly reduced the charge transfer resistance. The modification of BiVO4 with only Ti3C2-MXene resulted in significant surface recombination more likely due to the high concentration of the accumulated holes in the Mxene layer because of its low electrocatalytic activity toward water oxidation reaction. Employing the Co3O4/Ti3C2-MXene hybrid material significantly reduced the rate constant of surface recombination and dramatically enhanced the charge transfer efficiency (i.e. from 0.17% for BiVO4 to 14.0% for Co3O4/Ti3C2/BiVO4 at 0.65 VRHE). Under optimized conditions, the Co3O4/Ti3C2/BiVO4 photoanode was able to deliver 5.05 mA cm−2 at 1.23 VRHE and outperforms Ti3C2/BiVO4 and Co3O4/BiVO4 photoanodes. Moreover, it showed high stability for over 50 hours and reached a Faradaic efficiency of 90%. An applied bias photo-to-current efficiency (ABPE) value of 1.71% at 0.62 VPt was obtained for the overall water splitting. This work provides a facile method to reduce surface recombination and enhance the stability and PEC efficiency of BiVO4 photoanodes via the modification with dual-function oxygen evolution catalyst/hole transport layer hybrid material.

Charge Transfer Mechanism on a Cobalt-Polyoxometalate-TiO2 Photoanode for Water Oxidation in Acid.

We constructed a photoanode comprising the homogeneous water oxidation catalyst (WOC) Na8K8[Co9(H2O)6(OH)3(HPO4)2(PW9O34)3] (Co9POM) and nanoporous n-type TiO2 photoelectrodes (henceforth "TiO2-Co9POM") by first anchoring the cationic 3-aminopropyltrimethoxysilane (APS) ligand on a metal oxide light absorber, followed by treatment of the metal oxide-APS with a solution of the polyoxometalate WOC. The resulting TiO2-Co9POM photoelectrode exhibits a 3-fold oxygen evolution photocurrent enhancement compared to bare TiO2 in aqueous acidic conditions. Three-element (Co 2p, W 4f, and O 1s) X-ray photoelectron spectroscopy and Raman spectroscopy studies before and after use indicate that surface-bound Co9POM retains its structural integrity throughout all photoelectrochemical water oxidation studies reported here. Extensive charge-transfer mechanistic studies by photoelectrochemical techniques and transient absorption spectroscopy elucidate that Co9POM serves as an efficient WOC, extracting photogenerated holes from TiO2 on the picosecond time scale. This is the first comprehensive mechanistic investigation elucidating the roles of polyoxometalates in POM-photoelectrode hybrid oxygen evolution reaction systems.

Surface plasmon enhancement of 1D Ag nanowires modified electro-treated BiVO4 photoanode with abundant oxygen vacancies for solar water oxidation

The high carrier complexation rate is still one of the main obstacles for bismuth vanadate (BiVO4) to be used in photoelectrochemical (PEC) water oxidation. In this work, we used a simple method for dropping one dimension (1D) silver nanowires (Ag NWs) on the surface of electro-treated BiVO4 (Ag NWs/E-BiVO4). The photocurrent of Ag NWs/E-BiVO4 under AM 1.5 G illumination reaches to 3.12 mA/cm2 which is about 6.2 times than the pure BiVO4 photoelectrode. Meanwhile, the photogenerated charges injection efficiency of Ag NWs/E-BiVO4 photoelectrodes increasing from 12.7 % to 77.5 % than the pure BiVO4 photoelectrode. This is because the surface plasmon resonance (SPR) effect of 1D Ag nanowires enhanced the visible light absorption and provide more Ag active sites. Furthermore, the density functional theory (DFT) calculations show that the Ag NWs/E-BiVO4 have the best adsorption capacity for OH– and the electro-treated BiVO4 with rich oxygen vacancies greatly improves the electron transport efficiency which is benefit for improving the PEC catalytic performance. Thus, this work provides a simple method to enhance the PEC water splitting performance of BiVO4.

