X-ray structural studies on solubility of Fe substituted CuO

Abstract

CuO is a promising material for the spintronic industry for which lattice distortions/defects play an important role in determining its magnetic and various other physical properties. The ionic radii and charge of Cu²⁺[VI] (0.73 Å) and Fe³⁺[VI] (0.64 Å) are quite different. Hence high Fe substitution in CuO in place of Cu may generate strain/distortions. Fe substitution may enhance magnetic properties, even at room temperature, making such materials interesting for device applications. A detailed structural study on Fe incorporated CuO lattices to confirm phase purity, supported by evidence of the absence of a secondary phase is absolutely essential especially when considering a considerable substitution of up to ∼12.5%. The electronic valence state, fine structure and local neighborhood/geometry of constituent elements need to be investigated using synchrotron based X-ray absorption spectroscopy (XAS). We report, for the first time, such a detailed study on understanding this magnetically and electronically important material: Cu_1−xFe_xO, 0 ≤ x ≤ 0.125.

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1. Introduction

Dilute magnetic semiconductors (DMS's) have attracted intense attention from researchers due to their potential applications in spintronics,^1,2 such as spin transistors and non-volatile storage devices, in which we can control spin injection by a weak magnetic field.³ Such multifunctional spin based devices will be advantageous over conventional charge based devices in terms of higher integration densities, non-volatility, data processing speed, etc., potentially by simultaneous use of charge and spin.⁴

Transition metal oxides, such as CoO, MnO, NiO and CuO reveal complicated magnetic/electronic structure.⁵ Monoclinic CuO, tenorite⁶ is a strongly size dependent, tunable direct band gap, p-type semiconductor (E_g ∼ 1.85 eV)⁷ and is a promising material for photo-catalysis,⁸ humidity and gas sensors,^9,10 as a cathode in solar cells,¹¹ electrode material for supercapacitors¹² and lithium ion batteries,¹³ pigments,¹⁴ etc. Structural and magnetic properties of CuO, which is a strongly correlated electron system,⁵ have been in debate since a long time due to its controversial magnetic behaviour.^15–19 It exhibits antiferromagnetic and Mott insulating behaviour near 229 and 215 K. The interest in cupric oxide has been mainly governed due its close relation to high-temperature cuprate superconductors. CuO is a parent composition having CuO₂ planes, the number of which plays an important role in all high T_c superconductors.^20,21

Compared to their bulk counterparts, CuO nanostructures exhibit some fascinating properties, such as superparamagnetism and increased magnetic susceptibility at low temperature due to modification of structural and electronic properties at the surface.^22,23 Fine tuning of magnetism in CuO nanoparticles (NPs) has been reported with Mn,²⁴ Ti,²⁵ Cd,²⁵ Zn²⁵ and Fe^26–29 doping in CuO. A complex phase formation is encountered due to preparation technique restricting this material for potential applications.^26–28 The solubility of Fe in CuO, i.e. replacement of Cu by Fe in large quantities, is not well discussed in literature. It has been reported that 0.3% of Fe substitution by ceramic method results in phase isolation.²⁹ Few reports on higher Fe doped CuO^30,31 exist, but detailed studies on the structure and electronic properties of Fe-substituted CuO is still lacking.

We demonstrate here that Fe introduction to the CuO lattice by sol–gel technique may provide solubility upto ∼12.5%. We discuss the detailed structural and electronic properties of the substituted pure phase materials in this report. Oxidation states of Cu or Fe had been estimated using X-ray photoelectron spectroscopy (XPS) in literature.^32–35 Keeping in mind that XPS is mainly a surface sensitive technique good for thin films and involves a cumbersome fitting process because of a very narrow energy range containing the peaks of Cu⁰, Cu⁺ and Cu²⁺ at the Cu 2p XPS edge, we have used X-ray absorption spectroscopy (XAS), which is an element specific technique and yields information regarding valence state, local neighborhood/geometry, coordination number and hybridization of both the dopant and host atoms.

