Grain boundary effect on structural,optical,and electrical properties of sol–gel synthesized Fe-doped SnO2 nanoparticles

Archana V, Lakshmi Mohan,2,?, Kathirvel P, and Saravanakumar S

1Department of Sciences,Amrita School of Engineering,Coimbatore,Amrita Vishwa Vidyapeetham,India

2Research and Development Center,Bharathiar University,Coimbatore-641046,Tamilnadu,India

3GRD Center for Materials Research,Department of Physics,P S G College of Technology,Coimbatore-641004,Tamilnadu,India

4Department of Physics,N S S College,Pandalam-689501,Kerala,India

Keywords: sol–gel,compressive strain,grain boundary,AC conductivity

1. Introduction

Metal–oxide nanostructures are a part of advances in material science due to their remarkable optoelectronic, thermal, catalytic, photochemical, electrical, and mechanical properties.[1]Among the metal oxides,tin oxide holds a place as an important material. The tin oxide exists in two oxidation states, i.e., +4 as stannic oxide (SnO2) and +2 as stannous oxide (SnO) respectively. The SnO2is the stable form which can be synthesized through low-temperature easier routes. It is an n-type semiconductor having a wide band gap (3.6 eV).[2]It is one of the most promising metal oxides and found to have variety of applications in sensing,liquid crystal displays, solar cells as transparent conducting electrodes,lithium ion batteries,photo catalysis,etc.[3–8]The SnO2nanostructure has also got relevance as the electrical and structural properties of SnO2and can be tuned to enhance the properties. SnO2can exist in three crystalline phases: tetragonal rutile phase, cubic phase, and orthorhombic phase, of which,the tetragonal rutile phase is the most stable crystalline phase and can be quite synthesized under normal laboratory conditions. The tetragonal rutile structure of SnO2has tin and oxygen atoms with 6 and 4 co-ordinations, respectively.Various synthesis methods have been adopted through aqueous routes, such as hydrothermal,[9,10]solvothermal,[11,12]green synthesis,[13]chemical precipitation,[14,15]and sol–gel methods.[16–18]Among these methods the sol–gel technique was adopted in the present study, for it provides an easy way to obtain better homogeneity, controlled stoichiometry, and high purity.[16]The properties of metal oxides can be tailored through various techniques such as controlling morphology,calcination,surface modification,doping,etc.[19,20]Considerable enhancement in the structural, electrical, and electronic properties of SnO2has already been reported by being doped with appropriate materials.[21]Doping with transition metal is found to have a deep influence on the properties of the tin oxide.[22–28]One of the most commonly observed consequence of doping is the segregation of dopants or impurities at the grain boundaries.[29]This results in the segregation of defects at the grain boundaries which in turn can affect material properties.[30,31]

The goal of the present research reported in this paper is to investigate the structural,optical,morphological,compositional, vibrational, and conductivity characteristics of the undoped SnO2and Fe-doped SnO2nanoparticles synthesized by using a simple sol–gel synthesis method. The grain boundary influence on the properties of the doped SnO2is discussed in detail.

2. Materials and methods

2.1. Experimental procedure

Stannous chloride dihydrate (SnCl2.2H2O), ferric nitrate nonahydrate (Fe(NO3)3.9H2O), and 25% aqueous ammonia are chemicals for synthesis with polyvinylpyrrolidone((C6H9NO)n) as surfactant were of AR grade and was directly used without any purification. The iron-doped tin-oxide nanoparticles were synthesized by the sol–gel method. A transparent solution was prepared by mixing a 0.2-M aqueous solution of stannous chloride dihydrate(SnCl2·2H2O)which was first dissolved in HCl and 5 ml of ethanol under constant stirring.After the stannous chloride was completely dissolved,a small amount of ferric nitrate nonahydrate(Fe(NO3)3.9H2O)was added followed with the addition of 0.2-g PVP into in 4-ml aqueous solution. Aqueous ammonia solution was added dropwise in order to maintain a pH of 8 and continued stirring for 1 h.After that,the solution was allowed to settle overnight.The resulting gel was filtered,washed with water and ethanol to remove impurities,and dried at 100?C for 12 h. Finally,the sample was crushed and annealed at 600?C for 4 h forming the iron-doped tin oxide nano powders. The same procedure was repeated but by changing the concentration of the ferric nitrate by 5 wt%and 10 wt%,separately. The chemical reaction equation is as follows:

Tin hydroxide is calcined to obtain the tin oxide as follows:

2.2. Material characterization

Several characterization techniques were employed to analyze the structural, optical, morphological, and electrical properties of undoped SnO2and 5-wt%, 10-wt% Fe-doped SnO2. The x-ray diffractogram analysis was performed by XRD Philip PW1700 with source Cu Kα radiation of wavelength 1.54 ?A. The UV-diffused reflectance spectra recorded in a wavelength range of 200 nm–900 nm by JASCO UVVis spectrometer (model name: –V-700) were used to study the optical reflectance and absorbance of the undoped and doped SnO2. The room-temperature analyses of photoluminescence(PL)of all samples were recorded by using a JASCO FLUROSCENCE spectroscope(model name:–FP-8300)with a xenon(Xe)lamp at an excitation wavelength of 300 nm. Raman spectrum was recorded by using Bruker RFS 27 with a laser source Nd:YAG(1064 nm). The field emission scanning electron microscope(FESEM)imaging and energy dispersive analysis of x-ray (EDAX) spectra were conducted by using the CARL ZEISS microscope(model name:ZEISS,SIGMA).The conductivity studies was carried out by using Keithely electrometer 6517B/E and Hioki 3532-50 LCR Meter in a frequency range from 50 Hz to 5 MHz.

3. Results and discussion

3.1. Structural analysis

The x-ray diffractogram of SnO2and Sn1?xFexO2(x=0.05 wt%, 0.10 wt%)samples were recorded in θ–2θ geometry from a scan range from 20?to 70?. The x-ray diffraction patterns of SnO2(shown in Fig.1)are used to identify the crystalline structure and related lattice parameters.Comparing with the standard JCPDS data(71-0652)lattice planes(110),(101), (200), (111), (211), (220), (002), (310), (112), (301),(202),(321)oriented in the direction of observed peaks are indexed and the peaks correspond to rutile tetragonal structure of tin oxide nanoparticles.

Fig.1. The XRD pattern of undoped SnO2(curve a),Sn0.95Fe0.05O2(curve b),and Sn0.90Fe0.10O2 (curve c)nanoparticles.

The sharp and intense diffraction peaks indicate the polycrystal line natures of the samples. No traces of impurities are observed, which is attributed to the formation of the pure phase. The average crystalline size is estimated from the Scherrer’s formula[32](Eq.(1))and tabulated in Table 1.

Table 1. Lattice parameters of SnO2 and Fe-doped SnO2 nanoparticles.

These grain boundaries as well as the difference between ionic radii of Fe3+ion and Sn4+ion can also induce a strain in the lattice,[38,39]which can induce the XRD peaks to broaden. The micro strain in the lattice and other microstructural evaluations are analyzed by using various models based on Williamson hall plots.

3.2. Peak profile analysis

The x-ray peak broadening arises due to the deviation from ideal crystalline lattice which is infinitely large and has a perfectly ordered crystalline array. Different atoms in the unit cell have divergent scattering power,hence the diffracted wave undergoes the phase shift and changes its amplitude. All imperfections of the sample and instrumental errors contribute to the noticeable broadening of the observed peak profiles.Broadening due to sample related contribution arises from the sample imperfections, crystalline size, micro strain and other defects.

The net broadening due to size broadening (βD) and strain broadening(βs)given by Eq.(4)is known as the Williamson–Hall equation[40,41]

