Nanostructured Hematite Thin Films Research Articles

Rapid Single-Step Deposition of Nanostructured Hematite Thin Films Produced by PECVD Using Ferrocene as Precursor

Dense cauliflower type nanostructured hematite (<inline-formula> <tex-math notation="LaTeX">$\alpha $ </tex-math></inline-formula>-Fe₂O₃) thin films were grown by plasma-enhanced chemical vapor deposition (PECVD) using ferrocene (Fe(C₅H₅)₂) as precursor. The influence of the plasma parameters on the layer composition and growth rate was investigated and correlated with the plasma used during the deposition process. Optical emission spectroscopy (OES) data from the plasma in conjunction with scanning electron microscopy (SEM), energy dispersive spectrometry (EDX) and Raman spectroscopy showed that plasma power of about 50&#x2013;120 W with an O₂ content of 10&#x0025;&#x2013;15&#x0025; in the carrier gas Ar favors the growth of hematite rather than maghemite. The thickness of the thin films is position dependent due to the plasma reactor geometry. The growth rate can reach a very high value, up to 440 nm/min.

Simple solvothermal approach to highly nanostructured hematite thin films

In this work, we present a solvothermal method for the synthesis of hematite thin films on fluorine-doped tin oxide substrates. This simple method uses a precursor solution of iron(III) 2,4-pentanedionate dissolved in ethanol with a microliter-scale amount of water and yields hematite ∼500 nm thick films after annealing. The synthesized films were characterized using an array of methods, including scanning electron microscopy, energy-dispersive X-ray spectroscopy, diffuse reflectance, and powder X-ray diffraction. Incorporating water into the precursor solution provides nucleation sites for the reaction and results show that by altering the amount of water used in the synthesis, it is possible to generate nanocrystalline films of different morphologies, nanocrystal size distributions, and surface areas. This synthetic procedure therefore provides control over the films’ physical and electrochemical characteristics. Doping of hematite thin films is also possible using this synthesis, as exemplified by doping with tin by adding tin(II) 2,4-pentanedionate to the precursor solution. To demonstrate utility, we build prototype photoelectrochemical cells using the synthesized hematite as the photoanode.

Role of morphology, composition, and structure on the optical response of nanostructured hematite thin films

Nanostructured hematite films are promising for photoactive materials applications. One of the main challenges in this matter is to improve the understanding about the behavior of the optical properties affected by the nanostructure. This work presents a robust ellipsometric analysis (supported by AFM, XRD, and XPS characterization) of the optical response of nanostructured hematite films synthesized by an alternative methodology based on chemical bath deposition. The good match between the measured and calculated Δ and Ψ angles after the fitting processes is an evidence of the reliability of the ellipsometric model constructed here and leads to a well-agreement between the thicknesses obtained from ellipsometry and those from AFM and profilometry. The analysis of the fitted model parameters made possible to determine the existence of two types of porosity in the films. On a microscopic scale, cracks or gross porosity were quantified directly from the v% of air in an effective medium layer of the model. A fine porosity with nanometric size persisting among the solid-phase that constitutes the nanostructured hematite films, was inferred from their lower refractive index with respect to bulk hematite. The extinction coefficient reveled a strong electronic transition centered at ~550 nm associated with the optical bandgap of the films, determined in approx. 2 eV (620 nm), which correspond with bulk hematite. All results shown here reveal a complex relationship between the morphology and the optical response, which introduce relevant aspects about the nanostructured hematite films, like density and porosity, that could be used to propose performance improvements in their potential applications as photoactive material. • Nanostructured hematite thin films synthesized by a chemical-bath based procedure. • Morphological, structural, and chemical composition supports the optical analysis. • Ellipsometry describes complex correlation between morphology and optical response. • Development of an alternative route toward viable photocatalytic films systems.

