Combined Covalent and Supramolecular Polymerization to Reinforce Perylenebisimide Photosynthetic "Quantasomes"

PSII-inspired quantasomes have emerged as promising artificial photosystems evolving oxygen from water due to their integrated multi-chromophore asset, hierarchical architecture, and efficient light-harvesting capabilities. In this study, we adopt a combined covalent and supramolecular strategy by implementing a poly-styrene backbone that reinforces proximity and pairing between adjacent perylenebisimide (PBI) quantasome units. The covalent fixation of the quantasome network results in a significant enhancement of the photoelectrocatalytic performance on engineered IO-ITO photoanodes, with up to 290 % photocurrent increase (J up to 100 μA cm-2, λ >450 nm, applied bias <1.23 V vs RHE, F.E.O2 >80 %) compared to the non-polymerized analog. Moreover, the direct PBI-quantasome polymerization on the photoanode surface was performed by light irradiation of the radical initiator 2,2'-Azobis(2-methylpropionamidine), improving the photoelectrode robustness under high solar irradiance (>8 suns) and limiting the photocurrent loss (<20 %) at 1.52 V vs RHE compared to the non-polymerized system.

Strong Interactions between Au Nanoparticles and BiVO4 Photoanode Boosts Hole Extraction for Photoelectrochemical Water Splitting

Strong metal-support interaction (SMSI) is widely proposed as a key factor in tuning catalytic performances. Herein, the classical SMSI between Au nanoparticles (NPs) and BiVO4 (BVO) supports (Au/BVO-SMSI) is discovered and used innovatively for photoelectrochemical (PEC) water splitting. Owing to the SMSI, the electrons transfer from V4+ to Au NPs, leading to the formation of electron-rich Au species (Auδ-) and strong electronic interaction (i.e., Auδ--Ov-V4+), which readily contributes to extract photogenerated holes and promote charge separation. Benefitted from the SMSI effect, the as-prepared Au/BVO-SMSI photoanode exhibits a superior photocurrent density of 6.25 mA cm-2 at 1.23 V versus the reversible hydrogen electrode after the deposition of FeOOH/NiOOH cocatalysts. This work provides a pioneering view for extending SMSI effect to bimetal oxide supports for PEC water splitting, and guides the interfacial electronic and geometric structure modulation of photoanodes consisting of metal NPs and reducible oxides for improved solar energy conversion efficiency.

Bias distribution and regulation in photoelectrochemical overall water-splitting cells

The water oxidation half-reaction at anodes is always considered the rate-limiting step of overall water splitting (OWS), but the actual bias distribution between photoanodes and cathodes of photoelectrochemical (PEC) OWS cells has not been investigated systematically. In this work, we find that, for PEC cells consisting of photoanodes (nickel-modified n-Si [Ni/n-Si] and α-Fe2O3) with low photovoltage (Vph < 1 V), a large portion of applied bias is exerted on the Pt cathode for satisfying the hydrogen evolution thermodynamics, showing a thermodynamics-controlled characteristic. In contrast, for photoanodes (TiO2 and BiVO4) with Vph > 1 V, the bias required for cathode activation can be significantly reduced, exhibiting a kinetics-controlled characteristic. Further investigations show that the bias distribution can be regulated by tuning the electrolyte pH and using alternative half-reaction couplings. Accordingly, a volcano plot is presented for the rational design of the overall reactions and unbiased PEC cells. Motivated by this, an unbiased PEC cell consisting of a simple Ni/n-Si photoanode and Pt cathode is assembled, delivering a photocurrent density of 5.3 ± 0.2 mA cm-2.

Quantum Dots Mediated Heterojunction Coupling MoSe2 Photoanode for Photoelectrochemical Water Splitting

Graphene quantum dots (GQDs) possess the photosensitive absorption for photoelectrochemical hydrogen evolution owing to special band structures, whereas they usually confront with photo-corrosion or undesired charge recombination during photoelectrochemical reactions. Hence, we establish the heterojunction between GQDs and MoSe2 sheets via a hydrothermal process for improved stability and performance. Photoanodic water splitting with hydrogen evolution boosted by the heteroatom doped N,S-GQDs/MoSe2 heterojunction has been attained due to the abundant active sites, promoted charge separation and transfer kinetics with reduced energy barriers. Diphasic 1T and 2H MoSe2 sheet-hybridized quantum dots contribute to the Schottky heterojunction, which can play a key role in expedited carrier transport to inhibit accumulative photo-corrosion and increase photocurrent. Heteroatom dopants lead to favored energy band matching, bandgap narrowing, stronger light absorption and high photocurrent density. The external quantum efficiency of the doped heterojunction has been elevated twofold over that of the non-doped pristine heterojunction. Modification of the graphene quantum dots and MoSe2 heterojunction demonstrate a viable and adaptable platform toward photoelectrochemical hydrogen evolution processes.

Improvement in Photoelectrochemical Water Splitting Performance of GaN-nanowire Photoanode Using MXene

The photoelectrochemical water splitting (PEC-WS) performance of a photoanode consisting of GaN nanowires (NWs) is significantly improved using a Ti3C2-MXene coating as an intermediate layer to promote carrier transfer toward the electrolyte. The maximum current density and applied-bias photon-to-current efficiency of the photoanode comprising GaN NWs coated with Ti3C2-MXene (MGNWs) are measured to be 34.24 mA/cm2 and 14.47% at 1.2 and 0.4 V versus a reversible hydrogen electrode (RHE), respectively. These values are much higher than those of the GaN-NW photoanode without Ti3C2-MXene (4.04 mA/cm2 and 1.95%) and also markedly exceed those of previously reported photoanodes. After 8 days of PEC-WS, the current density was measured to be 31.07 mA/cm2, which corresponds to 97.58% of that measured immediately after the reaction started. Based on the time dependence of the current density, the hydrogen evolution rate over the reaction time is calculated to be 0.58 mmol/cm2·h. The results confirm that the PEC-WS performance of the optimized MGNW photoanode is superior to and more stable than those of previously reported photoanodes.

Synergistic Effect of Co3(HPO4)2(OH)2 Cocatalyst and Al2O3 Passivation Layer on BiVO4 Photoanode for Enhanced Photoelectrochemical Water Oxidation

Bismuth vanadate (BVO) is regarded as an exceptional photoanode material for photoelectrochemical (PEC) water splitting, but it is restricted by the severe photocorrosion and slow water oxidation kinetics. Herein, a synergistic strategy combined with a Co3(HPO4)2(OH)2 (CoPH) cocatalyst and an Al2O3 (ALO) passivation layer was proposed for enhanced PEC performance. The CoPH/ALO/BVO photoanode exhibits an impressive photocurrent density of 4.9 mA cm-2 at 1.23 VRHE and an applied bias photon-to-current efficiency (ABPE) of 1.47% at 0.76 VRHE. This outstanding PEC performance can be ascribed to the suppressed surface charge recombination, facilitated interfacial charge transfer, and accelerated water oxidation kinetics with the introduction of the CoPH cocatalyst and ALO passivation layer. This work provides a novel and synergistic approach to design an efficient and stable photoanode for PEC applications by combining an oxygen evolution cocatalyst and a passivation layer.

Morphology-Controlled ZnO@ZnWO4 Hetero-Nanostructures for Efficient Photooxidation of Water in Near-Neutral pH

One-dimensional ZnO nanorods (NRs) have been extensively studied as photoanodes because of their unique optical properties, high electron mobility, and suitable band positions for water oxidation. However, their practical efficiency is often compromised by chemical instability during water oxidation and high carrier recombination rates. To overcome this issue, precise morphological control of ZnO@ZnWO4 core-shell structured photoanodes, featuring a ZnO core and a ZnWO4 shell was used. This was accomplished by depositing WO3 onto hydrothermally grown ZnO NRs using the thermal chemical vapor deposition process. The photoelectrochemical performance of ZnO@ZnWO4 with an optimized morphology outperforms that of pristine ZnO NRs. Systematic optical and electrochemical analyses of ZnO@ZnWO4 demonstrated that the enhancement is attributed to the enhanced charge transfer efficiency facilitated by the optimized ZnWO4 shells.

