Fluorine-doped Tin Oxide Glass Research Articles

Crystallization management and defect passivation via additive engineering for efficient and stable carbon-based CsPbI2Br perovskite solar cells

Inorganic CsPbI2Br perovskite solar cells (PSCs) have attracted enormous interest due to their excellent thermal stability and immense potential for application in tandem solar cells. However, the rambunctious crystallization and poor film quality of inorganic CsPbI2Br perovskite are the main factors limiting their performance improvement. Herein, dibenzoylmethane (DBM), containing electron-rich and dual CO functional groups, as an efficient precursor additive, is first introduced to regulate the crystallization of CsPbI2Br perovskite while passivating its associated defects. After DBM modification, nucleation is accelerated and the crystallinity of CsPbI2Br film is improved, thus promoting the phase stability. Additionally, DBM can passivate uncoordinated Pb2+ defects, thereby significantly reducing non-radiative recombination, and can also decrease the energy level difference between CsPbI2Br/Carbon. As a result, when DBM additive is incorporated in the CsPbI2Br, the power conversion efficiency (PCE) of the device with the fluorine doped tin oxide glass (FTO)/SnO2/CsPbI2Br/Carbon structure is significantly improved to 13.46 %, an increase of nearly 13 % compared to the control device (11.72 %). The unpackaged devices exhibit excellent air and thermal stability, still remaining over 90 % of its initial PCE after aging for 500 h under ambient conditions, or continuous heating at 65 °C in glovebox for 500 h, respectively.

Recent advancement in the synthesis of zinc sulphide thin films

This research work has given an overview on the preparation of zinc sulphide thin films on different types of substrates (quartz, soda lime glass, silicon, polyethylene terephthalate film, fluorine-doped tin oxide glass, indium tin oxide coated glass, silica, stainless steel, mica, microscopy glass slide and (100) GaAs substrate) using different types of deposition methods. XRD data showed information about the different phases (cubic, wurtzite and hexagonal) presented in the obtained as-deposited films and annealed samples. Based on optical studies, the band gap was observed in the range of 2.6 eV to 3.87 eV with good absorption coefficient value (105 to 106 cm-1). Experimental results indicated these films could be used in solar cell applications, anti-reflection coating, photocatalysts, photo detector devices and laser-oriented coating applications.

Precursor-concentration-controlled Morphology of TiO2 Nanorod/Nanoflower Films for Enhanced Photoelectrochemical Water Splitting and Investigating Their Growth Mechanism

Titanium dioxide (TiO2) has been considered as one of the most promising photocatalysts for photoelectrochemical (PEC) water splitting. Therefore, numerous efforts have been devoted to improving its PEC water splitting performance. In this study, TiO2 nanorod/nanoflower (NRF) films with controlled morphology were synthesized on fluorine-doped tin oxide (FTO) glass substrates by following a facile one-step hydrothermal method. The TiO2 NRF films were characterized by X-ray diffraction (XRD), Raman spectroscopy, field emission scanning electron microscopy (FE-SEM), atomic force microscopy (AFM), energy-dispersive X-ray spectrometer (EDS), and ultraviolet-visible (UV-Vis) spectrophotometer. FE-SEM showed that the TiO2 films are composed of a simultaneous growth of a primary layer of TiO2 nanorod arrays (NRAs) and a second layer of TiO2 nanoflowers (NFs). The proposed growth mechanism highlighted the influence of precursor concentration on nucleation sites, affecting the preferred crystallographic plane growth of rutile TiO2 and nanorod alignment on the FTO substrate. Intriguingly, TiO2 NRF films prepared with 1.0 mL of titanium butoxide exhibited a maximum photocurrent density of 3.58 mA.cm−2 at 1.23 V versus (vs.) the reversible hydrogen electrode (RHE), along with a maximum photoconversion efficiency of 0.69%. The enhanced photocurrent density and photoconversion efficiency were attributed to the optimum thickness in the range of 4.52-7.31 µm, which caused the film to be formed with a unique morphology of the primary layer with well-vertically aligned nanorods and the second layer of flowers consisting of numerous rods stacked on top of one another. This study demonstrates the importance of designing semiconductors with 1D nanorod/3D nanoflower structures as high-performance photoelectrodes for PEC water splitting. Copyright © 2024 by Authors, Published by BCREC Group. This is an open access article under the CC BY-SA License (https://creativecommons.org/licenses/by-sa/4.0).

