nanomaterials

Article Additive-Free Rice Starch-Assisted Synthesis of Spherical Nanostructured for Degradation of Dye Contaminant

Juan Matmin 1,* , Irwan Affendi 1, Salizatul Ilyana Ibrahim 1 and Salasiah Endud 2 1 Centre of Foundation Studies UiTM, Universiti Teknologi MARA (UiTM), Cawangan Selangor, Kampus Dengkil, 43800 Dengkil, Selangor, Malaysia; [email protected] (I.A.); [email protected] (S.I.I.) 2 Chemistry Department, Faculty of Science, Universiti Teknologi Malaysia (UTM), 81310 Skudai, Johor, Malaysia; [email protected] * Correspondence: [email protected]; Tel.: +60-389-245-449

 Received: 20 August 2018; Accepted: 5 September 2018; Published: 8 September 2018 

Abstract: Nanostructured hematite materials for advanced applications are conventionally prepared with the presence of additives, tainting its purity with remnants of copolymer surfactants, active chelating molecules, stabilizing agents, or co-precipitating salts. Thus, preparing nanostructured hematite via additive-free and green synthesis methods remains a huge hurdle. This study presents an environmentally friendly and facile synthesis of spherical nanostructured hematite (Sp-HNP) using rice starch-assisted synthesis. The physicochemical properties of the Sp-HNP were investigated by Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), thermogravimetric analysis (TGA), field emission scanning electron microscopy (FESEM), energy dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM), UV-Vis diffuse reflectance spectroscopy (DR UV-Vis), and adsorption–desorption analysis. The Sp-HNP showed a well-crystallized structure of pure rhombohedral phase, having a spherical-shaped morphology from 24 to 48 nm, and a surface area of 20.04 m2/g. Moreover, the Sp-HNP exhibited enhanced photocatalytic degradation of methylene blue dye, owing to the large surface-to-volume ratio. The current work has provided a sustainable synthesis route to produce spherical nanostructured hematite without the use of any hazardous agents or toxic additives, in agreement with the principles of green chemistry for the degradation of dye contaminant.

Keywords: iron oxide; rice starch; templating method; hematite; additive-free synthesis; spherical structures; nanoparticles; nanostructured materials

1. Introduction Iron oxide nanoparticles (IONPs) in the form of magnetite, maghemite, lepidochrocite, goethite, akageneite, and hematite have emerged in advanced applications of medical processes, biotechnology, sensors, data storage, pigments, fine ceramics, photo/electrochemical cells, wastewater treatment, and catalysts [1–4]. Amongst all IONPs, hematite nanoparticles (HNPs) show the most prominent catalytic activity for organic transformation or degradation reactions, depending on the shape synthesized. The potential of a HNP can be further enhanced as a high-performance catalyst, which delivers good conversion activity, specific selectivity, and reliable stability simply by changing the exposed facet via shape tailoring [5]. In recent years, a variety of synthetic methods have been investigated to produce HNPs using different approaches of physical synthesis (laser ablation arc discharge, combustion, ultrasound irradiation, electrodeposition, and pyrolysis) [6], biological synthesis (plant extracts, bacterium, yeast, fungus, and algae) [7] and chemical methods (sol–gel

Nanomaterials 2018, 8, 702; doi:10.3390/nano8090702 www.mdpi.com/journal/nanomaterials Nanomaterials 2018, 8, 702 2 of 16 synthesis, the reverse micelle method, and hydrothermal methods) [8]. However, these methods have mainly yielded disordered or irregular-shaped nano/macro-sized HNP morphologies that create severe agglomeration, such as bitruncated polygons [8,9] and pseudo-peanut shapes [10]. Therefore, significant efforts have been devoted to controlling the morphology of HNPs by employing different synthesis and processing routes. Today, HNPs are prepared in different regular shapes and defined morphologies, such as nanocuboids, nanowhiskers, nanorods, ovals, cubes, rhombohedral shapes, and spherical morphologies [11,12]. Nevertheless, most of the preparation methods are less favored due to the complexity of the optimization process and the presence of multiple variables. Preferably, HNPs with ordered morphologies should be prepared from the co-precipitation method (a wet chemical synthesis), which is simple, efficient, and tractable, in which the size, shape, and even morphology of the particles can be tailored [6]. To obtain well-crystallized HNPs with ordered morphologies, this method strongly depends on the use of additives such as copolymer surfactants (pluronic, F88, F127, P123, and polyvinylpyrrolidone), surface active structures from chelating compounds (thiols-based molecules), and stabilizing agents (triethylamine and (NH2)2CO of urea), as well as co-precipitating salts (sodium acetate or NaAc) [13–16]. Unfortunately, the presence of additives significantly affects not only the morphology and percentage yield, but also drastically decreases the HNPs’ purity due to the large amounts of byproducts, such as b-FeOOH, Fe3O4, or g-Fe2O3 [17], and the salts’ impurities, such as with ferric and ferrous salts [18]. Alternatively, to overcome such problems, additive-free syntheses of HNPs have been proposed [19], using greener and more benign reagents from biomaterials templates, such as starch. These methods avoid the use of additives, which commonly promote the presence of impurities from the incomplete conversion of the salts’ precursors into HNPs. The use of starch as a template has been reported in a number of previous studies [19]. This includes the use of starch as a template by using starch-coordinated Fe as the mixture of capping agents and, subsequently, the removal of the template by controlled heat treatment. In spite of that, the approach only produces highly agglomerated and disordered hematite macroparticles (greater than 1000 nm) with limited functionality [20]. This is due to the presence of organic remnants, originating from the mixtures of the capping agent used, which is strongly bound to the HNPs. It is well-known that the use of starch-assisted synthesis to obtain well-crystallized HNPs with ordered nanostructured morphologies remains a great challenge. As mentioned previously, ordered nanostructured HNP morphologies are prepared in various shapes. Notably, HNPs with spherical shapes are regarded as the optimized morphology for high packing density, balanced electron distribution, and charge migration due to its ideal surface-to-volume ratio, which promises good catalytic activity. The intrinsic properties of spherical nanostructured particles can also be fine-tuned by changing certain parameters such as diameter, bulk structure, surface chemistry, chemical composition, and crystallinity, which have led to continuous research interests [21]. Nevertheless, the currently available method for the synthesis of HNPs with a spherical morphology using pluronic surfactant shows that the nanostructure is easily disrupted, leading to the presence of mixed phases such as magnetite, wustite, and hematite with a sharp decrease in Barrett–Emmett–Teller (BET) surface area [22,23]. Evidently, it is challenging to retain spherical-shaped HNPs using the conventional surfactant-templated preparation methods. Therefore, the search for an environmentally friendly route, simple approach, and cost-effective method to prepare stable HNPs with spherical nanostructures for large-scale applications, particularly as a catalyst, is urgently required. To the best of our knowledge, the synthesis of spherical nanostructured HNPs using a native biomaterials template of rice starch without any additives in an aqueous solution has not been reported yet. In continuation of our work on the green synthesis of nanomaterials using rice starch [24], here, we report an additive-free method of rice starch-assisted synthesis for the fabrication of HNPs having a spherical morphology and a size of less than 50 nm. Owing to the spherical shape and porous networks that led to a larger surface area, the HNPs synthesized from this method were successfully utilized as a catalyst for the degradation of methylene blue, a dye contaminant. Nanomaterials 2018, 8, 702 3 of 16

2. Materials and Methods

2.1. Preparation of Spherical Nanostructured Hematite (Sp-HNP) All the starting materials were analytical grade and used as received (i.e., without further purification). Rice starch (RS) of 99.9% purity (with ~30% of amylose and ~70% of amylopectin content) was purchased from Euramco (M) Ptd Ltd., Johor, Malaysia. Methylene blue (MB), ethanol, and hydrochloric acid (HCl) were purchased from MERCK (M) Sdn Bhd, Bandar Sunway, Selangor, Malaysia, and acetone was purchased from HmbG Chemical, Ludolfstr., Hamburg, Germany. Iron(II) sulphate heptahydrate (FeSO4·7H2O, 99.9%) was purchased from Sigma-Aldrich (M) Sdn Bhd, Subang Jaya, Selangor, Malaysia. In a typical RS-assisted synthesis, the HNPs were synthesized according to a calculated molar composition of H2O/RS/FeSO4·7H2O at 1:1:4. Firstly, an appropriate amount of ◦ FeSO4·7H2O and RS were added to double distilled water (

