The precursor solution of an iron oleate complex used for film deposition was obtained as described in our previous report (Kyesmen et al., 2019). FTO substrates of size 3.1 × 1.1 cm were first cleaned using a previously described procedure (Lee et al., 2014). Using the precursor solution prepared, α-Fe₂O₃ films were deposited on FTO substrates by dip, spin and combined dip/spin coating techniques. The precursor solution obtained was used to deposit α-Fe₂O₃ films on FTO using dip, spin and combined dip-spin coating methods. For dip coating, the FTO substrates were dipped for 2 min in the precursor, removed at a withdrawal speed of 30 mm/min, dried for 15 min at 70°C and annealed for 30 min at 500°C. The process was repeated thrice to obtain four layers of dip coated α-Fe₂O₃ films and the samples were denoted as D30. To obtain dip coated films of increased thickness, additional samples were prepared at 60 and 110 mm/min while maintaining all other experimental conditions and labelled as D60 and D110 respectively. In the spin coating process, few drops of the precursor complex were dropped on the substrate, made to spin for 30 s at 4000 rpm, dried/annealed under the same conditions as the dip coated films. The process was also repeated thrice to obtain four layers of spin coated α-Fe₂O₃ films on FTO substrate and labelled as S4000. In order to obtain spin coated films of increased thickness, additional samples were prepared at 900 and 600 rpm while maintaining all other experimental conditions and labelled as S900 and S600 respectively. Lastly, another three sets of samples were prepared using the combined dip/spin coating method consisting of two spin coated layers between dip coated ones. The first film was made with the two layers of spin coated films prepared at 4,000 rpm rotation speed while the dip coated layers were deposited at 30 mm/min withdrawal rate and labelled as C30. The second sample was prepared with the two spin coated layers prepared at the rotation speed of 900 rpm, while the dip coated films was made at the withdrawal rate of 60 mm/min and labelled as C60. Finally, the two spin coated layers of the third film were prepared at the rotation speed of 600 rpm, while its dip coated layers were made at 110 mm/min withdrawal rate and labelled as C110. The films were dried/annealed under the same conditions as described for the other films. Figure 1A–C presents an illustration that summarizes the rotation speeds and/or withdrawal rates used while preparing the different layers of dip, spin and dip/spin coated samples respectively.

X-ray diffraction (XRD) studies were performed on the prepared α-Fe₂O₃ films to investigate their structural properties using Bruker D2 PHASER-e diffractometer of Cu-Kα radiation at 0.15418 nm wavelength. Field emission gun scanning electron microscopy (FEG-SEM) conducted using the Ultrafast 540 instrument to obtain the cross-sectional images of the films. ImageJ software was employed in analyzing the cross-sectional micrographs of the films to obtain their approximate thicknesses.

A three-electrode PEC cell having a quartz window and coupled to a VersaSTAT 3F potentiostat from Princeton Applied Research was used to investigate the electrochemical behaviour of the films. 1 M NaOH (pH of 13.6) was used as the electrolyte while silver/silver chloride (Ag/AgCl) in 3M KCl, platinum meshed wire of 2 × 2 cm dimension and FTO/α-Fe₂O₃ films were used as the reference, counter and working electrodes respectively. About 1.80 cm² surface area of α-Fe₂O₃ photoanode was dipped into the electrolyte and linear sweep voltammetry (LSV) were performed in dark and under illumination conditions at a scan rate of 0.05 V/s to extract the photocurrent response of the films. The Newport Oriel^® LCS −100™ solar simulator was utilized as the light source, a Newport 91150 V reference cell was employed to calibrate the solar simulator to 1 Sun and the surface area of the photoanode exposed to light was 0.49 cm². EIS measurements of the photoanodes were performed at 0.23 V vs Ag/AgCl, in the frequency range of 10, 000–0.1 Hz at an amplitude of 10 mV, in dark and under illumination conditions. ZView software was engaged in fitting the raw data collected from the EIS measurements to a modelled equivalent circuit. M-S measurements of the photoanodes were performed in dark at a fixed frequency of 1000 Hz, in the potential range of −1.2 to 0.5 V and an amplitude of 10 mV.