NiO thickness measurement using a rectangular-type Sagnac interferometer as the material transport layer in a perovskite solar cell

This work aims to utilize a phase-shifting technique in a rectangular-type Sagnac interferometer (RTSI) to measure the thickness of a thin film of nickel (II) oxide (NiO) in an electron transport layer (ETL) in perovskite solar cell preparation. The NiO layer is deposited on a fluorine-doped tin oxide (FTO) glass substrate. In the RTSI setup, the signal output from the interferometer is divided into the reference and testing arms using a nonpolarizing beam splitter (NPBS). The balanced photodetectors then detect the signal, with the FTO/NiO layer placed in the testing arm and pure FTO in the reference arm. By analyzing the signal intensities at polarization settings of 0° to 180°, the phase shift and thickness of the NiO layer can be determined. The thickness values of FTO and NiO films obtained through three different phase-shifting algorithms of three-, four-, and five-steps are calculated. The obtained NiO thickness values are validated against scanning electron microscopy (SEM). Finally, by considering the NiO thickness value that exhibits the lowest percentage error compared to one from SEM, it is confirmed that the three-step algorithm is the most suitable scheme for obtaining intensities at 0°, 45°, and 90°. Therefore, the proposed setup shows promise as a replacement for SEM in thickness measurements.

Effect of cerium oxide nanorods growth time on the electrochromic properties of WO3/CeO2 hybrid films

In this study, the cerium oxide nanorods (CeO2) were grown on fluorine-doped tin oxide (FTO) glass substrates by using a hydrothermal process at growth times of 4hrs, 6hrs and 12hrs and annealed at temperature 3500C. The grown samples were characterized by using Scanning electron spectroscopy (SEM), Energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), Raman spectroscopy, and UV–Vis spectrometer. Tungsten oxide (WO3) film was deposited, on these hydrothermally grown nanorods (NRs), by using the DC sputtering process to develop WO3/CeO2 hybrid films. From the SEM and XRD analysis, CeO2 nanorods were observed at different growth times, and crystalline structures were observed respectively. The optical transmittance decreases with the increase in synthesis time, mainly due to the increase in nanorods' thickness. The diffusion coefficient of 11.24 × 10−11 cm2/s and the cathodic peak current of −10.1 mA was observed for 12-h WO3/CeO2 films and the coloration efficiency of 12 cm2/C was observed for 4-h WO3/CeO2 hybrid film.

Enhancement of Photo-Electrical Properties of CdS Thin Films: Effect of N2 Purging and N2 Annealing

The impact of N2 purging in the CdS deposition bath and subsequent N2 annealing is examined and contrasted with conventional CdS films, which were deposited without purging and annealed in ambient air. All films were fabricated using the chemical bath deposition method at a temperature of 80 °C on fluorine-doped tin oxide glass slides (FTO). N2 purged films were deposited by introducing nitrogen gas into the deposition bath throughout the CdS deposition process. Subsequently, both N2 purged and un-purged films underwent annealing at temperatures ranging from 100 to 500 °C for one hour, either in a nitrogen or ambient air environment. Photoelectrochemical (PEC) cell studies reveal that films subjected to both N2 purging and N2 annealing exhibit a notable enhancement of 37.5% and 27% in ISC (short-circuit current) and VOC (open-circuit voltage) values, accompanied by a 5% improvement in optical transmittance compared to conventional CdS thin films. The films annealed at 300 °C demonstrate the highest ISC, VOC, and VFB values, 55 μA, 0.475 V, and −675 mV, respectively. The improved optoelectrical properties in both N2-purged and N2-annealed films are attributed to their well-packed structure, enhanced interconnectivity, and a higher sulfur to cadmium ratio of 0.76 in the films.

