Synthesis and Characterization of Iron-Doped Tio2 Nanoparticles Using Ferrocene from Flame Spray Pyrolysis

catalysts

Article Synthesis and Characterization of Iron-Doped TiO2 Nanoparticles Using Ferrocene from Flame Spray Pyrolysis

Mohamed Anwar Ismail 1,* , Mohamed N. Hedhili 2 , Dalaver H. Anjum 2,3, Venkatesh Singaravelu 4 and Suk Ho Chung 5

1 Mechanical Power Engineering Department, Faculty of Engineering, Zagazig University, Zagazig 44519, Egypt 2 Imaging and Characterization Core Lab, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia; [email protected] (M.N.H.); [email protected] (D.H.A.) 3 Department of Physics, Khalifa University, Abu Dhabi 127788, United Arab Emirates 4 Physical Sciences and Engineering Division, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia; [email protected] 5 Clean Combustion Research Center (CCRC), King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia; [email protected] * Correspondence: [email protected]; Tel.: +20-109-526-1960

Abstract: Iron-doped titanium dioxide nanoparticles, with Fe/Ti atomic ratios from 0% to 10%, were synthesized by ﬂame spray pyrolysis (FSP), employing a single-step method. Ferrocene, being nontoxic and readily soluble in liquid hydrocarbons, was used as the iron source, while titanium tetraisopropoxide (TTIP) was used as the precursor for TiO2. The general particle characterization and phase description were examined using ICP-OES, XRD, BET, and Raman spectroscopy, whereas Citation: Ismail, M.A.; Hedhili, M.N.; the XPS technique was used to study the surface chemistry of the synthesized particles. For particle Anjum, D.H.; Singaravelu, V.; Chung, morphology, HRTEM with EELS and EDS analyses were used. Optical and magnetic properties S.H. Synthesis and Characterization were examined using UV–vis and SQUID, respectively. Iron doping to TiO2 nanoparticles promoted of Iron-Doped TiO2 Nanoparticles Using Ferrocene from Flame Spray rutile phase formation, which was minor in the pure TiO2 particles. Iron-doped nanoparticles Pyrolysis. Catalysts 2021, 11, 438. exhibited a uniform iron distribution within the particles. XPS and UV–vis results revealed that 2+ 3+ https://doi.org/10.3390/catal Fe was dominant for lower iron content and Fe was common for higher iron content and the

11040438 iron-containing particles had a contracted band gap of ~1 eV lower than pure TiO2 particles with higher visible light absorption. SQUID results showed that doping TiO2 with Fe changed the material Academic Editor: to be paramagnetic. The generated nanoparticles showed a catalytic effect for dye-degradation under Anna Zieli´nska-Jurek visible light.

Received: 26 February 2021 Keywords: ﬂame synthesis; ﬂame spray pyrolysis; titanium dioxide; iron-doping; dye degradation Accepted: 25 March 2021 Published: 29 March 2021

Publisher’s Note: MDPI stays neutral 1. Introduction with regard to jurisdictional claims in published maps and institutional afﬁl- Flame spray pyrolysis (FSP) has emerged as a cost-effective method for large-scale iations. synthesis of nanoparticles [1] and a number of large-scale pilot projects based on FSP have been recently demonstrated [2]. The Johnson-Matthey Company announced the use of FSP for the exploration of next-generation materials related to catalytic applications [3]. Wegner et al. [2], demonstrated that FSP can be used to produce nanoparticles at a rate of a few kilograms per hour at a cost below 100 EUR/kg, where chemical precursors account Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. for the majority of the cost. It is critical to explore research in different precursors for use This article is an open access article in FSP, where future innovation may occur at the juncture of combustion, material, and distributed under the terms and aerosol engineering. conditions of the Creative Commons Titanium dioxide (TiO2) is widely produced using ﬂame-based methods [4–6] for vari- Attribution (CC BY) license (https:// ous applications in catalysis, water-treatment, and solar cells. Doping TiO2 nanoparticles creativecommons.org/licenses/by/ with transition metals, such as Fe and Co, can result in better photocatalytic conversions 4.0/). due to a reduction in the band-gap and the recombination rate of the electron hole pair [7–9].

Catalysts 2021, 11, 438. https://doi.org/10.3390/catal11040438 https://www.mdpi.com/journal/catalysts Catalysts 2021, 11, 438 2 of 16

Additionally, such transition metals can be used to produce TiO2 nanoparticles with dilute magnetic semiconductor properties [10] that are important for spintronics applications. Wet-chemistry methods have been utilized to synthesize iron-doped titania nanoparticles [11–14]. To obtain crystallized nanoparticles from such methods, many steps are involved during the synthesis process in addition to post annealing, which may result in phase segregation at higher iron content (Fe 3 at.% and more) [13]. Single-step production of Fe-TiO2 nanoparticles is the main advantage in adopting gas-phase techniques of nanoparticle synthesis. Wang et al. [13] produced iron-doped titania with iron content from 0% to 20% using an radiofrequency (RF) plasma. Their major finding was that iron- doping advanced the change from anatase to rutile nanoparticles. Teoh et al. [15] utilized a flame spray pyrolysis method in generating iron-doped titania. They used titanium tetraisopropoxide as the titanium precursor and iron naphthenate as the iron precursor. FSP increased the iron-doping up to five times higher than conventional methods with annealing. Increasing Fe concentration results in the transformation from anatase to rutile phase nanoparticles. Fe-TiO2 nanoparticles were active under visible light and the rate of oxalic acid mineralization is 6.4 times greater when compared to pure TiO2 and Degussa P25. Magnetic properties were not reported for the FSP-made Fe-TiO2 nanoparticles. Ferrocene has many properties, which make it a suitable source of iron for iron-doping in titania nanoparticles such as nontoxicity, stability, and solubility in liquid hydrocarbons [16]. Ferrocene, as a fuel additive for soot reduction, has been broadly considered in the combustion community [16,17]. Ferrocene, as a catalyst, has been utilized in synthesis of carbon nanotubes [18,19] and silica nanoparticles with a controlled surface area in diffusion flames [20]. Ferrocene was used in FSP as a co-precursor for the production of carbon nano-tubes (CNTs) [21,22], boron nitride/carbon nano-tube composite (BN/CNTs) [23], and metal oxides as Al2O3 [24], Fe2O3 [25], and thin films [26]. However, as far as the authors’ aware, ferrocene has not been used as an iron source in generating iron-doped titania from FSP technique. In this work, ferrocene was utilized as an iron precursor and titanium tetraisopropoxide (TTIP) was utilized as a titanium precursor to synthesize iron-doped TiO2 nanoparticles. Various characterization techniques were performed to study the phase, morphology, and surface structure of the nanoparticles. Optical and magnetic properties were measured. Finally, the doped-particles were used as a photocatalyst for dye-degradation experiments under visible light.

2. Results and Discussion 2.1. General Nanoparticle Characterization

To estimate the accurate amount of iron doped in TiO2 samples, inductively-coupled plasma-optical emission spectrometry (ICP-OES) was implemented (Table1). Direct pro- portionality was noticed between amounts of ferrocene added to the iron content found in each Fe-TiO2 sample. The percentage of iron in the doped samples ranged from 0.87% to 5.24 at.%. The speciﬁc surface area results utilizing Brunauer–Emmett–Teller (BET) nitrogen adsorption experiments are shown in Table1. A speciﬁc surface area (SSA) for the 2 iron-free TiO2 sample was the highest and had a value of 151 m /g, while it was dropped to 118 m2/g when the Fe/Ti atomic ratio was 1%. Then it was raised monotonically with iron content in the doped-sample. The average primary particle diameter dBET was calculated from BET results while assuming spherical nanoparticles [27]. It was found that dBET was around 10 nm for most samples. Catalysts 2021, 11, 438 3 of 16

Table 1. Fe-doped TiO2 synthesis conditions and properties.

