Synthesis of Oxygen Deﬁcient TiO 2 for Improved Photocatalytic Efﬁciency in Solar Radiation

: The photocatalytic activities of TiO 2 have been limited mainly to absorbing in the ultraviolet spectrum which accounts for only 5% of solar radiation. High energy band gap and electron recombination in TiO 2 nanoparticles are responsible for its limitations as a photocatalyst. An oxygen deﬁcient surface can be artiﬁcially created on the titanium oxide by zero valent nano iron through the donation of its excess electrons. A visible light active TiO 2 nanoparticle was synthesized in the current investigation through simple chemical reduction using sodium boro-hydride. The physical and textural properties of the synthesized oxygen deﬁcient TiO 2 photocatalyst was measured using scanning/ transmission electron microscopy while FTIR, XRD and nitrogen sorption methods (BET) were employed for its further characterizations. Photochemical decoloration of orange II sodium dye solution in the presence of the synthesized TiO 2 was measured using an UV spectrophotometer. The resulting oxygen deﬁcient TiO 2 has a lower energy band-gap, smaller pore sizes, and enhanced photo-catalytic properties. The decoloration (88%) of orange (II) sodium salt solution (pH 2) under simulated solar light was possible at 20 min. This study highlights the effect of surface oxygen defects, crystal size and energy band-gap on the photo-catalytical property of TiO 2 nanoparticles as impacted by nano zero valent iron. It opens a new window in the exploitation of instability in the dopant ions for creation of a visible light active TiO 2 photocatalyst.


Introduction
The oxides of semiconductors are capable of absorbing a substantial quantity of (UV) radiation from a sunlight source and thus generate electron/hole pairs. This forms the basis of their utilization in photocatalysis and several other applications [1]. The generated electrons are capable of using the available excess energy to gain promotion (excitation) from the valence band (VB) to the conduction band (CB) and therefore leave behind positive holes. These holes are capable of breaking down water molecules into hydrogen and hydroxyl radicals as the electrons react with oxygen to produce the superoxide anion (O −2 ) [2,3]. Thereafter, both radicals and super-oxides react with persistent chemical or biological contaminants to generate more biodegradable end products [4]. Under solar radiation, the degradation of water contaminants is possible at room temperature and

Results and Discussion
The electronic configurations for TiO2 (unit cell) and Titanium doped nZVI (Yb, 2 × 2 × 1 supercell) are presented in Figure 1A,B. The red, gray, and purple spheres represent oxygen, titanium, and iron atoms, respectively. The calculated band structure of a unit cell of anatase TiO2 displays a band gap of 2.242 eV as calculated from computational simulation. When the doped supercell structure with 2 × 2 × 1 cell containing 48-atom of anatase TiO2 was used, and a Ti atom in the vicinity of the center of the supercell was replaced by a nZVI atom, the calculated band gap of zero valent nano iron-doped TiO2 (TD) was 2.083 eV. The injection of electron surplus nZVI increases the number of electrons on the titanium atoms and its conversion from Ti4+ to a lower oxidation state (Ti3+). The induced structural disorder in zero valent nano iron-doped TiO2 (TD) was primarily responsible for the lower energy band gap [25]. The calculated band structure of a unit cell of anatase TiO 2 displays a band gap of 2.242 eV as calculated from computational simulation. When the doped supercell structure with 2 × 2 × 1 cell containing 48-atom of anatase TiO 2 was used, and a Ti atom in the vicinity of the center of the supercell was replaced by a nZVI atom, the calculated band gap of zero valent nano iron-doped TiO 2 (TD) was 2.083 eV. The injection of electron surplus nZVI increases the number of electrons on the titanium atoms and its conversion from Ti4+ to a lower oxidation state (Ti3+). The induced structural disorder in zero valent nano iron-doped TiO 2 (TD) was primarily responsible for the lower energy band gap [25].

