Effects of Fe and Al doping on Titania: Structural, Morphological, Magnetic and Optical Investigations

Dilute magnetic semiconductors (DMS) are those semiconductors that are doped with transition metal atoms. The doping level directly affects the Curie temperature. DMS are promising materials for spintronics applications [1]. The semiconductors which are mostly used in transistors, lasers and integrated circuits are Gallium (Ga), Silicon (Si) and Titanium (Ti) that are nonmagnetic while, magnetic metals which are used in digital storage devices are Fe, Co, Ni, Al, and their compounds are not semiconductors. Moreover, the lattice crystal structures of magnetic ions are usually changed from the semiconductors which are used in the Electronics industry, making it difficult to combine the two in functional heterostructures [2]. Also, the dissimilarity of conductivity between metals and semiconductors limits the efficiency of spin-injection [3]. The other way is then to utilize materials that combine semiconducting and ferromagnetic behavior [4]. Development of magnetic semiconductors intended the manipulation of charge among spin in a single material without interrupting the semiconducting estates of the host [5]. DMSs have a wide band gap and high Curie temperature (T_c). A vacancy of oxygen is considered as a significant explanation behind RTFM in these semiconductors [6]. Spintronics operation is based on the manipulation, transport, and exposure of spin-polarized carriers to form semiconductors from a ferromagnetic material. Spintronics are used to enhance the traditional electronic and photonic devices because it provides faster device operation with low power consumption and innovate the information consumption [7, 8]. In solid-state devices, Spin based electron transit phenomena was discovered in the 1980s [9]. Spin-polarized LEDs, spin RTD, spin-FET (field-effect transistor), modulators, encoders, decoders are the possible spintronics devices [10]. TiO₂ based DMS nanomaterials possess the RTFM without changing its semiconducting properties which can be utilized as an application of spintronics semiconducting devices.

In recent years, numerous research on magnetism of oxide-based DMSs including transition metal-doped ZnO, TiO₂ and SnO₂, have been published [11, 12, 13, 14]. Titanium oxide (TiO₂) is a n-type semiconductor and is currently being used as a dilute magnetic semiconductor [15]. Titanium oxide occurs in several forms that are Anatase (Tetragonal), brookite (orthorhombic), and Rutile (Tetragonal) [16]. Anatase and rutile crystallize into Tetragonal structure (a = 3.78 Å and c = 9.52 Å) and (a = 4.59 Å and c = 2.96 Å), respectively. The band gap of anatase is slightly greater (3.2 eV) than that of rutile (3 eV) [17]. Anatase is more stable in Nanosized materials than in bulk-sized [18]. TiO₂ can be considered as a multifunctional material having the qualities of both magnetic materials and semiconductors [19]. The possible utilization of TiO₂ as a DMS material is ascribed to its great optical transmission in the UV-visible and close to the infrared region [20]. Many techniques are used for the preparation of transition metal-doped Titania: Hydrothermal method [21], sol-gel synthesis [22], pulsed laser deposition [23], RF plasma synthesis [24], ion beam induced chemical vapor deposition (IBICVD) [25], wet chemical synthesis [26].

