Organic Upgrading through Photoelectrochemical Reactions: Toward Higher Profits

Aqueous photoelectrochemical (PEC) cells have long been considered a promising technology to convert solar energy into hydrogen. However, the solar-to-H2 (STH) efficiency and cost-effectiveness of PEC water splitting are significantly limited by sluggish oxygen evolution reaction (OER) kinetics and the low economic value of the produced O2 , hindering the practical commercialization of PEC cells. Recently, organic upgrading PEC reactions, especially for alternative OERs, have received tremendous attention, which improves not only the STH efficiency but also the economic effectiveness of the overall reaction. In this review, PEC reaction fundamentals and reactant-product cost analysis of organic upgrading reactions are briefly reviewed, recent advances made in organic upgrading reactions, which are categorized by their reactant substrates, such as methanol, ethanol, glycol, glycerol, and complex hydrocarbons, are then summarized and discussed. Finally, the current status, further outlooks, and challenges toward industrial applications are discussed.

Interface engineering of sandwich SiO@α-FeO@COF core-shell S-scheme heterojunctions for efficient photocatalytic oxidation of gas-phase HS

Hydrogen sulfide (H₂S) is considered to be a broad-spectrum toxicant, and it is crucial to address this problem due to its serious health and climate change impacts. Photocatalysis can be effectively applied for the reduction of H₂S molecules to S and other products. We synthesized sandwich-structured composite materials with internally immobilized SiO₂ nanospheres and externally wrapped COF layers co-modified with iron oxide nanoparticles. Furthermore, originally looked at the efficiency of photocatalysis in reducing hydrogen sulfide to sulfur. In this paper, a sandwich structure of core-shell composite photocatalysts based on SiO₂ was prepared by a multi-step method including Stöber and double ligand-regulated solvent heat, and these sandwich core-shell structures exhibited high hydrogen sulfide reduction and stability in applications. In addition, characterization, degradation studies, active substance trapping studies, and energy band structure analysis showed that S-type heterojunctions could effectively increase photo-generated carrier separation. This research advanced knowledge of photocatalytic hydrogen sulfide reduction and offered a novel approach for catalysts in COF sandwich core-shell structures.

TiO2-HfN Radial Nano-Heterojunction: A Hot Carrier Photoanode for Sunlight-Driven Water-Splitting

The lack of active, stable, earth-abundant, and visible-light absorbing materials to replace plasmonic noble metals is a critical obstacle for researchers in developing highly efficient and cost-effective photocatalytic systems. Herein, a core–shell nanotube catalyst was fabricated consisting of atomic layer deposited HfN shell and anodic TiO2 support layer with full-visible regime photoactivity for photoelectrochemical water splitting. The HfN active layer has two unique characteristics: (1) A large bandgap between optical and acoustic phonon modes and (2) No electronic bandgap, which allows a large population of long life-time hot carriers, which are used to enhance the photoelectrochemical performance. The photocurrent density (≈2.5 mA·cm−2 at 1 V vs. Ag/AgCl) obtained in this study under AM 1.5G 1 Sun illumination is unprecedented, as it is superior to most existing plasmonic noble metal-decorated catalysts and surprisingly indicates a photocurrent response that extends to 730 nm. The result demonstrates the far-reaching application potential of replacing active HER/HOR noble metals such as Au, Ag, Pt, Pd, etc. with low-cost plasmonic ceramics.

Ethylene Glycol Electrochemical Reforming Using Ruthenium Nanoparticle-Decorated Nickel Phosphide Ultrathin Nanosheets