Absorption of Zero Valent Nano Iron-Doped TiO 2 under Electromagnetic Spectrum
The UV spectra of a reference TiO 2 (T R ) was compared to that of the synthesized oxygen deficient TiO 2 (T D ) in order to investigate the change in optical absorption characteristic. Additionally, the band gap of T R and T D were calculated from the UV-Vis spectra.
As shown in Figure 2, the light absorption edges of reference T R and that of T D were approximately 321 nm and 334 nm, respectively. Redshift to the visible region (longer wavelength) in UV absorption of T D is the direct effect of nZVI on the surface of TiO 2 . This is a clear indication of an enhanced solar absorption in T D [26,27]. An alternative energy level was created as a consequence of interactions among the electrons of titanium oxide and the nZVI. Consequently, a stronger absorbance yield was observed in the synthesized oxygen deficient TiO 2 (T D ) in relative to the reference TiO 2 (T R ). The implication of this (αhν) 1/n = C(hv − E g (1) where: α is the absorption coefficient, h is Planck's constant, ν is frequency (ν = c/λ, c is the light speed, λ is the wavelength), n = 1 2 and 2 for direct and indirect optical band gap, respectively, C is proportionality constant and E g is band gap. ygen deficient TiO2 (TD) in order to investigate the change in optical absorption character-istic. Additionally, the band gap of TR and TD were calculated from the UV-Vis spectra.
As shown in Figure 2, the light absorption edges of reference TR and that of TD were approximately 321 nm and 334 nm, respectively. Redshift to the visible region (longer wavelength) in UV absorption of TD is the direct effect of nZVI on the surface of TiO2. This is a clear indication of an enhanced solar absorption in TD [26,27]. An alternative energy level was created as a consequence of interactions among the electrons of titanium oxide and the nZVI. Consequently, a stronger absorbance yield was observed in the synthesized oxygen deficient TiO2 (TD) in relative to the reference TiO2 (TR). The implication of this change in visible light activity of the TD on its energy band gab variation can be further investigated using a Tauc's plot (1).
where: is the absorption coefficient, h is Planck's constant, is frequency (ν = c/λ, c is the light speed, λ is the wavelength), n = ½ and 2 for direct and indirect optical band gap, respectively, C is proportionality constant and Eg is band gap.  The synthesized oxygen deficient TiO 2, (T D ) resulted in a lower energy band gap (2.24 eV) in comparison with the reference TiO 2, (T R ) (2.95 eV), Figure 3.
The band gap variation of 0.71 eV obtained between the T D and T R is as a result of the improvement in electrical conductivity and mobility of charge carriers in the oxygen deficient TiO 2 (T D ). This can lead to a reduction of bulk recombination in the photo generated electron-hole pairs [28]. The higher transmittance intensity as observed on the FT-IR spectra of synthesized T D in comparison to that of T R is an indication of an enhanced infrared activity in the synthesized oxygen deficient TiO 2 ( Figure 4). The peak at 1040 due to Ti-O stretching vibration was only detected in the FT-IR spectra of T D . Likewise, the peculiar O-Ti-O lattice bonding of the anatase TiO 2 was pronounced in the FT-IR spectrum of the T D at 1382 cm −1 [29] and not the FT-IR of T R . However, the FT-IR peak at 1643 cm −1 is an indication of ferric oxyhydroxide (FeOOH) caused by oxidation of surface nZVI [30]. Likewise, FT-IR peak at 3322 cm −1 is related to the O-H bending vibrations spectra of TiO 2 (T D ) as a result of added hydroxyl groups due to water absorption on the nZVI surfaces  [30]. Apparently, the FT-IR result as presented in the current investigation showed lack of infrared activity in the reference TiO 2 (T R ) compared to the synthesized oxygen deficient TiO 2 (T D ). The band gap variation of 0.71 eV obtained between the TD and TR is as a resu improvement in electrical conductivity and mobility of charge carriers in the ox ficient TiO2 (TD). This can lead to a reduction of bulk recombination in the photo g electron-hole pairs [28]. The higher transmittance intensity as observed on the FT tra of synthesized TD in comparison to that of TR is an indication of an enhanced activity in the synthesized oxygen deficient TiO2 ( Figure 4). The peak at 1040 due stretching vibration was only detected in the FT-IR spectra of TD. Likewise, the O-Ti-O lattice bonding of the anatase TiO2 was pronounced in the FT-IR spectru TD at 1382 cm −1 [29] and not the FT-IR of TR. However, the FT-IR peak at 1643 c indication of ferric oxyhydroxide (FeOOH) caused by oxidation of surface nZVI [3 wise, FT-IR peak at 3322 cm −1 is related to the O-H bending vibrations spectra of T as a result of added hydroxyl groups due to water absorption on the nZVI su reported by Mragui et al. (2019) [30]. Apparently, the FT-IR result as presente current investigation showed lack of infrared activity in the reference TiO2 (TR) co to the synthesized oxygen deficient TiO2 (TD).  The adsorbed OH ions have a propensity for trapping charge carriers and reactive hydroxyl (OH) radicals [31]. The produced OH radicals are capable of the active sites of stable compounds or initiating nonspecific degradation of pollu ecules [32]. It can be confirmed through the current FT-IR results that the sy TiO2 is oxygen deficient and highly hydrogenated. These characteristics can r state of strong localization between the valency band and conduction bands The adsorbed OH ions have a propensity for trapping charge carriers and produce reactive hydroxyl (OH) radicals [31]. The produced OH radicals are capable of attacking the active sites of stable compounds or initiating nonspecific degradation of pollutant molecules [32]. It can be confirmed through the current FT-IR results that the synthesized TiO 2 is oxygen deficient and highly hydrogenated. These characteristics can result in a state of strong localization between the valency band and conduction bands [33,34]. Further studies of the interaction of nZVI on TiO 2 can be carried out by assessing the change in crystallite sizes of the titanium (IV) oxide.

