Synthesis and Characterization of SnO₂/α-Fe₂O₃, In₂O₃/α-Fe₂O₃, and ZnO/α-Fe₂O₃ Thin Films: Photocatalytic and Antibacterial Applications

Abstract

The fabrication of metal oxide semiconductor heterostructures is a major way to enhance their properties in photocatalytic and antibacterial applications. In the present work, ZnO/α-Fe₂O₃, In₂O₃/α-Fe₂O₃, and SnO₂/α-Fe₂O₃ are chosen to create the heterostructure of thin films using the spray pyrolysis method. This paper compares the experimental results of the structural and morphological properties of the prepared thin layers using XRD, Raman and SEM. The X-ray diffraction shows that the obtained thin film heterostructures crystallize in a hexagonal phase of ZnO, a cubic phase of In₂O₃ and a tetragonal structure of SnO₂, with all of the preceding phases positioned on the rhombohedral phase of the hematite α-Fe₂O₃. In addition, the SEM study provided the morphology and surface structure and confirmed the presence of a highly folded, rough, uneven surface with imperfections of 20 and 65 nm for In₂O₃/α-Fe₂O₃ and SnO₂/α-Fe₂O₃. The photoactivity of the prepared materials was tested via the photocatalytic degradation of methylene blue (MB) dye. Consequently, our findings demonstrate that the cracked surface improves the rapid absorption of contaminants and allows water to easily pass through the surface of the thin layers. Finally, the antibacterial abilities of ZnO/α-Fe₂O₃, In₂O₃/α-Fe₂O₃, and SnO₂/α-Fe₂O₃ thin films were investigated by using the agar well-diffusion technique, comparing the results to the Gram-negative of Pseudomonas aeruginosa and Gram-positive of Bacillus subtilis, and these thin films were found to have high antibacterial activity.

1. Introduction

Recent years have seen a significant increase in interest in thin film technology because of its many uses, including in gas sensors [1,2], energy storage [3,4,5], photovoltaic solar cells [6,7,8], heterogeneous photocatalysis for water splitting processes [9,10,11], and optoelectronic devices [12,13,14]. The amount of wastewater polluted with undesirable and dangerous dye has remarkably increased. The colored dyes that are frequently employed in the textile sector are one of the primary causes of pollution. Water’s transparency and gas solubility can be impacted by even minute amounts of dye, which can also give the water an incredibly vibrant color [15,16].

As a result, it is now crucial to explore the possibility of functionalizing various surface types without altering their optical characteristics. There exist numerous techniques for obtaining thin films, such as electroplating [17], anodic treatment [18], chemical vapor deposition (CVD) [19,20], atomic layer deposition (ALD) [21,22], and spin coating [23]. The spray pyrolysis process is the most commonly used because it is a simple way to make films with any material in any amount by adding the film to the spray precursor solution.

Owing to its low cost, small bandgap, environmental friendliness, and thermodynamic stability, hematite (Fe₂O₃) is a promising semiconductor material for use as an efficient photocatalyst under visible light irradiation. To increase the photoinduced electron–hole separation and, consequently, the photocatalytic activity of Fe₂O₃, heterojunction construction is used. Because it is an environmentally beneficial technology, the photocatalytic reduction of methylene blue has been applied extensively in a variety of semiconductor materials, including Fe₂O₃, In₂O₃, SnO₂, and ZnO, achieved via exposing them to UV and visible light. In this context, Alofi et al. [24] describe the mechanism of photogenerating radical species through a particulate film, such as OH^• (instead of h⁺) and super oxide O₂^–• (instead of e⁻) radicals. However, it is unclear why these species would not react quickly with one another as they pass through the film from the bulk to the surface. Different kinds of dyes are also investigated as different types of pollution; Singh et al. [25] used doped ZnO as a photocatalyst to decompose methyl orange (MO), methylene blue (MB), and congo red (CR), respectively. The insertion of another material into the oxide semiconductor lattice can alter its optical, morphological, and structural characteristics, among other physical and chemical modifications. These modifications might alter the semiconductor’s photocatalytic activities and shift them toward a visible area of the spectrum. Semiconductor heterojunctions, which comprise two or more semiconductors arranged in delayed energy bands, enable the separation of photogenerated electron–holes and the retention of redox capacity. R. M. Mohamed et al. [26] studied the effect of increasing the solar light-driven photocatalytic hydrogen evolution of different semiconductor photocatalysts; this technique might be employed when using organic and non-metal-based new-age materials, especially if combined with numerous modification procedures. The present paper aims to provide a well-founded direct comparison of three thin film heterostructures (ZnO/α-Fe₂O₃, In₂O₃/α-Fe₂O₃, and SnO₂/α-Fe₂O₃) prepared using a simple chemical technique. The photodegradation of methylene bule dye MB and the antibacterial application of the prepared materials were tested and compared. We need to point out that no previous research has been conducted to date using this method to create these thin film heterostructures.

