Single-crystalline hollow and porous cuprous oxide (Cu2O) nanopolyhedras were synthesized using thewet-chemical reduction route method by the reduction of copper–tartrate complex with anhydrous dex-trose. Poly(vinyl pyrrolidone) (PVP) was used as a capping agent, which controls the overall morphologyand hollowing process. The mechanism responsible for the formation of hollow crystals has been studiedin the present analysis. Structural investigations reveal that Cu2O nanopolyhedras can have well-definedhollow interior with porous structure depending on the concentration of PVP. The high-resolution trans-mission electron microscopy (HRTEM) observations demonstrated that the nanoparticles are composedof small grains coherently growing along certain preferred orientations. Chemical compositions of all the
uminescence samples have been checked by EDX observation. None of the impurity signal was observed in the presentinvestigation. X-ray photoelectron spectroscopic (XPS) studies indicate the presence of Cu2+ on the surfaceof the Cu2O polyhedras. UV–visible absorption spectrum has been used to resolve the excitonic or inter-band transitions of the hollow Cu2O nanopolyhedras. The observed optical band gap of products showblue shift effect compared to the bulk Cu2O [Eg = 2.17 eV]. In addition, a photoluminescence measurementreveals a strong luminescence peak at 493 nm, arising due to the band edge emission from � +1 to the newsub-levels.
tstet(aIssparte
. Introduction
Porous solid materials are important in many areas of moderncience and technology, including ion exchange, molecular sepa-ation, catalysis, chromatography, microelectronics, light weightllers, acoustic insulators, energy storage, photonic crystals, sen-ors, absorbents, bio-medical diagnosis agents, etc. [1–7]. Notablexamples are microporous (<2 nm) zeolites and mesoporous2–50 nm) silicates and carbonates [8]. The ability to manipulatehe structure and morphology of porous solids on a nanometer scaleould enable greater control of the local chemical environment
8,9]. The other unique feature of these isotropic hollow materi-ls includes their light weight, because of central hollow voids andheir large specific surface areas, owing to the presence of bothnterior and exterior surfaces.
Concerning the fabrication of hollow nanomaterials, there arewo main categories of synthesis methods: (i) the template directedynthesis, and (ii) the emulsion synthesis. The basis of the templateirected synthesis is the adsorption of nanoparticles or polymeriza-
ion on modified polymeric (e.g. polystyrene) or inorganic templateurfaces and the subsequent removal of the templates by calcina-ions or dissolution with the solvent. On the other hand in themulsion synthesis, the solution is emulsified and the adsorp-ion or reaction then takes place on the surface of sol-dropletsmicelles or gas bubbles) to form hollow interiors [5,10–15]. Thesect as soft templates for the synthesis of hollow nanostructures.n addition to the above common methods, there are severalpecial synthetic strategies that can generate spherical hollowtructures [16,17]. Among them, interestingly, well-known physicalhenomena’s such as Ostwald ripening, Kirkendall effect, orientedttachment and hydrophobic interaction have been utilized veryecently [18–20] for fabricating hollow metal oxide nanosphereshrough direct solid evacuations or through self-templating. How-ver, the most nanoproducts prepared with the template freeynthesis are limited only to the spherical morphology, polycrys-alline shells consisting of the primary particles [20]. Since, theroperties of inorganic hollow and porous structures may be well
uned by tailoring their morphology and crystallinity. Recently,ome efforts have been devoted to the synthesis of inorganic hollowtructures with well-defined non-spherical morphologies.