Morphology, Surface Area and Size Distributions
Diffraction patterns of reference and the synthesized TiO 2 (T R and T D ) were as presented in Figure 5. The XRD of T R and T D nanoparticles gave 2 Theta (2θ) peaks at 20. This result is in agreement with the standard XRD pattern (JCPDS-21-1272) and the peaks can be indexed as anatase phases of TiO2 (in the majority of cases) with body center tetragonal shape [35]. All orientations due to anatase phases of TiO2 were observed in both samples with higher intensity on TD which means higher crystallinity [36]. The observed 2theta peaks at 41.20 and 56.62 were attributed to traces of elemental iron present as nZVI particles [30,37,38]. The nZVI particles may be responsible for oxygen deficiency on titanium surfaces leading to an improved crystallinity in TiO2 nanoparticle [39]. The crystallite sizes of the synthesized and reference TiO2 nanoparticles were obtained by using Debye-Scherrer's Equation (2).
D is the crystal size while the wavelength of the X-ray radiation λ = 0.15406 nm for CuKα. K = 0.9 and β (FWHM radians) is the line width at half-maximum height and θ the peak position (radian). According to Debye-Scherrer's formula, the obtained crystallite sizes for TiO2 nanoparticles TR and TD were 17.91 nm and 15.31 nm, respectively. The derived crystallite sizes reflect the surface reductions and bulk defects in the synthesized zero valent nano iron-doped TiO2 [40]. This result is in agreement with the standard XRD pattern (JCPDS-21-1272) and the peaks can be indexed as anatase phases of TiO 2 (in the majority of cases) with body center tetragonal shape [35]. All orientations due to anatase phases of TiO 2 were observed in both samples with higher intensity on T D which means higher crystallinity [36]. The observed 2theta peaks at 41.20 and 56.62 were attributed to traces of elemental iron present as nZVI particles [30,37,38]. The nZVI particles may be responsible for oxygen deficiency on titanium surfaces leading to an improved crystallinity in TiO 2 nanoparticle [39]. The crystallite sizes of the synthesized and reference TiO 2 nanoparticles were obtained by using Debye-Scherrer's Equation (2).
D is the crystal size while the wavelength of the X-ray radiation λ = 0.15406 nm for CuKα. K = 0.9 and β (FWHM radians) is the line width at half-maximum height and θ the peak position (radian). According to Debye-Scherrer's formula, the obtained crystallite sizes for TiO 2 nanoparticles T R and T D were 17.91 nm and 15.31 nm, respectively. The derived crystallite sizes reflect the surface reductions and bulk defects in the synthesized zero valent nano iron-doped TiO 2 [40].
Transmission Electron Microscope (TEM) and Scanning Electron Microscope (SEM) images were used respectively to define the morphologies, surface areas and sizes of TiO 2 nanoparticles T R and T D ( Figure 6) The average sizes and distributions in the TiO 2 nanoparticles were determined using image J, software (NIH, Media Cybernetics Inc. Rockville, MD, USA) to process the derived images from TEM analysis. The average size with high frequency was between 16-20 nm for TiO 2 nanoparticles T R and T D . SEM micrographs show that the synthesized TiO 2 nanoparticles (T D ) were slightly smaller than the reference TiO 2 (T R ) nanoparticles on average. The SAED ring pattern displayed d-spacing values and hkl planes with a characteristic tetragonal anatase phase of TiO 2 nanoparticles, which was equally reported for TiO 2 nanoparticle synthesized by Hengerer et al. (2000) [41]. A remarkable decrease in the crystal sizes of TiO 2 nanoparticle T D relative to the reference TiO 2 nanoparticle T R was noted in the histogram, as seen in Figure 6A,B. This has been reported for the provision of the corresponding positive effect on the dispersion and prevention of agglomeration in the nanoparticles [42].  [41]. A remarkable decrease in the crystal sizes of TiO2 nanoparticle TD relative to the reference TiO2 nanoparticle TR was noted in the histogram, as seen in Figure 6A,B. This has been reported for the provision of the corresponding positive effect on the dispersion and prevention of agglomeration in the nanoparticles [42]. The SEM-EDS elemental analysis of TiO2 nanoparticles as presented in Figure 7 confirmed the polycrystallinity nature of the reference (TR) and synthesized TiO2 nanoparticles (TD). The SEM-EDS elemental analysis of TiO 2 nanoparticles as presented in Figure 7 confirmed the polycrystallinity nature of the reference (T R ) and synthesized TiO 2 nanoparticles (T D ). Major identified elements were oxygen (O), titanium and iron (Fe) while traces of sodium (Na) were confirmed on SEM-EDS of TD as impurities. The oxygen/ titanium ratio (O/T) in the EDS was determined to confirm the oxygen deficient impact of the synthesized TiO2 nanoparticles (TD). It was discovered that TD has a higher oxygen content (O/T = 2.39) in comparison with TR (O/T = 1.57). The observed higher oxygen content in the synthesized oxygen deficient TiO2 (TD) was a resultant effect of the oxidized nZVI on the TiO2 nanoparticle. The oxygen deficiency was created when iron shared its excess electron with the titanium oxide nanoparticles. Recently, Ti 3+ spice production has been reported when the excess electron on nZVI migrated into the empty spaces in titanium ions [7], [43].
The adsorption-desorption isotherms of the reference and synthesized TiO2 (TR and TD) exhibit type IV isotherm with a hysteresis loop at a relative pressure in the range 0.8-1.0 ( Figure 8). This is characterized by capillary condensation, indicating mesopores solids with weak interaction features [44]. Detailed observation of isotherms revealed a narrower desorption in the TD compared to TR. A reduction of pore volume of the sample TD compared to TR (from 0.19 to 0.18 cm 3 /g) as presented in Table 1, indicated the withdrawal of oxygen from TiO2 nanoparticles. This led to an enlargement of pore sizes as observed in the BET pore diameter of TD presented in the current investigation. The reduction in porosity of TD leads to a corresponding reduction in the BET surface area of the synthesized TiO2 samples (TD). The reduction in the BET surface area of the synthesized TiO2 nanoparticles (TD) can prevent electron recombination through the alteration in migration path of electron charges to the surface of the photocatalyst. Major identified elements were oxygen (O), titanium and iron (Fe) while traces of sodium (Na) were confirmed on SEM-EDS of T D as impurities. The oxygen/ titanium ratio (O/T) in the EDS was determined to confirm the oxygen deficient impact of the synthesized TiO 2 nanoparticles (T D ). It was discovered that T D has a higher oxygen content (O/T = 2.39) in comparison with T R (O/T = 1.57). The observed higher oxygen content in the synthesized oxygen deficient TiO 2 (T D ) was a resultant effect of the oxidized nZVI on the TiO 2 nanoparticle. The oxygen deficiency was created when iron shared its excess electron with the titanium oxide nanoparticles. Recently, Ti 3+ spice production has been reported when the excess electron on nZVI migrated into the empty spaces in titanium ions [7,43].
The adsorption-desorption isotherms of the reference and synthesized TiO 2 (T R and T D ) exhibit type IV isotherm with a hysteresis loop at a relative pressure in the range 0.8-1.0 ( Figure 8). This is characterized by capillary condensation, indicating mesopores solids with weak interaction features [44]. Detailed observation of isotherms revealed a narrower desorption in the T D compared to T R . A reduction of pore volume of the sample T D compared to T R (from 0.19 to 0.18 cm 3 /g) as presented in Table 1, indicated the withdrawal of oxygen from TiO 2 nanoparticles. This led to an enlargement of pore sizes as observed in the BET pore diameter of T D presented in the current investigation. The reduction in porosity of T D leads to a corresponding reduction in the BET surface area of the synthesized TiO 2 samples (T D ). The reduction in the BET surface area of the synthesized TiO 2 nanoparticles (T D ) can prevent electron recombination through the alteration in migration path of electron charges to the surface of the photocatalyst.