2.2. Characterization A Fourier-transform infrared (FTIR) spectrometer (Perkin-Elmer, Spectrum One, Waltham, MA, USA) was used to determine the vibrational wavenumber of the functional groups in the range from 4000 to 450 cm−1. The nitrogen adsorption–desorption measurement was performed using an AUTOSORB-1 Quantachrome volumetric adsorption analyzer (Boynton Beach, FL, USA) with nitrogen as the adsorbate at 77.35 K for full-scale adsorption–desorption isotherms. The samples were degassed at 363 K for 3 h and held at 433 K for 12 h before analysis. The specific surface area was calculated using a Barrett–Emmett–Teller (BET) model. Additionally, a Barrett–Joyner–Halenda (BJH) model was used to calculate the pore volume distribution and the average pore size. X-ray diffraction (XRD) was carried out using a Bruker Advance D8 X-ray powder diffractometer (Karlsruhe, Germany) with Cu Kα radiation (λ = 1.54 Å, kV = 40, mA = 40). The morphology of the particles was observed using field emission scanning electron microscopy (FESEM, JSM-6700F, JEOL, Tokyo, Japan), attached with energy dispersive X-ray spectroscopy (FESEM-EDX), operated at 3.0 kV to determine the elemental composition of the samples. The sample was pre-coated with platinum (Pt) at 10−1 mbar using a Bio-Rad SEM system. Transmission electron microscopy (TEM) micrographs of the samples were recorded by a JEM-2100F Electron Microscope (JEOL, Tokyo, Japan) at 160 kV accelerated voltage. The samples were ultrasonically dispersed in acetone and trapped on holey membranes. The thermogravimetric analysis (TGA) of the RS-HNP sample was carried out using a Mettler Toledo TGA-SDTA 851e thermal analyzer (Columbus, OH, USA), with a heating rate of 10 ◦C min−1 from room temperature to 1200 ◦C under a nitrogen atmosphere. For solid samples, the UV-Vis diffuse reflectance spectra (DR UV-Vis) were measured by a Perkin Elmer Lambda 900 ultraviolet-visible/near-infrared (UV-Vis/NIR, Waltham, MA, USA) using a polytetrafluoroethylene polymer as a standard background, scanning in the wavelength ranges from 250 to 900 nm against absorbance. For liquid samples, the UV-Vis spectra were measured using a Thermo Scientific GENESYS 10S (Madison, WI, USA) from 500 to 800 nm.

2.3. Catalytic Activity

Methylene blue (MB), with the chemical formula of C16H18ClN3S, is a thiazine dye commonly used in the textile and printing industries [25]. Based on the structural analysis in Figure S1, the MB molecules consist of thiazines with a heterocyclic center attached to 2 aromatics molecules that provide Nanomaterials 2018, 8, 702 4 of 16 extra stability and hinder its degradation under visible light. Its industrial effluents are harmful to living organisms and are a major source of water contamination, which is caused by the blocking of sunlight that, in turn, decreases the dissolved oxygen capacity. In humans, MB leads to vomiting, nausea, and permanent damages to eyes on acute exposure [26]. Therefore, the treatment of water effluents containing MB is one of the dire concerns in the field of environmental chemistry. In this study, MB was selected as a model of organic water contaminants to confirm the catalytic degradation activity of the synthesized Sp-HNP. In these experiments, 2.0 mL of 0.1 mM MB was mixed with 1.0 mL of H2O2 (35% v/v) in a cuvette and stirred for 30 min in dark conditions to reach the adsorption–desorption equilibrium. Afterwards, 0.05 g of the Sp-HNP was added to the reaction mixture and constantly stirred. The sample was then irradiated with a UV hand lamp (6 W, λ = 365 nm, intensity = 0.8 µW cm−2), set perpendicular at a distance of 2 cm. The absorption spectra were recorded through time-dependent UV-Vis at 30-min intervals over the scanning wavelength from 200 to 600 nm, at room temperature (25 ± 2 ◦C). The absorption band of MB was taken at 506 nm, and the degradation percentage was calculated using the following equation:

Degradation (%) = [(Co − Ct)/(Co)] × 100% (1) where Co is the initial concentration and Ct is the concentration at time t.

3. Results and Discussion

3.1. Characterization of Spherical Hematite (Sp-HNP) To monitor the progress of the reaction, FTIR spectroscopy was used to identify the vibrational spectra corresponding to the functional groups of the RS powder, RS-HNP paste, and Sp-HNP, as shown in Figure1. In Figure1a, the vibrational band at 2927 cm −1 was assigned to C–H stretches, whereas the IR peaks for RS at 1079 and 1021 cm−1 were assigned to the anhydroglucose ring of the O–C stretch. Moreover, there was a broad band due to the hydrogen-bonded hydroxyl groups (O–H) at around 3400 cm−1, attributed to vibrational stretches of free, intermolecular-, and intramolecular-bound hydroxyl groups [27]. The vibrational bands at 1154, 1079, 1021 and 930 cm−1, were attributed to C–O stretching in the fingerprint region [28,29]. Other vibrational bands in the fingerprint region, at 526, 575, 609, 700 and 764 cm−1, were due to the skeletal mode vibrations of the pyranose ring in the glucose unit [27–29]. Intrinsically, based on the FTIR spectra in Figure1a, the RS showed vibrational bands of an intact glucose polymer. In the case of the RS-HNP, the FTIR spectra were measured in Figure1b. It can be observed that the salt precursor regulated by the RS polymer with the presence of Fe-ions’ adsorption from the peak around 1032 cm−1, was due to the C–OH bending and vibration of C–O stretching [10,20,28]. In addition, the COOH asymmetric and symmetric stretching bands, which were found around 1623 and 1406 cm−1, also supported the presence of Fe-ions–starch [10]. It is suggested that the adsorption of the hematite on the RS involved the interactions with the O–H group attributed to the stretching band at 3420 cm−1. The adsorption characteristic of the –FeOOH intermediates was not found at around 905 cm−1 due to its low concentration in the mixture of the RS-HNP. It can, therefore, be inferred from Figure1b that the Fe-ions were absorbed onto the RS polymer through physical interactions and chemical adsorptions from the existence of van der Waals forces [30]. Figure1c shows the FTIR spectra for the Sp-HNP with −1 strong vibrational bands at 594 and 474 cm , which were attributed to the hematite, α-Fe2O3 [10,20]. Judging from the intensity of these two peaks, it can be asserted that band 594 was greater than −1 474 cm at a 3:1 ratio, hence confirming the formation of α-Fe2O3 [20]. Both the vibrational bands corresponded to the Fe–O bond in the hematite. To clarify, the presence of very weak vibrational bands −1 at 1640 and 3430 cm signified the presence of water molecules adsorbed on α-Fe2O3 due to the hygroscopic nature of the metal oxide [31]. Nanomaterials 2018, 8, 702 5 of 16 Nanomaterials 2018, 8, x FOR PEER REVIEW 5 of 16

FigureFigure 1. Fourier-transformFourier-transform infrared infrared (FTIR) (FTIR) spectra spectra of the of (a the) rice (a )starch rice starch(RS); (b (RS);) RS-HNP, (b) RS-HNP, and (c) andspherical (c) spherical nanostructured nanostructured hematite hematite (Sp-HNP). (Sp-HNP).

