studies on the synthesis and characterization of pdtio2sio2 nanocomposite for electroanalytical...

Studies on the Synthesis and Characterization of Pd�TiO2�SiO2

Nanocomposite for Electroanalytical Applications

P. C. Pandey,* Arvind Prakash

Department of Applied Chemistry, Institute of Technology, Banaras Hindu University, Varanasi-221005, India*e-mail: [email protected]

Received: February 14, 2011;&Accepted: May 30, 2011

AbstractA nanocomposite of Pd�TiO2�SiO2 is developed through a sol-gel process from the reaction products of titaniumisopropoxide followed by mixing the same with palladium linked 3-glycidoxypropyltrimethoxysilane. The reactionproduct is sonicated and calcinated to obtain the nanocomposite of Pd�TiO2�SiO2. The calcination at 600 8C yieldedan amorphous structure whereas at 900 8C it resulted into a nanocrystalline structure. The nanocomposite of palladi-um was further characterized by TEM, XRD, IR and EDS. The material acts as an efficient electrocatalyst. Electro-catalysis of ascorbic acid is observed at 0.1 V vs. Ag/AgCl, shows linearity between 1 mM and 1 mM in 0.1 M phos-phate buffer (pH 7.0).

Keywords: Electrocatalysis, Modified graphite paste electrode, Nanocomposite, Phase transformation, Sol-gelprocess

DOI: 10.1002/elan.201100083

1 Introduction

The requirement of nanomaterial useful in electrochemi-cal sensor design has great significance from technologicalangles, because these materials have better physical andchemical properties such as low melting point, high cata-lytic activity, specific optical and mechanical properties[1]. Nano-sized titanium oxide have efficient photocata-lytic properties for pollution degradation [2, 3], watersplitting [4], antibacterial activity [5], decomposition ofdyes [6], solar cells [7], sensors [8–10] and electrochromicdevices [11] and also used in self-cleaning windows [12].TiO2/SiO2 nanocomposite also used as catalysts and pho-tocatalyst for various reactions [13], annealed TiO2 andSiO2 used as anti-reflecting coating in high efficiencysolar cell [14]. Incorporation of nano-size palladium (Pd)significantly increases the catalytic activity of materialsand has been one of the attractive requirements for gen-erating materials for electrochemical sensors design andhas been one of the active areas of material preparation[15–17]. Accordingly it was planned to prepare compositeof Pd�TiO2�SiO2 useful in electrochemical sensing.

We have studied the interaction of palladium chlorideand 3-glycidoxypropyltrimethoxysilane where Pd was re-duced resulting Pd-linked 3-glycidoxypropyltrimethoxysi-lane [18–20] and has been used for making an organicallymodified silicate based electrocatalytic sensors. The reac-tion product of palladium chloride and 3-glycidoxypropyl-trimethoxysilane is found compatible with the reactionproduct of titanium isopropoxide and could be used formaking a nanocomposite of Pd/TiO2/SiO2 through sol-gel

processing. Accordingly it was planned to prepare a com-posite of hydrolyzed reaction product of titanium iso-propoxide and Pd-linked 3-glycidoxypropyltrimethoxysi-lane via sonication and calcination at optimum tempera-ture that may form a Pd/TiO2/SiO2 nanocomposite suita-ble for designing a modified electrode as such materialmay facilitate electrocatalysis while used in electrochemi-cal sensing which has been undertaken in the present in-vestigation.

The biocompatibility and electrocatalysis of the Pd�TiO2�SiO2 nanocomposite has directed our attention tounderstand the electroanalysis of a biologically active an-alyte. Since the determination of ascorbic acid (AA) inbiological fluids using electrochemical sensors is the sub-ject of wide interest [21–23] due to its consumption on alarge scale as an antioxidant in food, beverages and inmedicines [24,25], it was planned to understand the elec-trocatalysis of ascorbic acid at a Pd�TiO2�SiO2 modifiedelectrode which is reported in the present investigation.

2 Experimental

2.1 Materials

All reagents used were of analytical grade and used with-out further purification including titanium isopropoxide(Aldrich), PdCl2 (Aldrich), 3-glycidoxypropyltrimethoxy-silane (United Chem. Technol. Inc. Pertach), isopropanol[Thomas Baker (Chemical) Ltd. Mumbai], HNO3 (Merk)and l-ascorbic acid (Sisco Research Laboratories Pvt.Ltd. Mumbai), graphite powder (particle size 1–2 mm) and

Electroanalysis 2011, 23, No. 8, 1991 – 1997 � 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 1991

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Nujol oil (density 0.838) were obtained from AldrichChemical Co. The water used in experiments is doubledistilled-deionized water (Alga water purificationsystem). All electrochemical experiment were conductedin 0.1 M phosphate buffer solution (pH 7.0) containing0.5 M KCl.