An Interface-cascading Silicon Photoanode with Strengthened Built-in Electric Field and Enriched Surface Oxygen Vacancies for Efficient Photoelectrochemical Water Splitting

To promote interfacial charge transfer process and accelerate surface water oxidation reaction kinetics for photoelectrochemical (PEC) water splitting over n-type Silicon (n-Si) based photoanodes, herein, starting with surface stabilized n-Si/CoOx , a NiOx /NiFeOOH composite overlayer was coated by atomic layer deposition and spray coating to fabricate the multilayer structured n-Si/CoOx /NiOx /NiFeOOH photoanode. Encouragingly, the obtained n-Si/CoOx /NiOx /NiFeOOH photoanode exhibits much increased PEC activity for water splitting, with onset potential cathodically shifted to ~0.96 V vs. RHE and photocurrent density increased to 22.6 mA cm-2 at 1.23 V vs. RHE for OER, as compared to n-Si/CoOx , even significantly surpassing the counterpart n-Si/CoOx /NiOx /FeOOH and n-Si/CoOx /NiOx /NiOOH photoanodes. Photophysical and electrochemical characterizations evidence that the deposited CoOx /NiOx /NiFeOOH composite overlayer would create large band bending and strong built-in electric field at the introduced cascading interfaces, thereby producing a large photovoltage of 650 mV to efficiently accelerate charge transfer from the n-Si substrate to the electrolyte for water oxidation. Furthermore, the surface oxygen vacancy enriched NiFeOOH overlayer could effectively catalyze the water oxidation reaction by thermodynamically reducing the energy barrier of rate determining step for OER.

Controlled Growth of WO3 Photoanode under Various pH Conditions for Efficient Photoelectrochemical Performance

Herein, the effects of the precursor solution's acidity level on the crystal structure, morphology, nucleation, and growth of WO3·nH2O and WO3 nanostructures are reported. Structural investigations on WO3·nH2O using X-ray diffraction and Fourier-transform infrared spectroscopy confirm that the quantity of hydrate groups increases due to the interaction between H+ and water molecules with increasing HCl volume. Surface analysis results support our claim that the evolution of grain size, surface roughness, and growth direction on WO3·nH2O and WO3 nanostructures rely on the precursor solution's pH value. Consequently, the photocurrent density of a WO3 photoanode using a HCl-5 mL sample achieves the best results with 0.9 mA/cm2 at 1.23 V vs. a reversible hydrogen electrode (RHE). We suggest that the improved photocurrent density of the HCl-5 mL sample is due to the efficient light absorption from the densely grown WO3 nanoplates on a substrate and that its excellent charge transport kinetics originate from the large surface area, low charge transfer resistance, and fast ion diffusion through the photoanode/electrolyte interface.

Enhanced Carrier Dynamics of CsPbBr3 Nanocrystals Enabled by Short-Ligand Ethanedithiol for Efficient Photoelectrocatalytic Photoanodes

Photoelectrochemical (PEC) water splitting is a potential solution for a low-carbon society and clean energy storage due to its ability to produce hydrogen and oxygen. However, the slow oxidation half-reaction of the process has limited its overall efficacy, necessitating the development of an efficient photoanode. Colloidal CsPbBr3 nanocrystals (NCs) have been identified as promising candidates due to their high light absorption and valence band position. However, the presence of the electrical insulator, long-chain oleate molecules, on the surface of the CsPbBr3 NCs has hindered efficient charge carrier separation and transport. To solve this problem, short-chain 1,2-ethanedithiol (EDT) ligands were used to replace the oleate ligands on the surface of the CsPbBr3 NCs through a solid-state ligand exchange method. This resulted in a reduction of the nanocrystal spacing and a cross-linking reaction, which improved the photogenerated carrier separation and transport while still passivating the dangling bonds on the CsPbBr3 NC surface. Ultimately, this led to a remarkable photocurrent density of 3.34 mA cm-2 (1.23 VRHE), which was 5.2 times higher than that of the pristine oleate-CsPbBr3 NC (0.64 mA cm-2)-based device. This work presents an efficient way of developing inorganic lead halide perovskite colloidal nanocrystal-based photoanodes through surface ligand engineering.

Strong Intermixing Effects of LFO1-x/STOx toward the Development of Efficient Photoanodes for Photoelectrocatalytic Applications

Aiming to improve the photocatalytic properties of transition metal perovskites to be used as robust photoanodes, [LaFeO3]1-x/[SrTiO3]x nanocomposites (LFO1-x/STOx) are considered. This hybrid structure combines good semiconducting properties and an interesting intrinsic remanent polarization. All the studied samples were fabricated using a solid-state method followed by high-energy ball milling, and they were subsequently deposited by spray coating. The synthesized compounds were demonstrated to possess orthorhombic (Pnma) and cubic (Pm3¯m) structures for LFO and STO, respectively, with an average grain size of 55-70 nm. The LFO1-x/STOx nanocomposites appeared to exhibit high visible light absorption, corresponding to band gaps of 2.17-3.21 eV. Our findings show that LFO0.5/STO0.5 is the optimized heterostructure; it achieved a high photocurrent density of 11 μA/cm2 at 1.23 V bias vs. RHE and an applied bias photo-to-current efficiency of 4.1 × 10-3% at 0.76 V vs. RHE, as demonstrated by the photoelectrochemical measurements. These results underline the role of the two phases intermixing LFO and STO at the appropriate content to yield a high-performing photoanode ascribed to efficient charge separation and transfer. This suggests that LFO0.5/STO0.5 could be a potential candidate for the development of efficient photoanodes for hydrogen generation via photoelectrocatalytic water splitting.

Architecture design of TiO2 with Co-doped CdS quantum dots photoelectrode for water splitting

Photoelectrochemical hydrogen production is a critical key to solving the carbon-zero goal of countries due to renewable sources of solar light and combustion products of hydrogen-only water. Here, an architecture design for an n-type nano rosettes-rod TiO2 (RT) surface using CdS and Co-doped CdS quantum dots (QDs) is carried out utilizing the SILAR (simple ionic layer adsorption and reaction) method. Furthermore, the photocatalytic behaviour of Co-doped CdS QDs SILAR cycles deposition is investigated in various cycles, including 5, 8, 10, and 12. The FESEM, Raman XRD, Uv-Vis spectrometer, and vibration modes are used to evaluate the photoelectrode surface structure, crystal structure, and solar light absorption, respectively. FESEM images and XRD pattern revealed successive CdS QDS and Co-doped CdS QDs deposition on the RT boundary and rising SILAR cycles of Co-doped CdS QDs lead to further coverage of RT surface. UV-vis spectrometer indicated shifting solar light absorption to the visible region by applying more SILAR cycles of Co-doped CdS QDs deposition. The electrochemical parameters obtained from EIS showed total polarization resistance (Rp) of the RT electrode dramatically decreased with 10 SILAR cycle Co-doped CdS QDs deposition (5093 Ω cm2 and 617 Ω cm2). Linear sweep voltammetry (LSV) and chronoamperometric photocatalytic performance measurements indicated Co-doped CdS QDs on RT extremely enhanced photoresponse under solar irradiation and 10 SILAR cycle Co-doped CdS QDs improved photocurrent density about fourfold according to blank RT electrode.

Integration of Ag Plasmonic Metal and WO3/InGaN Heterostructure for Photoelectrochemical Water Splitting

In this study, a Ag/WO₃/InGaN hybrid heterostructure was successfully developed by sputtering and molecular beam epitaxy techniques, to obtain unique Ag nanospheres adorned with cauliflower-like WO₃ nanostructure over the InGaN nanorods (NRs). Exploiting the localized surface plasmon resonance of Ag, the Ag/WO₃/InGaN heterostructure exhibited superior photoabsorption ability in the visible region (400-700 nm) of the solar spectrum, with a surface plasmon resonance band centered around 440 nm. Comprehensive analysis through photoluminescence spectroscopy, photocurrent measurements, and electrochemical impedance spectroscopy revealed that the Ag/WO₃/InGaN hybrid heterostructure significantly enhances the charge carrier separation and transfer kinetics leading to improved overall photoelectrochemical (PEC) performance. The photocurrent density of the Ag/WO₃/InGaN photoanode is 1.17 mA/cm², which is about 2.72 times higher than that of pure InGaN NRs under visible light irradiation. The photoanode exhibited excellent stability for about 12 h. From the study, it has been found that the maximum applied bias photon-to-current efficiency (ABPE) is ∼1.67% at the applied bias of 0.6 V. The improved PEC water splitting efficiency of the Ag/WO₃/InGaN photoanode is attributed to the synergistic effects of localized surface plasmon resonance (LSPR), efficient charge carrier separation and transport, and the presence of a Schottky junction. Consequently, the plasmonic metal-assisted heterojunction-based semiconductor Ag/WO₃/InGaN demonstrates immense potential for practical applications in photoelectrochemical water splitting.