Sol-gel immobilization of lipase enzyme on FTO modified with silica-zirconia and gold nanoparticles films for design of triglyceride impedimetric biosensor

A novel electrochemical sensing platform was created using the formation of a sequence of films, over a conductive fluorine-doped tin oxide glass (FTO), obtained using the sol-gel method and the dip coating technique. Firstly, thin films of silica-zirconia mixed oxides were deposited, in sequence an ionic silsesquioxane film stabilizing and controlling the size of gold nanoparticles (6.5 ± 2.4 nm) was settled, and after the lipase enzyme, obtained from Candida rugosa, was entrapped into a silica film maintaining its operational stability. The films were characterized by scanning electron microscopy (SEM) with EDX, UV–Vis spectroscopy and X-ray photoelectron spectroscopy (XPS). The thickness of each film on the FTO glass was evaluated by optical profilometry. The presence of the enzyme on the platform was confirmed by cyclic voltammetry with the ρ-nitrophenyl palmitate method. This platform was applied successfully as a biosensor for tributyrin (TB) determination by electrochemical impedance spectroscopy EIS, showing low limit of detection (LD) of 1.86 μmol L−1 and high sensitivity of 5.37 μΩ μmol−1 L. The biosensor presented a low KMapp value of 22.69 μmol L− 1 and the Vmaxapp value of 85.57 μmol L−1 min−1 suggesting that the enzyme, immobilized with this method retained activity promoting a fast enzymatic reaction with the TB.

Solar to hydrogen conversion by a 25 cm2-photoelectrochemical cell with upscaled components

An innovative tandem photoelectrochemical cell with a scaled-up active area from 0.25 to 25 cm2 based on low-cost and non-critical raw materials was employed to produce green hydrogen by water splitting. It uses a photoanode/membrane/photocathode tandem cell configuration in which a hematite-based photoanode is layered on a fluorine-doped tin oxide glass for the oxygen evolution reaction, CuO is deposited on a hydrophobic gas diffusion layer as the photocathode for the hydrogen evolution reaction and an anion exchange membrane is used as the electrolyte and gas separator. The solid membrane's low hydrogen and oxygen crossover guarantees the operational stability of the tandem photoelectrochemical cell and the efficient separation of water-splitting products. Significant efforts have been focused on scaling up the cell. It has allowed obtaining, for the first time, a 25 cm2 unit cell prototype. Appropriate design approaches have been considered to optimise water/gas management, current collection, gas-tightness and clamping. The adopted architecture allowed for the reduction of the bias-potential from −1.23 V, generally employed to investigate PEC cells, up to −0.6/-0.4 V 20 h-durability tests demonstrated good resistance to corrosion, showing a constant photocurrent. Efficiency at −0.6 V was about 0.2%. Selective hydrogen production was demonstrated by mass spectrometric analysis.

Porous NiMoO4 Nanosheet Films and a Device with Ultralarge Optical Modulation for Electrochromic Energy-Storage Applications.

Reducing building energy consumption, improving aesthetics, and improving occupant privacy as well as comfort by dynamically adjusting solar radiation are important application areas for electrochromic (EC) smart windows. However, the current transition metal oxides still cannot meet the requirements of neutral coloration and large optical modulation. We report NiMoO4 nanosheet films directly grown on fluorine-doped tin oxide glasses. The as-grown NiMoO4 film not only achieves neutral coloration from transparent to dark brown but also shows an ultralarge optical modulation (86.8% at 480 nm) and excellent cycling stability (99.4% retention of maximum optical modulation after 1500 cycles). Meanwhile, an EC device demonstrating good EC performance was constructed. These results will greatly promote the research and development of binary transition metal oxides for both EC and energy-storage applications, and NiMoO4 films may be an excellent candidate to replace NiO films as ion-storage layers in complementary EC devices with WO3 films as EC layers.

