As-prepared Thin Films Research Articles

Enhancement of crystallinity and the optical properties in gamma irradiated and thermally annealed cobalt doped MgTiO3 thin films

We report the crystal structure, surface morphology and optical properties of gamma (γ) irradiated, and thermally annealed cobalt (5 %) doped magnesium titanate thin films. Mg0.95Co0.05TiO3 (MCT) thin films were fabricated on quartz substrates by Radio Frequency magnetron sputtering while substrate temperature was maintained at 300 °C. Firstly, the as-prepared MCT thin films were irradiated by 1.33 MeV 60Co γ – rays with varying doses in the range of 0 – 125 kGy with the intervals of 25 kGy and post-annealing process at 700 °C. The X-ray diffraction pattern of irradiated thin films is amorphous, whereas annealed films are crystalline, which are refined with the Rietveld refinement method by considering the R3¯space group. Dominated crystallite size (39 nm) with low strain is identified for the samples exposed at 75 kGy. In addition to the γ-irradiation of 75 kGy, the optical characteristics revealed that an enhanced bandgap (∼ 4.27 eV) and refractive index (2.26) had been observed for an annealed sample as compared with the bandgap (∼ 3.68 eV) and refractive index (2.10) of as-deposited sample. The influence of γ-irradiation dose and annealing on MCT thin films' structural and optical properties has been discussed. This study demonstrates that the γ–ray irradiated MCT films are potential candidates for anti-reflection coating applications.

Electrosynthesis-Induced Pt Skin Effect in Mesoporous Ni-Rich Ni-Pt Thin Films for Hydrogen Evolution Reaction.

A Pt skin effect, i.e., an enrichment of Pt within the first 1-2 nm from the surface, is observed in as-prepared electrodeposited Ni-rich Ni-Pt thin films. This effect, revealed by Rutherford backscattering (RBS), is present for both dense thin films and mesoporous thin films synthesized by micelle-assisted electrodeposition from a chloride-based electrolyte. Due to the Pt skin effect, the Ni-rich thin films show excellent stability at the hydrogen evolution reaction (HER) in acidic media, during which a gradient in the Pt/Ni ratio is established along the thickness of the thin films, while the activity at the HER remains unaffected by this structural change. Further characterization by elastic recoil detection with He ions analysis shows that hydrogen profiles are similar to those of Pt: a surface hydrogen peak coincides with the Pt skin, and a gradient in hydrogen concentration is established during HER in acidic media, together with a considerable uptake in hydrogen. A comparative study shows that in alkaline media, hydrogen evolution has little to no effect on the structural properties of the thin films, even for much longer times of exposure. The mesoporous thin films, in addition to their higher efficiency at HER compared to dense thin films, also show lower internal stress, as determined by Rietveld refinement of grazing incidence X-ray diffraction patterns. The latter also reveal a fully single-phase and nanocrystalline structure for all thin films with varying Ni contents.

Advancing spin-coating technique for semi-crystalline low-density polyethylene thin films

Polymeric thin films are widely used for coating substrates or as freestanding materials. They are prepared using physical/chemical vapor deposition, dip coating, solution casting, or spin coating. Among these, spin-coated films offer uniform thickness and reproducible results. These films are typically made from amorphous polymers, which are soluble in solvents at ambient temperature, making it easier to coat the polymer solution on the substrate. Despite the advantages of this technique, it has hardly been used for semi-crystalline polymers. In this work, we report the synthesis of freestanding porous thin films using semi-crystalline polymers, specifically low-density polyethylene (LDPE), through the utilization of spin coating. The effect of polymer concentration and porosity on thickness and strength was investigated. The as-prepared thin film was characterized by XRD, XPS, FTIR, and SEM techniques. The increase in crystallinity (14 % change) of the thin film was attributed to the rearrangement and uniform alignment of the polymer chains, which further resulted in relatively higher tensile strength.

