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Showcasing research by Neravathu G. Divya and

Prof. M. Junaid Bushiri from Department of Physics,

Cochin University of Science & Technology, India.

High index facets bounded α-Fe2O

3 pseudocubic nanocrystals

with enhanced electrochemical properties: Zn2+ ions assisted solvo-hydrothermal synthesis

Zn2+ ions assisted, solvo-hydrothermally grown, high index facets {012}, {014

_} and {2

_10} bounded, α-Fe

2O

3 pseudocubic

nanocrystals having low impedance, shows good sensitivity for the electrochemical detection of aqueous ammonia.

rsc.li/crystengcomm

As featured in:

See Neravathu G. Divya and M. Junaid Bushiri, CrystEngComm, 2019, 21, 1378.

CrystEngComm

PAPER

Cite this: CrystEngComm, 2019, 21,

1378

Received 26th October 2018,Accepted 7th January 2019

DOI: 10.1039/c8ce01837e

rsc.li/crystengcomm

High index facet bounded α-Fe2O3 pseudocubicnanocrystals with enhanced electrochemicalproperties: Zn2+ ion assisted solvo-hydrothermalsynthesis

Neravathu G. Divya and M. Junaid Bushiri *

Pseudocubic α-Fe2O3 nanocrystals were grown by a surfactant-free, low temperature, solvo-hydrothermal

process and characterised by XRD, FESEM, TEM, FTIR, Raman, XPS and UV-vis analysis. These α-Fe2O3

nanocrystals were bounded by high index facets {012}, {014̄} and {2̄10} having an average edge length of

120 nm and a dihedral angle of 85°. The growth of high index facet bounded α-Fe2O3 pseudocubic nano-

crystals was contributed by the structure inducing properties of Zn2+ cations (ZnCl2) added to the mother

solution. This work also sheds light into the low impedance and enhanced electrochemical activities of

high index facet bounded α-Fe2O3 pseudocubic nanocrystals, which exhibited good electrochemical sens-

ing response towards aqueous ammonia.

Introduction

Iron oxide nanoparticles has gained continued research inter-est over the past several decades due to their unique electri-cal, optical and magnetic properties. Apart from this, theirstability, biocompatibility, nontoxicity and abundance havealso added to their attractive benefits and they are suitablefor applications in areas such as diagnostics,1 biomedicines,2

catalysis,3,4 sensing,5 water treatment,6 magnetic resonanceimaging,7 ferrofluids,8 pigments9 etc. Among the differentpolymorphic forms of iron oxide such as α-Fe2O3, β-Fe2O3,γ-Fe2O3 etc., the most stable form under ambient conditionsis the well-known n-type semiconducting hematite nanoparti-cle (α-Fe2O3).

10,11 It has a rhombohedrally centred hexagonalstructure of close-packed oxygen lattices and two-thirds of itsoctahedral sites are occupied by Fe3+ ions.11

Controlling the size, shape and dimensions of α-Fe2O3

nanoparticles is a prerequisite for their technological applica-tion, because of the strong correlation of these parameterswith their chemical and physical properties and reaction effi-ciencies.12,13 This is due to the fact that most of these proper-ties are originating from the size, shape, morphologies anddifferent exposed crystal facets.14,15 In particular, increasingthe density of catalytically or electrochemically active crystal-lographic facets on the surfaces of various nanomaterials isthe most promising route to enhance the catalytic or electro-chemical properties of these materials.16,17 Hence, α-Fe2O3

nanoparticles with different morphologies and exposed facetshave been synthesized by a hydrothermal process,18,19 sol–gelsynthesis,20 sonochemical reactions,21 electrospinning,22

coprecipitation23 etc., which resulted in the formation of par-ticles in micro to nano size ranges with different morphol-ogies. The different synthesis procedures can also induce se-lective exposure of reactive crystal facets, and hencedrastically promote their reaction efficiencies.15 As a result,researchers reported the growth of α-Fe2O3 nanowires,24

nanorings,25 nanotubes,25 spheroids,26 pseudocubic micro-structures26 etc. However, the development of new strategiesfor the preparation and modification of their size and mor-phologies with alterations in the exposed crystallographicfacets have been intensively pursued, due to their scientificinterest and many other relevant applications of these kindsof new functional nanomaterials.

The majority of α-Fe2O3 nanoparticles grown with differ-ent techniques consisted of exposed low-index facets havingMiller indices {111}, {100}, {110} etc.27 This is because of thefact that the nucleation and subsequent growth process fa-vour the minimization of their surface energies during thecourse of crystal growth.27 However, it has been reported thathigh energy surfaces with high-index facets represented by aset of Miller indices {hkl}, with at least one index beinggreater than unity, generally exhibit much higher chemical-reaction activities due to their enhanced surface energies.28,29

Such surfaces with higher energies usually grow faster com-pared to other facets, which make them disappear ultimatelyduring the course of crystal growth in accordance with theprinciple of minimum energy.30,31 However, it is very

1378 | CrystEngComm, 2019, 21, 1378–1388 This journal is © The Royal Society of Chemistry 2019

Department of Physics, Cochin University of Science and Technology, Kochi,

Kerala-682022, India. E-mail: junaidbushiri@gmail.com; Tel: +91 9495348631

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important to develop and retain exposed high energy surfaceshaving higher reactivity in α-Fe2O3 nanocrystals to enhancetheir reaction efficiencies.32 Hence, a great challenge to pre-serve the exposed high-index facets, with high surface ener-gies, still persists due to the lack of knowledge regarding theentire crystal growth mechanisms.33

Semiconducting α-Fe2O3 nanocrystals with exposed highindex facets are particularly useful for applications in electro-chemistry, chemical sensing, gas sensing, heterogeneous ca-talysis, ion detection, photocatalysis, energy conversion etc.,because of their enhanced properties.34,35 Hence, in order tostabilize the high energy surfaces of α-Fe2O3 with enhancedchemical or electrochemical reaction activities for many tech-nological applications, the use of potential additives or cap-ping reagents such as halogen ions, was reported as benefi-cial, which could effectively modulate the nucleation centresand govern the crystal growth process under controlled exper-imental conditions.33,35 However, a few reports are also there,revealing the effectiveness of metal ions in controlling thenanostructures' morphologies,12–14,27,30,31,35,36 but no reportsreveal the promoted or enhanced electrochemical propertiesof high index facet bounded α-Fe2O3. Therefore, exploringsimple synthetic methods for developing α-Fe2O3 nano-crystals bounded by high-index facets is very crucial to facili-tate their potential electrochemical applications.