2. Experimental Section

2.1. Film Preparation

2.1.1. ZnO Thin Film Preparation

Zinc acetate (C₄H₆O₄Zn, 2H₂O) was chosen as the primary precursor and was dissolved in isopropyl alcohol at a concentration of 10⁻² mol/l to create the precursor solution.

Using the chemical spray approach, ZnO thin films were applied to a glass substrate at 460 °C [27].

2.1.2. In₂O₃ Thin Film Preparation

An aqueous solution of indium chloride (InCl₃) with a concentration of 0.01 M was sprayed onto glass substrates and heated to 350 °C using a N₂ gas pressure of 0.5 bar to create an In₂O₃ thin layer.

2.1.3. SnO₂ Thin Film Preparation

To create SnO₂ thin films, a precursor was obtained in a 0.05 M solution by dissolving tin chloride pentahydrate (SnCl₄.5H₂O) in pure ethanol (C₂H₅OH). This solution was sprayed at 0.5 bar of N₂ gas pressure onto glass substrates that were heated to 450 °C on a hot plate.

2.1.4. ZnO/α-Fe₂O₃, In₂O₃/α-Fe₂O₃, and SnO₂/α-Fe₂O₃ Thin Film Preparation

To prepare ZnO/α-Fe₂O₃, In₂O₃/α-Fe₂O₃, and SnO₂/α-Fe₂O₃ thin film heterostructures, an aqueous solution containing iron (III) and chloride dehydrate (FeCl₃.2H₂O) was utilized. After preparing this solution, we sprayed it at 350 °C onto the ZnO, In₂O₃, and SnO₂ substrates for use as a precursor. Using a nozzle with a 0.5 mm diameter, 20 mL of the resultant solution was sprayed at a rate of 4 mL/min. There was a 30 cm distance between the substrate and the spray gun.

2.2. Characterization Techniques

Initially, to study the structural properties using Cu Kα radiation (λ = 0.15418 nm) and 2θ varying from 10° to 70°, the X-ray diffraction spectra of the produced thin films were examined using a copper-source diffractometer (Analytical X Pert PROMP D, Sakaka, Jouf). In addition, at room temperature, Raman scattering studies were recorded using the Jobin Yvon Horibra LABRAM-HR micro-Raman system (Sakaka, Jouf), which was detectable within the 200–1200 cm⁻¹ range. Subsequently, a scanning electron microscope (SEM) of the JEOL-JSM 5400 type (Sakaka, Jouf) was used to examine the films’ morphology. An energy-dispersive X-ray (EDX) spectrometer (Sakaka, Jouf) connected to a thermal field emission scanning electron microscope (FESEM, Sakaka, Jouf) was used to analyze the elements with an electron gun. Electrical tests were carried out, and resistivity, hall mobility, and carrier concentration were determined at room temperature in a light magnetic field of around 0.554 T, using a hall measurement device that employed the van der Pauw method.

