Hematite Photoanodes Research Articles

Tuning the surface states by in-situ Zr/Hf co-doping and MoO3 hole transport layer modification for boosting photoelectrochemical performance of hematite photoanode

Hematite (α-Fe2O3) is a potential photoanode material for photoelectrochemical (PEC) water splitting; nevertheless, its PEC performance is constrained by substantial bulk and surface charge recombination rates. Herein, the effect of in-situ Zr/Hf dual-ion doping and the MoO3 hole transport layer (HTL) on regulating the surface states of α-Fe2O3 photoanodes for effective water oxidation is investigated. The co-doping improves bulk properties by enhancing the electrical conductivity, thereby facilitating hole transfer via intermediate surface states (i-SS). Furthermore, the MoO3 HTL passivates the recombination surface states (r-SS), thus alleviating the Fermi level pinning, resulting in an improved open-circuit photovoltage. As a result, a novel Zr/Hf-HT:MoO3 photoanode attains a photocurrent density of 2.34 mA cm−2 at 1.23 V vs. RHE (1.23 VRHE), which is 123% higher than that of Bare-HT. The Zr/Hf-HT:MoO3 photoanode achieves 86 and 61.3% of surface charge separation and charge transfer efficiencies at 1.23 VRHE, respectively, representing 26 and 100% enhancements over those of Bare-HT. Lastly, the FeNi(OH)x cocatalyst coated Zr/Hf-HT:MoO3 photoanode reaches a photocurrent density of 2.62 mA cm−2 at 1.23 VRHE with 98% stability and generates 47.8 and 22 μmol h−1 of H2 and O2 gases, respectively.

Ti doped hematite photoanodes: Protective coverage by titania overlayers

In this work, the photoresponse of Ti doped hematite electrodes was optimized by adjusting the thickness and the level of doping. The electrodes were then covered by a TiO2 overlayer by dip coating (DC) or by atomic layer deposition (ALD) to improve chemical stability. Coverage of hematite by a dip-coated sol–gel TiO2 film of thickness up to 135 nm resulted only in a small decrease in photocurrent (by about 30 %). A similar decrease in photocurrent (about 25 %) was observed after the coverage by a very thin (2 nm) ALD TiO2 film while the increase of the thickness to 8 nm led to almost complete suppression of photocurrent. This behaviour can be explained by the different morphology and structure of the overlayers. Although the surface morphology of sol–gel TiO2 films was very smooth and dense, penetration of such films by the electrolyte occurred even for a thickness of 135 nm. On the other hand, much thinner ALD TiO2 films (8 nm) exhibited almost no electrolyte penetration. Such different blocking properties of the two TiO2 overlayers are in direct agreement with the observed dissolution rate in acidic media – the better the blocking properties of an overlayer the better the chemical stability of stratified hematite/titania photoelectrode.

Ru-P pair sites boost charge transport in hematite photoanodes for exceeding 1% efficient solar water splitting

Fast transport of charge carriers in semiconductor photoelectrodes are a major determinant of the solar-to-hydrogen efficiency for photoelectrochemical (PEC) water slitting. While doping metal ions as single atoms/clusters in photoelectrodes has been popularly used to regulate their charge transport, PEC performances are often low due to the limited charge mobility and severe charge recombination. Here, we disperse Ru and P diatomic sites onto hematite (DASs Ru-P:Fe2O3) to construct an efficient photoelectrode inspired by the concept of correlated single-atom engineering. The resultant photoanode shows superior photocurrent densities of 4.55 and 6.5 mA cm-2 at 1.23 and 1.50 VRHE, a low-onset potential of 0.58 VRHE, and a high applied bias photon-to-current conversion efficiency of 1.00% under one sun illumination, which are much better than the pristine Fe2O3. A detailed dynamic analysis reveals that a remarkable synergetic ineraction of the reduced recombination by a low Ru doping concentration with substitution of Fe site as well as the construction of Ru-P bonds in the material increases the carrier separation and fast charge transportation dynamics. A systematic simulation study further proves the superiority of the Ru-P bonds compared to the Ru-O bonds, which allows more long-lived carriers to participate in the water oxidation reaction. This work offers an effective strategy for enhancing charge carrier transportation dynamics by constructing pair sites into semiconductors, which may be extended to other photoelectrodes for solar water splitting.