In the present experiment, synchrotron based soft X-ray absorption spectroscopy (SXAS), X-ray Absorption Near Edge Structure (XANES) and Extended X-ray Absorption Fine Structure (EXAFS) studies have been carried out at Cu, Fe, O K-edges and Cu and Fe L-edges of the Fe doped CuO samples. SXAS and XANES measurements have been carried out to investigate the oxidation state while EXAFS measurements have been done to reveal information about the local structure around the respective atoms in the samples.

2. Experimental method

2.1 Sample preparation

Nanocrystalline Cu_1−xFe_xO (x = 0, 0.027, 0.055, 0.097 and 0.125) powders were prepared by standard pechini sol–gel process followed by high temperature annealing to investigate structural and electronic properties and the solubility limit of the composition. Copper(II) oxide (99.7%, Alfa Aesar, metal basis) and iron(III) nitrate nonahydrate (98%, Alfa Aesar) were used as precursors. Solutions of CuO in HNO₃ and Fe(NO₃)₃·9H₂O in DI water were prepared separately and mixed together in appropriate stoichiometric ratio. A solution of citric acid and glycerol was prepared in a separate beaker to serve as gelling agent. The gelling agent was added to the mixed aqueous solution of precursors. The resulting solution was heated while stirring at 80 °C to form a gel which was further dried to produce fluffy black powders. To get rid of probable trapped carbonate and nitrate phases, these powders were heated at 450 °C followed by 600 °C.

2.2 Characterization

The X-ray diffraction (XRD) patterns of the nanoparticles were obtained using a Bruker D2 Phaser X-ray diffractometer. The lattice parameters were extracted from structural refinement of the XRD data using Fullprof program. HRTEM characterization was performed using a HRTEM (JEOL JEM-2100 LaB6) at accelerating voltage of 200 kV. The soft X-ray absorption spectroscopy (SXAS) of the samples at Cu and Fe L-edges and O K-edge have been carried out at the SXAS beamline (BL-01) of Indus-2, RRCAT, Indore in total electron yield (TEY) mode.

The XANES and EXAFS measurement of the samples at Cu and Fe K-edges have been carried out at room temperature at energy scanning EXAFS beamline (BL-9) at INDUS-2 synchrotron source (2.5 GeV, 100 mA) at RRCAT, Indore, India.^36,37 The beam line operates in the energy range of 4–25 keV. The beamline consists of an Rh/Pt coated meridional cylindrical mirror used for the collimation of the beam. The collimated beam is monochromatized by double crystal monochromator (DCM) using Si (111). Second crystal of the DCM (a sagittal cylindrical crystal) is used to focus the beam horizontally while another Rh/Pt coated post mirror is used for focusing the beam vertically at the sample position. XANES and EXAFS measurements were performed in transmission mode at the Cu K-edge (8979 eV) and in fluorescence mode at the Fe K-edge (7112 eV).

For the data collection in the transmission mode, two ionization chambers, each of 30 cm length have been used. The first ionization chamber measures the intensity of incident flux (I₀) and the second ionization chamber is used for measuring transmitted flux (I_t). The X-ray absorption co-efficient is determined as μ = ln(I₀/I_t). To improve the signal to noise ratio, an appropriate mixture of gases with optimum pressure were chosen to achieve 10–20% absorption in first ionization chamber and 70–90% absorption in second ionization chamber. The higher harmonics in the X-ray spectra were rejected by detuning of the second crystal (30%) using the piezo motor. In order to obtain a proper edge jump, samples of appropriate weight were taken in powder form and were palletized to 15 mm diameter by mixing with 100 mg cellulose and were kept in a Teflon tape. For EXAFS measurements in the fluorescence mode, sample were placed at 45° to the incident beam and the fluorescence signal (I_f) is detected by a Si drift detector (90° to incident beam). In this case, the X-ray absorption co-efficient is determined by μ = I_f/I₀, and the spectrum was obtained as a function of energy by scanning the monochromator over the specified range.