Coating effect of metal organic complex (Co-DTPMP) layer on enhancing PEC water oxidation performance of BiVO4 photoanode

Photoelectrochemical (PEC) water splitting is a promising method for transforming solar energy into clean and sustainable energy. However, PEC is severely limited, and they cannot achieve the predicted photocurrent density owing to the severe photochemical deterioration of the electrode and the recombination of photogenerated carriers. In this study, BiVO4/Co-diethylenetriamine Penta (methylene phosphonic acid) (BVO/Co-DTPMP) co-catalyst was successfully prepared as a nanoporous photoanode. Diethylenetriamine Penta (methylene phosphonic acid) was crosslinked with Co ions and coated on the BiVO4 surface by successive ionic layer adsorption and reaction (SILAR) to reduce fast recombination, which correlated with the BiVO4 photoanode. Various characterization and PEC measurements were conducted, revealing that the co-catalyst thin layer enhanced the charge separation and electrons transfer which significantly affected on the PEC performance of BiVO4, and the current density by BVO/Co-DTPMP was 4 mA cm−2 at 1.23 V vs. RHE. Furthermore, the co-catalyst exhibited improved charge transport and long-term stability.

Double oxygen evolution co-catalysts modified BiVO4 to boost photoelectrochemical water oxidation performance

Improving the kinetics of oxygen evolution is crucial for enhancing the performance of photoelectrochemical (PEC) water splitting. Herein, this study utilizes a chemical self-growth method to grow NiFe tannic acid complex (NFTA) and Co(OH)2 on the surface of BiVO4 photoanode (BiVO4/NFTA/Co). As a result, the synergistic effects of NFTA and Co(OH)2 layers promote the efficiency and stability of BiVO4 photoanode towards PEC water oxidation. The photocurrent density of the obtained BiVO4/NFTA/Co photoanode reaches 4.97 mA cm−2, which is significantly greater than those of BiVO4/NFTA (4.36 mA cm−2), BiVO4/Co (2.51 mA cm−2), BiVO4 (1.34 mA cm−2), respectively. Detailed analysis confirms that NFTA could provide an efficient way to hasten the transfer of photo-generated holes on the photoanode surface and diminish the surface charge transfer resistance. In other hand, Co(OH)2 could be served as a cocatalyst to accelerate charge transfer for efficient oxygen evolution reaction as well as a protective layer to maintain the long-term stability of NFTA on the surface of BiVO4 during water oxidation. Such double oxygen evolution co-catalysts decoration strategy paves an effective pathway to enhance the PEC water oxidation performance of BiVO4 photoanode.

Nonconjugated Polymers Enabled Solar Water Oxidation.

Wholly distinct from conjugated polymers which are featured by generic charge transfer capability stemming from a conjugated molecular structure, solid nonconjugated polymers mediated charge transport has long been deemed as theoretically impossible because of the deficiency of π electrons along the molecular skeleton, thereby retarding their widespread applications in solar energy conversion. Herein, we first conceptually unveil that intact encapsulation of metal oxides (e.g., TiO2, WO3, Fe2O3, and ZnO) with an ultrathin nonconjugated polyelectrolyte of branched polyethylenimine (BPEI) can unexpectedly accelerate the unidirectional charge transfer to the active sites and foster the defect generation, which contributes to the boosted charge separation and prolonged charge lifetime, ultimately resulting in considerably improved photoelectrochemical (PEC) water oxidation activities. The interfacial charge transport origins endowed by BPEI adornment are elucidated, which include acting as a hole-withdrawing mediator, promoting vacancy generation, and stimulating the directional charge flow route. We additionally ascertain that such charge transport characteristics of BPEI are universal. This work would unlock the charge transfer capability of nonconjugated polymers for solar water oxidation. The nonconjugated insulating polymer was utilized as a charge transport mediator for boosting charge migration and separation over metal oxides toward solar water oxidation.

