Micro- and nanostructures of photoelectrodes for solar-driven water splitting

Two-dimensional graphitic carbon nitride/N-doped carbon with a direct Z-scheme heterojunction for photocatalytic generation of hydrogen

Photocatalysts with a direct Z-scheme heterojunction are promising by virtue of the effectively enhanced separation of charge carriers, high retention of redox ability and the absence of backward photocatalytic reactions. Their activity depends on band alignment and interfacial configurations between two semiconductors for charge carrier kinetics and the effective active sites for photochemical reactions. Herein, a two-dimensional (2D) graphitic carbon nitride/N-doped carbon (C₃N₄/NC) photocatalyst is synthesized by a gas template (NH₄Cl)-assisted thermal condensation method. C₃N₄/NC has the synthetic merits of a direct Z-scheme heterojunction, 2D-2D interfacial contact, and enhanced specific surface area to improve charge separation kinetics and provide abundant active sites for photochemical reaction. It exhibits an over 46-fold increase of the photocatalytic hydrogen production rate compared to bulk C₃N₄ under visible light illumination. This work demonstrates the great potential of 2D Z-scheme heterojunctions for photocatalysis and will inspire more related work in the future.

Highly Efficient Degradation of Persistent Pollutants with 3D Nanocone TiO2-Based Photoelectrocatalysis

Photoelectrocatalytic (PEC) degradation of organic pollutants into CO₂ and H₂O is a promising strategy for addressing ever-growing environmental problems. Titanium dioxide (TiO₂) has been widely studied because of its good performance and environmental benignancy; however, the PEC activity of TiO₂ catalyst is substantially limited due to its fast electron-hole recombination. Herein, we report a TiO₂ nanocone-based photoelectrocatalyst with superior degradation performance and outstanding durability. The unique conical catalyst can boost the PEC degradation of 4-chlorophenol (4-CP) with 99% degradation efficiency and higher than 55% mineralization efficiency at a concentration of 20 ppm. The normalized apparent rate constant of a nanocone catalyst is 5.05 h^-1 g^-1 m², which is 3 times that of a nanorod catalyst and 6 times that of an aggregated particle catalyst, respectively. Further characterizations reveal that the conical morphology of TiO₂ can make photogenerated charges separate and transfer more efficiently, resulting in outstanding PEC activity. Moreover, computational fluid dynamics simulations indicate that a three-dimensional conical structure is beneficial for mass transfer. This work highlights that tuning the morphology of a photoelectrocatalyst at the nanometer scale not only promotes the charge transfer but also facilitates the mass transportation, which jointly enhance the PEC performance in the degradation of persistent pollutants.

3D Brochosomes-Like TiO2 /WO3 /BiVO4 Arrays as Photoanode for Photoelectrochemical Hydrogen Production

An ideal photoelectrochemical (PEC) anode should process effective light absorption, charge transport, and separation efficiency. Here, a novel 3D brochosomes-like TiO₂ /WO₃ /BiVO₄ array as an efficient photoanode by combining a colloid polystyrene sphere template and electrochemical deposition routes for PEC hydrogen generation is reported. The as-fabricated 3D TiO₂ /WO₃ /BiVO₄ brochosomes photoanode yields excellent PEC performance with photocurrent densities of ≈3.13 and ≈4.27 mA cm^-2 with FeOOH/NiOOH catalyst, respectively, measured in 0.5 m Na₂ SO₄ solution with 0.1 m Na₂ SO₃ at 1.23 V versus reversible hydrogen electrode (RHE) under simulated AM1.5 light illumination, which is ≈6 times the reference sample of a planar WO₃ /BiVO₄ film electrode. The significantly improved performance could be benefited from the ordered hollow porous structure that provides enhanced light absorption and efficient charge transport as well as improved charge separation efficiency by WO₃ /BiVO₄ "host-guest" heterojunctions.

Rational Design of Photoelectrodes with Rapid Charge Transport for Photoelectrochemical Applications

Photoelectrode materials are the heart of photoelectrochemical (PEC) cells, which hold great promise to address global energy and environmental issues by converting solar energy into electricity or chemical fuels. In recent decades, significant research efforts have been devoted to the design and construction of photoelectrodes for the efficient generation and utilization of charge carriers to boost PEC performance. Herein, insights from a literature study on the relationship between the architecture and charge dynamics of photoelectrodes are presented. After briefly introducing the fundamental theories of charge dynamics in nanostructured photoelectrodes, the development of photoelectrode design in 1D polycrystalline nanotube arrays, 1D single-crystalline nanowire arrays, and hierarchical and mesoporous nanowire arrays is reviewed with a focus on the interplay between architecture and charge transport properties. For each design, commonly used synthetic approaches and the corresponding charge transport properties are discussed. Subsequently, the applications of these photoelectrodes in PEC systems are summarized. In conclusion, future challenges in the rational design of photoelectrode architecture are presented. The basic relationships between the architectures and charge dynamics of photoelectrode materials discussed here are expected to provide pertinent guidance and a reference for future advanced material design targeting improved light energy conversion systems.