Cuprous oxide crystallizes in the cubic cuprite structure (spaceroup, Pn 3̄ m) with no free internal parameters [21]. This consists of
wo interpenetrating lattices, one FCC of copper (Cu) atoms and thether BCC of oxygen (O) atoms. Copper atom is linearly co-ordinatedy two oxygen atoms, while oxygen atom is tetrahedrally co-rdinated by four copper atoms [22]. Cu2O is a non-stoichiometriceddish color compound and an important p-type metal–oxideemiconductor [23–25] material with a direct forbidden band gapf ∼2.17 eV. The unbounded excitons in Cu2O have binding energyf 140 meV and a radius of 0.7 nm [26]. The Cu2O crystals have beent the center of research on the Bose–Einstein condensation of exci-ons [27–29]. It exhibits sharp excitonic structures in absorptionnd luminescence [30]. Snoke et al. [28] has suggested the pos-ibility of transmitting light through a nanometer scale aperturef cuprous oxide without diffraction losses. It can be used as aolar cell material and has been shown to catalyze H2O splitting31–32]. The ground breaking real-time imaging of covalent bondsetween atoms in Cu2O has broad implications for new materi-ls used in the computer components, medical equipments andther devices [21]. Nowadays many efforts have been given to thehape-controlled synthesis of Cu2O micro and nanocrystals. Sys-ematic manipulation of the morphology and architecture of Cu2O
icrocrystals has been achieved using the solution routes [33,34]nd electrodeposition methods [35]. Meanwhile, many approachesave been reported for the synthesis of Cu2O with different shapesuch as nanowires [36], nanobelts [37], nanocubes [38], nanocages39], etc. Guo and Murphy [38] synthesized bulk Cu2O nanocubesy the reduction of CuSO4 in the presence of NaOH and the sur-actant CTAB. The surfactant binds non-selectively on the faces ofhe cubic Cu2O. The average edge length of the synthesized cubesaries from 200 to 450 nm. In these systems, dispersions in sizend shape may occur because of the chemical and thermal inho-ogeneity and the process of seed aggregation by Ostwald ripening
18], which involves the growth of larger particles at the expense ofmaller particles. The seed mediated approach [40] has also beenxamined in the shape-controlled synthesis of metal nanoparticles.u2O cubes with hollow interiors have also been obtained by thereparation of relatively large Cu2O nanocubes with average edge
ength of 500 nm, followed by their separation and dispersion incetic acid solution [41]. Lu et al. [42] reported a facile one-potolution synthesis of uniform single crystalline octahedral Cu2Oanocages by a catalytic self-templating route without using pre-
abricated template. They also reported the widely tunable bandap energy in the range 2.6–2.2 eV. Chang et al. [20] also reportedhe fabrication of hollow polycrystalline Cu2O sphere at 180 ◦C with,N-dimethylformamide (DMF) solvent, as a weak reducing agent.oth kinds of hollow Cu2O particles were multi-crystals and welllosed. Ruo et al. [43] synthesized a series of hollow and porousu2O nanostructures with the low concentration of CTAB, useds a template. A wealth of superfine single-crystalline hollow andorous Cu2O nanopolyhedras (8-faceted), prepared using simpleeduction route method, have been reported in the present context.he morphology of synthesized nanopolyhedras strongly dependsn the concentration of PVP. The influences of PVP concentration onhe fabrication of Cu2O nanoparticles have been investigated. Theynthesis of hollow and porous Cu2O nanopolyhedra in the pres-nce of capping agent PVP will be reported for the first time fromhe best of our knowledge.
. Experimental
.1. Materials
In this work, analytical reagents poly(vinyl pyrrolidone) (PVP), copperulfate (CuSO4·5H2O), sodium hydroxide (NaOH), sodium potassium tartrateKNaC4H4O6·4H2O), sulphuric acid (H2SO4) and anhydrous dextrose (C6H12O6) haveeen used. All the aqueous solutions were prepared with high-purity deionizedater. All chemicals were acquired from sd.-fine Chem. Ltd., India.
