Synthesis and Behavior of Cetyltrimethyl Ammonium Bromide Stabilized Zn1+xSnO3+x (0 ≤ x ≤1) Nano-Crystallites

We report synthesis of cetyltrimethyl ammonium bromide (CTAB) stabilized Zn1+xSnO3+x (0 ≤ x ≤1) nano-crystallites by facile cost-effective wet chemistry route. The X-ray diffraction patterns of as-synthesized powders at the Zn/Sn ratio of 1 exhibited formation of ZnSn(OH)6. Increasing the Zn/Sn ratio further resulted in the precipitation of an additional phase corresponding to Zn(OH)2. The decomposition of these powders at 650°C for 3h led to the formation of the orthorhombic phase of ZnSnO3 and tetragonal SnO2-type phase of Zn2SnO4 at the Zn/Sn ratio of 1 and 2, respectively, with the formation of their mixed phases at intermediate compositions, i.e., at Zn/Sn ratio of 1.25, 1.50 and 1.75, respectively. The lattice parameters of orthorhombic and tetragonal phases were a ~ 3.6203 Å, b ~ 4.2646 Å and c ~ 12.8291Å (for ZnSnO3) and a = b ~ 5.0136 Å and c ~ 3.3055Å (for Zn2SnO4). The transmission electron micrographs revealed the formation of nano-crystallites with aspect ratio ~ 2; the length and thickness being 24, 13 nm (for ZnSnO3) and 47, 22 nm (for Zn2SnO4), respectively. The estimated direct bandgap values for the ZnSnO3 and Zn2SnO4 were found to be 4.21 eV and 4.12 eV, respectively. The ac conductivity values at room temperature (at 10 kHz) for the ZnSnO3 and Zn2SnO4 samples were 8.02 × 10−8 Ω-1 cm-1 and 6.77 × 10−8 Ω-1 cm-1, respectively. The relative permittivity was found to increase with increase in temperature, the room temperature values being 14.24 and 25.22 for the samples ZnSnO3 and Zn2SnO4, respectively. Both the samples, i.e., ZnSnO3 and Zn2SnO4, exhibited low values of loss tangent up to 300 K, the room temperature values being 0.89 and 0.72, respectively. A dye-sensitized solar cell has been fabricated using the optimized sample of zinc stannate photo-anode, i.e., Zn2SnO4. The cyclic voltammetry revealed oxidation and reduction around 0.40 V (current density ~ 11.1 mA/cm2) and 0.57 V (current density– 11.7 mA/cm2) for Zn2SnO4 photo-anode in presence of light.

Multi-cation material provides flexibility to engineer its physical and/ or chemical behavior by varying the composition [21]. The n-type bi-cation transparent conducting oxide such as ZnO-In 2 O 3 has revealed change in its work function, bandgap energy, resistivity and acid etching rate as the function of Zn/In content [22]. The abundance and tunable behavior make these multi-cation compounds interesting for continued research. The ZnSnO 3 , Zn 2 SnO 4 are wide bandgap n-type ternary semiconductor oxides with better corrosion resistance, faster charge injection and faster electron diffusion efficiency than anatase-TiO 2 used in conventional dyesensitized solar cell. The ZnSnO 3 , Zn 2 SnO 4 and/or intermediate mixed nano-crystalline phases are formed depending upon the Zn/Sn molar ratio of precursor compounds [1]. In general ZnSnO 3 precipitates in orthorhombic phase, while Zn 2 SnO 4 precipitates in cubic-spinel -type phase [14,23]. In ZnSnO 3 , Zn 2+ and Sn 4+ cations are distributed at tetrahedral and octahedral sites, respectively, while in Zn 2 SnO 4 , Zn 2+ are distributed equally at tetrahedral and octahedral sites, respectively, while Sn 4+ cations occupy the octahedral sites.
Limited number of studies are available in literature on utilizing Al 2 O 3 , NiO, ZnO and graphene/ TiO 2 for sensitization of perovskite solar cell [24]. The ternary oxides such as BaSnO 3 , ZnSnO 3 and Zn 2 Sn 2 O 4 have rarely been investigated [4,24]. In this paper, we report the synthesis of cetyltrimethyl ammonium bromide (CTAB) stabilized ZnSnO 3 , Zn 2 SnO 4 nano-crystallites and their intermediate compositions by facile cost-effective wet chemistry route. These nano-crystallites have been investigated for their structural, optical, dielectric and ac conductivity behavior. Here, an attempt has been made to synthesize tetragonal SnO 2 -type phase of Zn 2 SnO 4 , which has not normally been reported [25]. Many reports have revealed synthesis of ZnSnO 3 , Zn 2 SnO 4 nanostructures of various shapes, e.g., sphere, cube, anisotropic rods, etc. by the use of mineralizers and additives [16]. In present investigation, the particle size and nanocrystallite-type morphology has been realized using surfactant CTAB, which is vital for high absorption of dye and faster electron transport. The facile wet chemistry route utilized for the synthesis has advantage of precisely controlling thermodynamics and kinetic of nucleation and growth of nano-crystallites, which is otherwise difficult to control with other synthesis routes [16,26].  2 .6H 2 O solutions in desired amounts were mixed together to obtain another clear solution. Afterward, 5 mM solution of CTAB in the metal (Zn 2+ and Sn 2+ ) to CTAB molar ratio of 10:1, was added to the above solution drop-wise. These obtained five solutions were dried in oven at 100°C for 12 h, washed with ethanol afterward and subsequently dried for 6 h at 100°C, washed with ethanol again, and dried once more at 100°C for 6h. Now, these dried powders were decomposed at 650°C for 3h to obtain ZnSnO 3 , Zn 1.25 SnO 3+α, Zn 1.25 SnO 3+β , Zn 1.25 SnO 3+γ , Zn 2 SnO 4 (where α 0.25, β~0.5, γ~0.75) nano-crystallitess. These obtained five samples were coded as S1, S2, S3, S4 and S5, respectively for further investigation. Fig 1 shows the schematic of the synthesis process.