2.2 Equipments

Cyclic voltammetry and amperometry were performed onan electrochemical workstation CHI 660B (CH Instru-ments, USA) in a three-electrode cell configuration witha working volume of 3 mL. An Ag/AgCl electrode (3 MKCl saturated with Ag/AgCl) and a platinum plate elec-trode served as reference and counter electrode, respec-tively. All potentials given below were relative to the Ag/AgCl. The working electrode was a graphite paste elec-trode (GpE). Before each experiment the electrode wascleaned by acetone.

2.3 Preparation of the Pd�TiO2�SiO2 Nanocomposite

Preparation of TiO2 : Titanium isopropoxide was dilutedin isopropanol and added in 3 M HNO3 dropwise and letto be peptized 24 hours. The precipitate (titanic acid gel)was collected by filtration and washed with double dis-tilled-deionized water three times to remove the isopro-panol solvent. The obtained gel was dried at 120 8C fol-lowed by calcination at temperatures of 600 and 900 8Cfor 3 h [26].

Preparation of Pd�TiO2�SiO2 : 0.5 M solution of titani-um isopropoxide diluted in isopropanol was added drop-wise to 3 M HNO3 aqueous solutions. The resulting tita-nia sol (500 mL) was mixed with 75 mL of palladiumlinked 3-glycidoxypropyltrimethoxysilane and stirred for10 min followed by ultrasonication for 30 min. Palladiumlinked 3-glycidoxypropyltrimethoxysilane was made byadding 25 mL aqueous solution of palladium chloride(1 mg/mL) in 50 mL of 3-glycidoxypropyltrimethoxysilane.The resulting material was washed several times toremove the solvent and dried at 120 8C. The gel was calci-nated at 600 and 900 8C for 3 h.

2.4 Preparation of the Modified Electrode

The graphite paste electrode body used for the construc-tion of the modified electrode was obtained from Bioana-lytical Systems (West Lafayette, IN; (MF 2010)). For

modification of the electrode the well was filled with anactive paste of the composition given in Table 1 (Paste-1,Paste-2 and Paste-3). The desired amount of TiO2 andPd�TiO2�SiO2 nanocomposites was thoroughly mixedwith graphite powder (particle size 1–2 mm) in a blenderfollowed by addition of nujol oil. After homogenizationthe mixture was stored into a stoppered glass vial at roomtemperature when not in use. The paste surface was man-ually smoothened on a clean butter paper.

2.5 Characterization

Transmission electron microscopy (TEM) studies wereperformed using Hitachi 800 and 8100 electron micro-scopes (Tokyo, Japan) with an acceleration voltage of200 kV. Energy dispersive spectroscopic (EDS) analysiswas conducted with ZEISS SUPRA 40 (oxford) operatingat 20 keV to perform the quantitative analysis of materi-al. The X-ray powder diffraction patterns were obtainedon Rigaku miniflex diffractometer using nickel filteredCu Ka (l=0.15406 nm) radiation. Identification of thephase was made with the help of the JCPDS files. FTIRSpectra were performed on Perkin Elmer spectrum 100 inthe range of 400 cm�1 to 4000 cm�1. UV-visible spectrawere obtained through Ocean optics HR 2000 spectro-photometer assembled with DT 1000 CE power source.

3 Result and Discussion

3.1 Morphology and Chemical Nature of the Pd�TiO2�SiO2 Nanocomposite

The morphology of Pd�TiO2�SiO2 nanoparticles was ana-lysed by transmission electron microscopy (TEM). Char-acterisation shows the microstructure of Pd�TiO2�SiO2

nanoparticles calcinated at 900 8C. It can be seen that theaverage size of Pd�TiO2�SiO2 spherical nanoparticles wasabout 100 nm (Figure 1b) and TEM of TiO2 nanoparticlescalcinated at 900 8C shows an average particle size of50 nm (Figure 1a). The shape of TiO2 nanoparticles showsspherical geometry, however agglomeration of nanoparti-cles occur. The agglomeration decreases due to the pres-ence of SiO2 matrix [15] in Pd�TiO2�SiO2 nanoparticlesand shows well-ordered spherical geometry. Pd�TiO2�SiO2 nanoparticles showed the particular performance oflarge surface-to-volume ratio which was also confirmedfrom TEM. The chemical nature of material represented

Table 1. Composition of graphite paste electrodes. Paste-1: graphite paste bare; Paste-2: grapite paste with TiO2; Paste-3: graphitepaste with Pd�TiO2�SiO2 nanocomposite.