However, during the hydrothermal process, the crystalgrowth medium plays a significant role according to earlierreports of our research group. Satheesh et al. reported thegrowth of microspheres of various sizes in methanol, metha-nol–water and propanol solvent systems.26 However,according to their report, the propanol–water solvent combi-nation has resulted in the growth of pseudocubic α-Fe2O3

microcrystals.26 Moreover, ferric chloride, the precursor ofα-Fe2O3 in acetonitrile medium has been reported as an effi-cient antioxidant.37 This property is effectively exploited toenhance the structure inducing efficiency of Zn2+ ions in thepresent work. So, herein we report a simple strategy tosynthesize α-Fe2O3 nanocrystals bounded by exposed high en-ergy high index facets, by a one-step, surfactant-free, solvo-hydrothermal synthesis process in a water–acetonitrile sol-vent combination, in the presence of a minor quantity ofZnCl2 as the structure inducing additive and their electro-chemical properties were analysed along with their electro-chemical sensing efficiencies towards aqueous ammonia.

ExperimentalSynthesis of α-Fe2O3 pseudocubic nanocrystals

Pseudocubic nanocrystals of iron oxide (α-Fe2O3) having ex-posed high indexed facets were synthesized by a surfactant-free, solvo-hydrothermal process at a reaction temperature of180 °C. For this, 300 mM iron chloride hexahydrate (FeCl3·6H2O, Spectrum, India) was prepared in a deionized water–acetonitrile (C2H3N) solvent combination (1 : 1, v/v). Then,150 mM zinc chloride hexahydrate (ZnCl2·6H2O, Merck, In-dia) was added under continuous stirring and stirred to-

gether vigorously at room temperature for 30 minutes. Afterthe dropwise addition of the oxidizer (NaOH) dissolved in thesame solvent combination, the pH of the precursor solutionwas adjusted. Subsequently, it was transferred into a Teflon-lined autoclave (100 ml) and heated in an air atmosphere for6 hours. Finally, the reaction system was cooled down toroom temperature, and the resultant reaction product was fil-tered out and washed scrupulously with water and ethanol.Then, the resultant powder was dried in air to obtain a finepowdered sample of α-Fe2O3 (6FO).

In order to understand the effect of the duration of thesolvo-hydrothermal process on the morphological growth, time-dependent experiments were carried out under similar solvo-hydrothermal conditions at 180 °C for different durations of 12(12FO), 18 (18FO) and 24 (24FO) hours, respectively.

To understand the ability of Zn2+ ions to induce thepseudo cubic morphology, two reference samples were alsosynthesized via a similar solvo-hydrothermal process for 24hours under the same controlled experimental conditionsusing ZnIJNO3)2 instead of ZnCl2 and also without any zincsource, and the resultant samples were named as RS1 andRS2, respectively.

Characterization

The powder X-ray diffraction (XRD) measurements were car-ried out using an XPERT-PRO X-ray diffractometer(PANalytical), using Cu-Kα radiation (λ = 1.5405 Å). The surfacemorphologies and the elemental compositions of the synthe-sized samples were determined using a field emission scan-ning electron microscope (FESEM, Carl Zeiss, Supra 40VP)equipped with an energy dispersive X-ray spectrometer (EDX,Bruker). The transmission electron microscopy (TEM) images,high-resolution TEM (HRTEM) images and selected areaelectronic diffraction patterns (SAED) of the samples wereobtained by using a JEOL JEM-2100 with an accelerating volt-age of 200 kV. The Fourier transform infra-red spectroscopic(FTIR) data were gained from an IR Affinity-1-8400S, using theKBr pellet method, the Raman spectra were obtained with aHoriba Jobin Yvon Lab RAM HR system equipped with a He–Ne laser (514 nm wavelength) and the X-ray photoelectronspectra (XPS) were recorded on a Kratos AXIS Ultra spectrome-ter. Optical absorption measurements of the sample wereobtained using a UV-vis-NIR Jasco V-570 spectrometer.

All the electrochemical analyses were carried out on aCHI720 electrochemical workstation. Electrochemical AC im-pedance spectroscopy (EIS), cyclic voltammetry (CV) and am-perometric analysis were carried out using a three-electrodecell. A platinum wire was used as the counter electrode, Ag/AgCl was used as the reference electrode and an α-Fe2O3/RS2modified glassy carbon electrode was used as the workingelectrode for the electrochemical measurements.

Fabrication of electrodes

The working electrodes for obtaining the electrochemicalproperties and electrochemical sensing efficiencies towards

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aqueous ammonia were fabricated by dispersing α-Fe2O3

pseudocubic nanocrystals (0.01 g) and RS2 spherical nano-particles individually in ethanol (200 μl) in the presence ofNafion (5 wt%) as a conductive binder. The resultant slurries(10 μl) were deposited on two different glassy carbonelectrodes (GCE, 7.065 mm2 surface area and 3 mm diameter)by the drop casting method. The schematic representation ofthe α-Fe2O3 pseudocubic nanocrystal modified glassy carbonelectrode is shown in the inset of Fig. 11(a). A NaH2PO4 (0.1M, 39 ml) and Na2HPO4 (0.1 M, 61 ml) phosphate buffer solu-tion (PBS) at pH 7.4 was used as the electrolyte for all theelectrochemical analyses.

