Efficient Photoanode Research Articles

Cascaded multiple-step hole transfer for enhancing photoelectrochemical water splitting

Serious surface recombination and sluggish reaction kinetics hamper the photoelectrochemical (PEC) performance from being efficient. Although coupling oxygen evolution catalysts on photoanodes to accelerate electrode reactions has been demonstrated an effective strategy to enhance photocurrent, the surface recombination still limits the performance to an ideal level. Herein, a novel multi-step hole transfer strategy is developed for accelerating hole transport and thus suppressing the surface recombination. In this system, porphyrin and polyaniline both act as hole transfer layers, which construct an effective stair-stepping hole transfer channel to ensure much more holes take part in water oxidation process. This simple interface modulation system exhibits a remarkable photocurrent of 4.5 mA cm−2 at 1.23 V (vs. reversible hydrogen electrode (RHE), AM 1.5 G illumination) accompanied with good stability and provides a promising strategy for the design of efficient photoanodes for solar conversion.

Recent Developments in Visible‐Light‐Absorbing Semitransparent Photoanodes for Tandem Cells Driving Solar Water Splitting

The development of an efficient conversion system to transform solar energy into chemical energy, such as renewable hydrogen, is a promising way to overcome energy problems. Photoelectrochemical (PEC) water splitting is a promising means of obtaining renewable hydrogen directly from water utilizing sunlight. Recent reports have demonstrated that a PEC cell with a tandem configuration (tandem cell) has the potential to realize a high solar‐to‐hydrogen (STH) energy conversion efficiency by solar water splitting. However, there are still many obstacles to the development of practical and cost‐effective tandem cells. In particular, development of efficient photoanodes for the oxygen evolution reaction (OER) is a prerequisite for improving the STH efficiency. Herein, recent progress in developing (semi)transparent photoanodes for the OER, such as Fe2O3, BiVO4, and Ta3N5, is described based on the topics of preparation methods, semiconductor properties, and PEC performance. In addition, the strategies for enhancing the STH efficiency of tandem cells consisting of (semi)transparent photoanodes conjugated with photovoltaic (PV)‐based cathodes are summarized. This Review is expected to provide guidelines for the future development of tandem cells capable of highly efficient and stable water splitting.

Fabrication of dye-sensitized solar cells with ZnO nanorods as photoanode and natural dye extract as sensitizer

Highly oriented zinc oxide nanorods (ZnO NRs) have been grown on ITO substrates at various growth times using hydrothermal method. The necessary ZnO seed layers on ITO have been deposited using spray pyrolysis. Both the methods have been successfully applied to prepare efficient ZnO NRs-based photoanodes for DSSCs. FESEM images revealed the vertical growth of ZnO NRs from the substrates with a length and diameter ranges of 1.2–1.5 μm and 80–120 nm, respectively. The XRD and HRTEM results confirmed the hexagonal structure with preferential orientation (c-axis) of ZnO NRs. The high optical transmittance (90%) was observed in the visible region for the ZnO NRs grown for 9 h. A decrease in optical bandgap was noticed with the increase of growth times. The natural dye extract absorbed in to the ZnO NRs exhibited a broad absorption peak in UV–Vis spectra analysis. The power conversion efficiency of DSSCs prepared with ZnO NRs (9 h) as photoanode showed better performance compared to other growth times.

Surfactant assisted approach to development of efficient WO3 photoanode for natural dye sensitized solar cell

Herein, the single-step hydrothermal route has been successfully implemented to design a surfactant-assisted WO3 photoanode for dye-sensitized solar cell (DSSC). The influence of surfactant on growth and photoresponse properties of the dye-sensitized WO3 have been studied. Optical absorption study shows a direct allowed type of electronic transition with band gap energy lies in the range of 2.82–2.38 eV. The formation of pure hexagonal crystal with tuning in the surface morphology from nanospheres to nanoflowers were obtained in the structural and morphological investigation. The compositional analysis confirms the formation of stoichiometric films with respective valence states of W+6 and O2–. Photovoltaic performance of dye-sensitized WO3 demonstrates improved photocurrent from 1.53 to 2.61 mA/cm2 corresponding to photovoltage of 690–764 mV. The best photo-conversion efficiency of 3.52% was obtained for the W-SDS sample of DSSC. The IPCE measurement are well supported to PEC performance of the DSSC. The stability of the prominent W-SDS photoanode was performed to confirm the suitability and reproducibility of photoelectrode. The charge transfer properties of DSSC were analyzed by electrochemical impedance spectroscopy (EIS).