InIn viewview ofof thethe ,crystallography, thethe RSRS andand Sp-HNPSp-HNP werewere comparedcompared usingusing XRDXRD measurements,measurements, asas shownshown inin FigureFigure2 2.. ItIt isis worth worth notingnoting thatthat starchstarch is is a a semi-crystalline semi-crystalline biopolymer, biopolymer, containing containing both both amorphousamorphous and crystallinecrystalline structures, with three different crystal types (A, B andand C)C) accordingaccording toto theirtheir amylopectinamylopectin packingpacking [[32].32]. The XRD diffractogramdiffractogram in Figure2 2aa showedshowed observableobservable peakspeaks forfor ◦ ◦ ◦ ◦ thethe RSRS at 16°, 16 , 17°, 17 ,23° 23 andand 24°. 24 The. The broad broad and and ha hardlyrdly distinguishable distinguishable diffraction diffraction peaks peaks suggested suggested the thepresence presence of very of very small small crystallite crystallite sizes. sizes. Based Based on on the the XRD XRD pattern, pattern, the the RS RS in in its nativenative granulesgranules exhibitedexhibited aa typicaltypical pattern pattern of of the the type-A type-A crystal structure [33 [33].]. In contrast,In contrast, the the XRD XRD diffractogram diffractogram for thefor Sp-HNPthe Sp-HNP showed showed strong, strong, narrow, narrow, sharp, andsharp, intense and peaks,intense suggesting peaks, suggesting that the structures that the structures were well ◦ ◦ ◦ crystallized,were well crystallized, as shown in as Figure shown2b. in The Figure major 2b. XRD The peaks major were XRD measured peaks were at 24.14 measured, 33.24 at, 35.6124.14°,, ◦ ◦ ◦ ◦ ◦ ◦ 40.8133.24°,, 35.61°, 49.47 , 40.81°, 54.09 ,49.47°, 57.62 ,54.09°, 62.46 57.62°,and 64.13 62.46°and and assigned 64.13° and to theassigned (0 1 2), to (1the 0 (0 4), 1 (12), 1 (1 0), 0 (14), 1(1 3), 1 (00), 2 (1 4) ,1 (1 3), 1 6),(0 2 (0 4), 1 8),(1 (21 6), 1 4), (0 and1 8), (3 (2 0 0)1 4), planes, and (3 respectively. 0 0) planes, The respectively. diffractogram The wasdiffractogram indexed to was the theoreticalindexed to datathe theoretical of the pure data rhombohedral of the pure phase rhombo (a hexagonalhedral phase crystal (a hexagonal family) of crystal the hematite family) (JCPDS of the cardhematite no.33-0664) (JCPDS [ 10card,34 ]no.33-0664) and the lattice [10,34] parameters: and the alattice = b = parameters: 0.501 nm, c = a 1.373 = b = nm. 0.501 The nm, powder c = 1.373 pattern nm. correspondedThe powder pattern to α-Fe corresponded2O3 with all theto α reflections-Fe2O3 with of all the the similar reflections peak of width. the similar The space peak groupwidth. andThe spinelspace group structure and of spinel the Sp-HNP structure was of a the R-3c, Sp-HNP was system a R-3c, which crystals was insystem good agreementwhich was within good the literatureagreement [35 with]. Previously, the literature the identical[35]. Previously, XRD pattern the identical of the obtained XRD patternα-Fe2 Oof3 wasthe obtained also reported α-Fe2 byO3 otherswas also [10 ,36reported]. From theby XRDothers analysis, [10,36]. the From highly the crystalline XRD analysis, Sp-HNP suggestedthe highly that crystalline the particles Sp-HNP were freesuggested from other that impuritythe particles peaks were such free as b-FeOOH,from other Fe impurity3O4, or g-Fe peaks2O3 such[37]. as The b-FeOOH, high purity Fe indicated3O4, or g- thatFe2O the3 [37]. as-synthesized The high Sp-HNPpurity isindicated good for that catalytic the applications.as-synthesized Judging Sp-HNP from theis good intense for XRD catalytic peaks, itapplications. was evident Judging that the from Sp-HNP the wasintense pure, XRD and peaks, the diffraction it was evident lines were that considerablythe Sp-HNP broadenedwas pure, dueand tothe the diffraction very small lines size ofwere the hematiteconsiderably crystals. broadened due to the very small size of the hematite crystals.Furthermore, the average particle sizes of the Sp-HNP can be calculated using the Debye–Scherrer equation.Furthermore, Briefly, it givesthe average a relationship particle between sizes of the the peak Sp-HNP broadening can be in calculated the XRD and using the particlethe Debye– size accordingScherrer equation. to the following Briefly, equation:it gives a relationship between the peak broadening in the XRD and the particle size according to the following equation:d = kλ/( β × cosθ) (2) where d is the particle size of the crystal, k is thed = Scherrerkλ/(β  cos constantθ) for the shape factor (0.9 was used(2) for common crystallites), λ is the X-ray wavelength used (Cu Kα = 0.15406 nm), β is the full width at the where d is the particle size of the crystal, k is the Scherrer constant for the shape factor (0.9 was used half-height maxima (FWHM) of the concerned XRD peaks, and θ is the Bragg diffraction angle based on for common crystallites), λ is the X-ray wavelength used (Cu Kα = 0.15406 nm), β is the full width at the determined incident grazing angle. The three most prominent peaks with Miller indices of (1 0 4), the half-height maxima (FWHM) of the concerned XRD peaks, and θ is the Bragg diffraction angle (1 1 0), and (1 1 6) were considered for the estimation of the average crystallite size against Si (1 1 1) based on the determined incident grazing angle. The three most prominent peaks with Miller FWHM for the reflection (2θ = 28.44◦ with CuKα radiation) of 2θ = 0.10◦ as standard. The measured indices of (1 0 4), (1 1 0), and (1 1 6) were considered for the estimation of the average crystallite size against Si (1 1 1) FWHM for the reflection (2θ = 28.44° with CuKα radiation) of 2θ = 0.10° as standard. The measured 2θ values for the peaks of the Si (1 1 1) standard materials pattern was in

Nanomaterials 2018, 8, 702 6 of 16

NanomaterialsNanomaterials 2018 2018, ,8 8, ,x x FOR FOR PEER PEER REVIEW REVIEW 66 of of 16 16 2θ values for the peaks of the Si (1 1 1) standard materials pattern was in good agreement with the literature values [38]. Using the Debye–Scherrer, Equation (2), the average crystallite sizes of the goodgood agreement agreement with with the the literature literature values values [38]. [38]. Us Usinging the the Debye–Scherrer, Debye–Scherrer, Equation Equation (2), (2), the the average average Sp-HNP were calculated to be in the range of 26–40 nm. crystallitecrystallite sizes sizes of of the the Sp-HNP Sp-HNP were were calculated calculated to to be be in in the the range range of of 26–40 26–40 nm. nm.

FigureFigureFigure 2.2.2. X-ray X-rayX-ray diffraction diffractiondiffraction (XRD) (XRD)(XRD) patterns patternspatterns of the ofof (a ) the RSthe and (a(a) ) (RSbRS) Sp-HNP, andand (b(b) indexed ) Sp-HNP,Sp-HNP, to the indexedindexed rhombohedral toto thethe rhombohedralstructurerhombohedral of hematite structure structure JCPDS of of hematite hematite card No. JC JC 33-0664PDSPDS card card (vertical No. No. 33-0664 33-0664 lines). (vertical (vertical lines). lines).