Photoelectrochemical Valorization of Biomass Derivatives with Hematite Photoanodes Modified by Cocatalysts

The solar‐driven oxidation of biomass to valuable chemicals is rising as a promising anodic reaction in photoelectrochemical cells, replacing the sluggish oxygen evolution reaction and improving the added value of the energy conversion process. Herein, the photooxidation of 5‐hydroxymethylfurfural into furan dicarboxylic acid (FDCA) is performed in basic aqueous environment (borate buffer, pH 9.2), with the addition of 2,2,6,6‐tetramethylpiperidine‐1‐oxyl (TEMPO) as redox mediator. Because of its good stability, cost‐effectiveness, and nontoxicity, titanium‐modified hematite (Ti:Fe2O3) photoanodes are investigated to this aim, and their performance is tuned by engineering the semiconductor surface with a thin layer of Co‐based cocatalysts, i.e., cobalt iron oxide (CoFeO x ) and cobalt phosphate (CoPi). Interestingly, the electrode modified with CoPi shows improved efficiency and selectivity toward the final product FDCA The source of this enhancement is correlated to the effect of the cocatalyst on the charge carrier dynamics, which is investigated by electrochemical impedance spectroscopy and intensity‐modulated photocurrent spectroscopy analysis. In addition, the results of the latter are interpreted through a novel approach called Lasso distribution of relaxation time, revealing that CoPi cocatalyst is effective in the suppression of the recombination processes and in the enhancement of direct hole transfer to TEMPO.

Niobium‐doped Hematite Photoanodes Prepared through Low‐Cost Facile Methods for Photoelectrochemical Water Splitting

AbstractProducing solar fuels through semiconductor‐based photocatalytic water splitting is one of the main options for worldwide employment of non‐fossil energy sources. Nonetheless, the relatively low efficiency of the known photocatalysts, the high prices of composing elements, and the costly preparation methods render expensive devices that would not be economically competitive with fossil energy sources. Herein, we present a facile method for the preparation of both round hematite (Fe2O3) nanoparticles, with average diameter of (56±7) nm, and of niobium‐doped hematite nanoparticles with similar size and morphology. Thin films of these nanoparticles were assembled through electrophoretic deposition, resulting in homogeneous photoanodes with nanometer thickness that can split water, under simulated solar irradiation, to produce oxygen in a photoelectrochemical cell. The optimization of the film composition and thickness resulted in the improvement of the photocurrent from 352.3 to 1,342.5 μA cm−2 at 1.23 V vs RHE for the pristine hematite and Nb‐doped hematite, respectively, both with thickness of (499±63) nm. This enhancement in the oxidation reaction yield is a clear consequence of the improvement of charge carriers transport along the nanoparticle structure due to niobium doping and adequate light absorption/charge transfer balance of the photoelectrode under operation.

Surface Self-Transforming FeTi-LDH Overlayer in Fe2 O3 /Fe2 TiO5 Photoanode for Improved Water Oxidation.

Integrating hematite nanostructures with efficient layer double hydroxides (LDHs) is highly desirable to improve the photoelectrochemical (PEC) water oxidation performance. Here, an innovative and facile strategy is developed to fabricate the FeTi-LDH overlayer decorated Fe2 O3 /Fe2 TiO5 photoanode via a surface self-transformation induced by the co-treatment of hydrazine and NaOH at room temperature. Electrochemical measurements find that this favorable structure can not only facilitate the charge transfer/separation at the electrode/electrolyte interface but also accelerate the surface water oxidation kinetics. Consequently, the as-obtained Fe2 O3 /Fe2 TiO5 /LDH photoanode exhibits a remarkably increased photocurrent density of 3.54mAcm-2 at 1.23V versus reversible hydrogen electrode (RHE) accompanied by an obvious cathodic shift (≈140mV) in the onset potential. This work opens up a new and effective pathway for the design of high-performance hematite photoanodes toward efficient PEC water oxidation.

Roles of Cobalt-Coordinated Polymeric Perylene Diimide in Hematite Photoanodes for Improved Water Oxidation.

