Insight into the influence of high temperature annealing on the onset potential of Ti-doped hematite photoanodes for solar water splitting

The unfavorable onset potential shift of Ti:Fe₂O₃ photoanode induced by high temperature post-annealing was studied. The underlying mechanism is the surface Ti/Fe atomic ratio increase, leading to adsorption capacity change of surface hydroxide ions and the positive shift of flatband potential (i.e., theoretical onset potential).     

The Role of Surface States in the Oxygen Evolution Reaction on Hematite

Hematite (α‐Fe₂O₃) is an extensively investigated semiconductor for photoelectrochemical (PEC) water splitting. The nature and role of surface states on the oxygen evolution reaction (OER) remain however elusive. First‐principles calculations were used to investigate surface states on hematite under photoelectrochemical conditions. The density of states for two relevant hematite terminations was calculated, and in both cases the presence and the role of surface states was rationalized. Calculations also predicted a Nerstian dependence on the OER onset potential on pH, which was to a very good extent confirmed by PEC measurements on hematite model photoanodes. Impedance spectroscopy characterization confirmed that the OER takes place via the same surface states irrespective of pH. These results provide a framework for a deeper understanding of the OER when it takes place via surface states.     

提高氧化铁光电催化分解水效率的策略进展

为了解决能源危机与环境污染问题,发展一种可再生的清洁能源至关重要.太阳能是一种取之不尽用之不竭的清洁能源,而氢气是一种良好的能源载体.利用太阳能光电催化水分解制氢,是一项有望能够解决能源与环境问题的技术,具有很大的应用前景.其中,氧化铁因为具有合适的能带位置与带隙、良好的稳定性与廉价无毒等优点,成为一种理想的光阳极材料.但是,在实际的测试中,氧化铁仅仅只能得到一个较低的光电转换效率,这可能是因为其较短的空穴扩散距离、较低的电导率以及极度缓慢的水氧化反应动力学所致.整个光电催化水氧化可分为三个过程,即光吸收过程、电荷分离过程以及表面空穴注入过程.这三个过程的效率共同决定了器件的太阳能转化效率.鉴于此,本文将从如何提高这三个效率的角度出发,总结近期研究报道中提高氧化铁光电催化分解水效率的一些策略.光吸收过程是指半导体中价带的电子在吸收具有一定能量的光子后发生跃迁,产生空穴-电子对的过程.其光子的损失主要来源于光的反射、透射以及半导体吸收边的限制.提高光吸收效率的主要策略包括制备具有特定纳米结构的氧化铁电极、利用表面等离子体共振效应以及组成双光吸收系统和掺杂等.电荷分离过程指的是受光激发产生的空穴电子对,在内建电场的作用下发生电荷分离,即光生空穴流向电极表面,光生电子流向半导体内部并从外电路导出.电荷分离效率的损失主要来源于光生载流子在迁移过程中的复合.因此,为了提高电荷分离效率,常见的策略是提高载流子在电荷分离过程中的复合时间τ₁和减少电荷迁移到表面(空穴)或者基底(电子)的时间τ₂.具体的策略包括制备特定的纳米结构(缩短体表相距离,减少τ₂)、构建异质结(增强能带弯曲,提高τ₁和减少τ₂)、掺杂(减少τ₂)和钝化复合中心(提高τ₁)等.表面空穴注入是指到达表面的光生空穴发生水氧化反应生成氧气的过程.除了空穴注入外,表面还可能存在复合与逆反应过程.因此,为了提高表面空穴注入效率,我们既可以提高水氧化反应动力学,具体的手段包括引入水氧化催化剂、F掺杂和碱处理等;也可以采用减少复合反应的策略,具体的方法包括引入钝化层、酸处理和高温热处理等;还可以采用减少逆反应的方法,最常见的手段就是在基底与氧化铁层之间引入电子阻挡层.上述三种途径都能提高表面空穴注入效率.最后,通过结合上述的一些策略,目前得到的最高性能的氧化铁电极在1.23 V(相对于可逆氢电极)能够达到6 mA cm^?2的光电催化分解水电流,但这个值依然明显低于氧化铁的理论值(12.6 mA cm^?2).这可能是由于体相复合所致.除此之外,氧化铁表面的水氧化机理现在依然不清晰,这些都是需要我们在未来解决的问题.     

