TiO 2 Nanoparticles with Mixed Anatase-Rutile Phase Structure Doped with Different Concentrations of Iron for Photocatalytic Activity in Degrading Methylene Blue

doi:10.21203/rs.3.rs-5300138/v1

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TiO 2 Nanoparticles with Mixed Anatase-Rutile Phase Structure Doped with Different Concentrations of Iron for Photocatalytic Activity in Degrading Methylene Blue

https://doi.org/10.21203/rs.3.rs-5300138/v1

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Methylene blue as dye waste test was used to facilitate the photodegradation of iron-doped anatase-rutile mixed phase TiO2 nanoparticles (NPs) under visible light, which were synthesized using the co-precipitation method. These nanoparticles were characterized using X-Ray diffraction (XRD), UV-Vis, FTIR, field emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM), and energy-dispersive X-Ray spectroscopy (EDX). The XRD analysis revealed that the diffraction patterns exhibited two-phase structures, namely anatase and rutile phase structures, where the intensity of the rutile phase structures was greater than that of the anatase phase structures. The Ultraviolet-Visible Spectroscopy (UV-Vis) measurements indicated that there is a reduction of the bandgap energy of the Fe-TiO2 NPs. FESEM micrographs revealed that agglomerations formed clusters, and SEM results showed that the nanoparticles aggregate to create structures on the surface that resemble edelweiss flowers. Based on Kramers-Kronig analysis, the reduction in optical phonon (Δ(𝐿𝑂−𝑇𝑂)) cm-1 difference with a decrease in the rutile fraction as a function of Fe increased. The reduction in the rutile phase fraction correlated with a decrease in photocatalytic activity, indicating that the rutile phase has a crucial role in the photodegradation process (1wt% achieve 𝑘ads rate: 0.00273 min-1). These results suggest that iron-doped anatase-rutile mixed-phase TiO2 nanoparticles are suitable as photocatalysts.

Fe-TiO 2 nanoparticles

photocatalysts

anatase phase

Methylene blue is considered a coloured waste because it finds widespread use in many industries, including those of paper and textiles, which requires attention due to its potential toxicity that can harm aquatic life and disrupt ecosystems. Photo-catalysing is one solution to this problem, and nanoparticle materials are a promising candidate. Nanoparticles (NPs) are particles with sizes on the nanometer scale, ranging from 1 to 100 nanometers. These particles exhibit superior physical and chemical properties compared to bulk materials. The increased fraction of atomic surfaces due to the reduced particle size is the main reason behind these enhanced properties. One extensively studied nanoparticle is titanium dioxide (TiO₂). TiO₂ nanoparticles are n-type semiconductor materials widely employed in environmental purification due to their long-term stability, high photo-energy with oxidation potential, and non-toxic nature [1–4]. However, TiO₂ has limitations in in visible light absorption due to its wide bandgap that is 3.0 eV for rutile and 3.2 eV for anatase [1, 5]. Consequently, there are several restrictions on the further application of TiO₂ under visible light. Various techniques have been used, such as introducing impurities, to adjust the bandgap of TiO₂ into the visible range.

Impurities, such non-metals, have been added to TiO2 in order to lower the band gap energy and enhance photocatalytic activity [6–9]. However, the TiO2 crystal structure may be broken down by non-metal doping at high temperatures [10]. As a substitute, metal dopants like iron have been utilised because its comparable ionic radius to that of titanium, which aids in its integration into the crystal lattice. [10–12]. Adding Fe to TiO2 creates oxygen vacancies, which enables hydroxide ions to be adsorbed to the surface and change its chemical state [8], leading to a decrease in the band gap energy. Several investigations on iron-doped TiO₂ semiconductor NPs have reported on optical characterization using techniques like FTIR, Raman spectroscopy, XPS, and photoluminescence [5, 12]. However, few studies have been conducted on the optical characteristics of iron-doped TiO2 NPs using the Kramers-Kronig (K-K) method. Therefore, this study employs the K-K method to further investigate these optical properties and the surface features of Fe-doped TiO₂ nanoparticles, particularly visible light photocatalytic activity for methylene blue (MB: C₁₆H₁₈ClN₃S).

Here, iron (Fe)-doped titanium dioxide (TiO₂) NPs were prepared via the co-precipitation method, utilizing cold titanium tetrachloride (TiCl₄) as the primary precursor and iron (III) chloride (FeCl₃) as the dopant [6]. This method was selected for its simplicity, cost-effectiveness, and short processing time. The iron-doped anatase-rutile mixed-phase TiO₂ NPs were structurally characterized by evaluating crystallite size and strain utilizing the XRD method. Fourier Transform Infra-red was used to identify the functional group, and to assess changes in optical characteristics. Additionally, both FESEM and HRTEM techniques were employed to examine surface morphology. Since Fe-doped TiO₂ NPs are widely used as photocatalysts, UV-Vis absorption experiments were conducted by irradiating the nanoparticles with light in the presence of methylene blue for various time intervals.

