Electrical transport and optical properties of vanadyl phosphate—polyaniline nanocomposites

ARTICLE IN PRESS

Journal of Physics and Chemistry of Solids 68 (2007) 66–72 www.elsevier.com/locate/jpcs

Electrical transport and optical properties of vanadyl phosphate—polyaniline nanocomposites Sukanta De, Arup Dey, S.K. De Department of Materials Science, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India Received 7 October 2005; received in revised form 7 July 2006; accepted 6 September 2006

Abstract The nanocomposites of conducting polyaniline and layered vanadyl phosphate, VOPO4  2H2 O are synthesized by redox intercalation method. Water content decreases with insertion of polyaniline molecules. In scanning electron micrographs plate like structures are observed for both VOPO4  2H2 O and intercalated nanocomposites. Protonation of polyaniline and interaction with vanadyl phosphate are observed in infrared and UV absorption spectroscopy. Intercalation improves conductivity of pristine vanadyl phosphate. Thermally activated electrical dc conductivity at low temperature shows two distinct slopes around 210 K for both the nanocomposites. The optical band gap of vanadyl phosphate decreases from 4.0 to 3.7 eV due to insertion of polyaniline. r 2006 Elsevier Ltd. All rights reserved. Keywords: A. Nanostructures; A. Polymers; C. Thermogravimetric analysis (TGA); D. Transport properties; D. Optical properties

1. Introduction The assembly of organic molecules within inorganic frame-work is a novel technique to process nanoscale materials [1–3]. The co-operative interaction among their constituents results in a new hybrid inorganic–organic nanocomposite. Inorganic layered compounds are most attractive host systems to synthesize molecules by intercalation method [1,3]. Hydrated vanadyl phosphate, VOPO4  2H2 O is an important layered solid to undergo intercalation reactions with different guest elements [4–8]. In VOPO4  2H2 O, the distorted VO6 octahedra and PO4 tetrahedra are linked through corner sharing to form V–P–O sheets [9–12]. One water molecule is coordinated trans to an axial oxygen site of VO6 octahedra. A second water molecule occupies the space between PO4 tetrahedra of adjacent layers. The layers are held together by hydrogen bonding. A graphical representation about the structure of VOPO4  2H2 O is shown in Fig. 1. Being strongly acidic, the layers assist in accommodating foreign molecules within the host matrix. Such layered materials Corresponding author. Tel.: +91 33 24734971; fax: +91 33 24732805.

E-mail address: [email protected] (S. De). 0022-3697/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2006.09.001

have potential applications in catalysis, rechargeable battery and fuel cell. Vanadium atoms in vanadyl phosphate can be reduced from V5þ to V4þ which leads to redox intercalation. Metal cations are accommodated by redox reaction in the layer of VOPO4  2H2 O [10,13,14]. Various organic molecules such as amine [15], amides [16] and poly(oxyethylene) [6] compounds are incorporated within the interlamellar space of VOPO4  2H2 O. Such molecules are metal coordinated to the host lattice. Conducting polymers are extensively studied for their unique electronic properties. The conductivity of polyaniline can be improved by several orders of magnitude by protonation. Inorganic protonic acids such as HCl, H2 SO4 , H3 PO4 are commonly used to obtain free standing conducting polyaniline [17]. Bronsted acidic property of layered solids such as zirconium phosphate [18,19] and vanadium pentoxide [20,21] also induce polymerization and protonation of aniline. Some preliminary studies on intercalation of polyaniline into the interlamellar spaces of vanadyl phosphate have been reported [4,5,7,22,23]. In all these host compounds, it has been observed that intercalation and polymerization of aniline occurs through redox reaction. The guest polyaniline (PANI) molecules are ordered and are electronically

ARTICLE IN PRESS S. De et al. / Journal of Physics and Chemistry of Solids 68 (2007) 66–72

VO5

V

P

V-O-Player

H2O

PO4

O

H2O

VO6

V-O-Player

Fig. 1. Schematic diagram of the structure of VOPO4  2H2 O.

