Luminescence and electrochemical properties of rare earth (Gd, Nd) doped V₂O₅ nanostructures synthesized by a non-aqueous sol–gel route

Abstract

Vanadium pentoxide nanostructures have been obtained from an alkoxide sol–gel, prepared by a simple and inexpensive facile non-aqueous method. The progressive addition of rare earth (RE) ions (Gd³⁺, Nd³⁺) to pristine V₂O₅ and the structural, functional, morphological, optical and electrochemical properties were studied. XPS studies confirmed the presence of RE ions in the orthorhombic phase of pristine V₂O₅, which was supported by XRD. The doping of RE ions significantly altered the morphology of V₂O₅ into various nanostructures by the linkage of small V₂O₅ nanoparticles. A significant red shift from undoped V₂O₅ was observed from UV absorption and PL spectra. From the CV experiment, it was observed that the overall cell potential was increased for the doped samples. The specific capacity of the Gd³⁺ and Nd³⁺ doped V₂O₅ increased upto 10%, which is useful for secondary Li-ion rechargeable batteries.

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1 Introduction

In recent years, vanadium pentoxide (V₂O₅) has drawn much attention due to its outstanding properties that makes it a key element for a variety of technological and industrial applications.¹ Due to a lack of power generation cathode materials, possessing higher energy density, have been investigated extensively. Vanadium pentoxide is a typical intercalation compound because of the multiple valence states of vanadium and the rich structural chemistry, which enables redox-dependent properties.² Plenty of work has been carried out on the electrochemical properties of transition metal doped V₂O₅.^3–8 However, a detailed literature survey shows that only a few reports have discussed the electrochemical properties of RE ion (Ce³⁺, Sm³⁺ and Eu³⁺) intercalated V₂O₅ nanostructures with the aim of replacing the cost effective and eco-toxic Pb and Hg materials for secondary rechargeable batteries.^9,10 Cyclic performance of these materials has been improved greatly. However, the cycle performance is not stable and it needs to be prolonged, so it is important to understand the life of the reversibility of the cathode. With this motivation, both Gd³⁺ and Nd³⁺ were doped into a V₂O₅ matrix by a sol–gel method and characterised to understand the electrochemical properties. These oxides also have potential as host materials for rare earth ion (RE ion) implantation due to the suitability of oxygen inclusion and their wide band gap nature that enhance the photoluminescence (PL) emission of dopant ions. Due to the sharp and intense emission by the 4f intra shell transition of rare earth ions¹¹ the dopants enhance the luminescence property of V₂O₅ nanomaterials. Numerous techniques have been followed to synthesize RE doped vanadium pentoxide such as pulsed laser deposition, vacuum evaporation, solid-state synthesis and super/sub critical water one-step process etc.^9,12–14 Among these techniques, the non-aqueous sol–gel route is a robust technique to shrivel the particle size and shape, gain high crystallinity and control over the dimensionality of the product, with the non-requirements of post-thermal treatment and high monodispersity.¹⁵ This sol–gel method can also be extended to form self-assembled nanostructures.^16–20 In our work, V–O, V–Gd–O and V–Nd–O systems have been systematically studied with an emphasis on the morphology, surface composition, optical and electrochemical properties of the synthesized materials.

2 Experimental section

2.1 Materials synthesis

To prepare pure V₂O₅ nanoparticles, 0.5 ml of analytical grade vanadium oxytrichloride (VOCl₃) was slowly mixed with 20 ml of benzyl alcohol under vigorous stirring at room temperature. After 5 h stirring, a blue colored suspension was obtained and dried in an oven at 100 °C for 3 h. The resulting green gel was centrifuged and washed several times using ethanol and acetone to remove the ionic impurities. The final product was sintered at 400 °C for 2 h. RE (Gd, Nd) doped V₂O₅ nanostructures were synthesized using the above procedure, in addition to VOCl₃, suitable molar ratio of RE oxides (Gd₂O₃, Nd₂O₃) were individually mixed in the solvent.

A schematic illustration of the experimental procedure for the synthesis is depicted in Fig. 1. The chemical reaction formulae are given in eqn (1)–(3) for undoped, Gd and Nd doped V₂O₅ respectively.

2VOCl₃ + 3C₆H₅CH₂OH → V₂O₅ + 3C₆H₅CH₂Cl + 3HCl↑ (1)

Fig. 1 Schematic of non-aqueous sol–gel route for the preparation of pristine and rare earths (Gd, Nd) doped V₂O₅ nanostructures.

