Influence of Sb5+ as a Double Donor on Hematite (Fe3+) Photoanodes for Surface-Enhanced Photoelectrochemical Water Oxidation

Dual Functions of Fluorescence and Peroxidase Mimics for Hemin@NH2-UiO-66 and Ratiometric Fluorescence Sensing to l-Cysteine

The bifunctions of fluorescence activity and peroxidase mimics were well-integrated by a hemin chloride-modified Zr-based metal-organic framework (Hemin@NH2-UiO-66), which can catalyze the oxidation of o-phenylenediamine (OPD) to generate 2,3-diaminophenazine (DAP), presenting typical peroxidase mimic properties. The influence of the introduction of hemin chloride upon the enhanced peroxidase mimic activity was clarified, and the nanozyme catalytic parameters were optimized. Interestingly, the oxidized product of DAP presents strong fluorescence emission at 564 nm; combining it with the intrinsic fluorescence emission of NH2-UiO-66 at 452 nm, a dual-emission system could be built up by the resulting Hemin@NH2-UiO-66 in the presence of definite OPD and H2O2. Moreover, the fluorescence quenching effect (564 nm) was observed by adding l-cysteine (l-Cys); based on that, a straight-line dependence of the fluorescence intensity ratio (I452/I564) upon the concentrations of l-Cys was established. The detection limit was 0.21 μmol, and the analytical selectivity for l-Cys was also demonstrated. The work highlights the idea of combining the intrinsic fluorescence property and nanozyme catalytic activity in a functional MOF, and its special usability is found in the ratiometric fluorescence sensing analyses.

Revisiting strategies to improve the performance of hematite photoanodes for water photoelectrolysis

Photoelectrochemical (PEC) water splitting is emerging as a sustainable approach for producing green hydrogen. The design of efficient photoanodes is the key for the step forward technological application in which morphology optimization and defect engineering play central roles. In this perspective, the intricate interplay between morphology optimization, band engineering and chemical modifications is critically discussed. First, a brief introduction of the relevant aspects of semiconductors applied in PEC devices is provided. Then, a critical analysis of the influence of morphology and chemical modifications is presented, with hematite employed as a model system. Lastly, insights into future directions and outlooks for existing challenges on the development of photoelectrochemical devices for water splitting are displayed.

Parallel multi-stacked photoanodes of Sb-doped p-n homojunction hematite with near-theoretical solar conversion efficiency

Developing transparent and efficient photoanodes is a challenging but essential task in tandem photoelectrochemical cell for unassisted solar water splitting without an external bias. Here we report construction of p-n homojunction hematite photoanodes by hybrid microwave annealing-induced single antimony doping, which results in the gradually-increased valence states from the surface to the inside by the unique features of hybrid microwave annealing. The Sb-doped p-n homojunction hematite photoanode exhibits improved performance and displays a good transparency, achieving a stable photocurrent density of ~4.21 mA cm-2 at 1.23 VRHE under 100 mW cm-2 solar irradiation, which is comparable to the reported state-of-the-art hematite photoanodes. More importantly, a parallel-connected stack of six photoanodes of transparent p-n homojunction records a near-theoretical photocurrent density of ~10 mA cm-2 at 1.23 VRHE under standard photoelectrochemical water splitting conditions, which serves as a useful reference for hematite photoanodes and promises its practical application for unbiased photoelectrochemical water splitting.

Divergent repartitioning of antimony and arsenic during jarosite transformation: A comparative study under aerobic and anaerobic conditions

Jarosite is the host mineral of Sb(V) and As(V) in mining environments. However, the repartitioning of Sb and As during its transformation is poorly understood. Additionally, the mutual effect between the redistribution behavior of As and Sb during jarosite conversion remains unclear. Here, we investigated the transformation of Sb(V)-, As(V)- and Sb(V)-As(V)-jarosite at pH 5.5 under aerobic and anaerobic conditions without a reductant. The results indicated that co-precipitated Sb(V) promotes jarosite dissolution, and the final products were mainly goethite and hematite. In contrast, the co-precipitated As(V) retarded jarosite dissolution and altered the transformation pathway, mainly forming lepidocrocite, which might be attributed to the formation of As-Fe complexes on the jarosite surface. The inhibiting or promoting effect increased with the increase in co-precipitated As or Sb concentration. In the treatment with Sb(V)-As(V)-jarosite, the inhibition effect of co-precipitated As(V) on mineral dissolution was predominant, but the end-products were mainly goethite and hematite. Compared with the aerobic system, the dissolution and transformation of jarosite in treatments in the anaerobic system occurred faster, although without a reductant, which was possibly associated with the reduced CO2 content in the reaction solutions after degassing. In all treatments, the release of Sb(aq) and As(aq) into the solution was negligible during jarosite transformation. The transformation processes drove As into the surface-bound exchangeable and poorly crystalline phases, while Sb was typically redistributed in the poorly crystalline phase. During the transformation of Sb(V)-As(V)-jarosite, the co-existence of As significantly increased the proportion of Sb distributed on the solid surface and in the poorly crystalline phase. These findings are valuable for predicting the long-term fate of Sb and As in mining environments.