The models employed to collate and confirm the incorporation of Fe into the synthesized sample are as follows: (i) uniform deformation model (UDM), (ii) uniform stress deformation model(USDM),and(iii)uniform deformation energy density model (UDEDM)[42](Figs. 2–4). The slope values obtained from these methods are tabulated in Table 2.It can be seen that the values obtained for the slopes are negative indicating that the compressive strain in lattice might be due to the shrinkage of lattice compared with its standard values.As crystalline size decreases, higher lattice strain can be expected which leads the XRD peaks to broaden. Thus,the lattice parameter shrinkage and defects lead to compressive strain. The values of the six independent elastic compliance values for rutile tetragonal SnO2are calculated by the substitution of elastic stiffness constants(Cij(in unit GPa)).[32,43]As observed from the figures,all the samples (SnO2, Sn0.95Fe0.05O2, and Sn0.90Fe0.10O2)exhibit negative slopes, thus implying the compressive strain in lattice.The lattice expansion and shrinkage along with crystal defects are the responsible for this strain. The geometrical parameters of SnO2and(5 wt%,10 wt%)Fe-doped tin oxide are listed in Table 2. It can also be concluded from the micro strain that the presence of grain boundaries in the lattice leads to the compressive strain.

Fig.2. UDM plots of (curve a) undoped SnO2, (curve b) Sn0.95Fe0.05O2,and(curve c)Sn0.90Fe0.10O2 nanoparticles.

Table 2. Geometrical parameters of SnO2 and Fe-doped SnO2.

Fig.3. USDM plot of(curve a)undoped SnO2, (curve b)Sn0.95Fe0.05O2,and(curve c)Sn0.90Fe0.10O2 nanoparticles.

Fig.4. UDEDM plot of(curve a)undoped SnO2,(curve b)Sn0.95Fe0.05O2,and(curve c)Sn0.90Fe0.10O2 nanoparticles.

3.3. Optical analysis

3.3.1. UV–diffuse reflectance spectrum

The optical characterizations of the undoped and Fedoped SnO2nanoparticles are performed by using UV-DRS spectroscopic measurements recorded in a range from 200 nm to 700 nm. The reflectance and absorbance spectra of the undoped and doped samples obtained are shown in Figs. 5 and 6 respectively. The absorption edge located around 300 nm is found to get red-shifted as doping concentration increases.[38]

A broad absorption peak with a width of 500 nm is also observed which is due to incorporation of Fe3+into SnO2lattice and surface. It is observed from the XRD plot that the addition of Fe causes the crystallinity of sample to significantly decrease.[44]The precursor used has Fe in the oxidation state 3+ and the peak broadening clearly indicates the influence of Fe3+. The Fe3+ions occupy regular lattice sites causing the point defects and charge compensation effect which results in the distortion of crystal structure stoichiometry. The aggregation of Fe2O3also reduces the growth of SnO2nanoparticles with the increase of dopant concentration.The optical band gap of the sample is estimated by considering the Kubelka–Munk function F(R∞) for direct allowed transitions. Figure 7 shows the variation of(F(R∞)hν)2with photon energy hν, whose extrapolation of the linear portion gives the optical bandgaps 3.52 eV, 3.26 eV, and 2.28 eV respectively for undoped, 5-wt% doped, and 10-wt% Fe-doped SnO2nanoparticles.[45,46]

With the increase in dopant concentration a red shift is observed in the reflectance/ absorbance edge which confirms the incorporation of Fe3+ions into the tin oxide lattice.[47]The conduction band or valence band of tin-oxide and Fe3+ions can have charge transitions which might result in the electron concentration shift which can be corroborated with the red shift. The observed peak in the visible region centered around 550 nm corresponds to the characteristic peak of iron. As reported in Refs.[35,48]this can be attributed to the transitions between half-filled ‘d’ levels (d·d) of Fe3+ions(2T2g?→2A2g,2T1g)or by the charge transfer between interacting iron atoms(Fe3++Fe3+?→Fe4++Fe2+). Harve and Vandamme’s model[49]is employed to calculate the refractive index through using the estimated band gap. The calculated values are tabulated in Table 3 and are in agreement with the reported values.[50,51]

Fig.5. UV-reflectance spectra of (curve a) undoped SnO2, (curve b)Sn0.95Fe0.05O2,and(curve c)Sn0.90Fe0.10O2 nanoparticles.

Fig.6. UV-absorbance spectra of (curve a) undoped SnO2, (curve b)Sn0.95Fe0.05O2,and(curve c)Sn0.90Fe0.10O2 nanoparticles.

Fig.7. UV-DRS plot of(curve a)undoped SnO2,(curve b)Sn0.95Fe0.05O2,and(curve c)Sn0.90Fe0.10O2 nanoparticles.

