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Journal of Materials Science:Materials in Electronics ISSN 0957-4522 J Mater Sci: Mater ElectronDOI 10.1007/s10854-017-7207-9

Structural and optical properties ofnanosized ZnO/ZnTiO3 compositematerials synthesized by a facilehydrothermal technique

M. Jose, M. Elakiya & S. A. Martin BrittoDhas

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J Mater Sci: Mater Electron DOI 10.1007/s10854-017-7207-9

Structural and optical properties of nanosized ZnO/ZnTiO3 composite materials synthesized by a facile hydrothermal technique

M. Jose1 · M. Elakiya1 · S. A. Martin Britto Dhas1 

Received: 15 March 2017 / Accepted: 23 May 2017 © Springer Science+Business Media New York 2017

and chemical properties and hence have widespread appli-cations in optoelectronic and other related industries. Intensive efforts have been made to synthesis ternary nanocomposites which exhibit superior properties that are otherwise impossible by common binary composites. Recently, increasing attention has been paid to ternary zinc titanates because of their potential applications in many fields such as in paint pigments, gas sensors, catalytic sor-bents for removal of H2S, As, Se and other contaminants from hot coal gases, photocatalytic splitting of water and degradation of organic compounds, as an active anode material in Li-ion batteries, dielectric material for micro-wave devices and for low-temperature co-fired ceramics, DSSC and many more [1–10]. Moreover, in compari-son with pure ZnO, the composite ZnO/zinc titanate has enhanced green emission [11]. In addition, the coexistence of ZnO and Zn2TiO4 in ZnO/Zn2TiO4 core/shell nanostruc-tures are reported to be better catalysts in photocatalytic degradation of air pollutants [12, 13].

Zinc titanate manifests itself in three forms such as zinc orthotitanate (Zn2TiO4) with cubic spinel structure, zinc metatitanate (ZnTiO3) with rhombohedral ilmenite structure and Zn2Ti3O8 with cubic defect spinel structure. Zn2TiO4 has an fcc lattice wherein the tetrahedral sites are occupied by half of the Zn cations while the octahe-dral sites are filled by a stoichiometric mixing of Zn and Ti cations randomly with 56 atoms per unit cell contain-ing 32 oxygen atoms. Zn2Ti3O8 is a metastable form of ZnTiO3 which exist below 800 °C, while ZnTiO3 decom-poses to Zn2TiO4 and rutile TiO2 above 945 °C [14]. Opti-mising the experimental conditions and maintaining proper temperature is a prerequisite for the synthesis of zinc titan-ate in order to harness their potential applications. Quite a few methods such as the conventional solid-state reaction, ball-milling and sol–gel methods have been adopted for the

Abstract We report the structural and optical proper-ties of ZnO/ZnTiO3 nanocomposites synthesized at vari-ous calcination temperatures ranging from 500 to 900 °C by a simple hydrothermal process without using any dis-persant agents. The XRD results reveal the coexistence of ZnO and ZnTiO3 phases when calcined at 600–900 °C, however, only pure ZnO phase appears when calcined at 500 °C for 1 h. Functional groups were identified by FTIR spectroscopic technique, which exhibit characteristic infra-red absorption bands of ZnO/ZnTiO3 nanocomposites. The synthesized composites have absorbance at a wavelength of 300 and 370 nm and the band gap energy of the ZnO/ZnTiO3 nanocomposite system is tuned by varying the cal-cination temperature. The morphology of the particles were visualized by TEM analysis which shows the morphology changing from irregular particles to uniform spherical, rod and cubic structures while increasing the temperature from 500 to 900 °C. All the samples show interesting broad blue and green emission at about 490 and 530 nm. Incidentally, the emission intensity of the composite ZnO/ZnTiO3 phase is stronger than that of pure ZnO phase prepared at 700 and 900 °C.

1 Introduction

In the recent past, metal oxide semiconductor nanostruc-tures have attracted considerable attention of the research-ers due to their stupendous magnetic, electrical, optical

* M. Jose mjosh1231@gmail.com; jose@shctpt.edu

1 Department of Physics, Sacred Heart College (Autonomous), Tirupattur, Tamilnadu, India

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preparation of zinc titanate nanostructures; yet these meth-ods have various drawbacks like large particle size and lim-ited chemical homogeneity [5, 15–19].

