Enhanced Bulk and Interfacial Charge Transfer Dynamics for Efficient Photoelectrochemical Water Splitting: The Case of Hematite Nanorod Arrays

Enhanced photoelectrochemical performance of NiS-modified TiO2 nanorods with a surface charge accumulation facet

Photoelectrochemical (PEC) water splitting for hydrogen production technology is considered as one of the most promising solutions to energy shortage and environmental remediation. TiO2/NiS nanorod arrays were successfully prepared using hydrothermal deposition followed by the successive ionic layer adsorption and reaction (SILAR) method. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and photoluminescence (PL) spectra characterization studies indicate the successful deposition of NiS on TiO2 NRs. The NiS deposition on TiO2 was optimized by controlling the impregnation cycle. The optimal sample exhibits a photocurrent density of 1.16 mA cm-2 at 0.6 V vs. Ag/AgCl, which is a 1.9-fold enhancement over that of pristine TiO2 nanorod arrays. The enhanced photoelectrochemical performance can be attributed to two aspects. On the one hand, the (101) crystal plane of rutile TiO2 is the facet where photogenerated holes accumulate and is an efficient active plane for the oxygen evolution reaction; on the other hand, NiS is a narrow band gap semiconductor, and its deposition on TiO2 nanorods can further promote the separation and transport of photogenerated charge carriers.

How titanium and iron are integrated into hematite to enhance the photoelectrochemical water oxidation: a review

Hematite has been considered as a promising photoanode candidate for photoelectrochemical (PEC) water oxidation and has attracted numerous interests in the past decades. However, intrinsic drawbacks drastically lower its photocatalytic activity. Ti-based modifications including Ti-doping, Fe₂O₃/Fe₂TiO₅ heterostructures, TiO₂ passivation layers, and Ti-containing underlayers have shown great potential in enhancing the PEC conversion efficiency of hematite. Moreover, the combination of Ti-based modifications with various strategies towards more efficient hematite photoanodes has been widely investigated. Nevertheless, a corresponding comprehensive overview, especially with the most recent working mechanisms, is still lacking, limiting further improvement. In this respect, by summarizing the recent progress in Ti-modified hematite photoanodes, this review aims to demonstrate how the integration of titanium and iron atoms into hematite influences the PEC properties by tuning the carrier behaviours. It will provide more cues for the rational design of high-performance hematite photoanodes towards future practical applications.

Scavenger-Supported Photocatalytic Evidence of an Extended Type I Electronic Structure of the TiO2@Fe2O3 Interface

Heterostructures of TiO₂@Fe₂O₃ with a specific electronic structure and morphology enable us to control the interfacial charge transport necessary for their efficient photocatalytic performance. In spite of the extensive research, there still remains a profound ambiguity as far as the band alignment at the interface of TiO₂@Fe₂O₃ is concerned. In this work, the extended type I heterojunction between anatase TiO₂ nanocrystals and α-Fe₂O₃ hematite nanograins is proposed. Experimental evidence supporting this conclusion is based on direct measurements such as optical spectroscopy, X-ray photoemission spectroscopy, scanning electron microscopy, high-resolution transmission electron microscopy (HRTEM), and the results of indirect studies of photocatalytic decomposition of rhodamine B (RhB) with selected scavengers of various active species of OH^•, h^•, e^-, and ^•O₂^-. The presence of small 6-8 nm Fe₂O₃ crystallites at the surface of TiO₂ has been confirmed in HRTEM images. Irregular 15-50 nm needle-like hematite grains could be observed in scanning electron micrographs. Substitutional incorporation of Fe³⁺ ions into the TiO₂ crystal lattice is predicted by a 0.16% decrease in lattice parameter a and a 0.08% change of c, as well as by a shift of the Raman E_g(1) peak from 143 cm^-1 in pure TiO₂ to 149 cm^-1 in Fe₂O₃-modified TiO₂. Analysis of O 1s XPS spectra corroborates this conclusion, indicating the formation of oxygen vacancies at the surface of titanium(IV) oxide. The presence of the Fe³⁺ impurity level in the forbidden band gap of TiO₂ is revealed by the 2.80 eV optical transition. The size effect is responsible for the absorption feature appearing at 2.48 eV. Increased photocatalytic activity within the visible range suggests that the electron transfer involves high energy levels of Fe₂O₃. Well-programed experiments with scavengers allow us to eliminate the less probable mechanisms of RhB photodecomposition and propose a band diagram of the TiO₂@Fe₂O₃ heterojunction.