Interfacial charge recombination is a permanent issue that impedes the photon energy utilization in photoelectrochemical (PEC) water splitting. Herein, a conjugated polymer, urea linked perylene diimide polymer (PDI), is introduced to the designation of hematite-based composite photoanodes. On account of its unique molecule structure with abundant electronegative atoms, the O and N atoms with lone electron pairs can bond with Fe atoms at the surface of Zr4+ doped α-Fe2 O3 (Zr:Fe2 O3 ) and thus establish charge transfer channels for expediting hole separation and migration. Meanwhile, PDI molecules can passivate the surface states in Zr:Fe2 O3 , which is in favor of suppressing carrier recombination. Particularly, Co2+ is used to coordinate with PDI (Co-PDI) to accelerate hole extraction as well as utilization, and the as-obtained Co-PDI form type-II heterojunction with Zr:Fe2 O3 . Such a photoanode configuration takes advantage of the unique molecule structure of PDI, and the target Co-PDI/Zr:Fe2 O3 photoanodes eventually attain a photocurrent density of 2.17mAcm-2 , which is inspirational for unearthing the potential use of conjugative molecules in PEC fields.

Enhancement of the Solar Water Splitting Efficiency Mediated by Surface Segregation in Ti-Doped Hematite Nanorods.

Band engineering is employed thoroughly and targets technologically scalable photoanodes for solar water splitting applications. Complex and costly recipes are necessary, often for average performances. Here, we report simple photoanode growth and thermal annealing with effective band engineering results. By comparing Ti-doped hematite photoanodes annealed under nitrogen to photoanodes annealed in air, we found a strongly enhanced photocurrent of more than 200% in the first case. Using electrochemical impedance spectroscopy and synchrotron X-ray spectromicroscopy, we demonstrate that oxidized surface states and increased density of charge carriers are responsible for the enhanced photoelectrochemical (PEC) activity. Surface states are found to be related to the formation of pseudo-brookite clusters by surface Ti segregation. Spectro-ptychography is used for the first time at the Ti L3 absorption edge to isolate Ti chemical coordination arising from pseudo-brookite cluster contribution. Correlated with electron microscopy investigation and density functional theory calculations, the synchrotron spectromicroscopy data unambiguously prove the origin of enhanced PEC activity of N2-annealed Ti-doped hematite nanorods. Finally, we present here a handy and cheap surface engineering method beyond the known oxygen vacancy doping, allowing a net gain in the PEC activity for the hematite-based photoanodes.

A highly efficient hematite photoelectrochemical fuel cell for solar-driven hydrogen production

Hematite has demonstrated to be a promising photoanode for photoelectrochemical (PEC) hydrogen production because of its narrow band gap and outstanding stability. However, the sluggish water oxidation kinetics on hematite photoanode severely hinder the PEC hydrogen production performance. Herein, we report the construction of a Ti-doped porous hematite-based PEC fuel cell utilizing l-cysteine (L-Cys) oxidation to replace difficult water oxidation for efficient H2 production. The strong complexation interaction between hematite and L-Cys confers the direct photooxidation of L-Cys, producing additional electrons to participate in hydrogen production. As a result, the present Ti-doped porous hematite (Ti-PH) photoanode exhibits a current density of up to 4.45 mA cm−2 at 1.23 V vs. RHE under AM 1.5G illumination. This work provides a new strategy to utilize the additional electron contribution from photoinduced oxidation of organic chemicals for high-efficiency PEC fuel cell and solar fuel production.

Utilizing a Siloxane-Modified Organic Semiconductor for Photoelectrochemical Water Splitting

We explore the potential of employing diketopyrrolopyrrole (DPP) based π-conjugated OSs as a hole transport layer material in heteroatom-doped hematite (Ti–Fe2O3/Ge–Fe2O3) photoanodes for efficient photoelectrochemical water splitting. The siloxane-modified π-conjugated polymer (PSi) with a high carrier mobility and crystallinity revealed great potential to extract holes by forming a built-in potential with hematite photoanodes while showing high stability in an alkaline electrolyte for photoelectrochemical water oxidation. Because of the easy hole extraction and subsequent fast hole transport property of the PSi interlayer between NiFe(OH)x and Ge-doped porous Fe2O3(Ge-PH), NiFe(OH)x/PSi/Ge-PH showed a 1.8-fold increase in photocurrent density (4.57 mA cm–2 at 1.23 VRHE) with a cathodic shift of the onset potential (0.735 VRHE) and good stability for 65 h compared to Ge-PH. This study demonstrates the successful use of inherently unstable π-conjugated OSs as a hole extracting/transport medium in a photoanode, addressing the intrinsic recombination issues of hematite for efficient and stable water splitting.

