Photoelectrochemical Water Oxidation Activity Research Articles

Tailoring Non-Covalent Interaction Via Single Atom to Boost Interfacial Charge Transfer Toward Photoelectrochemical Water Oxidation.

Photoelectrochemical (PEC) water splitting for hydrogen generation holds immense potential for addressing environmental and energy crises. Tailoring non-covalent interaction via a single atom is anticipated to realize prominent hole extracting facilitating PEC performance, but it has never been reported. In this study, single atom Co-N4 is coordinated with 5-fluoroanthranilic acid (FAA) molecules, then used as a non-covalent hole-extracting layer on a BiVO4 substrate. Experiments including X-ray absorption fine spectra, Kelvin probe force microscopy, transient absorption, and theoretical calculation demonstrate the FAA coordination alters the local configuration of the central Co atom, adjusting the interfacial non-covalent interaction, thereby reducing the barrier of charge transfer between BiVO4 and the hole-extracting layer. Consequently, photogenerated carriers are more effectively separated, and the PEC water oxidation performance is significantly enhanced with the photocurrent density of 5.47mA cm-2 at 1.23V versus RHE, much higher than those of previously reported BiVO4 photoanodes composited with porphyrin-based compounds. Experiments and theoretical simulation confirm that the boosted PEC performance originates from exceptional interfacial charge transfer rather than surface catalysis dynamic. This study provides an efficient strategy for tailoring non-covalent interaction by regulating single-atom coordination and promoting hole extract to boost PEC water oxidation activity.

Enhancing Photoelectrochemical Water Oxidation on WO3 via Electrochromic Modulation: Universal Effects and Mechanistic Insights.

Although WO3 exhibits both electrochromic and photoelectrochemical (PEC) properties, there is no research conducted to investigate the correlation between them. The study herein reports the electrochromic enhancement of PEC activity on WO3. The electrochromic WO3 (e-WO3) exhibits a significantly enhanced activity for PEC water oxidation compared to raw WO3 (r-WO3), with a limiting photocurrent density three times that of r-WO3. The electrochromic enhancement of PEC activity is universal and independent of the type of cations inserted during electrochromism. Decoloring reduces the PEC activity but a simple re-coloring restores the activity to its maximum value. Electrochromism induces large amounts of oxygen vacancies and surface states, the former improving the electron density of WO3 and the latter facilitating the hole transfer across e-WO3/electrolyte interface. It is proved that the electrochromic enhancement effect is due to the significantly improved electron-hole separation efficiency and the charge transfer efficiency across the WO3/electrolyte interface.

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.

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.

Bifunctional NiCo-LDH cocatalyst with hole extraction and high catalytic activity for boosting photoelectrochemical water oxidation of Nd doped BiVO4 photoanode

The slow charge transfer kinetics and water oxidation dynamics hinder the efficient photoelectrochemical (PEC) water splitting of BiVO4. Here, Nd:BiVO4@NiCo-LDH photoanode was successfully prepared by introducing rare earth element Nd with a special 4 f energy level as a dopant and loading a bifunctional NiCo-LDH cocatalyst. The Nd:BiVO4@NiCo-LDH photoanode exhibits good PEC performance. The photocurrent density was 4.1 mA cm−2 at 1.23 V vs. reversible hydrogen electrode (RHE), which is about 5.1 times more than that of the unmodified BiVO4 electrode, and the onset potential was negatively shifted by 399 mV. Various physical characterizations and photoelectrochemical tests indicate that the Nd:BiVO4@NiCo-LDH photoanode has higher charge transfer rate and lower charge recombination rate than that of BiVO4. The NiCo-LDH catalyst serves as a bimetallic oxygen evolution reaction (OER) cocatalyst, where Co cation has hole extraction ability to reduce carrier recombination, and Ni addition increases the active site. The synergistic effect of the two significantly boosts the PEC water oxidation activity of the photoanode. This work has broad prospects for the construction of high-performance water oxidation systems in various photoanodes.