Understanding the varying mechanisms between the conformal interlayer and overlayer in the silicon/hematite dual-absorber photoanode for solar water splitting

Dual-absorber photoelectrodes have been proved to have great potential in the photoelectrochemical (PEC) water splitting application due to their broadband absorption and suitable energy-band position, while the surface/interface issues are still not clearly resolved and understood. Here, during the preparation of a silicon/hematite dual-absorber photoanode achieved via synthesizing a Sn-doped hematite film on the silicon nanowire (SiNW) substrate, we separately introduced the conformal overlayer and interlayer of an Al₂O₃ thin film by atomic layer deposition. With the thickness-optimized interlayer (overlayer) of the Al₂O₃ thin film, the photocurrent density at 1.23V_RHE can be enhanced from 0.85 mA cm^-2 to 1.51 mA cm^-2 (1.25 mA cm^-2), and the on-set potential has a cathodic shift of ∼0.32 V. Although both the overlayer and interlayer modification can substantially improve the PEC performance, the underlying mechanisms are obviously different. The overlayer can only reduce the carrier recombination on the top surface and in the bulk of the hematite film; in contrast, the interlayer not only passivates the SiNW surface and bottom surface of the hematite film, but also the top surface of the photoanode due to Al³⁺ thermal diffusion from the bottom to the top surface of the hematite film and the resultant Al₂O₃ formation. This work deepens our understanding for the roles of the surface and interface engineering in the achievement of high-performance PEC systems based on dual or more absorbers.

Multistep Surface Trap State Finishing Based on in Situ One-Step MOF Modification over Hematite for Dramatically Enhanced Solar Water Oxidation

Complex surface dynamics is the key to limit the photoelectrochemical performance of hematite, while its core content is the hole trapping and release by surface traps. Deep traps are accompanied by extremely fast capture rates and extremely slow release rates, which severely suppress the hole transport process. Herein, we proposed a unique method to progressively convert deep traps on the hematite surface for fast hole transfer via in situ one-step metal organic framework modification. This stepwise deep-trap passivation is achieved by hematite corrosion first on the surface and subsequent construction of a porous titanium layer. The gentle trap finishing helps prevent surface losses caused by excessively intense trap passivation. The hematite corrosion can initially passivate 80% of the surface deep traps, while the subsequent porous titanium layer can completely passivate the deep traps. In addition, the accurate optimization of the porous titanium layer can reconstruct the benign shallow traps on the surface, acting as superior oxygen evolution reaction active sites. This sophisticated surface-trap adjustment is accompanied by the rapid reduction of deep traps and the gradual increase of shallow traps, obtaining a superior surface state that is conducive to charge transport and interface catalysis. The obtained treated hematite yields a photocurrent density of 3.08 mA·cm^-2 at 1.23 V_RHE, increased by 570% compared to the pristine hematite.

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.

Designing efficient Bi2Fe4O9 photoanodes via bulk and surface defect engineering

An efficient Bi₂Fe₄O₉ photoanode can be fabricated through surface and bulk defect engineering to achieve a state-of-the-art photoresponse.