Morphology, Optical Properties and Photocatalytic Activity of Photo- and Plasma-Deposited Au and Au/Ag Core/Shell Nanoparticles on Titania Layers

Titania is a promising material for numerous photocatalytic reactions such as water splitting and the degradation of organic compounds (e.g., methanol, phenol). Its catalytic performance can be significantly increased by the addition of co-catalysts. In this study, Au and Au/Ag nanoparticles were deposited onto mesoporous titania thin films using photo-deposition (Au) and magnetron-sputtering (Au and Au/Ag). All samples underwent comprehensive structural characterization by grazing incidence X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Nanoparticle distributions and nanoparticle size distributions were correlated to the deposition methods. Light absorption measurements showed features related to diffuse scattering, the band gap of titania and the local surface plasmon resonance of the noble metal nanoparticles. Further, the photocatalytic activities were measured using methanol as a hole scavenger. All nanoparticle-decorated thin films showed significant performance increases in hydrogen evolution under UV illumination compared to pure titania, with an evolution rate of up to 372 μL H₂ h−1 cm−2 representing a promising approximately 12-fold increase compared to pure titania.

Enhanced solar water-splitting activity of novel nanostructured Fe2TiO5 photoanode by electrospray and surface F-modification

Fe2TiO5 is recognized as a novel photoanode material for solar water splitting. However, it has been seldom studied as a photoelectrode by itself, and its practical performance still needs to be improved. Herein, nanostructured Fe2TiO5 photoanode is prepared by the electrospray technique. The effects of the synthesis parameters on the photoelectrochemical water splitting activity are studied including the substrate temperature and film thickness. In addition, surface F-modification is applied on pure Fe2TiO5 to further improve its photoelectrochemical performance. Also, the water splitting photocurrent of F-treated Fe2TiO5 increases to 0.4 mA cm-2 at 1.23 VRHE, which is higher than that of pristine Fe2TiO5. X-ray photoelectron spectroscopy confirms the formation of surface Ti-F bonds after surface F-treatment, which facilitates the transfer of holes and the breakage of O-H bond under illumination. The enhanced performance can be attributed to a synergetic effect of nanoarchitecture and surface F-modification. Therefore, the nanoarchitecture assisted by surface F-modification offers an effective strategy to prepare high-efficiency nanostructured complex metal oxides for solar water splitting.

Controlled growth of vertically aligned ultrathin In2S3 nanosheet arrays for photoelectrochemical water splitting

This paper reports a facile solvothermal method for the in situ growth of vertically aligned In₂S₃ nanosheet arrays (NSAs) on fluorine-doped tin oxide substrates. The as-synthesized two-dimensional graphene-like In₂S₃ nanosheets show an ultrathin thickness down to 3.7 nm consisting of the duodenary interplanar spacing of the (222) plane and a tunable bandgap varying from 2.32 to 2.58 eV. The film thickness and nanosheet density of the In₂S₃ NSAs can be adjusted by varying the reaction time and precursor concentration. The In₂S₃ NSAs with a higher film thickness exhibit relatively higher photocurrent due to their stronger light absorption as well as larger surface area for sufficient charge separation and redox reaction. The photoelectrochemical performance of the In₂S₃ photoanodes can be greatly enhanced by constructing an effective heterojunction with ZnO to promote the photocarrier separation. The In₂S₃/ZnO NSAs have demonstrated an optimal photocurrent density of 349.1 μA cm^-2 at 1.2 V vs. RHE and a maximum incident photon to current efficiency of 10.26% at 380 nm, which are 13.5 and 38 times higher than those of the pristine In₂S₃ counterparts, respectively.