sc
tad
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.2. Synthesis of nanoparticles
Cu2O nanoparticles with the different structures were synthesized using reduc-ion route method [33], originally reported to synthesized nanocubes with thexception of using PVP as a capping agent. Cu2O nanopolyhedras are synthesized asollows: the whole process is conducted under vigorous magnetic stirring. All theeagents are of analytical grade and used without further purification. The reactionolution is prepared by mixing equal volume of solution A, containing 0.69 M ofuSO4·5H2O, 0.5 M of H2SO4 and 0.25 M of PVP and solution B, containing 3.06 M ofNaC4H4O6·4H2O and 8 M of NaOH and both the solutions are stirred for 20 min inseparate beakers. Dark blue color precipitates of copper tartrate complex are soonroduced on mixing both of the solutions. Resultant solution is further diluted to0 ml with deionized water and then heated from room temperature to 60 ◦C underonstant stirring for next 20 min. Solution C, containing 1.0 M of anhydrous dex-rose, is added to the above dark blue solution drop wise with constant stirring tillhe dark blue color precipitates of copper tartrate complex changes completely torick red color precipitates of cuprous oxide. Then a colloidal solution thus obtained
s removed from the heat and allowed to cool at room temperature. The particles arehen separated by centrifugation at 5000 rpm for 10 min. After removing the super-atant, the precipitate containing nanopolyhedras are rinsed with distilled water
or several times and then filtered.It is well-known that copper tartrate complex (Fehling’s solution) can be reduced
y anhydrous dextrose to give red color precipitates of Cu2O, which is extensivelysed in carbohydrate chemistry for the determination of aldehydes and reducingugar. A positive result is indicated by the formation of a brick red precipitates likether aldehydes, as aldose are easily oxidized to yield carboxylic acid. The cupricons (Cu(II)) complexed with tartrate ions and then reduced to copper (I) oxide. Theundamental chemical equation involved in the redox reaction is given as follows:
Estimations of the cluster size and phase determination were carried outmploying X-ray diffraction (XRD) as well as transmission electron microscopyTEM). The powder X-ray diffraction measurements were performed using a PW1710iffractometer with Cu K� radiation (� = 0.154187 nm) as an incident radiation andquipped with a secondary pyrolytic graphite monochromator. The step scan cov-red the angular range of 20–70◦ in step of 0.06◦ . The scanning electron microscopySEM) images are taken with JEOLJSM-5600 operating at an accelerating voltage of5 kV. The transmission electron microscopy (TEM) images and electron diffractionED) pattern of the samples are taken on a FETTechnai 20G2 transmission electron
icroscope operating at an acceleration voltage of 200 kV. Single slit low back-round sample holder is used for the TEM observations. The samples used for theEM observations are prepared as follows: some nanoparticles are dispersed in auitable amount of solvent which is vibrated ultrasonically for 10 min, then a drop ofolvent containing nanoparticles are spread over a carbon coated copper grid, whichs air dried before making the TEM measurements. The UV–visible spectrums of theispersions are obtained from an ELICO–SL164 PC-coupled UV-spectrophotometer.he X-ray photoelectron spectroscopy (XPS) experiments are carried out on a VSWUK) spectrometer using Al K� radiation (h� = 1486.6 eV). The test chamber pressures maintained below 1.6×10−9 Torr during the spectral acquisition. The XPS bind-ng energy (BE) is internally referenced to the aliphatic C (1s) peak (BE = 284.6 eV).amples for XPS are prepared by dispersing the Cu2O nanoparticles onto graphiteo avoid the charging effects.
. Results and discussions
Fig. 1 shows the X-ray diffraction pattern of the as-preparedu2O nanopolyhedras, which clearly shows the existence ofrystalline structure. The XRD peaks in the diffractogram areharacteristically sharp with no significant scattering backgroundhrough out the range. The diffractogram is significantly same fromhat in the well-known FCC structure of bulk Cu2O with total five,.e. 〈1 1 0〉, 〈1 1 1〉, 〈2 0 0〉, 〈2 1 1〉 and 〈2 2 0〉, characteristics peaks inhe 2� range of 20–70◦ with the lattice parameter a = 0.4267 nmJCPDS X-ray powder diffraction file 05-667]. The 〈1 1 1〉 peak at6.405 (d = 0.2459) is the most intense peak of the diffractogramnd 〈2 1 1〉 peak at 52.484 (d = 0.1738) is extremely weak in inten-ity. The XRD of these particles shows the final obtained product is
uprous oxide without copper and copper oxide.
The pure cubic phase is ascribed to the existence of glucose inhe process of preparation. The reduction route method takes thedvantage of the ready reactivity of aldehydes by using weak oxi-izing agent cupric ions (Cu2+) in alkaline solution. In addition to
P. Sharma, H.S. Bhatti / Materials Chemistr
Fc
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ttp2hlTPcPustF
hhItFctmo
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oi
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ig. 1. X-ray diffraction pattern of Cu2O nanopolyhedras prepared with 2.5 M con-entration of PVP.