Surface Oxygen Species in Metal Oxide Photoanodes for Solar Energy Conversion

Converting and storing solar energy directly as chemical energy through photoelectrochemical devices are promising strategies to replace fossil fuels. Metal oxides are commonly used as photoanode materials, but they still encounter challenges such as limited light absorption, inefficient charge separation, sluggish surface reactions, and insufficient stability. The regulation of surface oxygen species on metal oxide photoanodes has emerged as a critical strategy to modulate molecular and charge dynamics at the reaction interface. However, the precise role of surface oxygen species in metal oxide photoanodes remains ambiguous. The review focuses on elucidating the formation and regulation mechanisms of various surface oxygen species in metal oxides, their advantages and disadvantages in photoelectrochemical reactions, and the characterization methods employed to investigate them. Additionally, the article discusses emerging opportunities and potential hurdles in the regulation of surface oxygen species. By shedding light on the significance of surface oxygen species, this review aims to advance our understanding of their impact on metal oxide photoanodes, paving the way for the design of more efficient and stable photoelectrochemical devices.

Charge Transport Enhancement in BiVO4 Photoanode for Efficient Solar Water Oxidation

Photoelectrochemical (PEC) water splitting in a pH-neutral electrolyte has attracted more and more attention in the field of sustainable energy. Bismuth vanadate (BiVO₄) is a highly promising photoanode material for PEC water splitting. Additionally, cobaltous phosphate (CoPi) is a material that can be synthesized from Earth's rich materials and operates stably in pH-neutral conditions. Herein, we propose a strategy to enhance the charge transport ability and improve PEC performance by electrodepositing the in situ synthesis of a CoPi layer on the BiVO₄. With the CoPi co-catalyst, the water oxidation reaction can be accelerated and charge recombination centers are effectively passivated on BiVO₄. The BiVO₄/CoPi photoanode shows a significantly enhanced photocurrent density (J_ph) and applied bias photon-to-current efficiency (ABPE), which are 1.8 and 3.2 times higher than those of a single BiVO₄ layer, respectively. Finally, the FTO/BiVO₄/CoPi photoanode displays a photocurrent density of 1.39 mA cm^-2 at 1.23 V_RHE, an onset potential (V_on) of 0.30 V_RHE, and an ABPE of 0.45%, paving a potential path for future hydrogen evolution by solar-driven water splitting.

Temperature-Controlled Transformation of WO3 Nanowires into Active Facets-Exposed Hexagonal Prisms toward Efficient Visible-Light-Driven Water Oxidation

A unique transformation of WO₃ nanowires (NW-WO₃) into hexagonal prisms (HP-WO₃) was demonstrated by tuning the temperature of the (N₂H₄)WO₃ precursor suspension prepared from tungstic acid and hydrazine as a structure-directing agent. The precursor preparation at 20 °C followed by calcination at 550 °C produced NW-WO₃ nanocrystals (ca. <100 nm width, 3-5 μm length) with anisotropic growth of monoclinic WO₃ crystals to (002) and (200) planes and a polycrystalline character with randomly oriented crystallites in the lateral face of nanowires. The precursor preparation at 45 °C followed by calcination at 550 °C produced HP-WO₃ nanocrystals (ca. 500-1000 nm diameter) with preferentially exposed (002) and (020) facets on the top-flat and side-rectangle surfaces, respectively, of hexagonal prismatic WO₃ nanocrystals with a single-crystalline character. The HP-WO₃ electrode exhibited the superior photoelectrochemical (PEC) performance for visible-light-driven water oxidation to that for the NW-WO₃ electrode; the incident photon-to-current conversion efficiency (IPCE) of 47% at 420 nm and 1.23 V vs RHE for HP-WO₃ was 3.1-fold higher than 15% for the NW-WO₃ electrode. PEC impedance data revealed that the bulk electron transport through the NW-WO₃ layer with the unidirectional nanowire structure is more efficient than that through the HP-WO₃ layer with the hexagonal prismatic structure. However, the water oxidation reaction at the surface for the HP-WO₃ electrode is more efficient than the NW-WO₃ electrode, contributing significantly to the superior PEC water oxidation performance observed for the HP-WO₃ electrode. The efficient water oxidation reaction at the surface for the HP-WO₃ electrode was explained by the high surface fraction of the active (002) facet with fewer grain boundaries and defects on the surface of HP-WO₃ to suppress the electron-hole recombination at the surface.

Application of Covalent Organic Frameworks (COFs) as Dyes and Additives for Dye-Sensitized Solar Cells (DSSCs)

Five Covalent Organic Frameworks (COFs) were synthesized and applied to Dye-Sensitized Solar Cells (DSSCs) as dyes and additives. These porous nanomaterials are based on cheap, abundant commercially available ionic dyes (thionin acetate RIO-43, Bismarck brown Y RIO-55 and pararosaniline hydrochloride RIO-70), and antibiotics (dapsone RIO-60) are used as building blocks. The reticular innovative organic framework RIO-60 is the most promising dye for DSSCs. It possesses a short-circuit current density (J_sc) of 1.00 mA/cm², an open-circuit voltage (V_oc) of 329 mV, a fill factor (FF) of 0.59, and a cell efficiency (η) of 0.19%. These values are higher than those previously reported for COFs in similar devices. This first approach using the RIO family provides a good perspective on its application in DSSCs as a dye or photoanode dye enhancer, helping to increase the cell's lifespan.

Selective Valorization of Glycerol to Formic Acid on a BiVO4 Photoanode through NiFe Phenolic Networks

The integration of the glycerol oxidation reaction (GOR) with the hydrogen evolution reaction in photoelectrochemical (PEC) cells is a desirable alternative to PEC water splitting since a large quantity of glycerol is easily accessible as the byproduct from the biodiesel industry. However, the PEC valorization of glycerol to the value-added products suffers from low Faradaic efficiency and selectivity, especially in acidic conditions, which is beneficial for hydrogen production. Herein, by loading bismuth vanadate (BVO) with a robust catalyst composed of phenolic ligands (tannic acid) coordinated with Ni and Fe ions (TANF), we demonstrate a modified BVO/TANF photoanode for the GOR with a remarkable Faradaic efficiency of over 94% to value-added molecules in a 0.1 M Na₂SO₄/H₂SO₄ (pH = 2) electrolyte. The BVO/TANF photoanode achieved a high photocurrent of 5.26 mA·cm^-2 at 1.23 V versus reversible hydrogen electrode under 100 mW/cm² white light irradiation for formic acid production with 85% selectivity, equivalent to 573 mmol/(m²·h). Transient photocurrent and transient photovoltage techniques and electrochemical impedance spectroscopy along with intensity-modulated photocurrent spectroscopy indicated that the TANF catalyst could accelerate hole transfer kinetics and suppress charge recombination. Comprehensive mechanistic investigations reveal that the GOR is initiated by the photogenerated holes of BVO, while the high selectivity to formic acid is attributed to the selective adsorption of primary hydroxyl groups in glycerol on TANF. This study provides a promising avenue for highly efficient and selective formic acid generation from biomass in acid media via PEC cells.