Precursor Synthesized (control) (ICP) SSA (BET) dBET Anatase dXRD Catalysts 2021, 11, x FOR PEER REVIEW Fe/Ti Fe in Fe-TiO 15 of 15 2 %% [m2/g] [nm] % [nm] 0 0.0 151 9.9 91.8 16.2 1 0.87 1118 0.8712.6 85.8 118 12.618.8 85.8 18.8 3 1.56 137 10.9 64.2 16.3 3 1.56 5137 2.6110.9 64.2 139 10.416.3 42.1 12.2 5 2.61 10139 5.2410.4 42.1 140 10.212.2 20.9 15.9 10 5.24 140 10.2 20.9 15.9

Nanoparticle structureNanoparticle and crystallinity structure was andidentified crystallinity using the was X identiﬁed-ray diffraction using the X-ray diffraction (XRD) technique. Figure(XRD) 1 illustrates technique. the Figure XRD pattern1 illustrates for Fe the-doped XRD samples pattern forcompared Fe-doped to samples compared to ∼ pure TiO2 nanoparticlespure, revealing TiO2 nanoparticles, peaks at 2θ ≅ revealing 25.4 (for peaksanatase at 101) 2θ = and25.4 27.5 (for (for anatase rutile 101) and 27.5 (for rutile 110) crystalline phases.110) The crystalline dominant phases. phase found The dominant in the pure phase TiO found2 nanoparticles in the pure was TiO 2 nanoparticles was anatase, which is typicalanatase, for TiO which2 flame is typicalsynthesis for under TiO2 ﬂame lean conditions synthesis under[28], while lean conditionsFe- [28], while Fe- doping tended to promotedoping the tendedrutile phase to promote percentage the rutile and reduces phase percentage the anatase and percentage reduces the anatase percentage with increasing iron contentwith increasing in the sample iron content [29]. At in 10% the sample of iron [-29doping,]. At 10% the ofrutile iron-doping, phase the rutile phase was was dominant. By integradominant.ting the Byrespective integrating XRD the peak respective intensities, XRD anatase peak intensities, content could anatase content could be be calculated [30]. A calculatedcalculated anatase [30]. A percentage calculated for anatase various percentage samples forare variouslisted in samples Table are listed in Table1. 3+ 4+ Fe can occupy the Ti position in the TiO3+ lattice4+ because Fe and Ti have similar ionic 1. Fe can occupy the Ti position in the TiO2 lattice because Fe 2and Ti have similar ionic radii [31]. These results are in agreement with previously published data for Fe-TiO using radii [31]. These results are in agreement with previously published data for Fe-TiO2 using 2 gas-phase methods [13,15].gas-phase methods [13,15].

Figure 1. XRD spectraFigure for 1. ironXRD-doped spectra samples for iron-doped compared samples to pure compared TiO2 nano toparticles. pure TiO 2 nanoparticles.

Scherrer formula [32] isScherrer another formula method [ 32that] is gives another a good method indication that gives of the a goodcrystallite indication of the crystallite size dXRD by estimatingsize thed positionXRD by estimating of main anatase the position (101) and of main rutile anatase (110) peaks. (101) and It was rutile (110) peaks. It was observed that dXRD for allobserved analyzed that samplesdXRD for ranges all analyzed from 12 samples to 19 nm ranges, as shown from in 12 Table to 19 nm,1. as shown in Table1. To emphasize the variousTo emphasize phases existing the various in the phasessynthesized existing TiO in2 nanoparticles, the synthesized Raman TiO2 nanoparticles, Raman spectroscopy was carriedspectroscopy out, as shown was in carried Figure out, 2. Anatase as shown was in Figurethe dominant2. Anatase phase was for the dominant phase for iron-free TiO2 particles,iron-free whereas TiO the2 particles,rutile phase whereas was the the dominant rutile phase phase was for the iron dominant-doped phase for iron-doped particles. The Raman dataparticles. were Theconsistent Raman with data the were XRD consistent results. with the XRD results.

Figure 2. Raman spectra for Fe-doped TiO2 nanoparticles prepared by FSP with various Fe ratios (A, anatase. R, rutile).

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1 0.87 118 12.6 85.8 18.8 3 1.56 137 10.9 64.2 16.3 5 2.61 139 10.4 42.1 12.2 10 5.24 140 10.2 20.9 15.9

Nanoparticle structure and crystallinity was identified using the X-ray diffraction (XRD) technique. Figure 1 illustrates the XRD pattern for Fe-doped samples compared to pure TiO2 nanoparticles, revealing peaks at 2θ ≅ 25.4 (for anatase 101) and 27.5 (for rutile 110) crystalline phases. The dominant phase found in the pure TiO2 nanoparticles was anatase, which is typical for TiO2 flame synthesis under lean conditions [28], while Fe- doping tended to promote the rutile phase percentage and reduces the anatase percentage with increasing iron content in the sample [29]. At 10% of iron-doping, the rutile phase was dominant. By integrating the respective XRD peak intensities, anatase content could be calculated [30]. A calculated anatase percentage for various samples are listed in Table 1. Fe can occupy the Ti position in the TiO2 lattice because Fe3+ and Ti4+ have similar ionic radii [31]. These results are in agreement with previously published data for Fe-TiO2 using gas-phase methods [13,15].

Figure 1. XRD spectra for iron-doped samples compared to pure TiO2 nanoparticles.

Scherrer formula [32] is another method that gives a good indication of the crystallite size dXRD by estimating the position of main anatase (101) and rutile (110) peaks. It was observed that dXRD for all analyzed samples ranges from 12 to 19 nm, as shown in Table 1. To emphasize the various phases existing in the synthesized TiO2 nanoparticles, Raman Catalysts 2021, 11, 438 spectroscopy was carried out, as shown in Figure 2. Anatase was the dominant phase4 of for 16 iron-free TiO2 particles, whereas the rutile phase was the dominant phase for iron-doped particles. The Raman data were consistent with the XRD results.

Figure 2. Raman spectraspectra forfor Fe-dopedFe-doped TiOTiO22 nanoparticlesnanoparticles preparedprepared by by FSP FSP with with various various Fe Fe ratios ratios (A, Catalysts 2021, 11, x FOR PEER REVIEW(A, anatase . R, rutile). 15 of 15 anatase. R, rutile).

2.2. Fe-TiO2 Morphology

2.2.Bright-ﬁeld Fe-TiO2 Morphology TEM (BF-TEM) along with energy-dispersive X-ray spectroscopy (EDS)

analysesBright were-field carried TEM out(BF-TEM to examine) along with the overallenergy-dispersive elemental X composition-ray spectroscopy and ( qualityEDS) of the iron-dopedanalyses were TiO 2carriedparticles. out to Several examine electron the overall micrographs elemental composition at low magnification and quality and EDS spectraof the iron were-doped acquired TiO2 fromparticles various. Several locations electron on micrographs the grid for at alow good magnification representation of sizeand and EDS composition. spectra were Figure acquired3a–c from show various typical locations electron on the micrographs grid for a good for purerepre- TiO 2 and Fe-TiOsentation2 nanoparticles. of size and composition. Iron-free particles Figures have3a–3c an sho averagew typical size electron of 12 nm.micrograph The particles morphologyfor pure in TiO iron-doped2 and Fe-TiO cases2 nanoparticles (Fe/Ti ratio. Iron of-free 3% particles and 10%) have is similaran average to thatsize of of the 12 nm. The particle morphology in iron-doped cases (Fe/Ti ratio of 3% and 10%) is iron-free TiO2 case. The selected area electron diffraction (SAED) micrographs (Figure3a–c insets)similar contain to diffraction that of the iron rings-free that TiO confirm2 case. The the selected synthesized area electron nanoparticles diffraction were (SAED of anatase) and rutilemicrographs phases (also(Figure confirmeds 3a–3c insets) by XRDcontain and diffraction Raman rings results). that confirm Figure3 thed–f synthe- illustrates the highsized resolution nanoparticles TEM were (HRTEM) of anatase electron and rutile micrographs phases (also for pureconfirmed and iron-dopedby XRD and TiO Raman results). Figures 3d–3f illustrates the high resolution TEM (HRTEM) electron 2 nanoparticles. All analyzed samples showed high crystallinity as well as an ordered lattice micrographs for pure and iron-doped TiO2 nanoparticles. All analyzed samples structure. showed high crystallinity as well as an ordered lattice structure.