Synergistic selenium doping and colloidal quantum dots decoration over ZnIn2S4 enabling high-efficiency photoelectrochemical hydrogen peroxide production

Photoelectrochemical (PEC) water splitting to value-added hydrogen peroxide (H2O2) production is more promising than the traditional O2 evolution. However, enabling efficient PEC H2O2 generation is still challenging due to competitive two-/four-electron water oxidation pathways. In this work, ZnIn2S4 (ZIS) photoanodes were engineered by Se doping and colloidal quantum dots (QDs) modification to boost the PEC H2O2 evolution efficiency. Morphology and elemental studies testify the successful Se doping in ZIS and the anchoring of QDs on ZIS-Se photoelectrodes. Photoinduced carrier kinetics investigation verifies the optimized band bending, suppressed charge recombination, and promoted charge transfer in as-prepared ZIS-Se and ZIS-Se/QDs photoanodes, exhibiting inhibited four-electron O2 evolution and facilitated two-electron H2O2 production during PEC water oxidation, as evidenced by the density functional theory (DFT) calculations. Accordingly, the Se-doped and QDs-decorated ZIS photoelectrodes demonstrate a gradually less upward band bending and improved photo-excited hole transfer to the surface, delivering a maximum H2O2 production rate of 1.32 μmol min−1 cm−2 with a Faraday efficiency of 86 % at 1.23 V versus reversible hydrogen electrode (RHE) under one sun illumination.

Selective additions of dopants, Co-catalyst and surfactant on the hydrothermally synthesized BiVO4 semiconductors for outstanding photoelectrochemical water oxidation behavior

The present study describes the significant improvement of the PEC performance through modifications via co-doping of Mo/W and optimizing the level of surfactant (Triton-X 100; TX-100) addition to the hydrothermally synthesized BiVO4 photoanode. The photoelectrochemical performance is improved when the precursor solution contains an optimized 5 % TX-100 surfactant level. The visible light responsiveness of the semiconductor photocatalyst has been confirmed using linear sweep voltammograms under periodic chopped illumination of UV–vis and visible irradiation for H2O → O2 oxidation. Adding an optimized level of Mo and W to the precursor solution leads to modifications of the crystallites, which facilitates the water oxidation reaction and exhibits the highest photo-current of ∼0.61 mA cm−2. Selective loading of oxygen evolution catalyst (OEC) in the surface boosts the PEC performance to 1.05 mA cm−2 and significantly increases photon current conversion efficiency.

A hybrid MOFs/Ti-Fe2O3 Z-scheme photoanode with enhanced charge separation and transfer for efficient photoelectrochemical water oxidation

The construction of Z-scheme charge transfer pathways simulating natural photosynthesis is considered a promising method for improving reaction driving forces. Here, we modified the surface of titanium doped Fe2O3 (Ti-Fe2O3) nanorods with NH2-MIL-125(Ti) (Ti-MOFs) and a promising organic-inorganic hybrid Z-scheme NH2-MIL-125(Ti)/Ti-Fe2O3 was successfully prepared. At 1.23 V vs. RHE, the photocurrent density of the composite photoanode reaches 2.67 mA/cm2, which is 5 times higher than that of Ti-Fe2O3. The results of surface photovoltage, ESR and fs-TAS indicate that this improvement is mainly due to the effective Z-scheme charge transfer mechanism providing a strong driving force for charge separation and transport, greatly suppressing carrier recombination and allowing carriers with strong oxidation ability to participate in water oxidation. Meanwhile, NH2-MIL-125(Ti) can enhance light absorption and reduce the surface defect state of Ti-Fe2O3. This study not only provides a feasible approach for the photoanode water splitting of traditional inorganic semiconductor/MOF based heterostructures, but also provides rich and effective means for revealing Z-scheme charge transfer mechanism in depth.