Photoexcitation of Fe3 O Nodes in MOF Drives Water Oxidation at pH=1 When Ru Catalyst Is Present

Artificial photosynthesis strives to convert the energy of sunlight into sustainable, eco-friendly solar fuels. However, systems with light-driven water oxidation reaction (WOR) at pH=1 are rare. Broadly used [Ru(bpy)₃ ]²⁺ (bpy=2,2'-bipyridine) photosensitizer has a fixed +1.23 V potential which is insufficient to drive most water oxidation catalysts (WOCs) in acid, while Fe₂ O₃ , featuring the highly oxidizing holes, is not stable at low pH. Here, the key examples of Fe-based metal-organic framework (MOF) water oxidation photoelectrocatalysts active at pH=1 are presented. Fe-MIL-126 and Fe MOF-dcbpy structures were formed with 4,4'-biphenyl dicarboxylate (bpdc), 2,2'-bipyridine-5,5'-dicarboxylate (dcbpy) linkers and their mixtures. Presence of dcbpy linkers allows integration of metal-based catalysts via coordination to 2,2'-bipyridine fragments. Fe-based MOFs were doped with Ru-based precursors to achieve highly active MOFs bearing [Ru(bpy)(dcbpy)(H₂ O)₂ ]²⁺ WOC. Materials were analyzed with X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infra-red (FTIR) spectroscopy, resonance Raman, X-ray absorption spectroscopy, fs optical pump-probe, electron paramagnetic resonance (EPR), diffuse reflectance and electric conductivity measurements and were modeled by band structure calculations. It is shown that under reaction conditions, Fe^III and Ru^III oxidation states are present, indicating rate-limiting electron transfer in MOF. Fe₃ O nodes emerge as photosensitizers able to drive prolonged O₂ evolution in acid. Further developments are possible via MOF's linker modification for enhanced light absorption, electrical conductivity, reduced MOF solubility in acid, Ru-WOC modification for faster WOC catalysis, or Ru-WOC substitution to 3d metal-based systems. The findings give further insight for development of light-driven water splitting systems based on Earth-abundant metals.

Polymeric carbon nitride as a platform for photoelectrochemical water-splitting cells

Polymeric carbon nitride (CN) materials are promising low-cost photocatalysts that exhibit a combination of chemical and physical properties suitable for converting light into redox activity on their surface. In this perspective, we describe our experience with this family of materials as light absorbers that serve as an anode in photoelectrochemical cells toward water-splitting. We describe some of the CN deposition techniques and procedures established in our lab. The knowledge gained from powder-based photocatalysis is implemented in photoelectrochemical scenarios and is used to determine the merits and shortcomings of resulting layers. We show how the preparation methods are oriented based on these factors and how high photoelectrochemical water-splitting activity develops in photoanodes we developed where CN(s) act as photoabsorbers. Lastly, we present our view on the future prospects of this field.

Porphyrins-Assisted Cocatalyst Engineering with CoOV Bond in BiVO4 Photoanode for Efficient Oxygen Evolution Reaction

The application of photoelectrochemical (PEC) water splitting is limited by the sluggish surface oxygen evolution reaction (OER) kinetics. OER kinetics can be effectively improved through cocatalyst engineering. However, the tardy transfer process and serious recombination of carriers are the key factors restricting the cocatalyst development. Taking BiVO₄ as an example, a Co-modified heme film rich in large conjugated ring structures is introduced onto the photoanode surface using a solvothermal method. This film functions as an efficient cocatalyst. It considerably reduces the surface overpotential, promotes the transfer of photogenerated holes, and boosts the kinetics of OER by specifically affecting the formation of OOH*. Simultaneously, the formed CoOV bonds induce strong interaction at the photoanode/cocatalyst interfaces, reducing the recombination of photogenerated carriers. Consequently, the onset potential of the optimized photoanode decreases from 0.45 to 0.07 V and the photocurrent density at 1.23 V versus reversible hydrogen electrode boosts to 5.3 mA cm^-2 . This work demonstrates a facile strategy for designing cocatalysts to obtain rapid hole transfer capability and reduced carrier recombination for improved PEC performance.

Revealing the Essential Role of Iron Phosphide and its Surface-Evolved Species in the Photoelectrochemical Water Oxidation by Gd-Doped Hematite Photoanode

Phosphates are easily derived from transition metal phosphides under natural conditions, and the real roles of these two in catalytic reactions are not yet clear. Here, a multiphase FeP/Gd-Fe₂ O₃ shell-core structure photoanode was constructed and explored regarding the real role of FeP and its surface-reconstructed iron phosphate (Fe-Pi) in photoelectrochemical water oxidation. The FeP/Gd-Fe₂ O₃ photoanode exhibited an excellent photocurrent density of 2.56 mA cm^-2 at 1.23 V versus the reversible hydrogen electrode, up to 4 times greater than those of the pristine α-Fe₂ O₃ (0.64 mA cm^-2 ). Detailed studies showed that FeP could act as a photosensitizer to enhance light absorption and as a conductive layer to accelerate charge transfer. The FeP significantly enhanced the incident photon-to-current conversion efficiency of the photoanode and improved the electron transition within the photoanode. Naturally evolved Fe-Pi on the surface provided more active sites for water oxidation. They effectively passivated the surface capture state and synergistically inhibited the electron-hole recombination. Moreover, the in-situ constructed multiphase catalyst had a smaller interfacial contact resistance than the intentionally decorative cocatalyst. This work provides new insight into the understanding of the essential role of transition metal phosphides and their surface-reconstructed species in catalytic reactions.

Fabrication of an Efficient N, S Co-Doped WO3 Operated in Wide-Range of Visible-Light for Photoelectrochemical Water Oxidation

In this work, a highly efficient wide-visible-light-driven photoanode, namely, nitrogen and sulfur co-doped tungsten trioxide (S-N-WO₃), was synthesized using tungstic acid (H₂WO₄) as W source and ammonium sulfide ((NH₄)₂S), which functioned simultaneously as a sulfur source and as a nitrogen source for the co-doping of nitrogen and sulfur. The EDS and XPS results indicated that the controllable formation of either N-doped WO₃ (N-WO₃) or S-N-WO₃ by changing the n_W:n_(NH4)2S ratio below or above 1:5. Both N and S contents increased when increasing the n_W:n_(NH4)2S ratio from 1:0 to 1:15 and thereafter decreased up to 1:25. The UV-visible diffuse reflectance spectra (DRS) of S-N-WO₃ exhibited a significant redshift of the absorption edge with new shoulders appearing at 470–650 nm, which became more intense as the n_W:n_(NH4)2S ratio increased from 1:5 and then decreased up to 1:25, with the maximum at 1:15. The values of n_W:n_(NH4)2S ratio dependence is consistent with the cases of the S and N contents. This suggests that S and N co-doped into the WO₃ lattice are responsible for the considerable redshift in the absorption edge, with a new shoulder appearing at 470–650 nm owing to the intrabandgap formation above the valence band (VB) edge and a dopant energy level below the conduction band (CB) of WO₃. Therefore, benefiting from the S and N co-doping, the S-N-WO₃ photoanode generated a photoanodic current under visible light irradiation below 580 nm due to the photoelectrochemical (PEC) water oxidation, compared with pure WO₃ doing so below 470 nm.

Elucidating the Role of Hypophosphite Treatment in Enhancing the Performance of BiVO4 Photoanode for Photoelectrochemical Water Oxidation

Slow water oxidation kinetics and poor charge transport restrict the development of efficient BiVO4 photoanodes for photoelectrochemical (PEC) water splitting. Oxygen vacancy as an effective strategy can significantly enhance charge transport and improve conductivity in semiconductor photoanodes. Herein, we obtained BiVO4 photoanodes with appropriate oxygen vacancy by treating them with hypophosphite, which significantly improved the PEC performance. The synthesized photoanode exhibits a remarkable photocurrent density of 3.37 mA/cm2 at 1.23 V vs reversible hydrogen electrode with excellent stability. Interestingly, the performance improvement mainly originates from the oxygen vacancy rather than P doping. Our study provides insights in understanding the role of oxygen vacancy in PEC water splitting and strategies for designing more efficient photoelectrodes.

Dual co-catalysts activated hematite nanorods with low turn-on potential and enhanced charge collection for efficient solar water oxidation

Hematite (α-Fe₂O₃) photoanode suffers from significant photocarrier recombination and sluggish water oxidation kinetics for photoelectrochemical water splitting. To address these challenges, this work demonstrates the construction of dual co-catalysts modified Fe₂O₃nanorods photoanode by strategically incorporating CoPi and Co(OH)_xfor photoelectrochemical water oxidation. The Fe₂O₃/CoPi/Co(OH)_xnanorods photoanode exhibits the lowest ever turn-on potential of 0.4V_RHE(versus reversible hydrogen electrode) and a photocurrent density of 0.55 mA cm^-2at 1.23V_RHE, 358% higher than that of pristine Fe₂O₃nanorods. The dual co-catalysts modification enhances the light-harvesting efficiency, surface photovoltage and hole transfer kinetics of the hybrid photoanode. The dual co-catalyst coupling also increases the carrier density and significantly reduces the depletion width (1.9 nm), resulting in improved conductivity and favorable band bending, boosting photogenerated hole transfer efficiency for water oxidation.