We attempted hydrothermal route, a well-known tra-ditional wet chemical method for the reasons that this method offered several advantages such as simplicity, cost-efficiency and the capability for large-scale produc-tion. The structural and optical properties of ZnO/ZnTiO3 nanocomposites synthesized at various calcination tem-peratures ranging from 500 to 900 °C have been studied. Powder XRD results show only the pure ZnO phase when the powder is calcined at 500 °C for 1 h. When calcined at 600–900 °C for 1  h, the prepared material possesses both ZnO and ZnTiO3 phases. Interestingly, the number and intensity of diffraction peaks due to ZnTiO3 phase stead-ily increased implying longer calcinations temperature (900 °C) enhances the formation of ZnTiO3 phase. Further-more, we have demonstrated in the ensuing discussion that the band gap energy of the ZnO/ZnTiO3 nanocomposite system can be tuned by varying the calcination tempera-ture which could be useful for different optical applications. TEM images show the interesting morphology evolution from irregular particles to uniform spherical, rod and cubic structure with increasing calcination temperature. The PL analysis shows the emission intensity of the sample hav-ing ZnO/ZnTiO3 phase is stronger than the pure ZnO phase prepared at 700 and 900 °C, however the emission inten-sity of the other samples are moderately higher than that of composite ZnO/ZnTiO3 phase. The results obtained in this study indicate that this material could be useful for poten-tial applications in optoelectronic and nanoscale devices.

2 Experimental

2.1 Preparation of zinc titanate powder

Zinc nitrate hexahydrate, titanium tetrachloride, ammo-nium hydroxide and ethanol were purchased from Sigma Aldrich. Double distilled water was used for the prepara-tion of solutions and washings. Zinc titanatenano particles were prepared with stoichiometric amount of zinc nitrate hexahydrate and titanium tetrachloride taken in 1:1 ratio (0.05 M) by hydrothermal process without the addition of a dispersant agent. 0.2974 g of zinc nitrate hexahydrate and 0.2 ml of titanium tetrachloride were dissolved separately in 20 ml of distilled water at room temperature with vig-orous stirring for 20  min forming a clear solution. Then Zn(NO3)2·6H2O was added slowly to TiCl4 solution and stirred for 15 min to obtain homogeneous solution. The pH of the mixed solution was adjusted to 9 by adding NH4OH aqueous solution and stirring the resulting solution for 2 h at room temperature. Subsequently, the resulting mixture

was kept in 100 ml Teflon-lined stainless steel autoclave at 150 °C for 1 h and cooled naturally to room temperature. After completion of the reaction, the precipitates settled at the bottom of the autoclave were collected and centrifuged at 6000 rpm for 15 min. The obtained precipitates were fil-tered and washed three times with double distilled water and ethanol to eliminate the residual chloride and other ions. The resultant mixtures were separately divided into several parts and calcinated in an electric furnace at various temperatures ranging from 500 to 900 °C in steps of 100 °C for 1 h with a heating rate of 4 °C min−1, and then cooled in the furnace and finally white ZnO/ZnTiO3 precursor pow-ders were obtained.

2.2 Characterization

Powder XRD analysis was carried out using Enraf Non-ius CAD-F diffractometer with CuKa radiation source of wavelength (λ = 1.54056  Å) and the diffraction patterns were recorded by varying diffraction angles in the range of 20°–70°. The chemical-bond types of the prepared samples and molecular structure were investigated and confirmed by FT-IR spectra. FTIR spectral analysis was recorded using the KBr pellet technique on a Bruker Tensor 27 spectrom-eter from 400 to 4000 cm−1 with a total of 30 scans and a resolution of 1 cm−1 was employed in getting the spectra. The optical absorbance spectra was recorded using Perkin-Elmer 1355 UV–Vis spectrophotometer in the wavelength range 200–700 nm at 1 nm s−1 scan rate. The transmission electron microscopy (TEM) analysis was carried out using a Tecnai T20 G2 transmission electron microscope (TEM) and the photoluminescence (PL) emission spectra were recorded using Perkin Elmer 1355 fluorescence spectrom-eter at room temperature.

3 Results and discussion

3.1 Powder XRD analysis

The powder XRD pattern (Fig. 1a) of the prepared powder calcined at 500 °C for 1 h shows the presence of only pure phase ZnO due to the reflections located at (100), (002), (101), (102), (110), (103), (200), (112) and (201) (JCPDS card No 89-0510).