In Situ Construction of Ta:Fe2O3@CaFe2O4 Core–Shell Nanorod p–t–n Heterojunction Photoanodes for Efficient and Robust Solar Water Oxidation

In order to ameliorate the poor charge transfer characteristics of hematite (α-Fe2O3) photoanodes for photoelectrochemical (PEC) water splitting, heterojunction formation with p-CaFe2O4 is attempted. Here, we report the in situ construction of a highly crystalline p-CaFe2O4 shell on the surface of n-Ta:Fe2O3 nanorods to form Ta:Fe2O3@CaFe2O4 core–shell nanorod p–t–n heterojunction photoanodes with a transition layer (t) between them by a combined strategy of hybrid microwave annealing (HMA) and in situ Ta doping. The successful fabrication of the elaborate heterostructure is due to effective crystallization of p-CaFe2O4 by HMA and prevention of Ca diffusion by already doped Ta atoms in hematite. The optimized Ta:Fe2O3@CaFe2O4 photoanode loaded with the FeNiOx cocatalyst achieves a photocurrent density of 2.70 mA cm–2, a low onset potential of 0.63 VRHE, and long-time stability in PEC water oxidation at 1.23 VRHE under 100 mW cm–2 solar irradiation, which represent marked improvements over bare hematite photoanodes and already reported hematite-based heterojunction photoanodes.

Integration of cobalt-phosphate catalyst and titanium dioxide interlayer in the hematite photoanodes to improve photoelectrochemical water splitting for hydrogen production

Integration of cobalt-phosphate catalyst and titanium dioxide interlayer in the hematite photoanodes to improve photoelectrochemical water splitting for hydrogen production

Scanning transmission X-ray spectromicroscopy: A nanotool to probe hematite nanorods for solar water splitting

We report a scanning transmission X-ray microscopy (STXM) study of hematite nanorods, prototypical photoanode used in solar water splitting. Hematite nanorods were obtained by hydrothermal growth from aqueous solutions using FeCl3 as precursor. Potentials for onset of water splitting are smaller using this synthesis method, compared to values reported for hematite photoanodes obtained by epitaxial growth. STXM revealed the presence of a hexahydrate iron chloride phase at the surface of the nanorods, which is linked to the low onset potential values. We detail the quantification approach that revealed the specific microstructure of individual hematite nanorods.

Ti-doped Fe2O3 photoanodes on three-dimensional titanium microfiber felt substrate for photoelectrochemical oxygen evolution reaction

The macroporous structure of conductive substrates with large surface areas is crucial for fabricating semiconductor electrodes that offer better photoelectrochemical (PEC) performance in the water-splitting reaction. In this study, three-dimensional (3D) titanium microfiber felt (Ti felt) was used as a conductive substrate to prepare a Ti-doped Fe2O3 photoanode via a hydrothermal method with a two-step annealing treatment in air and argon atmospheres. Compared to conventional two-dimensional (2D) substrates, such as fluorine-doped tin oxide (FTO)-coated glass and Ti sheets, the loading amount of Fe2O3 on Ti felt was 2–5 times higher for a similar substrate geometric area. The thickness of Fe2O3 on the Ti felt (∼0.6 µm) was less than that on the 2D substrates. Ti-doped Fe2O3 on Ti felt (Ti-felt/Ti-Fe2O3) exhibits a higher photocurrent density than FTO/Ti-Fe2O3 and Ti-sheet/Ti-Fe2O3. Mott–Schottky analysis shows that the donor density of Ti-felt/Ti-Fe2O3 (8.58 × 1021 cm−3) was more than 10 times higher than that of FTO/Ti-Fe2O3 and Ti-sheet/Ti-Fe2O3. Analyses of the incident photon-to-current conversion efficiency (IPCE) and Faradaic efficiency indicated that Ti-felt/Ti-Fe2O3 showed higher PEC activity for the oxygen evolution reaction (OER) than FTO/Ti-Fe2O3. The enhanced PEC activity is ascribed to the higher donor density and moderate thickness of the Ti-doped Fe2O3 layer, which contribute to a short electron transport distance. Furthermore, the high loading of Fe2O3 on the Ti-felt substrate increased the photoabsorption in the UV and visible light regions and the photocurrent response at wavelengths near the band edge, even at low light intensities. This study provides new insights for preparing hematite photoanodes with higher PEC activity for the OER using a 3D porous substrate, rather than conventional 2D substrates.