Activation of α-Fe2O3 for Photoelectrochemical Water Splitting Strongly Enhanced by Low Temperature Annealing in Low Oxygen Containing Ambient

Photoelectrochemical (PEC) water splitting is a promising method for the conversion of solar energy into chemical energy stored in the form of hydrogen. Nanostructured hematite (α-Fe₂O₃) is one of the most attractive materials for a highly efficient charge carrier generation and collection due to its large specific surface area and the short minority carrier diffusion length. In the present work, the PEC water splitting performance of nanostructured α-Fe₂O₃ is investigated which was prepared by anodization followed by annealing in a low oxygen ambient (0.03 % O₂ in Ar). It was found that low oxygen annealing can activate a significant PEC response of α-Fe₂O₃ even at a low temperature of 400 °C and provide an excellent PEC performance compared with classic air annealing. The photocurrent of the α-Fe₂O₃ annealed in the low oxygen at 1.5 V vs. RHE results as 0.5 mA cm⁻², being 20 times higher than that of annealing in air. The obtained results show that the α-Fe₂O₃ annealed in low oxygen contains beneficial defects and promotes the transport of holes; it can be attributed to the improvement of conductivity due to the introduction of suitable oxygen vacancies in the α-Fe₂O₃. Additionally, we demonstrate the photocurrent of α-Fe₂O₃ annealed in low oxygen ambient can be further enhanced by Zn-Co LDH, which is a co-catalyst of oxygen evolution reaction. This indicates low oxygen annealing generates a promising method to obtain an excellent PEC water splitting performance from α-Fe₂O₃ photoanodes.     

A BiVO4 Photoanode with a VOx Layer Bearing Oxygen Vacancies Offers Improved Charge Transfer and Oxygen Evolution Kinetics in Photoelectrochemical Water Splitting

Sluggish oxygen evolution kinetics are one of the key limitations of bismuth vanadate (BiVO₄) photoanodes for efficient photoelectrochemical (PEC) water splitting. To address this issue, we report a vanadium oxide (VO_x) with enriched oxygen vacancies conformally grown on BiVO₄ photoanodes by a simple photo-assisted electrodeposition process. The optimized BiVO₄/VO_x photoanode exhibits a photocurrent density of 6.29 mA cm⁻² at 1.23 V versus the reversible hydrogen electrode under AM 1.5 G illumination, which is ca. 385 % as high as that of its pristine counterpart. A high charge-transfer efficiency of 96 % is achieved and stable PEC water splitting is realized, with a photocurrent retention rate of 88.3 % upon 40 h of testing. The excellent PEC performance is attributed to the presence of oxygen vacancies in VO_x that forms undercoordinated sites, which strengthen the adsorption of water molecules onto the active sites and promote charge transfer during the oxygen evolution reaction. This work demonstrates the potential of vanadium-based catalysts for PEC water oxidation.     

Annealing temperature dependent catalytic water oxidation activity of iron oxyhydroxide thin films

Nanostructured iron oxyhydroxide(Fe OOH) thin films have been synthesized using an electrodeposition method on a nickel foam(NF) substrate and effect of air annealing temperature on the catalytic performance is studied. The as-deposited and annealed thin films were characterized by X-ray diffraction(XRD),X-ray photoelectron spectroscopy(XPS), field emission scanning electron microscopy(FE-SEM) and linear sweep voltammetry(LSV) to determine their structural, morphological, compositional and electrochemical properties, respectively. The as-deposited nanostructured amorphous Fe OOH thin film is converted into a polycrystalline Fe_2O_3 with hematite crystal structure at a high temperature. The Fe OOH thin film acts as an efficient electrocatalyst for the oxygen evolution reaction(OER) in an alkaline 1 M KOH electrolyte. The film annealed at 200 °C shows high catalytic activity with an onset overpotential of 240 m V with a smaller Tafel slope of 48 m V/dec. Additionally, it needs an overpotential of 290 mV to the drive the current density of 10 m A/cm~2 and shows good stability in the 1 M KOH electrolyte solution.     