The potential values in Ag/AgCl reference utilized for all the electrochemical measurements were converted to the RHE scale using the Nernst relation in Eq. 1: V R H E = V A g / A g C l + ( 0.059 × P H ) + V A g / A g C l ° (1) where, V R H E is the potential in RHE, V A g / A g C l ° = 0.1976 V at 25C, and V A g / A g C l is the potential vs. the Ag/AgCl reference measured in the experiment (Tamirat et al., 2017).

The surface morphology of the films prepared using the different coating methods are presented in Figures 2A–I for samples D30, D60, D110, S4000, S900, S600 C30, C60 and C110, respectively. All the films revealed spherical nanoparticles and many agglomerations into larva-shaped nanostructures. The surface of all the dip coated samples (Figures 2A–C) appeared to be less uniform compared to spin and combined dip/spin coated films. The inset of Figures 2A–I shows the histogram of the particle size distribution of along the diameter of the nanoparticles for all the samples. The particle sizes estimated for the films ranged from 28.5 to 34.0, 26.9 to 29.7 and 27.9 to 30.3 nm with standard deviation (S.D) values of 3.5–5.7, 2.7–4.8, and 3.8–4.1 nm for dip, spin and combined dip/spin coated films respectively. The maximum particle size of 34.0 nm was estimated for dip coated sample D60 while sample S4000 prepared via the spin coating method yielded the least size of 26.9 nm. Increase in the particle size of hematite films can lead to the reduction of grain boundaries, promote charge transport and PEC performance of hematite films (Tamirat et al., 2016).

The cross-sectional views of the dip, spin and combined dip/spin coated films consisting of three samples for each of the coating technique used for film deposition are presented in Figure 3. Films prepared via the dip coating method revealed film thicknesses of 458 ± 43, 789 ± 42 and 924 ± 32 nm for samples D30, D60 and D110, presented in Figures 3A–C respectively. The films produced using the spin coating technique yielded films with thicknesses of 482 ± 53, 771 ± 35 and 941 ± 37 nm for samples S4000, S900 and S600, and given in Figures 3D–F respectively. Lastly, the films produced using the combined dip/spin coating approach disclosed film thicknesses of 473 ± 40, 750 ± 42 and 968 ± 47 nm for samples C30, C60 and C110, presented in Figures 3G–I respectively. Generally, samples with approximate film thicknesses in the range of 450–500 ± 30 nm, 740–800 ± 30 nm and 920–980 ± 30 nm were prepared using each of the coating methods respectively. The films were made to have similar thicknesses for each set of samples prepared using the different coating methods, through the optimization of the withdrawal rate and rotation speed for dip and spin coated layers respectively.

The withdrawal speed and rotation rate play critical roles in determining the thickness of films prepared using the dip and spin coating depositing methods, respectively. In the dip coating deposition method of thin films, the basic theoretical consideration that describes the final film thickness d for a specified precursor is expressed by the Landau-Levich relation in Eq. 2: d = c × ( η U ) 2 / 3 γ 1 / 6 ( ρ g ) 1 / 2 (2) where c = 0.944 representing a constant for Newtonian liquids, η represents the liquid viscosity, U stands for the withdrawal speed, γ represents surface tension of the liquid against air, ρ denote the liquid density and g is the gravitational acceleration. This theoretical concept neglects the normal features of the sol-gel film formation like the evaporation of the precursor and gelation; however, it presents a good approximation of the basic parameters governing the process of film formation. One of the conclusions of this theory is that as a result of the radical function in U, the film thickness increases with the withdrawal speed, which may not be thought of intuitively (Landau and Levich, 1942; Puetz and Aegerter, 2004). In the spin coating deposition of films, where the initial film is uniform and within appropriate velocity and flow boundary conditions, then the thickness of the film d is defined by the expression in Eq. 3 (Tyona, 2013): d = d o ( 1 + 4 K h o 2 t ) 1 / 2 (3) where d _o is the initial liquid film thickness, t represents the spin duration and K stands for the system constant given by K = ρ ω 2 3 η where ρ denote the liquid density, ω is the rotation rate in radians per second, and η represents the liquid viscosity. For a specified precursor, most of the parameters that can influence the thickness of the films will not vary except the spin duration and the rotation rate. Therefore, if the spin duration is kept constant, the film thickness will increase with increasing rotation rate and vice versa.