Results and discussion

The XRD patterns of α-Fe2O3 samples grown at different timeintervals are shown in Fig. 1(a–d), in which all the diffractionpeaks of samples 6FO, 12FO, 18FO and 24FO match with therhombohedral phase of α-Fe2O3 (JCPDS card no. 80-2377).The prominent diffraction peaks are seen at 2θ values of24.14° (012), 33.14° (104), 35.66° (110), 38.88° (006), 40.83(113), 43.9° (202), 49.51° (024), 54.08° (116), 57.78° (018),62.54° (214), 63.96° (300), 71.92° (1010), 82.36° (0210) and88.58° (226). The crystallinity of sample 6FO (Fig. 1(d)) is rel-atively lower as understood from its weak diffraction pattern,compared to 12FO, 18FO and 24FO, as depicted in Fig. 1(a–d). The grain sizes of samples 6FO, 12FO, 18FO and 24FOwere calculated from the intense XRD peaks at around 33.14°using the Debye–Scherrer formula and were obtained as 18,21, 20 and 24 nm, respectively.38

The XRD patterns of the reference samples grown in thepresence of ZnIJNO3)2 (RS1) and without any zinc source (RS2)are also identical to the same JCPDS card (JCPDS card no. 80-2377), confirming the formation of rhombohedral α-Fe2O3

(Fig. 1(e and f)). Diffractions from the lattice planes (012) areobserved in the XRD patterns of 6FO, 12FO, 18FO, 24FO andRS1 samples for which pseudocubic morphologies are ob-served. However, the diffraction from the (012) lattice planeis barely visible in the pseudo spherical α-Fe2O3 samplegrown in the absence of any zinc source (RS2) (Fig. 1(f)) com-pared to all other samples. These observations presumablyreflect the fact that (012) lattice planes are preferentiallygrown in samples which are synthesised in the presence ofzinc sources.

The relative intensities between the peaks at 24.14° corre-sponding to (012) lattice planes and those at 33.14° corre-sponding to (104) lattice planes (i.e. I(012)/I(104)) of samples6FO, 12FO, 18FO, 24FO, RS1 and RS2 were obtained, aspresented in Table 1. It is found that the intensity ratio(I(012)/I(104)) increases gradually from 0.02 in RS2 (grown inthe absence of zinc source) to 0.96 in 24FO (grown in thepresence of ZnCl2), as presented in Table 1. And the relativeintensity ratios (I(012)/I(104)) gradually increase from 6FO to24FO. Moreover, new peaks have also appeared at 43.83° in12FO, 18FO and 24FO along with prolonged solvo-hydrothermal treatment, compared to 6FO in which thesepeaks are almost completely absent (Fig. 1(a–d)). Hence, itcan be understood that the intensity due to (012) latticeplanes becomes relatively enhanced along with the progressof the hydrothermal duration in the presence of Zn2+ ions.This may be due to the enhancement in the volume of uni-form and homogenous pseudocubic nanocrystals of α-Fe2O3,as evidenced by its FESEM images (Fig. 3(d–f)). This kind ofincrease or decrease in peak intensities can be ascribed tothe lattice orientations or rearrangements of the crystal lat-tice under a prolonged solvo-hydrothermal process in thepresence of Zn2+ ions.39 This may also occur due to the mi-nor doping effects of Zn2+ ions or texture effects due to thetransformation from polycrystalline to single crystallinepseudocubic nanocrystals in 6FO, 12FO, 18FO, 24FO and RS1samples compared to the RS2 sample.40,41

The chemical composition of the grown 24FO pseudocubicnanocrystals was obtained from the FESEM-EDX analysis(Fig. 2), which could also help to confirm the presence of mi-nor quantities of Zn2+ ions on the surface of 24FO pseudo-cubic nanocrystals and to calculate their relative concentra-tion. Fig. 2(b and c) show the elemental mapping of Fe andO present in the 24FO sample taken from the area shown in

Fig. 1 XRD patterns of (a) 24FO (b) 18FO (c) 12FO (d) 6FO (e) RS1 and(f) RS2, synthesised by a solvo-hydrothermal process at 180 °C.

Table 1 Intensity ratios between XRD peaks (I(012)/I(104)) of RS2, RS1, 6FO,12FO, 18FO and 24FO samples

Sample

II012

014

Average edgelength/size

RS2 0.02 122 nmRS1 0.29 1.2 μm6FO 0.38 126 nm12FO 0.64 107 nm18FO 0.81 126 nm24FO 0.96 120 nm

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Fig. 2(a) and the EDX spectrum is presented in Fig. 2(d). TheEDX spectrum revealed that sample 24FO consists of ele-ments Fe and O along with minor quantities of Zn, as givenin the inset of Fig. 2(d). The carbon (C) peak observed inFig. 2(d) can be attributed to the carbon tape used for

FESEM-EDX analysis. The elemental signatures obtainedfrom the EDX spectrum are within the experimental accuracyand confirmed the presence of minor quantities of Zn asexpected.

In the EDX spectrum presented in Fig. 2(d), peaks due toFe are observed at 0.7, 6.4, and 7.1 keV, which is consistentwith previous reports on α-Fe2O3.