Synthesis, negative thermal expansion and optical performances of In0.5Sc1.5Mo3O12 thin films via sol-gel spin coating

Orthorhombic In0.5Sc1.5Mo3O12 thin films have been synthesized using the sol-gel spin coating. Influences of post-annealing temperatures on crystal structure, morphology, negative thermal expansion (NTE) and optical performances of the thin films were studied using XRD, SEM, XPS, high-temperature XRD and UV–Vis–NIR spectrophotometer. XRD and SEM results show that as-prepared thin film is in an amorphous state with a smooth and dense surface. Orthorhombic In0.5Sc1.5Mo3O12 thin films were obtained after post-annealing in 550–750 °C. The higher the annealing temperature in 550–750 °C, the better the crystallinity of orthorhombic In0.5Sc1.5Mo3O12 thin films. In addition, the grain size of the In0.5Sc1.5Mo3O12 thin film also grows as the annealing temperature increases. The resulting orthorhombic In0.5Sc1.5Mo3O12 thin films exhibit anisotropic NTE. Its volumetric coefficient of thermal expansion (CTE) is −3.44 × 10−6 °C−1, while the CTEs in a-, b- and c-axis directions are −4.70 × 10−6 °C−1, 5.37 × 10−6 °C−1, and -4.10 × 10−6 °C−1 in 25–700 °C, respectively. In addition, the transmittance of orthorhombic In0.5Sc1.5Mo3O12 thin films post-annealed at 750 °C is around 87.75 %–95.03 %.

Two-step synthesis of antimony sulfide thin films: enhancement in physical properties through sulfurization

Recently, there has been an increase in the use of antimony sulfide (Sb2S3) in Si-based tandem solar cells as a potential absorber material for top sub-cells. The choice of the material stems from the favoured properties such as appropriate bandgap, simple binary composition, nontoxic elements, and long-term stability. However, the physical properties and practical applicability of these materials depend largely on their synthesis conditions. In this work, we investigate the role of sulfurization on the structural, morphological, compositional, and optical properties of Sb2S3 thin films deposited on soda-lime glass via a thermal evaporation technique. Sulfurization was performed on the as-prepared thin films in a customized Chemical Vapor Deposition (CVD) chamber at five different temperatures. Analysis of the crystallinity of the film using the x-ray diffraction technique illustrates the transformation of the film from impure, poor crystalline phase to phase-pure, and highly crystalline orthorhombic structure due to sulfurization. Scanning electron microscopic investigations of the samples revealed better grains with nanorods on the surface at a temperature of 400 °C. For the samples investigated here, the energy values estimated via density functional theory (DFT) calculations agreed well with the experimental data obtained from UV-visible absorption spectral studies. Additionally, it was observed that the desired near-stoichiometric Sb2S3 thin films could be achieved via sulfurization, and the presence of Sb2S3 in all samples was confirmed via Raman spectroscopic studies. Additionally, the defects and trap states of the prepared films were investigated using photoluminescence studies, and donor and acceptor defects were identified. Our study revealed that sulfur rich Sb2S3 films prepared at a sulfurization temperature of 400 °C produced the desired structure, morphology, and optical qualities for future photovoltaic applications.

Highly oriented epitaxial Cu2O (011) thin film grown on MgO (001) substrate by dynamic aurora PLD method

The epitaxial Cu2O films are grown on MgO (001) substrates by the dynamic aurora pulsed laser deposition (PLD) method. The structure and epitaxial growth of the as-prepared thin films at different substrate temperatures (25 °C to 600 °C) were investigated. The thin film prepared at room temperature (RT, 25 °C) exhibits (111) plane of monoclinic CuO phase with poor crystallinity. The thin films deposited at 200 °C to 600 °C show single-phase cubic Cu2O with good crystallinity. The highly oriented epitaxial Cu2O thin film on the MgO substrate is achieved at low temperatures of 200 °C by applying an induced electromagnetic field during thin film growth. The quality and crystallinity of epitaxial Cu2O thin films remarkably improved with increasing temperature from 200 °C to 600 °C and all these thin films exhibited an out-of-plane epitaxial relationship of Cu2O (011) ‖ MgO (001). The CuO thin film grown on MgO substrate at RT shows a single domain structure, as observed by RHEED. In constrast, Cu2O films grown at 200 °C to 600 °C exhibit two different kinds of domain structure with corresponding in-plane epitaxial relationship of Cu2O [100] ‖ MgO [110] and Cu2O [100] ‖ MgO [11¯0]. This report reveals, for the first time, the feasibility of epitaxial growth of highly oriented Cu2O (011) thin film achieved at the low temperature of 200 °C by the dynamic aurora PLD method. The quality of epitaxial Cu2O (011) is significantly enhanced with increasing substrate temperature from 200 °C to 600 °C.