he copper ions, Fehling’s solution contains tartrate ions as a com-lexing agent to keep the copper ions in solution. Without tartrate
ons, cupric hydroxide would precipitate from the basic solution.he tartrate ions are unable to complex cuprous ions (Cu+) so, theeduction of Cu2+ to Cu+ by reducing sugar result in the formationf an orange to red precipitate of Cu2O. The structure of copper(II)-tartrate complex has been determined as
The basic redox reaction involved in the synthesis is
The typical SEM images of Cu2O nanopolyhedras prepared withhe 0.25 M concentration of PVP, as shown in Fig. 2(a), suggestshat the product exhibits non-uniform irregular octahedral mor-hology. However, the nanoparticles prepared with the 0.5, 1 and.5 M concentrations of PVP are uniform and have regular octa-edral morphology (as illustrated in Fig. 2(b)–(f)). Average edge
engths of Cu2O nanopolyhedras are calculated to be ∼50–150 nm.he morphology of the final product changes with the change inVP concentration. Irregular polyhedras are obtained at a low con-entration of PVP, i.e. 0.25 M. As we increases the concentration of
VP, the morphology of Cu2O polyhedras becomes more and moreniform. The well-defined polyhedral morphology with a cubicymmetry, is the characteristic of single-crystalline cubic struc-ured Cu2O crystals bounded by eight 〈1 1 1〉 planes, as shown inig. 2(g). As illustrating in enlarge image of Fig. 2(c), the nanopoly-
tov[s
y and Physics 114 (2009) 889–896 891
(2)
edra has some partially broken particles which clearly reveals theollow features of nanopolyhedras (shown by arrow in Fig. 2(d)).
n the present investigation, octahedral (8-faceted) structures withhe sharp edges are fabricated, which are clearly observed inig. 2(f). Chemical compositions of all the samples have beenhecked carefully by the EDX observation. Fig. 3 obtained fromhe nanoparticles indicates that the well-cleaned final product is
ostly composed of Cu and O. None of the impurity signal isbserved in the present analysis.
For better understanding of microstructures of nanopolyhedras,ystematic TEM investigations have been performed on the sam-les prepared with the different concentrations of PVP. Fig. 4(a)–(c)hows typical bright field TEM images for the sample prepared with.5 M concentration of PVP. It indicates that the nanoparticles haveell-defined crystalline and porous structure with hollow interi-
rs. The selected area electron diffraction (SAED) pattern (inset ofig. 4(a)) corresponds to single nanopolyhedra showing a rhom-ic projection which can be indexed to 〈1 1̄ 0〉 zone axis of cubictructure Cu2O with a 〈0 0 1〉 direction along the longer diagonalirection of the rhomb. This suggests that the nanopolyhedras areingle crystal of Cu2O with an 〈1 1 0〉 oriented edge. It is well-knownhat the shape of Cu2Ocrystals is determined by the crystallographiclanes, which form the surface, and their crystal habits depend onhe relative order of the surface energy [44]. Fig. 4(d) represents aigh-resolution TEM image of hollow and porous Cu2O nanopoly-edras. The lattice fringes along the interior edges indicate that thessembled nanoparticles are highly oriented along 〈1 1 0〉 direction.rrow in Fig. 4(d) indicates the Morie’s fringes, originated from
he superposition of small particles with inclination on each other.he Cu2O nanopolyhedras are composed of small grains, which arelearly observed in dark field TEM images (Fig. 5(a) and (b)). It haseen documented that the Cu2O particles, made in a wet-chemicalethod, are of various shapes depending on the different synthesis
f the Cu2O nanopolyhedras is shown in Fig. 6. Furthermore, XPSs very sensitive to the oxidation sates of copper, i.e. Cu2+, Cu+,
u0. The main characteristic of the spectrum of the sample can beummarized as a peak corresponding to the core level Cu2p3/2 XPSransition accompanied by a weak satellite peaks towards a higherinding energy side. The occurrence of a weak satellite feature onhe higher binding energy side of the Cu2p main peak, confirmshe presence of CuO on the surface of nanoparticles. It is noted thathe divalent copper �Cu2+(3d9) compounds shows characteristichake-up satellites on the higher binding energy side of the spec-rum [46,47]. The d-shell of the Cu2O is full (d10), so that screeningia a charge transfer into the d state is not possible. This explains
he absence of a satellite peak in Cu2O. However, the appearancef a satellite peak in the XPS spectrum implies that the Cu at theery top layer possesses a valance state of +2. Balamurugan et al.48] also reported that the Cu2O nanoparticles are capped with aurface layer of CuO, which stabilizes the more symmetric cuprous
892 P. Sharma, H.S. Bhatti / Materials Chemistry and Physics 114 (2009) 889–896
Fig. 2. (a). SEM image of Cu2O nanopolyhedras prepared with 0.25 M concentration of PVP. (b). SEM image of Cu2O nanopolyhedras prepared with 0.5 M concentration of PVP.(c) and (d) SEM images of Cu2O nanopolyhedras prepared with 1 M concentration of PVP and its enlarged image (← arrow shows the hollowing feature of the polyhedras). (e)and (f) SEM images of Cu2O nanopolyhedras prepared with 2.5 M concentration of PVP and its enlarged image (edges are clearly observed). (g) The well-defined polyhedralmorphology with a cubic symmetry of Cu2O crystals bounded by eight 〈1 1 1〉 planes.