Ultrathin MoSe2 nanosheet anchored CdS-ZnO functional paper chip as a highly efficient tandem Z-scheme heterojunction photoanode for scalable photoelectrochemical water splitting

Rational design of photoelectrode architectures is deemed as an effective solution to enhance efficiency of photoelectrochemical water splitting for conquering increasingly prominent environmental problems. Herein, a programmable flexible photoanode with built-in tandem Z-scheme configuration fabricated by in-situ growth of MoSe2-CdS-ZnO array with ultrathin nanosheet-interlaced-nanorod structure on the Au paper electrode, is firstly reported. By taking advantages of the interwoven nanonetwork structure, high conductivity as well as larger porosity of the as-fabricated Au paper electrode, a stable light-harvesting paper chip with fast transfer/separate mediators of the charge/carriers is thus integrated. By virtue of double internal electric fields (IEFs) and photothermal effect, the developed MoSe2-CdS-ZnO photoanode exhibits incident-photon-to-current efficiency of 52 % under 460 nm of light irradiation, and the current density reaches 2.4 mA cm−2 at 0.3 V versus normal hydrogen electrode, which is comparable with respect to current state-of-the-art “type-II-scheme”, “surface-scheme”, and cocatalyst-free photocatalysts (0.03–2.4 mA cm−2). Consequently, the paper-based photoanode achieves admirable photoelectrocatalytic hydrogen generation property (39.7 μmol cm−2 h-1). The synergistic effect of tailorable tandem Z-scheme configuration with double IEFs and delineating of 3D cross-linked network structure of the proposed photoanode in this work may open an avenue for constructing high-performance photoelectrochemical water splitting devices.

Graphitic carbon nitride (g-C3N4) incorporated TiO2–B nanowires as efficient photoanode material in dye sensitized solar cells

Graphitic carbon nitride (g-C3N4) nanosheets laminated TiO2–B nanowire composites (TNW/GCN) were synthesized adopting a simple pyrolysis process (500 °C), using melamine and hydrothermally grown TiO2–B NWs at different weight ratios. The TNW/GCN composites were used as efficient photoanode material in dye sensitized solar cells (DSSCs) and the systematic changes in the cell performance were investigated. The experimental results demonstrated a realistic improvement in the overall DSSC efficiency after modification of TiO2–B NWs with g-C3N4. Incorporation of optimum amount of g-C3N4 in the composite material could enhance the short circuit photocurrent density (JSC) as well as open circuit voltage (VOC). The DSSC based on TNW/GCN-10 photoanode enhanced the photocurrent conversion efficiency to 5.12% compared to 3.71% with TNW/GCN-0 photoanode (using pure TiO2–B NWs). This increased cell performance could be attributed to the suppression of recombination of the photogenerated electrons/hole charge pairs initiated by optimum g-C3N4 lamination of TiO2–B NWs photoanodes, in spite of subsequent lowering in the dye loading capacity.

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.

3D interconnected nanoporous Ta3N5 films for photoelectrochemical water splitting: thickness-controlled synthesis and insights into stability

Solar-driven photoelectrochemical (PEC) water splitting is a promising technology for sustainable hydrogen production, which relies on the development of efficient and stable photoanodes for water oxidation reaction. The thickness and microstructure of semiconductor films are generally crucial to their PEC properties. Herein, three-dimensional (3D) interconnected nanoporous Ta3N5 film photoanodes with controlled thickness were successfully fabricated via galvanostatic anodization and NH3 nitridation. The porous Ta3N5 nanoarchitectures (NAs) of 900 nm in thickness showed the highest PEC performance due to the optimal light-harvesting and charge separation. Compared with the hole-induced photocorrosion, the electrochemical oxidation at high anodic potentials resulted in severer performance degradation of Ta3N5. Although the surface oxide layer on deteriorated Ta3N5 photoanodes could be removed by NH3 re-treatment, the PEC performance was only partially recovered. As an alternative, anchoring a dual-layer Co(OH)x/CoOOH co-catalyst shell on the porous Ta3N5 NAs demonstrated substantially enhanced PEC performance and stability. Overall, this work provides reference to controllably fabricate 3D nanoporous Ta3N5-based photoanodes for efficient and stable PEC water splitting via optimizing the light absorption, hole extraction, charge separation and utilization.