However, the XRD spectrum of the sample calcined at 600 °C (Fig.  1b) reveals the existence of mixed ZnO and ZnTiO3 phases. It is confirmed from the appearance of reflection peaks at (100), (002), (101), (102), (110), (103), (112) and (201) corresponding to ZnO phase and (220), (311), (400) and (422) reflection peaks of the ZnTiO3 phase (JCPDS card No: 39-0190).

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However, the XRD pattern of zinc titanate powder calcined at 700 °C (Fig.  1c) reveals the emergence of the reflection peaks corresponding to ZnTiO3 phase as the major phase due to the peaks located at (220), (311), (400) and (422), nevertheless, the emergence of two more addi-tional peaks located at (440) and (442) shows that the phase content of the ZnTiO3 increases. It is evident from the XRD pattern of the powder calcined at 800 °C for 1 h (Fig. 1d) that the relative intensity of the peaks corresponding to ZnTiO3 phase is increased compared to ZnO peaks. Fur-thermore, the number and intensity of diffraction peaks due to ZnTiO3 phase steadily increased implying longer calci-nations temperature (900 °C) enhances the formation of ZnTiO3 phase. However, there still exist 40% of ZnO phase though the composition was controlled to some extent. The strong and sharp reflection peaks suggest that both products are well crystallized. In addition, the XRD patterns show no significant changes in the 2θ values indicating no differ-ence in the crystal structure for both the phases. The con-tent of the ZnO and ZnTiO3 phases in each XRD pattern are estimated by the following equations [21, 22].

where, IZn (101) and IZT (311) denote the relative integral intensities of (101) and (311) for the phases of ZnO and ZnTiO3 respectively. The percentage of the phase content of

(1)ZnO (%) =IZn(101)

IZT (311) + IZn(101)× 100

(2)ZnTiO3(%) =

IZT (311)

IZn(101) + IZT (311)× 100

ZnO and ZnTiO3 in each sample, calculated using Eqs. (1) and (2) is shown in Fig. 2.

It can be seen that the ZnO content decreases while the ZnTiO3 content increases when the calcination tem-perature is increased. The crystalline size is calculated using the Scherrer equation and the variation of the mean crystallite size of the ZnO/ZnTiO3 nanocompos-ites calcined at various temperatures for 1 h is depicted in Fig. 3. As the calcination temperature is increased from 500 to 900 °C, the crystalline size increases from 16 to 21  nm which could be attributed to decrease in surface area associated with partial diffusion of neighbouring crystallites.

Fig. 1 XRD patterns of ZnO/ZnTiO3 nanocomposites synthesized at pH 9 and calcined at various temperatures (a) 500 °C, (b) 600 °C, (c) 700 °C, (d) 800 °C and (e) 900 °C for 1 h

Fig. 2 Histogram showing the phase content with respect to the dif-ferent calcination temperatures

Fig. 3 The crystallite size of the samples calcined at various temper-atures for 1 h

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3.2 FT‑IR analysis

The Fig. 4a–e shows the FTIR spectra of the ZnO/ZnTiO3 nanocomposites prepared at different calcination tempera-tures (500, 600, 700, 800 and 900 °C) for 1  h. The sharp bands observed in all samples at around 3490  cm−1 were attributed to O–H stretching vibrations coming from the surface OH groups and the peak at 1620 cm−1 corresponds to water molecules adsorbed on the ZnO nanoparticles sur-face. This may be due to the possible adsorption of few water molecules from air as the handling of the nanopow-der was done in the ambient atmosphere.

The weak band observed at 2350  cm−1 is ascribed to the N–H stretching modes. At the same time, we can also observe absorption peaks around 1394 and 1194  cm−1 which correspond to C–OH in-plane-bending and C–OH out-of-plane bending respectively. The appearance of the band at 1123  cm−1 was assigned to the bending mode of Ti–O–C. The absorption bands at 1046 and 910  cm−1 could be attributed to C–O stretching and N=O stretch-ing vibrational modes. The strong absorption peaks in the range 400–750 cm−1 could be attributed to ZnO stretching modes. From the FT-IR spectra, we infer that all the five samples exhibit similar characteristics of infrared absorp-tion bands, which are quite similar to the ZnO/ZnTiO3 nan-oparticles reported in previous literatures [22, 23].

3.3 UV–Vis spectral analysis

The UV–Vis DRS is used to study the effect of different calcination temperatures on light absorption and the DRS spectra of the prepared samples calcinated at different tem-perature 500, 600, 700, 800 and 900 °C are shown in Fig. 5.