On the Theoretical and Experimental Control of Defect Chemistry and Electrical and Photoelectrochemical Properties of Hematite Nanostructures

Hematite (α-Fe₂O₃) is regarded as one of the most promising cost-effective and stable anode materials in photoelectrochemical applications, and its performance, like other transition-metal oxides, depends strongly on its electrical and defect properties. In this work, the electrical and thermomechanical properties of undoped and Sn-doped α-Fe₂O₃ nanoscale powders were characterized in situ at controlled temperatures ( T = 250 to 400 °C) and atmospheres ( pO₂ = 10^-4 to 1 atm O₂) to investigate their transport and defect properties. Frequency-dependent complex impedance spectra show that interfacial resistance between particles is negligible in comparison with particle resistance. Detailed defect models predicting the dependence of electron, hole, and iron and oxygen vacancy concentrations on temperature and oxygen partial pressures for undoped and doped α-Fe₂O₃ were derived. Using these defect equilibria models, the operative defect regimes were established, and the bandgap energy of undoped α-Fe₂O₃ and oxidation enthalpy of Sn-doped α-Fe₂O₃ were obtained from the analysis of the temperature and pO₂ dependence of the electrical conductivity. On the basis of these results, we are able to explain the surprisingly weak impact of donor doping on the electrical conductivity of α-Fe₂O₃. Furthermore, experimental means based on the results of this study are given for successfully tuning hematite to enhance its photocatalytic activity for the water oxidation reaction.

Facile Synthesis of Ultrafine Hematite Nanowire Arrays in Mixed Water-Ethanol-Acetic Acid Solution for Enhanced Charge Transport and Separation

Nanostructure engineering is of great significance for semiconductor electrode to achieve high photoelectrochemical performance. Herein, we report a novel strategy to fabricate ultrafine hematite (α-Fe₂O₃) nanowire arrays in a mixed water-ethanol-acetic acid (WEA) solvent. To the best of our knowledge, this is the first report on direct growth of ultrafine (∼10 nm) α-Fe₂O₃ nanowire arrays on fluorine-doped tin oxide substrates through solution-based fabrication process. The effect of WEA ratio on the morphology of nanowires has been systematically studied to understand the formation mechanism. Photoelectrochemical measurements were conducted on both Ti-treated α-Fe₂O₃ nanowire and nanorod photoelectrodes. It reveals that α-Fe₂O₃ nanowire electrode has higher photocurrent and charge separation efficiencies than nanorod electrode if the carrier concentration and space-charge carrier width are in the same order of magnitude. Normalized by electrochemically active surface area, the Ti-treated α-Fe₂O₃ nanowire electrode obtains 6.4 times higher specific photocurrent density than nanorod electrode. This superiority of nanowires arises from the higher bulk and surface charge separation efficiencies, which could be partly attributed to reduced distance that holes must transfer to reach the semiconductor-liquid junction.

Insights into the enhanced photoelectrochemical performance of hydrothermally controlled hematite nanostructures for proficient solar water oxidation

A controlled hydrothermal reaction time showed an improvement in the PEC performance of 1D α-Fe₂O₃ nanorods due to an optimum aspect ratio and Sn⁴⁺ diffusion.

Low temperature fabrication of Fe2O3 nanorod film coated with ultra-thin g-C3N4 for a direct z-scheme exerting photocatalytic activities

We engineered high aspect ratio Fe₂O₃ nanorods (with an aspect ratio of 17 : 1) coated with g-C₃N₄ using a sequential solvothermal method at very low temperature followed by a thermal evaporation method. Here, the high aspect ratio Fe₂O₃ nanorods were directly grown onto the FTO substrate under relatively low pressure conditions. The g-C₃N₄ was coated onto a uniform Fe₂O₃ nanorod film as the heterostructure, exhibiting rational band conduction and a valence band that engaged in surface photoredox reactions by a direct z-scheme mechanism. The heterostructures, particularly 0.75g-C₃N₄@Fe₂O₃ nanorods, exhibited outstanding photocatalytic activities compared to those of bare Fe₂O₃ nanorods. In terms of 4-nitrophenol degradation, 0.75g-C₃N₄@Fe₂O₃ nanorods degraded all of the organic pollutant within 6 h under visible irradiation at a kinetic constant of 12.71 × 10⁻³ min⁻¹, about 15-fold more rapidly than bare Fe₂O₃. Further, the hydrogen evolution rate was 37.06 μmol h⁻¹ g⁻¹, 39-fold higher than that of bare Fe₂O₃. We suggest that electron and hole pairs are efficiently separated in g-C₃N₄@Fe₂O₃ nanorods, thus accelerating surface photoreaction via a direct z-scheme under visible illumination.