Ultrathin layer TAFC on BiVO4 with ligand-to-metal charge transfer enhances built-in electric field for boosting photoelectrochemical water oxidation

To reveal the mechanism of charge transfer between interfaces of BiVO4-based heterogeneous materials in photoelectrochemical water splitting system, the cocatalyst was grown in situ using tannic acid (TA) as a ligand and Fe and Co ions as metal centers (TAFC), and then uniformly and ultra-thinly coated on BiVO4 to form photoanodes. The results show that the BiVO4/TAFC achieves a superior photocurrent density (4.97 mA cm−2 at 1.23 VRHE). The charge separation and charge injection efficiencies were also significantly higher, 82.0 % and 78.9 %, respectively. From XPS, UPS, KPFM, and density functional theory calculations, Ligand-to-metal charge transfer (LMCT) acts as an electron transport highway in TAFC ultrathin layer to promote the concentration of electrons towards metal center, leading to an increase in the work function, which enhances the built-in electric field and further improves the charge transport. This study demonstrated that the LMCT pathway on TA-metal complexes enhances the built-in electric field in BiVO4/TAFC to promote charge transport and thus enhance water oxidation, providing a new understanding of the performance improvement mechanism for the surface-modified composite photoanodes.

Electrochemical versus Photoelectrochemical Water Oxidation Kinetics on Bismuth Vanadate (Photo)anodes.

This study reports a comparison of the kinetics of electrochemical (EC) versus photoelectrochemical (PEC) water oxidation on bismuth vanadate (BiVO4) photoanodes. Plots of current density versus surface hole density, determined from operando optical absorption analyses under EC and PEC conditions, are found to be indistinguishable. We thus conclude that EC water oxidation is driven by the Zener effect tunneling electrons from the valence to conduction band under strong bias, with the kinetics of both EC and PEC water oxidation being determined by the density of accumulated surface valence band holes. We further demonstrate that our combined optical absorption/current density analyses enable an operando quantification of the BiVO4 photovoltage as a function of light intensity.

Ternary Photoanodes with AgAu Nanoclusters and CoNi-LDH for Enhanced Photoelectrochemical Water Oxidation.

Atomically precise metal nanoclusters (NCs) present new opportunities for creating innovative solar-powered photoanodes due to their extraordinary physicochemical properties. Nevertheless, ultrasmall metal NCs tend to aggregate and lack active sites under light irradiation, which severely limits their widespread application. We have developed a strategy to design efficient ternary photoanodes by successively modifying AgAu NCs and CoNi-LDH on BiVO4 substrates using versatile impregnation and electrodeposition. The electronic properties of AgAu NCs facilitate the rapid transfer of photogenerated carriers on BiVO4 and CoNi-LDH. Additionally, ultrathin CoNi-LDH acts as a hole-collecting layer, which quickly extracts holes to the electrode/electrolyte interface. The synergistic effect and the matched energy levels between the ternary heterostructures promote the OER process, which significantly improved the photoelectrochemical (PEC) water oxidation performance. This study presents a new idea for further exploration of metal nanocluster-based PEC systems.

Z-scheme S-modified Zn0.2Cd0.8S/α-Fe2O3 heterostructure for enhanced photoelectrochemical water oxidation

A Z-type heterojunction system is proven to be an efficient strategy to promote the separation of photo-generated carriers and maintain strong redox capacity for an enhanced photoanode. Herein, we synthesized a S-modified Fe2O3/Zn0.2Cd0.8S (Fe/ZCS) photoanode by one-pot hydrothermal treatment of FeOOH/Zn0.2Cd0.8S precursor film, followed by vapor deposition treatment. The as-synthesized S-modified Fe/ZCS displayed enhanced PEC water oxidation performance with a photocurrent density (2.76 mA cm−2) of about 2.63 times that of a pure phase Fe2O3 and 2.05 times that of Fe/ZCS at 1.23 VRHE. After 8 h of continuous operation, the photocurrent density retention rate was still 96.4 % demonstrating its satisfactory stability. Benefiting from the morphology of fine nanorods, the fabricated Z-scheme heterostructure exhibited strong oxidation–reduction ability and the well-distributed Fe(III)-SO42- bonds played key roles as the electron capture centers. The fabricated Z-scheme photoanode exhibited prospective applications in sustainable photoelectrochemical water oxidation.