Table 3. Refractive indexes of SnO2 and Fe-doped SnO2.

3.3.2. Photoluminescence spectra

The photoluminescence (PL) spectra are recorded at an excitation wavelength of 300 nm. The PL spectra of the synthesized nanoparticles shown in Fig.8 contain emissions in the visible range from 400 nm to 500 nm with peaks centered at 2.9 eV (425 nm) and 2.6 eV (465 nm). The maximum emission centered at 2.6 eV corresponds to the energy lower than the band gap (3.6 eV) of bulk SnO2. These peaks as well as the peaks centered on 2.9 eV can be ascribed to the defects or vacancies The intense emission in the visible region might be due to recombination of a large number of trapped states which result from solution-based synthesis process.[33]

Fig.8. The PL spectra of undoped SnO2 (curve a), Sn0.95Fe0.05O2 (curve b),and Sn0.90Fe0.10O2 (curve c)nanoparticles.

The emission centred at 3.2-eV peak can stem from the band edge emission and the peak corresponding to 2.9 eV originates from luminescence center formed by tin interstitials or dangling bonds in the SnO2nanoparticles.[52]As the doping concentration of Fe3+increases, non-radiative recombination will occur after the trapping of electrons and holes at the surface which can arise from the quenching of Pl intensity.Above the solubility limit,most of the Fe3+gets precipitated into ferrous oxide phase and the excess Fe3+phase resides at surface or grain boundary[53]as already explained in XRD analysis. The segregation of Fe3+induces more compressive strain,thereby reducing the crystallinity of lattice.From the PL spectrum it is quite visible that the surface defects dominate over others because of the nanometer particle size which is also indicated from the strain analysis.

3.3.3. FT-Raman spectra

Analyses of the vibrational properties and effects of doping on them are carried out by using FT Raman spectra in a wavenumber range of 400 cm?1–900 cm?1. The unit cell of SnO2comprised of 2 tin atoms and 4 oxygen atoms in the center of brillouin zone,giving rise to 18 modes of vibration. The representation in the mechanical form is given by[54]

Among these, A1g, B1g, B2g, and Egare the four modes that arise from the vibration of oxygen atom around the tin atoms.The active modes A1g, B1g, B2ghave vibrations of oxygen atoms with planes perpendicular to the C axis and A2uand Euare the modes that involve dipole moments and hence they are Raman-inactive.

Fig.9. Raman spectra of(curve a)undoped SnO2,(curve b)Sn0.95Fe0.05O2,and(curve c)Sn0.90Fe0.10O2 nanoparticles.

As reported in Ref. [55], the bulk SnO2shows peaks around 466, 635, 757 cm?1which can be ascribed to the Eg,A1g, and B2gmodes. It is observed that each of all the three bands with a shift is observed in the synthesized samples also,confirming rutile nature of SnO2. Apart from the fundamental vibrations, a few additional bands at 510, 549, 710, 740,781, 817, and 842 cm?1are also observed. These additional bands appear because the surface properties of the crystal depend on the oxygen vacancy, other disorder along with grain size and its distributions.[56]The bands in 549 cm?1is redshifted and 634 cm?1is blue-shifted,hence showing the effect of grain boundaries on the functional spectra and vibrational spectra of tin oxide. The band 679 cm?1and 700 cm?1observed in SnO2and Fe(5 wt%, 10 wt%)-doped SnO2correspond to IR-active A2umodes.[57]The continuous broadening in Raman peaks with the decrease in crystalline size can be ascribed to phonon confinement effect. As confirmed from the x-ray diffraction analysis a reduction in crystallinity occurs with the grain boundary formation which causes the phonon dispersions to occur near the zone center. Thus, the activation of inactive mode and shift of band can take place as a result of the Fe incorporation. The incorporation of Fe into the grain boundary results in trapped states and electron hole recombination which causes the Raman modes to decrease with the increase in Fe-dopant concentration. This is also in correlation with quenching observed in PL.[58]The spectral band transitions of 817 cm?1and 895 cm?1can be ascribed to the stretching vibration of double coordinated SnO2.[59]

3.3.4. Field emission scanning electron microscopy

The morphological and compositional analyses of SnO2,Sn0.95Fe0.05O2,and Sn0.90Fe0.10O2nanoparticles obtained by using FESEM and EDAX spectra,and the results are shown in Fig 10.