Effect of tantalum doping on the Fe2O3 toward photoelectrochemical water oxidation

Metal doping is an effective method to increase the photogenerated carrier concentration and improve the carrier transport performance of Fe2O3 for photoelectrochemical water oxidation. This work synthesized a series of Ta-doped Fe2O3 nanoparticles on FTO glasses using a hydrothermal-pyrolysis method. The XPS, XRD, and Raman spectra demonstrated that the Ta5+ dopants occupy the Fe3+ sites. The Ta-doped Fe2O3 photoanodes show noticeable improvement in photoelectrochemical water oxidation via enhanced electron separation and transfer competence. Doping Ta into Fe2O3 forces the Fermi level of Fe2O3 to move to the conduction band, leading to the decrease of charge transfer resistance in Fe2O3 and water oxidation on the surface. Moreover, the carrier concentration was increased. Whereas, to maintain the charge neutrality of the system, the Ta doping inhibits the formation of oxygen vacancies, which act as the effective defects. Consequently, 2%Ta-Fe2O3 performed the optimum enhanced photoelectrochemical water oxidation activity.

Facets control charge separation during photoelectrochemical water oxidation with strontium titanate (SrTiO3) single crystals

The photoelectrochemical water oxidation activity of SrTiO3 crystals is determined by the variable work functions of different facets. This produces facet dependent semiconductor–liquid junctions that can drive charge separation in photocatalysts.

A novel α-Fe2O3 photoanode with multilayered In2O3/Co–Mn nanostructure for efficient photoelectrochemical water splitting

Hematite (α-Fe2O3) is considered one of the most promising materials for PEC water splitting under solar light. However, the drawbacks of lower charge transfer efficiency and slow oxygen evolution reaction (OER) kinetics seriously limited the practical application of α-Fe2O3 photoanodes. In this work, a multilayer structure modified α-Fe2O3 photoanode is proposed. Firstly, the In2O3 nanolayers were loaded onto the surface of α-Fe2O3 nanorods, significantly increasing the photoelectrochemical (PEC) water oxidation activity. The heterojunction form by the In2O3 passivation layer with α-Fe2O3 effectively promotes charge separation. Next, an electrodeposition method deposited a nanosheet combining ultrathin non-crystalline Co(OH)x and Mn3O4 nanocrystals (Co–Mn) on the α-Fe2O3 thin film. As an efficient co-catalyst, the Co–Mn nanosheets assisted the performance of α-Fe2O3 PEC water oxidation. The optimized α-Fe2O3–In2O3–Co(OH)x-Mn3O4 photoanode produces a high photocurrent density of 6.5 mA/cm2 at 1.23 V (vs. RHE), a maximum IPCE of 57.9% at 400 nm can reach.

Metal-Organic Framework Glass Catalysts from Melting Glass-Forming Cobalt-Based Zeolitic Imidazolate Framework for Boosting Photoelectrochemical Water Oxidation.

Sluggish oxygen evolution kinetics and serious charge recombination restrict the development of photoelectrochemical (PEC) water splitting. The advancement of novel metal-organic frameworks (MOFs) catalysts bears practical significance for improving PEC water splitting performance. Herein, a MOF glass catalyst through melting glass-forming cobalt-based zeolitic imidazolate framework (Co-agZIF-62) was introduced on various metal oxide (MO: Fe2O3, WO3 and BiVO4) semiconductor substrates coupled with NiO hole transport layer, constructing the integrated Co-agZIF-62/NiO/MO photoanodes. Owing to the excellent conductivity, stability and open active sites of MOF glass, Co-agZIF-62/NiO/MO photoanodes exhibit a significantly enhanced photoelectrochemical water oxidation activity and stability in comparison to pristine MO photoanodes. From experimental analyses and density functional theory calculations, Co-agZIF-62 can effectively promote charge transfer and separation, improve carrier mobility, accelerate the kinetics of oxygen evolution reaction (OER), and thus improve PEC performance. This MOF glass not only serves as an excellent OER cocatalyst on tunable photoelectrodes, but also enables promising opportunities for PEC devices for solar energy conversion.