One-step hydrothermal growth of porous nickel manganese layered double hydroxide nanosheet film towards efficient visible-light modulation.

Electrochromic smart windows have attracted great attention due to their dynamic regulation of the solar spectrum. NiO and MnO2 are typical anodic coloration materials and widely investigated as complementary electrodes with WO3. However, NiO and MnO2 films often cannot be bleached to complete transparency, resulting in low transmittances and low optical modulations in the short-wavelength visible region. Herein, we report a porous nickel manganese layered double hydroxide (NiMn-LDH) nanosheet film directly grown on fluorine-doped tin oxide (FTO) glass using a one-step hydrothermal method, which demonstrates a high transmittance of 80.1% at 550 nm (without deduction of FTO glass). Induced by the double-redox couples of Ni2+/Ni3+ and Mn3+/Mn4+ associated synergistic electrochromic effect, the as-grown NiMn-LDH film electrode exhibits a large optical modulation of 68.5% at 550 nm, and a large solar irradiation modulation of 59.0% in the visible region of 400-800 nm. After annealing at 450 °C for 2 h, the NiMn-LDH film can be transformed into Ni6MnO8 film with a reduced optical modulation of 30.0% at 550 nm. Furthermore, the NiMn-LDH film electrode delivers an areal capacitance of 30.8 mF cm-2 at a current density of 0.1 mA cm-2. These results suggest that the as-prepared NiMn-LDH film electrode is a promising candidate for both electrochromic and energy storage applications.

Improving the charge separation efficiency by embedding the electron transfer layer of hematite photoanode for photoelectrochemical water oxidation

The photogenerated hole transfer from the bulk to the surface is highly valued in the rational design of advanced photoanodes. However, the transfer of photogenerated electrons to the FTO (fluorine-doped tin oxide glass) substrate has a vital influence on the separation behaviors of photogenerated charges as well, which would regulate the photoelectrochemical (PEC) performance of photoanodes in turn. Herein, the W-TiO2/Ti-Fe2O3 heterojunction was fabricated for the first time by inserting W-TiO2 as the electron transfer layer between the Ti-Fe2O3 photoanode and the FTO substrate. Benefitting from the unique electron transport channel of W-TiO2, the photogenerated electrons in Ti-Fe2O3 could smoothly migrate to FTO, while photogenerated holes water oxidation kinetics is promoted and then easily participate in water oxidation reaction. As expected, W-TiO2/Ti-Fe2O3 exhibits a remarkable negative onset potential and a slight improved photocurrent density.

Construction of SrTiO3/TiO2 heterojunction by in-situ ion exchange method to enhance photochemical performance

Construction of heterojunctions has been shown to be one of the most promising methods for the preparation of highly efficient catalysts, because it is feasible and effective for the separation of electron-hole pairs, prolonging the lifetime of photogenerated electrons. In this study, TiO2 was grown on fluorine-doped tin oxide glass (FTO), and then a strontium source was added to obtain heterojunctions between SrTiO3 and TiO2 using in-situ ion exchange hydrothermal method. The in-situ grown SrTiO3/TiO2 (TST) composite has a better absorption performance, a narrower band gap, and a lower carrier recombination rate compared to the pure TiO2 sample, which improves the photoelectrochemical performance. The photocurrent density of TST-10 (hydrothermally treated for 10 h) has the largest value of 0.06 mA·cm−2, which is about three times higher than that of pure TiO2. Then, the first-principles density functional theory (DFT) calculation was used to further explore the photocatalytic mechanism of TST. The exchange correlation energy was based on the Generalised Gradient Approximation (GGA) with Perdew–Burke–Ernzerhof (PBE) functional calculation method to study the electronic and optical properties of SrTiO3/TiO2 heterostructures. The results show that the carrier transfer rate and the optical absorption coefficient of the material increase significantly after the establishment of the in-situ heterojunction. This study offers insight into the heterostructures’ interface significance on photocatalytic activity.