Effect of annealing environment on the electrochemical performance of biosynthesized V2O5@C thin films

In the present work, the V2O5 thin films were prepared on flexible stainless steel mesh (FSSM) substrate using Bengal gram bean extract (BGBE) as a medium by reflux condensation method for its use as an electrode for supercapacitor application. The bio-molecules present in BGBE were in-situ converted into carbonaceous material after the annealing process. A systematic study of the influence of the annealing environment was investigated by heating the as-prepared V2O5 thin films in two different annealing atmospheres viz nitrogen and air. The annealing of the as-prepared V2O5 thin films under N2 atmosphere (V2O5-N) produces more carbonaceous material when compared to V2O5 thin films annealed in air atmosphere (V2O5-A). Interestingly, annealing in air atmosphere completely oxidizes the biomass, while annealing in nitrogen leaves traces of carbonaceous matrix in the thin film. This improves the conductivity and electrical properties of the material and its advantage is taken in fabricating supercapacitor devices. The obtained V2O5-N and V2O5-A thin films were characterized using X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), Raman, and X-ray Photoelectron Microscopy (XPS). Moreover, the electrochemical performance of V2O5-N and V2O5-A was investigated. The V2O5-N electrode exhibited a specific capacitance (Csp) of 633.3 F g−1 with an energy density (ED) of 87.9 wh kg−1 at 2 mA cm−2 current density and good cycles stability with capacitive retention of 86.2 % over the 10,000 continuous GCD cycles which is higher than the V2O5-A. A flexible symmetric solid state (SSC) device was assembled using V2O5-N electrodes with PVA-LiClO4 gel electrolyte. The device exhibited a Csp of 82.6 F g−1 with an ED of 22.4 wh kg−1 at 1 mA cm−2 current density. In summary, the V2O5-N demonstrated significantly improved electrochemical performance than the V2O5-A electrode and is supported with the appropriate data.

INFLUENCE OF ANNEALING TEMPERATURE ON ZINC OXYSULPHIDE THIN FILMS PROPERTIES DEPOSITED BY THERMAL SPRAY TECHNIQUE

This article studies the effects of annealing temperatures on the stoichiometry, structure, and optical characteristics of Zinc Oxysulphide thin films deposited on glass substrates using the thermal spray method. The annealing was done in the air under a temperature of 150 ºC, 250 ºC and 350 ºC. Field Emission Scanning Electron Microscopy (FESEM), X-ray diffractometry (XRD), and a UV-Vis Spectrophotometer were utilized to examine the thin films' morphological, structure, and optical characteristics as grown and annealed samples. It has been observed that the film thickness decreases as the annealing temperature increases. The XRD pattern demonstrates that as-prepared and annealed samples at 250C, and 350C are amorphous, while film annealed at 150 C is crystalline with the hexagonal phase and orientated along (110) plane. FESEM image of the as-prepared Zinc Oxysulphide thin films shows flower-like sheet nanostructures. However, for the annealed samples, the FESEM images show cone-like with a combination of sheets and aggregate nanoparticles. EDX analysis reveals the presence of Zn, O as well as sulfur. The transmittance of the film decreases from 63% to 37% and the band gap energy reduces slightly from 3.99 to 3.92 eV as the annealing temperature increase.

The advantages of methanol-amine solvent mixtures in solution processing of Ge-Sb-S chalcogenide glass thin films

We report on the substantial influence of methanol presence on the dissolution mechanism of Ge20Sb5S75 bulk glass with subsequent superior properties of thin films deposited from such solutions. Raman spectroscopy confirmed significant differences in structural features present in glass solutions prepared from pure amines (n-propylamine and n-butylamine) and their mixtures with methanol. The experiments with dissolved elemental sulfur and Ge25S75 glass analog proved that both antimony and methanol presence induce further splitting of the Ge4S104− cluster structure, which fundamentally affects the properties of deposited thin films. Significant structural and compositional differences were found not only in solutions and as-prepared samples, but also after thin films’ thermal treatment (hard baking up to 210 °C). The as-prepared thin films deposited from amine-methanol mixtures possessed the exact composition of source bulk glass while thin films of other solvent formulations exhibited sulfur deficiency. The annealing up to 210 °C only highlighted this difference. As a result of the different structure of the thin films prepared in this way, the other benefits of methanol addition were found, namely an increase in the refractive index by approx. 0.1 independent of the annealing temperature, or a lower thermally induced thickness contraction (up to 7.5 %).