Ultimately, the rate at which the methylene blue (MB) aqueous solution degraded in the presence of sunlight was used to evaluate the photocatalytic activity of the structures based on the obtained thin film heterostructures. The cylindrical batch reactor used in the research was opened to the air. For the photocatalytic tests, methylene blue MB (Aldrich) was selected as the model molecule. A steady stream of water flowing through the reactor maintained the MB solution at room temperature. The starting MB concentration was 4.5 mg/L. To achieve adsorption equilibrium, the MB solution was agitated at a magnetic stirrer for one hour in the dark. Under continuous stirring, the aqueous suspension comprising MB and the photocatalysts of ZnO/α-Fe₂O₃, In₂O₃/α-Fe₂O₃, and SnO₂/α-Fe₂O₃ thin films was exposed to solar radiation. Four analytical samples were taken from the MB solution every 30 min in order to determine the impact of sunlight irradiation. Each sample’s MB concentration was determined using a UV–Vis spectrophotometer.

Since the absorbance, A, and concentration, C, of MB are proportionate according to the Beer–Lambert law, the efficiency of MB degradation was determined using the following equation:

$$\mathrm{D}\mathrm{e}\mathrm{g}\mathrm{r}\mathrm{a}\mathrm{d}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n} \mathrm{e}\mathrm{f}\mathrm{f}\mathrm{i}\mathrm{c}\mathrm{i}\mathrm{e}\mathrm{n}\mathrm{c}\mathrm{y}\left(\mathrm{\%}\right)={\displaystyle \frac{{\mathrm{A}}_0-\mathrm{A}}{{\mathrm{A}}_0}}\times 100={\displaystyle \frac{{\mathrm{C}}_0-\mathrm{C}}{{\mathrm{C}}_0}}\times 100$$

(1)

where A₀, A and C₀, C represent the MB absorbance and concentration, respectively, corresponding to the start time and the variable time. Lastly, counting forming unity (CFU) was used to assess the bactericidal qualities of ZnO/α-Fe₂O₃, In₂O₃/α-Fe₂O₃, and SnO₂/α-Fe₂O₃ thin film heterostructures. The experimental strains of the Gram-negative bacteria Pseudomonas aeruginosa and Gram-positive bacteria Bacillus subtilis were employed. In Tryptic Lauria Bertani (LB) medium, the bacterial cells were cultivated at 37 °C until they reached an optical density (OD) of 0.2 at 600 nm. During the exponential growth phase, the culture was inoculated with 10⁶ CFU in a fresh LB medium, and 10 μL was placed onto the ZnO/α-Fe₂O₃, In₂O₃/α-Fe₂O₃, and SnO₂/α-Fe₂O₃ thin films. It was then incubated at 37 °C for 24 h in a dark environment. An identical process was applied to inert glasses devoid of thin coatings, which served as the control. Following incubation, bacterial suspensions were serially diluted and plated for CFU counting on Plate Count Agar (PCA) plates measuring 9 cm in diameter. The effect of thin films on bacterial growth was determined by looking at the decrease in CFU/mL. The residual bacterial viability was determined as follows:

$$\mathrm{\%} \mathrm{v}\mathrm{i}\mathrm{a}\mathrm{b}\mathrm{i}\mathrm{l}\mathrm{i}\mathrm{t}\mathrm{y}={\displaystyle \frac{\mathrm{C}\mathrm{F}\mathrm{U} \mathrm{w}\mathrm{i}\mathrm{t}\mathrm{h} \mathrm{t}\mathrm{h}\mathrm{i}\mathrm{n} \mathrm{f}\mathrm{i}\mathrm{l}\mathrm{m}\mathrm{s}}{\mathrm{C}\mathrm{F}\mathrm{U} \mathrm{o}\mathrm{f} \mathrm{c}\mathrm{o}\mathrm{n}\mathrm{t}\mathrm{r}\mathrm{o}\mathrm{l}}}\times 100$$

(2)