Fabrication of 3D hierarchical Fe2O3/SnO2photoanode for enhanced photoelectrochemical performance

Exploring and fabricating a suitable photoanode with high catalytic activity is critical for enhancing photoelectrochemical (PEC) performance. Herein, a novel 3D hierarchical Fe₂O₃/SnO₂photoanode was fabricated by a hydrothermal route, combining with an annealing process. The morphology, crystal structure were studied by scanning electron microscopy, transmission electron microscopy, x-ray photon spectroscopy, and x-ray diffraction, respectively. The results reveal the successful preparation of Fe₂O₃nanothorns on the surface of SnO₂nanosheets. The as-fabricated 3D Fe₂O₃/SnO₂photoanode yields obviously promoted PEC performance with a photocurrent density of approximate 5.85 mA cm^-2, measured in a mixture of Na₂S (0.25 M) and Na₂SO₃(0.35 M) aqueous solution at 1.23 V (versus reversible hydrogen electrode, RHE). This value of photocurrent is about 53 times higher than that of the bare SnO₂photoanode. The obvious improved PEC properties can be attributed to the 3D Fe₂O₃/SnO₂heterostructures that offer outstanding light harvesting ability as well as improved charge transport and separation. These results suggest that exploring a suitable 3D hierarchical photoanode is an effective approach to boost PEC performance.

Highly Efficient Photoelectrochemical Hydrogen Production Using Nontoxic CuIn1.5Se3 Quantum Dots with ZnS/SiO2 Double Overlayers

Quantum dots (QDs) are a promising material for photoelectrochemical (PEC) hydrogen (H₂) production because of their attractive optical properties including high optical absorption coefficient, band-gap tunability, and potential multiple exciton generation. To date, QDs containing toxic elements such as Cd or Pb have been mainly investigated for PEC H₂ production, which cannot be utilized in practice because of the environmental issue. Here, we demonstrate a highly efficient type II heterojunction photoanode of nontoxic CuIn_1.5Se₃ (CISe) QDs and a mesoporous TiO₂ film. In addition, ZnS/SiO₂ double overlayers are deposited on the photoanodes to passivate surface defect sites on the CISe QDs, leading to the enhancement of both photocurrent density and photostability. Due to a combination of a wide light absorption range of the CISe QDs and the reduced interfacial charge recombination by the overlayers, a remarkable photocurrent density of 8.5 mA cm^-2 (at 0.5 V_RHE) is obtained under 1 sun illumination, which is a record for the PEC sulfite oxidation based on nontoxic QD photoanodes.

Geometric Optimization of Bismuth Vanadate Core-Shell Nanowire Photoanodes using Atomic Layer Deposition

In this study, systematic geometric tuning of core-shell nanowire (NW) architectures is used to decouple the contributions from light absorption, charge separation, and charge transfer kinetics in photoelectrochemical water oxidation. Core-shell-shell NW arrays were fabricated using a combination of hydrothermal synthesis of ZnO and atomic layer deposition (ALD) of SnO2 and BiVO4. The length and spacing of the NW scaffold, as well as the BiVO4 film thickness, were systematically tuned to optimize the photoelectrochemical performance. A photocurrent of 4.4 mA/cm2 was measured at 1.23 V vs RHE for sulfite oxidation and 4.0 mA/cm2 at 1.80 V vs RHE for water oxidation without a cocatalyst, which are the highest values reported to date for an ALD-deposited photoanode. Electromagnetic simulations demonstrate that spatial heterogeneity in light absorption along the core-shell NW length has a critical role in determining internal quantum efficiency. The mechanistic understandings in this study highlight the benefits of systematically optimizing electrode geometry at the nanoscale when designing photoelectrodes.

Visible-light Bi-Fe-Nb-O photoanodes for solar-light driven hydrogen production

Currently, CO₂ emission is the main cause of climate change and its various related environmental impacts. Therefore, we have as a prime the development of clean sources of energy. The hydrogen economy is very attractive in this regard, however, when generated from the methane reform, there are also large-scale CO₂ emissions. Thus, this research aims to develop and characterize bismuth and iron niobate-based photoanodes for hydrogen production via water photoelectrolysis. Bi₂Fe_xNbO₇ films were synthesized by the sol-gel method and deposited on FTO coated glass plates by dip-coating technique. The influence of heat treatment (400, 500 and 600°C) and amount of iron on the structure (Bi₂Fe_xNbO₇, x = 0, 0.8, 1, 1.2) were evaluated. Optical, structural and morphological properties were performed, as well as photoanode efficiency in photocurrent assays. The results indicate that the increase of temperature as well as the amount of iron leads to a higher absorption capacity and hence to lower band gap values. Regarding the structural properties, it was possible to observe the BFNO phase in the samples treated at 500 and 600°C. The films heat-treated at 400°C had a heterogeneous texture and a good covering. At 600°C there were some cracks in films surface. Therefore, samples with more iron and treated at 400°C showed better responses in photocurrent assays. It can be concluded that bismuth-iron niobate has a great potential to be applied in photoelectrolysis hydrogen production.

Decoration of BiVO4 Photoanodes with Near-Infrared Quantum Dots for Boosted Photoelectrochemical Water Oxidation

Broadening light absorption and improving charge carrier separation are very critical to boost the water splitting efficiency in photoelectrochemical (PEC) systems. We herein reported a heterostructured photoanode consisting of BiVO₄ and eco-friendly, near-infrared (NIR) CuInSeS@ZnS core-shell quantum dots (QDs) for PEC water oxidation. The decoration of core-shell QDs concurrently extends the absorption range of BiVO₄ from the ultraviolet-visible to NIR region and promotes the effective separation and transfer of photo-excited electrons and holes. Without any sacrificial agents and co-catalysts, the as-fabricated NIR core-shell QDs/BiVO₄ heterostructured photoanodes exhibit an approximately fourfold higher photocurrent density than that of the bare BiVO₄, up to 3.17 mA cm^-2 at 1.23 V versus the reversible hydrogen electrode. It is revealed that both a suitable band alignment and an intimate interfacial junction between QDs and BiVO₄ are the main factors that result in enhanced charge separation and transfer efficiencies. We also highlight that the NIR CISeS QDs passivated with a ZnS shell can suppress the non-radiative recombination and enhance the stability of the QD photoanodes for optimized PEC performance. This work provides a facile and effective approach to boost the water oxidation efficiency of semiconductor photoanodes via utilizing NIR core-shell QDs as a light sensitizer and charge carrier separator.

Facile Fabrication of a Highly Crystalline and Well-Interconnected Hematite Nanoparticle Photoanode for Efficient Visible-Light-Driven Water Oxidation

Facile and scalable fabrication of α-Fe₂O₃ photoanodes using a precursor solution containing Fe^III ions and 1-ethylimidazole (EIm) in methanol was demonstrated to afford a rigidly adhered α-Fe₂O₃ film with a controllable thickness on a fluorine-doped tin oxide (FTO) substrate. EIm ligation to Fe^III ions in the precursor solution brought about high crystallinity of three-dimensionally well-interconnected nanoparticles of α-Fe₂O₃ upon sintering. This is responsible for the 13.6 times higher photocurrent density (at 1.23 V vs reference hydrogen electrode (RHE)) for photoelectrochemical (PEC) water oxidation on the α-Fe₂O₃ (w-α-Fe₂O₃) photoanode prepared with EIm compared with that (w/o-α-Fe₂O₃) prepared without EIm. The w-α-Fe₂O₃ photoanode provided the highest charge separation efficiency (η_sep) value of 27% among the state-of-the-art pristine α-Fe₂O₃ photoanodes, providing incident photon-to-current conversion efficiency (IPCE) of 13% at 420 nm and 1.23 V vs RHE. The superior η_sep for the w-α-Fe₂O₃ photoanode is attributed to the decreased recombination of the photogenerated charge carriers at the grain boundary between nanoparticles, in addition to the higher number of the catalytically active sites and the efficient bulk charge transport in the film, compared with w/o-α-Fe₂O₃.