Tin doped ZnFe2O4 thin film based ethanol sensor

In this work, tin doped ZnFe2O4 film was deposited onto microscope glass substrates by spray pyrolysis process followed by subsequent calcinations. The morphology and structure of the as-prepared thin films have been characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Temperature dependent current measurements were performed by two-probe method to analyze electrical properties, and electrical conductivity at room temperature. The effect of the tin component in ZnFe2O4 thin films on the gas sensing properties has been evaluated by the responses to ethanol vapor. The results have showed that the ZnFe2O4 thin films containing 2.5 wt% tin exhibit the best sensing properties to ethanol vapor. The response and recovery time are about 10 and 24 s, respectively. In addition, the as-prepared sensors exhibit excellent selectivity and stability. These results indicate that tin doped ZnFe2O4 thin films can be used in fabricating high performance gas sensors.

High dielectric temperature stability in the relaxor ferroelectric thin films via using a multilayer heterostructure

The temperature stability of the relaxor ferroelectrics (RFEs) mainly originates from the diffuse phase transition (DPT). Higher atomic disorder degrees, more inhomogeneous lattice distortion and polymorphic phases could increase the compositional inhomogeneity for a stronger DPT. This work aims to enhance this compositional inhomogeneity for a high dielectric temperature stability by preparing the multicomponent RFE-based multilayer heterostructure polycrystalline thin films. The as-prepared RFE thin films show a relatively high dielectric constant (∼165.14), and a very high dielectric temperature stability with a very low relative variation ratio of dielectric constant (∼1.7 %) from -55 °C to 150 °C which can be comparable to most linear dielectric thin films. Meanwhile, the dielectric loss and leakage current densities were also optimized. This work brings the potential application of the RFE thin films in the thin-film capacitor with high temperature stability requirements.

SILAR Synthesized Binder-Free, Hydrous Cobalt Phosphate Thin Film Electrocatalysts for OER Application: Annealing Effect on the Electrocatalytic Activity

Highly efficient and robust electrocatalysts intended for the oxygen evolution reaction (OER) are essential for energy conversion devices; the structural and morphological fractions of the electrocatalyst can also greatly influence the OER performance. Therefore, developing a high-performing electrocatalyst with desired properties is crucial through a simple and cost-effective chemical process. So, the binder-free, hydrous cobalt phosphate (Co3(PO4)2.nH2O) thin film electrocatalysts are prepared via the successive ionic layer adsorption and reaction (SILAR) method onto stainless steel (SS) substrates at ambient temperature. Additionally, the impact of annealing on the OER efficiency of thin film electrodes made of hydrous cobalt phosphate was observed by subjecting the electrocatalysts to different temperatures (200°C and 400°C). The SILAR synthesized hydrous Co3(PO4)2.nH2O with the short-range ordered agglomerated particles transformed into discrete nanoparticles with an annealing temperature. The as-prepared hydrous cobalt phosphate (CP) demonstrated outstanding OER performance with the least overpotential ( η ) of 265 mV at 10 mA cm-2 current density and the lowest Tafel slope of 37 mV dec-1, and the overpotential ( η 10 ) increased upon the annealing of catalysts (CP 200 and CP 400). Moreover, the as-prepared electrocatalyst demonstrated overall water splitting at the lowest potential of 1.56 V (@10 mA cm-2) in the alkaline electrolysis system (CP//Pt). The present study reveals that the electrocatalytic performance of the as-prepared cobalt phosphate thin film catalyst is significantly associated with the hydrous content present in catalysts and demonstrates the practical applicability of SILAR-synthesized binder-free cobalt phosphate thin film electrocatalysts.

Performance analysis of P-SnS thin films fabricated using CBD technique for photo detector applications

In the present work, SnS thin films were prepared using the CBD technique at room temperature and varying annealing temperaturesfrom300 to 450 °C for photo detector applications. The prepared samples were characterized using different techniques for analyzing the structural, optical, morphological, and photo sensing properties of the samples. From XRD analysis, the diffraction pattern of all the prepared thin films shows the pristine SnS phase of the samples possessing an orthorhombic phase without the presence of any impurity phases. Among the fabricated thin films, the SnS thin film annealed at a temperature of 350 °C reveals the highest crystallite size. The Raman results showed the vibrational modes of SnS films and with the increase in growth temperature, the peaks are slightly shifted towards the lower wavelength region. Morphological results show that the SnS thin films exhibit a uniform morphology of 2-D petal-like morphology with different sizes. The UV-vis spectroscopic study shows the decrease in the bandgap value of the samples with the increase in annealing temperature. The photo sensing properties of the fabricated samples show the SnS sample annealed at 400 °C has a higherresponsivityvalueof6.40×10⁻²AW⁻¹,externalquantumefficiency(EQE)valueof14.9%, and the detectivity value of 6.05 × 10⁹ Jones. Finally, the transient photo response results suggest that the SnS annealed at 350 °C shows a rise and fall time of 1.5 and 2.5 s compared to the othersampleswhichwouldbebettersuitedforphotodetectorapplications. The electrical conductivity and photo-conductivity of the films increase by more than two orders with increase of film thickness from 170 nm to 915 nm. Hall Effect measurements confirm the p-type nature of the as-prepared SnS thin films.

