CuFeO2 Thin Films Research Articles

CuFeO2/Cu Photoelectrode Modified with SrTiO3 Perovskite for Efficient Hydrogen Generation from Sanitation Water

We prepared a perovskite material, copper-doped strontium titanate (Cu-SrTiO3), using the chemical bath deposition method and cast it on a CuFeO2/Cu photoelectrode to generate hydrogen from sanitation water splitting. This preparation method considers a simple mass product and does not depend on complex techniques. The prepared perovskite materials had a compact nano-/microstructure. Both CuFeO2 and Cu-SrTiO3/CuFeO2 exhibited excellent optical properties, with bandgap values of 1.4 and 1.26 eV, respectively. Here, the prepared CuFeO2 and Cu-SrTiO3 thin films are used as photoelectrodes for hydrogen generation, and their current-voltage relationship is analyzed under various conditions, such as different light intensities, wavelengths, and temperatures. This approach is promising for using wastewater as a source of hydrogen gas without requiring any additional electrolyte, making it a dual-purpose approach for both hydrogen generation and wastewater treatment. Through the electrochemical study, increasing the light intensity from 25 to 100 mW.cm-2 resulted in a corresponding increase in the produced J ph values from -1.02 to -1.292 mA.cm-2. Similarly, the J ph values increased from -1.25 to -1.91 mA.cm-2 as the temperature increased from 30 to 70°C. We also calculated all thermodynamic parameters, the quantum efficiency (QE), and incident photon to current conversion efficiency (IPCE). For the Cu-SrTiO3/CuFeO2/Cu photoelectrode, the activation energy ( E a ) value was 14.14 kJ mol-1, while the Δ H ∗ and Δ S ∗ values were 11.46 kJ·mol-1 and 34.9 kJ-1·mol-1, respectively. Additionally, the IPCE value was 3.31%. The prepared photoelectrode showed high stability and low cost, making it a potential candidate for industrial applications.

Microstructural and transport properties of Mg doped CuFeO2 thin films: A promising material for high accuracy miniaturized temperature sensors based on the Seebeck effect

Delafossite type Mg doped CuFeO2 thin films have been deposited on fused silica by radio-frequency magnetron sputtering. As-deposited 300 nm thick films have been obtained and post-annealed between 350 and 750 °C under primary vacuum. The delafossite structure appears for the samples annealed above 550 °C. The microstructural analysis showed the presence of cracks and an inhomogeneous distribution of the dopant in the thickness. Only the sample annealed at 700 °C showed CuFeO2 stable phases, lower impurities amount, a high and constant Seebeck coefficient (+416 ±3 μV K−1) and good electrical conductivity (1.08 S cm−1 at 25 °C). High accuracy temperature sensors based on the Seebeck effect not only need high Seebeck coefficient without any drift with the temperature, but also a sufficient electrical conductivity and high phase stability. Thanks to its properties and also its low thermal conductivity (4.8 ±0.6 W m−1K−1 at 25 °C) due to the thin film configuration and the polaronic transport, the Mg doped CuFeO2 thin film annealed at 700 °C was found to be a very good p-type material for high accuracy miniaturized temperature measurement sensors based on the Seebeck effect in the medium temperature range.

Thickness-dependent microstructural properties of heteroepitaxial (00.1) CuFeO2 thin films on (00.1) sapphire by pulsed laser deposition