Materials and Methods
The phase(s) of the as-synthesized precursor powders and ZnSnO 3 , Zn 1.25 SnO 3+α, Zn 1.5 SnO 3+β , Zn 1.75 SnO 3+γ , Zn 2 SnO 4 (where α~0.25, β~0.5, γ~0.75) nano-crystallites with general formula Zn 1+x SnO 3+x (0 x 1) were analyzed by Rigaku Miniflex X-ray diffractometer using Cu K α1 radiation of 1.54056 Å at 30 kV and 15 mA. Morphological characterization of samples S1 and S5 was performed by using transmission electron microscopy Hitachi (model H-7500, 120 kV) equipped with CCD Camera. Diffuse reflectance spectra of powders were recorded by UV-Visible spectrometer; PG instruments Pvt. Ltd. T90+ in the spectral range of 300-1100 nm. Photoluminescence spectra were collected in the wavelength range of 330-450 nm with the excitation wavelength of 290 nm using Simadzu RF-530 spectroflurometer. The impedance analysis at different temperatures was performed using potentiogalvanostat, Biologic SP 240, in the frequency range between 100 Hz to 3MHz. For impedance investigation, the nano-crystalline samples were pelletized in 10 mm diameter pellets at a pressure of 5 ton, sintered at 900°C for 6h, polished subsequently to 1.4 mm thickness, and silver pasted afterward. The dye-sensitized solar cell (DSSC) devices were fabricated using zinc stannate photo-anode, and cyclic voltammetry studies were performed using potentiogalvanostat, Biologic SP 240.  6 and Zn(OH) 2 phases are also observed. The Zn(OH) 2 crystallites are presumably oriented in [11] preferred direction as Bragg's reflection planes other than (011) are hardly visible. The observed traces of Zn 2 SnO 4 cubic phase too are not visible in the XRD pattern of final compound (Fig 3,  . The Zn (OH) 4 2reacts with ZnSn(OH) 6 and forms Zn 2 SnO 4 as per the reaction, ZnSn(OH) 6 +Zn (OH) 4 2-! Zn 2 SnO 4 + 4H 2 O + 2OH -. It is obvious that when molar ratio of Zn/Sn is fractional number between 1 and 2, the precipitation of both ZnSnO 3 and Zn 2 SnO 4 phases will occur. The presence of the surfactant possibly alters the surface energy of the crystallites surfaces and, in turn, results in the anisotropic growth of nanoparticles. In the present work, the CTAB, which has been used as a surfactant, plays a pivotal role in monodispersion of the as synthesized nanocrystallites [27]. Fig 3 shows the X-ray diffraction patterns of the ZnSnO 3 , Zn 1.25 SnO 3+α, Zn 1.5 SnO 3+β , Zn 1.75 SnO 3+γ , Zn 2 SnO 4 (where α~0.25, β~0.5, γ~0.75) nano-crystallites obtained by decomposition of the gel product (formed through wet chemistry reaction at 100°C for 18 h) at 650°C for 3h. At Zn/Sn molar ratio of 1, it clearly indicates the formation of orthorhombic type-ZnSnO 3 (JCPDS # 28-1486) with the values of lattice parameters being as a~3.6203 Å, b4