2.2 Material characterization

The oxidation states of the products (V, O, Gd and Nd) and their binding energies were studied by conducting XPS analysis using an Omicron Nanotechnology XM1000 spectrometer, equipped with an excitation source of AlK_α in the range of 0–1100 eV. XRD patterns of the synthesized materials were recorded using a Bruker D8 Advance Powder diffractometer with CuK_α radiation (λ = 1.5406 Å) with 2θ in the range of 10–80°. The morphology and grain sizes were studied using scanning electron microscopy (HRSEM; FEI Quanta 200F microscope) operated at 20 kV. The chemical composition of the phases formed was elucidated using an energy dispersive X-ray (EDX) analyser with an ultra-thin window. HRTEM images were recorded with a JEOL JEM-2010F microscope at an accelerating voltage of 200 kV. FTIR spectra were recorded using an Alpha Bruker FTIR spectrometer in the range of 4000–400 cm⁻¹. UV-vis absorbance spectra were recorded between 300 nm and 650 nm at room temperature using a PG instrument T90+ UV-vis spectrophotometer. The PL spectra were recorded in the wavelength range from 450 nm to 750 nm by using a Fluoro Max-4 instrument. Cyclic voltammogram and discharge capacitive studies were performed using a computer-controlled electrochemical system (VSP-300, BioLogic Instruments, France) with a three electrode arrangement.

3 Results and discussion

The XPS technique is an effective tool to study the composition and ratio of the surface layer of materials (2–4 nm in depth).²¹ The XPS survey spectra of pristine and Gd, Nd doped V₂O₅ nanoparticles are shown in Fig. 2a–c. Great care has to be exercised to determine the different oxidation states of vanadium, since the core level binding energies (BE) of the V 2p₃ line (which is normally used) for vanadium oxides with various oxidation states of vanadium (V₂O₅, VO₂, and V₂O₃, corresponding to V⁵⁺, V⁴⁺, and V³⁺ respectively) are in a narrow range of about 515–530 eV. From Fig. 2d, the V 2p_3/2 and V 2p_1/2 peaks in vanadium pentoxide were observed at 517.5 eV (517.45 eV) and 525.01 eV (524.95 eV) respectively. The energy separation due to the spin orbit interaction 7.44 eV (7.5 eV) of the vanadium 2p levels suggests that the product composition of the sol–gel prepared pristine and RE doped V₂O₅ nanoparticles nearly approaches the stoichiometry with vanadium in its highest oxidation state. These binding energy (BE) values correspond to the V⁵⁺ state of vanadium and there is a close resemblance to the standard reference values.²² A clear O 1s line confirms that oxygen exists in the pristine and doped vanadium oxide samples. The oxygen signal is fitted with two peaks. The main typical O 1s peak centered at BE = 530.41 eV (530.3 eV) corresponds to O²⁻ ions in the metal oxide (V₂O₅).²³ The secondary oxygen peak with less intensity observed at 531.64 eV can be attributed to carbon–oxygen binding.²⁴ All binding energies have been corrected for the charging effect with reference to the adventitious carbon 1s peak at 284.6 eV.

Fig. 2 XPS survey spectra of (a) pristine, (b) Gd³⁺ doped (c) Nd³⁺ doped V₂O₅ nanostructures. Core level diagram of V 2p and O 1s state (d). High resolution XPS spectra of Gd 4d and Nd 3d states (e & f).

Bondarenka et al. have done a useful statistical analysis of XPS data on the binding energies of V 2p₃ lines for vanadium in different oxidation states.²⁵ The reported binding energies for V⁵⁺, V⁴⁺, and V³⁺ species were found to match with three Gaussian distributions centered at 517.3, 516.5 and 515.6 eV, respectively, with the statistical deviation of ±0.25 eV. The broad and intense peak observed between 140 and 150 eV is attributed to the 4d state of gadolinium as shown in Fig. 2e. The spin–orbit splitting of 4.25 eV occurred due to the doublet Gd 4d_3/2 and Gd 4d_5/2 at 148 and 143.75 eV respectively.