TheTheThe representative representativerepresentative thermogravimetric-differenti thermogravimetric-differentialthermogravimetric-differentialal thermal thermalthermal analysis analysisanalysis (TGA-DTA) (TGA-DTA) thermogram thermogramthermogram of ofof thethethe RS-HNP RS-HNP is is is shown shown shown in in in Figure Figure Figure 3. 3.3 .To To To confirm confirm confirm if if ifthe the the RS-HNP RS-HNP RS-HNP had had had completely completely completely converted converted converted into into into the the theSp- Sp- HNP,Sp-HNP,HNP, the the thedifferential differential differential thermal thermal thermal analysis analysis analysis (DTA) (DTA) (DTA) ther ther thermogrammogrammogram revealed revealed revealed heat heat heat variations variations variations associated associated with withwith differentdifferentdifferent endothermicendothermic reactions.reactions. For ForFor the thethe RS-HNP, RS-HNP,RS-HNP, the thethe mass massmass loss lossloss occurred occurredoccurred in inthreein threethree steps stepssteps following followingfollowing the ◦ theendothermicthe endothermic endothermic reactions. reactions. reactions. Based Based Based on on on the the the thermogram, thermogram, thermogram, the the the initial initial initial weight weight weightloss loss loss ofof of 2.8%2.8% 2.8% atat at 110110 110 °C °CC was was due duedue to toto thethethe removal removalremoval of of the the adsorbed adsorbed moisturemoisture moisture and and and otherother other volativolati volatilelele solventssolvents solvents in in in thethe the RS-HNP RS-HNP RS-HNP.. .In In In thethe the second second second step, step, step, a a ◦ weightaweight weight loss loss loss of of of13.8% 13.8% 13.8% at at at200 200 200 °C °C was Cwas was measured, measured, measured, indica indica indicatingtingting the the the decomposition decomposition decomposition of of an ofan anorganic organic organic component component component of of ◦ theofthe the polymericpolymeric polymeric RSRS RS inin inthethe the RS-HNP.RS-HNP. RS-HNP. TheThe The finalfinal final weightweight weight lossloss loss ofof of 18.1%18.1% afterafter 250 250 °C°CC corresponded correspondedcorresponded to to the thethe removalremovalremoval of ofof the thethe adsorbed adsorbedadsorbed oxygen oxygenoxygen species speciesspecies trapped trappedtrapped in inin the thethe nanoparticles nanoparticles [39]. [[39].39]. The TheThe final finalfinal residual residualresidual mass massmass was waswas α foundfoundfound to to be be 64.7%, 64.7%, 64.7%, which which which corroborated corroborated corroborated the the the presence presence presence of of α of α-Fe-Fe-Fe2O2O32 3for Ofor3 the forthe Sp-HNP. theSp-HNP. Sp-HNP.

FigureFigure 3. 3. Thermogravimetric-differential Thermogravimetric-differential thermal thermal anal analysisanalysisysis (TGA-DTA) (TGA-DTA) thermogram thermogramthermogram of ofof the thethe RS-HNP. RS-HNP.RS-HNP.

FigureFigureFigure 4 4a,b4a,ba,b showshow the thethe FESEM FESEMFESEM micrographmicrograph forfor for thethe the Sp-HNP,Sp-HNP, Sp-HNP, havinghaving having aa sphericalaspherical spherical morphology,morphology, morphology, atat differentatdifferent different magnifications.magnifications. magnifications. AsAs As suggestedsuggested suggested inin in thethe the mimi micrographcrographcrograph images,images, images, thethe the Sp-HNPSp-HNP revealed revealed almost almostalmost monodispersedmonodispersedmonodispersed spherical-shaped spherical-shapedspherical-shaped nano nanoparticles.nanoparticles.particles. Based Based on on the the morphological morphologicalmorphological analysis, analysis,analysis, the thethe spherical sphericalspherical shapeshapeshape of ofof the thethe Sp-HNP Sp-HNPSp-HNP was waswas also alsoalso supported supportedsupported by byby ImageJ ImageJImageJ software softwaresoftware (version (version(version 1.52e, 1.52e,1.52e, ImageJ2, ImageJ2,ImageJ2, University UniversityUniversity of ofof Wisconsin-Madison,Wisconsin-Madison, WI, WI, USA), USA), as as shown shown in in Figure Figure S2. S2. Precisely, Precisely, the the hematite hematite particles particles were were found found toto nucleatenucleate andand growgrow inin aa uniformuniform direction.direction. ThThee steadysteady growthgrowth ofof thethe hematitehematite waswas consistent,consistent, whichwhich eventuallyeventually developeddeveloped well-orderedwell-ordered crystalcrystalliteslites nanoparticlesnanoparticles andand thethe orderedordered sphericalspherical

Nanomaterials 2018, 8, 702 7 of 16

Wisconsin-Madison, WI, USA), as shown in Figure S2. Precisely, the hematite particles were found to nucleate and grow in a uniform direction. The steady growth of the hematite was consistent, which eventually developed well-ordered crystallites nanoparticles and the ordered spherical structure Nanomaterials 2018, 8, x FOR PEER REVIEW 7 of 16 of the Sp-HNP. However, a small agglomeration that affected the Sp-HNP dispersity was deemed present.structure The of composition the Sp-HNP. of However, the particles a small was agglom confirmederation that by the affected energy the dispersiveSp-HNP dispersity X-ray spectrum was (EDX)deemed as shown present. in FigureThe composition4c. The EDXof the spectrum particles was of the confirmed Sp-HNP by indicated the energy that dispersive there were X-ray only Fe (atspectrum 6.5 and (EDX) 0.7 keV)as shown and in O Figure (at 0.2 4c. keV) The EDX measured spectrum in theof the samples Sp-HNP due indicated to the that surface there were plasmon resonanceonly Fe [ 40(at]. 6.5 This and indicated 0.7 keV) and that O highly(at 0.2 keV) purified measured Fe2O 3innanoparticles the samples due were to the prepared surface plasmon by using the RS asresonance a green [40]. template. This indicated It is worth that highly mentioning purified that Fe2 theO3 nanoparticles peaks around were 6.5 prepared keV were by measuredusing the for RS as a green template. It3+ is worth mentioning that the peaks around 6.5 keV were measured for the the binding energies of Fe related to the as-synthesized RS-Fe2O3 colloids [13,41]. As mentioned 3+ in thebinding characterization, energies of Fe the related Pt element to the as-synthesized measured at 2.2RS-Fe keV2O3 in colloids the EDX [13,41]. was As due mentioned to the pre-coating in the characterization, the Pt element measured at 2.2 keV in the EDX was due to the pre-coating of the of the sample. As shown in Figure4d, the Sp-HNP was distributed as spherical nanoparticles with sample. As shown in Figure 4d, the Sp-HNP was distributed as spherical nanoparticles with diametersdiameters ranging ranging in in sizes sizes of of 24–48 24–48 nm nm andand appearedappeared to to be be well well substantiated substantiated with with the theestimated estimated valuevalue of theof the XRD. XRD. Table Table1 represents 1 represents the the EDX EDX depth depth analysisanalysis data data obtained obtained from from selected selected area area in in FigureFigure4b. In4b. Table In Table1, the 1, the elemental elemental composition composition for for thethe Sp-HNPSp-HNP was was confirmed confirmed to tobe beFe Fe(at (at35.89%) 35.89%) andand O (at O 58.27%),(at 58.27%), with with no no other other impurities impurities beingbeing detected. The The calculated calculated atomic atomic ratio ratio of Fe/O of Fe/O was was aboutabout 1:1.62 1:1.62 (Fe2 O(Fe3.252O),3.25 which), which agreed agreed well well with with the the expected expected stoichiometry stoichiometry (Fe (Fe2O23O).3). It It should should be be noted thatnoted the EDX that depththe EDX analysis depth analysis measured measured several severa micronsl microns in depth in depth from from the surfacethe surface of manyof many HNPs, whichHNPs, might which contribute might contribute to the experimental to the experimental uncertainty. uncertainty.

(a) (b)

(c) (d)

FigureFigure 4. ( a4.) ( Fielda) Field emission emission scanning scanning electronelectron microsco microscopypy (FESEM) (FESEM) micrograph micrograph of the of theSp-HNP Sp-HNP at at 20,00020,000× magnification magnification (the (the dotted dottedcircle circle suggestingsuggesting a a spherical spherical structure); structure); (b) (FESEMb) FESEM micrograph micrograph of of the Sp-HNP at 5000 magnification (with corresponding selected-area electron diffraction (SAED)); the Sp-HNP at 5000× magnification (with corresponding selected-area electron diffraction (SAED)); (c) energy dispersive X-ray spectroscopy (EDX) analysis spectra on SAED; and (d) histograms (c) energy dispersive X-ray spectroscopy (EDX) analysis spectra on SAED; and (d) histograms showing showing the particle size distributions. the particle size distributions.