Controlled Photoanode Properties for Large-Area Efficient and Stable Dye-Sensitized Photovoltaic Modules

Nowadays, a dye-sensitized solar cell (DSSC) attracts attention to its development widely due to its several advantages, such as simple processes, low costs, and flexibility. In this work, we demonstrate the difference in device structures between small size and large size cells (5 cm × 5 cm, 10 cm × 10 cm and 10 cm × 15 cm). The design of the photoanode and dye-sensitized process plays important roles in affecting the cell efficiency and stability. The effects of the TiO₂ electrode, using TiCl_4(aq) pretreatment and post-treatment processes, are also discussed, whereas, the open-circuit voltage (Voc), short-circuit current density (Jsc), and module efficiency are successfully improved. Furthermore, the effects on module performances by some factors, such as dye solution concentration, dye soaking temperature, and electrolyte injection method are also investigated. We have demonstrated that the output power of a 5 cm × 5 cm DSSC module increases from 86.2 mW to 93.7 mW, and the module efficiency achieves an outstanding performance of 9.79%. Furthermore, enlarging the DSSC modules to two sizes (10 cm × 10 cm and 10 cm × 15 cm) and comparing the performance with different module designs (C-DSSC and S-DSSC) also provides the specific application of polymer sealing and preparing high-efficiency large-area DSSC modules.

Anatase-Wrapped Rutile Nanorods as an Effective Electron Collector in Hybrid Photoanodes for Visible Light-Driven Oxygen Evolution

Arrays of single crystal TiO2 rutile nanorods (RNRs) appear highly promising as electron-collecting substrates in hybrid photoanodes as the RNRs offer direct charge carriers transport pathways, contrary to the conventional electrodes prepared from TiO2 powders that suffer from the numerous charge traps at the grain boundaries. However, the specific surface area of the nanorods is highly limited by their smooth morphology, which might be detrimental in view of utilizing the RNR as a substrate for immobilizing other functional materials. In this study, we developed a novel anatase-wrapped RNR (ARNR) material fabricated by a facile seed layer-free hydrothermal method. The ARNR comprises polycrystalline anatase nanoparticles formed on the surface of RNR, resulting in a large surface area that provides more deposition sites compared to the bare nanorods. Herein, we functionalize ARNR and RNR electrodes with polymeric carbon nitride (CNx) coupled with a CoO(OH)x cocatalyst for dioxygen evolution. The anatase wrapping of the rutile nanorod scaffold is found to be crucial for effective deposition of CNx and for improved photoanode operation in visible light-driven (λ > 420 nm) oxygen evolution, yielding a significant enhancement of photocurrent (by the factor of ∼3.7 at 1.23 V vs. RHE) and faradaic efficiency of oxygen evolution (by the factor of ∼2) as compared to photoanodes without anatase interlayer. This study thus highlights the importance of careful interfacial engineering in constructing photoelectrocatalytic systems for solar energy conversion and paves the way for the use of ARNR-based electron collectors in further hybrid and composite photochemical architectures for solar fuel production.

Enhanced Photoelectrochemical Water Oxidation Performance in Bilayer TiO2 /α-Fe2 O3 Nanorod Arrays Photoanode with Cu : NiOx as Hole Transport Layer and Co-Pi as Cocatalyst

Efficient charge transfer and excellent surface water oxidation kinetics are key factors in determining the photoelectrochemical (PEC) water splitting performance in photoelectrodes. Herein, a bilayer TiO₂ /α-Fe₂ O₃ nanorod (NR) arrays photoanode was prepared with deposited Cu-doped NiO_x (Cu : NiO_x ) hole transport layer (HTL) and Co-Pi oxygen evolution reaction (OER) cocatalyst for PEC water oxidation. The hierarchical TiO₂ /α-Fe₂ O₃ composite obtained by a secondary hydrothermal process exhibited an inapparent bilayer structure by embedding the underlayer TiO₂ NR arrays at the bottom part of the post-grown α-Fe₂ O₃ NR arrays. The underlayer TiO₂ NRs acted as an effective shuttling pathway for transferring photoelectrons generated in the upper hematite light absorber layer. A p-type inter-Cu : NiO_x HTL was introduced to form a build-in p-n electric field between Cu : NiO_x and α-Fe₂ O₃ NRs, which improved the hole extraction from α-Fe₂ O₃ to Co-Pi OER catalyst. As expected, the as-engineered TiO₂ /α-Fe₂ O₃ /Cu : NiO_x /Co-Pi photoanode displayed an excellent photocurrent density of 2.43 mA cm^-2 at 1.23 V versus the reversible hydrogen electrode (V_RHE ), up to 4.05 and 2.23 times greater than those of the bare α-Fe₂ O₃ (0.60 mA cm^-2 ) and TiO₂ /α-Fe₂ O₃ , respectively. The results demonstrate that the bottom-up engineering of electron-hole transport channels and cocatalyst modification is an attractive maneuver to enhance the PEC water oxidation activity in hematite and other photoanodes.

Sol-Gel Processing of Water-Soluble Carbon Nitride Enables High-Performance Photoanodes*

In spite of the enormous promise that polymeric carbon nitride (PCN) materials hold for various applications, the fabrication of high-quality, binder-free PCN films and electrodes has been a largely elusive goal to date. Here, we tackle this challenge by devising, for the first time, a water-based sol-gel approach that enables facile preparation of thin films based on poly(heptazine imide) (PHI), a polymer belonging to the PCN family. The sol-gel process capitalizes on the use of a water-soluble PHI precursor that allows formation of a non-covalent hydrogel. The hydrogel can be deposited on conductive substrates, resulting in formation of mechanically stable polymeric thin layers. The resulting photoanodes exhibit unprecedented photoelectrochemical (PEC) performance in alcohol reforming and highly selective (∼100 %) conversions with very high photocurrents (>0.25 mA cm-2 under 2 sun) down to <0 V vs. RHE. This enables even effective PEC operation under zero-bias conditions and represents the very first example of a 'soft matter'-based PEC system capable of bias-free photoreforming. The robust binder-free films derived from sol-gel processing of water-soluble PCN thus constitute a new paradigm for high-performance 'soft matter' photoelectrocatalytic systems and pave the way for further applications in which high-quality PCN films are required.

Conjugated π Electrons of MOFs Drive Charge Separation at Heterostructures Interface for Enhanced Photoelectrochemical Water Oxidation

Photoanode material with high efficiency and stability is extensively desirable in photoelectrochemical (PEC) water splitting for green/renewable energy source. Herein, novel heterostructures is constructed via coating rutile TiO₂ nanorods with metal organic framework (MOF) materials UiO-66 or UiO-67 (UiO-66@TiO₂ and UiO-67@TiO₂ ), respectively. The π electrons in the MOF linkers could increase the local electronegativity near the heterojunction interface due to the conjugation effect, thereby enhancing the internal electric field (IEF) at the heterojunction interface. The IEF could drive charge transfer following Z-scheme mechanism in the prepared heterostructures, inducing photogenerated charge separation efficiency increasing as 156% and 253% for the UiO-66@TiO₂ and UiO-67@TiO₂ , respectively. Correspondingly, the UiO-66@TiO₂ and UiO-67@TiO₂ enhanced the photocurrent density as approximate two- and threefolds compared with that of pristine TiO₂ for PEC water oxidation in universal pH electrolytes. This work demonstrates an effective method of regulating the IEF of heterojunction toward further improved charge separation.

Engineering Nanostructure-Interface of Photoanode Materials Toward Photoelectrochemical Water Oxidation

Photoelectrochemical (PEC) water oxidation based on semiconductor materials plays an important role in the production of clean fuel and value-added chemicals. Nanostructure-interface engineering has proven to be an effective way to construct highly efficient PEC water oxidation photoanodes with good light capture, carrier transport, and water oxidation kinetics. However, from theoretical and application perspectives, the relationship between the nanostructure and interface of photoanode materials and their PEC performance remains unclear. In this review, the PEC water oxidation reaction mechanism and evaluation criteria are briefly presented. The theoretical basis and research status of the nanostructure-interface engineering on constructing high-performance PEC water oxidation photoanodes are summarized and discussed. Finally, the current challenges and the future opportunities of nanostructure-interface engineering for the PEC reactions are pointed out.