Nanograss-Assembled NiCo2S4 as an Efficient Platinum-Free Counter Electrode for Dye-Sensitized Solar Cell.

Dye-sensitized solar cells (DSSCs) are often viewed as the potential future of photovoltaic systems and have garnered significant attention in solar energy research. In this groundbreaking research, we introduced a novel solvothermal method to fabricate a unique "grass-like" pattern on fluorine-doped tin oxide glass (FTO), specifically designed for use as a counter electrode in dye-sensitized solar cell (DSSC) assemblies. Through rigorous structural and morphological evaluations, we ascertained the successful deposition of nickel cobalt sulfide (NCS) on the FTO surface, exhibiting the desired grass-like morphology. Electrocatalytic performance assessment of the developed NCS-1 showed results that intriguingly rivaled those of the acclaimed platinum catalyst, especially during the conversion of I3 to I- as observed through cyclic voltammetry. Remarkably, when integrated into a solar cell assembly, both NCS-1 and NCS-2 electrodes exhibited encouraging power conversion efficiencies of 6.60% and 6.29%, respectively. These results become particularly noteworthy when compared to the 7.19% efficiency of a conventional Pt-based electrode under similar testing conditions. Central to the performance of the NCS-1 and NCS-2 electrodes is their unique thin and sharp grass-like morphology. This structure, vividly showcased through scanning electron microscopy, provides a vast surface area and an abundance of catalytic sites, pivotal for the catalytic reactions involving the electrolytes in DSSCs. In summation, given their innovative synthesis approach, affordability, and remarkable electrocatalytic attributes, the newly developed NCS counter electrodes stand out as potent contenders in future dye-sensitized solar cell applications.

Morphological, structural, optical and photocurrent characteristics of Bi(TixMn(1-x))O3 perovskite nanorods grown by hydrothermal synthesis

This study focuses on the hydrothermal synthesis of Bi(TixMn(1-x))O3 (x = 0.3–0.9) perovskite nanorods on fluorine-doped tin oxide glass substrates and investigates the influence of Ti4+ on their morphological, structural, optical properties, and photocatalytic behavior. The high-resolution XPS spectra results have proved that Ti has been properly substituted into the Bi(TixMn(1-x))O3 nanorods. The characterization of morphology reveals a decrease in average diameter from 222 to 115 nm as the Ti concentration increases, while the band-gap energy experiences a slight reduction from 3.158 to 3.132 eV. X-ray and Raman spectroscopy analyses of the Bi(TixMn(1-x))O3 nanorods indicate that Ti ions substitute not only Mn3+ but also Bi3+ ions, leading to structural distortions in the crystal lattice. Moreover, the Bi(TixMn(1-x))O3 systems demonstrate enhanced photocurrent densities at 1.5 V potential, ranging from 3.19 to 5.54 mA/cm2, as the Ti concentration increases. This enhancement can be attributed to the combined effects of increased surface area, accelerating electron transfer at the interface of the electrode, and decreased optical band-gap energy. It is anticipated that this novel system will pave the way for further advancements in the field of photocatalytic water splitting and solar energy conversion applications.

Medium-vacuum thermal evaporation of CdS:O thin films towards industrial CdS/CdTe solar cells: Temperature and O2% optimization

Oxygenated CdS thin films were deposited on Fluorine-doped Tin Oxide glasses by thermal evaporation technique at medium vacuum. The substrate temperature, source temperature and oxygen concentration at the ambient were varied to obtain optimum characteristics. The post-heat-treated thin films were subjected to optical, morphological and structural characterizations to achieve optimum deposition conditions. The optical bandgaps of this study were varied within 2.41–2.47 eV, and the highest average bandgap value of 2.47 eV was obtained from the CdS layer deposited at the substrate temperature of 150 °C, source temperature of 500 °C and 20% oxygen ambient. Crystallite size, microstrain and lattice parameters were obtained using XRD line profile analysis and the Pawley refinement technique. CdS/CdTe/Cu/Au solar cells were fabricated, and the highest average efficiency of 8.32% was observed from the CdS layers deposited with the above conditions.

