Influence of coating techniques on the optical and structural properties of hematite thin films

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Influence of coating techniques on the optical and structural properties of hematite thin films Pannan I. Kyesmen , Nolwazi Nombona , Mmantsae Diale PII: DOI: Reference:

S2468-0230(19)30381-5 https://doi.org/10.1016/j.surfin.2019.100384 SURFIN 100384

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Surfaces and Interfaces

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4 July 2019 10 September 2019 11 September 2019

Please cite this article as: Pannan I. Kyesmen , Nolwazi Nombona , Mmantsae Diale , Influence of coating techniques on the optical and structural properties of hematite thin films, Surfaces and Interfaces (2019), doi: https://doi.org/10.1016/j.surfin.2019.100384

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Influence of coating techniques on the optical and structural properties of hematite thin films Pannan I. Kyesmen1, Nolwazi Nombona2 and Mmantsae Diale1 1

Department of Physics, University of Pretoria, Private Bag X20, Hatfield 0028, South Africa Department of Chemistry, University of Pretoria, Private Bag X20, Hatfield 0028, South Africa

2

Abstract Hematite (α-Fe2O3) thin films were deposited by dip, spin and combined coating techniques on fluorine-doped tin oxide (FTO)/glass substrates. Structural properties observed from X-ray Diffraction (XRD) and Raman spectra analysis suggest better crystallinity for films prepared by dip and combined coating techniques as compared to the ones prepared by spin coating. The Raman spectra of films prepared by spin coating and combined techniques showed red shifted and more broadened peaks relative to the ones prepared by dip coating. The red shifts were attributed to increased strain and defects in the films. Field emission scanning electron microscopy (FE-SEM) showed spherical nanoparticles with some agglomeration into small larvae-shape nanostructures for all samples. Cross-sectional images of the films revealed that samples prepared by dip and spin coating had the lowest and highest thicknesses of 452 ± 28 and 622 ± 30 nm respectively. Ultraviolet-visible (UV-Vis) spectroscopy studies revealed that the films absorb light in the visible region due to their bandgap of 1.98 ± 0.03 eV. However, films prepared using combined coating technique showed better light absorption than the ones produced by spin coating at wavelengths below 485 nm despite having less film thickness. This study showed that coating techniques influences crystallization and optical absorption behaviour of hematite films which may have impact on their performance when applied towards photoelectrochemical (PEC) water splitting. Keywords: hematite, dip coating, spin coating, optical properties, structural properties 1

Introduction

The continuous need for clean energy has formed a major motivation leading research towards the development of green renewable sources. PEC water splitting, a means of converting solar energy to hydrogen, has attracted global research interest since 1972, when it was first demonstrated by Fujishima and Honda [1]. The main component of a PEC device is a semiconductor material such as iron(III) oxide (Fe2O3), tungsten trioxide (WO3), zinc oxide (ZnO) and titanium dioxide (TiO2), which converts incident photons to electron-hole pairs when exposed to sunlight [2, 3]. Hematite (α-Fe2O3) is a promising material for use as a photoelectrode in PEC water splitting due to its low band gap, availability, low cost, nontoxicity and stability in aqueous environments. It is able to absorb light in the visible region due to its band gap of ~2.0 eV. α-Fe2O3 promises a maximum theoretical photocurrent and solar to hydrogen (STH) efficiency of ~14 mA/cm2 and ~17% respectively [2, 4]. In addition, α-Fe2O3 is the most common crystal structure of the oxides of iron, and it’s easy and cheap to process [5]. Several methods such as solution-based colloidal approach, spray pyrolysis, hydrothermal