Metal‐Organic Framework Glass Catalysts from Melting Glass‐Forming Cobalt‐Based Zeolitic Imidazolate Framework for Boosting Photoelectrochemical Water Oxidation

AbstractSluggish oxygen evolution kinetics and serious charge recombination restrict the development of photoelectrochemical (PEC) water splitting. The advancement of novel metal–organic frameworks (MOFs) catalysts bears practical significance for improving PEC water splitting performance. Herein, a MOF glass catalyst through melting glass‐forming cobalt‐based zeolitic imidazolate framework (Co‐agZIF‐62) was introduced on various metal oxide (MO: Fe2O3, WO3 and BiVO4) semiconductor substrates coupled with NiO hole transport layer, constructing the integrated Co‐agZIF‐62/NiO/MO photoanodes. Owing to the excellent conductivity, stability and open active sites of MOF glass, Co‐agZIF‐62/NiO/MO photoanodes exhibit a significantly enhanced photoelectrochemical water oxidation activity and stability in comparison to pristine MO photoanodes. From experimental analyses and density functional theory calculations, Co‐agZIF‐62 can effectively promote charge transfer and separation, improve carrier mobility, accelerate the kinetics of oxygen evolution reaction (OER), and thus improve PEC performance. This MOF glass not only serves as an excellent OER cocatalyst on tunable photoelectrodes, but also enables promising opportunities for PEC devices for solar energy conversion.

Enhancing photoelectrochemical water oxidation activity of BiVO4 photoanode through the Co-catalytic effect of Ni(OH)2 and carbon quantum dots

The CQDs + Ni(OH)2/BiVO4 photoanode was fabricated by introducing CQDs with fluorescence and optical properties. The CQDs + Ni(OH)2/BiVO4 photoanode exhibited a low water oxidation onset potential (∼0.2 V vs. RHE) and a photocurrent density of 3.05 mA cm−2 at a voltage of 1.23 V vs. RHE, which is about 2.5 times than that of the unmodified BiVO4 electrode, and the charge injection efficiency is 65%. Surface dynamics test showed that CQDs + Ni(OH)2/BiVO4 photoanode had higher charge transfer rate and lower charge recombination rate than BiVO4 electrode. As a cheap and stable catalyst for water oxidation, Ni(OH)2 combined with CQDs can effectively improve the migration of photogenerated electrons and holes, thereby improving the photoelectrochemical (PEC) water oxidation activity of the photoanode.

Immobilization of cobalt-based polyoxometalates on titanium dioxide nanorod for enhancement in photoelectrochemical water oxidation

In this study, an effective hole scavenger layer of polyoxometalates (POMs) with excellent redox properties and high durability is coated on titanium dioxide (TiO2) nanorods to enhance the photoelectrochemical (PEC) water oxidation activity. Nanocomposite films, composing of cobalt-based POMs (Co-POMs) and TiO2 nanorod, were first fabricated by hydrothermal reaction, following by layer-by-layer (LbL) growth of Co-POMs on the TiO2 surface. It can be observed that the onset potential of Co-POMs immobilized TiO2 photoanode was lower and the induced photocurrent at the applied potential of 1.23 V vs. reversible hydrogen electrode (RHE) was almost two times that of pristine TiO2 under neutral conditions. Co-POMs layers would retard the recombination of photoinduced electron-hole pairs by extracting and transporting the generated holes, thus ameliorate the overall PEC performance. There was an optimal number of Co-POM layers beyond which the produced photocurrent density started to decline. This work provides a facile LbL method to deposit Co-POMs layers on TiO2 nanorod to promote PEC water oxidation under a neutral environment, which is proved to be more favorable for long-term application.

Revealing the enhanced photoelectrochemical water oxidation activity of Fe-based metal-organic polymer-modified BiVO4 photoanode

Metal-organic polymers (MOPs) can enhance the photoelectrochemical (PEC) water oxidation performance of BiVO4 photoanodes, but their PEC mechanisms have yet to be comprehended. In this work, we constructed an active and stable composite photoelectrode by overlaying a uniform MOP on the BiVO4 surface using Fe2+ as the metal ions and 2,5-dihydroxyterephthalic acid (DHTA) as ligand. Such modification on the BiVO4 surface yielded a core-shell structure that could effectively enhance the PEC water oxidation activity of the BiVO4 photoanode. Our intensity-modulated photocurrent spectroscopy analysis revealed that the MOP overlayer could concurrently reduce the surface charge recombination rate constant (ksr) and enhance the charge transfer rate constant (ktr), thus accelerating water oxidation activity. These phenomena can be ascribed to the passivation of the surface that inhibits the recombination of the charge carrier and the MOP catalytic layer that improves the hole transfer. Our rate law analysis also demonstrated that the MOP coverage shifted the reaction order of the BiVO4 photoanode from the third-order to the first-order, resulting in a more favorable rate-determining step where only one hole accumulation is required to overcome water oxidation. This work provides new insights into the reaction mechanism of MOP-modified semiconductor photoanodes.