Boosting photoelectrocatalytic oxygen evolution activity of BiVO4 photoanodes via caffeic acid bridged to NiFeOOH

Bismuth vanadate (BiVO4) is a promising n-type photoanode material for photoelectrochemical (PEC) water splitting. However, its activity is impeded by poor charge carrier transport and sluggish oxygen evolution kinetics. In this study, we reports a Fe doping and caffeic acid (CA)/NiFeOOH (NiFe) modification with BiVO4 photoanode (NiFe/CA/Fe-BiVO4) to enhance the PEC water oxidation performance and stability. The modified BiVO4 demonstrates significant advantages in promoting the PEC performance and stability. Specifically, Fe doping via in-situ electro-deposition results in smaller crystal sizes, larger specific surface areas, and more exposed active sites. Furthermore, co-catalyst NiFe connects with Fe-BiVO4 through CA bridge facilitates a more uniform self-assembly distribution of NiFe on BiVO4 surface. The NiFe/CA/Fe-BiVO4 exhibits an excellent photocurrent density of 6.2 mA/cm2 at 1.23 VRHEand demonstrates good stability at 0.8 VRHE. In addition, the composite photoanode shows a high applied bias photon-to-current efficiency (ABPE) value of 2.15% at 0.65 VRHE. EPR and in-situ FTIR confirm the generation of superoxide active species (O2- and ·O2-) during the PEC water oxidation reaction.

High-performance Bi2S3/ZnO photoanode enabled by interfacial engineering with oxyanion for efficient photoelectrochemical water oxidation

High-performance Bi2S3/ZnO photoanode enabled by interfacial engineering with oxyanion for efficient photoelectrochemical water oxidation

Coupling hole-transporting polyaniline layer with a cobalt phthalocyanine cocatalyst to boost photoelectrochemical water oxidation of bismuth vanadate photoanodes

Though bismuth vanadate (BiVO4) exhibits a promising potential as a semiconductor photoanode toward photoelectrochemical (PEC) water oxidation, its PEC performance is still limited by the poor charge carrier transport and sluggish oxygen evolution kinetics. Herein, the polyaniline (PANI) and cobalt coordinated-phthalocyanines (CoTcPc) were introduced onto the BiVO4 semiconductor substrate, constructing the integrated CoTcPc/nPANI/BiVO4 (n = 30, 60 and 90, n represents the deposition time for PANI with the unit of s) composite photoanodes. It is demonstrated that the PEC water oxidation activity of BiVO4 is significantly boosted by the incorporation of PANI and CoTcPc, stemming from the improved electron/hole transfer, enhanced charge separation efficiency, and accelerated carrier mobility and water oxidation kinetics. CoTcPc/60PANI/BiVO4 composite photoanode with the optimized compositions exhibits an enhanced photocurrent density of 4.03 mA/cm2 at 1.23 V vs. RHE under illumination, ranking among the best records reported recently. Furthermore, the roles of electrodeposition time to generate PANI are also distinguished through experimental studies. These results reveal the crucial role of engineering the hole transfer layer to optimize the semiconductor photoanodes toward efficient PEC water oxidation.

Tuning Surface Electronics State of P-Doped In2.77S4/In(OH)3 toward Efficient Photoelectrochemical Water Oxidation.

