Characterisation of nanostructured silver orthophosphate

October 2002

Materials Letters 56 (2002) 386 – 392 www.elsevier.com/locate/matlet

Characterisation of nanostructured silver orthophosphate Marykutty Thomas a, S.K. Ghosh b, K.C. George c,* a

Department of Physics, B.C.M. College, Kottayam 686 001, Kerala, India b Structural Ceramics Division, RRL, Trivandrum 695 019, Kerala, India c Department of Physics, S.B. College, Changanacheri 686 101, Kerala, India Received 14 August 2001; received in revised form 20 August 2001; accepted 12 December 2001

Abstract Fractal aggregates of nanoparticle silver orthophosphate were prepared by chemical precipitation. Structural characterisation was done using XRD, FTIR and TEM. Material characterisation is done by studying the dielectric behaviour at low frequency (100 Hz – 10 MHz). It is found that nano-Ag3PO4 is a dielectric relaxor material of high dielectric constant. The effects of reactant concentration on cluster morphology and dielectric behaviour were also under investigation. D 2002 Elsevier Science B.V. All rights reserved. PACS: 61.46; 61.43H; 77.22.G Keywords: Silver orthophosphate; Fractals; Nanoparticle; Reactant concentration; Dielectric relaxor

1. Introduction Nanophase materials and ultrafine particles have caught great attention during the last decade [1 – 4]. The production of ultrafine particles is nowadays one of the most important challenges of new technologies. These materials have properties that are often significantly different and considerably improved relative to those of their coarser grained counterparts [3,4]. Today, phosphate ceramics are getting great attention due to the variety of applications in electrical, optical, prosthetics, structural, etc. fields [5]. Also, transition metal phosphates are important since they are used as inorganic and biomaterials finding appli-

*

Corresponding author. Tel.: +91-481-428246; fax: +91-481411472. E-mail address: [email protected] (K.C. George).

cation as catalysts, ion exchangers and in low thermal expansion ceramic materials [6]. It has been proved that phosphate ceramics are more advantageous compared to silicate and some other systems that are used as hosts of impurity doped optical materials [7,8]. Cutroni et al. [9,10] have conducted studies on ionic conduction and dielectric behaviour of silver phosphate glasses. As far as the authors know, studies on nanoparticle silver phosphate have not been reported. Hence, it would be interesting to study the characterisation of nano-Ag3PO4. One of the conventional methods of metal phosphate preparation is chemical precipitation, which has the advantage of improved compositional homogeneity since the reactant constituents are mixed at a molecular level [6,11]. The reactant concentration has an obvious effect on the size of the particles prepared and its morphology [12]. Aggregation of particles is a major topic of concern in the chemical synthesis, which dramatically alter the

0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X ( 0 2 ) 0 0 4 9 6 - 2

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properties of the materials. Recent developments in the aggregation field have made it feasible to quantitatively characterise the aggregates, despite their very random and disordered appearance [13]. The random, tenuous clusters that are produced when colloids aggregate can be termed as fractals [14]. TEM imaging of the powder samples is the most direct and convenient method to see and analyse the structure of aggregates, and it has been shown that the fractal dimension obtained by TEM analysis corresponds to that of the clusters formed in the as prepared state from the solution [15]. The fractal structure, which is less dense and porous, has a considerable role in determining the physical, mechanical, electrical, etc. properties of a material. The morphology and, in particular, the connectivity of random networks influence both the mechanical restoring forces and the transport properties [13]. It has been shown that alternating current response as a function of frequency gives valuable information about the dynamic response of the system and makes it possible to characterise many materials [16]. Hence, it would be interesting to investigate the dielectric behaviour of fractal structured nano-Ag3PO4. In this article, we report the synthesis of nanoAg3PO4 for two reactant concentrations, their structural characterisation using XRD, FTIR and TEM imaging. The dielectric behaviour at low frequencies was studied which may be useful for the better understanding of their physical properties and in turn useful for the devices.