In this work, ruthenium nanoparticle-decorated ultrathin nickel phosphide nanosheets on nickel foam substrate (Ru/Ni₂P/NF) nanocomposites are synthesized conveniently by a cyanogel-NaBH₄ method and a subsequent phosphating process, which displays excellent electroactivity for both the hydrogen evolution reaction (HER) and ethylene glycol electro-oxidation reaction (EGEOR) in an alkaline solution. Concretely, at Ru/Ni₂P/NF nanocomposites, only 1.37 and -0.13 V potentials are required to obtain a current density of 100 mA cm^-2 for EGEOR and HER, respectively. Meanwhile, Ru/Ni₂P/NF nanocomposites also exhibit pre-eminent electrocatalytic performance of the long-running process for both EGEOR and HER. Density functional theory calculations demonstrate that the introduction of Ru nanoparticles results in an optimization of the surface adsorption energy and construction of a synergistic catalysis interface, which improve the electrocatalytic performance of nickel phosphide nanosheets. Notably, a symmetric Ru/Ni₂P/NF||Ru/Ni₂P/NF ethylene glycol electrolyzer needs only 1.14 V electrolysis voltage to obtain 10 mA cm^-2 for hydrogen production, which effectively eliminates the H₂/O₂ explosion risk and highlights an energy-saving mode for electrochemical hydrogen production.

Harvesting Hot Holes in Plasmon-Coupled Ultrathin Photoanodes for High-Performance Photoelectrochemical Water Splitting

The harvesting of hot carriers produced by plasmon decay to generate electricity or drive a chemical reaction enables the reduction of the thermalization losses associated with supra-band gap photons in semiconductor photoelectrochemical (PEC) cells. Through the broadband harvesting of light, hot-carrier PEC devices also produce a sensitizing effect in heterojunctions with wide-band gap metal oxide semiconductors possessing good photostability and catalytic activity but poor absorption of visible wavelength photons. There are several reports of hot electrons in Au injected over the Schottky barrier into crystalline TiO₂ and subsequently utilized to drive a chemical reaction but very few reports of hot hole harvesting. In this work, we demonstrate the efficient harvesting of hot holes in Au nanoparticles (Au NPs) covered with a thin layer of amorphous TiO₂ (a-TiO₂). Under AM1.5G 1 sun illumination, photoanodes consisting of a single layer of ∼50 nm diameter Au NPs coated with a 10 nm shell of a-TiO₂ (Au@a-TiO₂) generated 2.5 mA cm^-2 of photocurrent in 1 M KOH under 0.6 V external bias, rising to 3.7 mA cm^-2 in the presence of a hole scavenger (methanol). The quantum yield for hot-carrier-mediated photocurrent generation was estimated to be close to unity for high-energy photons (λ < 420 nm). Au@a-TiO₂ photoelectrodes produced a small positive photocurrent of 0.1 mA cm^-2 even at a bias of -0.6 V indicating extraction of hot holes even at a strong negative bias. These results together with density functional theory modeling and scanning Kelvin probe force microscope data indicate fast injection of hot holes from Au NPs into a-TiO₂ and light harvesting performed near-exclusively by Au NPs. For comparison, Au NPs coated with a 10 nm shell of Al₂O₃ (Au@Al₂O₃) generated 0.02 mA cm^-2 of photocurrent in 1 M KOH under 0.6 V external bias. These results underscore the critical role played by a-TiO₂ in the extraction of holes in Au@a-TiO₂ photoanodes, which is not replicated by an ordinary dielectric shell. It is also demonstrated here that an ultrathin photoanode (<100 nm in maximum thickness) can efficiently drive sunlight-driven water splitting.

One-Dimensional CdS/Carbon/Au Plasmonic Nanoarray Photoanodes via In Situ Reduction-Graphitization Approach toward Efficient Solar Hydrogen Evolution

Photoelectrochemical (PEC) hydrogen evolution has been acknowledged as a promising "green" technique to convert solar energy into clean chemical fuel. Photoanodes play a key role in determining the performance of PEC systems, spurring numerous efforts to develop advanced materials as well as structures to improve the photoconversion efficiency. In this work, we report the rational design of a plasmonic hierarchical nanorod array, composed of oriented one-dimensional (1D) CdS nanorods decorated with a uniformly wrapped graphite-like carbon (C_PDA) layer and Au nanoparticles (Au NPs), as highly efficient photoanode materials. An interfacial in situ reduction-graphitization method has been conducted to prepare the CdS/C_PDA/Au nanoarchitecture, where polydopamine (PDA) coating was used as a C source and a reductant. The CdS/C_PDA/Au nanoarray photoanode demonstrates superior photoconversion efficiency with a photocurrent density of 8.74 mA/cm² and an IPCE value (480 nm) of 30.2% (at 1.23 V vs RHE), under simulated sunlight irradiation, which are 12.7 and 13.5 times higher than pristine CdS. The significant enhancement of PEC performance is mainly benefited from the increase of the entire quantum yield and efficiency due to the formation of a Schottky rectifier, localized surface plasmon resonance (LSPR)-enhanced light absorption, and promoted hot-electron injection from interlayered graphene-like carbon. More importantly, thanks to the inhibited charge carrier recombination process and transferred oxidation reaction sites, the fabricated CdS/C_PDA/Au photoelectrode exhibits lengthened electron lifetimes and better photostability, illustrating its wonderful potential for future PEC application.