The origin of ferromagnetism in DMSs is also a controversial issue. To explain it, several models including the carrier-mediated exchange, super-exchange magnetic polarons, F-center exchange mechanism etc. have been employed [27]. M. Yeganeh et al. [28] synthesized iron-doped titania nanoparticles at different molar ratios from 1% to 5% by sol-gel technique. Rongfang Yuan et al. [29] Synthesized Fe and Al co-doped TiO₂ nanotubes by hydrothermal method for attaining the low band gap energy and large specific area. Jian-ping et al. [30] prepared Fe doped TiO₂ by sol-gel route. Iron-doped TiO₂ films exhibit room temperature ferromagnetism which indicated the strong correlation between the oxygen vacancies or defects and ferromagnetism in Fe doped anatase TiO₂ films. X.H. Wang et al. [31] synthesized iron (III) doped TiO₂ by RF thermal plasma. This experiment showed that due to the presence of high oxygen pressure, no iron clusters were formed, which confirmed the paramagnetic nature of as-synthesized samples. Xianjie et al. [32] synthesized Al-doped TiO₂ by PLD, producing coupled Ti³⁺ moments due to electron defect associated with the vacancy of oxygen. Prajapati et al. [33] have synthesized Fe - doped TiO₂ (Ti_1-xFe_xO₂, 0.005 ≤ x ≤ 0.05) nanoparticles by sol-gel technique, RTFM was observed which was induced due to the presence of oxygen voids and defects in as-synthesized nanoparticles. To examine the photocatalytic activity Crisan et al. [34] prepared TiO₂ nano powders that are iron doped at 0, 0.5, 1, 2, and 5 wt% of Fe/Ti ratio using sol-gel route. By measuring the positive magnetic susceptibility at RT of as prepared nanoparticles they found that undoped TiO₂ shows diamagnetic behavior while the doped nanoparticles show paramagnetic. Y. Anatoly et al. [35] investigated the electron paramagnetic response and photoelectron spectroscopy of Fe and Al-doped TiO₂ nano crystallites by hydrothermal synthesis. Room temperature ferromagnetism could not be observed due to the intrinsic and extrinsic effects of materials.

Many researchers have reported numerous preparations [36, 37], various dopants [38], and surface modifications to broaden the light absorption of TiO₂ into the visible range and limit the recombination of excited electrons and holes for highly active photocatalysts. A variety of metal ions, especially transition metal ions such as Iron [39], Cobalt [40], and Aluminum [41], have been used as dopants for TiO₂ in order to improve absorption into the visible light range. Among these elements, iron (Fe) has received enormous attention. Schrauzer and Guth [42] were among the first to report on various Fe-free and Fe-doped TiO₂ powders obtained by impregnation and annealed at 1000⁰C.

In the present work, Fe⁺³ and Al⁺³ are doped into TiO₂ because the majority of researches revealed that doping Fe ions can strengthen the saturation magnetization of TiO₂ powders. The magnetic moment of Fe⁺³ is 5.9 μB [43] which induces magnetic properties in titania. Xu et al. [44] demonstrated that the defects in TiO₂ mediated by a trivalent dopant also led to good outcomes. Al⁺³ ions behave like an electron or hole trapper and decrease the rate of recombination of electrons and holes when doped in titania [45]. The hydrothermal method is used for the synthesis of nanoparticles. It basically is a geological process, relevant to the natural process; based on the combined effect of water and heat under high pressure [46]. This synthesis is versatile and favorable to the environment, no further processing or organics are needed in it [47]. That’s why it is known as a widely used method for a wide range of materials [48]. In spite of this, a little attention has been paid to synthesize DMS materials by hydrothermal method. The novelty of this project is that we have synthesized nanoparticles by this method and have investigated their structural, Morphological, magnetic, and optical properties.

Materials and Methods

Titanium oxide TiO₂ (Sigma-Aldrich), Iron Nitrate III Fe (NO₃)₃ (Sigma-Aldrich), Aluminum Chloride AlCl₃ (Sigma-Aldrich), Sodium Hydroxide NaOH (Riedel- DeHaan), Hydrochloric Acid HCl, Distilled Water were used for the synthesis of nanoparticles by Hydrothermal method.

3g powder of TiO₂ was added into a solution of 0.5 mol/ltr NaOH. Then the mixture was stirred for 2 hours to form a suspension at room temperature. Afterwards, the mixture was transferred to the Teflon vessel and heated at 110⁰ C for 24 hours in the oven then cooled at room temperature. Precipitates were washed with distilled water to maintain pH at 7 and 0.1 mol/L of HCl was added into it. For homogenized distribution of nanoparticles, the precipitate was put into an ultrasonicator, after sonication, the precipitate was washed with distilled water and filtered. Then the obtained material dried at a suitable temperature and crushed into nanoparticles. By following the aforementioned procedure, the second sample was prepared in which 0.1 mole of Fe (NO₃)₃.9H₂O was used as a precursor of dopant,while the quantity of TiO₂ (3g)remained fixed in remaining three samples. In third sample Fe and Al co-doped TiO₂ nanoparticles were prepared whereas the quantity of Fe (NO₃)₃.9H₂O and Aluminum Chloride AlCl₃ was 0.05 moles .In fourth sample the dopant was single i.e AlCl₃ and its quantity was 0.1 mole. All as-synthesized samples were annealed at 550⁰ C for two hours. The flow chart diagram of hydrothermal synthesis is shown in Fig.1.