3. Results and discussions

3.1 Structural analysis

3.1.1 X-ray diffraction (XRD). X-ray diffraction (XRD) patterns shown in Fig. 1 reveal monoclinic crystalline phase of the Cu_1−xFe_xO nanoparticles for 0 ≤ x ≤ 0.125. For x ≤ 0.055, samples are single phase without any impurity peak. However, with further increase in Fe concentration, i.e. for compositions with x = 0.097 and 0.125, two nominal additional peaks at 43° and 64°, relevant to CuFe₂O₄ are observed. These may arise from some regular local structural modifications giving rise to d-spacings matching with the CuFe₂O₄ phase. Shifting of diffraction peaks to higher angle [Fig. 1; inset (a)] indicates changes in lattice constants [Fig. 1; inset (c)]. Rietveld refinement shows that lattice parameters, a and b, decrease with increasing x, while c shows a rapid decrease upto x = 0.027 and thereafter becomes independent of substitution. Similar trends have been observed and reported indicating the influence of Fe ions on the crystalline structure.³⁸ The broadening of the peaks with substitution indicates reduction in size of the particles [Fig. 1; inset (b)]. These evidences clearly demonstrate that Fe is indeed incorporated in monoclinic structure of CuO. The average crystallite size (∼5–8 nm) is estimated from the FWHM of prominent diffraction peak (111̄) using Scherer equation after subtracting instrumental broadening

D = 0.9λ/β cos θ (1)

where, D is the crystallite size, λ = 1.5406 nm is the X-ray wavelength, β line broadening at half the maximum intensity (FWHM) and, θ is Braggs diffraction angle.

Fig. 1 X-ray diffraction pattern of Cu_1−xFe_xO (0 ≤ x ≤ 0.125) samples: inset (a) shows a systematic variation in (11−1) peak; inset (b) shows the broadening of peak and average crystallite size with substitution, x; and (c) the plot showing the variation of lattice parameters a, b and c with x.

The ionic radii of Cu²⁺[VI] is 0.73 Å, while that of Fe²⁺[VI] is 0.74 Å and Fe³⁺[VI] is 0.64 Å. As we observe a decrease in lattice constants, a more probable Fe³⁺[VI] substitution is expected in CuO. However, Fe³⁺ being in a more positive state than Cu²⁺ will increase the O affinity of the lattice reducing the O-vacancies and providing scope of O-interstitial points. For every two Fe³⁺ substitutions, an oxygen atom interstitial may be necessary. This will increase the chances of a structural breakdown. Such behavior may be reflected by the fact that for x ≥ 0.097, reflection peaks such as (110) at 33°, (201̄) at 49°, (020) at 54° and (113̄) at 63° starts to split indicating that certain planes are preferential to strain in the lattice and are responsible for modification of the structure from a simple CuO to some other structure. We have observed that by prolonged heating at higher temperatures these splitting vanish but several additional peaks appear belonging to the CuFe₂O₄ structure. This means that the lattice is at an intermediate condition of phase transition or a solubility limit. However, it is noteworthy that if the entire substitution is Fe³⁺ then the solubility limit would have been yet less. Hence the chances of partial Fe²⁺ substitution cannot be absolutely ruled out especially when the ionic radii of Fe²⁺ and Cu²⁺ are very close.

3.2 Soft X-ray absorption spectroscopy (SXAS)

Soft X-ray absorption spectroscopy (SXAS) of Cu_1−xFe_xO samples at the Cu 2p, Fe 2p and O 1s-edges [Fig. 2] were investigated to estimate any small change in the valence state and measure the density of empty/partially filled electronic states.^39,40 Cu L-edge XAS spectrum represents the dipole transitions of Cu 2p_3/2 and 2p_1/2 electrons into empty d-states.^41–43 The 3d character of Cu in our samples were analyzed by investigating the XAS spectra of Cu L_3,2-edge [Fig. 2(a)]. The prominent features, L₃ and L₂ are separated by 20 eV. They correspond to transitions from 2p_3/2 and 2p_1/2 (spin–orbital coupled states) to unoccupied 3d states, respectively. One signature of the Cu²⁺ state is that Cu 3d⁹ satellites as observed in cuprate superconductors.⁴⁴ We observe signatures of such Cu 3d⁹ satellites in the all samples. However, these satellites become less significant in the higher doped samples. The satellite feature and the energy separation between the L₃ and L₂ edges (∼20 eV) signify the formation of the CuO phase, i.e. Cu²⁺ states for all substitutions.