2.1. Synthesis of anatase-rutile mixed phase Fe-doped TiO₂ NPs

Anatase-rutile mixed-phase TiO₂ nanoparticles (NPs) doped iron with different ratios of (Fe/Ti), ranging from 1 wt% to 4 wt%, were prepared using the co-precipitation technique. This method involved dissolving 0.211 grams of iron (III) chloride (FeCl₃) in 200 mL of distilled water, which was then filtered twice. A small quantity of hydrochloric acid (37%) was incorporated to the solution to ensure FeCl₃ dissolution in distilled water. Simultaneously, a 5 mL cold solution of titanium tetrachloride (TiCl₄) was prepared and added dropwise to the FeCl₃ solution. The mixed solution was agitated for 2 hours via a magnetic stirrer at a rate of 250 rpm. The mixed solutions were then continuously stirred at 50°C for 16 hours to obtain a precipitate solution. The obtained precipitate was dialyzed until no chloride ions were detected. Finally, the precipitate was heated in a furnace at 100°C for 5 hours. All chemicals in this study were obtained and taken from Merck brand. The illustration of the synthesis TiO₂ NPs doped with iron as illustrated in Fig. 1.

2.2. Material Characterization

The structural properties of anatase-rutile mixed-phase Fe-doped TiO₂ NPs with different iron concentrations were studied using several analytical techniques. X-Ray diffraction (X-RD) spectroscopy (Shimadzu XRD 7000) was used with CuKα radiation with a wavelength of 1.5406 Å, operated at 30 mA and 40 kV, respectively. Diffraction angles were recorded in the range of 15^o to 70^o at a speed rate of 2^o/min. The X-RD results were used to assess material properties, including crystallite size and strain, using the Debye-Scherrer, UDM, and SSP methods. The FTIR spectroscopy (IR Prestige-21 Shimadzu) was employed to analyze functional groups of FTIR spectra, recorded at wavenumbers between 350 and 4000 cm^− 1. The results from FTIR were used to further study the optical properties, including refractive index (n), extinction coefficient (k), and the dielectric layer using the Kramers-Kronig (K-K) method. The Ultraviolet-Visible (UV-Vis) spectroscopy (Shimadzu Spectrophotometer 1800) was employed to determine the band gap energy of the NPs under study. The surface morphology of Fe-doped TiO₂ NPs was studied using the FESEM (JIB-4610F, JEOL). Further, the FESEM was equipped with energy-dispersive Energy Dispersive X-ray Spectroscopy (EDX) to determine the chemical composition of the nanoparticles. Moreover, the HRTEM was performed to confirm the surface morphology observed from the FESEM results.

2.3. Analysis of activity measurements

To assess photocatalytic activity, MB: C₁₆H₁₈ClN₃S was used to examine the photocatalytic degradation of anatase-rutile mixed phase structures Fe-doped TiO₂ NPs. The % solution of MB that will be utilized was prepared by dissolving 40 mg of MB into 100 mL of distilled water. By incorporating 0.2 g of NPs into 100 mL of the pre-made MB solution, the photocatalytic activity was investigated. The prepared solution was exposed to light using a 300-watt Osram Tungsten halogen lamp. The solution was agitated at 200 rpm during irradiation for different time intervals. At 5-minute intervals, 5 mL of illuminated MB was extracted and filtered using Whatman paper no. 42. The photo-degradation of the MB and its absorption spectra were assessed using a UV-Vis spectrophotometer.

3.1. X-RF and X-RD

The oxide and element content of the anatase-rutile mixed phase Fe-TiO₂ NPs were determined using X-Ray fluorescence (XRF) analysis with an ARL Quant’X EDXRF Analyzer. Table 1 shows the percentage of oxide and element content in the nanoparticles with different Fe and Ti dopant ratios. As seen in Table 1, there was an increase in iron oxide (Fe₂O₃) and Fe content in Fe-TiO₂ NPs as the Fe/Ti dopant ratio increased from 1 wt % to 4 wt %. However, the titanium dioxide and Ti content slightly decreased. The Fe₂O₃ content was 0.24% at a dopant ratio of 1 wt %, increasing to 1.24% with a dopant ratio of 4 wt %.