3. Results and Discussions

3.1. Structural Properties

XRD was used to evaluate the crystal phase of the prepared ZnO/α-Fe₂O₃, In₂O₃/α-Fe₂O₃, and SnO₂/α-Fe₂O₃ thin film heterostructures. Firstly, all peaks observed in the crystal structure of ZnO/α-Fe₂O₃ (Figure 1a) were located at 2$\theta$ values of 24.3°, 31.4°, 33.4°, 34.9°, 40.6°, 44.5°, 54.2°, and 57.7°, which are related to the (012), (100), (104), (110), (113), (024), (110), and (122) planes, respectively. These peaks indicate the hexagonal phase of ZnO and the rhombohedral phase of the hematite α-Fe₂O₃ according to the JCPDS card numbers 036-1451 and 01-1053, respectively. These findings are consistent with those of Noukelag et al., who synthesized zincite ZnO and hematite α-Fe₂O₃ for the first time from an aqueous extract of rosemary [28].

The XRD of In₂O₃/α-Fe₂O₃ is shown in Figure 1b, with the primary diffraction peaks corresponding to the cubic phase of In₂O₃ (JCPDS No: 06-0416) and the rhombohedral phase of α-Fe₂O₃ (JCPDS No: 01-1053). The absence of further impurity peaks or mixed oxides suggests that the different metal oxides did not chemically interact with each other [29].

The XRD pattern of the SnO₂/α-Fe₂O₃ thin film is shown in Figure 1c. The patterns reveal many peaks at (110), (101), (200), (211), and (301), which match the lattice planes of the tetragonal structure JCPDS (no. 41–1445). In addition, weak peaks are revealed at (012), (104), (110), (116), (214), and (300) that are compatible with the JCPDS (no. 01-1053) associated with rhombohedral hematite α-Fe₂O₃ [30].

3.2. Raman Measurements

Raman spectroscopy is a dependable method for characterizing materials. It distinguishes various material phases using discrete vibrational modes. The phase type and purity of the material are confirmed using the numerous unique phonon modes revealed in the Raman spectra. Figure 2 shows the Raman spectrum analysis of the ZnO/α-Fe₂O₃, In₂O₃/α-Fe₂O₃, and SnO₂/α-Fe₂O₃ thin film heterostructures. The Eg modes of α-Fe₂O₃ are represented by the peaks at 243, 292, 404, and 604 cm⁻¹, whereas the A1g modes are associated with the peak at 497 cm⁻¹. These peaks appeared in all the different thin films. These results enhance the production of pure hematite α-Fe₂O₃ [10,11,12].

3.3. Morphological Properties

SEM analysis was used to evaluate the morphology of ZnO/α-Fe₂O₃, In₂O₃/α-Fe₂O₃, and SnO₂/α-Fe₂O₃ thin film heterostructures presented in Figure 3.

ZnO/Fe₂O₃ has a homogeneous and smooth surface with a very small particle size. However, the thin film SEM pictures of SnO₂/Fe₂O₃ and In₂O₃/Fe₂O₃ have a completely different shape compared to ZnO/Fe₂O₃. Figure 3b,c show a highly folded, rough, uneven surface with imperfections confined between 20 and 65 nm. The elaboration process and the formation of thin layers in the heterostructure may cause these irregularities. This cracked surface improves the rapid absorption of contaminants and allows water to easily pass through the surface of our thin layers.

3.4. EDX Measurments

EDX was used to support the aforementioned findings; the atomic percentages (at.%) of Zn (from ZnO), Sn (from SnO₂), In (from In₂O₃), and Fe (from the Fe₂O₃) found on thin films are presented in Figure 4a–c, which display the corresponding mapping structure of the obtained thin films.

Only three components were found in the spectrum for these films: O, Fe, and Zn for ZnO/α-Fe₂O₃; O, Fe, In for In₂O₃/α-Fe₂O₃; and O, Fe, and Sn for SnO₂/α-Fe₂O₃. No other elements were discovered within the apparatus’ sensitivity range, which shows the great purity of the thin films.

On the other hand, we noted that the distribution of Fe and O signals was uniform throughout the entire structure. Seldom were the Zn, In, and Sn signals found outside of the core region; they were mostly found inside. The unique characteristics of the ZnO/α-Fe₂O₃, In₂O₃/α-Fe₂O₃, and SnO₂/α-Fe₂O₃ heterostructure nanocomposites were validated by these results, which align with the findings documented in the previous literature [13,14].