Photo-Catalytic Application of the Synthesized Titanium Oxide Nanoparticle
The % decoloration of orange II sodium salt during photocatalysis using solar simulator in the presence of the TR and TD were compared (Figure 9). Orange II sodium salt was used in the current investigation because of its stability, structural resemblance to common organic pollutant and the ease of monitoring the decoloration using an Ultraviolent Spectro-photometer instrument. It was observed that TD had a higher activity as a photocatalyst in the visible region under solar light irradiation compared to the TR. The lower extent (%) of orange II sodium salt decoloration at dosage of 50 mg may be as a result of solar light path blockage by the high concentration of photocatalyst (500 mg/L).

Photo-Catalytic Application of the Synthesized Titanium Oxide Nanoparticle
The % decoloration of orange II sodium salt during photocatalysis using solar simulator in the presence of the T R and T D were compared (Figure 9). Orange II sodium salt was used in the current investigation because of its stability, structural resemblance to common organic pollutant and the ease of monitoring the decoloration using an Ultraviolent Spectro-photometer instrument. It was observed that T D had a higher activity as a photocatalyst in the visible region under solar light irradiation compared to the T R . The lower extent (%) of orange II sodium salt decoloration at dosage of 50 mg may be as a result of solar light path blockage by the high concentration of photocatalyst (500 mg/L).
The photo-catalytical decoloration of orange II sodium salt solution can also be influenced by its pH (Figure 10). This was tested by applying the optimum number of titanium oxide nanoparticles (T R or T D at 250 mg/L) in the decoloration of orange II sodium salt under the influence of simulated solar light. It was observed that the rate of decoloration increased as the solution became more acidic with the optimum decoloration achieved at pH 2 for both the reference TiO 2 (T R ) and synthesized oxygen deficient TiO 2 nanoparticles (T D ). Catalysts 2021, 11, x FOR PEER REVIEW 11 of 16 The photo-catalytical decoloration of orange II sodium salt solution can also be influenced by its pH (Figure 10). This was tested by applying the optimum number of titanium oxide nanoparticles (TR or TD at 250 mg/L) in the decoloration of orange II sodium salt under the influence of simulated solar light. It was observed that the rate of decoloration increased as the solution became more acidic with the optimum decoloration achieved at pH 2 for both the reference TiO2 (TR) and synthesized oxygen deficient TiO2 nanoparticles (TD).  The photo-catalytical decoloration of orange II sodium salt solution can also be influenced by its pH (Figure 10). This was tested by applying the optimum number of titanium oxide nanoparticles (TR or TD at 250 mg/L) in the decoloration of orange II sodium salt under the influence of simulated solar light. It was observed that the rate of decoloration increased as the solution became more acidic with the optimum decoloration achieved at pH 2 for both the reference TiO2 (TR) and synthesized oxygen deficient TiO2 nanoparticles (TD).