42 The peaks that appearedat 0.5 and 0.27 keV can be attributed to O and C (carbon tapeused for FESEM-EDX analysis), respectively.42 The minorpeaks that appeared at around 1 and 8.6 keV can be ascribedto the presence of minor quantities of Zn.43 The results illus-trate the presence of minor quantities of Zn2+ structure in-ducing additives on the surface of the nanocrystals of the24FO sample, as expected (the atomic percentage is 0.64).These ions might have been probably introduced into theα-Fe2O3 (doping) lattice in minor quantities, during thegrowth of these nanocrystals (24FO) in the presence of a Znsource as the structure inducing additive.

FESEM images of 6FO, 12FO, 18FO and 24FO (Fig. 3(a–d))illustrate that all these samples consist of pseudocubic nano-crystals with slight morphological variations. The samples6FO, 12FO and 18FO are having small granular nanocrystalsof α-Fe2O3 with size <10 nm along with pseudocubic nano-crystals. The average edge lengths of pseudocubic nano-crystals of 6FO, 12FO, 18FO and 24FO are obtained from theFESEM images taken from different locations, which is foundto be 126, 107, 126 and 120 nm (aspect ratio 1) respectively.And the average dihedral angle obtained for 24FO sample is85° (Fig. 3(d–f)).

The growth of the pseudocubic nanocrystal morphologycan be influenced by the pH, the viscosity of the precursor so-lution, the reaction temperature, and the pressure developedwithin the solvo-hydrothermal system. However, in the pres-ent case, all these parameters are kept constant and the dura-tion of the solvo-hydrothermal process alone is varied in or-der to obtain preferentially oriented nanocrystals withexposed high index facets, as seen in the FESEM images ofthe 24FO sample (Fig. 3(d–f)). This indicates that a certainperiod of reaction time is required for the formation ofuniform α-Fe2O3 pseudocubic nanocrystals with exposed highindex facets. One can also see a higher number of granularnanocrystals in sample 6FO, which was subjected to 6 h ofsolvo-hydrothermal treatment. However, the number of gran-ular structures is lower in samples 12FO and 18FO, grownwith relatively more time, such as 12 and 18 h, respectively.These observations indicate that the nano granular structuresmight have been formed initially in all the samples duringthe early hours of solvo-hydrothermal reaction for the synthe-sis of α-Fe2O3. These initially formed granular morphologicalstructures might have acted as nucleation centres and trans-formed into homogenous, pseudocubic nanocrystals withshape selectivity and homogeneity, due to their slow and pre-cise growth in the presence of Zn2+ ions of ZnCl2 during thecourse of the prolonged solvo-hydrothermal process. This canbe attributed to the structure inducing properties of Zn2+

ions, which might have efficiently influenced the nucleation

Fig. 2 (a) FESEM image α-Fe2O3 (24FO) pseudocubic nanocrystals (theinset shows the particle size distribution histogram of 24FO). FESEM-EDX elemental mapping of (b) O and (c) Fe elements present in 24FOpseudocubic nanocrystals and the (d) EDX spectrum of the 24FO sam-ple (the inset shows the atomic percentage of elements Fe, O and Zn).

Fig. 3 FESEM images of (a) 6FO, (b) 12FO, (c) 18FO, and (d) 24FOpseudocubic crystals, grown in 150 mM ZnCl2 solution (structureinducing additive) under solvo-hydrothermal conditions at 180 °C for6, 12, 18 and 24 h, respectively; (e) and (f) are 24FO pseudocubic nano-crystals at different magnifications.

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centres of all the granular nanocrystals seen in 6FO, 12FOand 18FO and made them grow into highly homogenouspseudocubic nanocrystals, as seen in the FESEM images ofthe 24FO sample (Fig. 3(d–f)).

The proposed chemical reactions for the formation ofα-Fe2O3 can be written as44

FeCl3·6H2O + 3NaOH → Fe(OH)3 + 3NaCl + 6H2O (1)

Fe(OH)3 → FeOOH + H2O (2)

2FeOOH → α-Fe2O3 + H2O (3)

The formation of an iron precursor, akaganeite (β-FeOOH), is a significant step in the solvo-hydrothermal pro-cess for the development of α-Fe2O3, since it determines thegrain size or the particle size.45 Akaganeite usually grows onferric hydroxide (FeIJOH)3), and it is gradually converted intoα-Fe2O3 during solvo-hydrothermal growth of the nano-crystals because heating is carried out in a closed system hav-ing pressure greater than 1 atm.44–46 In this reaction process,the autoxidation of Zn2+ ions derived from zinc chloridehexahydrate (ZnCl2·6H2O) is highly suppressed due to the freeradical scavenging property of ferric chloride in a water–ace-tonitrile solvent system.47 The free radical scavenging poten-tial of ferric chloride might have prevented the oxidation ofZn2+ ions by scavenging the highly reactive superoxide anion(O2˙−) and hydroxyl radicals formed during the solvo-hydrothermal process.47 These results indicate that the autox-idation of Zn2+ ions is strongly inhibited by the antioxidantactivities of FeCl3 in a water–acetonitrile solvent combinationwhich is consistent with previous reports.37 This might haveenhanced the role of Zn2+ ions as the structure inducing addi-tive during the solvo-hydrothermal process by effectively con-trolling and modulating the nucleation centres during thesolvo-hydrothermal growth process. This inference is furthersupported by the absence of any oxide phases of Zn (ZnO orZnIJOH)2) in the as-synthesized samples, as understood fromour XRD data. Hence, these Zn2+ ions are efficiently employedin inducing a pseudocubic morphology to the as preparedα-Fe2O3 nanoparticles of the 24FO sample, as expected.