Identification of Exciton–Exciton Annihilation in Hematite Thin Films

Hematite, a common type of iron oxide, is a promising material for solar technologies because of its small band gap that allows for solar radiation absorption in the visible region, low toxicity in aqueous solutions, easy synthesis, and natural abundance. However, fast electron–hole recombination has been hampering full applicability of hematite in solar technologies. In this study, we used visible femtosecond transient absorption spectroscopy to investigate the excited-state decay and electron–hole recombination dynamics of nanostructured hematite thin films. By varying the pump excitation fluence and performing global and target data analysis, we identified the presence of nonlinear decay processes during the initial picosecond after photoexcitation, which have a non-negligible contribution at pump fluences of >∼1 mJ/(pulse·cm2). Calculations show exciton–exciton annihilation to be the dominant nonlinear process, with an average rate constant of 7.09 × 10–9 cm3 s–1. Annihilation calculations also allowed us to estimate the annihilation radius to be 2.3 nm, thus explaining the rapid exciton–exciton annihilation in the immediate aftermath of photoexcitation. Probe wavelength-dependent decay dynamics points to excitation energy redistribution and involvement of traps in the recombination dynamics. We finally present a kinetic model, verified by performing target data analysis, showing the rates and channels of the dominant processes involved with electron–hole recombination upon relatively high excitation rates. The extremely fast electron–hole recombination process in hematite is one of the main reasons hindering the full applicability of the material in solar water splitting. Measures to limit these ultrafast recombination processes should, therefore, be incorporated into device fabrication and preparation to further improve the material.

Nanostructured hematite thin films for photoelectrochemical water splitting

Nanostructured hematite thin films prepared by dip coating technique were investigated for their photoelectrochemical activity for generation of hydrogen from water splitting. Structural, morphological and optical analyses of the doped/undoped films were performed by X-ray diffraction, high resolution field emission-scanning electron microscopy, UV–vis spectrophotometry and Raman spectroscopy. The photoelectrochemical measurements of the films showed enhanced photoresponse and cathodic shift of the onset potential upon Ti doping indicating improved transfer of photoholes at the semiconductor-electrolyte interface. Films doped with 1at% Ti produced 0.72mA/cm2 at 1.23V vs RHE which is 2 times higher than current density for the pure film (0.30mA/cm2, at 1.23V vs RHE). Gas chromatography analysis of the films also showed enhanced hydrogen evolution at 1at% Ti with respect to pure film.

Employing electrochemical reduced graphene oxide as a co-catalyst for synergistically improving the photoelectrochemical performance of nanostructured hematite thin films

Abstract In this study, a series of electrochemical reduced graphene oxide (eRGO)-hematite nanocomposites were developed through a facile and environmental benign two-step electrodeposition method with high photoelectrochemical (PEC) performance. The resulting nanocomposites formed an intimate contact between the eRGO and hematite interface as supported by the field emission-scanning electron microscopy (FE-SEM) analysis. A remarkable 8-fold enhancement in the photocurrent density was observed on the eRGO-hematite-4 nanocomposite (using 2.0 mg/ml GO precursor) relative to the bare hematite under light irradiation. This improvement is ascribed to the finely controlled eRGO sheets that enhance the light absorption, increase PEC active surface area of hematite, improve efficient transfer of the photoinduced electrons from the conduction band of hematite to eRGO sheets and as a result leads to a minimised electron–hole pairs recombination rate. This was further evidenced with impedance characteristics, where the obtained surface charge resistance values of eRGO-hematite-4 nanocomposite are much lower than the bare hematite, revealing an efficient charge transfer step to impede the charge recombination. Lastly, a postulated mechanism for the PEC process associated with eRGO-hematite nanocomposite was presented.

Understanding the synergistic between optimum dopant loading and charge transfer kinetics in platinum-mediated nanostructured hematite thin films

Photoelectrochemical (PEC) water splitting process has received immense attention owing to its ability in sustainably producing solar hydrogen from water. The main aim of this study was to systematically optimise the Platinum (Pt) dopant loadings and understand its synergistic role in enhancing the photoactivity in nanostructured hematite (α-Fe2O3) thin films. Through the optimisation study, it was found the 3mol% Pt-doped nanostructured hematite thin film exhibited the highest photocurrent density of 0.91mA/cm2 at 0.7V vs Ag/AgCl in 1M NaOH. In comparison with the bare hematite film, this was a 3-fold enhancement in terms of measured photocurrent density. Such an enhancement in the measured photocurrent density was attributed to the increased donor density caused due to the incorporation of optimum Pt dopant. Further EIS analysis revealed that the flat-band potential of hematite was shifted to a more negative potential by 30mV while the charge transfer resistance was considerably reduced through the incorporation of Pt dopant. All these suggested that the successful doping of optimum Pt loading will lead to a visible active, lower charge transfer resistance and an enhanced separation efficiency of the photogenerated charge carriers in nanostructured hematite thin films.