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Table 1. EDX analysis for the spherical hematite (Sp-HNP) with Pt coating. Table 1. EDX analysis for the spherical hematite (Sp-HNP) with Pt coating. Elements Atomic Percentage PlatinumElements (Pt) Atomic 5.47 Percentage OxygenPlatinum (O) (Pt) 58.27 5.47 OxygenIron (Fe) (O) 35.89 58.27 Iron (Fe) 35.89

The TEM micrograph images for the Sp-HNP in Figure 5 confirmed that the spherical nanostructuresThe TEM micrographinterlinked over images a rhombohedral for the Sp-HNP material in Figure[42] to5 give confirmed ordered thatshape the and spherical different nanostructuressized particles interlinkedof less than over 50 a nm. rhombohedral These resu materiallts strongly [42] tosupported give ordered both shape the andFESEM different and sizedXRD particlesmeasurements. of less thanFigure 50 nm.5a reveals These results that the strongly nanost supportedructured bothSp-HNP the FESEM was composed and XRD measurements.of many tiny Figurenanoparticles,5a reveals with that thesizes nanostructured around 20 nm, Sp-HNP which was self-assembled composed of into many regular tiny nanoparticles, spheres, which with might sizes aroundlead to 20the nm, formation which self-assembled of a cavity or pores. into regular The regular spheres, lattice which fringes might can lead be to clearly the formation seen in ofFigure a cavity 5b. orBased pores. on TheFigure regular 5b, the lattice measured fringes interplanar can be clearly distance seen in or Figure lattice5b. spacing Based onin Figurethe TEM5b, image the measured was 0.25 interplanarnm, which corresponds distance or latticeto the (1 spacing 1 0) plane in the of TEMthe hexagonal image was phase 0.25 of nm, Fe2O which3, matching corresponds well with to thethe (1XRD 1 0) patternplane of for the d hexagonal110 spacing phase (2θ of= 35.61°) Fe2O3, matchingof the pure well hexago with thenal XRDhematite. pattern The for fast d110 spacingFourier- ◦ (2transformθ = 35.61 (FFT)) of the pattern pure hexagonal (the inset) hematite. of the Theselect fasted-area Fourier-transform electron diffraction (FFT) pattern (SAED) (the in inset) Figure of the5a selected-areaindicated the electron crystalline diffraction nature (SAED)of the inSp-HNP. Figure5 aFr indicatedom the results, the crystalline it is also nature suggested of the Sp-HNP.that the Fromnanoparticles the results, grew it is as also un suggestediform monocrystals that the nanoparticles of α-Fe2O3 grew without as uniform any coalescence monocrystals effects of α -Feon 2theO3 withoutnanoparticles any coalescence after the removal effects on of the the nanoparticles RS by the afterhydrothermal the removal treatment of the RS to by obtain the hydrothermal the ordered treatmentspherical nanostructure. to obtain the ordered spherical nanostructure.

(a) (b)

FigureFigure 5.5.TEM TEM micrograph micrograph of (ofa) hematite(a) hematite nanoparticles nanoparticles (HNPs), (HNPs), inset: corresponding inset: corresponding fast Fourier-transform fast Fourier- (FFT)transform pattern (FFT) from pattern the SAED from and the (SAEDb) lattice and fringes (b) lattice at the fringes (1 1 0) at plane. the (1 1 0) plane.

ToTo quantifyquantify thethe surfacesurface area,area, thethe nitrogennitrogen (N(N22)) adsorption–desorptionadsorption–desorption measurementsmeasurements andand correspondingcorresponding porepore size size distribution distribution (PSD) (PSD) obtained obtained by by the the Barrett–Joyner–Halenda Barrett–Joyner–Halenda (BJH) (BJH) method method for thefor Sp-HNPthe Sp-HNP are presented are presented in Figure in Figure6. Based 6. onBase thed measurements,on the measurements, the calculated the calculated Brunauer–Emmett– Brunauer– TellerEmmett–Teller (BET) surface (BET) area surface was foundarea was to be found 20.04 to m be2/g. 20.04 The m obtained2/g. The surfaceobtained area surface for the area Sp-HNP for the was Sp- relativelyHNP was 20 relatively times higher 20 times than mosthigher natural than ironmost powders natural (0.1–0.4iron powders m2/g) and(0.1–0.4 4 times m2/g) higher and than4 times the commercialhigher than hematite the commercial (5 m2/g) hematite [43]. Compared (5 m2/g) with [43]. other Compared synthetic with , other thesynthetic surface hematites, area acquired the fromsurface the area starch-assisted acquired from synthesis the starch-assisted was also considerably synthesis higherwas also than considerably that in the workhigher presented than that byin Parkthe work et al. (15.9presented m2/g) by [23 Park] and et Wu al. et (15.9 al. (15.8 m2/g) m 2[23]/g) [and44]. Wu According et al. (15.8 to the m International2/g) [44]. According Union of to Pure the andInternational Applied ChemistryUnion of (IUPAC)Pure and classification, Applied Chemistry the N2 adsorption–desorption(IUPAC) classification, isotherms,the N2 adsorption– as shown indesorption Figure6a, isotherms, exhibited aas typical shown type-IV in Figure isotherm 6a, exhibited with a a H3-type typical hysteresistype-IV isotherm loop ( P/Pwith0 >a 0.6H3-type)[45]. Thehysteresis type-IV loop isotherm (P/P0 > was 0.6) caused[45]. The by type-IV the capillary isotherm condensation was caused in by the the mesopores capillary networkscondensation of the in Sp-HNP,the mesopores while networks the H3-type of the hysteresis Sp-HNP, loop while was the attributed H3-type to hysteresis slit-shaped loop pores. was Theattributed results to were slit- consistentshaped pores. with thoseThe results of other were works consistent producing with HNPs those from of biomaterialsother works templates producing [46 ,HNPs47]. As from can bebiomaterials seen, the H3-typetemplates hysteresis [46,47]. As loop can appeared be seen, the inthe H3-type high-pressure hysteresis region loop appeared at a relative in the pressure high- pressure region at a relative pressure higher than 0.6. This proposed the existence of voids, pores, or

Nanomaterials 2018, 8, 702 9 of 16

Nanomaterials 2018, 8, x FOR PEER REVIEW 9 of 16 higher than 0.6. This proposed the existence of voids, pores, or spaces in extra networks between spacesthe interconnected in extra networksα-Fe between2O3 nanoparticles the interconnected on the Sp-HNPα-Fe2O3 nanoparticles [48]. It is worth on the mentioning Sp-HNP [48]. that It theis worthcontrolled mentioning heat treatments that the controlled and subsequent heat treatmen removalts and of the subsequent RS polymers remova enabledl of the the RS synthesis polymers of enabledspatially the separated synthesisα-Fe of 2spatiallyO3 nanoparticles. separated As α shown-Fe2O3 nanoparticles. in Figure6b, the As Sp-HNP shown showed in Figure PSD 6b, at the 21–38 Sp- Å HNPwith showed predominant PSD at pore 21–38 diameters Å with at predominant 22 Å. Notably, pore a broad diameters peak centeredat 22 Å. atNotably, 29 Å was a broad attributed peak to centeredsecondary at mesopores29 Å was attributed [44], which to were secondary contributed mesopores by the [44], Sp-HNP which agglomeration. were contributed by the Sp- HNP agglomeration.

(a) (b)

FigureFigure 6. 6. NitrogenNitrogen adsorption–desorption adsorption–desorption measurements measurements (a ()a Isotherms) Isotherms plot; plot; and and (b ()b Barrett–Joyner–) Barrett–Joyner– HalendaHalenda (BJH) (BJH) models. models.

