Heterojunction Photoanode Research Articles

(Invited) Materials and Systems for Stand-Alone Solar Water Splitting

Sunlight is a clean, renewable and abundant energy source on the earth. Its conversion to hydrogen has been considered an ideal solution to counter the depletion and environmental problems of fossil fuels. Photoelectrochemical (PEC) water splitting is an ideal technology for the purpose, since H2 could be produced directly from abundant and renewable water and solar light from the process. The key to the technology is photoelectrodes made of small band gap semiconductors of photocatalytic properties. The materials should have high efficiency, high stability, and low cost. In addition of the discovery of new materials, the structure and morphology of the known materials could be controlled to enhance the performance of the photoelectrodes. In this presentation, the concepts of materials design and their examples are proposed for efficient photoelectrodes of PEC cells for visible light water splitting. About 400 semiconductor solids are known to have photocatalytic activity for water splitting. Yet there is no single material that could satisfy all the requirements for desired photocatalysts: i) suitable band gap energy (1.7 eV< Eg < 2.3 eV) for high efficiency, ii) proper band position for reduction and/or oxidation of water, iii) long-term stability in aqueous solutions, iv) low cost, v) high crystallinity, and vi) high conductivity. Hence, in the selection of photocatalytic materials, we better start from intrinsically stable materials made of earth-abundant elements. The band bap energy is also the primary consideration to absorb ample amount of solar energy of wide wavelength spectrum. It sets the limit of theoretically maximum efficiency and it could also be extended by band engineering techniques. Upon selection of the candidate materials, we can also modify the materials for full utilization their potentials. The main path of efficiency loss in PEC water splitting process is recombination of photoelectrons and holes. We discuss the material designs including i) p-n heterojunction photoanodes for effective electron-hole separation, ii) electron highway to facilitate interparticle electron transfer, iii) metal or anion doping to improve conductivity of the semiconductor and to extend the range of light absorption, iv) one-dimensional nanomaterials to secure a short hole diffusion distance and vectoral electron transfer, and v) loading co-catalysts for faci;e charge separation. High efficiency has been demonstrated for all these examples due to efficient electron-hole separation. Modern material processing techniques have been explored to materialize these concepts.

Titanium dioxide/zinc indium sulfide hetero-junction: An efficient photoanode for the dye-sensitized solar cell

A facile strategy is developed for the fabrication of titanium dioxide/zinc indium sulfide (TiO2/ZnIn2S4) hetero-junction photoanode with an adjustable ZnIn2S4 doping content and application in the dye-sensitized solar cell (DSSC). Comparing to the pure TiO2, TiO2/ZnIn2S4 hetero-junction materials demonstrate an enhanced light utilizing efficiency, a reduced recombination rate of electron-hole pairs, and an accelerated migration process of photoinduced electrons. Due to above merits, DSSC based on TiO2/ZnIn2S4 hetero-junction photoanode achieves a greatly enhanced short-circuit current density, leading to an improved photoelectric conversion efficiency of 8.09% under full sunlight illumination (100 mW cm−2, AM 1.5 G), which is almost 14.43% higher than that of the pure TiO2-based DSSC (7.07%).

Conformally coated BiVO4 nanodots on porosity-controlled WO3 nanorods as highly efficient type II heterojunction photoanodes for water oxidation