Hydrothermal Cobalt Doping of Titanium Dioxide Nanotubes towards Photoanode Activity Enhancement

Doping and modification of TiO2 nanotubes were carried out using the hydrothermal method. The introduction of small amounts of cobalt (0.1 at %) into the structure of anatase caused an increase in the absorption of light in the visible spectrum, changes in the position of the flat band potential, a decrease in the threshold potential of water oxidation in the dark, and a significant increase in the anode photocurrent. The material was characterized by the SEM, EDX, and XRD methods, Raman spectroscopy, XPS, and UV-Vis reflectance measurements. Electrochemical measurement was used along with a number of electrochemical methods: chronoamperometry, electrochemical impedance spectroscopy, cyclic voltammetry, and linear sweep voltammetry in dark conditions and under solar light illumination. Improved photoelectrocatalytic activity of cobalt-doped TiO2 nanotubes is achieved mainly due to its regular nanostructure and real surface area increase, as well as improved visible light absorption for an appropriate dopant concentration.

Single-Atom-Based Heterojunction Coupling with Ion-Exchange Reaction for Sensitive Photoelectrochemical Immunoassay

Benefiting from the maximum atom-utilization efficiency and distinct structural features, single-atom catalysts open a new avenue for the design of more functional catalysts, whereas their bioapplications are still in their infancy. Due to the advantages, platinum single atoms supported by cadmium sulfide nanorods (Pt SAs-CdS) are synthesized to build an ultrasensitive photoelectrochemical (PEC) biosensing platform. With the decoration of Pt SAs, the PEC signal of CdS is significantly boosted. Furthermore, theory calculations indicate the positively charged Pt SAs could change the charge distribution and increase the excited carrier density of CdS. Meanwhile, it also suggests that Cu²⁺ can severely hinder the photoexcitation and electron-hole separation of CdS. As a proof of concept, prostate-specific antigen is chosen as the target analyte to demonstrate the superiority of the Pt SAs-CdS-based PEC sensing system. As a result, the PEC biosensor based on Pt SAs-CdS exhibits outstanding detection sensitivity and promising applicability.

Interfacial Engineering at Quantum Dot-Sensitized TiO2 Photoelectrodes for Ultrahigh Photocurrent Generation

Metal oxide semiconductor/chalcogenide quantum dot (QD) heterostructured photoanodes show photocurrent densities >30 mA/cm² with ZnO, approaching the theoretical limits in photovoltaic (PV) cells. However, comparative performance has not been achieved with TiO₂. Here, we applied a TiO₂(B) surface passivation layer (SPL) on TiO₂/QD (PbS and CdS) and achieved a photocurrent density of 34.59 mA/cm² under AM 1.5G illumination for PV cells, the highest recorded to date. The SPL improves electron conductivity by increasing the density of surface states, facilitating multiple trapping/detrapping transport, and increasing the coordination number of TiO₂ nanoparticles. This, along with impeded electron recombination, led to enhanced collection efficiency, which is a major factor for performance. Furthermore, SPL-treated TiO₂/QD photoanodes were successfully exploited in photoelectrochemical water splitting cells, showing an excellent photocurrent density of 14.43 mA/cm² at 0.82 V versus the Reversible Hydrogen Electrode (RHE). These results suggest a new promising strategy for the development of high-performance photoelectrochemical devices.

In Situ Synthesis of α-Fe2O3/Fe3O4 Heterojunction Photoanode via Fast Flame Annealing for Enhanced Charge Separation and Water Oxidation

Hematite (α-Fe₂O₃) is a promising photoanode material in photoelectrochemical (PEC) water splitting. To further improve the catalytic activity, a reasonable construction of heterojunction and surface engineering can effectively improve the photoanode PEC water-splitting performance via improving bulk carrier transport and interfacial charge-transfer efficiency. As Fe₃O₄ has an excellent conductivity and a suitable energy band position, α-Fe₂O₃/Fe₃O₄ heterojunction can be an ideal structure to improve the activity of α-Fe₂O₃. However, only few studies have been reported on α-Fe₂O₃/Fe₃O₄ heterojunctions as photoanodes. In this work, a holey nanorod Fe₂O₃/Fe₃O₄ heterojunction photoanode with oxygen vacancies was fabricated using a rapid and facile flame reduction treatment. Compared with pure Fe₂O₃, the water oxidation performance of the Fe₂O₃/Fe₃O₄ photoanode is improved by ninefold at 1.23 V_RHE. Our study revealed that the porous nanorod structure providing more active sites and oxygen vacancies as the hole transfer medium, together improve the interface charge transfer performance of the photoanode. At the same time, Fe₃O₄ can form a Fe₂O₃/Fe₃O₄ heterojunction to improve the carrier separation efficiency. More importantly, Fe₃O₄ can serve as active sites, solving the slow water oxidation kinetic problem of hematite to enhance the catalytic activity. Our work shows that when flame acts on precursors containing oxygen or hydroxide, it is easy to form compounds with different microstructures or compositions in situ.

Highly Efficient Photoelectrochemical Water Splitting Using GaN-Nanowire Photoanode with Tungsten Sulfides

In the present study, we have achieved high-performance photoelectrochemical water splitting (PEC-WS) using GaN nanowires (NWs) coated with tungsten sulfide (W_xS_1-x) (GaN-NW-W_xS_1-x) as a photoanode. The measured current density and applied-bias photon-to-current efficiency were 20.38 mA/cm² and 13.76%, respectively. These values were much higher than those reported previously for photoanodes with any kind of III-nitride nanostructure. The amount of hydrogen gas formed was 1.01 mmol/cm² from 7 h PEC-WS, which was also much higher than the previously reported values. The drastic improvement in the PEC-WS performance using the GaN-NW-W_xS_1-x photoanode was attributed to an increase in the number of photogenerated carriers due to the highly crystalline GaN NWs, and acceleration of separation of photogenerated carriers and consequent suppression of charge recombination because of nitrogen-terminated surfaces of NWs, sulfur vacancies in W_xS_1-x, and type-II band alignment between NW and W_xS_1-x. The degree of impedance matching, evaluated from Nyquist plots, was considered to analyze charge transfer characteristics at the interface between the GaN-NW-W_xS_1-x photoanode and 0.5-M H₂SO₄ electrolyte. Considering the material system and scheme for the PEC-WS, our approach provides an efficient way to improve hydrogen evolution reaction.

Enhancement of Titania Photoanode Performance by Sandwiching Copper between Two Titania Layers

Vacancies in semiconductors can play a versatile role in boosting their photocatalytic activity. In this work, a novel TiO2/Cu/TiO2 sandwich structure is designed and constructed. Abundant vacancies were introduced in TiO2 lattice by Cu reduction under heat treatment. Meanwhile, Cu atom could diffuse into TiO2 to form Cu-doped TiO2. The synergistic effect between oxygen vacancies and Cu atoms achieved about 2.4 times improved photocurrent of TiO2/Cu/TiO2 sandwich structure compared to bare TiO2 thin film. The enhanced photoactivity may be attributed to regulated electron structure of TiO2 by oxygen vacancies and Cu dopant from experimental results and density functional theory calculations. Oxygen vacancies and Cu dopant in TiO2 formed through copper metal reduction can introduce impurity levels and narrow the band gap of TiO2, thus improve the visible light response. More importantly, the Cu2+ and oxygen vacancies in TiO2 lattice can dramatically increase the charge density around conduction band and promote separation of photo-induced charge carriers. Furthermore, the oxygen vacancies on the surface may serve as active site for sufficient chemical reaction. This work presents a novel method to prepare doped metal oxides catalysts with abundant vacancies for improving photocatalytic activity.