Typical low-temperature frustrated triangular antiferromagnet CuFeO2 is attracting extensive interest due to its narrow-band-gap semiconductor properties. High-quality and impurity-free CuFeO2 epitaxial thin films would be preferable for fundamental studies on the physical and chemical properties. However, the heteroepitaxial growth of impurity-free CuFeO2 thin films has been a significant challenge due to its narrow formation window in the Cu–Fe–O system as well as the metastable nature of the Cu1+ cations. This work reports for the first time the fabrication and characterization of high-quality and impurity-free (00.1)-oriented CuFeO2 epitaxial thin films grown with relaxed interfaces on (00.1) sapphire substrates by pulsed laser deposition. Below the critical thickness of around 16 nm, the films exhibit a rhombohedral structure with relatively good crystalline quality where all Cu ions appear to be in the 1+ oxidation state, while the rocking curves display a narrow full width at half maximum of about 0.11°. Increasing the thickness, the (111)-oriented γ-Fe2O3 nanograins grow embedded in the CuFeO2 films. Here, an excess Fe3+-assisted growth mechanism is proposed to explain the iron oxide grain formation. This study provides insight into the heteroepitaxial growth of relaxed CuFeO2 thin films with high purity and crystalline quality as an ideal sample design to characterize the fundamental properties of this material in view of potential device applications.

Synthesis of conducting single-phase CuFeO2 thin films by spray pyrolysis technique

P-type conducting CuFeO2 thin films were synthesized on fused silica substrates by a chemical deposition spray pyrolysis technique followed by a heat treatment in a nitrogen (N2) atmosphere for 120 min at annealing temperature 850 °C. The effects of spray solution quantity on the structural, morphological, optical and electrical were studied. The post-annealed CuFeO2 films are single-phase, polycrystalline with rhombohedral structure and (012) preferred orientation. The optical transmittances varied from 70 to 30% in the VIS-IR spectrum and the direct band gap value was decreased from 1.88 eV to 1.48 eV with thickness increase. The electrical conductivity of CuFeO2 thin films increased from 9 Х 10-5 S/cm to 1.5 Х 10-1 S/cm and the carrier concentration increased from 5.5 Х 1014 cm-3 to 7.4 Х 1018 cm-3 while the film’s crystalline quality improved.

Facile Two‐Step Synthesis of Delafossite CuFeO2 Photocathodes by Ultrasonic Spray Pyrolysis and Hybrid Microwave Annealing

AbstractDelafossite CuFeO2 photocathodes have recently attracted attention for water splitting due to their suitable band gap (∼1.5 eV) and high stability in aqueous media. The preparation of CuFeO2 usually requires long and energy‐intense treatments in an inert atmosphere for the full conversion of spinel CuFe2O4 to delafossite CuFeO2. Herein, we report the preparation and characterization of highly uniform and stable CuFeO2 thin films obtained via a combination of inexpensive ultrasonic spray pyrolysis followed by a short hybrid microwave treatment (∼4 min). The resulting films show good stability in alkaline media and produce a photocurrent of ∼650 μA/cm2 under 1.5 AM simulated sunlight and with oxygen bubbling. The effect of the rapid transformation from the spinel to the delafossite phase induced by hybrid microwave annealing was investigated with synchrotron‐based X‐ray absorption spectroscopy (XAS) and X‐ray photoelectron spectroscopy (XPS).

Communication—Lithium-Doped CuFeO2 Thin Film Electrodes for Photoelectrochemical Reduction of Carbon Dioxide to Methanol

Li-doped CuFeO2 thin films are fabricated by electrodeposition and their photoelectrocatalytic performance for CO2 reduction is examined. CuFeO2, LiFeO2, Cu2O and CuO are observed in Li-doped CuFeO2 thin films. At Li-doped CuFeO2 thin film photocathodes, CO2 can be reduced to methanol with high selectivity. At the overpotential of 110 mV, methanol concentration is up to 153.75 μM and faradaic efficiency of methanol is as high as 96.7% in 1.5 h at the photocathode electrodeposited at −1.00 V. Since adsorbed CO has low mobility on LiFeO2 surface, CO coupling on the electrode surface is inhibited, thus no C2+ products are formed.