Indium sulfide with a two-dimensional layered structure offers a platform for catalyzing water oxidation by a photoelectrochemical process. However, the limited hole holders hinder the weak intrinsic catalytic activity. Here, the nonmetallic phosphorus atom is coordinated to In2.77S4/In(OH)3 through a bridge-bonded sulfur atom. By substituting the S position by the P dopant, the work function (surface potential) is regulated from 445 to 210 mV, and the lower surface potential is shown to be beneficial for holding the photogenerated holes. In2.77S4/In(OH)3/P introduces a built-in electric field under the difference of Fermi energy, and the direction is from the bulk to the surface. This band structure results in upward band bending at the interface of In2.77S4/In(OH)3 and P-doped sites, which is identified by density functional theory calculations (∼0.8 eV work function difference). In2.77S4/In(OH)3/P stands out with the highest oxidation efficiency (ηoxi = 70%) and charge separation efficiency (ηsep = 69%). Importantly, it delivers a remarkable water oxidation photocurrent density of 2.51 mA cm-2 under one sun of illumination.

Alternating 3rd- to 2nd-Order Charge Reaction Kinetics on Bismuth Vanadate Photoanodes with Ultrathin Bismuth Metal-Organic-Frameworks.

The most challenging obstacle for photocatalysts to efficiently harvest solar energy is the sluggish surface redox reaction (e. g., oxygen evolution reaction, OER) kinetics, which is believed to originate from interface catalysis rather than the semiconductor photophysics. In this work, we developed a light-modulated transient photocurrent (LMTPC) method for investigating surface charge accumulation and reaction on the W-doped bismuth vanadate (W : BiVO4) photoanodes during photoelectrochemical water oxidation. Under illuminating conditions, the steady photocurrent corresponds to the charge transfer rate/kinetics, while the integration of photocurrent (I~t) spikes during the dark period is regarded as the charge density under illumination. Quantitative analysis of the surface hole densities and photocurrents at 0.6 V vs. reversible hydrogen electrode results in an interesting rate-law kinetics switch: a 3rd-order charge reaction behavior appeared on W : BiVO4, but a 2nd-order charge reaction occurred on W : BiVO4 surface modified with ultrathin Bi metal-organic-framework (Bi-MOF). Consequently, the photocurrent for water oxidation on W : BiVO4/Bi-MOF displayed a 50 % increment. The reaction kinetics alternation with new interface reconstruction is proposed for new mechanism understanding and/or high-performance photocatalytic applications.

Anthraquinone-based polymer modified BiVO4 photoanode with strong electron-withdrawing functional groups for boasting photoelectrochemical water oxidation

The photoelectrochemical (PEC) performance ofBiVO4 is limited by sluggish water oxidation kinetics and severe carrier recombination. Herein, a novel high-performance BiVO4/NiFe-NOAQ photoanode is prepared by a simple one-step hydrothermal method, using BiVO4 and 1-Nitroanthraquinone (NOAQ) as raw materials. The BiVO4/NiFe-NOAQ photoanode has an excellent photocurrent density of 5.675 mA cm−2 at 1.23 VRHE, which is 3.35 times higher than that of the pure BiVO4 (1.693 mA cm−2) photoanode. The BiVO4/NiFe-NOAQ shows a significant improvement in charge separation efficiency (86.12 %) and charge injection efficiency (87.86 %). The improvement is ascribable to the NiFe-NOAQ form a type II heterojunction with BiVO4 to inhibit carrier recombination. More importantly, the kinetic isotope experiment suggests that the proton-coupled electron transfer (PCET) process can enhance the charge transfer of BiVO4/NiFe-NOAQ. The contact angle measurements show that modifying functional groups enhanced the hydrophilicity of BiVO4/NiFe-NOAQ, which can further accelerate the PCET process. The XPS and PL results as well as the tauc plot indicate that the strong electron-withdrawing ability of –NO2 which can promote the extension of π conjugation, results in more π electron delocalization and produces more efficient active sites, thus achieving efficient photoelectrochemical water oxidation.