In the case of Nd, the Nd 3d core level showed complex and asymmetrical features between 970 and 1020 eV, corresponding to the 3d_5/2 and 3d_3/2 doublet (shown in Fig. 2f). The peak asymmetry originated from the final state effects (with Nd 4f level) as well as from overlapping with O KLL Auger lines that are observed in the same region. The BE for Nd 3d_5/2 are 983.3 ± 0.2 eV (4f₃) and 975.3 ± 2.3 eV (4f₄), which is in agreement with the available data for Nd³⁺.²⁶ Because of the significant error for the calculation of atomic ratios, due to the overlapping between Nd 3d and O KLL lines, the Nd 4d core level is used for quantification. It presents a more simple shape at a BE of about 122 eV. Another less intense peak observed at 1006.7 eV is ascribed to the core level spectra of Nd 3d_3/2.

The typical XRD patterns for the pristine and RE (Gd & Nd) doped V₂O₅ nanomaterials are shown in Fig. 3. It is clearly observed that the synthesized samples exhibit a crystalline nature and the indexed planes are well matched with the standard JCPDS data (card no: 41-1426; space group: Pmmn). From the XRD patterns, no characteristic peaks of impurity phases were observed. It ensures that the structure of the host material is not modified by dopant elements. The most intense diffraction peaks of the doped samples (in Fig. 3b and c) show a slight shift towards a higher angle compared to that of the undoped V₂O₅ sample. This shifting in XRD lines of the doped V₂O₅ suggests that Gd³⁺ and Nd³⁺ have been successfully doped into the pure V₂O₅ host structure at the V⁵⁺ site and it is ascribed to the decrease in the values of ‘d’ spacing.²⁷

Fig. 3 Powder XRD patterns of pure (a), Gd³⁺ (b) and Nd³⁺ (c) doped V₂O₅ nanostructures.

The estimated lattice parameters are depicted in Table 1. When compared to the ‘d’ spacing of pristine V₂O₅, both the doped samples possess smaller ‘d’ spacing values for all the diffraction angles. This is purely due to the larger atomic radius of the dopant cations (Gd = 180 pm & Nd = 185 pm) than the host cation (V = 135 pm). Besides the larger atomic radius, the dopant cations are trivalent (Gd³⁺ and Nd³⁺). When the impurity ion has a larger radius than the radius of the host lattice, the lattice constant will decrease and the particle size will increase.²⁸ But the host material is a pentavalent ion (V⁵⁺), so, it may be concluded that there is a reduction in the lattice fringes (‘d’ spacing) of the host material, caused by a deficiency of dopant ions to accumulate in the vacancy position of the oxygen ions in the pentavalent host lattice.²⁹ The average crystallite sizes of the pristine and doped samples were estimated using Scherrer's formula and are listed in Table 1. These results substantiate that the decrement in lattice fringes caused the increment of particle sizes.

Table 1 Comparison of diffraction angle (2θ), ‘d’ spacing, lattice parameters, volume (V) and avg. crystallite size (D) of pure V₂O₅ with Gd and Nd doped V₂O₅ nanoparticles

(hkl) planes	Pure V2O5		Gd:V2O5		Nd:V2O5
	2θ (deg)	‘d’ spacing (Å)	2θ (deg)	‘d’ spacing (Å)	2θ (deg)	‘d’ spacing (Å)
200	15.46	5.729	15.53	5.701	15.56	5.692
001	20.32	4.367	20.40	4.349	20.42	4.346
110	26.21	3.397	26.28	3.389	26.31	3.385
400	31.08	2.875	31.15	2.869	31.18	2.866
310	34.38	2.606	34.44	2.602	34.48	2.599
a (Å)	11.458		11.402		11.384
b (Å)	3.566		3.569		3.567
c (Å)	4.367		4.352		4.350
V (Å^3)	179.83		177.076		176.624
Avg. crystallite size ‘D’ (nm)	16		19		27

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V₂O₅ has orthorhombic symmetry and a layered structure: each vanadium atom is connected to five oxygen atoms to create pyramids that share their corners to build a double chain.³⁰ The chains are connected along the edges to form layers that are then stacked to form the bulk structure. There are three structurally different oxygen atoms in each layer. One is coordinated to one vanadium atom, the second is found in a bridging position, and the third has a threefold-coordinated position. The binding of the atoms inside a layer is strong whereas the interactions that keep the layers stacked are weaker, resulting in V₂O₅ being easily cleaved (shown in Fig. 4a). When the dopant material with a higher ionic radii (Gd³⁺) intercalates into the interstitial layer of the V₂O₅ lattice it causes an increase in the distance between layers (shown in Fig. 4b).^31,32

Fig. 4 Crystal structure depicting (a) the orthorhombic phase of V₂O₅ (Pmmn) (b) insertion of Gd ions into the V₂O₅ layered structure.