An overview on spray pyrolysis deposition of metal oxide thin films

This work provided an overview on the synthesis of metal oxide thin films on different types of substrates (soda lime glass, fluorine-doped tin oxide glass, silica glass, stainless steel, quartz, silicon and microscopic glass slide) using spray pyrolysis method. The fundamentals, advantages and limitations of this method have been described. Experimental results indicated that the properties of films were strongly depending on the specific process parameters (nozzle type, distance between nozzle and substrate, solvent type, solution flow rate, precursor concentration, compressed air pressure, spray time and substrate temperature). It was noted that deposition temperature could control the surface chemical composition based on the experimental results. The presence of amorphous or polycrystalline structure could be observed for the films prepared at different deposition times as indicated in XRD studies. The obtained films indicated excellent absorption coefficient value (104 to 105 cm-1) and high transmittance value (more than 82 %) in the visible and infrared portion.

Optimization of the MA/FA Ratio in the Structure of Absorber Layers Based on MA(1-x)FAxPbI3 Perovskites for Stable and Efficient Solar Cells

Herein, we investigate the mixture of methylammonium lead iodide (MAPbI3) and formamidinium lead iodide (FAPbI3). The perovskite films were coated onto fluorine-doped tin oxide (FTO) glass substrates using a spin-coating method, with the spin-coater set at 4,000 rpm for 20 s. We studied the influence of incorporation in MA(1-x) FA(x)PbI3 films. The crystal structure of the perovskite films was characterized using X-ray diffraction (XRD). Optical properties were assessed using UV-Vis spectroscopy with a spectrophotometer, and photoluminescence (PL) was characterized using a He-Cd Si-CCD laser source and a Hamamatsu detector. Images depicting the characteristic morphology of the films were captured with a scanning electron microscope (SEM). Our measurements reveal that the crystallinity of the MAPbI3 thin film improved with the incorporation of FAPbI3. In the case of 70% FAPbI3, the morphology of the MA30FA70PbI3 mixture also improved, exhibiting a rough surface with pores. The optical properties were enhanced, and the mixed thin film demonstrated better stability compared to pure MAPbI3 and FAPbI3 thin films. These film characteristics indicate that the mixtures are particularly suitable for use in photovoltaic applications.

Effects of mesoporous TiO2 with addition of V2O5 and rinsing with various solvents on the properties of dye-sensitized solar cells

Dye-sensitized solar cells (DSSC) have received much attention since they are simple, easily manufactured, inexpensive, and offer reasonable conversion efficiency. Mesoporous TiO2 with the addition of V2O5 nano-particles (0, 0.5, 1.0, 1.5 and 2.0 wt%) as a photoanode in DSSC have been successfully fabricated using a spin-coating method. Titanium tetrachloride (TiCl4) was used to synthesize a thin TiO2 blocking layer to enhance the performance of TiO2 photoanode in DSSC. After annealing treatment at 450 °C, the TiO2 blocking layer showed an amorphous structure. Performance was examined after soaking N719 dye and rinsing with five different solvents (ethanol, isopropanol, acetone, methanol, and diethyl ether). The results show that removing unanchored dye molecules using acetone solution can increase the specific surface area and dye adsorption amount of TiO2, resulting in good Jsc and photovoltaic efficiency. A conversion efficiency of 8.5% was achieved using fluorine doped tin oxide glass, a thin blocking layer, mixing 0.5 wt% V2O5 with mesoporous TiO2 and rinsing with an acetone solution.

