Influence of annealing temperature on optical, anticandidal, photocatalytic and dielectric properties of ZnO/TiO2 nanocomposites Wasi Khan, Suboohi Shervani, Swaleha Naseem, Mohd. Shoeb, J.A. Khan, B.R. Singh, A.H. Naqvi  Abstract— We have successfully synthesized ZnO/TiO2 nanocomposite using a two step solochemical synthesis method. The influence of annealing temperature on microstructural, optical, anticandidal, photocatalytic activities and dielectric properties were investigated. X-ray diffraction (XRD) and scanning electron microscopy (SEM) show the formation of nanocomposite and uniform surface morphology of all samples. The UV-Vis spectra indicate decrease in band gap energy with increase in annealing temperature. The anticandidal activity of ZnO/TiO 2 nanocomposite was evaluated against MDR C. albicans 077. The in-vitro killing assay revealed that the ZnO/TiO2 nanocomposite efficiently inhibit the growth of the C. albicans 077. The nanocomposite also exhibited the photocatalytic activity for the degradation of methyl orange as a function of time at 465 nm wavelength. The electrical behaviour of composite has been studied over a wide range of frequencies at room temperature using complex impedance spectroscopy. The dielectric constants, dielectric loss and ac conductivity (σac) were studied as the function of frequency, which have been explained by ‘Maxwell Wagner Model’. The data reveals that the dielectric constant and loss (tanδ) exhibit the normal dielectric behavior and decreases with the increase in frequency. Keywords—ZnO/TiO2, nanocomposites, SEM, photocatalytic activity, Dielectric properties. cells are promising devices for large scale, economical and environmentally friendly solar energy conversion. Moreover ZnO/TiO 2 nanocomposites have strong physical and chemical interaction. Low cost of precursors, simplicity and high efficiency of its chemical synthesis method encourage scientists to investigate various properties of nano-ZnO/TiO2 composites. ZnO/TiO 2 nanocomposites have strong physical and chemical interaction with adsorbed species and thereby these nano-composites have a variety of applications as gas sensing materials, antistatic films, surface acoustic wave devices, catalysts and anti reflecting coatings in solar cells [15]. Zhang et al. and Chen el al. [16] synthesized TiO2/ZnO through different methods and observed significant enhancement in photo-oxidation of phenol and photocurrent with several orders of magnitude higher than TiO 2 nanoparticles respectively. In this paper we have reported the microstructural, optical, anticandidal, photocatalytic activities and dielectric properties properties of ZnO/TiO2 nanocomposites synthesized using two step solochemical method at three different annealing temperatures 600°C, 800°C and 900°C. II. EXPERIMENTAL DETAILS I. INTRODUCTION Metallic oxides nanocomposites have attracted much attention because of their potential use in electronic, optoelectronic and spintronic devices [1-3]. Particularly zinc oxide are used in applications such as catalyst and additive in many products because of its good optical properties and electrical properties [4,5]. It has the high excitonic binding energy (60 meV) and the wide band gap (3.37 eV) make its great potential for UV light emitting and laser devices at room temperature [6,7]. It is also used in the fabrication of solar cells [8], gas sensors [9,10]. TiO 2 is widely used for water and photocatalytic air purification and other purposes based on decomposition of organic pollutants and photocatalytic oxidation [11,12]. This material can also be used for organic syntheses [13] of solar energy storage and conversion devices [14]. Excitonic solar Wasi Khan, Swaleha Naseem, M. Shoeb, J.A. Khan, B.R. Singh, A.H. Naqvi are with the Centre of Excellence in Materials Science (Nanomaterials), Department of Applied Physics, Faculty of Engineering & Technology, Aligarh Muslim University, Aligarh-202002, India (corresponding author contact no.: +91-9897532717; e-mail: wasiamu@gmail.com). Suboohi Shervani is with Indian Institute of Technology (IIT) Kanpur, India TiO2 sol has been prepared by adding titanium tetra isopropoxide (TTIP) in 40 ml 2-propanol and it is mixed in the solution of 10 ml 2 propanol with 10 ml H2O. ZnO sol has been synthesized by adding 0.5M zinc acetate in to 100 ml distilled water and it is stirred at 50°C. More detail of the sample preparation is given elsewhere [17]. The synthesized nanocomposite was annealed at three different temperatures 600°C, 800°C and 900°C. The morphology and particle size of the synthesized nanocomposite was examined by scanning electron microscopy (SEM) using JEOL JSM-6510LV microscope. UV-visible absorbance was carried out using UV-visible, Perkin Elmer spectrophotometer (model Lambda 35) in the 200-800 nm range. Moreover, the photocatalytic and anticandidal activities of as synthesized ZnO/TiO2 nanocomposite at 900 0C were performed according to the methods describe by the Moradi et al. [18] and Liong et al. [19] respectively. Dielectric