2. Experimental Fine powders of Ag3PO4 were prepared by chemical reaction between AR grade silver nitrate and disodium hydrogen phosphate [17]. For colloidal precipitation, the concentration of the reactants should be less than 0.05 mol l 1. Here, we prepared the precipitate of Ag3PO4 for two different concentrations (0.005 and 0.015 mol l 1), then washed a number of times using distilled water, filtered, dried in an oven at 100 jC for 3 h. Since Ag3PO4 is light-sensitive, care should be taken to keep it safe against light radiations. The finely prepared powder is used for the study of XRD, FTIR and TEM analyses. XRD profiles were taken by Philips (1710 PW) powder X-ray diffrac-

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tometer using Cu Ka radiation over a wide range of Bragg angle. The FTIR spectrum was taken by Nicolet Avatar 360-ESP spectrometer. TEM imaging was carried out in a Philips CM-200-Analytical Transmission electron microscope working at 120 kV. The powder samples were supported on conventional carbon-coated film on copper grid. For dielectric measurements, the powder is made into pellets of diameter 12 mm and thickness 1 – 2 mm by applying pressure using a hydraulic press. The pellets were annealed at 350 jC for 3 h and both the faces were coated with air drying silver paste for dielectric measurements, and the studies were carried out using 4192 A LF impedance analyser.

3. Results and discussion Figs. 1 and 2 show the XRD patterns of 0.005 and 0.015 mol l 1 Ag3PO4 respectively. Both patterns suggest the formation of single phase, crystalline (cubic) compounds (JCPDS, card no.: 6-505). The peaks corresponding to different crystallographic planes against an almost flat base line suggest the formation of polycrystalline compounds. When the molar concentration is changed, there is no significant change in the crystalline nature as is evident from the XRD patterns. The FTIR spectrum of 0.005 and 0.015 mol l 1 are shown in Fig. 3a and b, respectively. For 0.005 and 0.015 mol l 1, an increase in frequency of 42 and 35 cm 1, respectively, compared with bulk Ag3PO4 frequency 975 cm 1 (Fig. 3c) are observed. Increase in frequency (blue shift) is a general phenomenon observed in nanostructured materials due to its small size effect [18,19]. Also, the frequency shift is higher for low molarity sample, indicating smaller size for 0.005 mol l 1 particles. For nanoparticles, small changes in the environment of a chemical group (PO43) will lead to small changes in the characteristic vibrational frequencies for this group [19]. The morphology of the nanocrystalline Ag3PO4 was studied by TEM and is shown in Figs. 4 and 5. The TEM micrograph shows that the clusters are in the form of fractal aggregates, which may be formed due to diffusion-limited aggregation (DLA) or reaction-limited aggregation (RLA). The fractal aggregates of 0.015 mol l 1 (Fig. 5) is more open and

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Fig. 1. XRD pattern of 0.005 mol l

less dense falling in the DLA regime whereas 0.005 mol l 1 (Fig. 4) is more dense falling in the RLA regime [13,20]. In the DLA regime, the aggregation rate is maximum and the reaction rate is solely determined by the time needed for the clusters to encounter each other by diffusion [13]. Hence, in the case of 0.015 mol l 1 which is more concentrated, the

1

Ag3PO4.

clusters find it more easy to come close to each other, take less time and the DLA regime results. For 0.005 mol l 1 (Fig. 4) which is less concentrated, the slow process, RLA regime in which the cluster – cluster repulsion has to be overcome by thermal activation process results [13,20]. From the TEM image, the particle size of low concentrated sample (0.005 mol

Fig. 2. XRD pattern of 0.015 mol l

1

Ag3PO4.

M. Thomas et al. / Materials Letters 56 (2002) 386–392

Fig. 3. FTIR spectrum of (a) 0.005 mol l

1

Ag3PO4, (b) 0.015 mol l

1

Ag3PO4 and (c) bulk Ag3PO4.