Preparation Method
The oxygen deficient TiO2 was prepared by dissolving 500 mg of nano-powder TiO2 and 20 mg FeCl3.6H2O in 50 mL deionized water (as presented in Figure 11). The solution was poured into a three necked round bottom flask and stirred (magnetically) for 30 min in an inert (nitrogen) environment (Figure 1). The prepared solution was purged with nitrogen to ensure the removal of dissolved oxygen. A 20 mL sodium borohydride solution (5000 mg/L) was added to the contents of the (three necked) round bottom flask and stirred for 30 min for the production of oxygen deficient TiO2 photocatalyst. Ethanol (99%) was added to prevent reversed oxidation of the produced nanoparticles. The solution was

Preparation Method
The oxygen deficient TiO 2 was prepared by dissolving 500 mg of nano-powder TiO 2 and 20 mg FeCl 3 .6H 2 O in 50 mL deionized water (as presented in Figure 11). The solution was poured into a three necked round bottom flask and stirred (magnetically) for 30 min in an inert (nitrogen) environment ( Figure 1). The prepared solution was purged with nitrogen to ensure the removal of dissolved oxygen. A 20 mL sodium borohydride solution (5000 mg/L) was added to the contents of the (three necked) round bottom flask and stirred for 30 min for the production of oxygen deficient TiO 2 photocatalyst. Ethanol (99%) was added to prevent reversed oxidation of the produced nanoparticles. The solution was subsequently washed through pressure filtration using filter paper (0.45 micron) connected to a Bucher flask vacuum filter system and ethanol was repeatedly added to ensure thorough washing. subsequently washed through pressure filtration using filter paper (0.45 micron) connected to a Bucher flask vacuum filter system and ethanol was repeatedly added to ensure thorough washing. Figure 11. Diagram of experimental setup during the synthesis of zero valent nano iron-doped TiO2.

Application Method for the Synthesized Titanium (IV) Oxide Nanoparticle
The photocatalytic properties of reference titanium oxide (TR) and the synthesized titanium oxide (TD) were evaluated by the decoloration of orange II sodium salt under a solar light simulator. The source of visible light set-up included an ultra-high efficiency solar simulator equipment (Sciencetech class AAA uhe-nl-150, 1450 Global Drive London, Ontario, Canada). A quantity of photocatalyst sample (TR or TD) equal to 5 mg was added to a prepared 10 mg/L orange II sodium (pH 6). The sample was mixed (vortex mixer) and kept in the dark for 5 min. A portion of the prepared sample was subsequently transferred into 1.5 mL cuvette and subsequently subjected to decoloration under the solar simulator for 20 min. The dosage of TiO2 nanoparticles (TR or TD) were varied (50 mg/L, 100 mg/L, 250 mg/L, and 500 mg/L) to obtain an optimum amount. The optimum medium pH was also determined by applying the TR or TD (at 250 mg/L been the optimum) at a varied orange II sodium dye solution pH (pH 2, 4, 7, 10 and 11). A control experiment (K) was equally set up (at pH 6) and kept in the dark cupboard for 20 min.
The decoloration of orange II sodium salt (10 mg/L) was monitored with the aid of a PerkinElmer lambda 25 spectrophotometer (absorption measurement). The calculation of the initial (C0) and final (C) concentration of orange II sodium salt was derived through the absorbance calibration curve. The % decoloration of orange II was calculated as presented in Equation (3)

Sample Characterization
Information on structure and crystallite size of the synthesized TiO2 was obtained with the aid of an X-ray diffractometer (Schimadzu model: XRD 6000) operated with CuKα radiation in the range of 20-80° (λ = 0.154 nm). The functionalization of the TiO2 was determined using an attenuated total reflectance Nicolet 380 FTIR spectrometer