Reversible Intercalation of Protons in Tungsten Oxide Films for Multi-Transduction Sensor Applications

Tungsten oxide films under electrochemical bias, can form tungsten oxide bronzes (MxWO3 with M being small cations such as H+ or Li+) featuring high electronic conductivity and change in coloration to deep blue1. This doping process occurs through the reduction of W6+, as described by the following equation:WO3 + x H+ + xe- -> HxWO3The bronze phase can also be formed through exposure of tungsten oxide to H2 at high temperatures (> 200 oC) or in presence of catalysts2, as described by the following equation:WO3 + x/2 H2 -> HxWO3 Because of the significant changes in both optical and electrical proprieties of the films, electrochemical control of proton doping can be used as a strategy for the development of ion-gated transistors3,4, hydrogen sensors and smart windows5.In this work, we report the use of thin films prepared by sol-gel synthesis using polyperoxotungstic acid (PPTA) as a precursor. We used three different types of substrates (conductive transparent fluorine-doped tin oxide (FTO) glass, Si/SiO2 patterned with Au source-drain electrodes and Interdigitated electrodes patterned on transparent glass) to study the changes of color and conductivity following the formation of the bronze in both electrochemical and dry environments. In electrochemical environment, we show that gating by ionic liquid in transistor configuration can be used to propose an iontronic sensor to detect H2 and H2O6. In dry environment, we show that simultaneous monitoring of visible light transmittance and electrical conductivity can be used a double transducer for H2 sensors6.1 M. S. Whittingham, in Solid State Ionics, 2004, vol. 168, pp. 255–263.2 C. C. Chan, W. C. Hsu, C. C. Chang and C. S. Hsu, Sensors and Actuators, B: Chemical, 2011, 157, 504–509.3 M. Barbosa, X. Meng, F. Soavi, C. Santato and M. O. Orlandi, J Mater Chem C Mater, 2018, 6, 1980–1987.4 G. V. de Oliveira Silva, A. Subramanian, X. Meng, S. Zhang, M. S. Barbosa, B. Baloukas, D. Chartrand, J. C. Gonzáles, M. O. Orlandi, F. Soavi, F. Cicoira and C. Santato, J Phys D Appl Phys,5 G. Cai, J. Wang and P. S. Lee, Acc Chem Res, 2016, 49, 1469–1476.6 M. S. Barbosa, R. A. da Silva, C. Santato and M. O. Orlandi, Journal of Vacuum Science & Technology A, 2022, 40, 013202.

Bifunctional Lead–Iron Oxyfluorides, 3D Cubic PbFeO2f and 2D Layered Pb3Fe2O5F2, Driving Photoelectrochemical and Electrochemical Water-Oxidation