ZnO/BiOI heterojunction photoanodes with enhanced photoelectrochemical water oxidation activity

ZnO/BiOI heterojunction photoanode thin films were prepared by aerosol-assisted chemical vapour deposition, and the impact of growth temperature and film thickness on the water oxidation functionality was systematically investigated. A top ZnO layer with a thickness of 120 nm (deposited at 350 °C) and a 390 nm thick BiOI layer (deposited at 300 °C) were found to achieve the best photoelectrochemical performance of the heterojunction. The ZnO/BiOI heterojunction exhibited a significant increase in photoelectrochemical activity, with a photocurrent of 0.27 mA·cm−2 observed at 1.1 VRHE (350 nm, 2.58 mW·cm−2), which is ~ 2.2 times higher than that of single-layer ZnO and far higher than that of BiOI. Photoluminescence spectroscopy and transient absorption spectroscopy measurements showed that there was effective charge transfer across the heterojunction which spatially separated charge carriers and increased their lifetime and ability to drive photoelectrochemical water oxidation.

Nanocrystalline Iron Pyrophosphate-Regulated Amorphous Phosphate Overlayer for Enhancing Solar Water Oxidation

HighlightsInducing a localized crystalline iron pyrophosphate in amorphous iron phosphate overlayer.Enhanced photoelectrochemical water oxidation activity with long-term durability.The heterogeneous hybrid structure overcoming the energy barrier in water oxidation.

Effect of SnO2 incorporation on the photoelectrochemical properties of α-Fe2O3–SnO2 nanocomposites prepared by hydrothermal method

In this work, we report the effect of tin oxide (SnO2) incorporation on the photoelectrochemical (PEC) water oxidation activity of hematite-tin oxide (α-Fe2O3–SnO2) nanocomposites prepared by hydrothermal method. During synthesis of α-Fe2O3–SnO2, the proportions of SnO2 in the composites were varied from 5 wt% to 50 wt%. The samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Transmission electron microscopy (TEM), Fourier-transform infrared spectroscopy (FTIR), and UV–visible spectroscopy. XRD results showed diffraction signals from both α-Fe2O3 and SnO2 in the composite structure. The surface morphology and size features of α-Fe2O3 altered drastically with addition of SnO2. TEM analysis revealed that small-sized SnO2 were coated on the surface of α-Fe2O3, forming heterogeneous grain boundary interfaces. The α-Fe2O3–SnO2 nanocomposites were tested as photoanode for PEC water oxidation under standard 1 Sun illumination condition in 1 M NaOH. Among the various compositions, α-Fe2O3–SnO2 (50 wt%) displayed promising water splitting photocurrent density of 0.32 mAcm−2 at 1.4 V versus RHE, which is a 5-fold higher as compared to the photocurrent density of pure α-Fe2O3 (0.06 mAcm−2). This enhancement in photocurrent density can be attributed to the reduced electron-hole recombination as a result of better charge separation at local heterojunction between α-Fe2O3 and SnO2 in the composite.

Boosting photoelectrochemical activity of bismuth vanadate by implanting oxygen-vacancy-rich cobalt (oxy)hydroxide

Surface charge recombination is regarded as a detrimental factor that severely downgrades the photoelectrochemical (PEC) performance of bismuth vanadate (BiVO4). In this work, we demonstrate defect-rich cobalt (oxy)hydroxides (Co(O)OH) as an excellent cocatalyst nanolayer sheathed on BiVO4 to substantially improve the PEC water oxidation activity. The self-transformation of metal–organic framework produces an ultrathin Co(O)OH layer rich in oxygen vacancies, which could serve as a powerful hole extraction engine to promote the charge transfer/separation efficiency as well as an excellent oxygen evolution reaction catalyst to accelerate the surface water oxidation kinetics. As a result, the BiVO4/Co(O)OH hybrid photoanode achieves remarkably inhibited surface charge recombination and presents a prominent photocurrent density of 4.2 mA cm−2 at 1.23 V vs. RHE, which is around 2.6-fold higher than that of the pristine BiVO4. Moreover, the Co(O)OH cocatalyst nanolayer significantly reduces the onset potential of BiVO4 photoanodes by 200 mV. This work provides a versatile strategy for rationally preparing oxygen-vacancy-rich cocatalysts on various photoanodes toward high-efficient PEC water oxidation.