The engineered high aspect ratio of Fe₂O₃ nanorods coated with g-C₃N₄ demonstrates z-scheme mechanism, showing the best performance in 4-nitrophenol photodegradation and H₂ evolution.

Surface electron dynamics in hematite (α-Fe2O3): correlation between ultrafast surface electron trapping and small polaron formation

Spectroscopically following charge carrier dynamics in catalytic materials has proven to be a difficult task due to the ultrafast timescales involved in charge trapping and the lack of spectroscopic tools available to selectively probe surface electronic structure. Transient extreme ultraviolet reflection-absorption (XUV-RA) spectroscopy is able to follow surface electron dynamics due to its element, oxidation-state, and surface specificity, as well as the ultrafast time-resolution which can be achieved with XUV pulses produced by high harmonic generation. Here, we use ultrafast XUV-RA spectroscopy to show that charge localization and small polaron formation in Fe₂O₃ occur within ∼660 fs. The photoexcitation of hematite at 400 nm initially leads to an electronically-delocalized ligand-to-metal charge transfer (LMCT) state, which subsequently evolves into a surface localized LMCT state. Comparison of the charge carrier dynamics for single and polycrystalline samples shows that the observed dynamics are negligibly influenced by grain boundaries and surface defects. Rather, correlation between experimental results and spectral simulations reveals that the lattice expansion during small polaron formation occurs on the identical time scale as surface trapping and represents the probable driving force for sub-ps electron localization to the hematite surface.

High temperature activation of hematite nanorods for sunlight driven water oxidation reaction

Here we show that chlorine species originating from commonly used iron precursors annihilate the hematite nanorod photocurrent by providing recombination pathways. Although hematite nanorod films could be obtained by thermal decomposition of the iron oxyhydroxide phase (β-FeOOH), indistinguishable photocurrent responses under dark and sunlight irradiation conditions were observed until the nanorods were annealed (activated) at 750 °C. The annealing led to the elimination of observable chlorine species and allowed photocurrent responses of 1.3 mA cm^-2 at 1.23 V vs. RHE, which is comparable to the best results found in the literature, suggesting that residual chlorine species from the synthesis can act as electron traps and recombination sites for photogenerated holes.

A Facile Electrochemical Reduction Method for Improving Photocatalytic Performance of α-Fe2O3 Photoanode for Solar Water Splitting

Electrochemical reduction method is used for the first time to significantly improve the photo-electrochemical performance of α-Fe₂O₃ photoanode prepared on fluorine-doped tin oxide substrates by spin-coating aqueous solution of Fe(NO₃)₃ followed by thermal annealing in air. Photocurrent density of α-Fe₂O₃ thin film photoanode can be enhanced 25 times by partially reducing the oxide film to form more conductive Fe₃O₄ (magnetite). Fe₃O₄ helps facilitate efficient charge transport and collection from the top α-Fe₂O₃ layer upon light absorption and charge separation to yield enhanced photocurrent density. The optimal enhancement can be obtained for <50 nm films because of the short charge transport distance for the α-Fe₂O₃ layer. Thick α-Fe₂O₃ films require more charge and overpotential than thinner films to achieve limited enhancement because of the sluggish charge transport over a longer distance to oxidize water. Electrochemical reduction of α-Fe₂O₃ in unbuffered pH-neutral solution yields much higher but unstable photocurrent enhancement because of the increase in local pH value accompanied by proton reduction at a hematite surface.

On the role of metal atom doping in hematite for improved photoelectrochemical properties: a comparison study

A systematic comparison study of various metal atoms doped hematites was performed to look into the effect of metal element doping on the photoelectrochemical (PEC) properties of hematite photoelectrodes.