Fig.10. FESEM micrograph and EDAX spectra of (a) undoped SnO2 nanoparticles,(b)Sn0.95Fe0.05O2,and(c)Sn0.90Fe0.10O2 nanoparticles.

The micrograph clearly displays the influence of dopant concentration on the surface morphology of the nano powder.The structure shows homogeneously well dispersed spherical nanoparticles aggregated into the irregularly shaped tin oxide nanoparticles. Grains of each sample consist of large aggregates which transform into the fine aggregates with the increase in Fe dopant concentration. The average grain size obtained by using the Image J analysis of the undoped SnO2is around 10 nm accounting for 5 wt%and around 13 nm occupying 10 wt%,indicating the aggregation of particles and grain boundaries. The formation of irregular shape can be ascribed to the grain rotation.[60]

The grain rotation leads the neighboring grains to coalesce by eliminating the common grain boundary between them. It is one of the mechanisms for growth. The driving force for rotation is the net torque which arises from the variation in rate of diffusion as observed in XRD.The grains embedded in a polycrystal are subjected to a torque which can be reduced by the grain rotation. This rotation creates a void in other region because of the overlap of grains. This gives rise to the regions of compression and tension along the grain perimeter,[61]as explained in Williamson–Hall method which set up diffusion fluxes to accommodate the rotation. The increase in density of particles confirms the segregation of particles from boundaries. It is also reported that surfactant plays a major role in this coagulation and segregation after the nucleation.

The compositional analysis of the sample is conducted by energy dispersive x-ray spectroscopy(EDAX)(Fig.10). It is clear from the EDAX spectra that Sn and O are the only main elemental species in as-prepared SnO2, though additional Fe peaks are observed in the spectra of doped samples. The elemental compositions obtained by the sol-gel method show the values approximately equal to that estimated from synthesis conditions.

4. Conductivity studies

4.1. Impedance analysis

The variation in the dielectric nature of the SnO2with Fe-doping concentration is analyzed by using LCR meter in a frequency range of 50 Hz–5 MHz. The electrical impedance(Z),dielectric loss factor(tanδ),AC conductivity of undoped and doped SnO2samples are analyzed. The frequency dependence of dielectric constant and the influence of doping on it are studied in detail by taking the real and imaginary components of dielectric constant[62]

The phase difference,with the applied field leading to the drop in the dielectric value,is known as loss factor tangent[65]

Fig.11. Variations of dielectric constant(εh)with frequency for(a)undoped SnO2,(b)Sn0.95Fe0.05O2,and Sn0.90Fe0.10O2 nanoparticles.

Fig.12. Frequency dependence of dielectric constant (εhh) of (a) undoped SnO2,(b)Sn0.95Fe0.05O2,and Sn0.90Fe0.10O2 nanoparticles.

The variations of loss tangent with log frequency in Fig.13 show that the loss factor decreases invariably for all samples from in a frequency range from 50 Hz to 5 MHz. The Fe doping results in the formation of grain boundaries which are electrically resistive in nature,Based on Koop’s theory,the dielectric value should be high at low frequencies,and dielectric relaxation occurs beyond characteristic frequency. It is also observed that dielectric loss decreases with Fe doping concentration increasing, hence Fe doping concentration above the solubility limit in SnO2can get a material suitable for high frequency applications.[64]

The variations of alternating current (AC) conductivity with log frequency for SnO2and 5-wt%and 10-wt%Fe-doped SnO2are shown in Fig.14. The AC conductivity of SnO2and Fe-doped SnO2are analyzed using the relation[11]

The charge carriers in the trapped states overcome the potential barrier with the increase of frequency,thus contributing to conduction. The rapid increase in conductivity occurs when charge carriers gain enough energy to overcome the potential barrier.[66]

Fig.13. Variations of dielectric loss factor (tanδ) with log frequency for undoped SnO2,Sn0.95Fe0.05O2,and Sn0.90Fe0.10O2 nanoparticles.

Fig.14. Frequency-dependent AC conductivity of (a) undoped SnO2, (b)Sn0.95Fe0.05O2,and Sn0.90Fe0.10O2 nanoparticles.