Fig. 1: Schematic of the hydrothermal synthesis of Fe/Al co-doped TiO₂ ultra-fine powder.

The as-synthesized nanoparticles were characterized by using different characterization techniques. The structural properties were studied using D8 ADVANCE. The functional structural groups of nanoparticles were studied using IR Tracer 100. Magnetic properties at room temperature are studied using a vibrating sample magnetometer, lakeshore 7400. UV-visible spectroscopy is used to study optical properties. The UV-Vis spectra were recorded at room temperature in the range of 200–800 nm using. Spectrophotometer (U-2800). The results obtained are discussed below in brief.

Results and Discussion:

3.1 Structural analysis:

The XRD micrographs of all as-synthesized samples are shown in Fig.2. According to the figure, it is found that the samples show diffraction peaks at 25.48⁰ (101), 38.04⁰ (004), 48.20⁰ (200), 54.14⁰ (105), 55.22⁰ (211), 62.80⁰ (204), 68.95⁰ (116) respectively. The peaks identified are of the anatase phase with some secondary peaks of the Rutile phase, as confirmed by JPCDS. Card no. 21-1272, 21-1276 [49]. By adding the dopants the intensity of peaks increases indicating that the crystallinity of the samples is increasing. The tetragonal structure of anatase with lattice parameters a=b= 3.77 Å and c=9.543 Å was confirmed from evaluation of lattice parameters which indicated that Fe doping does not affect the tetragonal structure of titanium [50].

Lattice parameters (a, b, c) were calculated using first higher intensity peak (101) for values of a, b and neighboring higher intensity peak of anatase phase (004) for determining c through following equation [51] and are tabulated in Table 1.

1/d² = h²/a² + k²/b² + l²/c²

A slight variation in lattice parameters is expected because, Ionic radii of Al³⁺, Fe³⁺ is 0.54Å and 0.64Å, respectively, which is less than the host material TiO₂ (0.68 Å). Due to this reason, no peaks of Al³⁺ and Fe³⁺ could be ascribed [52]. Al and Fe act as substitutional atoms in titania crystal lattice [53]. Minor peaks of rutile may appear due to the hydrothermal process because it relies on high pressure principle.

Crystallite size (D) [54] is evaluated using Scherer formula and its values are given in Table 1.

D=(kλ)/(βcosθ)D=(kλ)/(βcosθ)

Unit cell volume (V) of all samples is evaluated by using the following relation:

V = a²c

The occurrence of a small change in the value of lattice parameters, indicates the presence of strain. Strain occurred due to the change in morphology, shape and size of Nano-particles [55]. Strain is calculated by using the following relation [56].

ε=(β)/(4tanθ)ε=(β)/(4tanθ)

Fig. 2: XRD patterns of undoped, Fe-doped, Al-Fe co-doped, and Al-doped TiO₂ powders.

3.2. FTIR Analysis:

The functional group of nanoparticles were identifying by FTIR spectroscopy. FTIR spectra having wavenumber ranges from 4000-550 cm^-1 of undoped TiO₂, Fe doped TiO₂, Fe-Al doped TiO₂, Al-doped TiO₂ are shown in Fig. 3.

FTIR spectra of anatase phase detect broad Stretching vibrations peaks of Ti-O and Ti-O-Ti bonds occurring from 400 to 800 cm^-1, in the spectrum of fingerprint region [57]. This FT-IR spectra show the anatase phase in the fingerprint region. The band observed between 2100- 2300 cm^-1 is due to the CO₂ molecule in air. At 3600-4000 cm^-1, spectra indicate the presence of vibrational bending and stretching bands of hydroxyl group [58, 59].