Systems TiO2/TiO2�Pd�SiO2

(% w/w)Graphite powder(% w/w)

Mineral oil(% w/w)

Paste-1 – 70 30Paste-2 1 69 30Paste-3 1 69 30

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in EDS and described in Table 2 from elemental compo-sition of the composite material have Pd 0.46% w/w.

3.2 Structure Characterization

3.2.1 XRD Analysis

XRD was used for the identification of the crystallinestructure of the TiO2 and Pd�TiO2�SiO2 nanocomposites.The gel of TiO2 calcined at 600 8C exhibited well knownpeaks at 2q=25.228, 37.788, 47.828, 53.828, 54.868, 62.58which is denoted to the (101), (004), (200), (105), (211),

(204) planes of anatase TiO2 as reported [27], and at900 8C peaks at 2q=278, 368, 41.58, which are denoted tothe (110), (101), (111), planes of rutile TiO2. The gel ofPd�TiO2�SiO2 nanocomposite calcined at 600 8C showsamorphous nature of the composite material and at900 8C exhibited all the peaks of anatase but the peaks ofthe rutile phase of TiO2 were diminished and representthe phase transformation of the composite material. Thecharacteristic peaks of Pd nanoparticles at 40.18, 46.68and 68.18, which corresponded to the (111), (110) and(100) crystalline plane of Pd, were merged with the char-acteristic peaks of TiO2. The XRD represent the sameamorphous nature of the composite material due to thepresence of SiO2 content.

3.2.2 IR Spectra

IR spectra represent characteristics peak of crystallineTiO2 (anatase or rutile) at 768 cm�1. IR spectra of Pd�TiO2�SiO2 composite represent the peaks at 815 cm�1 cor-responding to symmetrical stretching vibration of theSiO2, Ti�O stretching in TiO2 correspond to peak at713 cm�1 and vibration of the Si�O�Ti and Si�OH at952 cm�1 [28]. The Pd�TiO2�SiO2 composite was madehaving similar ratio of TiO2 and SiO2, the correspondingpeaks at 952 and 1026 cm�1 represent equal compositionand is confirmed by EDS data (Table 2). The peaks at1600 and 3400 cm�1 correspond to bending vibration ofadsorbed H2O molecule.

3.2.3 UV-Visible Spectra

A UV-visible spectrum of TiO2 represents transparencybetween 365 nm to 800 nm. Absorption below 370 nm isdue to the excitation of electrons from the valence bandto the conduction band of TiO2. Pure anatase crystallinepowder has an intense absorption band at 335 nm [29].Absorbance of TiO2 represents total transparency be-tween 368–800 nm whereas the composite shows absorb-ance within same range of wavelength due the presenceof Pd and Pd2+. The spectrum consists of a linear increas-ing absorption at decreasing wavelength [30].

3.3 Electrocatalytic Oxidation of Ascorbic Acid over theModified Electrode

3.3.1 Cyclic Voltammetry

The nanocomposite of Pd�TiO2�SiO2 does not show re-versible redox electrochemistery in buffer, however, thematerial acts as an efficient electrocatalyst for several an-alytes of biological significance. As an example we inves-tigated the sensing ability of the Pd�TiO2�SiO2 nanocom-posite modified graphite paste (Paste-3) electrode to-wards AA and compared with bare (Paste-1) and onlyTiO2 modified GpE (Paste-2). The voltammograms re-corded for these three modified electrodes are shown inFigure 2A in the absence (a) and on the addition of

Table 2. EDS data of Pd�TiO2�SiO2 nanocomposite.

Element Weight (%) Atomic (%)

O K 59.18 73.61Si K 21.52 15.25Ti K 27.00 11.22Pd L 0.46 0.06Totals 108.16

Fig. 1. TEM image of TiO2 (a) and Pd�TiO2�SiO2 (b) nano-composite calcinated at 900 8C.

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Synthesis of Pd�TiO2�SiO2 Nanocomposite


1 mM ascorbic acid (b). The values of peak currents inthe presence of 1 mM ascorbic acid are found to be 3.65,9.93 and 11.38 mA respectively for Paste-1, Paste-2 andPaste-3 respectively. The occurrence of remarkable varia-tion in peak current values justifies the introduction ofelectrocatalysis of titanium oxide nanoparticle and Pd�TiO2�SiO2 nanocomposite. In the above electrocatalysissurface reaction takes place where Pd enhances the cata-lytic nature of TiO2. We studied the electrochemistry ofPd�TiO2�SiO2 and pure Pd electrodes in 1 M H2SO4 and

the results are shown in Figure 3 a,b. Figure 3a shows thatof the pure Pd whereas Figure 3b shows the cyclic voltam-mogram (CV) of Pd�TiO2�SiO2. In both cases CV wasperformed over a carbon paste electrode containing 1 %of the respective materials as modifiers. The voltammo-grams (Figure 3a, b) justify that CV of pure Pd shows awell defined redox couple of oxygen adsorption (at~0.538 V) and stripping (at ~0.418 V) during anodic andcathodic scans respectively. These results are consistentwith that reported for electrodeposited Pd in H2SO4 [31].