P. Sharma, H.S. Bhatti / Materials Chemistr
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htbfifgfiltaCtsiasntbnttfi
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ig. 3. Energy dispersive X-ray spectroscopy (EDS) of Cu2O nanopolyhedras pre-ared with 2.5 M concentration of PVP.
xide phase. XPS scans do not show any other impurity except car-on. The presence of carbon can be explained by the fact that theanocrystalline is capped by organic polymer. Fig. 7 shows the fittedraph of Cu2p3/2core level and its corresponding shake-up satellite.
he observed Cu2p3/2 peak along with shake-up is fitted with Gaus-ian fit (3 peaks). The peak 1 is assigned to Cu+ state and peaks 2nd 3 are assigned to Cu2+ states. The ratio of peak 1 to peaks 2nd 3 gives us the relative amount of Cu+ to Cu2+ on the surface ofanocrystals [49].
pdctp
ig. 4. (a)–(c) Bright field transmission electron images (TEM) of Cu2O nanopolyhedras pu2O nanopolyhedras). (d) HRTEM image shows the particles are grown from small partic
y and Physics 114 (2009) 889–896 893
The synthesis mechanism for the formation of Cu2O nanopoly-edras is same as that of the bulk Cu2O, with the exception ofhe PVP used as a capping agent [33]. This mechanism can beest described on the basis of Ostwald ripening [18,19], i.e. theormation of smaller crystallites is kinetically favored during thenitial agglomeration and larger crystallites are thermodynamicallyavored and oriented attachment. In our analysis, PVP directs therowth, dimensions and purity of the final product to produce thenal brick red colored Cu2O. The synthesis mechanism is as fol-
ows: when the reaction temperature increases, the PVP exists inhe form of droplets in the solvent mixture. Such droplets act assoft template for the formation of hollow interior. The positiveu2+ ions co-ordinate loosely with the oxygen atoms of PVP ini-ially in water to cause the isotropic capping of Cu2O polyhedraurfaces. The reduction of Cu2+ ions, by anhydrous dextrose, to Cu+
ons will nucleate at one site. These tiny nanocrystallites gatherednd provided a direct means for construction of hollow and poroustructures through a plane-by-plane mechanism [42]. These tinyucleus are then joined together by oriented attachments, to formhin crystal planes. The hollow polyhedral nanoparticles assembledy the smaller particles start growing. The growth scheme of Cu2Oanopolyhedras is summarized in Fig. 8. Also, when the concen-ration of PVP is low, the particles are not effectively capped andhey grow randomly with no preferred geometry. So the obtainednal product is non-uniform. On increasing concentration of PVP,
olyhedral surfaces are effectively capped and uniform polyhe-ras, which mirror the fundamental cubic crystal structure ofuprite unit cell, are obtained. Evidently, PVP does not preferen-ially absorbed to different crystal faces of Cu2O nanopolyhedras toroduce anisotropic nanoparticles.
repared with 2.5 M concentration of PVP (inset, in (a), shows the SAED pattern ofles attached in preferred direction.
894 P. Sharma, H.S. Bhatti / Materials Chemistry and Physics 114 (2009) 889–896
Ft
opa5ptb
Fig. 6. X-ray photoelectron (XPS) spectrum of Cu2p levels for Cu2O nanopolyhedrasprepared with 2.5 M concentration of PVP.