In order to further understand that the pseudocubic mor-phological growth of 24FO is controlled by Zn2+ ions, FESEMimages of samples RS1 and RS2 synthesized via a similarsolvo-hydrothermal process using ZnIJNO3)2 (instead of ZnCl2)and without any zinc sources were also obtained (Fig. 4). Ref-erence sample RS1 synthesized in the presence of ZnIJNO3)2also shows a cubic morphology similar to 24FO. The repre-sentative edge length of these nanocrystals (RS1) is 1.2 μm,as shown in Fig. 4(a). However, the edge length of high indexfacet bounded α-Fe2O3 (24FO) is relatively lower (120 nm)with respect to that of RS1 (1.2 μm). For RS1, the basic crystalgrowth medium having a ZnIJNO3)2 additive would havefavoured the rapid growth of α-Fe2O3 nanoparticles along itsedges compared to 24FO. This rapid growth rate of micro-cubes of the RS1 sample can be attributed to the availability

of excess oxygen radicals during the solvo-hydrothermal pro-cess, originating from the nitrate groups of ZnIJNO3)2. How-ever, it is also observed that the crystals grown without anyZn sources (RS2) have a pseudo spherical morphology, asshown in Fig. 4(b). Hence, one can conclude that the Zn2+

ions from ZnCl2 and ZnIJNO3)2 are the key factors for thegrowth of α-Fe2O3 with a pseudocubic morphology, ratherthan the corresponding anions (Cl− ions of ZnCl2 or NO3

− ofZnIJNO3)2). This can be ascribed to the fact that the surfaceenergies of low index facets of rhombohedral α-Fe2O3 areclose to each other and hence the stability of these facets canbe easily changed by the preferential adsorption of additiveslike Zn2+ ions during the solvo-hydrothermal process.48 Theseadditives (Zn2+ ions) are observed to have induced a pseudo-cubic morphology to the α-Fe2O3 nanocrystals of the 24FOsample compared to the spherical α-Fe2O3 nanoparticles ofthe RS2 sample. Hence, one can conclude that the pseudospherical α-Fe2O3 nanocrystals obtained in water–acetonitrilemedium in the absence of any zinc source (RS2) change intopseudocubic nanocrystals (24FO) when they are grown undersimilar solvo-hydrothermal conditions under the influence ofstructure inducing transition metal (Zn2+) ions. Moreover, thesize of the pseudocubic nanocrystals of the 24FO sample isvery small compared to the previously reported α-Fe2O3 hav-ing a quasicubic morphology obtained with Zn2+ and Ni2+

ions, for which the edge length obtained is greater than orequal to 200 nm.12,27,30,31,35,36

In order to better understand the crystal morphology andstructure of pseudocubic nanocrystals, TEM, HRTEM andSAED investigations were carried out in detail. The 2D TEMimages (Fig. 5) of 3D pseudocubic nanocrystals of the 24FOsample further confirmed the formation of a pseudocubicmorphology, which is consistent with its FESEM images. Theaverage dihedral angle of these nanocubes was obtained fromthe TEM images, which is found to be 85° (±0.02) with an av-erage edge length of 122 nm. This suggests that thesepseudocubic nanocrystals might have not been enclosed bythe usual {100} facets as observed for cubic nanocrystals.31

For standard hexagonal iron oxide crystals, the dihedral anglebetween (012) and (014̄) lattice fringes is 85° and they areperpendicular to (2̄10) lattice fringes.31 This is consistentwith the coherent lattice fringes seen in the HRTEM image ofthe 24FO sample (the inset of Fig. 5(d)), which shows contin-uous fringes with a period of 0.37 and 0.27 nm (d-spacing),

Fig. 4 FESEM images of (a) RS1 and (b) RS2, grown under solvo-hydrothermal conditions at 180 °C for 24 h in the presence of 150 mMZnIJNO3)2 and in the absence of a Zn source, respectively.

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corresponding to (012) and (014̄) lattice planes and a dihe-dral angle of 85°. Hence these nanocrystals are bounded by{012}, {014̄} and {2̄10} facets, which is consistent with previ-ous reports on pseudocubic α-Fe2O3 nanocrystals.35 The se-lected area electron diffraction (SAED) pattern of 24FOpseudocubic nanocrystals (Fig. 5(c)) shows diffraction spotsfrom (012) and (014̄) lattice planes of single crystallineα-Fe2O3 pseudocubic nanocrystals.

The UV-vis absorption spectrum of 24FO at 300 K in thewavelength range of 220–800 nm is presented in Fig. 6(a). Theabsorption spectrum shows intense visible range optical ab-sorption with a maximum centred at 536 nm for 24FO pseudo-cubic nanocrystals. The optical bandgap of this sample isobtained from the Tauc plot (Fig. 6(b)), for which a graph isplotted for (αhυ)2 vs. hυ, where α is the absorption coefficientand hυ is the energy of the incident photons.49 The bandgapenergy (Eg) of 24FO pseudocubic nanocrystals is obtained as 2eV from the Tauc plot, which is in agreement with the bandgapenergy of previously reported α-Fe2O3 nanoparticles.

50–52

Fig. 7 depicts the FTIR spectrum of 24FO pseudocubicnanocrystals in the wavelength range of 400–4000 cm−1. TheIR bands at around 480 and 564 cm−1 in 24FO can be attrib-uted to the vibrations of Fe–O bonds and the band observedat 1129 cm−1 might have originated from the organic compo-nents contributed by the acetonitrile solvent medium.26,53

The appearance of characteristic IR bands of α-Fe2O3 at 480and 564 cm−1 indicates that the grown pseudocubic nano-crystals of the 24FO sample are α-Fe2O3 nanocrystals, whichis consistent with its XRD analysis.