FigureFigure 3. TEM 3. TEM (a–c ()a with–c) with SAED SAED pattern pattern (inset) (inset) and and HRTEM HRTEM (d (–df–)f for) for samples samples with with anan Fe/TiFe/Ti ratio ratio of of:: (a (a, ,dd)) 0%, 0%, ( (bb, ,e)) 3%, 3%, and (c,f) 10%.and (c, f) 10%. Figures 4a–4c show higher magnification of HRTEM images and illustrates the inter- planar spacing (d-spacing) of the planes for TiO2 phases of selected samples. It indicates the decrease in d-spacing with an increase of the iron content in the sample. For pure TiO2 nanoparticles, the d-spacing of 0.354 nm is related to (101) anatase planes, whereas the value of 0.327 nm for iron doped sample with Fe/Ti ratio of 10% is related to (110) rutile planes [33]. This phase change from anatase to rutile when increasing the iron content is in good agreement with the previous results. The statistical analysis of particle size distribution obtained from TEM results was based on more than 150 nanoparticles and the size distribution histograms are provided in Figure 4 (d–f). It could be concluded that more than 95% of the particles have a diameter between 7 and 19 nm. The particle size distribution does not change much with doping iron in the TiO2 samples.

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Figure4a–c show higher magniﬁcation of HRTEM images and illustrates the interpla- nar spacing (d-spacing) of the planes for TiO2 phases of selected samples. It indicates the decrease in d-spacing with an increase of the iron content in the sample. For pure TiO2 nanoparticles, the d-spacing of 0.354 nm is related to (101) anatase planes, whereas the value of 0.327 nm for iron doped sample with Fe/Ti ratio of 10% is related to (110) rutile planes [33]. This phase change from anatase to rutile when increasing the iron content Catalysts 2021, 11, x FOR PEER REVIEW is in good agreement with the previous results. The statistical15 analysis of 15 of particle size

distribution obtained from TEM results was based on more than 150 nanoparticles and the size distribution histograms are provided in Figure4d–f. It could be concluded that Catalysts 2021, 11, x FOR PEER REVIEWmore than 95% of the particles have a diameter between 7 and 19 nm. The particle15 of 15 size

distribution does not change much with doping iron in the TiO2 samples.

Figure 4. HRTEM images and particle size distribution for samples with an Fe/Ti ratio of: (a, d) 0%, (b, e) 3%, and (c, f) 10%. FigureFigure 4. HRTEM 4. HRTEM images images and and particle particle size size distribution distribution for for samples samples with with an anFe/Ti Fe/Ti ratio of of:: (a, (a ,dd)) 0%, 0%, (b, (b ,ee)) 3%, 3%, and and (c, (c f,f) ) 10%. 10%. Figure5 shows the data revealed from EDS spectra, which conﬁrms the existence Figure 5 shows the dataFigure revealed 5 shows from the data EDS revealed spectra from, which EDS confirms spectra, which the existence confirms ofthe existence of of both Ti and O elements2 in the iron-free TiO2 sample. For the 3% (Fe/Ti atomic ratio) both Ti and O elementsboth in Ti the and iron O elements-free TiO in sample.the iron- freeFor TiOthe 23% sample. (Fe/Ti For atomic the 3% ratio) (Fe/Ti sam- atomic ratio) sample, the Fe peak startedsample,ple, to the appear the Fe Fepeak and peak started its started intensity to appear to appearincreased and its and intensity for its higher intensity increased iron increased- fordoping higher (Fe/Ti foriron higher- doping iron-doping (Fe/Ti = 10%) with an additional(Fe/Ti= 10%) = Fe 10%)with peak an with, additionalwhich an additional indicate Fe peaks a, Fe whichhigher peak, indicate iron which concentrations a indicates higher iron a higher inconcentration the na- iron concentration in the na- in noparticles. In additionthenoparticles nanoparticles., a small. InCu addition peak In addition, is, aobserved small a Cu small inpeak the Cu is sample peakobserved is observedwith in the higher sample in theiron with sample con- higher with iron higher con- iron centration. Additionalconcentration.centration. Cu peaks Additional observed Additional Cu in Cupeaksthe peaks spectrum observed observed are in the attributed in spectrum the spectrum to are the attributed Cu are- attributedgrid. to the Cu to- thegrid. Cu-grid.

Figure 5. EDS spectra acquired from the region of Figure 4, revealing the existence of Fe in Fe-TiO2 nanoparticles. Figure 5. EDS spectraFigure acquired 5. EDS from spectra the region acquired of Figure from 4, the revealing region of the Figure existence4, revealing of Fe in the Fe existence-TiO2 of Fe in Fe-TiO 2 nanoparticles. nanoparticles.The presented electron energy-loss spectroscopy (EELS) spectra in Figure 6 contain the energy loss edges of Ti, O, and Fe elements. The energy loss edge of Ti at the energy The presented electronloss value energy of 456-loss eV is spectroscopy Ti-L23 and it (representsEELS) spectra electron in beamFigure-induced 6 contain electronic transi- the energy loss edgestions of Ti, in O,Ti atomsand Fe from elements. the initial The states energy of 2P loss1/2 and edge 2P3/2 of to Ti final at thestates energy of 3S or 3P orbitals. loss value of 456 eV isSimilarly, Ti-L23 theand energy it represents of edge ofelectron Fe at the beam energy-induced loss value electronic of 710 eV transi- is Fe-L23 and it also represents the same electronic transitions in Fe atoms as of Ti atoms. Whereas the energy tions in Ti atoms from the initial states of 2P1/2 and 2P3/2 to final states of 3S or 3P orbitals. Similarly, the energy lossof edge edge of of FeO atat thethe energy energy value loss ofvalue 532 eVof 710is the eV O is-K Fe edge-L23, it andrepresents it also the electronic transitions from 1S1/2 initial states to 2P final states in O atoms. The intensity of signals of represents the same electronic transitions in Fe atoms as of Ti atoms. Whereas the energy these energy loss edges is proportional to the elemental composition of nanomaterials loss edge of O at the andenergy, therefore value, haveof 532 been eV utilized is the Oto- Kmap edge the, spatialit represents distributions the electronic of these elements in the transitions from 1S1/2 initial states to 2P final states in O atoms. The intensity of signals of these energy loss edges is proportional to the elemental composition of nanomaterials and, therefore, have been utilized to map the spatial distributions of these elements in the

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The presented electron energy-loss spectroscopy (EELS) spectra in Figure6 contain the energy loss edges of Ti, O, and Fe elements. The energy loss edge of Ti at the energy loss value of 456 eV is Ti-L23 and it represents electron beam-induced electronic transitions in Ti atoms from the initial states of 2P1/2 and 2P3/2 to ﬁnal states of 3S or 3P orbitals. Similarly, the energy of edge of Fe at the energy loss value of 710 eV is Fe-L23 and it also represents the same electronic transitions in Fe atoms as of Ti atoms. Whereas the energy loss edge of Catalysts 2021, 11, x FOR PEER REVIEW 15 of 15 Catalysts 2021, 11, x FOR PEER REVIEW O at the energy value of 532 eV is the O-K edge, it represents the15 electronicof 15 transitions from

1S1/2 initial states to 2P ﬁnal states in O atoms. The intensity of signals of these energy loss edges is proportional to the elemental composition of nanomaterials and, therefore, samples.have been More utilized detailed to map analysis the spatial for EDS distributions and EELS ofresults these can elements be found in the in Reference samples. More [34]. samples. More detailed analysis for EDS and EELS results can be found in Reference [34]. Thedetailed acquired analysis elemental for EDS maps and EELSof the results10% sample can be are found presented in Reference in Figure [34]. 7 The. The acquired results The acquired elemental maps of the 10% sample are presented in Figure 7. The results demonelementalstrated maps that of Ti, the O, 10% and sample Fe were are present presented and inreasonably Figure7. Theuniform, results implying demonstrated that Fe demonstrated that Ti, O, and Fe were present and reasonably uniform, implying that Fe wasthat uniformly Ti, O, and doped Fe were in present the TiO and2 nanoparticles reasonably and uniform, did not implying contain thatsegregated Fe was iron uniformly oxide was uniformly doped in the TiO2 nanoparticles and did not contain segregated iron oxide particles.doped in the TiO2 nanoparticles and did not contain segregated iron oxide particles. particles.