3.5. Hall Effect Study

The Hall effect measurements provide important information, such as the type of material (n-type or p-type), carrier concentration, electrical resistivity, and Hall mobility. These findings are detailed in

Table 1. Hall effect results of In₂O₃/α-Fe₂O₃ and SnO₂/α-Fe₂O₃ thin films.

	Conductivity Type	Carrier Concentration (cm−3)	Resistivity ρ (Ω.cm)	Mobility μ (cm2/V.S)
In2O3/α-Fe2O3	n	−1.87 × 1021	5.71 × 10−4	5.82
SnO2/α-Fe2O3	n	−1.88 × 1020	1.49 × 10−3	22.20

.

Firstly, the results indicate that the conductivity of the ZnO/α-Fe₂O₃, In₂O₃/α-Fe₂O₃, and SnO₂/α-Fe₂O₃ thin films was n-type in nature and all the information shows notable changes between the obtained thin film heterostructures. The increase in the carrier concentration and the mobility of SnO₂/α-Fe₂O₃ can be attributed to the increase in the number of the grain boundary on the surface of layer and the corresponding increase in the dislocation density across the boundaries.

On the other hand, the heterostructure of the ZnO/α-Fe₂O₃ thin film has a high resistivity. Consequently, the results showed no changes.

3.6. Photocatalytic Test

Adsorption is a crucial step in every catalytic reaction process, regardless of the type of photodegradation employed. More effective adsorption enables improved pollutant–catalyst interactions. In this work, MB was adsorbed on the synthesized thin film heterostructures at room temperature, as plotted in Figure 5.

During photoelectrochemical water-splitting, the observed photocurrent is adversely affected by the consumption of photoexcited electrons. Under sun irradiation, ZnO/α-Fe₂O₃, In₂O₃/α-Fe₂O₃, and SnO₂/α-Fe₂O₃ are excited. The photogenerated electrons quickly move from ZnO, In₂O₃, and SnO₂ CB to α-Fe₂O_3’s VB, allowing for holes to collect in ZnO, In₂O₃, and SnO₂ VB, and for electrons to gather in high-potential α-Fe₂O₃ CB, where they can reduce H⁺ in H₂.

Morphology becomes important at this point. The photocatalyst’s precise engineering and design may be able to solve the issues with charge carrier transit and recombination. Indeed, there are numerous indications of the existence of a relationship between reduced-dimension structures and enhanced photocurrent density, which is linked to the occurrence of defects in nanostructures. Under visible light irradiation, we discovered that the concentration of MB decreases and the peak almost vanishes after six hours, but only for SnO₂/α-Fe₂O₃. This good response is due to the presence of a cracked surface, which increases the rapid absorption of contaminants and allows water to easily pass through the surface of our thin layers. Because of its improved nanoarchitecture, which facilitates dye molecule diffusion and oxygen species transport during the photoactivity process, the porous sample exhibits a higher photocatalytic performance than the pure one. Electron–hole pairs (e⁻, h⁺) are produced when photon energy (hv) is absorbed by the thin film heterojunctions during radiation.

After photogeneration occurs close to the surface, the electrons combine with oxygen to make superoxide (^⋅O₂⁻) (Equation (3)), which then reacts with water to form hydroxyl radicals (^⋅OH). In parallel with this, the water and photogenerated holes react to produce ^⋅OH radicles.

Intermediate reactions also result in the formation of ^⋅OH radicals and the powerful oxidants finally convert MB into CO₂ and H₂O.