FTIR spectra of pristine and rare earth ion (Gd & Nd) doped V₂O₅ nanostructures are shown in Fig. 5. For all samples, a very broad and high intense peak observed at 3450–3350 cm⁻¹ is attributed to the O–H (hydroxyl group) symmetric and bending vibrational modes. The position of the C–O stretch can be used to determine the type of alcohol. The peak observed around 1050 cm⁻¹ confirms the presence of primary alcohols (M–CH₂–OH), which is ascribed to the presence of the benzyl alcohol solvent, even after washing several times with ethanol. The terminal oxygen symmetric stretching mode of V=O and the bridge oxygen asymmetric and symmetric stretching modes (ν_as and ν_s) of V–O–V were observed at 997, 759 and 553 cm⁻¹, respectively.

Fig. 5 FTIR spectra of pristine (a) Gd³⁺ (b) and Nd³⁺ (c) doped V₂O₅ nanoparticles.

The positions of the absorption bands in the RE doped V₂O₅ samples are similar to those observed in the undoped sample, which implies that the layer structure is not significantly altered by intercalation. The peak at 997 cm⁻¹ in the high frequency region is due to vibration of V=O atoms, which also gives information about the structural quality of the product.³³ The vibration band, around 997 cm⁻¹ for pristine V₂O₅ was shifted to 1014 and 1018 cm⁻¹ in the Gd and Nd doped V₂O₅ spectra respectively.

The frequency shift to a higher wavenumber indicates the presence of impurity ions in the lattice, such as rare earth oxides (REO) in the vicinity of oxygen, and distortion of the V=O bond.³⁴ The absorption peaks around 1000 cm⁻¹ are related to the vibration of isolated V=O vanadyl groups in [VO₅] trigonal bipyramids.³⁵ The ν_s(V–O–V) and ν_as(V–O–V) modes are shifted to a higher wavenumber. These chemical shifts could be related to the reduction of the oxidation state of vanadium from V⁵⁺ to V⁴⁺ and an increase of electronic conductivity.³⁶ The shifting of triply coordinated oxygen (V₃–O) from 553 cm⁻¹ to a lower wavenumber in the vanadium oxide lattice may be caused by a distortion in the coordination geometry.³⁷ In addition, some minor spectroscopic differences and displacements were observed, showing the physical interaction between trivalent dopant cations (Gd³⁺ & Nd³⁺) and V⁵⁺ ions.

The morphological evolution by doping was examined by high resolution scanning electron microscopy (HRSEM). HRSEM images of undoped V₂O₅, Gd:V₂O₅ and Nd:V₂O₅ are shown in Fig. 6a–c respectively. It can be seen that the pure sample consists of uniformly dispersed spherical particles within the nanoscale regime in the order of 10–15 nm (Fig. 6a). The Gd:V₂O₅ sample has a morphology with randomly oriented nanoflakes (Fig. 6b). The nanoparticle diameter was found to be about 20 ± 5 nm. The HRSEM image of the Nd:V₂O₅ sample showed a sugar cube-like, interlinked and porous nanostructure with particle size in the range of 20–25 nm. These suggest that the change in particle size is attributed to the doping with rare earth elements, which is consistent with the XPS and XRD results.

Fig. 6 HRSEM images of undoped V₂O₅ (a), Gd:V₂O₅ (b) and Nd:V₂O₅ (c); EDAX spectra of undoped V₂O₅ (d), Gd:V₂O₅ (e) and Nd:V₂O₅ (f); TEM images of undoped V₂O₅ (g), Gd:V₂O₅ (h) and Nd:V₂O₅ (i); SAED patterns of undoped V₂O₅ (j), Gd:V₂O₅ (k) and Nd:V₂O₅ (l); HRTEM images of undoped V₂O₅ (m), Gd:V₂O₅ (n) and Nd:V₂O₅ (o).