and atmospheric pressure chemical vapour deposition (APCVD) have been used to prepare hematite thin films [2, 6]. The solution-based colloidal approach is one of the easiest ways of preparing hematite films. This approach involves coating a dispersion of particles and a porogen onto a conductive substrate; then the film is dried and sintered in air or oxygen, leaving a porous structure of interconnected nanoparticles [7]. Spin and dip coating are popular techniques used to deposit thin films. Dip coating has been widely used to prepare hematite films for PEC water splitting [8-10], because it is a low cost preparation process. A substrate is dipped into a precursor solution for a period of time and then withdrawn at a controlled speed in dip coating method. In this method, the process of film formation is based on a fluid mechanical equilibrium between the entrained film and the receding coating solution which is influenced by viscous drag, gravity, surface tension and inertial forces. The challenge with this method is that during withdrawal, turbulence in the atmosphere can easily lead to inhomogeneities in the film properties. Also, surface defects may appear in form of craters and dots typically arising from particles that may hit the wet surface [11]. Spin coating is also another technique that has been used to prepare hematite thin films [3, 12, 13]. Spin coating involves deposition of a small puddle of precursor solution onto the centre of a substrate, which is made to spin at a high and controlled speed. In this technique, factors such as centrifugal force acting on the precursor solution and the substrate, viscosity of solution, volatility of the solvent, turbulence and atmospheric humidity can affect the properties of the thin film produced [14]. Theoretically, dip and spin coating techniques influence the properties of films produced in different ways [11, 14]. Information on the impact of both spin and dip coating methods on the properties of the films can be obtained by comparing the techniques. Further knowledge could also be obtained by observing the properties of films prepared using a combined dip-spin coating technique on a single substrate. In this work, we prepared hematite thin films using dip, spin and combined coating techniques and studied their optical and structural properties. The results showed that the films prepared using the combined coating technique showed better light absorption and crystallinity than the ones prepared by spin coating. To the best of our knowledge, the combined dip-spin coating technique used to prepare hematite films has not been reported in literature. 2

Experimental

Hematite thin films were prepared by dip and spin coating techniques of a precursor complex on FTO substrates. The substrates were first cleaned in an ultrasonic bath with acetone, ethanol and deionized water for 15 min each, then dried in nitrogen gas. A procedure described elsewhere [9] was modified and used to prepare the precursor complex for the deposition process. A mixture of iron(III) nitrate nonahydrate (Fe(NO3)3.9H2O) and oleic acid in a ratio of 2:1 was heated at 110ºC for 2.5 hrs to obtain a reddish brown mass of ironoleate complex. The mass obtained was treated with tetrahydrofuran, sonicated for 15 min and centrifuged at 5000 rpm for 3 min. The supernatant solution was collected and used as the precursor complex for the deposition process by dip, spin and combined coating techniques to produce hematite thin films on FTO.

Four layers of α-Fe2O3 thin films were deposited on FTO substrate by dip coating. The substrate was dipped into the precursor complex for 2 min, withdrawn at the rate of 30 mm/min, dried at 70ºC for 15 min and annealed at 500ºC for 2 hrs at ramping rate of 10ºC/min. The sample was labelled D to denote dip coating. Using the same precursor solution, four layers of hematite films were spin coated on FTO at 1000 rpm for 30 s, dried and annealed at the same conditions as for dip coating except for the ramping rate during annealing which was 4ºC/min. The ramping rate was reduced to avoid physical defects in form of cracks during calcination [15]. The sample was labelled S to denote spin coating. Lastly, four layers of hematite thin films were prepared using dip and spin coating approaches on a single substrate following the procedure for both techniques, producing two samples. The first sample contains two spin coated layers between dip coated layers and labelled C1 while the second contains alternates of spin and dip coated layers, labelled C2. Fig. 1 shows a schematic illustration of the experimental process.

Fig. 1. Schematic illustration showing: (a) experimental procedure, (b) coating approach used to prepare samples D, S, C1 and C2, and (c) architecture of samples (this is not to scale). The surface morphology of the samples and their cross-sectional view were investigated with FE-SEM, using Zeiss Crossbeam 540. The Zeiss Crossbeam 540 was coupled with Energy Dispersive X-ray Spectroscopy (EDS) which was used for elemental composition analysis of the films. XRD measurements were done with Bruker D2 PHASER-e diffractometer using

Cu-Kα radiation at 0.15418 nm wavelength to study the structural properties of thin films. Raman spectroscopy measurements were done using WiTec alpha300 RAS+ Confocal Raman Microscope with 532 nm excitation laser at 5 mW. UV-Vis absorption and transmittance of the films were measured using CARY 100 BIO UV-Vis spectrometer. 3

Results and Discussion

3.1 Surface morphology, film thickness and elemental composition The surface morphologies of samples D, S, C1 and C2 obtained from FE-SEM studies are shown in Fig. 2. The images reveal spherical nanoparticles with some agglomeration of individual particles into small larvae shape nanostructures. The films prepared by spin and combined coating techniques (S, C1 and C2) appeared to be more homogeneous than the ones prepared by dip coating. ImageJ software was used to approximate the particle diameter along the width of the nanoparticles. The highest and lowest estimated average particle diameter was 36.25 and 31.97 nm for films prepared by dip and spin coating techniques with standard deviations of 5.6 and 6.2 nm respectively. Particle diameter of 33.85 and 33.41 nm were estimated for samples C1 and C2 with standard deviations of 5.9 and 4.9 nm respectively. Coating techniques may have influenced the particle size of samples. A similar observation was made in another study, where higher average particle sizes were estimated for tungsten oxide films prepared by dip coating compared to the ones prepared by spin coating [16].