FigureFigure 6 6.. CoreCore-loss-loss EELS analysis of TiO22 and Fe-TiOFe-TiO22 samples.samples. SpectraSpectra containcontain edgesedges of of all all elements ele- Figure 6. Core-loss EELS analysis of TiO2 and Fe-TiO2 samples. Spectra contain edges of all ele- mentspresent present in these in samples.these samples. ments present in these samples.

Figure 7. Elemental mapping of 10% Fe in TiO2 nanoparticles indicating (a) raw TEM image, (b, c, Figure 7. Elemental mapping of 10% Fe in TiO2 nanoparticles indicating (a) raw TEM image, (b, c, dFigure) are (O, 7. ElementalFe, and Ti) mappingelemental of maps, 10% Ferespectively. in TiO2 nanoparticles indicating (a) raw TEM image, (b, c, d) are (O, Fe, and Ti) elementaland d) aremaps, (O, Fe,respectively. and Ti) elemental maps, respectively. 2.3. Surface Chemistry 2.3. Surface Chemistry 2.3. Surface Chemistry Chemical composition was analyzed using X-ray photoelectron spectroscopy (XPS). Chemical compositionChemical was analyzed composition using X was-ray analyzedphotoelectron using spectroscopy X-ray photoelectron (XPS). spectroscopy (XPS). Survey spectra from pure TiO2 and from Fe-doped TiO2 samples are shown in Figure 8. Survey spectra from pureSurvey TiO spectra2 and from from Fe pure-doped TiO TiOand2 samples from Fe-doped are shown TiO insamples Figure 8 are. shown in Figure8. Ti, O, C, and Fe elements are 2detected for the iron-doped2 samples whereas Fe was not Ti, O, C, and Fe elementsTi, O, are C, anddetected Fe elements for the iron are- detecteddoped samples for the whereas iron-doped Fe sampleswas not whereas Fe was not detected from pure TiO2 sample. It should be noted that the synthesized samples do not detected from pure TiOdetected2 sample. from It should pure TiO be2 notedsample. that It shouldthe synthesized be noted samples that the synthesizeddo not samples do not contain any carbon while the carbon peak in the survey scan comes from the background. contain any carbon whilecontain the carbon any carbon peak while in the the survey carbon scan peak comes in the from survey the background. scan comes from the background. High resolution XPS spectra of Ti 2p and O 1s core levels from pure TiO2 are shown in High resolution XPS Highspectra resolution of Ti 2p and XPS O spectra 1s core of levels Ti 2p from and O pure 1s core TiO levels2 are shown from purein TiO2 are shown in Figure 9. The Ti 2p3/2 core level is fitted using two components. The dominant peak cen- Figure 9. The Ti 2p3/2 core level is fitted using two components. The dominant peak centered at 458.6 eV is associated with Ti ions with a formal valence 4+ (Ti4+), while the peak tered at 458.6 eV is associated with Ti ions with a formal valence 4+ (Ti4+), while the peak at the lower binding energy of ~456.6 eV is associated with Ti ions with a reduced charge at the lower binding energy of ~456.6 eV is associated with Ti ions with a reduced charge state (Ti3+) [35]. The O 1s core level is centered at 529.8 eV, corresponding to the metal state (Ti3+) [35]. The O 1s core level is centered at 529.8 eV, corresponding to the metal oxide (TiO2) [35]. These Ti 2p and O 1s peaks are also observed for all the Fe-doped TiO2 oxide (TiO2) [35]. These Ti 2p and O 1s peaks are also observed for all the Fe-doped TiO2 samples. samples.

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Figure9. The Ti 2p 3/2 core level is ﬁtted using two components. The dominant peak centered at 458.6 eV is associated with Ti ions with a formal valence 4+ (Ti4+), while the Catalysts 2021, 11, x FOR PEER REVIEW peak at the lower binding energy of ~456.6 eV is associated with Ti ions15 with of 15 a reduced charge state (Ti3+)[35]. The O 1s core level is centered at 529.8 eV, corresponding to the Catalysts 2021, 11, x FOR PEER REVIEW metal oxide (TiO )[35]. These Ti 2p and O 1s peaks are also observed15 of for 15 all the Fe-doped 2 TiO2 samples.

FigureFigure 8. Survey 8. Survey scan spectra scan spectra of pure of TiO pure2 and TiO Feand-doped Fe-doped nanoparticles. nanoparticles. Figure 8. Survey scan spectra of pure TiO2 and2 Fe-doped nanoparticles.

Figure 9. High resolution spectra of TiO2 nanoparticles showing (a) Ti 2p peak and (b) O 1s peak for pure TiO2. FigureFigure 9. 9.High High resolution resolution spectra spectra of TiO of2 TiOnanoparticles2 nanoparticles showing showing (a) Ti 2p (a peak) Ti 2p and peak (b) O and 1s peak (b) O for 1s pure peak TiO 2.

for pureF igureTiO2. 10 illustrates high resolution XPS spectra for iron-doped TiO2 nanoparticles Figure 10 illustrates high resolution XPS spectra for iron-doped TiO nanoparticles resulting from the Fe 2p core level. High resolution data was fitted using eight compo- 2 resulting from the Fe 2p core level. High resolution data was ﬁtted using eight components nentsFigure located 10 atillustrates 710.8, 709.1, high 718.3, resolution 719.4, 722.3, XPS spectra724.2, 728.5 for , ironand -732.9doped eV. TiO The2 nanoparticles Fe 2p3/2 located at 710.8, 709.1, 718.3, 719.4, 722.3, 724.2, 728.5, and 732.9 eV. The Fe 2p doublet resultingdoublet frompeak atthe 710.8 Fe 2peV , coreand itslevel. corresponding High resolution satellite data peak was at 719.4 fitted eV using, together eight with compo- 3/2 peak at 710.8 eV, and its corresponding satellite peak at 719.4 eV, together with the Fe 2p1/2 the Fe 2p1/2 doublet peak at 724.3 eV, and its corresponding satellite peak at 732.9 eV, are nents located at 710.8,doublet 709.1, peak 718.3, at 724.3 719.4, eV, and 722.3, its corresponding724.2, 728.5, and satellite 732.9 peak eV. at The 732.9 Fe eV, 2p are3/2 the main the main signature of the Fe3+ oxidation state of Fe [36,37]. Another signature of Fe3+ ions doublet peak at 710.8signature eV, and of the its Fe corresponding3+ oxidation state satellite of Fe [ 36peak,37]. at Another 719.4 eV signature, together of Fe with3+ ions are the are the satellite peaks located at 8.6 eV from the main peaks. the Fe 2p1/2 doubletsatellite peak peaksat 724.3 located eV, and at 8.6 its eV corresponding from the main peaks.satellite peak at 732.9 eV, are The Fe 2p3/2 doublet peak at 709.1 eV and its corresponding satellite peak at 715.3 eV, the main signature of the Fe3+ oxidation state of Fe [36,37]. Another signature of Fe3+ ions together with the Fe 2p1/2 doublet peak at 722.3 eV and its corresponding satellite peak at are the satellite peaks located at 8.6 eV from the main peaks. 728.5 eV are signatures of the Fe2+ oxidation state of Fe [37,38]. Other signatures of Fe2+ The Fe 2p3/2 doublet peak at 709.1 eV and its corresponding satellite peak at 715.3 eV, ions are the satellite peaks located at 6.2 eV from the main peaks. The area ratio of Fe 2p3/2– togetherFe 2p1/2 withcomponents the Fe 2pequal1/2 doublets 2:1 with peak their at doublet 722.3 eVseparation and its correspondingof 13.5 and 13.2 eVsatellite for Fe peak3+ at 728.5and eVFe2+ ,are respectively signature. Thes of same the Fevalues2+ oxidation are used forstate their of correspondingFe [37,38]. O thersatellites. signature Increas-s of Fe2+ ionsing are the the percentage satellite ofpeaks the Fe located dopant at leads 6.2 eV to froman increase the main of the peaks. Fe3+/Fe The2+ ratioarea fromratio 0.5of Feat 2p3/2– Fe3% 2p (Fe/Ti1/2 components ratio) to 1.8 equal at 10%s 2:1(Fe/Ti with ratio). their doublet separation of 13.5 and 13.2 eV for Fe3+ and Fe2+, respectively. The same values are used for their corresponding satellites. Increas- ing the percentage of the Fe dopant leads to an increase of the Fe3+/Fe2+ ratio from 0.5 at 3% (Fe/Ti ratio) to 1.8 at 10% (Fe/Ti ratio).