Interfacial insight in multi-junction metal oxide photoanodes for water-splitting applications

Photoelectrochemical (PEC) properties of nanostructured hematite (Fe2O3) thin films prepared using plasma-enhanced chemical vapor deposition (PE-CVD) were investigated against the influence of processing parameters and post-synthesis heat-treatment procedures. Annealing at high temperatures (>500°C) was found to substantially affect the micro-structure (grain growth and densification) and electronic (interdiffusion at the film/substrate interface) concomitantly manifested in an enhancement in the PEC behavior. The Sn impurity level in hematite films was found to increase with the annealing temperature with highest values achieved in samples heat-treated at 750°C, due to the interdiffusion and substitution of Sn(IV) species at Fe(III) sites. Sn:Fe2O3 films exhibited significantly high photocurrent density of 1.33mAcm−2 at the water oxidation level of 1.23V vs. RHE. The diffusion of Sn ions into iron oxide lattice altered the electronic properties of hematite films due to electron–donor behavior of the dopants that was verified by X-ray photoelectron spectroscopy and secondary ion mass spectroscopy (SIMS) analyses. Deposition of a thin overlayer of TiO2 (10nm) on hematite films by atomic layer deposition (ALD) was found to further improve the photocurrent density to 1.8mAcm−2 at 1.23V vs. RHE. Ab-initio calculations on the effect of substitutional Sn(IV) dopants in the Fe2O3 lattice on the electronic structure and the band alignment between hematite and the TiO2 over layer revealed that Sn-dopants led to the generation of localized Fe(II) centers augmenting the n-type behavior of hematite. No effect of the Sn-doping on the electrostatic potential was found on a macroscopic scale. However, the charge transfer from the Sn-doping to the Fe(II) centers would cause high electric fields on the nanometer scale and might hence play an important role in the efficient separation of electron and holes. The simulations showed that the hematite band edges are enclosed by the TiO2 band edges and therefore electron depletion at the surface–liquid interface is enhanced. This might lead to reduced recombination rates near the surface and consequently to increased photocurrents, since the Fe2O3/TiO2 interface constitutes a barrier for hole transport.

Nanostructure-Preserved Hematite Thin Film for Efficient Solar Water Splitting.

High-temperature annealing above 700 °C improves the activity of photoelectrochemical water oxidation by hematite photoanodes by increasing its crystallinity. Yet, it brings severe agglomeration of nanostructured hematite thin films and deteriorates electrical conductivity of the transparent conducting oxide (TCO) substrate. We report here that the nanostructure of the hematite and the conductivity of TCO could be preserved, while the high crystallinity is attained, by hybrid microwave annealing (HMA) utilizing a graphite susceptor for efficient microwave absorption. Thus, the hematite thin-film photoanodes treated by HMA record 2 times higher water oxidation photocurrents compared to a conventional thermal-annealed photoanode. The enhanced performance can be attributed to the synergistic effect of a smaller feature size of nanostructure-preserved hematite and a good electrical conductivity of TCO. The method could be generally applied to the fabrication of efficient photoelectrodes with small feature sizes and high crystallinity, which have been mutually conflicting requirements with conventional thermal annealing processes.

Effects of annealing temperature on the physicochemical, optical and photoelectrochemical properties of nanostructured hematite thin films prepared via electrodeposition method

Hematite (α-Fe2O3) is a promising photoanode material for hydrogen production from photoelectrochemical (PEC) water splitting due to its wide abundance, narrow band-gap energy, efficient light absorption and high chemical stability under aqueous environment. The key challenge to the wider utilisation of nanostructured hematite-based photoanode in PEC water splitting, however, is limited by its low photo-assisted water oxidation caused by large overpotential in the nominal range of 0.5–0.6V. The main aim of this study was to enhance the performance of hematite for photo-assisted water oxidation by optimising the annealing temperature used during the synthesis of nanostructured hematite thin films on fluorine-doped tin oxide (FTO)-based photoanodes prepared via the cathodic electrodeposition method. The resultant nanostructured hematite thin films were characterised using field emission-scanning electron microscopy (FE-SEM) coupled with energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), UV-visible spectroscopy and Fourier transform infrared spectroscopy (FTIR) for their elemental composition, average nanocrystallites size and morphology; phase and crystallinity; UV-absorptivity and band gap energy; and the functional groups, respectively. Results showed that the nanostructured hematite thin films possess good ordered nanocrystallites array and high crystallinity after annealing treatment at 400–600°C. FE-SEM images illustrated an increase in the average hematite nanocrystallites size from 65nm to 95nm when the annealing temperature was varied from 400°C to 600°C. As the crystallites size increases, the grain boundaries reduce and this suppresses the recombination rate of electron–hole pairs on the nanostructured hematite thin films. As a result, the measured photocurrent densities of nanostructured hematite thin films also increased. The highest measured photocurrent density of 1.6mA/cm2 at 0.6V vs Ag/AgCl in 1M NaOH electrolyte was achieved for the nanostructured hematite thin film annealed at 600°C. This study had confirmed that strong interdependencies exist between the average hematite nanocrystallites size and grain boundaries with annealing temperature on the eventual PEC water splitting performance of nanostructured hematite thin films. The annealed hematite thin films at a higher temperature will enhance the nanocrystals growth and thus, suppressing the electron–hole pairs recombination rate, lowering the grain boundary resistance and enabling higher photocurrent flow at the molecular level. As a result, the photocurrent density and thus, the overall PEC water splitting performance of the nanostructured hematite thin films are significantly enhanced.