Si-doped Fe2O3 nanotubular/nanoporous layers for enhanced photoelectrochemical water splitting

The present work reports the enhancement of the photoelectrochemical water splitting performance of in-situ silicon (Si)-doped nanotubular/nanoporous (NT/NP) layers. These layers were grown by self-organizing anodization on Fe-Si alloys of various Si content. The incorporation of Si is found to retard the layer growth rates, leads to a more pronounced nanotubular morphology, and most importantly, an improved photoelectrochemical behavior. By increasing Si content from 1, 2 to 5 at.% in the iron oxide NT/NP photoanodes, the photocurrent onset potential shifts favorably to lower values. At 1.3 V vs. RHE, hematite layer with 5 at.% Si shows a 5-fold increase of the photocurrent, i.e. 0.5 mA cm^− 2 in comparison to 0.1 mA cm^− 2 for the undoped samples. The study also reveals that a suitable layer thickness is essential to achieve a beneficial effect of the Si doping.     

Chemical bias of electrochemical and photoelectrochemical water splitting using a hydrogel separator

Resorcinol-formaldehyde hydrogels are shown to be adequate separators in electrochemical and photoelectrochemical water splitting cells. Combined with concentrated buffer electrolytes, they allow the maintenance of relatively stable pH gradients between basic anolytes and acidic catholytes. The water splitting potential at a current density of 3.0 mA/cm² applied between two Pt electrodes and a pH bias of 8.1 units retained a value of 2.7 V for several hours. Using iron foam/hematite as photoanode and Pt as cathode, the water splitting photocurrents at an applied potential of 1.23 V reached values of 0.40 and 0.06 mA/cm² in the presence and absence, respectively, of this chemical bias.     

Enhanced Photoelectrochemical Water Oxidation from CdTe Photoanodes Annealed with CdCl2

Most CdTe photoanodes and photocathodes show positive and negative photocurrent onset potentials for water oxidation and reduction, respectively, and are thus unable to drive photoelectrochemical (PEC) water splitting without external applied biases. Herein, the activity of a CdTe photoanode having an internal p‐n junction during PEC water oxidation was enhanced by applying a CdCl₂ annealing treatment together with surface modifications. The resulting CdTe photoanode generated photocurrents of 1.8 and 5.4 mA cm^?2 at 0.6 and 1.2 V_RHE, respectively, with a photoanodic current onset potential of 0.22 V_RHE under simulated sunlight (AM 1.5G). The CdCl₂ annealing increased the grain sizes and lowered the density of grain boundaries, allowing more efficient charge separation. Consequently, a two‐electrode tandem PEC cell comprising a CdTe‐based photoanode and photocathode split water without any external bias at a solar‐to‐hydrogen conversion efficiency of 0.51 % at the beginning of the reaction.     

Tungsten doping of Ta3N5-nanotubes for band gap narrowing and enhanced photoelectrochemical water splitting efficiency

Ordered W-doped Ta₂O₅ nanotube arrays were grown by self-organizing electrochemical anodization of TaW alloys with different tungsten concentrations and by a suitable high temperature ammonia treatment, fully converted to W:Ta₃N₅ tubular structures. A main effect found is that W doping can decrease the band gap from 2 eV (bare Ta₃N₅) down to 1.75 eV. Ta₃N₅ nanotubes grown on 0.5 at.% W alloy and modified with Co(OH)_x as co-catalyst show ~ 33% higher photocurrents in photoelectrochemical (PEC) water splitting than pure Ta₃N₅.     

A co-activation strategy for enhancing the performance of hematite in photoelectrochemical water oxidation

Hematite(α-Fe_2O_3) is a promising photoanode for photoelectrochemical(PEC) water splitting.However,the severe charge recombination and sluggish water oxidation kinetics extremely limit its use in photohydrogen conversion.Herein,a co-activation strategy is proposed,namely through phosphorus(P)doping and the loading of CoAl-layered double hydroxides(CoAl-LDHs) cocatalysts.Unexpectedly,the integrated system,CoAl-LDHs/P-Fe_2O_3 photoanode,exhibits an outstanding photocurrent density of 1.56 mA/cm~2 at 1.23 V(vs.reversible hydrogen electrode,RHE),under AM 1.5 G,which is 2.6 times of pureα-Fe_2O_3.Systematic studies reveal that the remarkable PEC performance is attributed to accelerated surface OER kinetics and enhanced carrier separation efficiency.This work provides a feasible strategy to enhance the PEC performance of hematite photoanodes.     