Effective Visible Light Exploitation by Copper Molybdo-tungstate Photoanodes

The need for stable oxide-based semiconductors with a narrow band gap, able to maximize the exploitation of the visible light portion of the solar spectrum, is a challenging issue for photoelectrocatalytic (PEC) applications. In the present work, CuW1-x Mo x O4 (E g = 2.0 eV for x = 0.5), which exhibits a significantly reduced optical band gap E g compared with isostructural CuWO4 (E g = 2.3 eV), was investigated as a photoactive material for the preparation of photoanodes. CuW0.5Mo0.5O4 electrodes with different thicknesses (80-530 nm), prepared by a simple solution-based process in the form of multilayer films, effectively exhibit visible light photoactivity up to 650 nm (i.e., extended compared with CuWO4 photoanodes prepared by the same way). Furthermore, the systematic investigation on the effects on photoactivity of the CuW0.5Mo0.5O4 layer thickness evidenced that long-wavelength photons can better be exploited by thicker electrodes. PEC measurements in the presence of NaNO2, acting as a suitable hole scavenger ensuring enhanced photocurrent generation compared with that of water oxidation while minimizing dark currents, allowed us to elucidate the role that molybdenum incorporation plays on the charge separation efficiency in the bulk and on the charge injection efficiency at the photoanode surface. The adopted Mo for W substitution increases the visible light photoactivity of copper tungstate toward improved exploitation and storage of visible light into chemical energy via photoelectrocatalysis.

Electrochemical Oxidation of Metal-Catechol Complexes as a New Synthesis Route to the High-Quality Ternary Photoelectrodes: A Case Study of Fe2TiO5 Photoanodes

A new electrochemical, solution-based synthesis method to prepare uniform multinary oxide photoelectrodes was developed. This method involves solubilizing multiple metal ions as metal-catechol complexes in a pH condition where they are otherwise insoluble. When some of the catechol ligands are electrochemically oxidized, the remaining metal complexes become insoluble and are deposited as metal-catechol films on the working electrode. The resulting films are then annealed to form crystalline multinary oxide electrodes. Because catechol can serve as a complexing agent for a variety of metal ions, the newly developed method can be used to prepare a variety of multinary oxide films. In the present study, we used this method to prepare n-type Fe₂TiO₅ photoanodes and investigated their photoelectrochemical properties for use in a photoelectrochemical water-splitting cell. We also performed a computational investigation with two goals. The first goal was to investigate small electron polaron formation in Fe₂TiO₅. Charge transport in most oxide photoelectrodes involves small polaron hopping, but small polaron formation in Fe₂TiO₅ has not been examined prior to this work. The second goal was to investigate the effect of substitutional Sn doping at the Fe site on the electronic band structure and the carrier concentration of Fe₂TiO₅. The combined experimental and theoretical results presented in this study greatly improve our understanding of Fe₂TiO₅ for use as a photoanode.

Highly Efficient Polymeric Carbon Nitride Photoanode with Excellent Electron Diffusion Length and Hole Extraction Properties

Polymeric carbon nitride (CN) has emerged as a promising semiconductor in photoanodes for photoelectrochemical cells (PECs) owing to its suitable electronic structure, tunable band gap, high stability, and low price. However, the poor electron diffusion within the CN layer and hole extraction to the solution still limit its applicability in PECs. Here, we report the fabrication of a CN photoanode with excellent electron diffusion length and remarkable hole extraction properties by careful design of its electronic interfaces. We combine complementary synthetic approaches to grow tightly packed CN layers forming a type-II heterojunction, which results in a CN photoanode with excellent charge separation, high electronic conductivity, and remarkable hole extraction efficiency. The optimized CN photoanode displays excellent PEC performance, reaching up to 270 μA cm^-2 in a 0.1 M KOH solution at 1.23 V vs RHE, extremely low onset potential (∼0.0012 V), and long-term stability up to 18 h.

Black-Si as a Photoelectrode

The fabrication and characterization of photoanodes based on black-Si (b-Si) are presented using a photoelectrochemical cell in NaOH solution. B-Si was fabricated by maskless dry plasma etching and was conformally coated by tens-of-nm of TiO₂ using atomic layer deposition (ALD) with a top layer of CoOx cocatalyst deposited by pulsed laser deposition (PLD). Low reflectivity R<5% of b-Si over the entire visible and near-IR (λ<2 μm) spectral range was favorable for the better absorption of light, while an increased surface area facilitated larger current densities. The photoelectrochemical performance of the heterostructured b-Si photoanode is discussed in terms of the n-n junction between b-Si and TiO₂.

Patterning of BiVO4 Surfaces and Monitoring of Localized Catalytic Activity Using Scanning Photoelectrochemical Microscopy

There is a lot of interest in understanding localized catalytic activities at the micro and nanoscale and designing robust catalysts for photoelectrochemical oxidation of water to address the pressing energy and environmental challenges. Here, we demonstrate that scanning photoelectrochemical microscopy (SPECM) can be effectively employed as a novel technique (i) to modify a photocatalyst surface with an electrocatalyst layer in a matrix fashion and (ii) to monitor its localized activity toward the photoelectrochemical (PEC) water oxidation reaction. The three-dimensional SPECM image clearly shows that the loading of the FeOOH electrocatalyst on the BiVO₄ semiconductor surface strongly affects its local PEC reaction activity. The optimal photoelectrodeposition time of FeOOH on the BiVO₄ photocatalyst was found to be ∼20 min when FeOOH was employed as the electrocatalyst. The electrocatalyst optimization process was conducted on a single photoanode electrode surface, making the optimization process efficient and reliable. The morphology of the formed photocatalyst/electrocatalyst hybrid, inclusive of its localized activity toward the water oxidation reaction, was simultaneously probed. A photoanode surface comprising CuWO₄/BiVO₄/FeOOH was further prepared in this study and investigated. It was found that the localized photoactivity truly reflects the activity of the local area, differs from region to region, and is contingent on the morphology of the surface. Moreover, the Pt UME is determined as an efficient probe to analyze the photoactivity of the PEC water splitting reaction. This work highlights the novel SPECM technique for enhancement and examination of the catalytic activity of the nanostructured materials.

Fabrication of ZnO@MoS2 Nanocomposite Heterojunction Arrays and Their Photoelectric Properties

In this paper, ZnO@MoS₂ core-shell heterojunction arrays were successfully prepared by the two-step hydrothermal method, and the growth mechanism was systematically studied. We found that the growth process of molybdenum disulfide (MoS₂) was sensitively dependent on the reaction temperature and time. Through an X-ray diffractometry (XRD) component test, we determined that we prepared a 2H phase MoS₂ with a direct bandgap semiconductor of 1.2 eV. Then, the photoelectric properties of the samples were studied on the electrochemical workstation. The results show that the ZnO@MoS₂ heterojunction acts as a photoanode, and the photocurrent reaches 2.566 mA under the conditions of 1000 W/m² sunshine and 0.6 V bias. The i-t curve also illustrates the perfect cycle stability. Under the condition of illumination and external bias, the electrons flow to the conduction band of MoS₂ and flow out through the external electrode of MoS₂. The holes migrate from the MoS₂ to the zinc oxide (ZnO) valence band. It is transferred to the external circuit through the glass with fluorine-doped tin oxide (FTO) together with the holes on the ZnO valence band. The ZnO@MoS₂ nanocomposite heterostructure provides a reference for the development of ultra-high-speed photoelectric switching devices, photodetector(PD) devices, and photoelectrocatalytic technologies.

Electrophoretic Deposition of Quantum Dots and Characterisation of Composites

Electrophoretic deposition (EPD) is an emerging technique in nanomaterial-based device fabrication. Here, we report an in-depth study of this approach as a means to deposit colloidal quantum dots (CQDs), in a range of solvents. For the first time, we report the significant improvement of EPD performance via the use of dichloromethane (DCM) for deposition of CQDs, producing a corresponding CQD-TiO₂ composite with a near 10-fold increase in quantum dot loading relative to more commonly used solvents such as chloroform or toluene. We propose this effect is due to the higher dielectric constant of the solvent relative to more commonly used and therefore the stronger effect of EPD in this medium, though there remains the possibility that changes in zeta potential may also play an important role. In addition, this solvent choice enables the true universality of QD EPD to be demonstrated, via the sensitization of porous TiO₂ electrodes with a range of ligand capped CdSe QDs and a range of group II-VI CQDs including CdS, CdSe/CdS, CdS/CdSe and CdTe/CdSe, and group IV-VI PbS QDs.