Fig. 3: FTIR spectra of undoped, Fe-doped, Al-Fe co-doped, and Al-doped TiO₂ powders.

Table 1: Structural parameters obtained from XRD spectrum of all as prepared samples:

3.3 SEM Analysis:

Scanning Electron Microscope (SEM) analysis is used to explore the morphology of as-synthesized nanoparticles. The SEM images are shown below in Fig. 4. The figure shows that the particle size decreases as 0.9 μm (900 nm), 0.8μm (800 nm), 0.7 μm (700 nm), 0.6 μm (600 nm), as the dopants are added to TiO₂ as well as the number of voids increases by the doping of metal ions. Agglomerates are distinct in all samples. Agglomerates induce due to particles held together by surface forces [60].

Fig. 4: SEM micrographs of (a) undoped TiO₂, (b) Fe-doped TiO₂, (c) Al-Fe co-doped TiO₂, and (d) Al-doped TiO₂ powders (500μm).

3.4.Magnetic properties:

By applying a two Tesla magnetic field at room temperature the magnetic properties of samples have been evaluated. M-H hysteresis loops of all as prepared nanoparticles are shown in Fig. 5. Coercivity (H_c) and retentivity (M_r) and Saturation Magnetization (M_s) are the magnetic parameters that are determined by the M-H curves, are tabulated in Table 2. M-H curve of Undoped TiO₂ shows diamagnetic behavior, according to the electronic states of TiO₂ as Ti⁴⁺ has paired electrons in d-d transition orbitals which enhance its diamagnetic behavior [61]. M-H curves of metal ion-doped TiO₂ show the ferromagnetism because it is already known fact that when the magnetic field is zero, each sample shows a certain value of magnetization which is a characteristic of ferromagnetic behavior. In the case of doped TiO₂, Al shows weak ferromagnetism as compared to others because Al has weak attraction toward Magnetic Field at room temperature but becomes ferromagnetic below Curie temperature [62].

The value of magnetization increases by the addition of dopants, it may be due to an increase of oxygen vacancies [63]. The Coercivity of ferromagnetic samples decreases. Coercivity has the following relation with particle size [64].

H_c∝1/D

D represented as the crystallite size. But in our case, it did not follow.

The ferromagnetism source has always been a debatable issue due to various conflicting magnetic sources in the literature and is not clear yet. The origin of magnetic behavior in DMS is complex and it relies on a number of factors such as magnetic ions spacing, carrier density and dopants type. These elements interfere with electronic band structure which in turn affect their magnetic properties.

In order to find the source of ferromagnetism in our samples, different aspects of ferromagnetism were taken into consideration. One of the reasons for ferromagnetism is impurity phases in XRD patterns [65]. In our XRD analysis, the secondary phase of rutile was observed at 2θ = 27⁰ and 43⁰. But if it would be the cause then undoped TiO₂ which is showing diamagnetic behavior should also show ferromagnetism. So this reason can be ruled out.

Another source of ferromagnetism is Sp-d exchange interactions which might be applicable as samples are showing different behavior with different dopants i.e. Undoped TiO₂ – diamagnetic, Fe doped TiO₂- ferromagnetic, Fe-Al doped TiO₂- ferromagnetic, Al-doped- weak ferromagnetic.[66]

Oxygen vacancies are also considered as a source of ferromagnetism that are generated to compensate the charge neutrality whenTi⁺⁴ is replaced by Fe⁺³ or Al⁺³ [67]. Additionally, It has been reported [67] that TiO₂ has a potent polaronic effect, whereby the interaction between electrons and phonons increases the effective mass of carriers. The polaronic electron consequently spends the majority of its time near to an oxygen vacancy. An F center is created when this electron is trapped by an oxygen vacancy. The d shell of magnetic impurities overlaps with the trapped electron’s orbit. As a result, ferromagnetism could potentially be caused by F- center bound magnetic polaron. When several magnetic polarons begin to overlap and form a continuous chain over the entire material, ferromagnetism is produced. In this way ferromagnetism is percolated in DMS [68, 69].