Platinum-like oxidation of nickel surfaces by rapidly switching voltage to generate highly active bifunctional catalysts

Recently the importance of catalyzing the water splitting step of the hydrogen evolution reaction (HER) was highlighted. We demonstrate here a treatment to modify a nickel surface into a highly effective bifunctional HER catalyst (i₀ = 0.18 A/m², Tafel Slope = 106 mV/dec) that has a good distribution of both water splitting sites and H_ads combination sites. The resulting surface is characterized electrochemically, and with SEM, EDX, XPS and AFM. The data is found to be consistent with the treatment oxidizing the Ni surface in a novel way creating the hypothesized “Ni(OH)_x” structure (x between 0 and 2).     

One-step fabrication of chitosan–hematite nanotubes composite film and its biosensing for hydrogen peroxide

This work reports on a novel chitosan–hematite nanotubes composite film on a gold foil by a simple one-step electrodeposition method. The hybrid chitosan–hematite nanotubes (Chi–HeNTs) film exhibits strong electrocatalytic reduction activity for H₂O₂. Interestingly, two electrocatalytic reduction peaks are observed at −0.24 and −0.56 V (vs SCE), respectively, one controlled by surface wave and the other controlled by diffusion process. The Chi–HeNTs/Au electrode shows a linear response to H₂O₂ concentration ranging from 1 × 10⁻⁶ to 1.6 × 10⁻⁵ mol L⁻¹ with a detection limit of 5 × 10⁻⁸ mol L⁻¹ and a sensitivity as high as 1859 μA μM⁻¹ cm⁻².     

Preparation of anatase phase titanium dioxide film by non-aqueous electrodeposition

The electrodeposition–annealing route to fabricating thin film of the promising photocatalyst material anatase-titanium dioxide (anatase-TiO₂) has been studied. The sample was deposited with a solution of N,N-dimethylformamide containing titanium compound by controlled-potential technique. SEM image showed the annealed sample at 600 °C for 1 h under air provided a continuous film with a thickness of ca. 350 nm. In this sample, X-ray photoelectron spectrum corresponding to the Ti 2p peak assigned to a chemical bond of TiO₂ and X-ray diffraction peaks assigned to the anatase phase were observed, respectively. Electrochemical oxidation in sodium sulfate solution on this annealed film was enhanced in the presence of UV light radiation. These results confirm the successful synthesis of photocatalytic anatase-TiO₂ film by the electrodeposition and annealing process.     

Highly Efficient Photoelectrochemical Water Splitting with an Immobilized Molecular Co4O4 Cubane Catalyst

Molecular Co₄O₄ cubane water oxidation catalysts were combined with BiVO₄ electrodes for photoelectrochemical (PEC) water splitting. The results show that tuning the substituent groups on cobalt cubane allows the PEC properties of the final molecular catalyst/BiVO₄ hybrid photoanodes to be tailored. Upon loading a new cubane complex featuring alkoxy carboxylato bridging ligands ( 1 h ) on BiVO₄, an AM 1.5G photocurrent density of 5 mA cm⁻² at 1.23 V vs. RHE for water oxidation was obtained, the highest photocurrent for undoped BiVO₄ photoanodes. A high solar‐energy conversion efficiency of 1.84 % was obtained for the integrated photoanode, a sixfold enhancement over that of unmodified BiVO₄. These results and the high surface charge separation efficiency support the role of surface‐modified molecular catalysts in improving PEC performance and demonstrate the potential of molecule/semiconductor hybrids for efficient artificial photosynthesis.     