In order to further confirm that the as-synthesized nano-particles consist of α-Fe2O3 rather than its polymorphs suchas β-Fe2O3, γ-Fe2O3 and ε-Fe2O3, the Raman spectra of 24FOpseudocubic nanocrystals and RS2 spherical nanoparticleswere obtained and are shown in Fig. 8. It is evident from thefigure that the Raman spectra of these samples consist ofcharacteristic bands of α-Fe2O3, in the range of 200 to 700cm−1, as expected.54 The bands obtained at 212 and 485 cm−1

in 24FO and at 221 cm−1 in RS2 can be ascribed to A1g sym-metric stretching vibrations of Fe–O bonds.50,51 The Ramanbands observed at 275, 385 and 590 cm−1 in 24FO pseudo-cubic nanocrystals can be attributed to Eg symmetric bendingmodes of vibrations of Fe–O bonds in α-Fe2O3, while thesebands are observed at 287, 406 & 604 cm−1 in RS2 sphericalnanoparticles.54,55

Fig. 6 (a) Optical absorption spectrum and (b) Tauc plot extrapolatingthe optical band gap energy of α-Fe2O3 (24FO) pseudocubicnanocrystals grown in 150 mM ZnCl2 solution (structure inducingadditive) under solvo-hydrothermal conditions at 180 °C for 24 h.

Fig. 7 FTIR spectrum of α-Fe2O3 (24FO) pseudocubic nanocrystalsgrown in 150 mM ZnCl2 (structure inducing additive), in a water–acetonitrile solvent combination under solvo-hydrothermal conditionsat 180 °C for 24 h.

Fig. 8 Raman spectra of (a) RS2 spherical nanoparticles (b) 24FOpseudocubic nanocrystals.

Fig. 5 (a) and (b) TEM analysis showing 2D images of α-Fe2O3 (24FO)pseudocubic nanocrystals at different magnifications, (c) SAED patternof 24FO pseudocubic nanocrystals presenting diffraction spots from(012) and (014̄) lattice planes of the 24FO sample and (d) HRTEM imageof 24FO pseudocubic nanocrystals (the inset shows an enlarged view).

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The Raman shift towards the lower energy side in 24FOwith respect to RS2 along with the broadening of the Ramanprofile is probably related to the surface stress caused by theexposure of high energy, high index facets of pseudocubicα-Fe2O3 nanocrystals (24FO) compared to spherical α-Fe2O3

(RS2).12 The observed red shift of Raman bands in 24FOnanocrystals implies that the stress associated with their sur-face is tensile stress.57 Moreover, the broadening of Ramanbands can be ascribed to the minor substitution of Zn2+ ionsinto the α-Fe2O3 lattice and the subsequent lattice disordersconsistent with the EDX analysis.58 The appearance of Ramanbands at 1310 cm−1 in RS2 and at 1278 cm−1 in 24FO pseudo-cubic nanocrystals can be attributed to the magnon modes,and the observed shift and peak broadening of this band inthe 24FO sample is due to the phonon confinement.56

XPS analysis is used to obtain the elemental and chemicalstate of the investigated sample 24FO pseudocubic nano-crystals, which could be made approximately 10 nm from thesurface of these nanocrystals. It is also helpful to detect theions and ligand energies attached to the sample.

The XPS survey spectrum in the binding energy range of0 to 1200 eV revealed the presence of strong peaks due to Feand O along with minor peaks of Zn, as demonstrated inFig. 9(a) (C from the atmosphere). The high resolution XPSspectrum (Fig. 9(b)) shows the doublets of Fe at 710.6 and724.4 eV corresponding to 2p3/2 and 2p1/2, respectively, due tothe characteristic Fe3+ oxidation states of α-Fe2O3.

35,36 TheO1s peak of α-Fe2O3 is deconvoluted into two, in which thepeak at 529.7 eV is due to the presence of oxygen in the metaloxide and the other one which appeared at around 531.6 eVcan be ascribed to the crystal defect oxygen.35,36 And the highresolution XPS spectrum of Zn demonstrated in Fig. 9(d)gives the binding energies of Zn 2p3/2 and Zn 2p1/2 as1021.22 and 1044.22 eV, respectively, with very weak intensi-ties as is evident in the XPS survey spectrum.59 This is closeto the values of divalent zinc ions, indicating that Zn existsin the Zn2+ oxidation state rather than Zn°, which in turnconfirms the presence of minor quantities of structure induc-ing Zn2+ metal ions on the surface of the pseudocubic nano-crystals of this sample. Probably, a minor quantity of theseions might have been incorporated into the α-Fe2O3 nano-crystal lattices of the 24FO sample, as understood from itsEDX analysis.

The atomic concentrations of elements Fe, O and Zn werealso obtained from the XPS analysis, as depicted in Table 2.This indicates that the atomic concentration of Zn is very low(3.32%). However, it is slightly higher compared to the dataobtained from the EDX analysis. Hence, XPS analysis also con-firms the presence of minor quantities of Zn2+ ions on the sur-face and the minor incorporation of these ions into 24FOpseudocubic nanocrystals consistent with the EDX analysis.

In order to understand the electrochemical behaviour ofthe as-synthesised 24FO pseudocubic nanocrystals, electro-chemical impedance spectroscopic analysis (EIS), cyclicvoltammetry analysis (CV) and determination of aqueous am-monia sensing efficiencies were carried out.

Electrochemical AC impedance spectroscopic analysis(EIS) of the glassy carbon electrode (GCE), 24FO pseudocubicnanocrystals (24FO/GC) and RS2 spherical nanoparticle modi-fied GC (RS2/GC) electrodes was carried out within a fre-quency range of 106 to 0.1 Hz. The obtained Nyquist plots areshown in Fig. 10, where Z′ and Z″ are the real and imaginaryparts of the impedance spectra (Z), which could probe theelectron transfer kinetics of 24FO pseudocubic nanocrystalsand RS2 spherical nanoparticles in comparison with the GCelectrode.