Table 1

The oxide and the element content in Fe-doped TiO₂ nanoparticles
Dopant Ratio (Fe/Ti) (%)	Oxide content (%)
Dopant Ratio (Fe/Ti) (%)	TiO₂	Fe₂O₃	P₂O₅	others	Ti	Fe	Cl	others
1	98.84	0.24	0.10	0.82	98.50	0.32	1.09	0.09
2	98.07	0.71	0.11	1.11	97.47	0.93	1.49	0.11
3	98.01	0.85	0.14	1.00	97.43	1.11	1.30	0.16
4	97.72	1.24	0.29	0.75	97.17	1.63	1.00	0.20

Figure 1 shows the X-RD curves of Fe-doped TiO₂ NPs at different ratios of Fe/Ti doping concentrations from 1 to 4 wt %. It can be seen in Fig. 1, the X-RD pattern are mixed of anatase and rutile phase structures in which the anatase phase fraction is inferior compared to the rutile phase fraction. The position of diffraction peak 2θ was seen at 24.53^o, which corresponds to the anatase phase of (101) planes with the lattice parameter of a = b = 3.789 Å, and c = 9.537 Å (JCPDS Card no. 96-900-9087). On the other hand, for the rutile structures, the position of diffraction peak 2θ was observed at 27.24, 36.07, 41.29, 43.65, 54.26, 56.25, 62.93, 69.03^o which is assigned to the reflection plane of (110), (101), (111), (120), (211), (220), (130), and (112), respectively, with the lattice parameter of a = b = 4.603 Å, and c = 2.966 Å (JCPDS Card no. 96-900-4145). It is noteworthy that the main peak of rutile structures (110) at iron concentration for 2 wt% and 3 wt% is shifted to the right in comparison to iron concentration of 1 wt % and 4 wt %. Moreover, a small diffraction peak related to the hematite (Fe₂O₃) at (104) plane was observed in this X-RD curves indicating that the iron dopants are not well dispersed within the TiO₂ matrix although they have similar atomic radius of Ti = 0.64 Å and Fe = 0.68 Å. The presence of the hematite in X-RD results was also confirmed from X-RF results. The observed hematite peak is also shown in the previous study by Ganesh et al [8]. It is interesting to note in Fig. 1, the entire diffraction pattern is similar at various concentrations of iron, indicating a mixture of both anatase and rutile phase structures.

As shown in Fig. 1, for all the samples the X-Ray diffraction curves have both anatase and rutile structures, therefore we estimate the percentage of phase structures on Fe-doped TiO₂ NPs. The Spurr and Myers approach is used to estimate the fraction of phase structure, as follows [13].

$$\:{X}_{A}=1-{X}_{R}=1-\left(\frac{1.26{I}_{110}}{{I}_{101}+1.26{I}_{110}}\right)$$

The subscripts (A) and (R) denote the anatase and rutile phases, with I₁₁₀ and I₁₀₁ representing the higher intensities of the rutile (110) and anatase (101) peaks, respectively. The obtained results are listed in Table 2. In general, there is a steady rise in the anatase phase fraction as the iron content rises from 1–4% by weight. The rutile phase fraction reduces as the iron concentration increases, and at an iron concentration of 1 wt %, the anatase phase composition is around 29.08%, and it raises to 39.92% when the iron concentration raises to 4 wt %. In contrast, the rutile phase composition reduces as the iron concentration increases. At an iron concentration of 1 wt %, the rutile phase is around 70.92% and decreases to around 67.08% at an iron concentration of 4 wt%. The outcome significantly diverges from prior research [14, 15] where they found that the crystalline structure of Fe-TiO₂ NPs was not mixed between anatase and rutile phase structure. The X-RD pattern showed the anatase phase composition from 1 wt % to 10 wt%.

Figure 1 shows the diffraction peak 1 wt% is quite broad. This broadening peak is probably due to the instrumental effects and sample-dependent factors. Gaussian correction can be used to adjust instrumental broadening and accurately determine the full width at half maximum (β_hkl) from standard data. This data has facilitated the calculation of instrumental broadening ($\:{\beta\:}_{instrumental}$). By using peak broadening, one can approximate the size of the crystallites and the strain in the lattice by applying the following formula:

$$\:{\beta\:}_{hkl}^{2}={\beta\:}_{measures}^{2}-{\beta\:}_{instrumental}^{2}$$

$$\:D=\frac{k\lambda\:}{{\beta\:}_{hkl\:}\text{cos}\theta\:}$$

Previous studies [16, 17], has shown that the microstrain (ε) had a role in the broadening as well, leading to the following modification of the Scherrer formula:

$$\:\beta\:={\beta\:}_{hkl}+{\beta\:}_{strain}=\frac{k\lambda\:}{D\:cos\theta\:}+4ϵtan\theta\:$$

where $\:D$ is the crystalline size (nm), $\:\epsilon\:$ is a microstrain, $\:\lambda\:$ is the radiation wavelength (1.5406 Å for CuKα radiation), $\:\beta\:$ is full-width half maximum, and $\:k$ is a constant. Several methods estimate the high precision of structural characteristics using quantitative XRD spectra at high diffraction angles. However, the size strain plot (SSP) approach is more accurate at low diffraction angles. This model uses Gaussian and Lorentzian functions for strain profile and crystalline size. The SSP model is described in the following equation:

$$\:{\left({d\beta\:}_{hkl}cos\theta\:\right)}^{2}=\frac{k}{D}\left({d}^{2}{\beta\:}_{hkl}cos\theta\:\right)+{\left(\frac{ϵ}{2}\right)}^{2}$$

where d is the spacing between the atoms (Å), $\:\epsilon\:$ is a microstrain that is determined using the Gaussian function, and D is the crystalline size obtained from Lorentz function. For different concentrations of iron, the UDM and SSP outcomes of the composite are shown in Fig. 2. As presented in Table 2, the Scherrer and SSP methods were used to quantitatively analyze the XRD spectra. Figure 2 and Table 2 show that the crystallite size is larger for the Scherrer technique compared to the SSP approach. This discrepancy is likely caused by the different number of factors used in the computations. The effects of microstrain, atomic distance, and full-width half maximum were all included into the SSP method's computations, but not the Scherrer method's.

3.2. Optical Properties

3.2.1. Fourier Transform Infra-Red (FTIR)

The FTIR spectra of anatase-rutile mixed phase Fe-doped TiO₂ NPs with various concentrations of iron from 1 wt% to 4 wt % are shown in Fig. 3(a, b). Figure 3(a) presents the transmittance spectra of Fe-doped TiO₂ NPs from λ = 4000 to 480 cm^− 1, while Fig. 3(b) displays the FTIR spectra from λ = 1000 − 250 cm^− 1. Figure 3(a) shows a clear transmittance band at 3393 cm^− 1, attributed to the stretching vibration of O-H groups from H₂O adsorbed on the TiO₂ nanoparticle surface. The intensity of this band diminishes with increasing the iron-doped concentration, from 1–4%. Also, the observed peak at 1627 cm^− 1 can be attributed to the bending vibration of the adsorbed H₂O molecules on the surface of TiO₂ [18, 19]. The intensity of this band correlates directly with Fe concentration. Since the O-H groups were crucial in degrading MB, their presence in Fe-doped TiO2 may enhance its photocatalytic activity. Additionally, a broad transmittance peak was observed at 637 cm^− 1, which is related to the stretching vibration mode of TiO₂ [20]. This peak was shifted to the left at 789 cm^− 1 when the iron concentration increased to 4 wt %.

An expansion of the FTIR spectra was performed in order to detect the absorbance peak associated with the vibration of TiO2 in the 1000 cm^− 1 to 250 cm^− 1 wavenumber region. In Fig. 3(b), the transmittance band appears at 393 and 368 cm^− 1, which is associated with the stretching vibration mode of TiO₂ [20]. Previous studies by Reddy, et al. reported that characteristic peaks at 616 until 480 cm^− 1 are assigned TiO₂ band with stretching vibration [20]. Furthermore, Kramers-Kronig (K-K) relations are applied to study the absorption band at 480–616 cm^− 1. K-K relation was used to FTIR spectra to determine optical parameters such the refractive index ($\:n$), the extinction coefficient ($\:k$), the dielectric, and the energy loss function.

3.2.2. Refractive index (n) and extinction coefficient (k)

This study further examines the optical properties of the anatase-rutile mixed phase of Fe-doped TiO₂ NPs such as the refractive index ($\:n$), the extinction coefficient ($\:k$), the electric function, and the energy loss function (ELF) using the Kramers-Kronig (K-K) model. In this model, the FTIR data were used to determine these properties [21–23]. To examine the optical properties, data from FTIR transmittance spectra were transformed to reflectance spectra by the following equations [24–26]:

$$\:A\left(\omega\:\right)=2-log\left|T\left(\omega\:\right)\%\right|$$

$$\:R\left(\omega\:\right)=100-\left|T\left(\omega\:\right)+A\left(\omega\:\right)\right|$$

where $\:A\left(\omega\:\right)$, $\:T\left(\omega\:\right)$, and $\:R\left(\omega\:\right)$ are the absorbance, transmittance, and reflectance spectra, respectively. The complex quantity's refractive index ($\:n$) is expressed as $\:\widehat{n}\left(\omega\:\right)=n\left(\omega\:\right)+ik\left(\omega\:\right)$, where $\:n\left(\omega\:\right)$ is the refractive index for the real part, and $\:k\left(\omega\:\right)$ is the extinction coefficient for the imaginary part. Both these parameters can be expressed in the following equation [27, 28]:

$$\:n\left(\omega\:\right)=\frac{1-R\left(\omega\:\right)}{1+R\left(\omega\:\right)-2\sqrt{R\left(\omega\:\right)}cos\varnothing\:\left(\omega\:\right)}$$

$$\:k\left(\omega\:\right)=\frac{2\sqrt{R\left(\omega\:\right)}\:sin\varnothing\:\left(\omega\:\right)}{1+R\left(\omega\:\right)-2\sqrt{R\left(\omega\:\right)}cos\varnothing\:\left(\omega\:\right)}$$