Fig. 2 XAS (a) Cu L₂,₃-edge spectra of Cu_1−xFe_xO for (0 ≤ x ≤ 0.125) samples; (b) the Fe L_2,3-edge spectra with reference to Fe₂O₃, shows predominantly Fe³⁺ state with evolution of A₁ and A₂ features with increased substitution; and (c) the O K-edge confirms enhancement of Fe³⁺.

Two intense peaks of Fe L₃ and L₂ edges [Fig. 2(b)] are assigned to Fe 2p → 3d states.⁴⁵ The intensity of these peaks is a measure of unoccupancy of Fe 3d states. The L₃ (∼710 eV; Fe 2p_3/2) and L₂ (∼723 eV; Fe 2p_1/2) features resemble that of Fe₂O₃. The feature L₃, is composed of a low intense peak marked as A₁ (Fe 2p_3/2 – t_2g) and a main peak denoted as A₂ (Fe 2p_3/2 – e_g).⁴⁶ In fact, in general L₃ is actually composed of four multiplets.^47,48 In most Fe³⁺ states the multiplets appear as a doublet. However the profile and position changes depending on the presence of other valence states of Fe. We observe that the peak profile and position of the L₃ and L₂ edge features matches considerably with Fe₂O₃ but consistently evolve with increased Fe substitution hinting a mixed valence state along with a dominating Fe³⁺ valence state in all the substituted samples.

O K-edge XAS features [Fig. 2(c)] appear due to the transition of O 1s electron to the different partially occupied and unoccupied states of various transition metal oxides.⁴⁰ The O K-edge XAS spectra is generally discussed in the light of two regions; R₁: 528 eV assigned to a 1s → hybridized O p states with metal 3d states^41,49,50 and R₂: 532 eV to 550 eV assigned to 1s → hybridized O p states with metal 4s and 4p states.³⁹ In the Cu_1−xFe_xO samples we observe these two regions, with modifications in both the regions. For x ≤ 0.055, the R₁ is a single peak feature following the trend of reported CuO samples.⁴¹ However, for x ≥ 0.097, R₁ becomes an un-split doublet type feature hinting the effect of Fe ions.⁴¹ The doublet may be a result of a mixed phase of Fe²⁺ and Fe³⁺ contributions. On the other hand careful investigation of the R₂ region clearly shows resemblance with the CuO features rather than Fe₂O₃ and Fe₃O₄ features. This is a strong evidence of a CuO like structure rather than Fe₂O₃ and Fe₃O₄ structure.

3.3 X-ray absorption near edge structure (XANES)

To further confirm the valence states of the Cu and Fe ions in the samples, we have performed the X-ray absorption near edge spectroscopy of the samples at Cu and Fe K-edges.

3.3.1 Cu K-edge. The normalized XANES spectra of the Fe doped CuO samples measured at Cu K-edge [Fig. 3(a)] apparently matches with that of pure CuO. Two regions appear in the spectra; (a) a shoulder/edge at 8980–8990 eV attributed to a forbidden 1s → 4s transition but allowed due to mixing of 4s and 4p orbitals, and (b) the main peak ∼9000 eV due to allowed 1s → 4p transition which merges with the continuum.⁵¹ The absorption feature between 9000 and 9020 eV originate from multiple scattering processes. The oscillation between 9030 and 9080 eV is mainly due the single scattering from nearest neighbor oxygen. No pre-edge peak appears in the XANES spectra which agree with the results reported by other workers also.^52–54

Fig. 3 XANES of Cu_1−xFe_xO for (0 ≤ x ≤ 0.125) samples at (a) Cu and (b) Fe K-edges.