Bismuth vanadate (BiVO4) has become a hot topic for solar hydrogen generation by water oxidation due to its appropriate band edges, and narrow band gap. However, the actual conversion efficiency achieved with BiVO4-based photoanodes is considerably less than the theoretical values because of drawbacks such as poor electron transportation and slow kinetics of oxygen evolution. In these respects, formation of nanostructured heterojunctions is considered to have a marked effect in terms of improving the photoactivities of BiVO4 photoanodes since it is possible to enhance the charge transfer at the interface and increase light absorption significantly. Herein, we report the synthesis and photoelectrochemical (PEC) properties of various BiVO4-based heterojunction anodes based on stoichiometric BiVO4 by pulsed electrodeposition. We show that WO3 is the optimum bottom layer for a type II band system with BiVO4 among various metal oxides. It is revealed that the photocurrent density of bare WO3 nanorods can be changed drastically by controlling the porosity and aspect ratio and thus BiVO4/WO3 heterojunction anodes show as similar tendency to bare WO3 nanorods. Under the optimum conditions, a BiVO4/WO3 heterojunction anode can lead to a high photocurrent density of 4.55mA/cm2 and an incident photon to current conversion efficiency of 80% at 1.23V versus a reversible hydrogen electrode without additional catalyst. Cross-sectional transmission electron microscopy shows that dot-like BiVO4 is well decorated on the entire surface of WO3 nanorods. This systematic study provides a viewpoint on the crucial role of the active area of the bottom layer for high-performance BiVO4-based type II heterojunction photoanodes, and the proposed concept is applicable to various photoelectrode systems.

Fabrication of an Efficient BiVO4-TiO2 Heterojunction Photoanode for Photoelectrochemical Water Oxidation.

In this work, a simple planar BiVO4/TiO2 heterojunction photoanode was prepared on a fluorine-doped tin oxide (FTO) substrate for photoelectrochemical (PEC) water oxidation. The measurements of surface photovoltage, photocurrent transient behavior, and hole-scavenger-assisted PEC performance indicate that charge separation efficiency is improved compared to that of the BiVO4/FTO photoanode. This improvement is caused by the formation of the staggered BiVO4/TiO2 heterojunction. However, the photocurrent densities of the BiVO4/TiO2/FTO photoanode are higher than those of the BiVO4/FTO one only at potentials >1.2 V vs reversible hydrogen electrode, although the two BiVO4 layers with comparable light harvesting efficiencies were prepared by the same method. The hole-scavenger-assisted PEC measurements reveal that the hole injection efficiency of the BiVO4/TiO2/FTO photoanode is inferior to that of the bare BiVO4/FTO anode for oxygen evolution. It shows that the surface property of the BiVO4 layers is altered as they are deposited on different substrates. On the basis of these characterizations, the cocatalyst cobalt phosphate (Co-Pi) was further deposited on the surface of BiVO4/TiO2/FTO photoanode to improve the hole injection efficiency. Subsequently, the photocurrent density and stability of the Co-Pi/BiVO4/TiO2/FTO photoanode were significantly improved compared to those of the bare BiVO4/FTO photoanode.

WO3@α‐Fe2O3 Heterojunction Arrays with Improved Photoelectrochemical Behavior for Neutral pH Water Splitting

AbstractPhotoelectrochemical (PEC) water splitting on illuminated semiconductors plays an increasing role in alternative energy devices because of the need for clean and sustainable energy. However, the overall efficiency of the process is still very low and requires further improvement for widespread application. Herein, we report that the PEC performance of WO3 can be optimized by constructing a WO3@α‐Fe2O3 heterojunction nanosheet array to expand the spectral range of light absorption and promote photogenerated electron–hole separation/transfer. This heterojunction photoanode exhibit a pronounced photocurrent response of 1.66 mA cm−2, a high incident photon‐to‐current conversion efficiency of ∼73.7 % at 390 nm, and an excellent photostability of 100 %, all at 1.23 V versus reversible hydrogen electrode. Therefore, our approach has great potential for designing efficient hybrid photoanodes for PEC water‐splitting systems.

ZnO/TiO2 nanohexagon arrays heterojunction photoanode for enhancing power conversion efficiency in dye-sensitized solar cells

TiO2 nanohexagon arrays (TNHAs) were grown by anodization of Ti foil in an electrolyte containing NH4F, and then the TNHAs were transferred onto FTO glass by using a drop of sol containing Ti(OBu)4 for fabricating a front-side illumination cell. In order to improve the power conversion efficiency (PCE) of the as-fabricated dye-sensitized solar cells (DSSCs), ZnO was introduced into the TNHAs by a facile immersing method. The as-obtained ZnO/TiO2 nanohexagon heterojunction was used as the photoanode for DSSCs, which is expected to have a low recombination rate and high absorption of light. Results indicated that the DSSCs based on the ZnO/TNHAs exhibit an enhanced current density and high PCE of 6.85% under 100 mW cm−2 illumination. Furthermore, electrochemical impedance spectroscopy showed that the ZnO/TNHAs heterojunction-based DSSCs have optimized electron properties, including longer electron lifetime, lower resistance to electron transport, and smaller probability for electron recombination as compared to the DSSCs fabricated from pure TNHAs as photoanode.