Fig. 2. A) Cyclic voltammograms of bare GpE (1a), TiO2 modified electrode (2a), Pd�TiO2�SiO2 modified electrode (3a) in 0.1 Mphosphate buffer, pH 7.0 at 25 8C and their response in 1 mM ascorbic acid 1b, 2b,3b respectively. B) Cyclic voltammograms of barePd�TiO2�SiO2 modified electrode (a) in 0.1 M phosphate buffer, pH 7.0 at 25 8C and their response in 1 mM ascorbic acid (b) and re-sponse in 1 mM uric acid (c).




The CV of Pd�TiO2�SiO2 (Figure 3b) also shows redoxbehaviour at identical potential values as that in Figure 3abut this is not as prominent as that of the former whichmay be due to the interaction of Pd with TiO2 and SiO2

followed by composite formation. However, in both thecases the redox process observed is extremely sensitive topH of the medium and disappears at neutral or evenlower acidic pH. We did not observe this redox process atpH 7 which was the working medium for the analysis ofAA.

3.3.2 Amperometry

In order to study the quantitative determination of AA,the amperometric responses curve for the three systemsof the modified graphite paste electrode were recordedon the addition of varying concentrations (1 mM to1 mM) of AA in phosphate buffer (0.1 M, pH 7.0) atworking potential of 0.1 V vs. Ag/AgCl (Figure 2a). Itshould be noted that at such an operating potential(0.1 V) the interfering analyte like uric acid does not un-dergo electrochemical oxidation and its contributionduring amperometric sensing of ascorbic acid is negligible(Figure 2B). It is very clear from Figure 4A that the re-sponse was highest in case of Paste-3 followed by Paste-2and Paste-1 again suggesting the excellent electrocatalyticbehaviour of composite material towards the oxidation ofAA. The calibration curves for AA detection by amper-ometry at graphite paste electrodes modified with Paste-1, Paste-2 and Paste-3 were constructed using averagecurrents recorded at three individual electrodes for eachconcentration point. Figure 4B shows the calibrationcurves for AA for Paste-1 (curve a), Paste-2 (curve b)

and Paste-3 (curve c). The sensitivities towards AA wasfound to be 0.19 mA/mM for Paste-1 (curve a), 0.54 mA/mM for Paste-2 (curve b) and 2.72 mA/mM for Paste-3(curve c) modified electrodes. The Figure 4B also showsthe linear range for AA detection from 1 mM–1 mM.Moreover, the Pd�TiO2�SiO2 nanocomposite sensor ex-hibited a linear dependence on AA concentration (R2 =0.9993) with a linear range of 1 mM–1 mM. A comparisonon the analytical performance of the material is given inTable 3 and justifies the advantages of the present nano-composite in electrocatalysis.

3.3.3 Stability and Reproducibility

The reproducibility of the electrode was examined bycyclic voltammetry of AA at the surface of the Pd�TiO2�SiO2 electrode (Paste-3) after twenty repetition cycles at10 mV s�1. Results show that the oxidation peak potentialof AA was not changed and the anodic peak current wasdecreased by less than 2.8%. The storage stability of thesensor was evaluated over a period of 30 days by storingthe sensor in phosphate buffer (0.1 M, pH 7.0) at roomtemperature. No obvious amperometric changes were ob-served after storing. It retained 91.6 % of its initial cur-rent after 30 days storage.

4 Conclusions

We report herein a novel method for the preparation ofPd�TiO2�SiO2 nanocomposite having a particle size ofapproximate 100 nm with spherical morphology. Phasetransform of TiO2 (rutile to anatase) at 900 8C in compo-

Fig. 3. Cyclic voltammograms of pure Pd (a), Pd�TiO2�SiO2 (b) modified electrode in 1 M H2SO4.




site material preparation has been observed forming Pd�TiO2�SiO2 nanocomposite. The resulting composite mate-rial showed excellent electrocatalytic activity for the oxi-dation of the biologically significant analytes. As an ex-ample the electrocatalytic oxidation of AA is reported.The modified electrode exhibit a good sensitivity towardsAA (2.7 mA/mM) with a linear range of 1 mM–1 mM.

Acknowledgement

Authors are thankful to UGC, New Delhi for financialsupport.

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