Film thickness is an important factor that influences the photocatalytic efficiency of α-Fe₂O₃ during PEC water splitting. α-Fe₂O₃ often requires films with a thickness of over 400 nm to absorbs sufficient solar radiation for PEC applications due to its low absorption coefficient (Sivula et al., 2011). On the other hand, if the films are too thick, the electron-hole pairs generated in the bulk of the films during PEC water splitting will recombine before getting to the surface to oxidize water as a result of the short hole diffusion length of 2–4 nm of α-Fe₂O₃ (Ahn et al., 2014). In view of these, the sets of films of similar thicknesses prepared using the different coating methods will provide a more comprehensive perspective on the effects of coating techniques as well as film thickness on the PEC behavior of the α-Fe₂O₃ films produced.

XRD studies were performed to examine the influence of coating techniques and film thickness on the structural properties of the α-Fe₂O₃ films produced. The XRD pattern of dip, spin and combined dip/spin coated films are given in Figures 4A–C respectively. The XRD pattern disclosed pronounced diffraction peaks at (104) and (110) planes with other reflections at (012), (113), (024), (018), (214) and (300) which correlates to those of rhombohedral crystal structure of hematite based on the JCPDS file no. 33-0664. The spin coated films yielded the most crystal growth in the (104) plane compared to other films and the intensity of the plane increases with film thickness. On the other hand, the dip coated films produced the least growth in the (104) plane and the intensity did not increase despite increasing film thickness. Supplementary Figure S3 of the supplementary data present the reference XRD pattern of the hematite films for better comparison of their intensities. It clearly shows that an increase in the intensities of (104) peaks of spin and combined dip/spin coated films relative to the ones prepared via dip coating, confirming a preferential growth of the films in that plane. Crystal growth of α-Fe₂O₃ in the (104) plane has been associated with the presence of oxygen vacancies acting as a location for electron trapping and charge recombination. Also, increased intensity of (104) peak has been related to low charge mobility leading to reduced conductivity of α-Fe₂O₃ films (Kment et al., 2015; Shinde et al., 2016; Quiterio et al., 2020). The high intensity of the (104) peak observed for spin coated films can limit their photocatalytic efficiency during water splitting due to poor charge mobility and increased electron-hole recombination (Quiterio et al., 2020).

The Debye-Scherrer approximation was used to estimate the crystal sizes of the α-Fe₂O₃ films using the (110) peaks of the films. Table 1 presents the full width at half maximum (FWHM) and crystal sizes estimated for the α-Fe₂O₃ films of various thicknesses prepared using different coating methods. The maximum crystal size value of 28.5 nm was obtained for the dip coated sample D60 while the least value of 21.8 nm was estimated for spin coated sample S4000. The films with 740–800 nm thickness range gave the maximum crystal size for each coating method used for samples preparation as shown in Figure 5. All dip coated films yielded higher crystal sizes while the ones produced via spin coating gave the least values across films of various thickness ranges (Figure 5). PEC water splitting can be promoted by improved crystallization due to the consequent reduction in electron scattering centers and improved conductivity associated with enhancement of crystal size (Aegerter et al., 1997; Schuler and Aegerter, 1999).