conducting. The formation of polymer between the two consecutive layers leads to low dimensional physical property. Moreover, the polymer chains are isolated from each other and consequently interchain coupling is almost absent. The redox intercalation introduces the mixed valence behavior of vanadium atoms. Acidic protons of vanadyl phosphate contribute to ionic conduction. Different types of charge carriers originating from vanadyl phosphate and conducting polymer make the heterogenous system more complicated. Electrical and optical properties of vanadyl phosphate—polyaniline nanocomposite have not yet been studied in detail. The present paper reports the electrical transport and optical properties of PANI intercalated vanadyl phosphate nanocomposites. 2. Experimental Dihydrate vanadyl phosphate VOPO4  2H2 O was prepared by refluxing a mixture of V2 O5 (24 g) and concentrated H3 PO4 (136 ml) in a solution of distilled water (580 ml) for 16 h in air [9]. The product was isolated by vacuum filtration, washed with distilled water and ethanol several times. Yellow powder was obtained after drying in air. PANI intercalated vanadyl phosphate nanocomposites were prepared as described previously [7]. Intercalation of aniline monomer was carried out in presence of dry ethanol. In order to prepare polyaniline intercalated VOPO4  2H2 O nanocomposites, 0.05 and 0:1 mol dm3 aniline in 50 ml dry ethanol were poured in 50 ml of dry ethanol containing 5 mmol VOPO4  2H2 O, respectively. The solution was stirred by magnetic stirrer at room temperature for 24 h. The products were filtered, washed with dry ethanol and dried in air at room temperature. Elemental analysis (CHN) was done using Perkin Elmer-2400 Series-II CHN analyzer. The X-ray powder

67

diffraction studies were carried out with Philips Diffractometer (PW 1710) in the range 2–40 using Cu–Ka radiation. Fourier transform infrared (FTIR) spectra were recorded from pressed KBr pellets using a Perkin–Elmer1600 FTIR spectrometer. Thirty two background scans and sample scans were performed for each spectrum. UV study were done using UV-2401PC (Shimadzu, Japan) spectrometer. Thermogravimetric analysis (TGA) was conducted from room temperature to 500 1C at the heating rate of 10  C min1 with a Mettler Toledo Stare System. Scanning electron microscope (SEM) were done on a JEOL-JSM 35CF microscope operated at 20 KV. Samples were pelletized using hydraulic pressure (applied pressure 5 ton). Pellets of 0.1 cm thickness and 0.7854 sq-cm area were used for detail characterization. Low temperature (down to liquid nitrogen temperature) dc conductivity was measured using Keithley 2000 multimeter and Lakeshore DRC - 91CA temperature controller.

3. Results and discussion Vanadium V5þ ions in VOPO4  2H2 O have empty 3d orbital. Electrons transfer within V–OH bonds associated with water molecules give rise to acidic dissociation of V–OH bond at water—vanadium oxygen interface. Aniline monomer is converted into anilinium cation C6 H5 NHþ 3 in contact with acidic VOPO4  2H2 O. This cation is inserted within the layers of vanadyl phosphate via proton exchange. It is polymerized into polyaniline by reducing vanadium ions [5,20]. The chemical compositions and the amount of water of two nanocomposites, ðPANIÞx VOPO4  nH2 O were determined by elemental carbon, hydrogen and nitrogen (CHN) analysis and TGA. The contents of PANI(x) and water(n) are shown in Table 1. The X-ray diffraction (XRD) patterns of crystalline VOPO4  2H2 O, and intercalated nanocomposites are shown in Fig. 2. The first peak indicates (0 0 1) reflections. The first peak in Fig. 2(b) shifts to lower angle which corresponds to the lattice expansion from 7.64 A˚ to 14.12 A˚. For both the nanocomposites the lattice expansion is same. The VOPO4  2H2 O phase with d ¼ 7:64 A˚ is present in nanocomposite P1, which is completely absent in the maximum loading nanocomposite P2. As the molecular dimension of water is about 2.8 A˚, the interlayer expansion of 6.48 A˚ is sufficient to accommodate PANI and water molecules.

Table 1 Amount of aniline intercalated (y) and chemical compositions of ðPANIÞx VOPO4  nH2 O (value of x and n, calculated from CHN data and TGA) Sample

y ðmol dm3 Þ

C(%)

H(%)

N(%)

x

n

P1 P2

0.05 0.1

7.64 9.98

2.24 2.49

1.49 1.93

0.28 0.42

1.71 1.44

ARTICLE IN PRESS S. De et al. / Journal of Physics and Chemistry of Solids 68 (2007) 66–72 (001)

68

(003)

(001) * (001)*

(004)

(003)

(b) (002)*

(001)

intensity (arbitrary)

(002)

(c)

(002)*

(a)

5

10

15

20

25

30

35

40

2θ (degree)

Table 2 FTIR bands ðcm1 Þ with proposed assignment before and after PANI intercalation Before intercalation

After intercalation

Band assignment

678 940; 998

678 900; 963

1090

1073

1606

1600

3000–3600 — —

3000–3600 1240,1306 1495

V–O–P bending V–O stretching of the vanadyl group ðV ¼ OÞ P–O stretching of the PO4 tetrahedral group. H–O–H bending of water molecule O–H stretching of water C–N stretching C–C stretching

1073

900 963

1240

678

1495

1306

1600

Fig. 2. X-ray diffraction pattern of (a)VOPO4  2H2 O, (b) nanocomposite P1 and (c) nanocomposite P2.