Where, $\:\varnothing\:\left(\omega\:\right)$ represent the phase difference between the incident and reflected photon signals in the FTIR spectroscopy:

$$\:\varnothing\:\left(\omega\:\right)=-\frac{\omega\:}{\pi\:}\underset{0}{\overset{\infty\:}{\int\:}}\frac{lnR\left({\omega\:}^{{\prime\:}}\right)-lnR\left(\omega\:\right)}{{\omega\:}^{{\prime\:}2}-{\omega\:}^{2}}$$

Utilizing the K-K relation, the ∅(ω) is expressed as: $\:\varnothing\:\left({\omega\:}_{j}\right)=-\frac{4{\omega\:}_{j}}{\pi\:}x\:\varDelta\:\omega\:\:x\:\sum\:_{i}\frac{ln\left(\sqrt{R\left(\omega\:\right)}\right)}{{\omega\:}_{i}^{2}-{\omega\:}_{j}^{2}}$ (11)

j represent a series of wavenumbers, if j is an odd number, then i takes the values 2,4,6,8, …j-1, j + 1. Conversely, when j is an even number, then i is 1,3,5,7, … j-1, j + 1, … Δω_i+1 - Δω_i. Figure 4(a) shows the experimental results for the refractive index (n) and extinction coefficient (k) of the investigated NPs with different iron concentrations. In Fig. 4(a), the black solid line shows where 𝑛 and 𝑘 intersect, which relates to the optical properties of transverse optical (TO) and longitudinal optical (LO) phonon vibrations. The lower wavenumber point corresponds to the TO mode, while the higher wavenumber point corresponds to the LO mode. From Fig. 4(a), at iron concentrations of 1 wt % to 4 wt % the $\:TO$ mode appears at 603, 605, 611, and 725 cm⁻¹ meanwhile, the $\:LO$ is observed at 956, 975, 966, and 1037 cm⁻¹, respectively. It was clearly seen that the optical mode $\:TO$ and $\:LO$ increase as function iron concentration increases. Data from $\:LO\:$and $\:TO$ was used to determine the optical phonon difference $\:\varDelta\:(LO-TO)$ which is 353, 370, 355 and 312 cm⁻¹. The optical phonon difference reduces when iron concentration increases, which could be associated with a decrease in the rutile fraction composition.

3.2.3. Dielectric layer

Another method to identify the $\:TO$ and $\:LO$ modes involves analysing the main peak position of the dielectric function and the energy loss function. Dielectric function is defined as $\:\epsilon\:\left(\omega\:\right)={\epsilon\:}_{1}\left(\omega\:\right)+i{\epsilon\:}_{2}\left(\omega\:\right)$, where $\:{\epsilon\:}_{1(}\omega\:)$ and $\:{\epsilon\:}_{2}\left(\omega\:\right)$ means the real and the imaginary components of dielectric function, which are calculated using the relations:

$$\:{\epsilon\:}_{1}\left(\omega\:\right)={n}^{2}\left(\omega\:\right)-{k}^{2}\left(\omega\:\right)$$

$$\:{\epsilon\:}_{2}\left(\omega\:\right)=2n\left(\omega\:\right)k\left(\omega\:\right)$$

Using E.q. 12 and 13, FTIR spectra are quantitatively analysed and shown in Fig. 4(b). The main peak position of the real ($\:{\epsilon\:}_{1(}\omega\:)$) and imaginary parts ($\:{\epsilon\:}_{2(}\omega\:)$) of the dielectric function are shifted to a higher wavenumber position after the iron concentration increases from 603 cm^− 1 (1 wt %) to 725 cm^− 1 (4 wt %). The $\:TO$ mode is derived from the electric function is a good agreement with $\:TO$ mode derived from the refractive index and coefficient extinction. Similar results in the refractive index and the extinction coefficient for $\:LO$ mode obtained from the energy loss function $\:Im\left(-1/{\epsilon\:}_{2}\left(\omega\:\right)\right)=\left({\epsilon\:}_{2}\left(\omega\:\right)\right)/\left({\epsilon\:}_{1}\left(\omega\:\right)\right)/\left({\epsilon\:}_{1}^{2}\left(\omega\:\right)+{\epsilon\:}_{2}^{2}\left(\omega\:\right)\right)$ as shown in Fig. 4(c) [29].