Catalysts 2021, 11, x FOR PEER REVIEWCatalysts 2021, 11, 438 15 of 15 8 of 16

Figure 10. High resolutionFigure spectra 10. High for iron resolution-doped spectra samples for iron-doped resulting samplesfrom Fe resulting 2p peaks from with Fe 2p an peaks with an Fe/Ti Fe/Ti ratio of (a) 3%, (b) 5%,ratio and of (a ()c 3%,) 10%. (b) 5%, and (c) 10%.

The Fe 2p3/2 doublet peak at 709.1 eV and its corresponding satellite peak at 715.3 eV, The valence band spectra from pure and Fe-doped TiO2 nanoparticles are shown in together with the Fe 2p1/2 doublet peak at 722.3 eV and its corresponding satellite peak at Figure 11. The spectrum728.5 of undoped eV are signatures TiO2 contained of the Fe2+ threeoxidation major state features of Fe [37 (marked,38]. Other as signaturesA, B, of Fe2+ and C) at binding energyions positions are the satellite around peaks ~7.5, located 5, and at1 eV 6.2, eVrespectively from the main. This peaks. is consistent The area ratio of Fe with the literature [39].2p Features3/2–Fe 2p 1/2A componentsand B are related equals 2:1to withthe O their-2pdoublet derived separation states and of 13.5are and 13.2 eV 3+ 2+ referred to as ‘bonding’for and Fe ‘nonand- Febonding, respectively. orbital The emissions same values,’ respectively. are used for theirFeature corresponding C cor- satellites. Increasing the percentage of the Fe dopant leads to an increase of the Fe3+/Fe2+ ratio from responds to the Ti3+ 3d defect state. After Fe doping, feature D (~2.5 eV) began to appear. 0.5 at 3% (Fe/Ti ratio) to 1.8 at 10% (Fe/Ti ratio). Such a shape, based on the literature, is assigned to a mixed Fe 3d and Ti 3d derived states The valence band spectra from pure and Fe-doped TiO2 nanoparticles are shown [39]. Note that the valencein Figure band 11 edge. The spectrumfrom the ofFe undoped-doped TiO TiO22 containedshifted by three ~1 eV major to a features lower (marked as binding energy, comparedA, B, andto undoped C) at binding TiO energy2, which positions could enhance around ~7.5, photo 5, and-catalytic 1 eV, respectively. prop- This is erties in the region of visibleconsistent light with [40]. the literature [39]. Features A and B are related to the O-2p derived states and are referred to as ‘bonding’ and ‘non-bonding orbital emissions,’ respectively. Feature C corresponds to the Ti3+ 3d defect state. After Fe doping, feature D (~2.5 eV) began to appear. Such a shape, based on the literature, is assigned to a mixed Fe 3d and Ti 3d derived states [39]. Note that the valence band edge from the Fe-doped TiO2 shifted by ~1 eV to a lower binding energy, compared to undoped TiO2, which could enhance photo-catalytic properties in the region of visible light [40].

Figure 11. Valance band for TiO2 and Fe-doped TiO2 samples, indicating structures A, B, and C for TiO2 and structure D for Fe.

2.4. Magnetic Properties To study the intrinsic magnetic property, temperature variations of magnetization measurements (moment M vs. temperature T) were performed. Initially, the sample was cooled to 5 K under a nominal zero field. Then a field of H = 1 kOe was applied. Subse- quently, M vs. T data was measured under the constant magnetic field, while warming the sample from 5 K to 300 K (Figure 12). Results showed that all the samples were typical paramagnetic and none showed any Curie transition (TC), within the temperature range,

Catalysts 2021, 11, x FOR PEER REVIEW 15 of 15

Figure 10. High resolution spectra for iron-doped samples resulting from Fe 2p peaks with an Fe/Ti ratio of (a) 3%, (b) 5%, and (c) 10%.

The valence band spectra from pure and Fe-doped TiO2 nanoparticles are shown in Figure 11. The spectrum of undoped TiO2 contained three major features (marked as A, B, and C) at binding energy positions around ~7.5, 5, and 1 eV, respectively. This is consistent with the literature [39]. Features A and B are related to the O-2p derived states and are referred to as ‘bonding’ and ‘non-bonding orbital emissions,’ respectively. Feature C corresponds to the Ti3+ 3d defect state. After Fe doping, feature D (~2.5 eV) began to appear. Such a shape, based on the literature, is assigned to a mixed Fe 3d and Ti 3d derived states Catalysts 2021[39]., 11 Note, 438 that the valence band edge from the Fe-doped TiO2 shifted by ~1 eV to a lower 9 of 16 binding energy, compared to undoped TiO2, which could enhance photo-catalytic properties in the region of visible light [40].

Figure 11. Valance band Figurefor TiO 11.2 andValance Fe-doped band TiO for2 TiOsamples,2 and indicating Fe-doped TiOstructures2 samples, A, B, indicating and C for structures A, B, and C for TiO2 and structure D for TiOFe. 2 and structure D for Fe.

2.4. Magnetic Properties2.4. Magnetic Properties To study the intrinsic Tomagnetic study theproperty, intrinsic temperature magnetic property, variations temperature of magnetization variations of magnetization measurements (momentmeasurements M vs. temperature (moment T) were M vs. performed. temperature Initially, T) were the sample performed. was Initially, the sample cooled to 5 K under a wasnominal cooled zero to field 5 K. under Then a field nominal of H zero = 1 kOe field. was Then applied. a field Subse- of H = 1 kOe was applied. Catalysts 2021, 11, x FOR PEER REVIEW 15 of 15 quently, M vs. T data Subsequently,was measured Munder vs. the T data constant was measuredmagnetic field, under while the constantwarming magnetic field, while the sample from 5 K towarming 300 K (Figure the sample 12). Results from 5 showed K to 300 that K (Figure all the 12 samples). Results were showed typical that all the samples were paramagnetic and nonetypical showed paramagnetic any Curie transition and none (TC), showed within any the Curie temperature transition (TC),range, within the temperature which implies that Fe range,doping which did not implies induce that a long Fe range doping of dida ferromagnetic not induce ainteraction long range of a ferromagnetic in these samples. interaction in these samples.

Figure 12. Temperature-Figuredependent 12. Temperature-dependentmagnetizations measured magnetizations under a constant measured magnetic under field of a constant1 magnetic ﬁeld of kOe. 1 kOe.

Isothermal magnetizationIsothermal measurements magnetization (magnetic measurements moment M (magneticvs. magnetic moment field H) M vs. magnetic ﬁeld H) were carried out. A necessarywere carried diamagnetic out. A necessary correction diamagnetic corresponding correction to the correspondingbare sample to the bare sample holder was applied afterholder magnetization was applied measurements after magnetization on each measurements sample. Figure on 1 each3 shows sample. Figure 13 shows the magnetic hysteresisthe (M magnetic-H) curves hysteresis of the (M-H)undoped curves and ofFe the-doped undoped samples and Fe-dopedtaken at 5 samples taken at 5 and and 300 K. The results 300showed K. The that results magnetization showed that increa magnetizationses monotonically increases with monotonically increased with increased Fe Fe content. At room temperature,content. At most room of temperature, the doped samples most of have the dopedsmall moment samples values, have small moment values, which is similar to the whichundoped is similar sample. to However, the undoped for the sample. 10% case, However, a higher for moment the 10% was case, a higher moment observed. The inset inwas Figure observed. 13b shows The a inset narrow in Figure opening 13b of shows the hysteresis a narrow loop opening near of the the hysteresis loop near origin for the 10% case,the indicating origin for a theparamagnetic 10% case, indicating behavior. aEarlier paramagnetic reports attributed behavior. the Earlier reports attributed absence of ferromagnetismthe absence (or paramagnetism of ferromagnetism in Fe-TiO (or2) to paramagnetism sample preparation in Fe-TiO methods2) to sample preparation and/or dopant concentrationsmethods [ and/or41] where dopant dopant concentrations Fe ions remain [41] as where isolated dopant ions Fe weakly ions remain as isolated ions interacting in the hostweakly TiO2. The interacting lack of magnetic in the host properties TiO2. The in lackprevious of magnetic works was properties also in previous works α attributed to the presencewas alsoof secondary attributed phases, to the presencesuch as α of-Fe secondary2O3 or Fe3O phases,4, since such iron asions-Fe 2O3 or Fe3O4, since may coexist in differentiron valance ions may states coexist [42, in43]. different For magnetic valance behavior, states [42 oxygen,43]. For defects magnetic behavior, oxygen [41,44,45] could inducedefects magnetic [41,44 interactions.,45] could induce Since magneticonly paramagnetic interactions. properties Since only were paramagnetic properties were observed in the present study, it was attributed to oxygen defects and Fe3+ ions in the observed in the present study, it was attributed to oxygen defects and Fe3+ ions in the TiO2 crystal. TiO2 crystal.