4000

678

1606

1090 998 940

absorption (arbitrary)

(b)

(a)

3500

3000

2500

2000

1500

1000

500

wavenumber (cm-1)

Fig. 3. Fourier Transform Infrared (FTIR) spectra of (a) VOPO4  2H2 O and (b) nanocomposite P2.

The FTIR spectra of VOPO4  2H2 O and PANI intercalated nanocomposites are shown in Fig. 3. The absorption peaks as presented in Fig. 3(a) at 678; 940; 998 and 1090 cm1 are the characteristics of VOPO4  2H2 O [9,11]. The bands at 998 and 940 cm1 are assigned to the V–O stretching of the vanadyl group ðV ¼ OÞ. The peak at 1090 cm1 corresponds to P–O stretching vibration of the PO4 tetrahedral group. The band at 678 cm1 can be ascribed to V–O–P bending vibration. The band around 1606 cm1 is due to H–O–H bending vibration of water molecule. A broad band between 3000 and 3600 cm1 arises from O–H stretching of water. The characteristic FTIR bands before and after PANI intercalation are depicted in Table 2. The FTIR spectrum of nanocomposite P2 as presented in Fig. 3(b) reveals the characteristics bands of PANI at 1240; 1306 and 1495 cm1 [24,25]. The bands at 1240; 1306 cm1 originate from C–N stretching and 1495 cm1 is from the stretching vibration of C–C bond. The band at 678 cm1 associated with V–O–P bond remains unchanged upon intercalation. After intercalation

of PANI, the vibrational peaks of VOPO4  2H2 O shift from 940 and 998 cm1 to 900 and 963 cm1 , respectively. Both V ¼ O and V–OH bonds become weak due to the appearance of IR peaks at lower wave numbers. These indicate the interaction of guest PANI within the host. These changes are attributed to the greater number of V4þ centers present in the nanocomposite. The peak at 1600 cm1 becomes broader and a new sharp peak appears at 3644 cm1 for water. The broad band between 3000 and 3600 cm1 due to O–H stretching of water becomes narrower after intercalation of PANI. The thermal stability of VOPO4  2H2 O and the nanocomposites P2 are tested by means of TGA applying heating rates of 10 1C/min and are exhibited in Fig. 4. The weight loss of vanadyl phosphate up to around 200 1C is due to water molecules. The weight loss of P2 is slightly higher than that of vanadyl phosphate. This may due to cummulative losses of water, unreacted monomer and dopant molecule. The weight loss above 200 1C is due to degradation of PANI main chain. The amount of water has been determined from the weight loss as shown in Table 1. The decrease of water content from 2.0 to 1.44 implies that the intercalation of aniline occurs by the replacement of water molecules. SEM images of VOPO4  2H2 O and nanocomposite (P2) are shown in Fig. 5(a) and (b). The nanocomposite exhibits a plate like morphology similar to the VOPO4  2H2 O. This result supports that the layered structure is preserved after intercalation of PANI. The room temperature dc conductivity (sRT ) for parent material VOPO4  2H2 O is 2:0  107 Scm1 . The values of sRT for nanocomposites P1 and P2 are 5:0  106 Scm1 and 1:06  105 Scm1 , respectively. The dc conductivity at room temperature increases by two orders of magnitude for the maximum loading of PANI. Seebeck coefficient [26] and impedance [27] measurements indicate that at room temperature protons are dominant charge carriers in hydrated vanadyl phosphates. The strong acidic character of V–P–O layers leads to the formation of H3 Oþ in the interlayer space. Protons associated with water molecules

ARTICLE IN PRESS S. De et al. / Journal of Physics and Chemistry of Solids 68 (2007) 66–72

standard Nernst–Einstein’s ionic conduction relation

100

Weight (%)

69

DCe2 , kB T

(1)

95

s¼

90

where D is the diffusion constant, C is the concentration of protons, e is electronic charge and kB is Boltzmann’s constant. Both D and C are thermally activated, so s can be described as

85 (a)

80

sT ¼ s0 exp ðE s =kB TÞ,

75 70

(b)

65 50

100

150

200

250

300

350

400

450

500

Temperature (°C) Fig. 4. Thermogravimetric analysis of (a) VOPO4  2H2 O and (b) nanocomposite P2.