TheThe UV-Vis UV-Vis diffuse diffuse reflectance reflectance spectra spectra of of the the RS RS and and Sp-HNP Sp-HNP samples samples are are shown shown in in Figure Figure 7a.7a. AsAs can can be be seen, seen, no no significant significant absorption absorption was was obtained obtained for for the the RS RS sample. sample. Typically, Typically, the the Sp-HNP Sp-HNP gavegave four four absorption absorption regions regions spectra spectra from from (i) (i) to to (iv) (iv) detected detected in in the the UV-Visible UV-Visible spectral spectral region region (near-UV(near-UV and and near-infrared)near-infrared) contributed contributed from from the theα-Fe α2O-Fe3 2nanoparticles.O3 nanoparticles. In the In same the mannersame manner reported reportedpreviously previously [43,49], the [43,49], absorption the absorption at 250–400 at nm 250–400 corresponding nm corresponding to the ligand-to-metal to the ligand-to-metal charge transfer chargetransitions transfer (LMCT) transitions was measured(LMCT) was at regionmeasured (i), attributedat region (i), to theattributed Fe–O ( πto-3d) the bondFe–O for(π-3d) somewhat bond 3+ 3+ 6 4 4 6 4 4 6 4 4 6 64 4 4 4 6 forfrom somewhat the Fe fromligand the field Fe transitions ligand fieldA transitions1→ T1( P) atA1 290–310→ T1( P) nm, at 290–310A1→ E( nm,D) andA1→ AE(1→D) Tand2( D)A1 at →360–3804T2(4D) at nm. 360–380 For region nm. For (ii), region the absorption (ii), the absorption from 400 tofrom 600 400 nm to was 600 attributed nm was attributed to the pair to excitation the pair 6 6 6 4 64 44 4 4 4 4 excitationprocesses processesA1 + A1 →A1T +1 ( AG)1→ + TT1(1(G)G) + atT 485–5501( G) at nm,485–550 partially nm, overlapped partially overlapped ligand field ligand transitions field at 6 4 4 64 4 4 4 transitions430 nm on at of 430A1 nm→ E,on Aof1 ( AG),1→ andE, charge-transferA1( G), and charge-transfer (CT) shoulder. (CT) The shoulder. absorption The at 600–750absorption nm at for 6 4 4 6 4 4 600–750region (iii)nm wasfor assignedregion (iii) to thewas Aassigned1→ T2( G)to transitionthe A1→ atT2( aboutG) transition 640 nm. Lastly,at about region 640 (iv)nm. measured Lastly, 6 4 4 6 4 4 regionfrom 750(iv) tomeasured 900 nm was from the 750A 1to→ 900T1( nmG) transitionwas the atA1 about→ T1( 900G) transition nm. Both regionsat about (iii) 900 and nm. (iv) Both were regionsmeasured (iii) forand an (iv) absorption were measured peak around for an 500–900 absorption nm, which peak couldaround be 500–900 deduced nm, from which the Fe–Fe could bond be deducedon d-to-d from transitions the Fe–Fe [50 ].bond Based on on d-to-d selection transition rules [49s [50].,50], theBased absorption on selection from therules charge-transfer [49,50], the absorptiontransitions from in regions the charge-transfer (i) and (ii) was transitions stronger than in thatregions from (i) the and ligand (ii) was field stronger transitions than in regionsthat from (iii) theand ligand (iv), as field indicated transitions from the in UV-Visregions absorption (iii) and intensity(iv), as indicated [43,49,50]. from More importantly,the UV-Vis theabsorption bandgap 2 intensity(Eg) value [43,49,50]. can be estimated More importantly, using a Tauc the Mott bandgap plot, by (E plottingg) value ( αcanhυ )beversus estimated hυ, as using shown a Tauc in Figure Mott7b. plot,To obtainby plotting the desired (αhυ)2 bandgap,versus hυ, the as shown Tauc’s in region Figure was 7b. extrapolated To obtain the to desired (αhυ)2 bandgap,= 0. The relationshipthe Tauc’s regionbetween was the extrapolated absorption coefficientto (αhυ)2 (=α 0.hυ The)n and relationship incident photon between hυ isthe shown absorption based oncoefficient the Tauc formula(αhυ)n andas follows:incident photon hυ is shown based on the Tauc formula as follows: n αhυ = A(hυ − Eg) (3) αhυ = A(hυ − Eg)n (3) α υ wherewhere α isis the the absorption absorption coefficient, coefficient, hυ h isis the the photon photon energy, energy, A A is is constant, constant, n ncan can be be 2 2(indirect (indirect transition) or 1/2 (direct transition), and Eg is the allowed bandgap. As shown in Figure7b, the indirect transition) or 1/2 (direct transition), and Eg is the allowed bandgap. As shown in Figure 7b, the indirectbandgap bandgap values werevalues extrapolated were extrapolated at 2.38 eV. at The2.38obtained eV. The valueobtained was value comparable was comparable to those synthesized to those α-Fe O nanoparticles that absorb visible light [51], suggesting that the Sp-HNP has a medium synthesized2 3 α-Fe2O3 nanoparticles that absorb visible light [51], suggesting that the Sp-HNP has a mediumbandgap bandgap (at 1–3 eV)(at 1–3 and eV) promising and promising semiconductor semiconductor applications applications (i.e., as (i.e., a photocatalyst) as a photocatalyst) [52]. [52].

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(a) (b)

FigureFigure 7.7. OpticalOptical analysis for (a) UV-Vis diffusediffuse reflectancereflectance spectroscopyspectroscopy (DR UV-Vis) andand ((bb)) TaucTauc plotplot ofof thethe Sp-HNP.Sp-HNP.

3.2.3.2. PlausiblePlausible FormationFormation ofof SphericalSpherical HematiteHematite (Sp-HNP)(Sp-HNP) BasedBased onon the the characterization characterization of theof the Sp-HNP, Sp-HNP a schematic, a schematic formation formation could becould depicted be depicted in Figure 8in. TheFigure highlighted 8. The highlighted additive-free additive-free procedure was procedure mainly duewas tomainly the multifunctional due to the multifunctional RS that can act RS as boththat reducingcan act as and both capping reducing agents and (stabilizer)capping agents simultaneously. (stabilizer) simultaneously. During the synthesis, During the the iron(II) synthesis, sulfate the powder initially converted into Fe-ions of metal aquo complex [Fe(H O) ]2+ upon dissolving in water. iron(II) sulfate powder initially converted into Fe-ions of metal aquo2 6 complex [Fe(H2O)6]2+ upon Notably,dissolving RS in consists water. of Notably, linear amylose RS consists and branchedof linear amylose amylopectin and structures,branched isamylopectin rich with hydroxyl structures, (OH) is backbones,rich with hydroxyl and contains (OH) abundant backbones, alcohol and terminalscontains (COH),abundant as shownalcohol in terminals Figure S3. (COH), The OH as backbones shown in reasonablyFigure S3. The facilitated OH backbones the complexation reasonably of facilitated Fe-ions to the the complexation RS polymer, of while Fe-ions the to alcohol the RS terminals polymer, assistedwhile the in alcohol the reduction terminals of the assisted Fe-ions in (+2)the reduction to Fe (0) nanoparticles of the Fe-ions [53 (+2)]. Taking to Fe (0) starch nanoparticles retrogradation [53]. processTaking starch [54] into retrogradation consideration, process the retrograded [54] into co RSnsideration, became soluble the retrograded in hot water, RS indicatingbecame soluble that the in polysaccharideshot water, indicating granules that swelled the poly andsaccharides burst, destroying granules the swelled rigidity and of burst, the semi-crystalline destroying the structure rigidity asof thethe smallersemi-crystalline amylose startedstructure leaching as the outsmaller and leavingamylose behind started the leaching amylopectin out and in theleaving granules. behind It can,the therefore,amylopectin be inferredin the granules. that after It adding can, therefore, the iron(II) be sulfateinferred solution that after to the adding RS solution, the iron(II) the Fe-ionssulfate weresolution attracted to the RS by solution, the oxygen the ofFe-ions the OH were branches, attracted selectivelyby the oxygen on theof the polysaccharides OH branches, selectively backbone ◦ aton 70 theC, polysaccharides seeding inside the backbone granules at regions 70 °C, of highseeding concentrations inside the of granules amylopectin. regions The of smaller high amyloseconcentrations molecules of amylopectin. began to form The a gelatinizedsmaller amylose network molecules on continuous began to heating form a outside gelatinized the granules, network whichon continuous increased heating the mixture’s outside viscositythe granules, (Scheme whic 1h inincreased Figure8 ).the At mixture’s the same viscosity time, the (Scheme long-chain 1 in amylopectinFigure 8). At assistedthe same in time, the nucleationthe long-chain and amylopectin promoted the assisted initial in growth the nucleation of the hematite and promoted crystallites the insideinitial thegrowth granules, of the as wellhematite as controlling crystallites the inside self-assembly the granules, into nano-spherical as well as morphology.controlling the For self- the mostassembly part, theinto vannano-spherical der Waals interactions morphology. between For the surfacemost part, molecules the van of theder hematite Waals interactions crystallites (RS-metalbetween the ion surface interactions) molecules are recognized of the hematite as the crystallites driving force (RS-metal for self-assembly, ion interactions) favoring are the recognized formation ofas largerthe driving Sp-HNPs force spheres for self-assembly, due to extensive favoring self-assembled the formation nanocrystals. of larger The Sp-HNPs formation spheres of the RS-HNP due to pasteextensive indicates self-assembled the synthesis nanocrystals. of starch-stabilized The formation nanoparticles of the RS-HNP that is similar paste indicates to other reports the synthesis for ZnO, of Ag,starch-stabilized and Au nanoparticles nanoparticles [55–57 that]. In is the similar case ofto theother RS-HNP, reports the for long ZnO, alkyl Ag, chain and Au of RS nanoparticles resulted in stable[55–57]. homogeneous In the case nanoparticlesof the RS-HNP, and preventedthe long alkyl agglomeration chain of RS (Scheme resulted 2 in in Figure stable8). Whenhomogeneous further heated,nanoparticles the RS-HNP and prevented yielded the agglomeration crystallized Sp-HNP (Scheme (Scheme 2 in Figure 3 in Figure8). When8) and further byproducts heated, of the sulfur RS- dioxide, SO , and sulfur trioxide, SO , according to the following chemical equation: HNP yielded2 the crystallized Sp-HNP3 (Scheme 3 in Figure 8) and byproducts of sulfur dioxide, SO2, and sulfur trioxide, SO3, according to the following chemical equation: 2FeSO4 → Fe2O3 + SO2 + SO3. (4) 2FeSO4 → Fe2O3 + SO2 + SO3. (4) As for the role of RS, it did not only function in reducing and stabilizing the Sp-HNP to protect As for the role of RS, it did not only function in reducing and stabilizing the Sp-HNP to protect it from the sintering effect and agglomeration, but it also can be suggested as the nanostructured it from the sintering effect and agglomeration, but it also can be suggested as the nanostructured template to control the morphology, as well as the nanoparticles’ sizes. Intriguingly, the obtained template to control the morphology, as well as the nanoparticles’ sizes. Intriguingly, the obtained Sp-HNP showed a high-quality hematite, giving a clean spectrum in FTIR, intense XRD