Photoelectrochemical water splitting using a Cu(In,Ga)Se2 thin film

The effects of surface modification and reaction conditions on the photoelectrochemical properties of polycrystalline Cu(In,Ga)Se₂ (CIGS) thin films for water splitting were studied. CIGS modified with platinum particles (Pt/CIGS) generated a cathodic photocurrent at potentials up to + 0.4 V vs. RHE at pH = 9.5. The photocurrent was stable for 16 h, which resulted in a turnover number of over 500. A CdS-inserted film (Pt/CdS/CIGS) had significantly improved properties compared to Pt/CIGS: a 0.3 V higher onset potential of cathodic photocurrent and a three-fold increase in the quantum efficiency. Our results suggest the feasibility of CIGS as a photocathode for biphotoelectrochemical water splitting.     

Size-dependence of the heat capacity and thermodynamic properties of hematite (α-Fe2O3)

The heat capacity of a 13 nm hematite (α-Fe₂O₃) sample was measured from T = (1.5 to 350) K using a combination of semi-adiabatic and adiabatic calorimetry. The heat capacity was higher than that of the bulk which can be attributed to the presence of water on the surface of the nanoparticles. No anomaly was observed in the heat capacity due to the Morin transition and theoretical fits of the heat capacity below T = 15 K show a small T³ dependence (due to lattice contributions) with no T^3/2 dependence. This suggests that there are no magnetic spin-wave contributions to the heat capacity of 13 nm hematite. The use of a large linear term to fit the heat capacity below T = 15 K is most likely due to superparamagnetic contributions. A small anomaly within the temperature range (4 to 8) K was attributed to the presence of uncompensated surface spins.     

Design of a bioelectrocatalytic electrode interface for oxygen reduction in biofuel cells based on a specifically adapted Os-complex containing redox polymer with entrapped Trametes hirsuta laccase

The design of the coordination shell of an Os-complex and its integration within an electrodeposition polymer enables fast electron transfer between an electrode and a polymer entrapped high-potential laccase from the basidiomycete Trametes hirsuta. The redox potential of the Os^3+/2+-centre tethered to the polymer backbone (+ 720 mV vs. NHE) is perfectly matching the potential of the enzyme (+ 780 mV vs. NHE at pH 6.5). The laccase and the Os-complex modified anodic electrodeposition polymer were simultaneously precipitated on the surface of a glassy carbon electrode by means of a pH-shift to 2.5. The modified electrode was investigated with respect to biocatalytic O₂ reduction to H₂O. The proposed modified electrode has potential applications as biofuel cell cathode.     

Thermally-driven processes on rutile TiO2(1 1 0)-(1 × 1): A direct view at the atomic scale

The technological importance of TiO₂ has led to a broad effort aimed at understanding the elementary steps that underlie catalytic and photocatalytic reactions. The most stable surface, rutile TiO₂(1 1 0), in particular, has became a prototypical model for fundamental studies of TiO₂. In this critical review we have selected oxygen, water, and alcohols to evaluate recent progress relevant for applications in the areas of water splitting and oxidation of organic contaminants. We first focus on the characterization of defects and the distribution of excess charge that results from their formation. The subsequent section concentrates on the role of individual surface sites and the effect of available charge in the adsorption processes. The discussion of adsorbate dynamics follows, providing models for intrinsic and extrinsic diffusion processes as well as rotational dynamics of anchored alkoxy species. The final section summarizes our current understanding of TiO₂(1 1 0) catalyzed reactions between water, oxygen, and their dissociation products.     

Annealing effects on the morphology and luminescence of cubic boron nitride based ceramics

Cubic boron nitride based ceramics with silicon were sintered at 1350 °C under a pressure of 5.0 GPa. The effects of post-annealing on grain morphology, surface morphology, and photoluminescence of Si–cBN ceramics were investigated by scanning electron microscope and room temperature photoluminescence measurements. The results showed that the annealing treatment had great influence on cBN grain morphology, rather than the surface morphology. The luminescence intensity increased with annealing temperature and annealing time. The void-net structure formed by continuous distribution of SiO_x particulate on the ceramic surface resulted in the emission band peaking at about 701.2 nm, and the tense passivation of Si by SiO_x led to the peak's low intensity. The near ultraviolet emission band peaking at about 317 nm was attributed to the oxygen vacancies formed in cBN grain surface, caused by the scavenging of oxygen from the cBN grain surface by the added Si.