Unprecedented Ag-Cu2O composited mesocrystals with efficient charge separation and transfer as well as visible light harvesting for enhanced photocatalytic activity

Mesocrystals with highly ordered subunits can provide good charge transfer tunnels and more active sites for catalytic reactions. So far, single-component mesocrystals have been well-developed in metals or metal oxides in the past decades, but the construction of mesocrystals in nanocomposites has been a great challenge. Herein we demonstrated a simple, one-pot wet chemical strategy for the preparation of plate-like Ag-Cu2O composited mesocrystals (CMCs) without any organic capping agent, which broke through the traditional dependence on organic capping agents for the synthesis of mesocrystals. As expected, these unprecedented Ag-Cu2O CMCs displayed superior visible-light-driven photodegradation performance toward tetracycline solution compared to the core-shell Ag@Cu2O and pure Cu2O photocatalysts. The improved photocatalytic activity of Ag-Cu2O CMCs could be ascribed to the synergistic effect of an ordered crystallographic orientation, the Schottky barrier and localized surface plasmon resonance (LSPR) for simultaneously enhancing charge separation and transfer as well as visible light harvesting. This research might stimulate in-depth investigations on the exploration of new synthetic methods for the design and construction of novel composited mesocrystals.

A promising ternary sulfide bidirectional p-n heterojunction for unassisted tandem photoelectrochemical cells

There has been great demand for high-efficiency, low-cost, and abundant photoelectrode materials for photoelectrochemical (PEC) systems. Here, we studied the PEC performance of ternary sulfide photoelectrodes (ZnIn2S4 and CuInS2) with the same nanostructure and applied bidirectional p-n heterojunctions with energy levels that were well matched to the unassisted tandem PEC cell device. Moreover, ZnIn2S4/CuInS2 and CuInS2/ZnIn2S4 were used as a photoanode and photocathode, respectively, in the unassisted tandem PEC cell device, which achieved a relatively high photocurrent density of 0.06 mA cm-2. This work provides new ideas for the design and manufacture of high-efficiency unassisted tandem PEC cell devices.

Plasmon-Driven Electrochemical Methanol Oxidation on Gold Nanohole Electrodes

Direct methanol oxidation is expected to play a central role in low-polluting future power sources. However, the sluggish and complex electro-oxidation of methanol is one of the limiting factors for any practical application. To solve this issue, the use of plasmonic is considered as a promising way to accelerate the methanol oxidation reaction. In this study, we report on a novel approach for achieving enhanced methanol oxidation currents. Perforated gold thin film anodes were decorated with Pt/Ru via electrochemical deposition and investigated for their ability for plasmon-enhanced electrocatalytic methanol oxidation in alkaline media. The novel methanol oxidation anode (AuNHs/PtRu), combining the strong light absorption properties of a gold nanoholes array-based electrode (AuNHs) with surface-anchored bimetallic Pt/Ru nanostructures, known for their high activity toward methanol oxidation, proved to be highly efficient in converting methanol via the hot holes generated in the plasmonic electrode. Without light illumination, AuNHs/PtRu displayed a maximal current density of 13.7 mA/cm² at -0.11 V vs Ag/AgCl. Enhancement to 17.2 mA/cm² was achieved under 980 nm laser light illumination at a power density of 2 W/cm². The thermal effect was negligible in this system, underlining a dominant plasmon process. Fast generation and injection of charge carriers were also evidenced by the abrupt change in the current density upon laser irradiation. The good stability of the interface over several cycles makes this system interesting for methanol electro-oxidation.