Surface Rh-Boosted Photoelectrochemical Water Oxidation of α-Fe2O3 by Reduced Overpotential in the Rate-Determining Step

The photoelectrochemical behavior of Rh cluster-deposited hematite (α-Fe2O3) photoanodes (α-Fe2O3@Rh) was investigated. The interactions between Rh clusters and α-Fe2O3 nanorods were elucidated both experimentally and computationally. A facile UV-assisted solution casting deposition method allowed the deposition of 2 nm Rh clusters on α-Fe2O3. The deposited Rh clusters effectively enhanced the photoelectrochemical performance of the α-Fe2O3 photoanode, and electrochemical impedance spectroscopy (EIS) and Mott-Schottky analysis were applied to understand the working mechanism for the α-Fe2O3@Rh photoanodes. The results revealed a distinctive carrier transport mechanism for α-Fe2O3@Rh and increased carrier density, while the absorbance spectra remained unchanged. Furthermore, density functional theory (DFT) calculations of the oxygen evolution reaction (OER) mechanism corresponded well with the experimental results, indicating a reduced overpotential of the rate-determining step. In addition, DFT calculation models based on the X-ray diffraction (XRD) measurements and X-ray photoelectron spectroscopy (XPS) results provided precise water-splitting mechanisms for the fabricated α-Fe2O3 and α-Fe2O3@Rh nanorods. Owing to enhanced carrier generation and hole transfer, the optimum α-Fe2O3@Rh3 sample showed 78% increased photocurrent density, reaching 1.12 mA/cm-2 at 1.23 VRHE compared to that of the pristine α-Fe2O3 nanorods electrode.

Enhanced Oxygen Evolution Reaction Activity in Hematite Photoanodes: Effect of Sb-Li Co-Doping

Co-doping represents a valid approach to maximize the performance of photocatalytic and photoelectrocatalytic semiconductors. Albeit theoretical predictions in hematite suggesting a bulk n-type doping and a surface p-type doping would deliver best results, hematite co-doping with coupled cations possessing low and high oxidation states has shown promising results. Herein, we report, for the first time, Sb and Li co-doping of hematite photoanodes. Particularly, this is also a seminal work for the introduction of the highly reactive Sb⁵⁺ directly into the hematite thin films. Upon co-doping, we have a synergistic effect on the current densities with a 67-fold improvement over the standard. Via a combined investigation with profuse photoelectrochemical measurements, X-ray diffraction, X-ray photoelectron spectroscopy, and Raman analyses, we confirm the two doping roles of Sb⁵⁺ and Li⁺ as the substitutional and interstitial dopant, respectively. The improvements are attributed to a higher charge carrier concentration along with a lower charge transfer resistance at the surface.

Enhanced biological antimony removal from water by combining elemental sulfur autotrophic reduction and disproportionation

Antimony (Sb), a toxic metalloid, has serious negative effects on human health and its pollution has become a global environmental problem. Bio-reduction of Sb(V) is an effective Sb-removal approach. This work, for the first time, demonstrates the feasibility of autotrophic Sb(V) bio-reduction and removal coupled to anaerobic oxidation of elemental sulfur (S⁰). In the S⁰-based biological system, Sb(V) was reduced to Sb(III) via autotrophic bacteria by using S⁰ as electron donor. Meanwhile, S⁰ disproportionation reaction occurred under anaerobic condition, generating sulfide and SO₄^2- in the bio-systems. Subsequently, Sb(III) reacted with sulfide and formed Sb(III)-S precipitate, achieving an effective total Sb removal. The precipitate was identified as Sb₂S₃ by SEM-EDS, XPS, XRD and Raman spectrum analyses. In addition, it was found that co-existing nitrate inhibited the Sb removal, as nitrate is the favored electron acceptor over Sb(V). In contrast, the bio-reduction of co-existing SO₄^2- enhanced sulfide generation, followed by promoting Sb(V) reduction and precipitation. Illumina high-throughput sequencing analysis revealed that Metallibacterium, Citrobacter and Thiobacillus might be responsible for Sb(V) reduction and S⁰ disproportionation. This study provides a promising approach for the remediation of Sb(V)-contaminated water.