Figure 13. Magnetization (M-H) curves for iron-doped TiO2 nanoparticles compared to pure TiO2 measured under an external magnetic field (H), from −30 to 30 kOe at temperatures of (a) 5 K and (b) 300 K.

2.5. Optical Properties

The UV–vis absorption spectroscopy was adopted to determine light absorption from iron-free and iron-doped TiO2 nanoparticles. The spectra are shown in Figure 14.

Catalysts 2021, 11, x FOR PEER REVIEW 15 of 15

which implies that Fe doping did not induce a long range of a ferromagnetic interaction in these samples.

Figure 12. Temperature-dependent magnetizations measured under a constant magnetic field of 1 kOe.

Isothermal magnetization measurements (magnetic moment M vs. magnetic field H) were carried out. A necessary diamagnetic correction corresponding to the bare sample holder was applied after magnetization measurements on each sample. Figure 13 shows the magnetic hysteresis (M-H) curves of the undoped and Fe-doped samples taken at 5 and 300 K. The results showed that magnetization increases monotonically with increased Fe content. At room temperature, most of the doped samples have small moment values, which is similar to the undoped sample. However, for the 10% case, a higher moment was observed. The inset in Figure 13b shows a narrow opening of the hysteresis loop near the origin for the 10% case, indicating a paramagnetic behavior. Earlier reports attributed the absence of ferromagnetism (or paramagnetism in Fe-TiO2) to sample preparation methods and/or dopant concentrations [41] where dopant Fe ions remain as isolated ions weakly interacting in the host TiO2. The lack of magnetic properties in previous works was also attributed to the presence of secondary phases, such as α-Fe2O3 or Fe3O4, since iron ions may coexist in different valance states [42,43]. For magnetic behavior, oxygen defects [41,44,45] could induce magnetic interactions. Since only paramagnetic properties were Catalysts 2021, 11, 438 10 of 16 observed in the present study, it was attributed to oxygen defects and Fe3+ ions in the TiO2 crystal.

Figure 13. MagnetizationFigure 13. Magnetization (M-H) curves (M-H) for curves iron-doped for iron TiO-doped2 nanoparticles TiO2 nanoparticles compared compared to pure TiO to pure2 measured TiO2 under an external magneticmeasured ﬁeld under (H), froman external−30 to magnetic 30 kOe at field temperatures (H), from of −3 (0a )to 5 K30 andkOe ( bat) temperatures 300 K. of (a) 5 K and (b) 300 K. Catalysts 2021, 11, x FOR PEER REVIEW 15 of 15

2.5. Optical Properties2.5. Optical Properties The UV–vis absorption spectroscopy was adopted to determine light absorption from

The pure TheTiO 2 UVsample–visiron-free didabsorption not and show iron-doped spectroscopy any absorption TiO2 wasnanoparticles. in adoptedthe region to The of determine visible spectra light are light shown(>400 absorption nm) in Figure 14. The pure whereasfrom it iron manifested-free TiOand 2an sampleiron absorptio-doped didn notTiO edge show2 nanoparticles toward any absorptionthe UV. The region inspectra the (<400 region are nm). shown of This visible in absorp- Figure light (>400 14. nm) whereas tion peak could be assignedit manifested to anatase an absorption band-gap edge excitation toward, which the UV equals region 3.2 (<400 eV [3 nm).1]. The This absorption peak could be assigned to anatase band-gap excitation, which equals 3.2 eV [31]. The visible light visible light absorption (350–550 nm) was significantly increased by iron doping in TiO2 nanoparticles and monotonicallyabsorption (350–550 increase nm)d with was iron signiﬁcantly content in increased the sample by iron. This doping observa- in TiO2 nanoparticles tion is consistent withand data monotonically found in literature increased [ with46,47 iron]. The content powder in thecolor sample. converted This observationfrom is consistent with data found in literature [46,47]. The powder color converted from white (pure TiO ) white (pure TiO2) by passing through yellow (low iron content) to brown (high iron con- 2 by passing through yellow (low iron content) to brown (high iron content), indicating that tent), indicating that doping TiO2 nanoparticles with iron enhanced the visible light absorption. doping TiO2 nanoparticles with iron enhanced the visible light absorption.

Figure 14. UltravioletFigure–visible 14. (Ultraviolet–visibleUV–vis) spectra for (UV–vis) iron-free spectra and iron for-doped iron-free TiO and2 nanoparticles iron-doped. TiO 2 nanoparticles.

3. Photocatalytic Application3. Photocatalytic Application 3.1. Photocatalytic Activity Test 3.1. Photocatalytic Activity Test A photocatalytic reactor system was utilized to carry out the photodegradation tests A photocatalytic reactor system was utilized to carry out the photodegradation tests of Rhodamine B (Rh-B) solution with iron-doped TiO2 nanoparticles under visible-light of Rhodamine B (Rh-B) solution with iron-doped TiO2 nanoparticles under visible-light irradiation. A mass equals 40 mg of iron-doped TiO2 nanoparticles (as a catalyst) was irradiation. A mass equals 40 mg of iron-doped TiO2 nanoparticles (as a catalyst) was added to 100 mL of organic dye solution (20 mg/L). Then the mixture was stirred in added to 100 mL of organic dye solution (20 mg/L). Then the mixture was stirred in dark dark for 60 min to reach an adsorption-desorption equilibrium before performing the for 60 min to reachphotodegradation an adsorption-desorption experiments. equilibrium The reactor before was performing loaded up the with photo- the mixture of catalyst degradation experiments.and organic The reactor dye solution. was loaded Water up cooling with the was mixture used to of maintain catalyst the and temperature or- constant at ganic dye solution.298 Water K.Then, cooling aXenon was used lamp to (CX-04E)maintain withthe temperature an optical ﬁlter constant (>420 at nm) 298 passing only visible K. Then, a Xenon lamplight (CX was-04E) used with to irradiate an optical the filter suspension. (>420 nm) At passing a time intervalonly visible of 15 light min, about 2 mL of the was used to irradiatesuspension the suspension. was extracted At a time and interval centrifuged of 15 min, at 14,000 about rpm 2 mL for of a the period sus- of 5 min in order to pension was extractedget ridand of centrifuged the catalyst. at To 14,000 calculate rpm thefor newa period dye concentrationof 5 min in order in the to solution,get absorbance at rid of the catalyst. To calculate the new dye concentration in the solution, absorbance at λmax = 553 nm (maximum absorption peak of Rh-B) was measured and given the symbol C. The rate of dye degradation is recorded as C/C0 where C0 is the initial concentration after reaching the adsorption-desorption equilibrium.