properties of the samples were carried out in the frequency range 1 kHz to 1 MHz using LCR meter (model Agilent 4285A). III. RESULTS AND DISCUSSION A. Structural and morphological studies The crystal structure of the ZnO/TiO2 nanocomposites annealed at different temperature was investigated by X-ray diffraction (XRD) as described elsewhere [17]. The peak positions of samples show the anatase and rutile structure of TiO2, and wurzite structure of ZnO which confirmed from the ICDD card No. 820514, 782486 and 800075 respectively. No other impurity phases were detected in the XRD patterns which show the purity of the nanocomposite formation. depends on the type of transition and it may have values1/2, 2, 3/2 and 3 corresponding to the allowed direct, allowed indirect, forbidden direct and forbidden indirect transitions respectively [21]. The value of band gap was determined by extrapolating the straight line portion of (αhν)2 on the Y- axis versus photon energy (hν) on the x-axis gives the value of the optical band gap (Eg) [22]. Figure 1: SEM micrographs of the ZnO/TiO2 nanocomposites annealed at 600 0C, 800 0C and 900 0C. ZnO/TiO2 nanocomposite was characterized by SEM to evaluate the annealing temperature effect on preparation of ZnO/TiO2 nanocomposite. Samples were sputter–coated with gold prior to SEM observation and SEM micrographs shown in Fig. 1. It is obvious that the annealed product of homogeneous precipitation of titanium isopropoxide and zinc acetate consists of approximate spherical particles agglomerated with an average diameter ~ 21 nm for the sample annealed at 600 0C. These particles are formed by spherical nanoparticles conjoined to chains. The ZnO/TiO2 composites are formed with a mixture of single agglomerates of anatase, rutile, zincite or zinc titanium oxide. B. Optical Properties In order to describe the photo-absorption behaviour of the ZnO/TiO2 nanocomposite samples, a certain amount of samples were uniformly dispersed in ethanol, and then their UV–VIS absorption spectra were recorded at room temperature. The absorbance is expected to depend on several factors, such as band gap, oxygen deficiency, surface roughness and impurity centers. The absorption coefficient α can be calculated from the below relationship α = 2.303A/t where A is the absorbance and t is the thickness of the cuvett. It is well known that the theory of optical absorption gives the relationship between absorption coefficients α and photon energy hν for direct allowed transition [20]. The optical band gap of the nanopowder was determined by applying the Tauc relationship given by: αhν= B(hν –Eg )n where hν is the photon energy, α is the Absorption coefficient (α = 4πk/λ; k is the absorption index or absorbance, λ is wavelength in nm), Eg represents energy band gap and B is a constant, n=1/2 for allowed direct band gap. Exponent n Figure 2: UV-VIS spectra of ZnO/TiO 2 nanocomposite at different annealing temperatures and their band gap plots. Fig. 2 shows the UV–VIS absorption spectra of ZnO/TiO2 nanocomposites annealed at different temperatures. It can be seen that all the samples had an extremely strong absorption at the wavelength range from 250 to 300 nm, except a relatively strong absorption in the visible region (~ 400 nm) for ZnO/TiO2. It can be observed from Fig. that the UV–VIS absorption of the ZnO/TiO2 sample at 900°C is redshifted compare to that of 6000C and 8000C. The band gap at 600°C is 3.70 eV, band gap at 800°C is 3.62 eV and band gap at 900 0C is 3.61 eV. It is concluded that on increasing the annealing temperature band gap decreases. Therefore the decrease in band gap in the present study can also be explained on the basis of the increase in the crystallite size which is observed from XRD and SEM results. So, both results co-relate to each other. C. Anticandidal and photocatalysis activities Due to coupled ZnO/TiO2 oxide, generate a heterojunction between ZnO and TiO2 increase the lifespan of photogenerated electron-hole pairs, which increase the generation of reactive oxygen species (ROS), as a resultant enhance the anticandial and photocatalysis activity [23,24]. Therefore, the anticandidal and photocatalytic activities with the nanocomposite composed of two components, ZnO and TiO2 was investigated in this study. Due to multidrug resistance isolates of C. Albicans, necessity to find a new class of anticandidal agent is supreme importance [25]. However, the past record of rapid, widespread emergence of resistance to newly introduced antifungal agents indicates that even new families of antimicrobial agents will have a short life assurance. All the above write causes, need to attention towards nanomaterials, looking for new leads to develop better nano-antimicrobial drugs against multi drug resistant (MDR) C. albicans strains [26,27]. Therefore, in the current study we have assessed the anticandidal activity of ZnO/TiO2nanocomposite against MDR C. albicans077. The in-vitro killing assay revealed that the ZnO/TiO2nanocomposite at 500 µg/mL efficiently