Advances in Engineered Metal Oxide Thin Films by Low-Cost, Solution-Based Techniques for Green Hydrogen Production

Functional oxide materials have become crucial in the continuous development of various fields, including those for energy applications. In this aspect, the synthesis of nanomaterials for low-cost green hydrogen production represents a huge challenge that needs to be overcome to move toward the next generation of efficient systems and devices. This perspective presents a critical assessment of hydrothermal and polymeric precursor methods as potential approaches to designing photoelectrodes for future industrial implementation. The main conditions that can affect the photoanode's physical and chemical characteristics, such as morphology, particle size, defects chemistry, dimensionality, and crystal orientation, and how they influence the photoelectrochemical performance are highlighted in this report. Strategies to tune and engineer photoelectrode and an outlook for developing efficient solar-to-hydrogen conversion using an inexpensive and stable material will also be addressed.

Boosting Hole Transfer in the Fluorine-Doped Hematite Photoanode by Depositing Ultrathin Amorphous FeOOH/CoOOH Cocatalysts

The charge transfer is a key issue in the development of efficient photoelectrodes. Here, we report a method using F-doping and dual-layer ultrathin amorphous FeOOH/CoOOH cocatalysts coupling to enable the inactive α-Fe₂O₃ photoanode to become highly vibrant for the oxygen evolution reaction (OER). Fluorine doping is revealed to increase the charge density and improve the conductivity of α-Fe₂O₃ for rapid charge transfer. Furthermore, ultrathin FeOOH was deposited on F-Fe₂O₃ to extract photogenerated holes and passivate the surface states for accelerated charge carrier transfer. Moreover, CoOOH as an excellent cocatalyst was coated onto FeOOH/F-Fe₂O₃ with the photoassisted electrodeposition method remarkably expediting OER kinetics through an optional pathway of holes utilized by Co species. Ultimately, the CoOOH/FeOOH/F-Fe₂O₃ photoanode exhibits a satisfactory photocurrent density (3.3-fold higher than pristine α-Fe₂O₃) and a negatively shifted onset potential of 80 mV. This work showcases an appealing maneuver to activate the water oxidation performance of the α-Fe₂O₃ photoanode by an integration strategy of heteroatom doping and cocatalyst coupling.

Effect of Sn-self diffusion via H2 treatment on low temperature activation of hematite photoanodes

A simple approach of lowering the activation temperature for hematite photoanode was developed. H₂ treatment and air quenching allows Sn⁴⁺ diffusion from FTO to hematite which exhibited the photocurrent density of 1.17 mA cm⁻² at 1.23 V vs. RHE.

Oxygen-Vacancy-Dominated Cocatalyst/Hematite Interface for Boosting Solar Water Splitting

Surface suppression is one of critical issues for semiconductors in photoelectrochemical (PEC) water splitting. Deposition of oxygen evolution cocatalysts on photoanodes can improve the oxygen evolution rate, but still it has some limits in some cases. In this work, we propose a new and simple precipitation approach to transform the surface of hematite into iron phosphate (Fe-Pi). Further, Ar-plasma treatment on Fe-Pi/Fe₂O₃ introduces oxygen vacancies on the phosphorous and photoanode. A surface phosphate treatment accelerates the transfer of holes from the bulk to the surface. Besides, creating oxygen vacancy defects on Fe-Pi/Fe₂O₃ can significantly increase the reactivity of active sites, leading to the remarkable enhancement in oxygen evolution reaction activity and PEC performance. The resulting photoanode has a current density of 2.71 mA cm^-2 at 1.23 V_RHE and 3.5 mA cm^-2 at 1.50 V_RHE under simulated solar light condition. The reduced surface recombination by Fe-Pi layer and Ar-plasma treatment is confirmed by electrochemical analysis. These findings give a great potential of the use of a combination strategy for cocatalyst deposition and optimizing the performance of hematite.

Fe2O3 Blocking Layer Produced by Cyclic Voltammetry Leads to Improved Photoelectrochemical Performance of Hematite Nanorods

Hematite is a low band gap, earth abundant semiconductor and it is considered to be a promising choice for photoelectrochemical water splitting. However, as a bulk material its efficiency is low because of excessive bulk, surface, and interface recombination. In the present work, we propose a strategy to prepare a hematite (α-Fe2O3) photoanode consisting of hematite nanorods grown onto an iron oxide blocking layer. This blocking layer is formed from a sputter deposited thin metallic iron film on fluorine doped tin oxide (FTO) by using cyclic voltammetry to fully convert the film into an anodic oxide. In a second step, hematite nanorods (NR) are grown onto the layer using a hydrothermal approach. In this geometry, the hematite sub-layer works as a barrier for electron back diffusion (a blocking layer). This suppresses recombination, and the maximum of the incident photon to current efficiency is increased from 12% to 17%. Under AM 1.5 conditions, the photocurrent density reaches approximately 1.2 mA/cm2 at 1.5 V vs. RHE and the onset potential changes to 0.8 V vs. RHE (using a Zn-Co co-catalyst).