Back Electron–Hole Recombination in Hematite Photoanodes for Water Splitting

The kinetic competition between electron-hole recombination and water oxidation is a key consideration for the development of efficient photoanodes for solar driven water splitting. In this study, we employed three complementary techniques, transient absorption spectroscopy (TAS), transient photocurrent spectroscopy (TPC), and electrochemical impedance spectroscopy (EIS), to address this issue for one of the most widely studied photoanode systems: nanostructured hematite thin films. For the first time, we show a quantitative agreement between all three techniques. In particular, all three methods show the presence of a recombination process on the 10 ms to 1 s time scale, with the time scale and yield of this loss process being dependent upon applied bias. From comparison of data between these techniques, we are able to assign this recombination phase to recombination of bulk hematite electrons with long-lived holes accumulated at the semiconductor/electrolyte interface. The data from all three techniques are shown to be consistent with a simple kinetic model based on competition between this, bias dependent, recombination pathway and water oxidation by these long-lived holes. Contrary to most existing models, this simple model does not require the consideration of surface states located energetically inside the band gap. These data suggest two distinct roles for the space charge layer developed at the semiconductor/electrolyte interface under anodic bias. Under modest anodic bias (just anodic of flatband), this space charge layer enables the spatial separation of initially generated electrons and holes following photon absorption, generating relatively long-lived holes (milliseconds) at the semiconductor surface. However, under such modest bias conditions, the energetic barrier generated by the space charge layer field is insufficient to prevent the subsequent recombination of these holes with electrons in the semiconductor bulk on a time scale faster than water oxidation. Preventing this back electron-hole recombination requires the application of stronger anodic bias, and is a key reason why the onset potential for photocurrent generation in hematite photoanodes is typically ~500 mV anodic of flat band and therefore needs to be accounted for in electrode design for PEC water splitting.

Nanostructured hematite thin films produced by spin-coating deposition solution: Application in water splitting

Doped and undoped hematite films for photoelectrochemical hydrogen production were prepared by spin-coating deposition solution (SCDS). To understand the influence of the Si-doping and identify the critical parameters of the proposed SCDS method an extensive characterization was conducted. The Si-doped hematite exhibited higher photocurrent response when compared with undoped films. We have shown that the crystallographic orientation degree of the films appears to be a dominant factor affecting the photocurrent. The performance of our hematite electrodes is well below the maximum theoretical efficiency and the conceivable explanation could be given by the high value of recombination phenomena (electron/hole pair).

Structural, Morphological and Photoelectrochemical Behavior of Hematite Modified by 120 MeV Ag9+ Ions

AbstractNanostructured hematite thin film for photoelectrochemical (PEC) splitting of water has great potential in the design of low-cost, environmental friendly solar-hydrogen production. Presently, solar-to-hydrogen conversion efficiency of PEC cell using iron oxide is limited by its poor charge transport due to high recombination losses and mismatch of band edges position with the redox level of water. High energy heavy ion irradiation provides the researchers a new dimension to introduce the desired changes in the behaviour of the material, which largely influence their properties. In order to get efficient PEC system, spray-pyrolytically deposited nanostructured hematite thin films were modified by irradiating the samples with 120 MeV Ag9+ ions with fluences ranging from 5×1011 to 1×1013 ions/cm2. Irradiated samples exhibited a partial transition from the hematite to the magnetite phase and reduction in particle size as indicated by XRD and Raman analysis. SEM picture showed a decrease the thickness and porosity of the films after irradiation. These irradiated films, when used in PEC cell showed significantly higher photocurrent density than unirradiated α-Fe2O3.