3.2. Degradation of Rhodamine B

It was clear from the previous characterization of Fe-TiO2 nanoparticles that iron doping shifted the optical response of TiO2 nanoparticles from UV to the visible light region. The optical response was directly proportional to iron concentration in the iron- doped samples. This implied that these iron-doped TiO2 nanoparticles, unlike iron-free TiO2, could be active in the visible-light region and considered to be catalysts. Since Rhodamine B is considered a major source of pollution for industrial waste water, its degradation in an aqueous solution has been studied. Figure 15 illustrates that, in the presence of iron-doped nanoparticles with Fe/Ti = 10%, the absorbance (at λmax = 553 nm) of the Rh-B dye monotonically decreases with time. The degradation efficiency of Rh- B dye in the presence of iron-doped TiO2 catalysts have been calculated from the following equation. C D% = [1 − ] × 100 (1) C0

Catalysts 2021, 11, 438 11 of 16

Catalysts 2021, 11, x FOR PEER REVIEW 15 of 15 λmax = 553 nm (maximum absorption peak of Rh-B) was measured and given the symbol C. The rate of dye degradation is recorded as C/C0 where C0 is the initial concentration after reaching the adsorption-desorption equilibrium.

where C0 is the initial concentration of dye and C is the concentration of dye after irradia- 3.2. Degradation of Rhodamine B tion in a selected time interval. The Rh-B dye concentrationIt was clear in fromthe solution the previous and its characterization absorbance at λ ofmax Fe-TiO =553 nm2 nanoparticles are that iron directly proportionaldoping according shifted to theBeer’s optical law. responseC0 was considered of TiO2 nanoparticles to be the initial from concen- UV to the visible light tration of Rh-B dyeregion. in the solution The optical to make response sure wasthat only directly a chemical proportional reaction to ironchanges concentration the in the iron- dye concentration dopedand removes samples. any This effect implied of dark that adsorption. these iron-doped TiO2 nanoparticles, unlike iron-free Figure 16 illustratesTiO2, could the RH be-B active dye degradation in the visible-light rates in region the existence and considered of iron-free to be TiO catalysts.2 Since Rhodamine B is considered a major source of pollution for industrial waste water, and iron-doped TiO2 nanoparticles under visible-light radiation. When using pure TiO2 nanoparticles as a itscatalyst degradation for two inhours an aqueous of radiation, solution the dye has was been degraded studied. Figureby ~12% 15 and illustrates that, in the the degradation efficiencypresence was of iron-doped raised to 20% nanoparticles when using with an iron Fe/Ti-doped = 10%, sample the absorbance with Fe/Ti (at λmax = 553 nm) = 1%. Then it was monotonicallyof the Rh-B dye increased monotonically up to 53% decreases when withusing time. an iron The-doped degradation sample efﬁciency of Rh-B with Fe/Ti = 10%. Thedye degradation in the presence efficiencies of iron-doped of Rh- TiOB dye2 catalysts in an aqueous have been solution calculated in the from the following equation. current study are consistent with values in literature [14,48,49]. After absorbing light, the C excited dye (Rhodamine B) injected an electron intoD% the= conduction1 − band× 100 of TiO2 where it (1) C0 was captured by surface-adsorbed O2 to form O2•−. Then, the dye cation radicals were degraded via attack bywhere oxygen C0 is active the initial species concentration [50]. In addition, of dye and it was C is thenoticed concentration that no Rh of-B dye after irradiation dye degradation tookin a place selected under time visible interval.-light radiation without catalysts.

Figure 15. AbsorptionFigure spectra 15. Absorption of Rh-B dye spectra in the existence of Rh-B dyeof the in Fe/Ti the existence = 10% sample of the at Fe/Ti various = 10% sample at various irradiation times. irradiation times.

The Rh-B dye concentration in the solution and its absorbance at λmax = 553 nm are directly proportional according to Beer’s law. C0 was considered to be the initial concentration of Rh-B dye in the solution to make sure that only a chemical reaction changes the dye concentration and removes any effect of dark adsorption. Figure 16 illustrates the RH-B dye degradation rates in the existence of iron-free TiO2 and iron-doped TiO2 nanoparticles under visible-light radiation. When using pure TiO2 nanoparticles as a catalyst for two hours of radiation, the dye was degraded by ~12% and the degradation efﬁciency was raised to 20% when using an iron-doped sample with Fe/Ti = 1%. Then it was monotonically increased up to 53% when using an iron-doped sample with Fe/Ti = 10%. The degradation efﬁciencies of Rh-B dye in an aqueous solution in the current study are consistent with values in literature [14,48,49]. After absorbing light,

Figure 16. Rhodaminethe B excited concentration dye (Rhodamine vs. irradiation B) injectedtime in the an existence electron intoof Fe the-TiO conduction2 nanoparti- band of TiO2 where •− it was captured by surface-adsorbedcles. O2 to form O2 . Then, the dye cation radicals were degraded via attack by oxygen active species [50]. In addition, it was noticed that no Rh-B The first-orderdye rate degradation constant k took(min− place1) for underphotodegradation visible-light radiationof dyes has without been calcu- catalysts. lated by employing the following equation:

lnC = lnC0 − 푘t (2)

where C0 and C are dye concentrations initially and after time t, respectively. The photodegradation rate constant (k) could be calculated from the above kinetic model and found to have the value of 0.001, 0.0019, 0.0039, 0.0047, and 0.006 min−1 for samples Fe/Ti = 0%, 1%, 3%, 5%, and 10%, respectively. Previous studies showed that the rate constant of Rh-

Catalysts 2021, 11, x FOR PEER REVIEW 15 of 15

where C0 is the initial concentration of dye and C is the concentration of dye after irradiation in a selected time interval. The Rh-B dye concentration in the solution and its absorbance at λmax =553 nm are directly proportional according to Beer’s law. C0 was considered to be the initial concentration of Rh-B dye in the solution to make sure that only a chemical reaction changes the dye concentration and removes any effect of dark adsorption. Figure 16 illustrates the RH-B dye degradation rates in the existence of iron-free TiO2 and iron-doped TiO2 nanoparticles under visible-light radiation. When using pure TiO2 nanoparticles as a catalyst for two hours of radiation, the dye was degraded by ~12% and the degradation efficiency was raised to 20% when using an iron-doped sample with Fe/Ti = 1%. Then it was monotonically increased up to 53% when using an iron-doped sample with Fe/Ti = 10%. The degradation efficiencies of Rh-B dye in an aqueous solution in the current study are consistent with values in literature [14,48,49]. After absorbing light, the excited dye (Rhodamine B) injected an electron into the conduction band of TiO2 where it was captured by surface-adsorbed O2 to form O2•−. Then, the dye cation radicals were degraded via attack by oxygen active species [50]. In addition, it was noticed that no Rh-B dye degradation took place under visible-light radiation without catalysts.

Catalysts 2021, 11, 438 12 of 16 Figure 15. Absorption spectra of Rh-B dye in the existence of the Fe/Ti = 10% sample at various irradiation times.

Figure 16. RhodamineFigure B concentration 16. Rhodamine vs. Birradiation concentration time vs.in the irradiation existence time of Fe in- theTiO existence2 nanoparti- of Fe-TiO2 nanoparticles. cles. The ﬁrst-order rate constant k (min−1) for photodegradation of dyes has been calcu- The first-orderlated rate byconstant employing k (mi then−1) following for photodegradation equation: of dyes has been calculated by employing the following equation: lnC = lnC0 − kt (2)

lnC = lnC0 − 푘t (2) where C0 and C are dye concentrations initially and after time t, respectively. The photodegradation rate constant (k) could be calculated from the above kinetic model and found where C0 and C are dye concentrations initially and after time t, respectively. The photo- to have the value of 0.001, 0.0019, 0.0039, 0.0047, and 0.006 min−1 for samples Fe/Ti = 0%, degradation rate constant (k) could be calculated from the above kinetic model and found 1%, 3%, 5%, and 10%, respectively. Previous studies showed that the rate constant of Rh-B to have the value of 0.001, 0.0019, 0.0039, 0.0047, and 0.006 min−1 for samples Fe/Ti = 0%, photodegradation had reported a value of 0.0029 min−1 when using iron-doped titania and 1%, 3%, 5%, and 10%, respectively. Previous studies showed that the rate constant of Rh- 0.00016 min−1 when using P25 [48]. Higher rate constants for the decomposition of Rh-B of the synthesized materials than standard P25 and Fe-TiO2 NPs prepared using different methods suggested that the present materials are more active. Hence, the above ﬁndings clearly demonstrate that a higher amount of iron incorporation into the framework of TiO2 makes the catalyst more efﬁcient for photocatalytic degradation of organic dyes.