Hematite photoanode modified with inexpensive hole-storage layer for highly efficient solar water oxidation

Hematite is recognized as an excellent photocatalyst for photoelectrochemical photoanodes for water oxidation because of its favorable band gap, excellent anti-photocorrosion and structural stability in alkaline solution. However, slow charge transport and fast carrier recombination in the bulk and at the hematite photoanode/electrolyte interface, have limited its applications for water splitting. Herein, we report a highly efficient hematite/ferrhydrite (Fh) core-shell photoanode system, consisting of hematite (α-Fe₂O₃) semiconductor nanorods which dramatically enhance light harvesting, and ferrhydrite as the hole-storage shell. Our integrated hematite/ferrhydrite core-shell photoanode shows 2.7 times increased photo-current density under simulated sun light irradiation.

Fabrication of Dandelion-like p-p Type Heterostructure of Ag2O@CoO for Bifunctional Photoelectrocatalytic Performance

A novel three-dimensional purple dandelion-like hierarchical Ag₂O@CoO heterojunction with an appropriate redox potential was constructed by chemical precipitation of Ag₂O nanoparticle on flower-like CoO. By feat of this hierarchical structure, the Ag₂O@CoO photocathode showed significantly high photoelectroreduction activities toward p-nitrophenol (p-NP) and Cr(VI). The high performance of Ag₂O@CoO was mainly attributed to the specific structural characteristics and synergistic effect of each chemical component. This hierarchical structure could effectively increase the specific surface area, provide more exposed active edges, and be beneficial for multiple light reflection/scattering channels and light utilization efficiency. The introduction of Ag₂O optimized the composition and further improved the band structure, resulting in an improved separation of photogenerated electrons and holes. The unique photocathode achieves a removal efficiency of 86% for photoelectrocatalytic p-NP degradation after 120 min and 95% for Cr(VI) after 40 min under visible light irradiation with excellent stability. This research provided a simple way for the synthesis of photoelectrocatalytic material with potential applications in the field of environmental governance with visible light illumination.

Fabrication and high photoelectrocatalytic activity of scaly BiOBr nanosheet arrays

Bismuth oxybromide (BiOBr) nanosheet arrays (NSAs) were successfully prepared on the surface of indium tin oxide glass (hydrophilic pretreated) by solvothermal method using [C₁₆mim]Br ionic liquid as bromine source and template. The effects of different reaction temperatures on array synthesis were investigated. BiOBr NSA-160 (NSAs prepared at 160 °C for 8 h) had the best photoelectrocatalytic (PEC) activity. The removal rate of ciprofloxacin hydrochloride by BiOBr NSA-160 was 91.4% by applying a bias voltage of 0.9 V and irradiating under visible light for 180 min. Results of the analyses of the morphology, photoelectric properties, energy band structure, and degradation active species show that BiOBr NSA-160 is a p-type photocatalyst with a thickness of approximately 500 nm, a light response range of less than 440 nm, and photocurrent density of 69 μA/cm² at the optimal bias voltage is 0.9 V. The high PEC activity of BiOBr NSA-160 was deduced from two aspects: one is that the bias potential effectively improves the separation efficiency of photogenerated carrier, and the other is that the structure of the nanoarray increases light absorption and active sites. BiOBr NSAs are promising PEC material for application in pollutant removal.