Fig. 2. SEM micrograph of dip coated (D), spin coated (S), and combined dip-spin coated (C1 and C2) films with the insets showing the corresponding histograms of their particle size distribution. FE-SEM cross-sectional view and estimated film thicknesses for all the samples are shown in Fig. 3. ImageJ was also used to approximate the thicknesses of the films. Samples prepared using dip coating and spin coating techniques had the least and highest film thicknesses of 452 ± 28 and 622 ± 30 nm respectively. The films prepared using the combined techniques had thicknesses of 512 ± 34 and 521 ± 50 nm for samples C1 and C2 respectively. The thickness of spin coated films was higher than the dip coated films. This was due to the low spin speed of 1000 rpm used during spin coating and the slow withdrawal rate of 30 mm/min used for dip coating. Film thickness increases with increasing withdrawal rate in dip coating while in spin coating, it reduces with spin speed [11, 14].

Fig. 3. SEM cross-sectional view of dip coated (D), spin coated (S,) and combined dip-spin coated (C1 and C2) films. Energy Dispersive X-ray Spectroscopy (EDS) analysis was used to qualitatively confirm elemental composition and ascertain any impurities in the films and the results are shown in Fig. 4. The analysis indicates the presence of both iron (Fe) and oxygen (O) in all the samples which are the elements of our nanostructured hematite films. Tin (Sn) was also dictated in the films which was due to the tin oxide (SnO2) present in FTO substrates. Small amount of silicon (Si) were detected in the films which considered to have come from the glass content of glass/FTO substrate. In order to clarify these, we carried out EDS analysis on the glass/FTO and was observed to contain high amount of Si in the substrates. Since the

temperature of 500°C used for annealing the films was not enough to diffuse Si into our samples, the Si content detected was most likely from glass/FTO substrates.

Fig. 4. EDS analysis of dip coated (D), spin coated (S), combined dip-spin coated (C1, C2) films and glass/FTO substrates. 3.2 Structural analysis The XRD spectra of thin films done within a 2 theta range of 20 and 70 degrees are shown in Fig. 5. The study revealed Bragg reflections at (104) and (110) planes for all samples which confirmed the formation of the rhombohedral crystal structure of hematite with lattice parameters a = b = 5.032, c = 13.733; R c symmetry group. Weak reflections were also observed at (012) (113), (024), (122) and (310) planes corresponding to those of hematite, according to JCPDS no. 33-0664. Peaks corresponding to other iron oxide phases were not detected which implies high purity of α-Fe2O3. The XRD peak intensities of samples increases with increasing film thickness [17, 18].

Fig. 5. XRD pattern of hematite films prepared by dip, spin, and combined coating techniques. In order to understand the impact of coating techniques on the structural properties of the films, the peaks at (110) were analysed to obtain the full width at half maximum (FWHM) values and parameters such as the crystal size, microstrain and dislocation densities. The approximate crystal size (D) of the samples were calculated using the Debye-Scherrer formula given in equation 1 [6], (1) where k is the shape factor (0.9), λ is the wavelength of incident X-ray (λ = 0.15418 nm for Cu-Kα radiation), β is the observed FWHM. The microstrain (ɛ) of the hematite films was calculated using equation 2 [17]. (2) The dislocation density (ᶑ) of the films were calculated using the Williamson and Smallman’s relation in equation 3 [17].