PEC thrombin aptasensor based on Ag–Ag2S decorated hematite photoanode with signal-down effect of precipitation catalyzed by G-quadruplexes/hemin

A photoelectrochemical (PEC) aptasensor for thrombin detection was rationally designed based on the photoanode of one-dimensional hematite nanorods (α-Fe2O3 NRs) with several steps of modifications. Uniform α-Fe2O3 NRs were grown vertically on the surface of fluorine-doped tin oxide (FTO) conductive glass through a one-step hydrothermal method; then Ag was grown on the surface of α-Fe2O3 NRs through a photoreduction method followed by a partial in-situ transformation into Ag2S, conferring an improvement on the initial photocurrent. Two main critical factors, namely, the steric hindrance of thrombin, benzoquinone (BQ) precipitation oxidized by H2O2 under the catalysis of G-quadruplexes/hemin, contributed to the sensitive signal-down response toward the target. Photocurrent signals related with thrombin concentration was established for thrombin analysis due to the non-conductive complex as well as their competitive consumption of electron donors and irradiation light. The excellent initial photocurrent was combined with the signal-down amplification in the design of the biosensor, conferring a limit of detection (LOD) as low as 40.2 fM and a wide linear range from 0.0001 nM to 50 nM for the detection of thrombin. The proposed biosensor was also assessed in terms of selectivity, stability, and applicability in human serum analyses, which provided an appealing maneuver for the specific analysis of thrombin in trace amount.

Unveiling the Synchronized Effect of Bulk and Surface Dual Modification of In Situ Nb-Doping and Microwave-Assisted Co(OH)x Cocatalyst for Boosting Photoelectrochemical Water Splitting of Fe2O3 Photoanodes

Herein, in order to improve the conductivity and slack water oxidation kinetics of the hematite (α-Fe2O3) photoanode, we propose the Nb-doped and Co(OH)x cocatalyst-deposited α-Fe2O3 thin film photoanode (Nb-HT/Co(OH)x) via in situ diluted hydrothermal and microwave-assisted methods. The as-prepared Nb-HT/Co(OH)x thin-film photoanode exhibited a photocurrent density of 1.78 mA cm–2 at 1.23 V versus a reversible hydrogen electrode (RHE), which is 1.7-fold higher than that of the Bare-Fe2O3 photoanode. The dual effect of Nb-doping and Co(OH)x deposition markedly improved the PEC performance through enhancing the charge carrier mobility and donor density in hematite, as well as accelerating the interfacial charge transfer kinetics at the electrode/electrolyte interface of the α-Fe2O3 thin film photoanode. The Nb-HT/Co(OH)x photoanode displayed a high charge separation efficiency of 90% (at 1.23 VRHE) and excellent stability over a 10 h period without any decrease. Detailed electrochemical analyses using electrochemical impedance spectroscopy (EIS), open-circuit potential (OCP), and accumulated charge density techniques disclosed the charge separation and transfer processes. This strategy of bulk and surface modification highlights a new approach to constructing a stable photoanode for sustainable solar energy conversion.

Hetero phase modulated hematite photoanodes for practical solar water splitting

Hematite is an excellent catalyst for solar water splitting but its practical performance is still low. In this work, a hetero phase of β-Fe2O3 was firstly introduced in α-Fe2O3 to optimize the internal structure, which can synergistically work with oxygen vacancy to enhance the charge extraction, facilitate the charge transport, and suppress the electron-hole recombination. As a result, the obtained hematite exhibits a high initial photocurrent of 2.72 mA/cm2 at 1.23 V vs. RHE, and can be well coupled with various further modifications such as P, Hf, and FeNiOOH to achieve an excellent photocurrent of 5.24 mA/cm2 at 1.23 V vs. RHE with a good stability over 100 h. Moreover, it can realize a benchmark solar-to-hydrogen conversion efficiency of 5.3% when connected to a commercial Si solar cell. The low cost, efficient, and stable hematite-based photoanodes may pave the way for practical solar water splitting.