Structural and morphological characterization
.2646 Å and c~12.8291Å. On increasing the Zn/Sn molar ratio to 2, the structure revealed formation of single phase of tetragonal SnO 2 -type Zn 2 SnO 4 (JCPDS # 88-0287), with mixed phases of ZnSnO 3 and Zn 2 SnO 4 at intermediate compositions (i.e., at Zn/Sn molar ratio of 1.25, 1.50, 1.75). The crystallite sizes were estimated using the Bragg relation, i.e., D = 0.9λ/ βcosθ, where D is average crystallite size, λ wavelength of the X-ray used, β, corrected full width at half maximum (FWHM) of the respective peak belonging to diffraction angle 2θ. The estimated values of crystallite size for samples S1, S2, S3, S4 and S5 were found to be 47, 58, 48, 51, and 49 nm, respectively. At Zn/Sn molar ratio of 1, the average crystallite size was 47 nm, which increased to 58 nm at Zn/Sn molar ratio of 1.25. The coordination numbers of Zn 2+ and Sn 4+ ions in orthorhombic ZnSnO 3 are 4 and 6, respectively. Further, the ionic radii of Sn 4+ and Zn 2+ ions in six-coordination are 0.69 Å, and 0.74 Å, respectively. Obviously, the ionic radius of Zn 2+ is higher than Sn 4+ , therefore on occupying Sn 4+ lattice sites in the ZnSnO 3 crystal, it will put crystal under tensile stress, which, in turn, will give rise to increase crystallite size  [28][29]. On increasing the Zn/Sn molar ratio to 1.5, the precipitation of secondary phase, i.e., Zn 2 SnO 4 becomes significant to inhibit the growth of primary phase crystallites, i.e., ZnSnO 3 . Due to this reason the crystallite size of sample S3 has been found to be smaller than the sample S2. At Zn/Sn molar ratio of 1.75, i.e., for sample S4, the size increased slightly in comparison to sample S3, which may be due to the fact that sample S4 at Zn/Sn molar ratio of 1.75 will have higher amount of tetragonal Zn 2 SnO 4 phase formation than sample S3. In contrary to sample S2, where primary phase was orthorhombic type-ZnSnO 3 , in this case, the primary phase is Zn 2 SnO 4 . As discussed before, in case of sample S2, the lower amount of Zn 2 SnO 4 phase was not able to inhibit the growth of ZnSnO 3 . Similarly in present case the lower amount of ZnSnO 3 phase is not able to inhibit the growth of Zn 2 SnO 4 crystallites, and, in turn, exhibit increased crystallite size than sample S3. Further increase in Zn/Sn molar ratio (Zn/Sn = 2) reveals a slight decrease in crystallite size (~49 nm), possibly due to inherent oxygen vacancies in Zn 2 SnO 4 , which, put crystal under compressive stress. The values of lattice parameters for tetragonal Zn 2 SnO 4 were as a = b~5.0136 Å and c~3.3055Å. Fig 4 shows transmission electron micrographs of ZnSnO 3 and Zn 2 SnO 4 samples, i.e., for samples S1 and S5. The sample S1 exhibits nano-rod type morphology with average aspect ratio of~2; the length and thickness being as~24 nm and~13 nm, respectively. The sample S5 retains the similar morphology; the average crystallite length being~47 nm and thickness~22 nm, respectively. The average aspect ratio remains almost same. It is obvious that crystallite size obtained via transmission electron microscopy (TEM) observations are smaller than that of estimated from X-ray diffraction line broadening using Scherrer equation. The possible reason may be settling down of large particles during TEM sample preparation. .75) nanocrystallites obtained by decomposition of the gel product (formed through wet chemistry reaction at 100°C for 18 h) at 650°C for 3h. As per optical absorbance and Kubelka-Munk function, the pure diffuse reflectance of the sample can be expressed as [30],