Note that with increasing substitution the edge gradually shifts towards higher energy. This is also evident from the first order derivatives of the XANES spectra [inset (ii): Fig. 3]. This hints at the fact that in the pure CuO there is scope of lower valence states of Cu which further hints at oxygen vacancies or an oxygen-deficient lattice. However, with the introduction of Fe³⁺, the lattice is forced to accommodate more oxygen, thereby oxidize Cu towards a Cu²⁺ state resulting in the shift of the edge towards higher energies. The Cu³⁺ state is not very common because it requires a lot of energy to ionize to the 3+ state and hence may not be a logical solution to the above observation. The intensity and hence the areas under the curves of the white line peaks [Fig. 3(a)] in the substituted samples increase with increasing Fe doping. This indicates a higher 4p_σ density of states in the Fe doped samples compared to that in undoped CuO.⁵⁵

3.3.2 Fe K-edge. The main absorption peak around 7127 eV (1s → 4p electric dipole transition) of the normalized XANES Fe K-edge spectra [Fig. 3(b)] of all samples seem to be located in between FeO and Fe₂O₃ edges [inset (ii); Fig. 3(b)] but at energies closer to Fe₂O₃. This suggests that Fe is present in a mixture of majority 3+ and minority 2+ oxidation states in agreement to our observations in XAS and XRD studies. About 80–85% of Fe is in 3+ oxidation states as quantitatively estimated using linear combination fitting of the XANES spectra of the samples with that of FeO and Fe₂O₃ reference samples. However, there is not much change in the white line intensity between the undoped and doped samples unlike in the case of Cu K-edge XANES. We can notice that the Fe absorption edge features in Cu_1−xFe_xO samples do not match with the sharp white line of Fe₂O₃ (reference sample), hinting that Fe does not exist as a different phase of simple and complex oxides of Fe and Cu such as Fe₂O₃, CuFe₂O₄ etc. in CuO, rather it exists as Fe³⁺ in the CuO lattice. Thus the XANES study, support the substitution of Cu²⁺ with Fe³⁺ in the lattice and clearly rules out the presence of any form of iron oxides or Fe metallic clusters in the samples.

A very weak pre-edge feature [inset (i): Fig. 3(b)] at around 7115 eV is characteristic of Fe³⁺ (3d⁵) ion. In an ideal octahedron a corresponding transition is generally forbidden due to the electric dipole moment approximation. However, it becomes allowed due to few reasons: (a) electric quadruple transition, (b) hybridization of the vacant d orbital and (c) the presence of distorted octahedral or non-centrosymmetric tetrahedral forms.⁵⁶ It should be noted that the pre-edge peak positions of the samples do not coincide with that of FeO or Fe₂O₃ standards and are at least 1.5 eV higher than the later, neither do they exactly follow CuFe₂O₄ pre-edge feature.⁵⁷ This is yet another proof that Fe-substitution in CuO does not result in creation of any extra phase, however, the change in the intensity and width in the pre-edge region is the signature of change in the centrosymmetry around the dopant atom.

3.4 Extended X-ray absorption fine structure (EXAFS) analysis

From the Fourier transform of EXAFS data of the Cu and Fe K-edges, the metal–metal and metal–oxygen bond distances were estimated along with the coordination number of the metal ions. The energy dependent absorption coefficient μ(E) has been converted to absorption function χ(E) defined as follows:⁵⁸ where, E₀ is absorption edge energy, μ₀(E₀) is the bare atom background and Δμ₀(E₀) is the step in the μ(E) value at the absorption edge (difference between the pre edge and the post edge energy spectra). χ(E) was subsequently converted to χ(k) and weighted by k² [Fig. 4(a)] to amplify the EXAFS oscillations. Fourier transform of χ(k)k² provided χ(R) (or FT-EXAFS) vs. R plots [Fig. 4] which were finally fitted with theoretical models. A set of EXAFS data analysis program (available within the IFEFFIT software package) have been used for reduction and fitting of the experimental EXAFS data.⁵⁹ The bond distances (R) and co-ordination numbers (CN) have been determined from the best fit of the data along with the Debye–Waller, σ², which is a measure of structural disorder (mean-square fluctuations in bond distances). Theoretical scattering paths were generated by refining lattice constants and Wickoff positions of pure CuO monoclinic (space group-C2/C) structure⁶⁰ to fit χ(R), assuming the basic structure for the Cu K-edge data but with atoms being replaced by Fe.