3.2.4. UV-Vis absorbance spectra

Figure 5(a, b) shows the absorption spectra of Fe-TiO₂ nanoparticles at various iron concentrations, and the curve of $\:{\left(ahʋ\right)}^{2}$ against $\:\left(hʋ\right)$ to determine the value of the band gap energy of the nanoparticles at various iron concentration, respectively. The absorbance spectra display two peaks at 396 and 660 nm, respectively, as illustrated in Fig. 5(a). With an increase in iron concentration to 1 wt %, the absorbance peak at 660 nm is shifted to the right (indicating a redshift). Furthermore, with a concentration of 1 wt % iron, the band gap energies computed around 3.26 eV. The band gap decreases to 3.19, 2.88, and 2.83 eV with Fe concentrations of 2 wt %, 3 wt %, and 4 wt %, respectively. The XRD data reveal a notable reduction in the band gap energy, which might be correlated with increasing fraction of the anatase phase and the decreasing fraction of the rutile phase (Table 2). Previous research has also revealed similar outcomes where the narrowing of the band gap of TiO₂ and Fe-TiO₂ are observed is due to the increase of anatase phase when the iron concentration increased. [30, 31].

3.3. Surface Morphology

The surface morphology of Fe-doped TiO₂ NPs at 3 wt % and 4 wt % iron concentration was investigated using FESEM. The SEM micrographs of Fe-doped TiO₂ are depicted in Fig. 6(a, b). These images depict that the nanoparticles clump together to create structures that resemble edelweiss flowers on the surface. The surface of Fe-doped TiO₂ NPs forms the nano grass at the iron concentration of 3 wt %, and 4 wt %. The length sizes of the nano grass are around 36 nm at the iron concentration of 3 wt %. However, after the iron concentration was increased to 4 wt %, the length sizes of the nano grass increased to about 41 nm shown in Fig. 6(c, d). The length size obtained in this study is quite large in the range of the crystallite size obtained from the X-RD results.

Prolonged aging is the likely cause of nano grass production. Prior research using the same precursor, which combines the chemical elements FeCl₃ and TiCl₄, also found a similar surface morphology [32]. Furthermore, the EDS data shown in Fig. 6(e, f) indicate the existence of Ti, O, and Fe as chemical components of the Fe-doped TiO2 at 3 wt % and 4 wt %, respectively. The chemical composition obtained from the EDX study is tabulated in the inserted Table in Fig. 6(e, f). As shown in the inserted Table, the presence of the oxygen element increases as the iron concentration increases, while Ti elements reduce when the iron concentration increases. Also, the Cu elements are observed at 4 wt % of iron concentration.

The HRTEM was conducted on the sample to further investigate the surface morphology obtained from the FESEM results. Figure 7(a, b) represents the micrograph of TEM of Fe-doped TiO₂ at 3 wt % and 4 wt %. In these images, the nano grass exhibits considerable sharpness at 3 wt % and 4 wt %. Also, the fringe pattern is depicted in Fig. 7(c, d) at iron concentrations of 3 and 4 Wt %. Figure 7(c) shows the fringe pattern in one growth direction of anatase (011) with an interplanar distance of 0.26 nm. However, the iron concentration of 4 wt % as shown in Fig. 7(d), illustrates the fringe pattern has 2 growth directions; anatase (011) and rutile (110), corresponding to interplanar distances of 0.34 and 0.31 nm, respectively. The results might be due to the change of fraction composition as the iron concentration increases from 3 wt % to 4 wt %.

The SAED patterns of Fe-doped TiO₂ NPs with varying iron concentrations confirm the mixed-phase structure as illustrated by the circular rings at different indexed planes of anatase and rutile in Fig. 7(e, f). The rings, from inner to outer, correspond to the (101) planes of the anatase phase and the (110), (101), (111), and (120) planes of the rutile phase. These planes are consistent with planes obtained from X-RD results.

3.4. UV-Vis Spectroscopy

Figure 5 shows the absorption spectra of Fe-TiO2 nanoparticles subjected to 60 minutes of annealing at temperatures ranging from 200 to 500 ^oC. The inset figure displays the plot of $\:{\left(ahʋ\right)}^{2}$ against $\:\left(hʋ\right)$, is used to compute the band gap energy of the nanoparticles following the annealing. Figure 5 illustrates the absorbance spectra at iron concentration 1 wt % 386 reveal two absorbance peaks at the wavelength of 386 and 660 nm, respectively. The absorbance peak at 386 nm is shifted to right (a red shift) of 396 nm at an increasing iron concentration of 4 wt %. While the absorbance peak at 660 nm is still in the same position even the iron centration was increased at 4 wt %. In Fig. 5(b), the estimated band gap of Fe-doped TiO₂ from a plot of $\:{\left(ahʋ\right)}^{2}$ to $\:\left(hʋ\right)$ at an iron concentration of 1 wt % is around 3.26 eV. There is a steady decrease in this band gap to around 3.19, 2.88, and 2.83 eV at iron concentration of 2, 3, 4 wt %, respectively. The decrease of the band gap energy can be attributed to the reduction of the rutile phase fraction as indicated by X-RD results (Table 2). Previous research have also revealed similar outcomes where the narrowing of the band gap of TiO₂ and Fe-TiO₂ is observed due to the increase of the rutile phase after annealing [45]. According to ref [45], after annealing at 500 ^oC, the band gap energy dropped from 3.1 eV to 2.3 eV.