Delafossite CuFeO2 thin films via aerosol assisted CVD: Synthesis and characterization

The optimal synthesis conditions for the growth of delafossite CuFeO2 thin films via aerosol assisted CVD were studied. An iterative synthesis process was followed until the average atomic ratio of Cu/Fe in the synthesized thin films was close to 1. TiO2 coated borosilicate was initially used as substrate. Complete characterization of selected samples revealed the formation of polycrystalline CuFeO2 in a rhombohedral phase. Small amounts of tenorite (CuO) were also observed. Further investigation of the growth of CuFeO2 was achieved using ZnO coated borosilicate substrates. The synthesized thin films were in all cases homogeneous and had an average thickness of 45 ± 3 nm and 41 ± 3 nm when grown onto TiO2 or ZnO, respectively. Outcomes indicated an almost negligible influence of the buffer layer on the microstructure and optical properties of CuFeO2. Therefore, the direct synthesis via AACVD technique of this bimetal and visible light absorbing oxide was verified.

Influences of annealing temperature on sprayed CuFeO2 thin films

Abstract Delafossite CuFeO2 thin films were successfully prepared onto quartz substrates using simple spray pyrolysis technique. Post annealing under nitrogen atmosphere for 2 h was necessary to form delafossite CuFeO2 phase. The effect of alteration in annealing temperature (TA) 800, 850 and 900 °C was study on structural, morphology and optical properties. The XRD results for thin film annealed at TA = 850 °C show single phase CuFeO2 with rhombohedral crystal system and R 3 ¯ m space group with preferred orientation along (0 1 2). The prepared copper iron oxide thin films have an optical transmission ranged ∼40% in the visible region. The optical direct optical band gap of the prepared thin films was ranged ∼2.9 eV.

Delafossite CuFeO2 Photocathodes Grown by Direct Liquid Injection Chemical Vapor Deposition for Efficient Photoelectrochemical Water Reduction

Delafossite CuFeO2 (CFO) thin film photocathodes are fabricated by a direct liquid injection chemical vapor deposition (DLI-CVD) technique. CFO photocathodes grown on fluorine-doped tin oxide (FTO) substrates are investigated for photoelectrochemical (PEC) water reduction for hydrogen generation. CFO photocathode fabricated at 550°C with an optimal film thickness of ∼150 nm exhibit a maximum photocurrent density of ∼0.71 mA cm−2 at 0.6 V vs. RHE under a standard 1 SUN illumination (AM 1.5G, 100 mW cm−2) in 1 M NaOH electrolyte (pH 13.5) under ambient conditions. The performance is further improved in a O2-saturated solution yielding higher photocurrent density as high as ∼1.3 mA cm−2 at 0.6 V vs. RHE, with O2 serving as a sacrificial agent. CFO film absorbs over the entire visible wavelength range and delivers maximum incident photon-to-current conversion efficiency (IPCE) values of ∼18% and ∼28% at 0.6 V vs. RHE (at 400 nm) in unpurged and O2-saturated NaOH solution, respectively. The enhancement in PEC and IPCE with O2 purging is due to combined effects of water reduction and oxygen reduction reaction (ORR). Mott-Schottky study demonstrate an improvement in onset potential with higher carrier density for the CFO films as compared to previous reports. Overall, the enhanced PEC performance is attributed to the high-quality growth of crystalline CFO thin films on FTO substrates by DLI-CVD technique, exhibiting good stability in different electro-chemical environments.

Optical Study of Cuprous Oxide and Ferric Oxide Based Materials for Applications in Low Cost Solar Cells

One of the interesting forms of cuprous oxide and ferric oxide based materials is CuFeO2 which can be a delafossite-type compound and is a well known p-type semiconductor. This compound makes up an interesting family of materials for technological applications. CuFeO2 thin films recently gained renewed interest for potential applications in solar cell devices especially as absorption layers. One of the interesting facts is that CuFeO2 is made from cheap materials such as copper and iron. In this study, CuFeO2 thin films are intentionally deposited on corning glass and silicon substrates by the radio-frequency and direct current sputtering method with complicated and well developed co-sputtering recipes. The deposition was performed at room temperature which leads to an amorphous phase with extremely low roughness and high density. The films also were annealed at 500°C in 5% H2 in Ar for the passivation. A detailed optical study was performed on these thin films by spectroscopic ellipsometry and by ultra-violet visible near infrared spectroscopy. Depending on sputtering conditions, the direct band gap was extrapolated to be from 1.96 eV to 2.2 eV and 2.92 eV to 2.96 eV and the indirect band gap is about 1.22 eV to 1.42 eV. A good electrical conduction is also observed which is suitable␣for solar cell applications. In future more study on the structural properties will be carried out in order to fully understand these materials.