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can be omitted since it may be due to electrode polarisation [22]. The dielectric behaviour is controlled by three kinds of polarisation, namely space charge polarisation, rotation direction polarisation and electronic polarisation [21,22]. Since the nanoparticles contain a large number of defects such as dangling bonds, vacancies and micropores at the grain boundaries causing a change of positive and negative space charge distribution in interfaces. When an external electric field is applied, the space charge moves under this field and they are trapped by the defects in interfaces forming lots of dipole moments (space charge polarisation). Therefore, space charge polarisation is an important factor that gives a high value for dielectric constant at low frequencies. The fractal structure also affects the polarization as is evident from Fig. 6. The polarization is maximum for DLA clusters, indicating high value of dielectric constant. The variation of loss tangent with frequency is as

Fig. 4. TEM image of 0.005 mol l 1 Ag3PO4 showing RLA fractal structure and the corresponding electron diffraction pattern.

l 1) is in the range of 10 –15 nm whereas that of high concentrated sample (0.015 mol l 1) is 15 –25 nm. A significant feature noticed in the TEM image analysis of the two samples is that the reactant concentration shows considerable influence on the morphology and size of nanoparticles of Ag3PO4. The well-defined selected area electron diffraction (SAED) pattern shows spotty rings characteristic of polycrystalline pattern, suggesting that the as prepared Ag3PO4 powder is nanocrystalline, supporting the XRD pattern. The dielectric constant (e), loss tangent (tand) and a.c. conductivity (r) of these two samples were measured as a function of frequency (100 Hz – 10 MHz) at room temperature. The variation of e with frequency at room temperature is as shown in Fig. 6. We can observe high values of dielectric constant for these samples, which are characteristic of nanostructured materials [21]. The high value of e below 1 kHz

Fig. 5. TEM image of 0.015 mol l 1 Ag3PO4 showing DLA fractal structure and the corresponding electron diffraction pattern.

M. Thomas et al. / Materials Letters 56 (2002) 386–392

Fig. 6. Variation of dielectric constant with logarithm of frequency.

shown in Fig. 7. There are two peaks, one at 10 kHz and the other at 1 MHz, due to relaxation time for charge transport. Due to this relaxation time, the

Fig. 7. Variation of tangent loss with logarithm of frequency.

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energy loss called tangent loss is maximum at these two frequencies [22]. The peak at 10 kHz may be due to relaxation in space charge polarisation and the peak at 1 MHz is due to ion jump and dipole relaxation losses [22]. In the case of nano-Ag3PO4, the mobile Ag+ ions may be responsible for re-orientational and translational hopping motions based on jump relaxation model [10]. If the relaxation time for an ion jump is s, maximum energy loss occurs for a frequency equal to the jump frequency 1/s [22]. Therefore, here, the ion jump frequency is found to be about 106 Hz (Fig. 7) for RLA clusters and an increase in frequency is observed in DLA clusters at room temperature. It is reported that the region of 103 –106 Hz is important for dielectric applications [22]. The a.c. conductivity against frequency at room temperature (Fig. 8) shows conductivity maximum at 106 Hz, supporting hopping motions of mobile, Ag+ ions and dielectric relaxation in 0.005 mol l 1 sample [22]. It is seen that hopping motions of mobile Ag+ ions is enhanced in DLA clusters which is more porous and less dense, showing a higher value of r. From the studies of dielectric behaviour, we can observe that e, tand and r are maximum for 0.015 mol l 1 samples, showing DLA fractal behaviour. Therefore, we can assume that

Fig. 8. Variation of a.c. conductivity with logarithm of frequency.

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dielectric behaviour depends on particle size, structure of fractal aggregates and morphology. 4. Conclusion Nanosized silver orthophosphate particles have been obtained by chemical method. Here, one of the structures determining factor is reactant concentration. The fractal aggregates formed are due to diffusionlimited aggregation (DLA) or reaction-limited aggregation (RLA). The reactant concentration affects the morphology and particle size as is evidenced from TEM imaging. Nano-Ag3PO4 is found to be a dielectric relaxor material having high value of dielectric constant and the polarisation may be mainly due to ion jump polarisation at 106 Hz and space charge polarisation at 104 Hz. References [1] [2] [3] [4] [5]

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