The optical characterization of the samples was carried out by measuring their diffuse reflectance R at room temperature. R can be related with absorption Kubelka–Munk function which is proportional to the absorption co-efficient.

In general, it was known that prepared samples have strong absorption peaks in the range 300–400  nm. In the spectra recorded for the synthesised products, strong absorp-tions were observed at wavelengths 311, 299, 303, 308 and 309  nm associated with ZnTiO3 phase [19, 24], while the peaks located at 373, 371, 370, 371 and 373 nm corresponds to ZnO phase [25, 26] for the samples calcinated at 500, 600, 700, 800 and 900 °C respectively. There is a blue or red shift after increasing calcination temperature which may be related

(3)FKM(R) =(1 − R)2

2R

Fig. 4 FT-IR spectra of ZnO/ZnTiO3 nanocomposites synthesized at pH 9 and calcined at various temperatures (a) 500 °C, (b) 600 °C, (c) 700 °C, (d) 800 °C and (e) 900 °C

Fig. 5 UV–Vis diffuse reflectance spectra for the ZnO/ZnTiO3 nanocomposites synthesized at various temperatures (a) 500 °C, (b) 600 °C, (c) 700 °C, (d) 800 °C and (e) 900 °C

Fig. 6 Tauc plot for band gap calculation

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with the decrease or increase in particle size respectively [27]. Further, in this investigation, all the samples are com-pletely transparent above 400 nm making them better candi-dates as good transparent conducting oxide semiconductor for transparent electrodes [28].

The optical band gap energies of the samples [Eg] were estimated by plotting the function fKM= (FKMh�)

2versus pho-ton energy. The optical absorption band gap energy can be determined using the Tauc formula

(4)� =A(h� − Eg)

n

h�

where α is absorption coefficient, h is Planck’s constant and � =

c

� where, c is speed of light, λ is absorption wave-

length, Eg is band gap energy, A is related to the effective masses associated with the valence and conduction bands and n depends on the nature of transition in a semiconduc-tor. For allowed direct transition n =

1

2 and for indirect

transition n = 2. To calculate the band gap values of the prepared samples, (αhν)2 versus (hν) has been plotted and it is shown in Fig. 6. The values of the band gap of these materials were determined by extrapolating the straight line portion of the energy axis at zero absorption. From the above equation, we can see that (αhν)2 has a linear relation-ship with hν. The band gap energy increases slightly with increasing calcination temperatures as shown in Table  1. This result indicates that the band gap energy of the ZnO/ZnTiO3 nanocomposite system can be tuned by varying the calcination temperature for different applications. As shown in the table, the band gap was tuned from 3.14 to 3.20  eV by raising the calcinations temperature from 500 to 900 °C.

Table 1 The effect of calcination temperature on energy band gap of ZnO/ZnTiO3 nanocomposites

Sample Calcination temperature (°C)

Band gap (Eg)

ZnO 500 3.14ZnO/ZnTiO3 600 3.16ZnO/ZnTiO3 700 3.17ZnO/ZnTiO3 800 3.19ZnO/ZnTiO3 900 3.20

Fig. 7 TEM image of the pre-pared nanoparticles calcinated at 500° C

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3.4 TEM analysis

The particle size and the morphology evolution of the ZnO/ZnTiO3 nanoparticles were visualized by transmis-sion electron microscopy (TEM) analysis. Figure  7a–d shows the TEM images of the prepared samples cal-cinated at 500 °C. The morphology of the sample was found to be an aggregation of irregular particles with average particle size of about 65  nm. HR-TEM image (Fig.  7c) shows that the ZnO/ZnTiO3 nanoparticles are highly crystalline with lattice spacing of 0.278 nm corre-sponding to the (100) plane of ZnO. The size distribution is uniform and the crystallinity of the prepared nanoparti-cles seems to be relatively good and the Debye rings indi-cates the polycrystalline nature of the powder.

Figure  8a–d shows that the TEM images of the pre-pared nanoparticles calcinated at 600 °C. Figure  8a, b present a rod like structure which indicates the forma-tion of nanorods by combination of nanoparticles. The particle size estimated by TEM micrographs is about 300 nm. It is likely that particles seen in the micrographs are composed of smaller particles as average crystallite

size calculated by XRD was much smaller than deduced by TEM observations. The lattice fringes can be seen very clearly and the estimated lattice spacing 0.247 and 0.21 nm correspond to the (101) and (400) plane of ZnO and ZnTiO3 phases respectively.