The Nyquist plots of GCE, 24FO/GC and RS2/GC electrodesobtained in the frequency range of 106 to 0.1 Hz (Fig. 10) con-sist of semi-circular regions in the high frequency areas (lowimpedance region), followed by inclined straight lines towardsthe low frequency areas.60 As depicted in the figure, the diame-ter of the semi-circular region of 24FO/GCE, which corre-sponds to its charge transfer resistance, is lower compared tothat of RS2/GCE, but comparable to bare GCE. This shows thereduction of the charge transfer resistance of 24FO/GCE com-pared to RS2/GCE.52 This indicates that the electron transferefficiency of 24FO/GCE is comparable to that of GCE and rela-tively higher with respect to that of RS2/GCE. The zinc dopedα-Fe2O3 is reported to have less conductivity.61 Hence the en-hanced electron transfer efficiency of 24FO/GCE can be as-cribed to the exposure of electrochemically active facets on thesurface of 24FO nanocrystals or different defect densities com-pared to RS2 spherical nanoparticles.

Fig. 9 (a) XPS survey spectrum of 24FO pseudocubic nanocrystals.Deconvoluted high resolution XPS spectra of elements (b) Fe (c) O and(d) Zn, in 24FO pseudocubic nanocrystals, and (e) Table 2 depicting theatomic percentages of Fe, O and Zn in 24FO pseudocubic nanocrystals.

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The electrochemical process involves surface adsorptionand several kinds of interactions at the electrode/electrolyteinterface.60 Hence the exposed facets on the surfaces of thenanocrystals are important in maintaining good electro-chemical properties. Thus, in the present case, the pseudo-cubic surface morphology of 24FO with exposed high indexfacets might have played a crucial role in maintaining goodelectron transfer kinetics, which might have caused the re-duction of its impedance.60 These results revealed that thehigh index facet bounded α-Fe2O3 pseudocubic nanocrystals(24FO/GC) modified electrode is a promising candidate forelectrochemical applications due to the exposure of electro-chemically active high index facets in this material. In orderto further confirm the enhanced electrochemical propertiesof the exposed high index facets of the α-Fe2O3 pseudocubicnanocrystal (24FO/GC) modified electrode, the electro-chemical sensing efficiencies of this electrode towards aque-ous ammonia were also studied.

The cyclic voltammetry (CV) curves of the RS2/GCelectrode in the absence of aqueous ammonia and the 24FO/GC electrode, both in the absence and presence of 1 mMaqueous ammonia, were obtained, as depicted in Fig. 11(a).These analyses were carried out in 10 ml PBS electrolyte at ascan rate of 0.1 volt per s. Interestingly, the electrochemicalperformance of the 24FO/GC electrode is good compared tothe RS2/GC electrode, as depicted in Fig. 11(a). The CV curveof 24FO/GC electrode is nearly symmetrical and displaysa pair of strong redox peaks at 0.52 and −1.1 V vs. Ag/AgCl(Fig. 11(a, iii)), during both the anodic and cathodic sweepsin the absence of aqueous ammonia. These distinct redoxpeaks of the 24FO/GC electrode can be attributed to revers-ible redox processes:41

Fe2O3 + OH− ↔ Fe2O3OH + e− (4)

These cathodic and anodic peaks are characteristic oxida-tion reduction peaks of electroactive α-Fe2O3.

62,63 These kinds

of prominent oxidation reduction peaks indicate its en-hanced electrochemical behaviour, due to its good electrontransfer efficiencies between the surface of 24FO pseudocubicnanocrystals and the chosen electrolyte. However, theseredox peaks are slightly shifted to 0.48 and −0.95 V vs. Ag/AgCl (Fig. 11(a, ii)), in the presence of 1 mM of aqueous am-monia. Moreover, the redox peak current of the 24FO/GCelectrode is significantly higher in the presence of aqueousammonia owing to its electrochemical sensing response to-wards aqueous ammonia, as depicted in Fig. 11(a). This im-plies the surface accessibility of exposed high index facets ofthese nanocrystals, which is also crucial in maintaining itsenhanced electrochemical activity and catalytic efficiency to-wards the reduction of aqueous ammonia.64 The increase inthe cathodic and anodic peak current can be attributed tothe enhancement in carrier ion concentrations, emergingfrom the electrocatalytic dissociation of NH4OH into ammo-nium (NH4+) and hydroxide (OH−) ions with the applicationof electric potential.64 This carrier ion enhancement can in-ject excess electrons into the conduction band of α-Fe2O3

causing enhanced conductivity, which is a linear function ofthe concentration (correlation coefficient: R2 = 0.9861 for theanodic peak current and 0.9975 for the cathodic peak cur-rent) of aqueous ammonia present in the electrolyte until sat-uration is attained.64

To investigate the effects of scan rate (kinetics) on theelectrochemical properties of the 24FO/GC electrode, CVcurves were obtained at varying scan rates in the range of 25–150 mV s−1, in the presence of 2 mM aqueous ammonia, asshown in Fig. 11(b). The increase in the anodic and cathodicpeak current along with the scan rate is evident from this fig-ure. The increased scan rate can result in an enhancement inthe diffusion or depletion of charge carriers at the electrode/electrolyte interface resulting in enhanced peak current.Hence, the increase in the anodic and cathodic peak currentmight be caused by the diffusion confined electrode processin accordance with previous reports.65 Additionally, the insetof Fig. 11(b) shows the linear variations of the peak currentversus the square roots of the scan rates, for both anodic andcathodic peak current. All these indicate that the electro-chemical reaction occurring at the surface of the high indexfaceted 24FO is a diffusion controlled process.65,66

In order to further explore the electrochemical propertiesof α-Fe2O3 pseudocubic nanocrystals (24FO), electrochemicalsensing of these nanocrystals (24FO) towards aqueous ammo-nia was conducted through CV and chronoamperometic mea-surements. For this, CV measurements were carried out overa wide range of molar concentrations of aqueous ammonia (5M–0.1 nM) by adding the aqueous ammonia of the requiredmolarity into 10 ml of PBS electrolyte (Fig. 11(c)). A simulta-neous increase in the anodic peak current intensities alongwith the increase in concentration of aqueous ammonia inthe range of 1 nM to 1 M is observed, as depicted inFig. 11(c). It is observed that, there are significant variationsin the CV response, depending on the molar concentration ofaqueous ammonia up to a lower detection limit of 1 nM,

Fig. 10 Nyquist impedance spectroscopic plots of (a) GCE, GCmodified (b) 24FO (24FO/GC) and (c) RS2 (RS2/GC) electrodes within afrequency range of 106 to 0.1 Hz (the inset shows an enlarged view).