Accelerated photoelectron transmission by carboxymethyl β-cyclodextrin for organic contaminants removal: An alternative to noble metal catalyst

Applications of noble metal decorated photocatalytic nanomaterials are restricted by its high cost. In this study, carboxymethyl β-cyclodextrin (CM-β-CD), as an alternative to gold nanoparticle, was used to modified titanium dioxide (CM-β-CD-P25) to accelerate photoelectron transmission and enhance the organic contaminants removal from water. Several of emerging organic contaminants, such as bisphenol A (BPA), phenol and sulphanilamide (SA), were used to evaluate their photocatalytic activities. Carboxymethyl-β-cyclodextrin not only provide hydrophobic sites to entrap organic contaminants but also provide a "bridge" for accelerated transmission of photogenerated charges without introducing the recombination interface. Consequently, 91.6 % of BPA, 71.9 % of phenol and 97.1 % of SA could be removed by CM-β-CD-P25(2:1) under 1 h UV light irradiation. The photooxidation rate constant of BPA, phenol and SA by CM-β-CD-P25(2:1) were 0.039 min^-1, 0.021 min^-1 and 0.062 min^-1, respectively, which are much higher than that of pristine P25 and Au-P25. Moreover, the photocatalytic activity of CM-β-CD-P25(2:1) remains almost unchanged in repeated cycle test owing to its high stability. The reasonable mechanism of CM-β-CD-P25 were investigated. CM-β-CD-P25 hybrid nanoparticles completely surpasses Au-P25 in organic contaminants removal, and shows great potential to replace noble metal as mediator.

A novel immobilized Z-scheme P3HT/α-Fe2O3 photocatalyst array: Study on the excellent photocatalytic performance and photocatalytic mechanism

The Z-scheme photocatalyst is valuable for use in polluted water purification. To improve the practical application value of the Z-scheme, in this study, a nano α-Fe₂O₃ and P3HT composite photocatalyst array was synthesized by electrophoretic deposition for the first time. This material was named F1P. The EPR, VBXPS and in situ illumination XPS analyses indicated that the charge transfer path of F1P conforms to the Z-scheme, and F1P array has a high light energy conversion efficiency. Due to the advantages of the Z-scheme system and the good light harvesting ability, F1P exhibited a good degradation effect on different types of pollutants under visible light irradiation. The degradation efficiency of tetracycline and bisphenol A by F1P can reach 97 % and 99 %, respectively. Moreover, F1P showed a certain resistance to water quality changes. After four cycles, F1P can maintain the structural stability and good pollutant treatment effect. Due to the high redox ability of F1P, the complex structure of pollutants can be destroyed and the degradation path of pollutants in F1P system was also analyzed. This study provides a new way of synthesizing Z-scheme systems and immobilizing Z-scheme photocatalysts. F1P is a new material with high practicability.

Underlayer engineering into the Sn-doped hematite photoanode for facilitating carrier extraction

A semiconductor underlayer(s) has been extensively used to improve the performance of photoelectrochemical (PEC) cells. Unfortunately, in many cases, the incorporation of underlayers leads to degraded system performances. A comprehensive study on the functions and manipulations of underlayers is therefore of high significance for achieving high-performance PEC cells. This study indicates that Sn-doped hematite photoanodes decorated with various underlayer materials show substantially distinguished photocurrent responses, leading to qualitatively different PEC cells. With an optimized TiO2 (ITO, Al2O3) underlayer, the photocurrent density at 1.23 V versus RHE can be enhanced from 0.25 to 0.71 (0.59, 0.42) mA cm-2, while it is decreased to 0.14 mA cm-2 by using NiO. Our further analysis reveals that the performance differences come mainly from the distinguished bulk and surface carrier recombination effects, i.e., (1) metal doping (i.e., Ti4+, In3+ and Al3+) from the underlayers improves the conductivity of hematite film and thus reduces the bulk recombination; (2) the underlayers of TiO2, ITO and Al2O3 can effectively suppress the carrier recombination at the bottom/top surfaces of the hematite layer, while the NiO underlayer leads to a higher surface recombination. Our work provides a basis for selecting an underlayer and a general guideline for the interface engineering for high performance photoelectrodes.