(2)

where s0 is the characteristic conductivity and E s is the activation energy of a material. The plot of lnðsTÞ vs 1000=T is shown in Fig. 6 for two samples P1 and P2. The variation of dc conductivity with temperature of the pure VOPO4  2H2 O as shown in the inset of Fig. 6 indicates that the conductivity is much lower than that of nanocomposites. Two distinct slopes for the nanocomposites are found around a certain temperature T S as shown in Table 3. The best fitted parameters for high temperature ðT4T S Þ regimes are shown in Table 3. The corresponding values of s0 are 0.04 and 0:03 Scm1 and E s are 113 and 81 meV of P1 and P2, respectively, for ToT S . Both s0 and E s are quite different below and above T S . The charge carriers have low s0 and E s at low temperature. The most interesting fact is that the crossover temperature T S in the conductivity plot is almost the same in both the compositions. The values of s0 and E s for P1 are much higher than that of P2. Proton conduction is usually described by two mechanisms: Grotthus and Vehicle [28]. The protons ðHþ Þ are transported from one site to another in Grotthus process. In vehicle conduction, the mobile protons are attached to water molecule and diffuse as complex hydronium ðH3 Oþ Þ ions. In this case two mobile species H2 O and H3 Oþ are involved in comparison with single mobile specie in Grotthus mechanism. The cooperative motion of H3 Oþ and the neutral H2 O molecule gives rise to higher -10

ln ( σT) (Scm-1 K)

-6

-1

ln(σT) (Scm K)

-8

-16

3.25

3.50

3.75

4.00

1000/T (K

4.25 -1

4.50

4.75

)

-12

P1 P2

-16 2

occupy well defined V ¼ O sites of oxide layer. The layers become negatively charged to compensate protons. The diffusion process of mobile protons can be described by the

-14

-18

-10

-14

Fig. 5. Scanning electron micrograph (SEM) of (a) VOPO4  2H2 O and (b) nanocomposite P2.

-12

4

6

8

10

12

14

-1

1000/T (K )

Fig. 6. Temperature variation of dc conductivity of two nanocomposites. Inset shows the same for VOPO4  2H2 O.

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Table 3 The value of s0 and activation energy E s for T4T S as defined in Eq. (2) and the hopping energy W for ToT S and the value of c as defined in Eq. (3) Sample

T S ðKÞ

E s ðmeVÞ

W (meV)

so ðScm1Þ

c

P1 P2

210 207

186 129

113 81

1.98 0.47

0.15 0.24

activation energy than Grotthus proton conduction. Proton conduction above T S can be ascribed to vehicle mechanism. The lower activation energy below T S is associated with Grotthus mechanism. The discontinuity in conductivity at about 210 K is consistent with a transition from vehicle mechanism at higher temperature to Grotthus mechanism at lower temperature. Protons are generally less mobile at low temperature and it has been found that the hydrated vanadyl phosphate is a mixed electronic–protonic conductor [27]. Electrons may play important role in conduction at low temperature. Intercalation and polymerization of aniline within the interlayer spaces of VOPO4 occur by simultaneous reduction of V5þ to V4þ ions. In redox intercalation, concentration of V4þ increases with content of polyaniline. The host lattice of the nanocomposite, vanadyl phosphate contains mixed valency of vanadium atoms (V4þ and V5þ ). The electronic conduction within layers occurs between vanadium ions of different valence states. The charge transport in transition metal mixed valence is explained based on polaron theory [30] and the expression for conductivity is   e2 2R W  exp  s ¼ gcð1  cÞ , (3) L kB T RkB T ˚ average where R is the average intersite separation (7 A, V–V distance), T is the temperature, g is the optical–phonon frequency, c ¼ V4þ =ðV5þ þ V4þ Þ is the ratio of the transition-metal (TM) ion concentration in the low valence state to the total TM ion concentration, L is the localization length describing the localized state at each TM ion site and W is the activation energy for the hopping conduction. Assuming the dominant electronic conduction at low temperature, the values of c are determined by fitting Eq. (3) below T S . The best fitted values of L and g are 1 A˚ and 0:9  1013 Sec1 .The estimated value of c as shown in Table 3 increases with intercalation. According to the Mott’s model [30], we can write    R p 1=3 rp ¼ , (4) 2 6 where rp is the polaron radius. The calculated value of the ˚ It is very difficult to distinguish polaron radius is 2:83 A. between protonic and electronic contribution from simply dc measurements as a function of temperature. Intercalation of PANI increases electron concentration and decreases proton content. The higher conductivity of P2