A comparative study of photoelectric performance of Ga2O3 solar-blind photodetectors with symmetric and asymmetric electrodes

Gallium oxide (Ga2O3) thin films were deposited on quartz substrates under different oxygen flow ratios (O2/[O2+Ar], 0%−5%) by radio-frequency (RF) magnetron sputtering technique followed by thermal annealing at 900 °C for 2 h under N2 atmosphere. The influence of the oxygen concentration on the structural and optical characteristics of Ga2O3 thin films was investigated. The Ga2O3 thin films prepared under an oxygen flow ratio of 1% exhibited optimum crystal quality and the lowest oxygen vacancy (VO) concentration according to X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and photoluminescence (PL) investigations. All as-prepared polycrystalline Ga2O3 thin film samples exhibited high average transmittance above 87.8% in the visible spectral region and the 0% and 1% thin films had optical bandgap energies of about 4.9 eV. We also fabricated metal−semiconductor−metal (MSM) photodetectors based on 1% Ga2O3 thin films by employing pairs of Ni−Ni symmetrical or Ni−Al asymmetrical interdigitated electrodes. The Ni−Ni photodetector had a high on/off current ratio (Ion/Ioff > 105), a responsivity of 88.7 mA/W, and specific detectivity of 4.81×1010 J at 10 V bias under ultraviolet light-C (UVC) illumination, and the Ni−Al photodetector exhibited self-driven behavior along with short rise and fall times (0.45 s and 0.69 s, respectively).

Effect of variable thickness on liquefied petroleum gas sensing response of ZnO thin films

In the present study, transparent and crack free ZnO thin films were casted on silicon substrate by using spin coating technique to analyze the effect of variable thickness. The crystallographic structure of as-prepared thin films were characterized using X-ray diffraction (XRD) technique and it was revealed that all the samples exhibit hexagonal wurtzite-type crystal symmetry with Surface morphology and granular microstructure illustrated uniform distribution and well-interlinking of grains and film thickness of ZnO@1, ZnO@2, and ZnO@3 samples was found to be 80 nm, 155 nm, and 230 nm, respectively. The effect of liquefied petroleum gas (LPG) purging on varying thickness of ZnO thin films were carried out lab customized gas calibrated testing apparatus. The maximum gas sensing response was observed for the sample ZnO@2 with thickness 155 nm at an operating temperature of 180 °C.

Tailoring of the Structural, Optical, and Electrical Characteristics of Sol-Gel-Derived Magnesium-Zinc-Oxide Wide-Bandgap Semiconductor Thin Films via Gallium Doping.

The Ga-doped Mg0.2Zn0.8O (GMZO) transparent semiconductor thin films were prepared using the sol-gel and spin-coating deposition technique. Changes in the microstructural features, optical parameters, and electrical characteristics of sol-gel-synthesized Mg0.2Zn0.8O (MZO) thin films affected by the amount of Ga dopants (0-5 at%) were studied. The results of grazing incidence X-ray diffraction (GIXRD) examination showed that all as-prepared MZO-based thin films had a wurtzite-type structure and hexagonal phase, and the incorporation of Ga ions into the MZO nanocrystals refined the microstructure and reduced the average crystallite size and flatness of surface roughness. Each glass/oxide thin film sample exhibited a higher average transmittance than 91.5% and a lower average reflectance than 9.1% in the visible range spectrum. Experimental results revealed that the optical bandgap energy of the GMZO thin films was slightly higher than that of the MZO thin film; the Urbach energy became wider with increasing Ga doping level. It was found that the 2 at% and 3 at% Ga-doped MZO thin films had better electrical properties than the undoped and 5 at% Ga-doped MZO thin films.