Introduction Photoelectrochemical water splitting has attracted attention as a method of generating solar fuel such as hydrogen gas to be useful as a renewable energy carrier and a clean fuel of an internal‐combustion engine. For water oxidation of the half reaction in the water splitting, mixed-anion compounds have appeared as good candidates of the catalyst. In the mixed-anion compounds, a metal center is coordinated by several kind of anionic species. This environment induces their wide variety of physical and chemical properties not to be realized in single anion counterparts. The mixed-anion compounds possess visible-light responsibility and can utilize sunlight efficiently, therefore, which have fascinated photocatalyst researchers. The reason why they absorb visible light is the narrow bandgap with less electronegative anion species than oxygen. However, they are not very stable for photoelectrochemical water-oxidation because photogenerated holes oxidize the anion species except for oxygen in the compounds and cause degradation. Recently, we have reported an oxyfluoride Pb2Ti2O5.4F1.2 as a stable photoanode material that absorbs visible light at wavelengths up to ~500 nm.1 Oxyfluoride containing O2− and F− anions in the same phase is an example of the mixed-anion compounds. The other anion F− is less oxidized than O2−. Thus, the Pb2Ti2O5.4F1.2 photoanode showed essential stability toward the degradation by photogenerated holes exceptionally. However, this is the sole example of a visible-light-responsive oxyfluoride photoanode. Development of a new oxyfluoride photoanode absorbing visible light is important to gain the insight for water oxidation using the oxyfluorides. In the present work, another type of lead-iron oxyfluorides, PbFeO2F and Pb3Fe2O5F2, are applied to water oxidation as not only a photoanode but also an electrocatalyst.2,3 The lead-iron oxyfluoride, PbFeO2F, drives photoelectrochemical water-oxidation in visible light up to ~600 nm. In addition, this oxyfluoride functions as the electrocatalyst for water oxidation.2 In the electrochemical water-oxidation, another Pb3Fe2O5F2 shows higher performance for water oxidation than PbFeO2F. It seems that the crystal structure of two-dimensional layered perovskite Pb3Fe2O5F2 influences the superior performance to three-dimensional cubic perovskite PbFeO2F.3 Results and Discussion PbFeO2F and Pb3Fe2O5F2 powder were synthesized through a solid-state reaction and that in the evacuated glass tube, respectively.2,3 They were deposited on a fluorine-doped tin oxide (FTO) glass substrate by using electrophoresis method. The PbFeO2F or Pb3Fe2O5F2/FTO electrode was subjected to post-necking treatment to deposit TiO2 layer (TiO2/PbFeO2F or Pb3Fe2O5F2/FTO electrode), followed by electrodeposition of cobalt phosphate (Co-Pi) cocatalyst (Co-Pi/TiO2/PbFeO2F or Pb3Fe2O5F2/FTO electrode). Linear-sweep voltammetry for the as-prepared electrodes was conducted under intermittent visible-light irradiation (>400 nm) in 0.1 M K3PO4 solution (Figure 1A). The PbFeO2F/FTO electrode exhibited no photocurrent response. By contrast, a slight photocurrent response was observed for the TiO2/PbFeO2F/FTO electrode because of mainly improved interparticle conductivity in the electrode. Furthermore, a clear anodic photocurrent was observed for the Co-Pi/TiO2/PbFeO2F/FTO electrode. The Co-Pi cocatalyst, known as a water-oxidation promoter, improved the charge transfer at the electrode/water interface and charge separation in the electrode. Meanwhile, the Co-Pi/TiO2/PbFeO2F/FTO electrode exhibited an anodic photocurrent in visible light with longer wavelengths than ~600 nm. These tendencies were like those of Pb3Fe2O5F2 electrodes though the photocurrent were slightly higher than the PbFeO2F electrodes (Figure 1A, right). On contrary to the photoelectrochemical water-oxidation, a clear difference was observed when using those lead-iron oxyfluorides as an electrocatalyst. Under dark conditions, controlled-potential electrolysis with non-modified PbFeO2F and Pb3Fe2O5F2/FTO electrode was performed at +1.7 V vs. RHE (Figure 1B). The evolved O2 was detected by GC analysis in the measurement. The amount of that was almost one-fourth of electrons flowed during the electrolysis, which means the water oxidation was the major path for the electrochemical reaction. Besides, Pb3Fe2O5F2/FTO electrode showed 8 times higher activity than PbFeO2F. That reason has not been revealed but the difference is likely to be attributed to two-dimensional layered perovskite Pb3Fe2O5F2 and three-dimensional cubic perovskite PbFeO2F. References Hirayama, H. Nakata, H. Wakayama, S. Nishioka, T. Kanazawa, R. Kamata, Y. Ebato, K. Kato, H. Kumagai, A. Yamakata, K. Oka and K. Maeda, Solar-Driven Photoelectrochemical Water Oxidation over an n-Type Lead-Titanium Oxyfluoride Anode, J. Am. Chem. Soc., 141, 17158 (2019).Mizuochi, K. Izumi, Y. Inaguma and K. Maeda, A bifunctional lead–iron oxyfluoride, PbFeO2F, that functions as a visible-light-responsive photoanode and an electrocatalyst for water oxidation, RSC Adv., 11, 25616 (2021).Mizuochi, K. Oka, Y. Inaguma and K. Maeda, A two-dimensional perovskite oxyfluoride Pb3Fe2O5F2 as a catalyst for electrochemical oxidation of water to oxygen, Sustain. Energy Fuels, 6, 2423 (2022). Figure 1