O₂ + e⁻ → ^⋅O₂^‾

(3)

^⋅O₂‾ + H₂O → OH‾ + ^⋅HO₂

(4)

H₂O + h⁺ → ^⋅OH + H⁺

(5)

h⁺ + OH‾ → ^⋅OH

(6)

^⋅HO₂ + H₂O → H₂O₂ + ^⋅OH

(7)

H₂O₂ + e⁻ → ^⋅OH + OH^‾

(8)

MB + (^⋅OH + ^⋅O₂ ‾) → CO₂ + H₂O

(9)

3.7. Antibacterial Activity

The antibacterial effectiveness against Gram-negative Pseudomonas aeruginosa (Pa) and Gram-positive Bacillus subtilis (Bs) bacteria was also assessed using ZnO/α-Fe₂O₃, In₂O₃/α-Fe₂O₃, and SnO₂/α-Fe₂O₃ thin films. The In₂O₃/α-Fe₂O₃ thin film demonstrated a residual viability equal to 0.5% and 0.187% of bacterial cells against of Pa and of Bs, respectively. In addition, the SnO₂/α-Fe₂O₃ showed a viability of 0.65% and 0.62%. However, the results of the bacteria test in the presence of ZnO/α-Fe₂O₃ revealed higher values, as illustrated in

Table 2. Antibacterial activity of In₂O₃/α-Fe₂O₃, SnO₂/α-Fe₂O₃, and ZnO/α-Fe₂O₃ thin films against Pseudomonas aeruginosa and Bacillus subtilis bacteria.

		Pseudomonas Aeruginosa		Bacillus Subtilis
			% Viability		% Viability
Bacteria without sample		2 × 108	--	4 × 107	--
Bacteria in the presence of	In2O3/α-Fe2O3	106	0.5	7.5 × 104	0.187
	SnO2/α-Fe2O3	2 × 106	0.65	2.5 × 105	0.625
	ZnO/α-Fe2O3	1.3 × 106	1	4 × 106	10

. The generation of reactive oxygen species (ROS) and the electrostatic interaction between In₂O₃/α-Fe₂O₃ and the absorbed OH- and H₂O may be the cause of the antibacterial action. One possible explanation for the increased antibacterial properties of In₂O₃/α-Fe₂O₃ thin films could be their greater root mean square roughness (Rms) in relation to that of the other thin films.

The combined benefits of the photocatalytic activity and the antibacterial impact are crucial for a variety of applications (medical, food storage, etc.). To the best of our knowledge, thin films have never been used to deteriorate Pa and Bs.

4. Conclusions

This study presents the preparation of heterostructure ZnO/α-Fe₂O₃, In₂O₃/α-Fe₂O₃, and SnO₂/α-Fe₂O₃ thin films using the spray pyrolysis method. The XRD and Raman spectroscopy confirm the presence of a rhombohedral structure of α-Fe₂O₃ and hexagonal phase of ZnO, a cubic phase of In₂O₃, and a tetragonal structure of SnO₂. The surface morphology of the obtained thin films was analyzed via scanning electron microscopy (SEM). The results show a homogeneous and smooth surface with a very smaller particle size for ZnO/Fe₂O₃. Contrarily, the thin film SEM pictures of SnO₂/Fe₂O₃ and In₂O₃/Fe₂O₃ show a highly folded, rough, uneven surface with imperfections at 20 and 65 nm. The Hall effect indicates an increase in the carrier concentration and the mobility of SnO₂/α-Fe₂O₃, which can be attributed to the increase in the number of grain boundaries on the surface of the layer and corresponding increase in the dislocation density across the boundaries. When the photocatalytic activity of the sprayed thin films was evaluated, it was found that the paired In₂O₃/α-Fe₂O₃ photoatalysts had an impact on the photocatalytic activity and degradation mechanism of the MB dye. This fractured surface enhances the speed at which impurities are absorbed and facilitates water’s easy penetration into the surface of the thin layers. Additionally, when a thin coating of In₂O₃/α-Fe₂O₃ was used, only 5% of Gram-negative Pseudomonas aeruginosa (Pa) and 0.187% of Gram-positive Bacillus subtilis (Bs) bacterial cells were still viable. As this heterostructure can be produced using an easy spray pyrolysis approach, these results are very interesting. This study offers a path for additional research in the healthcare industry. The In₂O₃/α-Fe₂O₃ and SnO₂/α-Fe₂O₃ thin films yielded the most effective combined efficiency.