Fig. 5: Room-temperature M-H curves of Fe-doped and Al-Fe co-doped TiO₂. Insets: M-H curves of undoped TiO₂ (top left) and Al-doped TiO₂ (bottom right).

Table 2: Magnetic parameters of as synthesized samples

3.5.Optical analysis:

The absorption Spectra of as-synthesized nanoparticles range from 200-800 nm, as shown in Fig. 6. In the case of Undoped titania maximum absorption is observed at 200 nm whereas, in Fe-doped TiO₂, Fe-Al doped TiO_2, and Al-dopedTiO_2, maximum absorption shifted towards a longer Wavelength which indicates the redshift of absorption [70]. Redshift associated with the transition of charge transfer between the implanted ions CB or VB and TiO₂ tends to decrease the optical band gap [71].

The optical energy band gap of samples is calculated by using Tauc relation [72].

αhν = A(hν − E_g)ⁿ

A is a Tauc Parameter or constant, E_g is the Energy band gap, hv is the Photon Energy, α is the absorption coefficient, n is the density distribution of the states (n=1/2 for allowed direct transition and n=2 for indirect band gap) [73].

(αhv)^(2) vs E_g spectra of as synthesized nanoparticles is shown in Fig. 7. The intercept on abscissa of Tauc plot (αhv)^(2) vs E_g to α=0 is employed to attain the value of energy band gap of samples [74]. In Undoped TiO₂, the obtained value of the energy band gap is 3.2 eV which matches well with the literature [75]. The energy band gap value decreases as the dopants are added to TiO₂. For Fe doped TiO₂, Al-Fe doped TiO_2, and Al-doped TiO₂ samples, the estimated value of optical band gap showed by the Tauc plot is 2.87 eV, 2.82 eV and 2.71 eV, respectively. The reason for decreasing energy values with Fe-concentration is the Alloying Effect. The band gap energy of Fe₂O₃ is 2.2 eV which is less than the band energy of Anatase TiO₂ i.e. 3.2 eV.When doping Fe₂O₃ into TiO₂ matrix and forming an alloy, the band gap is expected to decrease according to the alloying effect. However, some other parameters must be taken to count when describing decrease in band gap energy such as the presence of defects, decrease in crystallite size and Sp-d exchange interactions [76, 77]

The band gap may also be decreased due to compression of lattice constant ‘a’ and elongation of the lattice constant ‘c’. These lattice constants cause stress which is another reason for the narrowing of the band-gap [78].

Fig. 6: UV-Vis absorption spectra of undoped, Fe-doped, Al-Fe co-doped, and Al-doped TiO₂ powders.

Fig. 7: Tauc plots for band gap estimation of (a) undoped TiO₂ (3.2 eV), (b) Fe-doped TiO₂ (2.87 eV), (c) Al-Fe co-doped TiO₂ (2.82 eV), and (d) Al-doped TiO₂ (2.71 eV).

Conclusions:

Four samples, undoped TiO₂, Fe doped TiO₂, Al-Fe doped TiO_2, and Al-doped TiO₂ nanoparticles were synthesized by autoclave-assisted hydrothermal method and calcined at 550⁰C. XRD analysis confirms the single-phase anatase. XRD calculations also showed that the crystallite size decreases in the case of doping as compared to undoped nanoparticles of TiO₂. UV visible absorption spectrum shows absorption shifting towards longer wavelength, in case of doping of magnetic ions. Energy band gaps of all samples were calculated by the Tauc plot, which showed that the band gap decreased by the addition of transition metal ions. FTIR spectrum of as synthesized nanoparticles confirmed the presence of functional bands. SEM images showed the agglomeration of particles. MH curve obtained by VSM analysis showed the diamagnetic nature of TiO₂. RTFM can be observed in metal-ion doped TiO₂ nanoparticles. All these concluded results confirm that optical and magnetic properties of TiO₂ are enhanced by the addition of metal ion dopants, they show both semiconducting and ferromagnetic nature at room temperature. All these obtained features may be valuable for expected applications in various memory devices with diminished measurements.

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