Microwave-assisted sequential Pt/Al attachment on FeOOH for fabrication of highly efficient hematite photoanodes: Synergistic effect of Pt/Al co-doping and Al2O3 passivation layer

This article presents the fabrication and systematic analyses of Pt and Al co-doped hematite photoanodes utilizing a simple microwave process to improve the photoelectrochemical (PEC) performance of hematite. The Pt/Al dual ions doping synergistically elevated the photocurrent of hematite by 48% at 1.23 V versus (vs.) the reversible hydrogen electrode (RHE) (1.55 mA/cm2), and resulted in an obvious decrease in the onset potential (compared to that of the solely Pt-doped hematite), demonstrating that Al contributes as both a bulk dopant and a surface passivation layer for water oxidation. The enhanced photoelectrochemical response is a result of the synergy of Pt/Al co-doping, which increases the donor density and decreases the electron transport time while the unintended formation of Al2O3 improves the surface charge transfer kinetics. The optimized Pt(1.2%)/Al(2.5 mM) co-doped hematite (Pt/Al-HT) photoanode achieved the highest incident photon-to-current efficiency (IPCE) of 33% at 350 nm at 1.23 Vvs. RHE. Further, we explored in detail the mechanism of microwave-assisted attachment of each dopant to FeOOH and charge transfer in Pt/Al-HT photoanodes. This highlights the potential of microwave-assisted dual ions doping to advance hematite PEC performance for solar water splitting.

Dual-suppression of bulk and surface charges recombination for hematite photoanode

Surface treatments are considered as one of the most effective strategies to overcome charges recombination of photoanodes. Herein, the E–Ti–Fe2O3 photoanode was prepared by hydrothermal, electrodepositon and subsequent KOH treatment methods on the surface of Ti–Fe2O3 photoanode. The photocurrent density of E–Ti–Fe2O3 reached 3.1 mA/cm2 at 1.23 V versus reversible hydrogen electrode (vs. RHE), which was about 6.5 times than that of Ti–Fe2O3 (0.48 mA/cm2 at 1.23 V vs. RHE). The photocurrent density of E–Ti–Fe2O3 was further improved by the modification of CoPi (4.1 mA/cm2 at 1.23 V vs. RHE). The excellent PEC activity can be attributed to the dual-suppression of bulk and surface charges recombination, which was proved by work function (WF) measurements, transient photovoltage (TPV) measurements and photoelectrochemistry (PEC) tests. This research provided a deep insight into the bulk and surface charge recombination of the photogenerated charges for photoanode.

Photoelectrochemical water oxidation and methylene blue degradation enhanced by Nb doping and CoPi modification for hematite photoanodes

Nanostructured α-Fe2O3 films doped with niobium and supported with CoPi cocatalyst (Nb-α-Fe2O3 @CoPi) are prepared by hydrothermal growth together with electrochemical deposition. The effect of Nb doping and CoPi cocatalyst modification on the performance of photoelectrochemical water oxidation and methylene blue degradation of α-Fe2O3 is studied in detail. It shows that Nb doping and CoPi modification toward α-Fe2O3 not only enhances its light absorption, but also improves its electrical properties, including conductivity, bulk charge separation efficiency and charge transfer efficiency at the electrode/electrolyte interface. When used as photoanode for photoelectrochemical water oxidation, the Nb-α-Fe2O3 @CoPi photoanodes show a negative shift of 540 mV in the onset potential in comparison with the pristine α-Fe2O3 films. At the same time, the photocurrent density (at 1.6 V, vs. RHE) of Nb-α-Fe2O3@CoPi is more than three times that of the pristine α-Fe2O3, reaching 1.44 mA·cm−2. In addition, the obtained Nb-α-Fe2O3 @CoPi photoanodes also show excellent performance in visible light-driven photoelectrochemical degradation of methylene blue, a common pollutant in wastewater. For example, a degradation rate of 98% is achieved in 2 h by the Nb-α-Fe2O3 @CoPi, which is 3.6 times that of the pristine α-Fe2O3 (26%).