Photorechargeable High Voltage Redox Battery Enabled by Ta3 N5 and GaN/Si Dual-Photoelectrode

Solar rechargeable battery combines the advantages of photoelectrochemical devices and batteries and has emerged as an attractive alternative to artificial photosynthesis for large-scale solar energy harvesting and storage. Due to the low photovoltages by the photoelectrodes, however, most previous demonstrations of unassisted photocharge have been realized on systems with low open circuit potentials (<0.8 V). In response to this critical challenge, here it is shown that the combined photovoltages exceeding 1.4 V can be obtained using a Ta₃ N₅ nanotube photoanode and a GaN nanowire/Si photocathode with high photocurrents (>5 mA cm^-2 ). The photoelectrode system makes it possible to operate a 1.2 V alkaline anthraquinone/ferrocyanide redox battery with a high ideal solar-to-chemical conversion efficiency of 3.0% without externally applied potentials. Importantly, the photocharged battery is successfully discharged with a high voltage output.

3D Cathodes of Cupric Oxide Nanosheets Coated onto Macroporous Antimony-Doped Tin Oxide for Photoelectrochemical Water Splitting

Cupric oxide (CuO), a narrow-bandgap semiconductor, has a band alignment that makes it an ideal photocathode for the renewable production of solar fuels. However, the photoelectrochemical performance of CuO is limited by its poor conductivity and short electron diffusion lengths. Herein, a three-dimensional (3D) architecture consisting of CuO nanosheets supported onto transparent conducting macroporous antimony-doped tin oxide (mpATO@CuONSs) is designed as an excellent photocathode for promoting the hydrogen evolution reaction (HER). Owing to the 3D structure affording superior light-harvesting characteristics, large contact areas with the electrolyte, and highly conductive pathways for separation and transport of charge carriers, the mpATO@CuONSs photocathode produces an impressively high photocurrent density of -4.6 mA cm^-2 at 0 V versus the reversible hydrogen electrode (RHE), which is much higher than that of the CuONSs array onto planar FTO glass (-1.9 mA cm^-2 ).

A Titanium-Doped SiOx Passivation Layer for Greatly Enhanced Performance of a Hematite-Based Photoelectrochemical System

This study introduces an in situ fabrication of nanoporous hematite with a Ti-doped SiOx passivation layer for a high-performance water-splitting system. The nanoporous hematite with a Ti-doped SiOx layer (Ti-(SiOx /np-Fe2 O3 )) has a photocurrent density of 2.44 mA cm(-2) at 1.23 VRHE and 3.70 mA cm(-2) at 1.50 VRHE . When a cobalt phosphate co-catalyst was applied to Ti-(SiOx /np-Fe2 O3 ), the photocurrent density reached 3.19 mA cm(-2) at 1.23 VRHE with stability, which shows great potential of the use of the Ti-doped SiOx layer with a synergistic effect of decreased charge recombination, the increased number of active sites, and the reduced hole-diffusion pathway from the hematite to the electrolyte.

Controlled Design of Functional Nano-Coatings: Reduction of Loss Mechanisms in Photoelectrochemical Water Splitting

Efficient water splitting with photoelectrodes requires highly performing and stable photoactive materials. Since there is no material known which fulfills all these requirements because of various loss mechanisms, we present a strategy for efficiency enhancement of photoanodes via deposition of functional coatings in the nanometer range. Origins of performance losses in particle-based oxynitride photoanodes were identified and specifically designed coatings were deposited to address each loss mechanism individually. Amorphous TiO2 located at interparticle boundaries enables high electron conductivity. A thin layer of amorphous Ta2O5 can be used as protection layer for photoanodes because of its hole conductivity and thermal and chemical stability. An amorphous layer of NiOx and Co(OH)2 reduces photocorrosion or increases water oxidation kinetics because they act as a hole-capture material or water oxidation catalyst, respectively. Crystalline CoOx nanoparticles increase photocurrent and reduce the onset potential due to enhanced charge separation. The combination of all coatings deposited by a scalable, mild, and reproducible step-by-step approach leads to high-performance oxynitride-based photoanodes providing a maximum photocurrent of 2.52 mA/cm(2) at 1.23 VRHE under AM1.5G illumination.