The SAED pattern of pristine V₂O₅ nanoparticles (Fig. 6j), illustrates the diffraction rings, which can be indexed to an orthorhombic structure. The SAED analysis of Gd and Nd doped vanadia samples indicates highly oriented nanocrystalline grains due to the ordered diffraction spots as compared to the X-ray diffraction results. The morphology and qualitative degree of dispersibility and agglomeration of the doped V₂O₅ nanoparticles were studied by TEM. Fig. 6g–i show the TEM overview images of pristine and Gd and Nd doped V₂O₅ nanostructures respectively. Improved dispersibility was achieved by using pure benzyl alcohol as the solvent. A slightly lower degree of agglomeration was observed in the case of Nd:V₂O₅. The particle sizes of all the samples are in accordance with the HRSEM results. Fig. 6d–f show the EDX spectra of pure and Gd and Nd doped V₂O₅ nanostructured materials. From Fig. 6d, it can be seen that the undoped V₂O₅ nanospheres contain only V and O. Apart from the vanadium and oxygen, an amount of gadolinium and neodymium was also present in the Gd and Nd doped samples respectively (Fig. 6e and f). HRTEM images reveal that the products are highly crystallized nanomaterials (Fig. 6m–o), and the lattice fringes corresponding to d spacings of 0.47 nm, 0.37 nm and 0.3 nm are in agreement with the d₀₀₁, d₁₁₀ and d₁₁₀ spacing in the XRD pattern of undoped and Gd and Nd doped V₂O₅ samples. These results demonstrate that, the inorganic material synthesized through organic medium is a highly preferential way to achieve the products in the nanoscale regime for potential use in a variety of applications.³⁸

UV-vis absorption spectra of pristine and RE doped V₂O₅ nanoparticles are shown in Fig. 7. The UV-vis absorption spectrum of pristine V₂O₅ shows a strong absorption in the UV region, which extends to the visible region, which is mainly associated to the charge transfer transition from the O 2p valence band to the empty V 3d orbitals. Also the UV-visible spectra of doped V₂O₅ nanoparticles exhibit a similar pattern to that of the V-oxide system and it is dominated by a broad envelope around 400 nm, ascribed to O²⁻ → V⁵⁺ charge-transfer transitions and V⁴⁺ → V⁵⁺ intervalence transitions in the near infrared region.³⁹ It was observed that the absorption edges of Gd and Nd doped V₂O₅ shift slightly towards the higher wavelength region compared to that of pristine V₂O₅, for which the samples were active under visible light of the solar spectrum.⁴⁰ This red shift attributed to the charge transfer transition between the f electron of rare earth ions and the V₂O₅ conduction or valence band.⁴¹ The optical band gap was obtained by plotting the relation of (αhν)² versus hν by using the relation,

αhν = A(hν − E_g)^m

where A is a constant, E_g is the energy band gap of the material, αhν is the incident photon energy and the exponent ‘m’ determines the type of electronic transitions causing the absorption, which takes the values of 1/2 and 2. The optical band gap calculated using Tauc's plot by extrapolating the straight line parts of the curves at (αhν)^m = 0, is shown in the inset of Fig. 7. The direct band gap for undoped and Gd and Nd doped V₂O₅ nanoparticles were found to be 2.18, 2.12 and 2.10 eV respectively.

Fig. 7 UV-vis absorbance spectra of pristine (a) and Gd³⁺ (b) and Nd³⁺ (c) doped V₂O₅ nanoparticles. Tauc's plot for direct allowed band gap transition (inset figure).

Fig. 8 shows the photoluminescence spectra of pristine and RE doped V₂O₅ nanoparticles. In the PL spectra, an excitation wavelength of 475 nm was used for all the samples.^42–44 The emission peaks were observed at 502, 528 and 532 nm for pristine and Gd and Nd doped samples respectively.⁴⁵ Peaks around 520 nm in V=M oxides are assigned to the V=O double bonds.⁴⁶ These results confirmed the shift in absorption edges towards the higher wavelength region (red shift), which is in good agreement with the UV-vis absorbance spectra. The total PL intensity decreased for doped samples, because of the increase in non-radiative traps produced by structural disorder. Furthermore, an absence of secondary emission peaks (defect peaks) in the PL spectra suggests that the dopant cations are successfully intercalated into the host lattice. The obtained results suggest that these materials are applicable to blue InGaN phosphor conversion.⁴⁷

Fig. 8 Room temperature photoluminescence spectra of pristine and Gd³⁺ and Nd³⁺ doped V₂O₅ nanoparticles.