Optical characterization
where R pd is the pure diffuse reflectance, K is absorption coefficient and S is scattering coefficient. The pure diffuse reflectance, F(R pd ) is proportional to the molar absorption coefficient (α). The relation between optical bandgap (E g ) and α can be given by well-known Tauc relation [30], where hυ is the energy of the absorbed photon, and C is proportionality constant. Also, from Eqs 1 and 2, following relations can be obtained [31][32][33], where n equals to ½ for allowed direct transition, 1 for non-metallic materials, 3/2 for direct forbidden transitions, 2 for allowed indirect transitions and 3 for indirect forbidden transitions, respectively [31][32][33].  [34][35][36][37][38]. Therefore, these samples have further been investigated using photoluminescence studies. Fig 6 show the emission spectra of the samples S1, S2, S3, S4 and S5, i.e., ZnSnO 3 , Zn 1.25 SnO 3+α, Zn 1.5 SnO 3+β , Zn 1.75 SnO 3+γ , Zn 2 SnO 4 obtained at the excitation wavelength of 290 nm. It is evident that in case of sample S1, i.e. ZnSnO 3 , the emission peak occurs at the wavelength of around 365 nm (energy~3.40 eV). The increase in Zn/Sn molar ratio to 1.25, leads to slight redshift in peak energy~3.39 eV, and at Zn/Sn molar ratio of 1.5 to 3.38 eV. Afterward, it retains its position till Zn/Sn molar ratio to 1.75. At Zn/Sn molar ratio of 2, the peak exhibits a little blue-shift in energy~3.39 eV.  drastic variation in bandgap. It indicates that somehow the diffused reflectance curves of intermediate samples which results due to overlapping of two phase compounds propagate more errors in bandgap estimation, i.e., the assumption of considering the intermediate samples as single phase and estimation of the bandgap does not fits well. As discussed before, the fact of dual phase formation in intermediate compounds has been verified through XRD investigation and is shown in Fig 3. To understand the these samples further, the excitation spectra of samples S1 and S5 were collected at the emission wavelength of 366 nm and are shown in S2 Fig. These show excitation at almost same wavelength as emission; indicating direct bandgap semiconductor nature of these samples. It is obvious that photoluminescence investigation (emission and absorption spectra) is more close to experimentally reported bandgap values [39][40][41] in comparison to estimated bandgap values using diffused reflectance spectra. Fig 7(a) and 7(b) shows variation of ac conductivity with frequency in the temperature range up to 473 K. The conductivity variation as a function of temperature can be shown as σ (ω) = ωε o ε'tan(δ) [42], where ε o, ε', and tan(δ) are permittivity of free space, relative permittivity of the sample, and loss tangent, respectively at the frequency ω. Obviously, the conductivity increases with frequency as well as with temperature (Fig 7a and 7b) for both the samples ZnSnO 3 and Zn 2 SnO 4 . The ac conductivity values for the sample ZnSnO 3 at the frequency of 10 kHz are 8.02×10 −8 Ω -1 cm -1 , 8.72×10 −8 Ω -1 cm -1 and 7.68×10 −7 Ω -1 cm -1 for the temperature of 300, 373 and 473 K, respectively. It is evident that the conductivity values increase with the increase in temperature as more charge carriers become available for conduction at increased Fig 7. (a, b) variation of ac conductivity with frequency at temperature range up to 473 K, top left corner inset shows variation of relative permittivity with frequency for samples ZnSnO 3 (Fig 7a) and Zn 2 SnO 4 (Fig 7b), bottom right corner insets show loss tangent as the function of frequency for ZnSnO 3 and Zn 2 SnO 4 , respectively.

Variation of ac conductivity, permittivity and loss tangent
doi:10.1371/journal.pone.0156246.g007 temperature. The ac conductivity values for Zn 2 SnO 4 nano-crystalline sample at the frequency of 10 kHz were found to be 6.77×10 −8 Ω -1 cm -1 , 8.06×10 −8 Ω -1 cm -1 and 1.00×10 −6 Ω -1 cm -1. Clearly, the conductivity values for the sample Zn 2 SnO 4 are lower than ZnSnO 3 upto the temperature of 373 K, and subsequently become higher at the temperature of 473 K. This may be ascribed to its different crystal structure (crystal structure of Zn 2 SnO 4 and ZnSnO 3 are tetragonal and orthorhombic, respectively), which leads to different crystal defects. The number of oxygen defects is possibly higher in Zn 2 SnO 4-Δ , (where Δ shows oxygen defects) in comparison to ZnSnO 3-▲ (where ▲ shows oxygen defects) samples, which, in turn lead to higher charge carrier density at elevated temperatures. This fact has also been discussed in structural analysis Section 3.1 for the explanation of reduced crystallite size at the Zn/Sn ratio of 2.
Top left corner insets of Fig 7(a)

Conclusions
The cetyltrimethyl ammonium bromide (CTAB) stabilized Zn 1+x SnO 3+x (0 x 1) nanocrystallites have successfully been synthesized using facile cost-effective wet chemistry route. The structural analysis confirmed the formation of orthorhombic ZnSnO 3 , tetragonal SnO 2type Zn 2 SnO 4 and/or their mixed phases depending upon the Zn/Sn ratio of the precursors. The morphological analysis exhibited ZnSnO 3 crystallites to be approximately of the half size that of Zn 2 SnO 4 crystallites (the length and thickness being 24, 13 for ZnSnO 3 and 47, 22 nm for Zn 2 SnO 4 ), respectively with aspect ratio of 2. The UV-visible diffuse reflectance together with photoluminescence data revealed these nano-crystallites to be direct wide bandgap semiconductors. The emission spectra exhibited bandgap of these nano-crystallites to be~3.40 eV. Further, the room temperature ac conductivity values for the ZnSnO 3 were found to be higher than Zn 2 SnO 4 samples. The cyclic voltammetry analysis of ZnSnO 3 and Zn 2 SnO 4 photo-anode based dye-sensitized solar cell revealed oxidation and reduction around 0.40 V and, 0.57V, respectively.