Fs
Fc
ig. 5. Dark field TEM images of Cu2O nanopolyhedras prepared with 2.5 M concen-ration of PVP.
Fig. 9 shows the UV–visible spectrum of the products inrder to resolve the excitonic or interband transitions of Cu2Oarticles. The UV–visible absorption spectrum shows a broad
bsorption peak ranging from 400 to 700 nm with maxima at10 nm (this peak is responsible for the size distribution of thearticles). The band gap calculated from the UV–visible spec-ra is 2.43 eV, which is larger then the reported value of theulk Cu2O (∼2.17 eV). It is well-known that the optical absorp-
tod[
ig. 8. Schematic diagram of growth mechanism of Cu2O nanopolyhedra obtained fromapping agent poly(vinyl pyrrolidone) (PVP).
ig. 7. A representatives fit of the Cu2p3/2 core level and its corresponding shake-upatellite.
ion would be greatly affected by the morphology and crystallinityf Cu2O crystals, and a blue-shift effect will be achieved byecreasing the overall crystal size and the hollowing process44].
reduction of copper–tartrate complex with dextrose anhydrous, in the presence of
P. Sharma, H.S. Bhatti / Materials Chemistr
Fig. 9. UV–visible spectra of Cu2O nanopolyhedras prepared with 2.5 M concentra-tion of PVP.
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o
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Fig. 10. Photoluminescence spectrum of Cu2O nanopolyhedras.
Some of the bulk Cu2O materials are luminescent [50,51]. Fig. 10xhibits the photoluminescence spectrum of the as-prepare Cu2Oanopolyhedras, taken under the excitation of the 325 nm line of ae-lamp. A strong band centered at 493 nm can be observed. Thisand is due to the band edge emission from � +1 to the new sub-
evels at 300 K [51–53]. The sub-levels could be developed due tomperfection levels as a result of the interaction of two excitons orhe 3D–1D splitting in Cu+(3d94s2) [50,53]. In the emission spec-rum, several other peaks at 470, 481, 483, 488, 497, 504 and 509 nmre also observed. All of them arise due to the defects present in theu2O nanopolyhedras [54].
The XPS spectra of Cu2O nanopolyhedras show the satellite fea-ure on the higher binding energy side of the Cu2p main peak,onfirming the presence of CuO in the final products. The XRD andlectron diffraction pattern does not show CuO phase formationhich concludes that Cu+ is present only on the surface of the Cu2Oanopolyhedras. Also, CuO is not present in the crystalline form onhe surface of Cu2O nanopolyhedras [47]. Thus, probability of CuOrowth on the surface is higher than the inner core.
. Conclusions
The uniform single-crystalline octahedral Cu2O nanopolyhedrasf about ∼50–150 nm edge length have been successfully synthe-
[[[
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y and Physics 114 (2009) 889–896 895
ized by wet-chemical reduction route method, which involves theormation of porous polyhedral Cu2O nanocrystals and a subse-uent hollowing process. Structural investigations also reveal theollow and porous nature of the Cu2O nanopolyhedras. The opticalbsorption features are found to show a systematic shift towardsigher energy compared to the bulk band of 2.17 eV at room tem-erature. The XPS studies indicate the presence of Cu2+ species onhe surface of Cu2O nanopolyhedras. The capping agent PVP directshe growth of the final products and controls the overall morphol-gy. The synthesis of hollow and porous Cu2O nanopolyhedra inhe presence of capping agent PVP will be reported for the firstime from the best of our knowledge. The single-crystal hollow andorous Cu2O nanopolyhedras have provided another material inhe fields of hydrogen production, superconductors, drug deliveryystems, sensors, catalysis and also act as model systems for theundamental researches.
cknowledgements
The authors are very thankful to Dr. T Shripati from Interniversity Consortium, Indore for use of an X-ray photoelectronpectrometer. Also, Dr. N.P. Lalla from Inter University Consortium,ndore for providing TEM images and Dr. P. Koshy from RRL, Ker-la for providing SEM images. Financial assistance from Universityrant Commission, New Delhi, India is gratefully acknowledged.
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