Enhanced Charge Separation through ALD-Modified Fe2 O3 /Fe2 TiO5 Nanorod Heterojunction for Photoelectrochemical Water Oxidation.

Hematite suffers from poor charge transport and separation properties for solar water splitting. This paper describes the design and fabrication of a 3D Fe2 O3 /Fe2 TiO5 heterojunction photoanode with improved charge separation, via a facile hydrothermal method followed by atomic layer deposition and air annealing. A highly crystallized Fe2 TiO5 phase forms with a distinct interface with the underlying Fe2 O3 core, where a 4 nm Fe2 TiO5 overlayer leads to the best photoelectrochemical performance. The favorable band offset between Fe2 O3 and Fe2 TiO5 establishes a type-II heterojunction at the Fe2 O3 /Fe2 TiO5 interface, which drives electron-hole separation effectively. The Fe2 O3 /Fe2 TiO5 composite electrode exhibits a dramatically improved photocurrent of 1.63 mA cm(-2) at 1.23 V versus reversible hydrogen electrode (RHE) under simulated 1 sun illumination (100 mW cm(-2) ), which is 3.5 times that of the bare Fe2 O3 electrode. Decorating the Fe2 O3 /Fe2 TiO5 heterojunction photoanode with earth-abundant FeNiOx cocatalyst further expedites surface reaction kinetics, leading to an onset potential of 0.8 V versus RHE with a photocurrent of 2.7 mA cm(-2) at 1.23 V and 4.6 mA cm(-2) at 1.6 V versus RHE. This sandwich photoanode shows an excellent stability for 5 h and achieves an overall Faradaic efficiency of 95% for O2 generation. This is the best performance ever reported for Fe2 O3 /Fe2 TiO5 photoanodes.

Facile fabrication of organic/inorganic nanotube heterojunction arrays for enhanced photoelectrochemical water splitting.

Organic/inorganic heterojunction photoanodes are appealing for making concurrent use of the highly photoactive organic semiconductors, and the efficient dielectric screening provided by their inorganic counterparts. In the present work, organic/inorganic nanotube heterojunction arrays composed of TiO2 nanotube arrays and a semiconducting N,N-(dicyclohexyl) perylene-3,4,9,10-tetracarboxylic diimide (PDi) layer were fabricated for photoelectrochemical water splitting. In this arrayed architecture, a PDi layer with a tunable thickness was coated on anodic TiO2 nanotube arrays by physical vapor deposition, which is advantageous for the formation of a uniform layer and an adequate interface contact between PDi and TiO2. The obtained PDi/TiO2 junction exhibited broadened visible light absorption, and an effective interface for enhanced photogenerated electron-hole separation, which is supported by the reduced charge transfer resistance and prolonged excitation lifetime via impedance spectroscopy analysis and fluorescence emission decay investigations. Consequently, such a heterojunction photoanode was photoresponsive to a wide visible light region of 400-600 nm, and thus demonstrated a highly enhanced photocurrent density at 1.23 V vs. a reversible hydrogen electrode. Additionally, the durability of such a photoanode can be guaranteed after long-time illumination because of the geometrical restraint imposed by the PDi aggregates. These results pave the way to discover new organic/inorganic assemblies for high-performance photoelectric applications and device integration.

Highly-oriented Fe2O3/ZnFe2O4 nanocolumnar heterojunction with improved charge separation for photoelectrochemical water oxidation.