Chemically Bonded N-PDI-P/WO3 Organic-Inorganic Heterojunction with Improved Photoelectrochemical Performance

The chemical bonding of bandgap adjustable organic semiconductors with inorganic semiconducting materials is effective in constructing a high-performance heterogeneous photoanode. In this study, a new asymmetric perylene diimide derivative molecule (N-PDI-P) was synthesized by connecting tert-butoxycarbonyl on an N-site at one end of a PDI molecule through methylene and connecting naphthalene directly onto the other end. This molecule was bonded onto the WO3 film surface, thereby forming the photoanode of organic-inorganic heterojunction. Under light illumination, the photocurrent density of chemically bonded N-PDI-P/WO3 heterojunction was twofold higher than that of physically adhered heterojunction for photoelectrochemical water oxidation at 0.6 V (vs. Ag/AgCl). Energy band structure and charge transfer dynamic analyses revealed that photogenerated electron carriers on the highest occupied molecular orbital (HOMO) of an N-PDI-P molecule can be transferred to the conduction band of WO3. The charge transfer and separation rates were accelerated considerably after the chemical bond formed at the N-PDI-P/WO3 interface. The proposed method provides a new way for the design and construction of organic-inorganic composite heterojunction.

Omnidirectional Au-embedded ZnO/CdS core/shell nanorods for enhanced photoelectrochemical water-splitting efficiency

The charge transfer mechanism that enhanced the photoconversion efficiency of omnidirectional Au-embedded ZnO/CdS core/shell nanorods.

Ligand-protected atomically precise gold nanoclusters as model catalysts for oxidation reactions

This feature article provides a systematic overview and outlook on the oxidation reactions catalyzed by gold nanoclusters.

Self-improvement of solar water oxidation for the continuously-irradiated hematite photoanode

Improving bulk- or surface-properties has been found as an effective route to regulate and enhance the photoelectrochemical (PEC) performances of some metal-oxide photoelectrodes. However, both bulk and surface self-improvement resulting from the photocharging (PC) effect is rarely reported and as a result the underlying mechanism of the PC effect is not fully understood. Here, we demonstrate that the hematite photoanode integrated with Sn doping and a TiO2 underlayer shows a substantial increase in the photocurrent density (i.e., from 0.69 to 1.12 mA cm-2 at 1.23 V relative to the standard hydrogen electrode) and a cathodic shift of the onset potential after being irradiated by a one-sun simulator for 12 h. The primary reasons for these can be categorized into two fundamental factors: (1) the enhanced bulk conductivity and the resulting decrease in carrier bulk recombination from the gradually increasing ratio of Fe2+ and Fe3+; (2) the reduced carrier surface recombination from the photogenerated passivation layer. Ultimately, both the bulk and surface electrical properties of the hematite photoanode are substantially self-improved under continuous irradiation. This work deepens the understanding of the PC effect and proves that it is a promising technique for the PEC-performance enhancement of the hematite photoanode.

Enhanced photoelectrocatalytic activity of direct Z-scheme porous amorphous carbon nitride/manganese dioxide nanorod arrays

Carbon nitride (C₃N₄) is a promising photocatalyst that can be applied in environmental remediation and energy conversion. However, the absorption range and charge separation efficiency of C₃N₄ are still severely restricted for its large-scale practical applications. Herein, we demonstrate a simple thermal polymerization and electrodeposition method, followed by partial etching strategy to synthesize direct Z-scheme porous zinc oxide/amorphous carbon nitride/manganese dioxide hybrid core-shell nanorod array (denoted as P-ZnO/ACN/MnO₂) by encapsulating amorphous carbon nitride layers (ACN) and manganese dioxide nanosheets (MnO₂) on the zinc oxide nanorod arrays (denoted as ZnO). Interestingly, ZnO serves as the collector of charge carriers and MnO₂ plays a significant role in protecting ACN from corrosion. The as-prepared Z-scheme P-ZnO/ACN/MnO₂ heterojunction exhibits high photocurrent density of 5.2 mA cm^-2 at 0.6 V vs. Ag/AgCl, high photoconversion efficiency 0.98%, and universal photoelectrocatalytic degradation activity for degradation of organic dyes under visible light irradiation. The band gap energy and conduction band position of ZnO, ACN and MnO₂ are calculated by UV-visible diffuse reflection and Mott-Schottky measurement, which strongly support the direct Z-scheme charge carrier migration mechanism. This finding provides an efficient strategy to construct highly active and stable C₃N₄-based Z-scheme photocatalytic system.