3.5. Photocatalytic Activity

Figure 8 displays the UV–Vis absorption spectra of photogenerated MB solution over the examined NPs that have been annealed at 1–4% iron content. A halogen light was used to illuminate all the samples for varying durations of irradiation. Two absorption peaks may be seen at 290 and 660 nm, as seen in Fig. 8(a). As the irradiation period increases to 150 minutes, the absorption strength is drastically reduced due to the blue shift, which moves these two peaks to a lower wavelength. Absorption of MB becomes more apparent as crystallite size rises, as seen by the decrease in absorption intensity [33]. It is worth mentioning that the 290 and 660 nm absorption peaks remain unchanged. Figure 9(a, b) shows the photocatalytic activity of the nanoparticles with different concentrations of iron and exposed to UV light. The samples that had a 1 wt % iron content showed the most photocatalytic activity compared to the other samples. The kinetic model proposed by Langmuir-Hinshelwood was used to examine the rate of photocatalytic degradation of MB over Fe-doped TiO2 nanoparticles as follows [34]:

$$\:ln\left(\frac{C}{{C}_{o}}\right)=-{k}_{ads}t$$

Where $\:{C}_{o}$ denotes the original concentration of MB, $\:C$ denotes the residual concentration of MB following the irradiation time (min), $\:{k}_{ads}$ represents the apparent kinetic constant. The $\:{k}_{ads}$ value was determined from the slope of the linear relationship of $\:-ln\left(\frac{C}{{C}_{o}}\right)$ and irradiation time (min) as seen in Fig. 9(c-d). The estimation of $\:{k}_{ads}$ value was 0.00273, 0.00241, 0.00243, and 0.00175 min^− 1 for the Fe-TiO₂ NPs with iron concentrations of 1, 2, 3, and 4 wt%, respectively. It can be noticed that the samples with an iron content of 1 wt% possess the greatest rate of photocatalytic degradation of MB. It is possible that the crystal fraction composition, with rutile being more advantageous than anatase phase fraction composition, is associated with the improved photocatalytic activity at a 1% iron concentration. Based on the X-RD results, at 1 wt % iron concentration, the rutile phase composition is around 71%, while the anatase phase composition is around 29% indicating that rutile is superior to anatase. As the iron concentration increases to 4 wt %, the rutile phase fraction decreases to 67%. According to these findings, the reduction of the photoactivity of Fe-TiO₂ NPs is caused by the reduction of the rutile phase fraction. This finding is supported by the previous results where the lowering of the photocatalytic active is caused by reducing of rutile phase fraction [35].

Table 2

The structural parameters of Fe-TiO₂ nanoparticles with dopant ratio (Fe/Ti) wt 1,2,3, and 4%, including the lattice parameters, crystalline size, strain, stress, and energy using Scherrer, UDM, and SSP method, respectively.
Dopant Ratio (Fe/Ti)	hkl	Crystalline Structures	Phase composition (%)	a = b (Å)	c(Å)	Scherrer		William-Hall		Size-Strain Plot
Dopant Ratio (Fe/Ti)	hkl	Crystalline Structures	Phase composition (%)	a = b (Å)	c(Å)	D(nm)	ε	D(nm)	ε	D(nm)	ε	σ (MPa)	U(Kj/m³)
	[101]*
	[110]					14.26	0.0112
	[101]					19.34	0.0063	.
	[111]					19.90	0.0054
	[120]	Anatase	29.08	3.7892	9.5370	21.13	0.0048	18.88	0.000695	10.59	0.00614	141.26	43.370
1%	[210]	Rutile	70.92	4.6030	2.9660	14.55	0.0057
	[220]					12.37	0.0065
	[130]					18.38	0.0039
	[112]					15.88	0.0042
2%	[101]*	Anatase Rutile	31.79 68.21	3.7300 4.6030	9.3700 2.9660	16.10	0.0216			13.99	0.0043	98.44	21.068
	[110]					12.96	0.0122
	[101]					17.89	0.0067
	[111]					18.17	0.0059
	[120]					18.40	0.0023	18.32	0.000367
	[210]					15.17	0.0054
	[220]					14.91	0.0053
	[130]					16.41	0.0044
	[112]					15.45	0.0043
3%	[011]*	Anatase Rutile	31.75 68.25	3.7300 4.6160	9.3700 2.9770	15.46	0.0225			10.41	0.0062	141.80	43.723
	[110]					12.54	0.0126
	[101]					17.88	0.0068
	[111]					19.71	0.0054
	[120]					17.15	0.0059	12.36	0.00190
	[210]					14.54	0.0057
	[220]					16.67	0.0048
	[130]					19.18	0.0038
	[112]					20.19	0.0033
4%	[011]*	Anatase Rutile	32.92 67.08	3.7300 4.6030	9.3700 2.9660	15.22	0.0231			10.41	0.0060	141.81	43.723
	[110]					13.75	0.0116
	[101]					18.47	0.0066
	[111]					19.90	0.0054
	[120]					18.55	0.0055	15.23	0.00056
	[210]					15.24	0.0054
	[220]					14.71	0.0054
	[130]					16.91	0.0043
	[112]					18.94	0.0035