This paper describes the design and synthesis of a heterojunction photoanode composed of highly-oriented Fe2O3/ZnFe2O4 nanocolumnar arrays with a well-defined morphology by reactive ballistic deposition and atomic layer deposition. This specific structure enhances the charge separation at the Fe2O3/ZnFe2O4 interface, leading to an improved photoelectrochemical performance for water oxidation.

Structure engineering of a core/shell Si@Ta3N5 heterojunction nanowires array for photoelectrochemical water oxidation

Structure-tuned core/shell n-Si@n-Ta3N5 nanowires array heterojunction photoanodes were prepared on a wafer scale by electroless metal-assisted etching of an Si wafer, spin coating of a Ta(OC2H5)5 sol precursor solution, and ammonia treatment to convert it to Ta3N5.

A highly efficient BiVO4/WO3/W heterojunction photoanode for visible-light responsive dual photoelectrode photocatalytic fuel cell

A highly efficient BiVO4/WO3/W heterojunction photoanode was fabricated based on the nanoporous WO3 film, which was prepared on the tungsten plate by anodic oxidation, in order to facilitate the electrons transfer from BiVO4 to WO3 by providing a natural connection between WO3 film and W substrate. Then, a visible-light responsive dual photoelectrode photocatalytic fuel cell (PFC) consisted of BiVO4/WO3/W photoanode and Pt modified commercial buried junction silicon (Pt/BJS) photocathode was constructed. The results showed that the optimized BiVO4/WO3/W photoanode obtained a photocurrent of 2.01mA/cm2 at 0.6V (vs Ag/AgCl) in 0.1M KH2PO4 (pH 7) electrolyte under simulated AM1.5 solar light, which was 180% and 205% higher than that of bare WO3 film and bare BiVO4 film, respectively. The established dual photoelectrode PFC showed high converting performance of organics into electricity. For example, a short-circuit current density of 0.26mAcm−2, which is higher than most of the reported visible-light responsive dual photoelectrode PFC systems, was obtained with the open-circuit voltage of 0.78V and maximum power output of 2.0×10−4 Wcm−2 in 20mgL−1 tetracycline hydrochloride. Furthermore, a removal ratio of 78% after 4h was achieved with a stable output photocurrent in the degradation process.

Photoelectrochemical water oxidation over fibrous and sponge-like BiVO4/β-Bi4V2O11 photoanodes fabricated by spray pyrolysis

Two different heterojunction photoanodes were made by one-pot spray pyrolysis deposition of BiVO4/Bi4V2O11 directly onto a conductive FTO substrate or on a previously deposited SnO2 layer onto an FTO substrate (sample BiVO4/Bi4V2O11/SnO2). The direct deposition onto FTO produced fibrous-like BiVO4/Bi4V2O11, whereas the deposition onto SnO2 gave sponge-like BiVO4/Bi4V2O11 morphologies. The photoactivity of these photoelectrodes was investigated for photoelectrochemical water oxidation under visible light irradiation. The fibrous-like BiVO4/Bi4V2O11 exhibited higher photoactivity than that of the sponge-like BiVO4/Bi4V2O11/SnO2 film. The transient photocurrent decay studies revealed that the time of e−/h+ recombination is higher in fibrous-like BiVO4/Bi4V2O11 than sponge-like BiVO4/Bi4V2O11/SnO2 film, which suggests that the electrons might accumulate in the SnO2 layer, thus causing a higher e−/h+ recombination and a lower photoactivity. Electrochemical impedance data showed that the hole transfer from the semiconductor to the water takes place more readily in fibrous-like BiVO4/Bi4V2O11 than sponge-like BiVO4/Bi4V2O11/SnO2 films. Consequently, the overall photoelectrochemical cell efficiencies for fibrous-like BiVO4/Bi4V2O11 and sponge-like BiVO4/Bi4V2O11/SnO2 films were 0.14 and 0.11%, respectively, without using oxygen evolution catalysts. These photoanodes prepared by spray pyrolysis are thus promising for use in PEC water splitting cells.

P-Type Transparent Conducting Oxide/n-Type Semiconductor Heterojunctions for Efficient and Stable Solar Water Oxidation.