Design and fabrication of ternary BiVO4/FeVO4/Cu2O nanorod array photoelectrode for boosting photoelectrochemical water oxidation

In this study, we demonstrate for the first time the construction of a ternary BiVO4/FeVO4/Cu2O nanorod array (NRA) photoelectrode via seed-assisted refluxing, hydrothermal and successive ionic-layer adsorption and reaction (SILAR) method and its photoelectrochemical (PEC) water oxidation performance. The energy band structures of the BiVO4, FeVO4 and Cu2O show that two type-II band alignments with a ladder-like structure are formed in the constructed ternary BiVO4/FeVO4/Cu2O NRA photoelectrode, enabling the photoelectrode to display substantially enhanced PEC performance. The constructed ternary photoelectrode loaded with the optimized Cu2O nanoparticles achieves a zero-bias photocurrent density of 50 μA cm−2, 5 times higher than that of the pure BiVO4 NRA photoelectrode. The measurements of optical property reveal that the constructed BiVO4/FeVO4/Cu2O nanorod arrays (NRAs) demonstrate improved light absorption capability. The electrochemical impedance and photoluminescence measurements confirm that the charge carriers of the BiVO4/FeVO4/Cu2O heterojunction NRAs are transferred and separated effectively. The substantially boosted PEC water oxidation activity of the constructed ternary BiVO4/FeVO4/Cu2O heterojunction NRA photoelectrode is ascribed to the improved light harvesting and effective charge separation.

Cd-Doped Polyoxotitanium Nanoclusters with a Modifiable Organic Shell for Photoelectrochemical Water Splitting.

Incorporating heterometal and chromogenic groups into the titanium oxo cluster (TOC) nanomaterials is one of the effective strategies for the development of new high-performance photoelectrically active materials. In this Article, we report the structures and photoelectrochemical (PEC) performances of a family of TOCs, including pure [Ti12O8(OEt)16L8] ({Me-Ti12}) and six Cd-doped clusters formulated as [H4Cd2Ti10O8(OEt)16(L)8(H2O)2] ({Cd2Ti10}; L = salicylic acid and their derivatives). The six Cd-doped clusters are isostructural, containing the same {Cd2Ti10O8} core, but are protected by salicylic ligands modified with different functional groups. The compositions, structures, and solution stability of these clusters have been studied in detail by single-crystal X-ray diffraction and electrospray ionization mass spectrometry measurements. The embedding of heterometallic Cd(II) and chemical modification of organic protective shells can effectively regulate the PEC water oxidation activity of those clusters, with {F-Cd2Ti10} having the highest turnover number of 518.55 and the highest turnover frequency of 172.85 h-1. Our work highlights the potential of using TOCs that do not contain noble metals as water oxidation catalysts, and their catalytic activity can be regulated by structural modification.

Pt‐Induced Defects Curing on BiVO4 Photoanodes for Near‐Threshold Charge Separation

AbstractPhotostability is one of the most essential properties for evaluating photoelectrochemical (PEC) water splitting performance on semiconductors. Herein, the oxygen‐deficiency conditions are applied to tune and activate BiVO4 photoanodes with a class of oxygen vacancies across the whole bulk material, and regulate the electronic occupancy of these states upon the charge carrier processes that determine PEC water oxidation activity. Through the experimental results and nonadiabatic molecular dynamics with time‐domain density functional theory calculations, the charge carrier lifetime can be influenced by the oxygen vacancies concentration on BiVO4, and the semiconductor can be flexibly photoactivated under oxygen‐sufficient and deficient atmospheres for enhancing the charge carrier density and photovoltage. The PEC performance of BiVO4 is further boosted by Pt doping, which exhibits a record photocurrent density of 5.45 mA cm–2 at 1.23 VRHE with solar conversion efficiency of 2.1% at 0.65 VRHE. The Pt can prevent the unnecessory charge recombination on the defected BiVO4, which also enhances the majority charge carrier density, resulting in one of the best charge separation efficiencies, close to 100%, among the steady‐state PEC performance for BiVO4. More importantly, the resulting Pt:BiVO4 presents long‐term stability over 50 h at 0.8 VRHE.