A microstructured p-Si photocathode outcompetes Pt as a counter electrode to hematite in photoelectrochemical water splitting

Herein, we communicate about an Earth-abundant semiconductor photocathode (p-Si/TiO2/NiOx) as an alternative for the rare and expensive Pt as a counter electrode for overall photoelectrochemical water splitting. The proposed photoelectrochemical (PEC) water-splitting device mimics the "Z"-scheme observed in natural photosynthesis by combining two photoelectrodes in a parallel-illumination mode. A nearly 60% increase in the photocurrent density (Jph) for pristine α-Fe2O3 and a 77% increase in the applied bias photocurrent efficiency (ABPE) were achieved by replacing the conventionally used Pt cathode with an efficient, cost effective p-Si/TiO2/NiOx photocathode under parallel illumination. The resulting photocurrent density of 1.26 mA cm-2 at 1.23VRHE represents a new record performance for hydrothermally grown pristine α-Fe2O3 nanorod photoanodes in combination with a photocathode, which opens the prospect for further improvement by doping α-Fe2O3 or by its decoration with co-catalysts. Electrochemical impedance spectroscopy measurements suggest that this significant performance increase is due to the enhancement of the space-charge field in α-Fe2O3.

Bandgap control of α-Fe2O3 nanozymes and their superior visible light promoted peroxidase-like catalytic activity

Iron oxide nanoparticles (NPs) possessing peroxidase-like catalytic activity have been widely explored in recent decades, owing to their high stability against harsh conditions, low cost, flexibility in structure design and composition, adjustable activities and excellent biocompatibility in comparison with natural enzymes. Recently, a lot of great achievements have been made in this field of iron oxide nanozymes, however, this research has now reached a bottleneck in that the maximum activity enhancement is difficult to achieve via a material design. Hence, in this work, visible light was introduced to improve the peroxidase-like activity of Fe₂O₃ NPs synthesized via a combination of electrospinning technology and hydrothermal reaction. Our results showed that with the assistance of visible light, Fe₂O₃ NPs exhibited at least 2.2-fold higher peroxidase activity than those tested under darkness, confirming the superiorly visible light promoted peroxidase-like catalytic activity of Fe₂O₃ NPs. Furthermore, the affinity and maximum reaction velocity of Fe₂O₃ nanoflowers (bandgap = 1.78 eV) towards 3,3',5,5'-tetramethylbenanozymeidine (TMB) were at least over 3.7 and 4.3 times greater than in Fe₂O₃ nanocubes (bandgap = 2.08 eV), suggesting that the reaction performance of semiconductors could be controlled by proper adjustment of the bandgap. Moreover, the Fe₂O₃ NPs were also successfully utilized to detect glucose. Herein, we believe that the present work exhibits a fascinating perspective for peroxidase-like catalytic fields.

X-ray Photoelectron Spectroscopy of Fast-Frozen Hematite Colloids in Aqueous Solutions. 6. Sodium Halide (F-, Cl-, Br-, I-) Ion Binding on Microparticles

Electrolyte ion binding at mineral surfaces is central to the generation of surface charge and key to electric double-layer formation. X-ray photoelectron spectroscopy of fast-frozen (-170 °C) mineral wet pastes provides a means to study weakly bound electrolyte ions at the mineral/water interface. In this study, we build upon a series of articles devoted to ion binding at hematite (α-Fe₂O₃) particle surfaces to resolve the nature of sodium halide ion binding. Measurements on micron-sized hematite particles terminated by the charged and amphoteric (012) and the relatively uncharged (001) faces point to the formation of salt loadings of similar composition to those of cryosalts of NaCl, NaBr, NaI, and NaF. These coatings could be likened to those of the better-known hydrohalite (NaCl·2H₂O) phase, one that typically forms under concentrated (≫0.1 M) aqueous solutions of NaCl under freezing conditions. As we have previously shown that these reaction products do not occur in nanosized hematite particles, our work points to the involvement of the basal (001) face and/or the juxtaposition of these faces in packed tabular microparticles of hematite (1-3 μm in width) in stabilizing these cryosalts. One possible formation pathway involves first-layer Na⁺ and Cl^- ions serving as an anchoring layer for a topotactic-like growth of amorphous to low-crystalline salt hydrates at the (001) face. Thus, by contrasting reaction products of four sodium halides at surfaces of tabular microparticles of hematite, this work revealed the formation of cryosalt-like solids. The formation of such solids may have especially important ramifications to ice nucleation mechanisms in the atmosphere, as well as in saline permafrosts on Earth and on planet Mars where salt-laden mineral particles prevail.