Achieving stable operation of photoanodes used as components of solar water splitting devices is critical to realizing the promise of this renewable energy technology. It is shown that p-type transparent conducting oxides (p-TCOs) can function both as a selective hole contact and corrosion protection layer for photoanodes used in light-driven water oxidation. Using NiCo2O4 as the p-TCO and n-type Si as a prototypical light absorber, a rectifying heterojunction capable of light driven water oxidation was created. By placing the charge separating junction in the Si using a np(+) structure and by incorporating a highly active heterogeneous Ni-Fe oxygen evolution catalyst, efficient light-driven water oxidation can be achieved. In this structure, oxygen evolution under AM1.5G illumination occurs at 0.95 V vs RHE, and the current density at the reversible potential for water oxidation (1.23 V vs RHE) is >25 mA cm(-2). Stable operation was confirmed by observing a constant current density over 72 h and by sensitive measurements of corrosion products in the electrolyte. In situ Raman spectroscopy was employed to investigate structural transformation of NiCo2O4 during electrochemical oxidation. The interface between the light absorber and p-TCO is crucial to produce selective hole conduction to the surface under illumination. For example, annealing to produce more crystalline NiCo2O4 produces only small changes in its hole conductivity, while a thicker SiOx layer is formed at the n-Si/p-NiCo2O4 interface, greatly reducing the PEC performance. The generality of the p-TCO protection approach is demonstrated by multihour, stable, water oxidation with n-InP/p-NiCo2O4 heterojunction photoanodes.

N-Fe₂O₃ to N⁺-TiO₂Heterojunction Photoanode for Photoelectrochemical Water Oxidation.

To improve the performance of the thin hematite photoanode for photoelectrochemical water oxidation, in this work, an nN(+) α-Fe2O3 (hematite)-TiO2 heterojunction photoanode is constructed on fluorine-doped tin oxide substrate to establish a built-in field in the space charge region for facilitating the charge separation in the hematite layer. Charge distribution in the hematite-TiO2 heterostructure is investigated using Kelvin probe force microscopy, which confirms the improvement of charge separation in hematite layer by the formation of energy-matched nN(+) α-Fe2O3-TiO2 heterojunction. Compared to the hematite photoanode, an eightfold enhancement of the photocurrent density at 1.23 V versus reversible hydrogen electrode is measured in the hematite-TiO2 heterojunction photoanode. By using hydrogen peroxide as a hole scavenger, it demonstrates that both charge separation and charge injection efficiencies in the hematite-TiO2 heterojunction photoanode are superior to those in the hematite photoanode. It results from the significant suppressions of the charge recombinations occurring within the hematite layer as well as at the interface of photoelectrode and electrolyte by the formation of the nN(+) α-Fe2O3-TiO2 heterojunction.

In situ formation of CuWO4/WO3 heterojunction plates array films with enhanced photoelectrochemical properties

CuWO4/WO3 heterojunction photoanode based on WO3 plates arrays and CuWO4 nanoparticles was fabricated to enhance the photoelectrochemical performance. WO3 plates array films were prepared through a simple hydrothermal method, then a facile dipping-annealing process was employed to fabricate CuWO4/WO3 films. The crystal structure, morphology, composition and optical properties of the samples were analyzed by X-ray diffraction, scanning electron microscope, transmission electron microscope, X-ray photoelectron spectroscopy and UV–vis spectrometry. As-prepared CuWO4/WO3 heterojunction films achieved a photocurrent density of 1.21 mA/cm2 at 1.5 V vs. Ag/AgCl, which was almost 2-fold higher than that of the pure WO3 film (0.64 mA/cm2) under illumination. The enhanced photocurrent was attributed to the extension of visible light response, increased carriers density, effective separation at the interface of heterojunction and better electron transport properties, which were confirmed by Mott–Schottky and electrochemical impedance spectroscopy. This study reveals that the low-dimensional morphological structure and heterojunction structure are expected to provide a promising photoanode for water splitting.