Structure, surface morphology and optical properties of post-annealed delafossite CuFeO2 thin films

Delafossite CuFeO2 (TCO) thin films have been successfully deposited onto quartz substrate by low-cost chemical spray pyrolysis at 450°C and post-annealed at 800°C for 2h. The XRD results for as deposited and post-annealed films show polycrystalline structure with Hexagonal (Rhombohedral) phase. The CuFe2O4 phase appeared as the film deposited and post-annealed. The preferred orientation was along (012) plane and the crystallite size increased from 20.89nm to 22.588nm after annealing process. The surface morphology of CuFeO2 thin film seems relatively smooth and plated regularly; the EDX spectra confirm the stoichiometry of the prepared films. The average roughness values obtained from AFM measurements have been decreases from and from (19.7 to 14.4nm) respectively after annealing process. The optical transmission of these films has been studied in the wavelength range 400–800nm and found to be more than 62%. The direct optical band gap of the CuFeO2 films was 2.66eV and decreased to 2.63eV after annealing process. The Urbach's energy was found to be increase from 0.247eV for as deposited film at 450°C to 0.2857eV after annealing process.

Influences of post-annealing temperature on the structural and electrical properties of mixed oxides (CuFeO2 and CuFe2O4) thin films prepared by spray pyrolysis technique

P-type mixed oxides (CuFeO2 and CuFe2O4) transparent conducting thin films have been successfully deposited on p-type Si (111) substrates at 450°C by spray pyrolysis deposition (SPD) and annealed at 800°C for 2h. The crystal structure, surface morphology and electrical property have been investigated. It is observed that the CuFeO2 and CuFe2O4 thin films as deposited and annealed, have polycrystalline hexagonal structure and the crystallite size increases by annealing processes. The electrical property of the Ni/CuFeO2/Si Metal–Semiconductor–Metal (MSM) photo detectors was investigated using the current–voltage (I–V) measurements. The barrier heights ϕΒ of Ni/CuFeO2/Ni MSM thin films of as deposited and annealed on Si substrates were calculated and its values are 0.478, 0.345eV, respectively with an applied bias voltage of 3V.

Characterization of amorphous p-type transparent CuFeO2 thin films prepared by radio frequency magnetron sputtering method at room temperature

Amorphous CuFeO2 thin films were prepared on Al2O3 (001) substrates by radio-frequency magnetron sputtering method at room temperature. Various oxygen partial pressures PO in the sputtering gas ambient (9.1%, 11.1% and 13.0%) were used in the deposition experiments. X-ray photoelectron spectroscopy analysis convinced the chemical state of Cu+ and Fe3+ in the films, and the chemical compositions of the thin films are close to the stoichiometry of CuFeO2. The content of oxygen composition increased systematically with increasing PO. From the optical transmission spectra, only one absorption edge could be observed during 200–1200nm for all the films with a band gap around 3.25eV. This result is different from the polycrystalline CuFeO2 films which have 3 absorption edges in the range of 200–1200nm. This interesting result indicates that amorphous CuFeO2 thin films have much different electronic energy band with polycrystalline ones. The resistivity of the films first decreases and then increases with increasing PO. The minimum resistivity of 4.2Ωcm with optical transmittance of 47% at 600nm is obtained for the film deposited in PO=11.1%.