(3) From the results obtained (Table 1), it was observed that the thin films prepared by dip coating (sample D) had the highest crystal size followed by samples C2 and C1 (prepared by combined coating). This is indicative of a more crystalline film for sample D. The crystal size values of samples C1, C2 and S were within the same error margin of ± 2 nm, however, the results suggest a more crystalline film for both C1 and C2 relative to sample S. High crystallization of films can help improve their electron mobility due to increase in mean free path and reduced scattering centers of electrons and this can enhance their performance in photocatalytic applications [19, 20]. Table 1. XRD analysis showing FWHM, crystal size, microstrain and dislocation density for samples D, S, C1 and C2. Sample

Bragg angle 2θ (degrees)

FWHM (degrees)

Crystal size (nm)

Strain ɛ х 10-3

Dislocation density ᶑ х 1014 (Lines/m2)

D S C1 C2

35.7532 35.6885 35.6203 35.6501

0.30 0.37 0.36 0.35

27.6 22.6 23.1 23.7

4.01 5.00 4.91 4.78

13.13 19.51 18.72 17.79

Films prepared by dip and spin coating had the lowest and highest microstrain of 4.01 х 10-3 and 5.0 х 10-3, and dislocation density of 13.13 х 1014 lines/m2 19.51 х 1014 lines/m2 respectively. Increase in microstrain and dislocation density for samples C1, C2 and S relative to sample D can be attributed to increase in defect levels, and grain boundaries as a result of decreased crystal sizes [17]. The increased defect levels for samples C1, C2 and S may have resulted from the calcination of spin coated layers of the films. During spin coating, strong centrifugal forces acting on the growing film may have created a higher tension in the film, which then collapsed during calcination leading to physical defects in form of cracks [15]. This may have occurred in spite of the reduced heating rate for spin coated layers. Raman spectra analysis was employed to study the chemical composition of the films and extract structural information. Raman spectra of hematite films are shown in Fig. 6A. The spectra of all samples revealed the seven symmetrical optical phonon modes of hematite at the first Brillion zone; two A1g and five Eg modes, given by group theory [21]. This further confirmed the integrity of hematite films in addition to the XRD results. Further analysis of the spectra shown in Fig. 6B revealed red shift of Raman peaks for films prepared by spin coating (S) and combined techniques (C1 and C2) when compared to the ones prepared by dip coating (D). To further reveal details of peak positions and broadening, Lorentzian fitting and deconvolution of all Raman peaks were carried out and the results are shown in Table 2 and Fig. 7. Analysis of the results show red shift and broadened peaks for films prepared by spin coating (S) and combined techniques (C1 and C2) relative to the films prepared by dip coating. The red shifts and peaks broadening observed for samples S, C1 and C2 are due to

increased strain and physical defects in the films as observed in XRD analysis [22, 23]. Raman peaks for C1 and C2 are more broadened and shifted to lower energies than S as seen in their FWHM and peak positions values given in Table 2, respectively, despite having similar lattice strain values. The cause of the more pronounced red shift and broadened peaks observed for C1 and C2 is yet to be fully ascertained, however, we suspect that this may be due to lattice disorder resulting from the combined approach employed during film deposition [23]. Higher peak intensities were observed for films prepared by dip and combined coating techniques over spin coated film, despite having less film thickness. This is an indication of improved crystallization for samples D, C1 and C2 films over the spin coated film (S). These results agree with XRD analysis which suggest higher crystallization for samples D, C1 and C2 relative to S. Direct comparison between the peak intensities of sample D with C1 and C2 could not be made due to the variation in film thickness.

Fig. 6. Raman spectra of hematite films A); showing optical phonon vibrational modes for samples prepared by dip, spin and combined coating techniques B); showing a red shift for samples prepared by spin and combined coating techniques (S, C1 and C2) with respect to the ones prepared by dip coating (D).

Fig. 7. Fitted and deconvoluted curves showing peak positions of the first two vibrational modes of the Raman spectra for samples D, S, C1 and C2. Table 2. Wavenumbers (ʋ) representing peak positions of the Raman spectra for samples D, S, C1 and C2 and their symmetrical phonon mode assignments. Sample D

3.3

Sample S

Sample C1

Sample C2

Symmetry Assignment

ʋ (cm-1)

FWHM ʋ (cm-1) (cm-1)

FWHM ʋ (cm-1) (cm-1)

FWHM (cm-1)

ʋ (cm-1)

FWHM (cm-1)

226.9

12.2

224.7

12.5

219.3

17.6

222.6

13.6

A1g(1)

246.9

10.6

243.0

19.4

229.2

21.2

241.8

22.3

Eg(1)

293.3

16.0

290.9

15.7

279.7

26.0

286.5

22.9

Eg(2)