Enhanced Photoelectrochemical Water Oxidation on Nanostructured Hematite Photoanodes via p-CaFe2O4/n-Fe2O3 Heterojunction Formation

In this paper, nanostructured hematite p-CaFe2O4/n-Fe2O3 heterojunction photoanodes have been fabricated employing a facile template-less film processing technique by controlling the chemical bath. Anisotropic growth of a β-FeOOH akaganeite film on FTO conductive glass from an aqueous FeCl3 solution containing CaCl2 followed by a two-step thermal annealing at 550 and 800 °C induces the formation of a p-CaFe2O4/n-Fe2O3 heterojunction. The structural, morphological, electronic states, and electrochemical characteristics of the films have been investigated by X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, and impedance spectroscopy, respectively. The heterojunction photoanode showed 100% higher photocurrent response than that obtained using a bare hematite electrode under simulated 1-sun illumination (100 mW/cm2). The photocurrent enhancement is attributed to the enhanced charge carrier separation and the reduced resistance in the charge transfer across the electrode and the electrolyte as revealed by electrochemical impedance spectroscopy analysis. The modification of the p-CaFe2O4/n-Fe2O3 heterojunction photoanode with CoPi cocatalyst further facilitates the electron transfer at the electrode/electrolyte interface and thus enhances the photoelectrochemical water oxidation. Since cheap and abundant materials have been employed for the synthesis of the heterojunction photoanode via a simple route, the current results have great importance, both from a scientific and an economical point of view.

Effect of the Si/TiO2/BiVO4 heterojunction on the onset potential of photocurrents for solar water oxidation.

BiVO4 has been formed into heterojunctions with other metal oxide semiconductors to increase the efficiency for solar water oxidation. Here, we suggest that heterojunction photoanodes of Si and BiVO4 can also increase the efficiency of charge separation and reduce the onset potential of the photocurrent by utilizing the high conduction band edge potential of Si in a dual-absorber system. We found that a thin TiO2 interlayer is required in this structure to realize the suggested photocurrent density enhancement and shifts in onset potential. Si/TiO2/BiVO4 photoanodes showed 1.0 mA/cm(2) at 1.23 V versus the reversible hydrogen electrode (RHE) with 0.11 V (vs RHE) of onset potential, which were a 3.3-fold photocurrent density enhancement and a negative shift in onset potential of 300 mV compared to the performance of FTO/BiVO4 photoanodes.

Preparation and enhanced photoelectrochemical performance of a p–n heterojunction CuFe2O4/WO3 nanocomposite film

CuFe2O4 was loaded on the surface of a WO3 film to form a p–n heterojunction photoanode with a better performance than WO3.

Unique 3D heterojunction photoanode design to harness charge transfer for efficient and stable photoelectrochemical water splitting

The hierarchically CoOxdecorated 2D C3N4nanosheet–1D/2D nanorod/nanosheet-assembled barium-doped TaON array as 3D heterojunction photoanode exhibited the enhanced photocurrent density and durable photostability for photoelectrochemical solar water splitting.

Improved photoelectrochemical activity of CaFe2O4/BiVO4 heterojunction photoanode by reduced surface recombination in solar water oxidation.

A bismuth vanadate photoanode was first fabricated by the metal-organic decomposition method and particles of calcium ferrite were electrophoretically deposited to construct a heterojunction photoanode. The characteristics of the photoanodes were investigated in photoelectrochemical water oxidation under simulated 1 sun (100 mW cm(-2)) irradiation. Relative to the pristine BiVO4 anode, the formation of the heterojunction structure of CaFe2O4/BiVO4 increased the photocurrent density by about 60%. The effect of heterojunction formation on the transfer of charge carriers was investigated using hydrogen peroxide as a hole scavenger. It was demonstrated that the heterojunction formation reduced the charge recombination on the electrode surface with little effect on bulk recombination. The modification with an oxygen evolving catalyst, cobalt phosphate (Co-Pi), was also beneficial for improving the efficiency of CaFe2O4/BiVO4 heterojunction photoanode mainly by increasing the stability.