Fig. 4 (a) k²χ(k) spectra at Cu and Fe K-edges of Cu_1−xFe_xO for (0 ≤ x ≤ 0.125); and (b) experimental χ(R) versus R plots along with the theoretical fitted plot of Cu_1−xFe_xO at both Cu and Fe K-edges respectively.

The χ(R) data of the Cu and Fe K-edges along with the best fit theoretical data [Fig. 4(b)] reveal similar local neighborhood of the Cu and Fe ions confirming that the Fe³⁺ ions are substituting Cu²⁺ ions without drastically changing the lattice in agreement with the XRD, SXAS and XANES results. We observe that in the Cu K-edge χ(R) vs. R plot, the first peak at ∼1.5 Å is, due to Cu–O scattering path, while the second peak at ∼2.4 Å is due to Cu–Cu coordination shell. We have fitted the data using three shell model. The 1^st co-ordination shell (i.e. Cu–O) was fitted using two assumptions, viz., (a) all the 6 oxygen atoms are at the same distance of 1.95 Å (proper octahedral structure) and (b) only 4 oxygen atoms are at same distance of 1.95 Å (planer) while, rest 2 oxygen atoms are at a distance of 2.78 Å (axial). The values of reduced χ² and R_factor improve drastically while fitted with the latter assumption. This shows that Cu atoms are in a distorted octahedral symmetry in CuO structure. The 8 Cu/Fe atoms at 2.90 Å in the 3^rd co-ordination shell contribute to the next scattering path. However, the 2 oxygen atoms between these co-ordination shells at 2.78 Å distance contribute very little to this and has higher σ² value which is a clear evidence of having distorted (Jahn–Teller distortion) octahedral structure. The monoclinic structure of CuO unit cell and their elongated octahedral structure have been shown in Fig. 5 using VESTA code.⁶¹

Fig. 5 VESTA generated structure of Cu_1−xFe_xO: (a) unit cell with atoms and bonds; (b) elongated octahedral structure with atoms and bonds.

The Fe K-edge spectra in the 1^st co-ordination shell (Fe–O) show a distorted octahedral structure which is in close agreement with pre-edge XANES spectra. The second intense peak between 2–2.5 Å is due to scattering from the 8 Cu/Fe atoms (3^rd shell) at a nominal distance of 2.90 Å from the absorbing core atom. This peak is also observed in Cu K-edge data.

It has been observed from the above EXAFS measurement that Fe–O bond lengths [Fig. 6(a)] are slightly higher than Cu–O bond lengths in the samples, though Cu–Cu1 bond distances are larger than Fe–Cu1. The coordination number [Fig. 6(b)] for Cu–O1 first increase for x = 0.027 and then decreases systematically for higher substitutions. Moreover, for Fe–O1, there is gradual increment in the oxygen coordination. In fact, coordination numbers for Cu–O1 and Fe–O1 form a mirror image to each other. Thus, oxygen vacancies are present (oxygen coordination are less than nominal values) in our system which decrease due to Fe³⁺ doping in place of Cu²⁺ to maintain charge balance. Debye–Waller factor decreases [Fig. 6(c)] with substitution for Fe–O and Fe–Cu bond lengths except for x = 0.55 substitution. Also, Debye Waller factor (σ²) of the 3^rd coordination shell (metal–metal) are significantly higher than that of 1^st and 2^nd shell (metal–oxygen) manifesting that this shell is significantly affected by the doping.

Fig. 6 The variation of (a) bond distances (b) coordination numbers and (c) Debye–Waller factor (σ²) with substitution, x.