with lower water content is mainly due to the more electrons in the system. The conductivity data are interpreted by considering both protons and electrons as charge carriers at low temperature. Polyaniline can exist in different chemical structures depending on the degree of oxidation and protonation. The insulating emeraldine base consists of equal number of oxidized (imine) and reduced (amine) units. The conductivity of the reduced base can be varied from 1010 to 10 2 Scm1 by protonation at imine ðN ¼Þ sites of polyaniline. The protonation occurs from chemical reaction with protonic acids of the form Hþ M ; M being the counter ion for balancing charge. The electronic configuration of vanadium atom is V5þ ðd0 Þ in vanadyl phosphate. Some electrons transfer from H2 O to the empty 3d orbitals of vanadium due to the strong polarising power of V5þ and Lewis acid properties of water molecules. The water molecules coordinated to V5þ become more acidic, V–OH þ H2 O()V–O þ H3 Oþ . The hydrated phase of VOPO4 acts as Bronsted solid acid which induces spontaneous protonation (doping) of the intercalated PANI. In this case, negatively charged layer of vanadyl phosphate acts as counter ion. Upon protonation, polyaniline is segregated into two crystalline and amorphous regions. The highly oriented PANI chains consist of conducting molecular nano wires along which charge transport occurs. Moreover the charge species associated with polyaniline are p-electrons [17]. The synergetic effects of mixed valency of vanadium atom, surface oxygen vacancy [29] of host VOPO4 and the conducting PANI enhance the electronic conductivity. This is also evident by comparing the conductivity of pure VOPO4  2H2 O as presented in Fig. 6. The UV-vis spectra of VOPO4  2H2 O and two nanocomposites (P1, P2) are shown in Fig. 7. The absorption spectrum of vanadyl phosphate shows a shoulder like structure around 4 eV as shown in Fig. 7. Three main

VOPO4.2H2O P1 P2

α (arbitrary unit)

70

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

h ν (eV)

Fig. 7. UV–VIS spectra of VOPO4  2H2 O and two nanocomposites.

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decreases due to the decrease of water content with intercalation. The formation of higher conducting polymer results in nanosized conducting path in the host solid. Arrhenius type behavior in conductivity is consistent with proton and electron conduction in transition metal mixed valence system. The structural rearrangement in the host lattice may lead to different activation energy. The intercalation of polyaniline reduces the optical band gap of vanadyl phosphate.

0.10

2

(αhν) (arbitrary unit)

0.08

71

P2

0.06

0.04

0.02

Acknowledgements

0.00 1.0

1.5

2.0

2.5

3.0

3.5

4.0

hν (eV)

Fig. 8. ðahnÞ2 versus hn for the nanocomposite P2.

features are found in P1 and P2 between 2 and 4 eV. The absorption coefficient a of the semiconductor can be described as [31] m

ahn ¼ Aðhn  E g Þ ,

(5)

where photon energy is hn, h being the plank constant and E g is optical band gap. Fig. 8 shows the relation between ðahnÞ2 and hn for the nanocomposite P2. This gives two straight line portions and the extrapolation of these two lines at ðahnÞ2 ¼ 0 yield the so-called optical band gaps. In this case m ¼ 1=2, so the interband transition is allowed direct. Absorption bands at 3.8 eV (325 nm) and 2.0 eV (625 nm) are found for p  p transitions of benzenoid and excitonic effect of quinoid rings of polyaniline emeraldine base. The estimated band gaps are 1.5 and 2.4 eV. These two band gaps are attributed to direct transitions from the highest and the second highest valence bands to the lower polaron band of protonated polyaniline [32,33]. The optical band gap of vanadyl phosphate calculated in a similar manner as shown in Fig. 8 is 4.0 eV. The valence and conduction bands of V2 O5 are mainly oxygen p and vanadium d states [34]. The basic difference between V2 O5 and VOPO4 is that one vanadium is replaced by phosphorous atom. The absorption around 4.0 eV may originate from occupied p bands of oxygen and phosphorous to unoccupied d bands of vanadium. The calculated band gap in the nanocomposites reduce to 3.7 eV. The interlayer lattice parameter increases with intercalation of polyaniline. This may lead to decrease in band gap in vanadyl phosphate. 4. Conclusion Significant shifting of V ¼ O and V–OH vibration bands of VOPO4  2H2 O to lower wavenumbers indicate the reduction of vanadium atoms with intercalation. This establishes a strong interaction between the host vanadyl phosphate and the guest polyaniline. Proton conductivity

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