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below which the CV curves begin to overlap and the innermost curve of Fig. 11(c) shows the overlapping of 1 and 0.1nM concentrations of aqueous ammonia. Hence, the lowerdetection limit is identified as 1 nM aqueous ammonia. Thelinear variation of the anodic peak current versus the concen-tration of aqueous ammonia in the upper concentrationrange of 1–5 M is shown in the inset of Fig. 11(c). The de-crease in the peak current above 4 M concentration of aque-ous ammonia indicate that the surface of the electrochemicalsensor (24FO/GC) is saturated with ammonia at 4 M concen-tration, above which detection is not possible. Hence the up-per detection limit is obtained as 4 M aqueous ammonia. So,the detection limit for the electrochemical sensor developedusing 24FO pseudocubic nanocrystals (the 24FO/GCelectrode) towards aqueous ammonia is constrained withinthe range of 1 nM–4 M.

Fig. 12 depicts the amperometric response of the 24FO/GCelectrode with three successive additions of different molarconcentrations of aqueous ammonia (1 nM, 1 μM, 1 mM and1 M) at an applied potential of +0.45 V vs. Ag/AgCl. The varia-tion of the amperometric current versus the concentration of

the aqueous ammonia in the lower concentration range of 1nM and the higher concentration range of 1 M is evidentfrom this figure.

Fig. 11 (a) CV curves of (i) RS2/GC in the absence of ammonia, (ii) 24FO/GC electrodes in the presence of 1 mM aqueous ammonia and the (iii)24FO/GC electrode in the absence of aqueous ammonia (the inset is the schematic representation of the 24FO/GC electrode), (b) CV curves ofα-Fe2O3 pseudocubic nanocrystals in the scan rate range of 25–150 mV s−1 in the presence 2 M aqueous ammonia (the inset shows the variationof the anodic and cathodic peak current vs. the square root of the scan rate), (c) CV curves of the 24FO/GC electrode at different molarconcentrations of aqueous ammonia (0.1–1 nM), and (d) amperometric response of the 24FO/GC electrode with successive addition of 0.1 mMaqueous ammonia at an applied potential of +0.45 V (the inset shows the current versus concentration curve).

Fig. 12 Amperometric response of the 24FO/GC electrode with threesuccessive additions of aqueous ammonia (1 nM, 1 μM, 1 mM and 1 M)at an applied potential of +0.45 V vs. Ag/AgCl (the inset shows anenlarged view of the lower concentration region).

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The sensitivity of the 24FO/GC electrode towards aqueousammonia was obtained from its amperometric response to-wards aqueous ammonia (Fig. 11(d)) which was evaluated byconducting a chronoamperometry experiment with successiveaddition of 0.1 mM aqueous ammonia into PBS electrolyteunder continuous stirring, at an applied potential of +0.45 Vvs. Ag/AgCl. A current versus concentration plot is also drawn(the inset of Fig. 11(d)) to demonstrate the current–ammoniaconcentration correlation. The linearity of this plot is demon-strated in the inset of Fig. 11(d). The sensitivity of this chemi-cal sensor towards aqueous ammonia is obtained from theslope of this current versus concentration curve, which is50.46 μA(mM)−1. This is relatively higher compared to the pre-viously reported aqueous ammonia sensitivity of α-Fe2O3.

64

This can be ascribed to the adsorption ability and electrocata-lytic activity of the as-synthesized α-Fe2O3 pseudocubic nano-crystals. However, the lower detection limit is slightly highcompared to the previously reported α-Fe2O3 nanoparticles.

64

Conclusions

The growth of high index facet bounded α-Fe2O3 pseudocubicnanocrystals in water–acetonitrile solvent medium by a solvo-hydrothermal synthesis process provided insight into the roleof Zn2+ metal ions in inducing a uniform pseudocubic mor-phology and retaining the exposed high energy facets ofα-Fe2O3. These α-Fe2O3 pseudocubic nanocrystals, boundedby high energy {012}, {014̄} and {2̄10} facets, have a band gapof 2 eV, indicating their semiconducting property.

The impedance spectroscopy and cyclic voltammetry anal-ysis revealed that the high index facet bounded α-Fe2O3

pseudocubic nanocrystals possess low impedance and en-hanced electrochemical properties compared to the sphericalα-Fe2O3 nanoparticles synthesised in the absence of a Znsource. This enhanced electrochemical performance of thehigh index facet bounded α-Fe2O3 nanocrystals is benefitedby their unique surface structure and the atomic arrange-ments of exposed electrochemically active high index facets.The cyclic voltammetry and amperometry analysis also re-vealed that these nanocrystals have good sensitivity towardsaqueous ammonia. Thus, these pseudocubic α-Fe2O3 nano-crystals are promising candidates for various electrochemicaland aqueous ammonia sensing applications.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors acknowledge DST FIST for FESEM analysis, DSTPURSE for EDX analysis and Prof. M. K. Jayaraj, Nano-photonics and Optoelectronic Device Laboratory, Departmentof Physics, CUSAT for XRD and Raman analysis.

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