With increasing Fe³⁺ replacing Cu²⁺ in CuO lattice, oxygen affinity is increased. This results in reduction of oxygen vacancies making the lattice less defective. Oxygen related defect states are located close to the conduction and valence bands. These states appear like edge tails known as Urbach tails. Reduction of such oxygen related defects result in an ideal bandgap which is reflected in the blue shift of absorption edge with Fe substitution [Fig. 7]. An estimation of the Urbach energy would have provided more evidence but due to limitations of our experimental data below 1.4 eV we could not estimate the same. However, from Debye Waller factor calculated from EXAFS fitting, we have pointed out that defect states decrease with increasing Fe substitutions in agreement with our UV-vis studies. A detailed study of opto-electronic properties will be reported elsewhere.

Fig. 7 Optical absorbance and Tauc's plot of Cu_1−xFe_xO samples; (inset right) shows increment in band gap with increased Fe substitution.

To investigate the effects of strain we did transmission electron microscopy (TEM) of the Cu_1−xFe_xO samples [Fig. 8]. TEM images of the nanoparticles reveal that the average particle size is decreased with Fe substitution (60 nm to 50 nm). The high resolution images and FFT of the lattice confirm that the pure CuO crystals are more ordered and crystalline than the higher substituted nanocrystals. FFT clearly shows that some extra reflections appear in x = 0.097. However one can easily recognize point defects in x = 0 sample representing oxygen defects while crystallographic shears in the x = 0.097 sample. Crystallographic shears are signatures of efforts of the lattice to reduce strains by rearrangement of the atoms when distortion goes beyond a limit. It is most likely due to these planes we observe extra reflections in the XRD data. A detailed TEM analysis will follow.

Fig. 8 High resolution TEM images of (a) x = 0 and (b) x = 0.097 for Cu_1−xFe_xO samples; insets in figure, show corresponding TEM and FFT images.

4. Conclusions

Simple sol–gel grown Cu_1−xFe_xO nanoparticles (x = 0, 0.027, 0.055, 0.097, 0.125) have been structurally studied. Rietveld refinement of XRD data shows that Cu_1−xFe_xO crystallizes in a monoclinic single phase (x ≤ 0.55). For x ≥ 0.097, few extra peaks appear which are due to creation of extra planes inside CuO structure. The presence of metallic Fe clusters, FeO and Fe₂O₃ phases is ruled out from SXAS and XANES studies suggesting that Fe³⁺ is incorporated in CuO lattice. As the ionic radii of Fe³⁺ ion is lesser than Cu²⁺, the crystallite size as well as the unit cell volume reduces with increasing Fe content. χ(R) data (EXAFS studies) reveals that the local neighborhood of Fe³⁺ matches with that of Cu²⁺ in CuO. Any similarity with oxides of Fe and other complex oxides of Cu and Fe was ruled out. To be noted is that from XRD studies some additional reflections belonging to few reflections from CuFe₂O₄ phase was observed. However, EXAFS studies do not support such a structure emphasizing on a single phase formation of the materials. EXAFS data analysis reveals reduction in oxygen vacancies with increasing Fe content possibly to due to extra charge of Fe³⁺ than Cu²⁺ ions within the same structure. The oxygen coordination is higher, and DW factor lower with Fe incorporation hinting at higher oxygen content in place of oxygen vacancies. Metal–metal interactions are significantly affected by the substitution than the metal–oxygen ones. This causes changes in the lattice supported by the fact that the Fe–O bonds are longer than Cu–O ones. Such stresses lead to crystallographic shear in the higher doped samples by rearrangement of atoms. The shears in the lattice are natural solutions to accommodate more oxygen ions and reduce lattice strain originated from oxygen vacancies. This increases bandgap by removing the Urbach tails.

Acknowledgements

The authors gratefully acknowledge Professor Pradeep Mathur, Director, IIT Indore for XRD facility. M Nasir is also thankful to UGC, New Delhi, for providing Maulana Azad fellowship.

Notes and references

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