Application Method for the Synthesized Titanium (IV) Oxide Nanoparticle
The photocatalytic properties of reference titanium oxide (T R ) and the synthesized titanium oxide (T D ) were eValuated by the decoloration of orange II sodium salt under a solar light simulator. The source of visible light set-up included an ultra-high efficiency solar simulator equipment (Sciencetech class AAA uhe-nl-150, 1450 Global Drive London, Ontario, Canada). A quantity of photocatalyst sample (T R or T D ) equal to 5 mg was added to a prepared 10 mg/L orange II sodium (pH 6). The sample was mixed (vortex mixer) and kept in the dark for 5 min. A portion of the prepared sample was subsequently transferred into 1.5 mL cuvette and subsequently subjected to decoloration under the solar simulator for 20 min. The dosage of TiO 2 nanoparticles (T R or T D ) were varied (50 mg/L, 100 mg/L, 250 mg/L, and 500 mg/L) to obtain an optimum amount. The optimum medium pH was also determined by applying the T R or T D (at 250 mg/L been the optimum) at a varied orange II sodium dye solution pH (pH 2, 4, 7, 10 and 11). A control experiment (K) was equally set up (at pH 6) and kept in the dark cupboard for 20 min.
The decoloration of orange II sodium salt (10 mg/L) was monitored with the aid of a PerkinElmer lambda 25 spectrophotometer (absorption measurement). The calculation of the initial (C 0 ) and final (C) concentration of orange II sodium salt was derived through the absorbance calibration curve. The % decoloration of orange II was calculated as presented in Equation (3)

Sample Characterization
Information on structure and crystallite size of the synthesized TiO 2 was obtained with the aid of an X-ray diffractometer (Schimadzu model: XRD 6000) operated with CuKα radiation in the range of 20-80 • (λ = 0.154 nm). The functionalization of the TiO 2 was determined using an attenuated total reflectance Nicolet 380 FTIR spectrometer (Thermo Scientific, Waltham, MA, USA). The FTIR spectra were collected in the 500-4000 cm −1 range, with a resolution of 4 cm −1 at room temperature. Characteristic surface morphologies in TiO 2 nanoparticle were measured with a high resolution scanning electron microscope (Hitachi S4160, Cold Field Emission, voltage 20 KV, Europark, Fichtenhain A12, 47807 Krefeld, Germany). Meanwhile, Energy-dispersive X-ray (EDX, voltage 20 KV, Take off angle 35.0 • ) was also applied for the identification of the constituent chemicals in nanoparticles using the SEM instrument. The effect of doping with zero valent nano iron on the surface functionalities and porosities of TiO 2 was investigated by using the nitrogen absorptions as applied in the Quantachrome Brunauer-Emmett-Teller Instrument, Old Pretoria Road Midrand, South Africa (adsorption-desorption isotherm at 77 K, degassing at 150 • ).

Computational Simulation
The protocol available in the literature was used for the execution of the computation [45]. The impact of nZVI on the TiO 2 was estimated in terms of variation in the band structure and partial density of states (PDOS) which were computed using the density functional theory (DFT) algorithm in the Cambridge Serial Total Energy Package [46], available in the Material Studio 2019 (Centre for High Performance Computing, Cape Town, South Africa). The general gradient approximation (GGA) exchange-correlational function available in the DFT protocols of the CASTEP was used and the Perdew-Burke-Ernzerhof (PBE) scheme was included. In the primary step, the geometry optimization was performed using Broyden-Fletcher-Goldfarb-Shanno (BFGS) algorithm with convergence of energy change per atom set at 5 × 10 −6 eV with stress of 0.02 GPa, displacement of atoms at 0.0005 Å, as well as residual force at 0.01 eV/Å. During the optimization, the FFT (Fast Fourier Transformation) grid was set at 25 × 25 × 64, with 4 × 4 × 2 k-point. The ultrasoft pseudo-potential was applied and the kinetic energy cutoff for the plane-waves basis was 381 eV.

Conclusions
A simple reduction process was employed for the novel synthesis of oxygen deficient TiO 2 . The excess electrons in nano zero valent iron enabled the creation of an artificial oxygen defect on TiO 2 surface which led to the formation of tetragonal anatase phase solar active TiO 2 . There is an enhancement in the photocatalytic properties of the resulting nanoparticles due to their higher absorption in the visible solar spectrum, reduced crystalline sizes and lower energy band gap. The synthesized photo-catalyst also showed optimum activity in the acidic solution of orange II sodium salt. The treatment of contaminated water can be achieved under solar irradiation using the TiO 2 , as currently synthesized.