Figure  9a–d shows the formation of well crystallized nanorods and looks similar to ice bars recorded for the sample prepared at 700 °C. Further, the areas of vari-ous dark shades give information about the presence of nanoparticles close to each other. Additional informa-tion about the structures of the nanoparticles was found through detailed analysis with HRTEM. The HRTEM image, Fig.  9c, shows clear lattice fringes with inter-planar distances of d = 0.285  nm and d = 0.253  nm cor-responding to miller indices of (100) and (311) crystal-lographic planes of hexagonal ZnO and cubic ZnTiO3. In addition, the regular succession of the atomic planes indicates that the nanocrystallites are structurally uniform and crystalline with almost no amorphous phase present.

The TEM images recorded for the samples synthesised at 800 °C presents a near uniform spherical nanoparti-cles with the biggest particles measuring 80 nm and the

Fig. 8 TEM image of the pre-pared nanoparticles calcinated at 600 °C

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smallest ones measuring 30  nm with their average size of 50 nm. The SAED pattern reveals the polycrystalline nature of the sample. The d spacing was measured as 0.26 and 0.21 nm from the HR-TEM image (Fig. 10c).

Figure  11 shows the TEM micrographs, SAED pat-terns and HRTEM images of the ZnO/ZnTiO3 nanocom-posites calcined at 900 °C for 1 h. Fig.  11a, b indicate that many nanocrystallites were incorporated and the crystallite size was measured about 55  nm. Figure  11c shows the HR-TEM image which reveals that the (102) and (220) d-spacing corresponding to the phases of ZnO and ZnTiO3. The SAED results of Fig. 11d are agreement with the XRD results.

3.5 Photoluminescence analysis

PL spectra (Fig.  12) were measured from 350 to 600  nm after exciting the samples at 380  nm at ambient condi-tions. It is understood that ZnO and ZnTiO3 nanoparticles

display near band gap emission due to the recombination of free photogenerated electrons and holes while intrinsic and extrinsic structural defects manifest themselves as deep level emission [28].

All the emission spectra display a strong and sharp emis-sion band starting from 470 to 520 nm with a peak maxi-mum at 490 nm corresponding to blue emission, along with a small hump at 510 nm and another relatively less intense broad peak at 530 nm corresponding to green emission. The blue emission is attributed to the electron transition from the shallow donor level of oxygen vacancies to the valence band and electron transition from the shallow donor level of zinc interstitials to the valence band [29]. However, oxy-gen vacancies and other vacancy related defects manifest themselves as a broad green emission band. Nevertheless, the presence of broad peaks showed that the nanocompos-ites are pure with only very less intrinsic defects [30]. It is evident that the emission intensity of the sample hav-ing only ZnO phase is stronger than the composite ZnO/

Fig. 9 TEM image of the pre-pared nanoparticles calcinated at 700 °C

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ZnTiO3 samples prepared at 600 and 800 °C, however the emission intensity of the other samples are stronger than that of ZnO phase. The ZnO/ ZnTiO3 composite materials with enhanced blue and green emission may have potential applications in displaying and lighting devices.

4 Conclusions

In summary, ZnO/ZnTiO3 nanocomposites have been pre-pared through a simple hydrothermal method without using any dispersant agent at different calcinations temperatures. The XRD pattern confirmed the existence of mixed ZnO

and ZnTiO3 phases for the samples calcined above 600 °C. FTIR analysis revealed the characteristic absorption bands of ZnO/ZnTiO3 nanocomposites. It is observed that all the samples under study are transparent above 400 nm and the optical energy gap of the ZnO/ZnTiO3 nanocomposite sys-tem is tuned by changing the calcination temperature. TEM images evidenced the interesting morphology changes of ZnO/ZnTiO3 nanocomposites while increasing calcination temperatures. All the samples exhibited broad blue and green emission centered around 490 and 530  nm respec-tively. The emission intensity of the sample having ZnO/ZnTiO3 phase is stronger than the pure ZnO phase prepared at 700 and 900 °C, though the emission intensity of the

Fig. 10 TEM image of the pre-pared nanoparticles calcinated at 800 °C

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other samples are moderately higher than that of composite ZnO/ZnTiO3 phase.

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