Initially, the green color of the as-prepared products suggests that the V⁵⁺ is partly reduced to V⁴⁺ during preparation in an organic environment. Later, it becomes yellow, due to the calcination at 400 °C. This is attributed to the oxidation during the calcination to V⁵⁺. The CV measurements were performed on the calcinated samples, using three electrode systems with a potential range from −2.0 to +2.0 V versus Li/Li⁺, in acetonitrile solution containing 0.1 M LiClO₄ (Fig. 9). In this solution media, the electrochemical process can be attributed to the V^V/V^IV redox pair (yellow⁵⁺ ↔ green^5+/4+ ↔ blue⁴⁺), with concomitant insertion and deinsertion of alkaline ions in order to maintain electroneutrality.^48,49

xe⁻ + xLi⁺ + V₂O₅ → Li_xV₂O₅ (4)

Fig. 9 The typical cyclic voltammogram curves of (a) undoped V₂O₅ (b) Gd:V₂O₅ and (c) Nd:V₂O₅ with a scanning rate of 10 mV⁻¹ between −2.0 and +2.0 V.

Fig. 9 compares the cyclic voltammograms (20^th cycle) of pure and Gd and Nd doped V₂O₅ nanostructured materials. All the prepared samples exhibited three well-defined reduction peaks at ∼−0.40, 0.20 and 0.63 V corresponding to the formation of ε, δ and γ phases, respectively.⁵⁰ However, two peaks appeared at ∼0.62 and 1.40 V in the anodic region. This indicates that the complex structural changes occurred in the cathode material during the oxidation process. The slight shift in the cathodic and anodic peaks depends upon the dopant content. The redox current is more for the doped samples than the pristine V₂O₅. Moreover, the cathodic/anodic peak shifts to a more positive voltage range, which leads to an increase of the overall average cell potential compared with pristine V₂O₅.

Fig. 10 illustrates the 25 complete galvanostatic discharge cycles of the battery. When the batteries were cycled between −2.0 to +2.0 V at a constant current density of 6 mA g⁻¹, the maximum discharge capacity of undoped V₂O₅ was 284 mA h g⁻¹ and 188 mA h g⁻¹ after 25 cycles, and the initial discharge capacities of Gd and Nd doped V₂O₅ were 302 mA h g⁻¹ and 305 mA h g⁻¹ respectively; and 247 mA h g⁻¹ and 251 mA h g⁻¹ after 25 cycles respectively. It was observed that the cyclic performance of the doped samples is obviously more stable after the 7^th cycle and the specific capacity is higher than that of the pristine sample, which is in agreement with the cyclic voltammetry results. These results are due to the increased active area and the flexible structure of doped samples, which can provide more Li⁺ ion intercalation sites and accommodate a large volume variation, supported by the XRD results.⁵¹ The specific capacity of the doped samples increased upto 10% and meanwhile, the capacity loss decreased upto 16% from the pristine V₂O₅ nanoparticles. The successful incorporation of rare earth ions within the host lattices also enhanced the electronic conductivity and reduced the resistance within the nanostructures, which facilitates the faster mobility of Li⁺ ion in the cathode material.

Fig. 10 Discharge curves of (a) undoped V₂O₅ (b) Gd:V₂O₅ and (c) Nd:V₂O₅ at a constant current density 6 mA g⁻¹ when sweeps between −2.0 and +2.0 V.

4 Conclusions

Single-phase pristine and RE:V₂O₅ nanostructures have been successfully prepared by employing the non-aqueous sol–gel process using metal alkoxide as a precursor. The XPS study deduced the V–O, Gd–O and Nd–O bonding in the synthesized nanostructures, which clearly revealed the substitution of RE-ions in the V site. Evidence has been obtained from XRD analysis, for 100% yield of an orthorhombic structure to all the samples. The doping of RE-ions significantly altered the morphology from spherical to flakes and cubes within the nanoscale regime. Optical studies of the synthesized product indicated a red shift with a band gap around 2.2 eV. Electrochemical studies exhibited a significant capacity retention for the doped samples. This approach will be very useful for the synthesis of semiconductor nanostructures in opto-electronic devices and prominent cathode materials for rechargeable Li-ion batteries.

Acknowledgements

One of the authors (A. V.) is grateful to National Centre for Nanoscience and Nanotechnology, University of Madras, Chennai, India for XPS analysis, and Dr P. Chinnadurai, Secretary and Correspondent, Panimalar Engineering College, Chennai, India for his moral support.

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