Nanomaterials 2018, 8, 702 11 of 16

Nanomaterials 2018, 8, x FOR PEER REVIEW 11 of 16 Sp-HNP showed a high-quality hematite, giving a clean spectrum in FTIR, intense XRD diffractogram diffractogrampeaks, and highly peaks, ordered and highly nanoparticles ordered nanoparticles in TEM, respectively in TEM, respectively (Figures2,3 (Figuresand5). In2, addition,3 and 5). In addition,the biomaterial the biomaterial features of features RS have of several RS have advantages several toadvantages its use in ato green-synthesis its use in a green-synthesis method. First, method.the environmentally First, the environmentally friendly approach friendly from theappr dispersionoach from of the Fe-ions–RS dispersion in Hof2 OFe-ions–RS to give the in RS-HNP H2O to giveimplies the benign RS-HNP reagents implies from benign biomaterials reagents without from bi theomaterials need of organic without solvents. the need Second, of organic the interaction solvents. Second,of the van the der interaction Waals forces of betweenthe van Fe-ions–RS der Waals is forc relativelyes between weak asFe-ions–RS compared withis relatively the interaction weak byas comparedthe hydrogen with bond the frominteraction the typical by the thiol-based hydrogen additives bond from [58]. Thethe typical weak interaction thiol-based enabled additives complete [58]. Theremoval weak of interaction the organic enabled component complete from theremoval metals of to producethe organic highly component ordered HNPs,from the as supportedmetals to produceby FTIR, highly TGA, ordered XRD, FESEM-EDX, HNPs, as supported and TEM by analysis. FTIR, TGA, It is XRD, of interest FESEM-EDX, that the and Sp-HNP TEM analysis. showed Ita magneticis of interest response that whenthe Sp-HNP applied showed with an externala magnetic magnet, response as shown when in applied Figure S4.with This an promisesexternal magnet,a prospect as forshown easy in separation, Figure S4. hence This promises increasing a itsprospect reusability for easy purpose. separation, Previous hence work increasing has shown its reusabilitythat HNPs havepurpose. been Prev separatedious work using has an shown external that magnet HNPs from have their been reacting separa mixturested using and an reused external to magnetdemonstrate from their their functional reacting mixtures stability [59and]. Thereused specific to demonstrateα-Fe2O3, having their rhombohedralfunctional stability repeating [59]. units The specificwith a hexagonal α-Fe2O3, having centered rhombohedral structure and repeating close-packed units oxygen with a in hexagonal the nano-sized centered array, structure allowed and the close-packedmagnetic response oxygen [60 in]. the nano-sized array, allowed the magnetic response [60].

Figure 8. Schematic formation of the Sp-HNP through RS-assisted synthesis. Figure 8. Schematic formation of the Sp-HNP through RS-assisted synthesis.

3.3. Catalytic Activity Although the focus of the study is the RS assisted-synthesisassisted-synthesis of the Sp-HNP for nanostructured hematites, the application of of the the Sp-HNP Sp-HNP as as a a photocatalyst photocatalyst represents represents a a further further remarkable remarkable feature. feature. The Sp-HNPSp-HNP was was expected expected to demonstrateto demonstrate promising promising catalytic catalytic capability, capability, especially especially when the when spherical the sphericalmorphology morphology can facilitate can the optimumfacilitate chargethe opti migrationmum charge without migration any diffusion without limitations any ofdiffusion reactant limitationsmoleculesdue of reactant to porous molecules networks. due As to reported porous bynetworks. Singh and As hisreported co-workers, by Singh IONPs and were his co-workers, used in the IONPsremoval were of toxic used ions in the and removal organic pollutantsof toxic ions due and to theorganic presence pollutants of porous due networks to the presence [61]. In of this porous study, networksthe Sp-HNP [61]. underwent In this astudy, catalytic the performanceSp-HNP un evaluationderwent a bycatalytic comparing performance the absorption evaluation spectra by of comparingthe degraded the MB, absorption which wasspectra irradiated of the degraded under UV MB, light which within was certain irradiated periods. under Figure UV9 light shows within the certainperformance periods. of the Figure prepared 9 shows catalysts, the examined performance by the of degradation the prepared of MB catalysts, for 270 min. examined From Figure by the9b, degradationMB was measured of MB for to degrade270 min. at From 17% Figure upon adding9b, MB Hwas2O 2measuredexposed underto degrade UV light at 17% irradiation upon adding after H302O min.2 exposed After aunder period UV of time,light MBirradiation further degradedafter 30 min. to 50% After and a period 99%, monitored of time, MB after further 120 and degraded 270 min, torespectively. 50% and 99%, The monitored degradation after process 120 and was 270 also min, supported respectively. by the The photo-captured degradation process images inwas Figure also supportedS5a. It is worth by the mentioning photo-captured that MB images was notin Figure significantly S5a. It degraded is worth inmentioning dark conditions, that MB indicating was not significantly degraded in dark conditions, indicating dependency on the UV light. Importantly, the process was chemical photo-degradation rather than physical adsorption. For comparison, the MB degradation experiment was also performed in presence of H2O2 and/or the UV light without the

Nanomaterials 2018, 8, 702 12 of 16 dependency on the UV light. Importantly, the process was chemical photo-degradation rather than physicalNanomaterials adsorption. 2018, For8, x FOR comparison, PEER REVIEW the MB degradation experiment was also performed in12 presenceof 16 of H2O2 and/or the UV light without the Sp-HNP (Figure S5). It can be seen that the degradation of MB is hardlySp-HNP initiated (Figure without S5). It the can Sp-HNP, be seen that which the confirmsdegradation its catalyticof MB is hardly activity. initiated Nonetheless, without thethe catalyticSp- HNP, which confirms its catalytic activity. Nonetheless, the catalytic activity of the Sp-HNP is activityNanomaterials of the Sp-HNP 2018, 8, x isFOR comparable PEER REVIEW to that of the previously reported HNPs as a catalyst12 of [ 1662 ]. comparable to that of the previously reported HNPs as a catalyst [62]. Sp-HNP (Figure S5). It can be seen that the degradation of MB is hardly initiated without the Sp- HNP, which confirms its catalytic activity. Nonetheless, the catalytic activity of the Sp-HNP is comparable to that of the previously reported HNPs as a catalyst [62].