Fig.15. Nyquist plot of (a) undoped SnO2, (b) Sn0.95Fe0.05O2, and (c)Sn0.90Fe0.10O2 nanoparticles.

To obtain the detailed information about the contribution of grain boundaries and grains,the capacitance and resistance components in the impedance are plotted in the complex plane.Figure 15 shows the Nyquist plots which indicate slight semicircle behaviors for all samples. The diameter of semicircular pattern increases with doping concentration increasing, indicating that the impedance increases with doping concentration increasing.[67]

4.2. DC conductivity

Using a two-probe model, as-prepared SnO2and irondoped SnO2samples are analyzed between room temperature and 120?C to obtain the parameters like conductivity (σdc),resistivity(ρ),and activation energy(Ea). Figure 16 shows I–V characteristic curves of SnO2and Fe(5 wt% and 10 wt%)-doped SnO2. With applied voltage increasing, the current value linearly increases for each of all samples. With the increase in dopant concentration, the current value decreases,but the samples show significant temperature dependence. For as-prepared SnO2sample, the current values show constant linear behavior with variation in temperature. As the dopant concentration increases, the current values change considerably with the temperature,showing a highest current value at 120?C.

Fig.16. The I–V characteristics of(a)undoped SnO2,(b)Sn0.95Fe0.05O2,and(c)Sn0.90Fe0.10O2 nanoparticles.

The DC conductivity is calculated from the following equation[68]

where t is pellet thickness,R is resistance,and A is pellet area.

From Table 4, it is observed that the calculated σdcvalues of the samples decrease gradually with the increase in Fedoping concentration in a range from 10?4S/cm to 10?7S/cm.

Initially the carrier concentration decreases with Fe3+-doping concentration increasing and then remains almost the same (with only small variations). The carrier concentration decreases because Fe3+creates a positively charged hole and segregates electrically inactive Fe atoms into the grain boundary.[69]

Table 4. Values of resistivity and conductivity of samples.

It is observed that conductivity of individual sample increases with temperature increasing, which shows the semiconducting nature of the sample. The increase in resistivity with Fe-doping concentration increasing is due to segregation of dopants into the boundary. Figure 17 shows the Arrhenius plot,i.e.,(1/T)(K?1)versus lnσdc(S/cm).As the dopant concentration increases beyond the solid solubility limit it will get clustered in the material boundaries,thus reducing the electron concentration, and the conductivity will fall at a certain doping concentration.[62]The semiconducting property is found to be enhanced with Fe doping at 10 wt%as the grain boundary blocking effect which reduces the conductivity is negated at high temperatures and higher doping concentrations.[62]

Fig.17. The Arrhenius plot of (cure a) undoped SnO2, (cure b)Sn0.95Fe0.05O2,and(cure c)Sn0.90Fe0.10O2 nanoparticles.

5. Conclusions

The SnO2and Fe-doped SnO2nanoparticles are synthesized by using the simple sol–gel method.

The decrease in crystallinity obtained from the XRD confirms the formation of grain boundaries. The decrease in the band gap is measured by using UV DRS analysis,the photoluminescence,and Raman spectra also confirm the grain boundary influence on the properties of SnO2nanoparticles. The field emission scanning electron microscope images reveale the Fe-doped inhomogeneous grain boundary formation. The conductivity studies performed on this material show that the Fe doping increases the impedance of the material and the dielectric loss tangent decreases with doping concentration increasing at high frequencies. The semiconducting natures of the samples are found to be enhanced as grain boundary blocking effect decreases at high temperature and higher doping concentration. Thus, the doping of Fe above the solubility limit forms the grain boundary, which can be used as a material for high-frequency detection application.

Acknowledgment

The authors thank the support rendered by Dr. A Karthega,Amrita Vishwa Vidyapeetham for the facilitation of experimental part,and also thank SRMV Advanced Research Instrumentation Center, Coimbatore for their helps with UV,PL, and dielectric measurements, and SAIF, IIT Madras for FT-Raman measurement.Authors also thank Karunya University for XRD measurements and SITRA and CIT,Coimbatore for FESEM and EDAX measurements.