301.3

13.5

296.6

15.7

291.4

24.1

296.1

17.4

Eg(3)

410.6

18.6

406.2

24.1

397.2

40.4

401.8

31.4

A1g(2)

501.2

27.6

497.9

24.0

492.8

43.9

497.1

38.5

Eg(4)

614.2

43.0

609.0

43.0

595.3

54.1

604.0

49.4

Eg(5)

Optical properties

The optical absorption of hematite films studied using UV-Vis spectroscopy is shown in Fig. 8A. The absorbance of the films increases with increasing film thickness, except that the film prepared by spin coating technique (S) exhibited poor absorption at wavelengths below 500

nm. The surface of the spin coated film appeared to be visually smoother compared to other samples which may have led to high surface reflection, leading to the observed poor absorption. Hematite films prepared by dip and combined coating techniques showed good absorption in the visible region with peaks at 533 and 408 nm. The bands at 533 and 408 nm have been assigned to 6A1→4E arising from spin-forbidden ligand field transitions [24] and 6A1→4T2 due to single electron transitions [25], respectively. Films prepared using combined coating technique (C1 and C2) showed enhanced light absorption over spin coated film at wavelengths below 485 nm, despite having less film thickness. The combined technique used to prepare samples C1 and C2 with their top layers being dip-coated may have led to reduced surface reflection which could have been high for spin coated film, thereby enhancing light absorption. Films prepared by dip and combined coating techniques both showed good absorption behaviour within the visible spectrum. However, we could not compare directly their absorbance due to the variation in their film thickness (sample concentration). Transmittance measurements shown in the inset of Fig. 8A with an expanded view in Fig. 8B showed that spin coated film transmits more light at lower wavelengths than the ones prepared by combined technique. This is in agreement with the absorption properties observed.

Fig. 7. A) UV-Vis absorption spectra of hematite films for samples D, S, C1 and C2 with the inset showing their transmittance measurements, B) expanded view of the transmittance spectra of the films and C) Tauc plot corresponding to the absorption spectra of the films.

The Tauc plot was used to examine the optical indirect band gap for all films as shown in Fig. 8C. The plot was derived from the Tauc model in equation 4, which gives the relation between band gap (Eg), absorption coefficient (α), and the photon energy (hν). (4) where A is an arbitrary constant and ?? is a constant depending on the type of transition (2 for allowed indirect and 1/2 for allowed direct transition) [26]. In this work, we determined the indirect transitions which are due to Fe3+ 3d→3d excitation [27]. Values of (??hν)1/2 were plotted against hν followed by extrapolation of the linear region of the curve to intersect (hν)axis whose value is the band gap. The estimated band gaps for all films were in the range of 1.96-2.01 eV. Depending on the method of preparation of hematite, the band gap value has been reported to be between 1.9 and 2.2 eV corresponding to absorption wavelength range of about 650 to 560 nm [7]. The band gap of the films confirmed the strong absorption exhibited by the films in the visible region and shows that they could be suitable materials for photocatalytic reactions. 4

Conclusion

Hematite thin films were prepared using dip, spin and combined coating techniques. FE-SEM study of the films reveal spherical nanoparticles with some agglomeration into small larvae shaped nanostructures for all films, however, films prepared by spin and combined coating techniques appeared to be more homogeneous. From XRD studies, the film prepared by dip coating (D) had the highest crystal size of 27.60 nm followed by C2 and C1 with crystal sizes of 23.11 and 23.70 nm, respectively. Raman spectra analysis also confirmed higher crystallinity for films prepared by dip and combined coating techniques over spin coated films. The Raman spectra of films prepared by spin coating and combined techniques showed red shifted and more broadened peaks relative to the ones prepared by dip coating which was due to increased strain and defect levels. The films exhibited strong optical absorption in the visible region due to their bandgap of 1.98 ± 0.03 eV. Hematite films prepared by combined coating techniques showed better light absorption at wavelengths below 485 nm than the ones prepared by spin coating, despite having less film thickness. This study showed that coating techniques influence crystallization and optical absorption behaviour of hematite films which may impact on their performance when applied towards PEC water splitting. Acknowledgements The authors acknowledge funding from the University of Pretoria, National Research Foundation (NRF), grant UID; 110814 and South African Research Chairs Initiative (SARCHI), UID; 115463.

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