inhibit the growth of the C. albicans 077 shown in Fig. 3(a). (A) (c ) O2 e- e- CB CB O2 ._ Zn O TiO 2 VB VB H 2 O / OH . h+ h+ . OH (a ) (B) Control 500 µg/mL D. Dielectric properties The complex dielectric permittivity ε*=ε'–iε″ of ZnO/TiO 2 nanocomposite was measured as a function of frequency at room temperature, where ε' is real part of dielectric constant and describes the stored energy while ε″ is imaginary part of dielectric constant, which describes the dissipated energy. The dielectric constant as a function of frequency is shown in Fig. 4(a). It can be seen from Fig. 4(a) that the dielectric constant decreases with the increase in frequency and becomes almost constant at high frequencies. However, its value decreases with decrease in annealing temperature. This behavior can be explained using Maxwell–Wagner interfacial model [30]. According to this model, a dielectric medium is considered to be composed of double layers, well conducting grains which are separated by poorly conducting or resistive grain boundaries. Under the application of external electric field, the charge carriers can easily migrate the grains but are accumulated at the grain boundaries. This process can produce large polarization and high dielectric constant. The higher value of dielectric constant can also be explained on the basis of interfacial/space charge polarization due to inhomogeneous dielectric structure. (C) (b) Control 1.50 100 1.00 250 0.50 500 100 75 Degradation (%) Absorbance 2.00 25 0.00 270 320 370 420 470 50 520 570 0 100 Wavelength (nm) 250 500 ZnO/TiO2 (µg/mL) Figure 3. ZnO/TiO 2 nanocomposite shows the anticandidal activity and photocatalysis of methyl orange dye through production of reactive oxygen species (ROS). (a) Anticandidal activity against Candida albicans 077 (b) Photocatalysis of methyl orange dye upon UV light exposure for 2h and (c) Plausible ROS production mechanism. Fig. 3(b) displays photocatalytic activity of the ZnO/TiO2nanocomposite for degradtion of MO as a function of time at λ=465nm [28]. The photocatalytic activity of the different concentrations of synthesized nanocomposite was investigated for degradtion of MO (25 µg/mL) in water under UV irradiation in a batch reactor. Figure 3(c) shows that the ZnO/TiO2 composite (500 µg/mL) had the highest photocatalytic degradation efficiency (77.48 %) at 2h exposure under the neutral pH condition (~ pH 7.2) [29]. In brief, the anticandidal and photocatalytic actions of the aqueous ZnO/TiO2 suspension system based on Fig. 3(c) hypothetical model are summarized as follows. / ℎ . + + . + + . + + → . . . + 2 . → → → ℎ → . → + ℎ + \ ℎ Fig. 4. Variation in (a) dielectric constant, and (b) dielectric loss with frequency of ZnO/TiO2 nanocomposites annealed at 600 0C, 800 0C and 900 0C. Loss tangent or loss factor tanδ represents the energy dissipation in the dielectric system. Fig. 4(b) shows the variation in dielectric loss factor with frequency at room temperature. It has been observed that tanδ decreases with the increase in frequency, which may be due to the space charge polarization. The grain boundary resistance Rgb is found to decrease, while capacitance Cgb is observed to increase with temperature. The ac conductivity shows the frequency dependent behaviour. The data reveals that the dielectric constant and tanδ exhibit the normal dielectric behaviour and decreases with the increase in frequency, which has been explained in the light of Maxwell–Wagner model. The pellets were coated on adjacent faces with silver paste, thereby forming parallel plate capacitor geometry. The value of dielectric constant (ε′) is calculated using the formula, as follows: ε′ = where, ε0 is the permittivity of free space, d is the thickness of pellet, A is the cross sectional area of the flat surface of the pellet, Cp is the capacitance of the specimen in Farad (F). The complex dielectric constant (ε″) of the samples was calculated using relation: ε″ = ε′ tanδ where, tanδ (=1/tanθ) is the dielectric loss which is proportional to the loss of energy from the applied field into the sample and is therefore called as dielectric loss. The ac conductivity ( ) of the samples was determined using the relation, as follows: = ε′ε0 ω tanδ where, ω (=2f ) is the angular frequency. The electrical behaviour of composite has been studied over a wide range of frequencies at room temperature using ac technique of complex impedance spectroscopy. This technique is widely used to separate the resistive and capacitive components of electrical parameters and hence provides a clear picture of the features of the material. When the impedance data of materials having capacitive and resistive components is plotted in a complex plane plot it appears in the form of a sequence of semicircles representing electrical phenomenon due to bulk (grain) material, grain boundary, and interfacial phenomenon if any. TABLE I VARIATION IN DIFFERENT ELECTRICAL PARAMETERS AS A FUNCTION OF ANNEALED TEMPERATURE Ann. Temp. (°C) Particle size (nm) 600 20 800 900 C gb ( F) × 10-14 ωgb ×105 τ gb (s) ×106 3.47 115.8 7.854 1.2 35 18.56 111.9 4.712 2.1 41 22.54 0.0273 4.290 2.3 Rgb (Ω) ×105 exhibited by the appearance of semicircular arcs in Nyquist plots. Fig. 5 shows the complex impedance plots (Nyquist plots) of ZnO/TiO2 composites at various temperatures. It is evident that both the samples show single semicircular behaviour, which suggests the predominance of grain boundary resistance over the grain resistance in each sample. In the literature it has been mentioned that the resistivity of a polycrystalline material in general increases with decreasing grain size [32]. Smaller grains imply a larger number of insulating grain boundaries which act as a barrier to the flow of electrons. Smaller grains also imply smaller grain– grain surface contact area and therefore a reduced electron flow. In both samples the grain size is not seen and the grain boundary contribution becomes dominant and grain contribution is not seen. This is the reason for which only single semicircular arc appears in Cole–Cole plots of both samples. In terms of impedance plots, each semicircular arc can be modelled by an equivalent circuit consisting of a resistor (R) and a capacitor (C) connected in parallel [33]. The impedance spectra can be interpreted by the equivalent circuit consisting of series connecting parallel resistance R and capacitance C as shown in Fig. 6. Figure 6: Equivalent circuit representation of impedance plots. The complex impedance of a system can be written as the sum of real and imaginary part, as follows: Z* = Z′ + j Z″, where, Z′ and Z″ are given by the following relations: Figure 5: Nyquist plots of ZnO/TiO2 nanocomposites annealed at 600 0C, 800 0C and 900 0C temperatures. Generally, the grains are effective in high frequency region while the grain boundaries are effective in low frequency region. Thus the semicircle appearing in the high frequency region corresponds to grain contribution while in low frequency region corresponds to the grain boundary contribution [31]. The electrical characteristic of a material is Z′= Z″= ( ( ) ) + + ( ) ( ) where Rg, Rgb, Cg and Cgb are the resistance and capacitance of the grain and grain boundary, respectively, while g andgb are the frequencies at the peaks of the semicircles for grain and grain boundary, respectively. The resistance values are obtained from the circular arc intercepts on Z-axis, while the capacitance values can be derived from the maximum height of the circular arcs. The capacitances and the relaxation times can be calculated for the grain and grain boundary by the expressions as follows: = = , = = and , = = The grain and grain boundary parameters like resistance and capacitance are obtained by analyzing the impedance data using nonlinear least square fitting (NLLS) method, which are shown in Table I. IV. CONCLUSION We have successfully synthesized ZnO/TiO 2 nanocomposite by two step solo chemical synthesis method. For microstructural studies we have used x-ray diffraction and scanning electron microscopy (SEM) techniques. Scanning electron micrographs confirmed the formation of nanocomposite ZnO/TiO2 and exhibited its uniform morphology. Decrease in band gap energy were observed with increase in annealing temperature and it has lowest value (Eg=3.61 eV) for the sample annealed at 900 0C. The in-vitro killing assay revealed that the ZnO/TiO2 nanocomposite efficiently inhibit the growth of the C. albicans 077. The nanocomposite also exhibited the photocatalytic activity for the degradation of methyl orange as a function of time at 465 nm wavelength. The dielectric behaviour of the nanocomposite suggested the dominance of grain boundary resistance. Moreover, the grain boundary resistance was found to increase, while capacitance was observed to decrease with increase in annealing temperature. The ac conductivity shows the frequency dependent behavior. The dielectric constant and loss exhibited normal behavior and decreases with increase in frequency, which has been explained in the light of Maxwell– Wagner model. ACKNOWLEDGMENTS Authors are grateful to the Council of Science & Technology (CST), Govt. of UP, India for financial support in the form of Center of Excellence in Materials Science (Nanomaterials). S. Naseem thanks to UGC New Delhi for the financial support in the form of Maulana Azad National Fellowship (MANF). REFERENCES [1] Fig. 7. Variation in ac conductivity with frequency annealed at different temperatures. [2] It has been observed that grain boundary resistance R gb increases while the capacitance Cgb decreases with temperature. Fig. 7 shows the variation in electrical conductivity with frequency for different temperatures. The ac conductivity increases with the increase in frequency in each sample. Total conductivity of the system is given by: [4] σ= (T) + σ (ω, T) Here, first term on R.H.S is dc conductivity which is independent of frequency. The second term is pure ac conductivity due to the electron hopping between the metal ions. 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