Delafossite CuFeO2 thin films electrochemically grown from a DMSO based solution

This study shows the results obtained in the direct electrodeposition of CuFeO2 thin films from a DMSO based solution. First, a detailed electrochemical study was carried out in order to determinate the best condition for the CuFeO2 electrodeposition. The films were obtained potentiostatically from a 0.01M CuCl2+0.005M Fe(ClO4)3+0.1M LiClO4 solution in the presence of molecular oxygen at 50°C onto FTO/glass substrates. In all cases, the time of electrodeposition was 1000s. The grown films presented a yellow-reddish color and exhibit an homogeneous aspect. Analyses of composition carried out through EDS, shown that a stoichiometric composition (atomic relation Cu:Fe=1:1) is obtained at a potential of –0.6V. However, as-grown films analyzed through XRD experiences did not evidence the presence of CuFeO2 compound presumably because it is amorphous. An annealing treatment at 650° C for 30minutes in an argon atmosphere was necessary to transform the solid phase of the as grown films in crystalline CuFeO2. Furthermore, the presence of CuFeO2 has been confirmed through XPS analyses. UV-vis analyzes shown a ladder-like appearance due to the presence of several absorption edges from the IR to the UV spectrum region. The most clearly determined were a direct-like edge at the IR: Eg(IR)=1.64eV. However, several absorption edges in the visible (Egdir(vis)=2.35eV, and Egind(vis)=2.03eV) and UV beginning (Egind(UV)=3.37eV) spectrum region also were observed.

The effect of oxygen partial pressure on the properties of CuFeO2 thin films prepared by RF sputtering

Single phase CuFeO2 thin films with c-axis orientation were prepared by RF sputtering method at room temperature under oxygen partial pressure PO from 5% to 15% and then post annealed at 900 °C for 4 h in flowing N2 atmosphere. The grains of the films show layer-by-layer structure and are closely gathered and densely arranged. The Eg at near-UV region shows a redshift with increment of PO which may due to the impurity levels induced by oxygen interstitials. The resistivity of the films first decreased and then increased with increasing PO. The minimum resistivity of 0.32 Ωcm at room temperature for the sample 10%-PA is nearly two-order magnitude smaller than that (18 Ωcm) for 5%-PA. Oxygen interstitial doping can effectively enhance the conductivity of CuFeO2 thin films.

Structural and magnetic properties of epitaxial delafossite CuFeO2 thin films grown by pulsed laser deposition

Growth of pure phase delafossite CuFeO2 thin films on Al2O3 (00.1) substrates by pulsed laser deposition was systematically investigated as a function of growth temperature and oxygen pressure. X-ray diffraction, transmission electron microscopy, Raman scattering, and x-ray absorption spectroscopy confirmed the existence of the delafossite phase. Infrared reflectivity spectra determined a band edge at 1.15 eV, in agreement with the bulk delafossite data. Magnetization measurements on CuFeO2 films demonstrated a phase transition at TC ≈ 15 ± 1 K, which agrees with the first antiferromagnetic transition at 14 K in the bulk CuFeO2. Low temperature magnetic phase is best described by commensurate, weak ferromagnetic spin ordering along the c-axis.

The morphologies and optoelectronic properties of delafossite CuFeO2 thin films prepared by PEG assisted sol–gel method

Single phase delafossite CuFeO2 thin films were synthesized by a simple sol–gel method. The influence of polyethylene glycol (PEG) on the morphology and optoelectronic properties of the films was studied by addition of 1.0 g PEG in 10 ml precursor solution. The crystal sizes of the derived CuFeO2 films with and without addition of PEG were 49 nm, but the sample with addition of PEG (labeled as CFO-PEG) showed weaker c-axis orientation growth. The sample without addition of PEG (labeled as CFO) showed a compact surface without detectable pores and had a thickness around 50 nm. However, the sample CFO-PEG exhibited a porous surface with worm-like grains in nanometric scale and had a thickness around 310 nm. Enhanced absorbance in UV–vis region was observed for the sample CFO-PEG which might ascribe to both the thickness and porous surface. The optical direct bandgaps at near-UV were estimated to be ~3.0 and 3.38 eV for the sample CFO-PEG and CFO, respectively. Though the porous surface of CFO-PEG has improved the absorbance in UV–vis region, the resistivity has also been increased due to the homogeneous distribution of interspaces between the worm-like grains, which makes the incident photon to current efficiency of CFO-PEG lower than that of CFO.