Reduced Graphene Oxide-Laminated One-Dimensional TiO2-Bronze Nanowire Composite: An Efficient Photoanode Material for Dye-Sensitized Solar Cells.

A facile one-step hydrothermal method was developed to prepare reduced graphene oxide-laminated TiO2–bronze (TiO2-B) nanowire composites (TNWG), which contain two-dimensional graphene oxide nanosheets and TiO2-B nanowires. In the hydrothermal process, the functional groups of graphene oxide were reduced significantly. Dye-sensitized solar cells (DSSCs) were fabricated using TNWG as the photoanode material. The effects of different reduced graphene oxide contents in TNWG on the energy conversion efficiency of the dye-sensitized solar cells were investigated using J–V and incident photon-to-current conversion efficiency characteristics. DSSCs based on a TNWG hybrid photoanode with a reduced graphene oxide content of 8 wt % demonstrated an overall light-to-electricity conversion efficiency of 4.95%, accompanied by a short-circuit current density of 10.41 mA cm–2, an open-circuit voltage of 0.71 V, and a fill factor of 67%, which were much higher than those of DSSC made with a pure TiO2-B NW-based photoanode. The overall improvement in photovoltaic performance could be associated to the intense visible light absorption and enhanced dye adsorption because of the increased surface area of the composite, together with faster electron transport due to reduced carrier recombination.

Reaction kinetics and interplay of two different surface states on hematite photoanodes for water oxidation

Understanding the function of surface states on photoanodes is crucial for unraveling the underlying reaction mechanisms of water oxidation. For hematite photoanodes, only one type of surface states with higher oxidative energy (S1) has been proposed and verified as reaction intermediate, while the other surface state located at lower potentials (S2) was assigned to inactive or recombination sites. Through employing rate law analyses and systematical (photo)electrochemical characterizations, here we show that S2 is an active reaction intermediate for water oxidation as well. Furthermore, we demonstrate that the reaction kinetics and dynamic interactions of both S1 and S2 depend significantly on operational parameters, such as illumination intensity, nature of the electrolyte, and applied potential. These insights into the individual reaction kinetics and the interplay of both surface states are decisive for designing efficient photoanodes.

Synthesis of transition metal oxide incorporated MOF-5 and NH2-MOF-5 as efficient photoanode for oxygen evolution reaction

Synthesis of transition metal oxide incorporated MOF-5 and NH2-MOF-5 as efficient photoanode for oxygen evolution reaction

Decoration of CdS nanoparticles on dense and multi-edge sodium titanate nanorods to form a highly efficient and stable photoanode with great enhancement in PEC performance

This study provided the preparation and application of a highly efficient photoanode having dense and efficient sodium titanate (NTO) nanorods decorated with CdS nanoparticles.

Preparation of ZnO/two-layer self-doped black TiO2 nanotube arrays and their enhanced photochemical properties.

Highly efficient TiO2 photoanodes can be achieved by enhancing electrical conductivity and improving charge separation and transfer. In this paper, Ti foils were used to fabricate TiO2 nanotubes by anodic oxidation and ZnO/two-layer self-doped black TiO2 nanotubes were prepared by electrochemical reduction and a hydrothermal method. The formed black TiO2 nanotubes have a better photoconversion efficiency and the maximum photoconversion efficiency increased by 59% compared with the pure nanotubes. The deposition of ZnO further improves the maximum photoconversion efficiency to 456% based on black TiO2. The photocurrent responses also increase by about 5 times in our results. This work is instructive for the development of highly robust and efficient photoanode materials in fields including photoelectrochemistry and photocatalysis.

Solvothermal phase change induced morphology transformation in CdS/CoFe2O4@Fe2O3hierarchical nanosphere arrays as ternary heterojunction photoanodes for solar water splitting

The design of efficient heterojunction photoanodes with appropriate band alignment and ease of charge separation has been one of the most highly focused research areas in photoelectrodes.

Vacancy defect engineering of BiVO4 photoanodes for photoelectrochemical water splitting.