A three-dimensional interconnected hierarchical FeOOH/TiO₂/ZnO nanostructural photoanode for enhancing the performance of photoelectrochemical water oxidation

A novel ZnO/TiO2/FeOOH hierarchical nanostructure has been synthesized by a low temperature chemical bath deposition method. The integrated three-dimensional (3D) nanostructure consists of one-dimensional (1D) ZnO/TiO2 core-shell nanowire arrays and two-dimensional (2D) interconnected FeOOH nanosheets. By applying such a hierarchical nanostructure as a photoanode for photoelectrochemical water reaction, higher photostability and photocurrent density are gained compared with the reported ZnO based nanostructures. It is concluded that the giant enhancement of the properties is because, in the process of photoelectrochemical reaction, electron-hole separation and transfer are enhanced efficiently through the ZnO/TiO2 heterojunction, and in the meanwhile, terminal interconnected FeOOH nanosheets play both the roles of a surface catalyst and a protective layer effectively to accelerate water splitting reaction and enhance photostability. Based on such an environmentally friendly hierarchical nanostructure, photoelectrochemical water splitting and other similar reactions could be performed effectively and economically.

Design Guidelines for High-Performance Particle-Based Photoanodes for Water Splitting: Lanthanum Titanium Oxynitride as a Model

Semiconductor powders are perfectly suited for the scalable fabrication of particle-based photoelectrodes, which can be used to split water using the sun as a renewable energy source. This systematic study is focused on variation of the electrode design using LaTiO2 N as a model system. We present the influence of particle morphology on charge separation and transport properties combined with post-treatment procedures, such as necking and size-dependent co-catalyst loading. Five rules are proposed to guide the design of high-performance particle-based photoanodes by adding or varying several process steps. We also specify how much efficiency improvement can be achieved using each of the steps. For example, implementation of a connectivity network and surface area enhancement leads to thirty times improvement in efficiency and co-catalyst loading achieves an improvement in efficiency by a factor of seven. Some of these guidelines can be adapted to non-particle-based photoelectrodes.

Joint Effects of Photoactive TiO2 and Fluoride-Doping on SnO2 Inverse Opal Nanoarchitecture for Solar Water Splitting

Inverse opal (IO) films of tin dioxide (SnO2) were fabricated on polystyrene (PS) beads (diameter=350 nm (±20 nm) with a spin coating method. To compensate for the large band gap (Eg=3.8 eV), a thin TiO2 shell was deposited on the SnO2-IO films with atomic layer deposition (ALD), which produced shells with thicknesses of 10-40 nm. The morphological changes and crystalline properties of the SnO2 and TiO2-coated SnO2 (herein after referred to as TiO2/SnO2) IO films were investigated with field-emission scanning electron microscopy and X-ray diffraction, respectively. The photoelectrochemical (PEC) behavior of the samples was tested in a 0.1 M KOH solution under 1 sun illumination (100 mW/cm2 with an AM 1.5 filter). The highest PEC performance was obtained with the TiO2(10 nm)/SnO2 IO films, which produced a photocurrent density (Jsc) of 4.67 mA/cm2 at 0.5 V (vs NHE) and was sequentially followed by the TiO2(20 nm)/SnO2-IO, TiO2(30 nm)/SnO2-IO, TiO2 (40 nm)/SnO2-IO and SnO2 IO films. Overall, the thin TiO2 shell covered on the SnO2-IO core enhanced Jsc by 3 orders of magnitude, which in turn the PEC activity. This is mainly ascribed to the extremely low charge-transfer resistance (Rct) in the photoelectrode/electrolyte and at the TiO2/SnO2 interface, as well as the contribution of the photoactive TiO2 layer, which has an Eg of 3.2 eV. Moreover, to improve the electrical conductivity of the core SnO2 IO film, the films were doped with 10 mol % of F. The F- doped films were labeled as the FTO IO film. The Rct of the FTO-IO films decreased because of the improved electronic conductivity, enhancing the PEC performance of the TiO2(10 nm)/FTO-IO films by approximately 20%. The core-shell nanowire mesh nanoarchitecture is therefore suggested to provide an insight for designing the peculiar structure based on the material's properties and the engineering of their band gap energy for highly efficient PEC performance.

Photoelectrochemical Hydrogen Production of TiO2 Passivated Pt/Si-Nanowire Composite Photocathode

Si nanowire (SiNW) arrays decorated with Pt nanoparticles are passivated with TiO2 surface layer using atomic layer deposition (ALD). The sandwich structure TiO2/Pt/SiNW shows superior photoelectrochemical performance to the control planar silicon electrodes, especially under the concentrated solar radiation. Pt nanoparticles separated from aqueous electrolyte by TiO2 layer of more than 15 nm still well catalyze surface photoelectrochemical hydrogen production without direct contact to the electrolyte. This structural configuration shows remarkable chemical stability and anodically shifted onset potential, suggesting great promise for applications in solar hydrogen production. The maximum photon-to-energy conversion efficiency of the TiO2/Pt/SiNW reaches 15.6%.