The effect of solvent parameters on properties of iron-based silica binary aerogels as adsorbents

The magnetic Fe₃O₄-SiO₂ binary aerogel and nonmagnetic α-Fe₂O₃-SiO₂ binary aerogels are obtained by adjusting the solvent type during the solvothermal reaction and varying the Fe/Si proportion in the sol-gel process. The microstructure, surface charge and the formation mechanism of iron-based silica binary aerogels are analyzed by SEM, zeta potential and BET. The influence of the Fe/Si proportion on the surface group and morphology of binary aerogels is also investigated by FTIR and TEM analysis. The adsorption behavior of the iron-based silica binary aerogels on the Congo Red (CR) dye is also discussed by adsorption kinetics model and adsorption isotherm model. In addition, the effects of pH and initial concentration of the solutions, adsorption time and the maximum adsorption capacities for CR of iron-based silica binary aerogels adsorbents are also discussed, respectively. Moreover, the maximum adsorption capacity of as-prepared magnetic Fe₃O₄-SiO₂ binary aerogels for dyes achieved 489.13 mg g^-1, the maximum adsorption capacity of nonmagnetic α-Fe₂O₃-SiO₂ reached 454.55 mg g^-1, respectively. Thus, the iron-based silica binary aerogels provides valuable clues for the study of other aerogel materials as adsorbents.

3D Branched Ca-Fe2 O3 /Fe2 O3 Decorated with Pt and Co-Pi: Improved Charge-Separation Dynamics and Photoelectrochemical Performance

The construction of junctions on hematite is an effective way to overcome the problems of slow charge separation and transfer kinetics, but constructing the junction is a significant challenge in photoelectrochemical (PEC) water splitting. Herein, a considerable improvement in PEC performance for α-Fe₂ O₃ was achieved following the introduction of a p-n homojunction between n-type α-Fe₂ O₃ and p-type Ca-doped α-Fe₂ O₃ through a facile hydrothermal method. The resultant 3D branched Ca-Fe₂ O₃ /Fe₂ O₃ enhanced the absorption intensity and reached a photocurrent density of 2.14 mA cm^-2 at 1.23 V vs. reversible hydrogen electrode (RHE). The merit of the desired lattice matching of the buried p-n homojunction structure built an internal electric field, which led to appropriate band alignment. These results were supported by a series of photoelectrochemical measurements, in particular, surface photovoltage (SPV) measurements. For further improvement of the charge-separation efficiency, a combination of separated cocatalysts was established on the homojunction structure, in which Pt acted as the electron collector and was deposited on the bottom, and Co-Pi as the hole-extraction cocatalyst was inserted to accelerate hole transfer on the surface of the photoanode. The resulting Co-Pi/Ca-Fe₂ O₃ /Fe₂ O₃ /Pt branched nanorods showed a significant improvement in charge-separation efficiency and photocurrent density (2.94 mA cm^-2 at 1.23 V vs. RHE). The present strategy, both the construction of the p-n homojunction and the coupling electron- and hole-transfer cocatalyst, could be expanded to many unstable or low-efficiency semiconductors for the design and fabrication of cost-effective photoanodes in PEC water splitting.

Synergy between Plasmonic and Electrocatalytic Activation of Methanol Oxidation on Palladium-Silver Alloy Nanotubes

Localized surface plasmon resonance (LSPR) excitation of noble metal nanoparticles has been shown to accelerate and drive photochemical reactions. Here, LSPR excitation is shown to enhance the electrocatalysis of a fuel-cell-relevant reaction. The electrocatalyst consists of Pd_x Ag alloy nanotubes (NTs), which combine the catalytic activity of Pd toward the methanol oxidation reaction (MOR) and the visible-light plasmonic response of Ag. The alloy electrocatalyst exhibits enhanced MOR activity under LSPR excitation with significantly higher current densities and a shift to more positive potentials. The modulation of MOR activity is ascribed primarily to hot holes generated by LSPR excitation of the Pd_x Ag NTs.