Photoelectrochemical (PEC) water splitting has been regarded as a promising technology for sustainable hydrogen production. The development of efficient photoelectrode materials is the key to improve the solar-to-hydrogen (STH) conversion efficiency towards practical application. Bismuth vanadate (BiVO4) is one of the most promising photoanode materials with the advantages of visible light absorption, good chemical stability, nontoxic feature, and low cost. However, the PEC performance of BiVO4 photoanodes is limited by the relatively short hole diffusion length and poor electron transport properties. The recent rapid development of vacancy defect engineering has significantly improved the PEC performance of BiVO4. In this review article, the fundamental properties of BiVO4 are presented, followed by an overview of the methods for creating different kinds of vacancy defects in BiVO4 photoanodes. Then, the roles of vacancy defects in tuning the electronic structure, promoting charge separation, and increasing surface photoreaction kinetics of BiVO4 photoanodes are critically discussed. Finally, the major challenges and some encouraging perspectives for future research on vacancy defect engineering of BiVO4 photoanodes are presented, providing guidelines for the design of efficient BiVO4 photoanodes for solar fuel production.

Boosting light harvesting and charge separation of WO3 via coupling with Cu2O/CuO towards highly efficient tandem photoanodes.

Photoanodes based on semiconductor WO3 have been attractive due to its good electron mobility, long hole-diffusion length, and suitable valence band potential for water oxidation. However, the semiconductor displays disadvantages including a relatively wide bandgap, poor charge separation and transfer, and quick electron–hole recombination at the interface with the electrolyte. Here we present a significantly improved photoanode with a tandem structure of ITO/WO3/Cu2O/CuO, which is prepared first by hydrothermally growing a layer of WO3 on the ITO surface, then by electrodepositing an additional layer of Cu2O, and finally by heat-treating in the air to form an exterior layer of CuO. Photocurrent measurements reveal that the prepared photoanode produces a maximum current density of 4.7 mA cm−2, which is, in comparison, about 1.4 and 5.5 times the measured values for ITO/WO3/Cu2O and ITO/WO3 ones, respectively. These enhancements are attributed to (1) harvested UV, visible, and NIR light of the solar spectrum, (2) accelerated charge separation at the heterojunction between WO3 and Cu2O/CuO, (3) better electrocatalytic activity of formed CuxO than pure Cu2O, (4) formation of a protective layer of CuO. This study thus may lead to a promising way to make high-performance and low-cost photoanodes for solar energy harvesting.

A hybrid bulk-heterojunction photoanode for direct solar-to-chemical conversion

The development of efficient and stable organic semiconductor-based photoanodes for solar fuel production is advanced by using a robustin situ-formed covalent polymer network together with a mesoporous inorganic film in a hybrid bulk heterojunction.

Carrier engineering of Zr-doped Ta3N5 film as an efficient photoanode for solar water splitting

Ta3N5 is regarded as a promising candidate material with adequate visible light absorption and band structure for photoelectrochemical water splitting.

The construction of a single-crystalline SbSI nanorod array-WO3 heterostructure photoanode for high PEC performance.

A novel kind of highly efficient photoanode was constructed with a SbSI/WO3 heterostructurefabricated through two hydrothermal reactions followed by an iodination reaction (WO3 → Sb2S3/WO3 → SbSI/WO3). After optimizing the solvent [carbon disulfide (CS2)] for SbI3, the SbSI(CS2)/WO3 photoanode shows high-density single-crystalline SbSI nanorods growing along the polar [001] direction on WO3 nanoplates, resulting in excellent photocurrent performance (∼2.1 mA cm-2@1.23 V vs. RHE) and an improved photostability. It is evidenced that the higher crystallinity of SbSI has a positive effect on the photostability of the constructed SbSI/WO3 photoanodes.

Highly sensitive and selective photoelectrochemical sensor for mercury(II) detection based on efficient Bi2MoO6 photoanode decorated with CuS

Stronger photocatalytic activity and better photoelectrochemical (PEC) performance can Bi2MoO6/CuS photocatalyst accomplish compared to that of pure Bi2MoO6. CuS was also conducive to strengthen the visible light absorption and inhibited the recombination of electron-hole pairs. The addition of glutathione (GSH) in the electrolyte led to a significant enhancement to the reactivity of the PEC sensor in the modified electrode. In contrast, the increase in photocurrent was restrained to a high degree for that the hole scavenging agents were consumed through the addition of mercury (Hg2+). Consequently, the electrode was able to be exploited for Hg2+ detection even though no specific identification component was introduced yet. The wide linear range of 0.5 nM–230 nM and low detection limit of 0.23 nM made the electrode adaptable to the detection of Hg2+ in practical samples. Thanks to a ameliorative PEC performance, thus, the Bi2MoO6/CuS sensor stands a good chance to play a useful part in the detection of Hg2+.