4. Experiment

FSP was applied to synthesize TiO2 and Fe-TiO2 nanoparticles. Figure 17 shows the schematic of experimental setup, composed of a spray system with an annular pilot flame, a precursor delivery system, and a particle collection system. The spray system was an air-assisted spray nozzle. The precursor flowed through a capillary tube while the dispersion gas passed through the annular gap. Dispersion gas of oxygen (10.6 L/min) was supplied to the spray nozzle for precursor atomization with a pressure drop across the nozzle of 2.5 bar. An annular slit (15.88 mm inner diameter and 1 mm thickness), surrounding the nozzle provided the supporting premixed pilot flame. Methane (CH4 with purity 99.995%) with a flow rate of 1.25 L/min was mixed with 2.5 L/min of oxygen (O2 with purity 99.9995%) for the pilot flame. Various precursors were used to synthesize pure and doped-TiO2 nanoparticles. Ti- tanium tetraisopropoxide (Sigma-Aldrich, 97% purity) for TiO2 nanoparticles (NP), and dissolving ferrocene (Sigma-Aldrich, 98%) in m-xylene (Sigma-Aldrich, 99%) for Fe doping. Table1 shows the Fe/Ti atomic ratios in synthesizing nanoparticles resulting from changing ferrocene concentration. A syringe pump was used to inject the precursor into the spray nozzle with a flow rate of 450 mL/hr. Synthesized nanoparticles were collected on a glass-fiber filter with the aid of a vacuum pump. Catalysts 2021, 11, x FOR PEER REVIEW 15 of 15

B photodegradation had reported a value of 0.0029 min−1 when using iron-doped titania and 0.00016 min−1 when using P25 [48]. Higher rate constants for the decomposition of Rh- B of the synthesized materials than standard P25 and Fe-TiO2 NPs prepared using different methods suggested that the present materials are more active. Hence, the above find- ings clearly demonstrate that a higher amount of iron incorporation into the framework of TiO2 makes the catalyst more efficient for photocatalytic degradation of organic dyes.

4. Experiment

FSP was applied to synthesize TiO2 and Fe-TiO2 nanoparticles. Figure 17 shows the schematic of experimental setup, composed of a spray system with an annular pilot flame, a precursor delivery system, and a particle collection system. The spray system was an air-assisted spray nozzle. The precursor flowed through a capillary tube while the dispersion gas passed through the annular gap. Dispersion gas of oxygen (10.6 L/min) was supplied to the spray nozzle for precursor atomization with a pressure drop across the nozzle of 2.5 bar. An annular slit (15.88 mm inner diameter and 1 mm thickness), surrounding the nozzle provided the supporting premixed pilot flame. Methane (CH4 with purity Catalysts 2021, 11, 438 13 of 16 99.995%) with a flow rate of 1.25 L/min was mixed with 2.5 L/min of oxygen (O2 with purity 99.9995%) for the pilot flame.

Figure Figure17. Schema 17. Schematictic of flame of spray flame pyrolysis spray pyrolysis (FSP) apparatus. (FSP) apparatus. Phase and iron-content in the synthesized nanoparticles were identified by X-ray Various precursors were used to synthesize pure and doped-TiO2 nanoparticles. Ti- diffraction (Bruker, D8) using radiation from CuKα (λ = 1.5406 Å), Raman spectroscopy tanium tetraisopropoxide (Sigma-Aldrich, 97% purity) for TiO2 nanoparticles (NP), and (Aramis, LabRAM HR Visible) with 473 nm excitation, and inductively-coupled plasma- dissolving ferrocene (Sigma-Aldrich, 98%) in m-xylene (Sigma-Aldrich, 99%) for Fe dop- optical emission spectrometry (ICP-OES; Varian, 720ES). Average particle diameter can be ing. Table 1 shows the Fe/Ti atomic ratios in synthesizing nanoparticles resulting from calculated from a specific surface area (SSA), which was measured by Brunauer–Emmett– changing ferrocene concentration. A syringe pump was used to inject the precursor into Teller (BET) nitrogen adsorption (Micromeritics, ASAP 2420). Surface chemistry was the spray nozzle with aidentified flow rate usingof 450 X-ray mL/hr. photoelectron Synthesized spectroscopy nanoparticles (XPS). were Lightcollected absorbance was measured on a glass-fiber filter withusing the ultraviolet-visible aid of a vacuum pump. spectroscopy (UV–vis, Varian, Cary 500) and a superconducting Phase and iron-contentquantum in the interference synthesized device-vibrating nanoparticles were sample identified magnetometer by X-ray dif- (SQUID-VSM, Quantum fraction (Bruker, D8) usingdesign, radiation USA) wasfrom used CuK forα (λ the = 1 magnetic.5406 Å ), properties.Raman spectroscopy (Ar- amis, LabRAM HR Visible) Thewith nanoparticle 473 nm excitation, morphology and inductively and size- werecoupled extracted plasma from-op- transmission electron tical emission spectrometrymicroscope (ICP-OES; results Varian (TEM,, 720ES). FEI Com., Average Titan particle 80-300ST) diameter by operating can be it at an accelerating calculated from a specificvoltage surface of 300area kV. (SSA) The, which microscope was measured was set in by various Brunauer modes,–Emmett such– as bright-field TEM (BF- Teller (BET) nitrogen adsorptionTEM), high (Micromeritics resolution TEM, ASAP (HRTEM), 2420). Surface selected chemistry area electron wasdiffraction iden- (SAED), core-loss tified using X-ray photoelectronelectron energy-loss spectroscopy spectroscopy (XPS). Light (EELS), absorbance and energy-filteredwas measured us- TEM (EFTEM) modes to ing ultraviolet-visible spectroscopyobtain specific (UV information–vis, Varian about, Cary the physical500) and properties a superconducting of Fe-doped TiO2 nanoparticles. quantum interference The device entire-vibrating TEM data sample were magnetometer obtained and resolved (SQUID by-VSM Digital-Micrograph, Quantum Software Package design, USA) was used(Gatan for the Inc., magnetic Version properties. GMS1.85).

5. Conclusions

Flame spray pyrolysis was utilized to synthesize pure and Fe-doped TiO2 nanoparticles with various iron concentrations in a single-step process. Ferrocene was mixed with

TTIP and used as the precursor for iron-doped TiO2. XRD and Raman conﬁrmed that the anatase phase was dominant in pure TiO2 nanoparticles (>90% anatase), with an average particle diameter of about 12 nm (based on HRTEM). The addition of iron substantially changed the anatase phase to rutile with the anatase percentage decreasing from 86% to 21% when the Fe/Ti atomic ratio in the sample was changed from 1% to 10%, respectively. Morphology of the generated nanoparticles was studied using HRTEM, which revealed the crystallinity of the spherical particles and the uniformity of Fe doping distribution across the samples. XPS conﬁrmed the formation of Fe3+ and Fe2+ within the Fe-doped 3+ 2+ TiO2 samples. The ratio of Fe /Fe varied from 0.5 at Fe/Ti ratio of 3% to 1.8 at a 10% ratio. The valence band edge for the Fe-doped TiO2 was also lowered by ~1 eV when compared to the pure TiO2 sample. Regarding magnetic properties, Fe doping of TiO2 nanoparticles changed the material from nonmagnetic to paramagnetic. Doping TiO2 particles with Fe remarkably increased light absorption and expanded it to the visible-light region due to their narrower band gap. The iron-doped samples showed much higher catalytic activity for dye degradation under visible-light radiation when compared to pure Catalysts 2021, 11, 438 14 of 16

TiO2 nanoparticles. Non-toxic, cost-effective photo-catalysts are vital for industrial waste water treatment. In summary, FSP is an attractive method for the production of Fe-doped TiO2 with ferrocene as the source of iron. The resulting material has desirable magnetic and optical properties, which can be used in various photocatalytic and electronic applications.

Author Contributions: Conceptualization, M.A.I. and S.H.C.; Data curation, M.A.I., M.N.H., D.H.A. and V.S.; Formal analysis, M.A.I., M.N.H., D.H.A. and V.S.; Funding acquisition, S.H.C.; Investigation, M.A.I., D.H.A.; Methodology, M.A.I., M.N.H., D.H.A. and V.S.; Supervision, S.H.C.; Visualization, M.A.I.; Writing—original draft, M.A.I.; Writing—review & editing, M.A.I., M.N.H., D.H.A., V.S. and S.H.C. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by King Abdullah University of Science and Technology (KAUST) under a CCF Grant. Data Availability Statement: The data presented in this study are available on request from the corresponding author. Acknowledgments: The authors are grateful to Nasir Memon for his fruitful discussions and com- ments during the experimental phase of this work. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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