(a) (b)

FigureFigure 9. Degradation 9. Degradation of methylene of methylene blue (MB) blue for(MB) 270 for min 270 under min (undera) UV-Vis (a) UV-Vis and (b )and plot ( ofb) degradationplot of (a) (b) (%) againstdegradation time. (%) against time. Figure 9. Degradation of methylene blue (MB) for 270 min under (a) UV-Vis and (b) plot of As shown in Figure 10, the rate of the photocatalytic degradation of MB was deduced from the As showndegradation in Figure (%) against 10, the time. rate of the photocatalytic degradation of MB was deduced from the plot of the concentration against the irradiation time for 180 min. The overall MB degradation plot of the concentration against the irradiation time for 180 min. The overall MB degradation showed showed both an exponential and linear relationship of pseudo-first-order kinetics, as indicated in both an exponentialAs shown in and Figure linear 10, the relationship rate of the photoc of pseudo-first-orderatalytic degradation kinetics, of MB was as deduced indicated from in the the inset theplot inset of the of Figureconcentration 10 for the against full degradation the irradiation time time up to for 270 180 min. min. For The all theoverall initial MB concentrations degradation of of FigureMB,showed 10 the for degradationboth the an full exponential degradation gave a andstraight linear time line relationship up that to fits 270 well of min. pseudo-first-order into For a logarithmic all the initial kinetics, function, concentrations as indicatedas commonly in of MB, the degradationobservedthe inset ofin gave Figure the aphotocatalytic straight10 for the line full degradation thatdegradation fits well reactitime into upons a to logarithmic of 270 most min. organic For function, all thecomp initialounds as commonlyconcentrations [63]. Hence, observed of the in the photocatalytickineticMB, the study degradation degradationon the rategave of a reactions thestraight degradation line of mostthat forfits organic MBwell in into the compounds apresence logarithmic of [ 63 thefunction,]. Sp-HNP Hence, as thecommonlyfollowed kinetic the study on the ratepseudo-first-orderobserved of the in degradationthe photocatalytic reaction. for The MBdegradation plot in for the line presence reacti (Ct/Cons0) ofagainstof themost Sp-HNPtime organic showed comp followed aounds linear the[63]. relationship pseudo-first-order Hence, the until 180kinetic min study of degradation on the rate time, of the so thedegradation rate constant for MB (k) incan the be presencedetermined of the from Sp-HNP the gradient. followed Here, the C0 reaction. The plot for line (Ct/C0) against time showed a linear relationship until 180 min of andpseudo-first-order Ct are the initial reaction. concentration The plot forof MBline and(Ct/C the0) against concentration time showed of MB a linear after relationship𝑡, irradiation until time, degradation time, so the rate constant (k) can be determined from the gradient. Here, C0 and Ct respectively.180 min of degradation Moreover, time,the calculated so the rate k constant for the degradation(k) can be determined reactions fromwas 5.68 the gradient. 10−3 min Here,−1 and C 0the are the initial concentration of MB and the concentration of MB after t, irradiation time, respectively. half-life,and Ct are the the t1/2 initialvalue, concentrationwas found to of be MB 122 and min, the suggesting concentration that ofthe MB Sp-HNP after 𝑡 ,causes irradiation good time, photo- −3 −1 Moreover,degradationrespectively. the calculated of Moreover, MB. k forthe calculated the degradation k for the degradation reactions was reactions 5.68 was× 10 5.68 min 10−3 minand−1 and the the half-life, the t1/2 half-life,value, wasthe t found1/2 value, to was be 122found min, to be suggesting 122 min, suggesting that the Sp-HNPthat the Sp-HNP causes goodcauses photo-degradationgood photo- of MB. degradation of MB.

Figure 10. Concentration of MB as a function of the irradiation time for 180 min. The inset shows the concentrationFigure 10. Concentration of MB for the of MBfull asirradiation a function time of the from irradiation 0 to 270 timemin forwith 180 the min. first-order The inset kinetic shows section the Figure 10. definedconcentrationConcentration in the ofdotted MB for ofbox. MBthe full as irradiation a function time of thefrom irradiation 0 to 270 min time with forthe 180first-order min. Thekinetic inset section shows the concentration defined ofin the MB dotted for the box. full irradiation time from 0 to 270 min with the first-order kinetic section defined in the dotted box. Nanomaterials 2018, 8, 702 13 of 16

4. Conclusions In conclusion, ordered nanoparticles and the monodispersed spherical iron oxide of Sp-HNP have been prepared by using an additive-free method, followed by controlled hydrothermal treatments. For the additive-free method, RS was successfully employed as a simple, environmentally friendly and low-cost multifunctional reducing and stabilizing agent. Briefly, the presence of RS protected the nucleation growth of the HNPs against sintering and agglomeration and allowed the avoidance of the use of unnecessary additives. The homogeneous nucleation process of Fe-ions happened during the retrogradation of the rice starch inside the granular networks to regulate the size and morphology of the Sp-HNP. Based on the physicochemical characterization, the synthesized Sp-HNP had a spherical-shaped, nano-sized morphology from 24 to 48 nm, a pure rhombohedral crystalline structure, and a good surface area of 20.04 m2/g, which is comparable to most conventional hematite nanoparticles. In addition, because the facile synthesis procedure does not require the introduction of any additives to the reaction system, this simple synthesis method has the potential to be suitable for the synthesis of other novel nanostructured metals. Based on its catalytic performance, the Sp-HNP successfully degraded MB at 99% (after 270 min irradiation under UV light) and obeyed the pseudo-first-order kinetics. In the future, the RS-assisted synthesis presented here will ensure a marked improvement in the studies of advanced nanomaterials that demand spherical nanostructured hematite, especially with respect to size-dependent particles for magnetization properties in biomedical imaging, lithium batteries, and gas sensors.

Supplementary Materials: The following are available online at www.mdpi.com/link, Figure S1: Structural analysis (a) methylene blue (MB) with the chemical formula of C16H18ClN3S; and (b) ball and stick model based on Molecular Mechanics-2 (MM2) for MB at different perspectives showing a sterically hindered central portion of molecules. The presence of Cl- is omitted to avoid bulky structures. Figure S2: Morphological analysis for Sp-HNP showing the spherical morphology on a surface plot analyzed using ImageJ from Java-based software version 1.52e. Figure S3: Rice starch (RS) granules model representing linear amylose and branched amylopectin chains. Figure S4: Photo-captured image on Sp-HNP (reddish-brown powder), showing a magnetic response when applied with an external magnet. Figure S5: Catalytic activity for the degradation of MB (a) Photo-captured images for MB/Sp-HNP/H2O2/UV; Ultraviolet–visible (UV-Vis) spectra for different conditions: (b) MB/UV/H2O2; (c) MB/H2O2; and (d) MB/UV without the presence of the Sp-HNP catalyst Author Contributions: J.M. and S.E. conceived of and designed the experiments; J.M. performed the experiments; J.M., I.A. and S.I.I. contributed reagents/materials/analysis tools; and J.M. and I.A. analyzed the data and wrote the paper. All authors have read and approved the final manuscript. Funding: This work was financially supported by Universiti Teknologi Mara (UiTM, Malaysia) under research grant cost centre No. 600-UiTMSEL (PI. 5/4) (044/2018). Acknowledgments: We highly appreciated the use of ImageJ software as our open source image processing program, obtained at https://imagej.net/Downloads. Conflicts of Interest: The authors declare no conflicts of interest.

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