Delafossite–CuFeO2 thin films prepared by atmospheric pressure plasma annealing

This study reports the use of atmospheric pressure plasma annealing to prepare the delafossite–CuFeO2 thin films. The pure delafossite–CuFeO2 phase was formed using atmospheric pressure plasma annealing at 5% O2, which had a large agglomerate morphology. The Cu-2p3/2 and the Fe-2p3/2 photoelectron peaks in the delafossite–CuFeO2 thin films were centered at 932.8eV and 710.2eV, indicating the chemical state of Cu and Fe were +1 and +3. The delafossite–CuFeO2 thin films had an optical bandgap of 3.1eV. The electrical conductivity of the thin films was 0.67±0.02Scm−1 with the carrier concentration of (2.56±1.24)×1018cm−3. Hence, an atmospheric pressure plasma annealing offers an effective tool and a feasible method for preparing the delafossite–CuFeO2 thin films.

Influences of post-annealing conditions on the formation of delafossite–CuFeO2 thin films

In this study, post-annealing conditions, the partial oxygen pressures (pO2) and temperatures, influence on the formation of delafossite–CuFeO2 thin films is studied. The sol–gel derived films were annealed at 500°C in air and post-annealed at 500–850°C in pO2=10−2 to pO2=5×10−5atm. The CuO and CuFe2O4 phases appeared when the sol–gel derived films were post-annealed below 800°C for 2h in pO2=10−2atm, 650°C for 2h in pO2=10−3atm, and 550°C for 2h in pO2=5×10−5atm. Pure delafossite–CuFeO2 phase was detected as specimens were post-annealed above 800°C for 12h in pO2=10−2atm, 650°C for 12h in pO2=10−3atm, and 550°C for 12h in pO2=5×10−5atm. The surface of post-annealed thin films exhibited a nanoparticle-like morphology when the specimens exhibited CuO and CuFe2O4 phases. However, the surface revealed granular features caused by the formation of the delafossite–CuFeO2 phase. The formation of the delafossite–CuFeO2 phase, which resulted from the chemical reaction of the CuO and CuFe2O4 phases in the post-annealing process, is consistent with thermodynamics. The optical bandgaps of delafossite–CuFeO2 thin films prepared using post-annealing ranged between 3.1 and 3.2eV. The electrical conductivities of delafossite–CuFeO2 thin films were (1.62–6.37)×10−1Scm−1 and the carrier concentrations were (1.52–8.84)×1017cm−3. The pO2 and temperatures in the post-annealing process played primary roles in the formation of delafossite–CuFeO2 thin films in this study.

Structure and optoelectronic properties of Mg-doped CuFeO2 thin films prepared by sol–gel method

CuFe1−xMgxO2 (0 ⩽ x ⩽ 0.05) thin films were synthesized by a simple sol–gel method. The structural and optoelectronic properties of the films were studied. Single phase CuFeO2 thin films with c-axis orientation were obtained on sapphire substrates and secondary phase was not detected by XRD measurements in the studied doping range. The resistivity of the films first decreased and then increased with increasing x. The minimum resistivity was observed as 3.23 Ω cm at room temperature for the doping amount of x = 0.02. The transmittance of all the films was around 37% in the wavelength of 600 nm and three absorb edges were observed in the wavelength range of 200–1100 nm. The optical direct bandgaps of CuFeO2 were estimated to be ∼3.38 eV, 2.04 eV and 1 eV, respectively.