Composition-dependent crystal structure, dielectric properties and temperature stability of the (1-x)CaZrO3-xSrTiO3 paraelectric thin films

The (1-x)CaZrO3-xSrTiO3 solid-solution thin films were prepared with different compositions from x = 0.05 to x = 0.45. The as-prepared thin films experienced such a crystal structure transition process from orthorhombic to pseudo cubic and then to cubic as the x increases. Among the as-prepared films, the smallest expansion ratio of lattice volume with temperatures, and the highest temperature stability of dielectric response and leakage current density at high voltage were found in the x = 0.3 thin films. The relationship between the tolerance factor and the temperature stability was analyzed from the crystal structure dependence of the absorbed thermal energy distribution. Finally, the x = 0.3 thin films were developed into thin-film capacitors, which showed high DC resistance and uniform capacitance properties. The result also shows that high temperature stability of the perovskite-type paraelectric thin films could be realized through the phase boundary design, and further improved by optimizing the temperature stability of the lattice structure.

An artificial dead-layer to protect the ferroelectric thin films from electron injection

For ferroelectric (FE) thin films, working in the harsh environment of a high electric field (E) or high temperature (T) remains a great challenge. As a post-treatment approach, dead-layer engineering exhibits a certain generality and could improve the dielectric strength (Eb) via depositing a specially designed artificial “dead-layer” on most of the as-prepared FE thin films. However, physical essence of the artificial dead-layer needs further analysis. Great challenges are the abundant and complicated conduction mechanisms in the FE-based thin films and the lack of adequate research on modulating these conduction mechanisms by this artificial dead-layer. As a vital part of the conduction mechanisms, electron injection can be easily triggered under E and thermal excitation that almost appeared in all FE-based thin films. Here, the ultrathin artificial dead-layer of Ca0.2Zr0.8O1.8 (CSZ) was used to restrain multiple electron injection in low-Eb LaNiO3/Ba0.58Sr0.42TiO3 (LNO/BST) FE thin films, including thermal emission and tunneling effect, under a high E and T, even under an opposite E. It was found that the suppressing effect on the multiple electron-injection mechanisms via the artificial dead-layer mainly comes from its wall-like energy-barrier structure, which is composed of two opposite and high interface energy barriers (BST/CSZ: 2.95 eV, CSZ/Au: 3.92 eV) and a wide depletion layer. The generality of protecting the ferroelectric thin films from electron injection via the artificial dead-layer was discussed.

Effect of annealing temperatures on the structural, optical, and electrical properties of Cu2Sb thin films

The Cu2Sb thin films were successfully synthesized by the chemical bath deposition method to form an as-prepared thin film and annealed at 300, 400, and 500 °C. The X-ray diffraction results of the Cu2Sb thin films were found with the CuSb2O6 phase for the as-prepared, 300 °C, and 400 °C films and exhibited crystallinity. Then, the thin film became amorphous after annealing at 500 °C. Field-emission scanning electron microscopy (FESEM) was used to examine surface morphologies. Linear optical, dispersion, optoelectrical, and nonlinear optical properties were evaluated. Furthermore, the energy bandgap (Eg), Urbach energy (Eu), and electrical properties were determined. It can be suggested that the optimum annealing temperature for our synthesized mixed-phase thin films should not exceed 400 °C. Finally, the majority of the carriers in the Cu2Sb thin film exhibited p-type conductivity. Thus, the distinctive parameters investigated for our thin-film materials show a further promising optical system in optoelectronic and nonlinear optical devices and a new absorber layer for solar cell applications.

Post-annealing of hematite films: The changes in surface chemistry, lattice dynamics, and photoelectrochemical properties

This research deals with tracking changes in the surface chemistry and lattice dynamics of α-Fe2O3 (hematite) thin films upon post-annealing and its correlation with the photoelectrochemical (PEC) properties. The hematite thin films were prepared by the electric field-assisted liquid phase deposition (EA-LPD) method. The as-prepared thin films were heated at three different heating rates: 10, 22, and 80 ℃/min to 350 ℃; and kept for 15 min and then quenched in the air. The structural characterizations showed that the post-annealing process induces oxygen vacancies and the formation of hydroxyl groups on the surface. The extent of the changes in the surface chemistry strongly depends on the heating rate. Such changes in the surface chemistry possess a synergic effect on the PEC properties. Here the post-annealed photoanodes with an 80 ℃/min heating rate showed significantly improved PEC activity and exhibited a maximum photocurrent density ⁓ 0.73 mA.cm−2 at 1.23 V vs. RHE, 2.5 times higher than that for pristine sample. The lattice dynamics investigations suggested that the as-prepared photoanodes are composed of α-FeOOH. Upon post-annealing, α-FeOOH was partially converted into a proto-hematite intermediate phase. The longitudinal optical (LO) modes for the post-annealed samples enhanced as an indicator for the development of lattice disorder, as the results of induced oxygen vacancy defects and hydroxyl groups on the surface.