Here, the discussion of the photodegradation of anatase-rutile mixed-phase TiO₂ nanoparticles (NPs) doped with iron (1, 2, 3, and 4) wt% under visible light and its properties has been discussed. The shift in phase composition was accompanied by a red shift at 660 nm in the absorbance peak and a decrease in the intensity of the O-H strain vibration peak in FTIR analysis. Based on Kramers-Kronig analysis, the reduction in optical phonon ($\:\varDelta\:\left(LO-TO\right)$) cm^− 1 difference with a decrease in the rutile fraction as a function of Fe increased. The HRTM confirm both anatase (011) and rutile (110) fringe patterns in the 4 wt% iron-doped sample confirms the coexistence of both phases, with a potential shift towards a higher rutile fraction. The reduction in the rutile phase fraction correlated with a decrease in photocatalytic activity, indicating that the rutile phase is crucial to the photodegradation process (1wt% achieve $\:{k}_{ads}$: 0.00273 min^− 1).

Author Statement

All Author declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Ethical Approval

Not applicable.

Author Contribution

PLG: Conceptualization, Methodology, Investigation, Resources, Data Curation, Writing - Original Draft; HH: Conceptualization, Investigation, Resources, Data Curation, Validation, Writing – Review & Editing; SS: Methodology, and Supervision; DT: Software; AA: Validation; PT: Visualization; DA: Formal analysis; AA: Writing – Review & Editing

Acknowledgment

The authors thank the LP2M-UNHAS throughout the Collaborative of Fundamental Research (PFK) scheme under contract number 00309/UN4.22/PT.01.03/2024 for the laboratory facilitator.

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TiO 2 Nanoparticles with Mixed Anatase-Rutile Phase Structure Doped with Different Concentrations of Iron for Photocatalytic Activity in Degrading Methylene Blue

Status:

Version 1

Abstract

Figures

1. Introduction

2. Experimental details

2.1. Synthesis of anatase-rutile mixed phase Fe-doped TiO₂ NPs

2.2. Material Characterization

2.3. Analysis of activity measurements

3. Results and Discussion

3.1. X-RF and X-RD

3.2. Optical Properties

3.2.1. Fourier Transform Infra-Red (FTIR)

3.2.2. Refractive index (n) and extinction coefficient (k)

3.2.3. Dielectric layer

3.2.4. UV-Vis absorbance spectra

3.3. Surface Morphology

3.4. UV-Vis Spectroscopy

3.5. Photocatalytic Activity

4. Conclusion

Declarations

Author Statement

Ethical Approval

Author Contribution

Acknowledgment

References

Additional Declarations

Status:

Version 1

TiO 2 Nanoparticles with Mixed Anatase-Rutile Phase Structure Doped with Different Concentrations of Iron for Photocatalytic Activity in Degrading Methylene Blue

Status:

Version 1

Abstract

Figures

1. Introduction

2. Experimental details

2.1. Synthesis of anatase-rutile mixed phase Fe-doped TiO2 NPs

2.2. Material Characterization

2.3. Analysis of activity measurements

3. Results and Discussion

3.1. X-RF and X-RD

3.2. Optical Properties

3.2.1. Fourier Transform Infra-Red (FTIR)

3.2.2. Refractive index (n) and extinction coefficient (k)

3.2.3. Dielectric layer

3.2.4. UV-Vis absorbance spectra

3.3. Surface Morphology

3.4. UV-Vis Spectroscopy

3.5. Photocatalytic Activity

4. Conclusion

Declarations

Author Statement

Ethical Approval

Author Contribution

Acknowledgment

References

Additional Declarations

Status:

Version 1

2.1. Synthesis of anatase-rutile mixed phase Fe-doped TiO₂ NPs