Ultrathin MoS2 nanosheets for high-performance photoelectrochemical applications via plasmonic coupling with Au nanocrystals

In this work, we prepared ultrathin MoS2 nanosheets with exposed active edge sites and high electric conductivity that can sufficiently absorb light in the visible region to enable solar energy conversion. The gold nanocrystal-decorated MoS2 nanosheets facilitate sufficiently enhanced photoelectrochemical water splitting in the UV-visible region. Different Au nanostructures, such as Au nanoparticles and nanorods, were modified on the surface of MoS2 nanosheets to promote photoelectrochemical water decomposition. By spin-coating a synthetic gold-modified MoS2 hybrid photoanode on a FTO substrate, the efficiency of photoelectrochemical water oxidation was significantly enhanced, by 2 times (nanorods) and 3.5 times (nanoparticles) in the visible-infrared region; furthermore, the average optical resistance was reduced by a factor of two compared to the MoS2 photoanode without Au, and the photocurrent increases exponentially when the system bias was greater than 0.7 volts. The Au-MoS2 metal-semiconductor interface plays an important role in studying the surface plasmon interactions, charge transfer mechanism, and electric field amplification. This rational design for such a unique hybrid nanostructure explains the plasmon-enhanced photoelectrochemical water splitting. This current contribution provides a new path for using the plasmonic metal/semiconductor heterostructure to effectively harvest UV-visible light for solar fuel generation.

Highly efficient ethylene glycol electrocatalytic oxidation based on bimetallic PtNi on 2D molybdenum disulfide/reduced graphene oxide nanosheets

In this paper, a two-dimensional (2D) hybrid material of molybdenum disulfide (MoS₂)/reduced graphene oxide (RGO) is facilely synthesized and used as an ideal support for the deposition of Pt nanoparticles. The as-prepared Pt/MoS₂/RGO composites are further worked as electrocatalysts towards ethylene glycol oxidation reaction (EGOR). In addition, when alloying with Ni, the composite shows obvious enhancement in electrocatalytic performance for EGOR. Specifically, the optimized molar ratio of Pt to Ni is 3:1, namely Pt₃Ni/MoS₂/RGO performs the strongest current density of 2062 mA mg^-1_Pt, which is 11.1, 5.80 and 2.40 times higher than those of Pt, Pt₃Ni and Pt/MoS₂/RGO electrodes, respectively. The systematically electrochemical measurements indicate that the largely promoted electrocatalytic performances of Pt₃Ni/MoS₂/RGO are mainly attributed to the synergistic effect of Ni and Pt, and 2D sheets of MoS₂/RGO. This excellent performance indicates that the reported electrocatalytic material could be an efficient catalyst for the application in direct ethylene glycol fuel cell and beyond.

SnS2 Nanosheets/H-TiO2 Nanotube Arrays as a Type II Heterojunctioned Photoanode for Photoelectrochemical Water Splitting

Improving the separation efficiency of photogenerated electron-hole pairs and the conductivity of electrons to photoanode substrates are critical to achieve high-performance photoelectrochemical (PEC) water splitting. Here, a SnS₂ /H-TiO₂ /Ti heterojunction photoanode was fabricated with SnS₂ nanosheets vertically grown on hydrogen-treated TiO₂ (H-TiO₂ ) nanotube arrays on a Ti substrate. It showed a significantly enhanced photocurrent of 4.0 mA cm^-2 at 1.4 V (vs. reversible hydrogen electrode) under AM 1.5 G illumination, 70 times higher than that of SnS₂ /TiO₂ /Ti. Kelvin probe force microscopy measurements indicated that photogenerated electrons could be easily transported through the SnS₂ /H-TiO₂ interface but not through the SnS₂ /TiO₂ interface. Through hydrogen treatment, defects were created in H-TiO₂ nanotubes to convert type I junctions to type II with SnS₂ nanosheets. As a result, a high efficiency of electron-hole separation at the SnS₂ /H-TiO₂ interface and a high electron conductivity in H-TiO₂ nanotubes were achieved and improved PEC performance. These findings show an effective route towards high-performance photoelectrodes for water splitting.

Exposing the photocorrosion mechanism and control strategies of a CuO photocathode

A CuO photocathode modified with TiO₂ and Pt displays superior photocorrosion stability in PEC water splitting.