J Mater Sci: Mater Electron DOI 10.1007/s10854-017-7959-2

Effect of pH and annealing temperature on the properties of tin oxide nanoparticles prepared by sol–gel method Mohana Priya Subramaniam1 · Geetha Arunachalam1 · Ramamurthi Kandasamy1 · Pandiyarasan Veluswamy2 · Ikeda Hiroya2 

Received: 6 June 2017 / Accepted: 3 October 2017 © Springer Science+Business Media, LLC 2017

Abstract  Tin oxide ­(SnO2) nanoparticles were synthesized employing simple sol–gel method. Modification in the structural, morphological and optical properties of the as-synthesized tin oxide nanoparticles due to various solution pH (6–12) and thermal annealing at 400 °C (Experiment 1) was studied. X-ray diffraction results of the tin oxide nanoparticles prepared from the precursor solution pH 8 and annealed at 400 °C showed the formation of tin oxide tetragonal phase ­(SnO2-t) and the surface morphology of the ­SnO2-t nanoparticles studied by scanning electron microscope revealed the formation of spherical shaped agglomerations. Hence, the tin oxide nanoparticles prepared from the solution pH 8 were annealed at 200, 400, 600 and 800 °C in order to study the effect of annealing at various temperatures on the structural, morphological, optical and vibrational properties of tin oxide nanoparticles (Experiment 2). When the annealing temperature was increased to 600 and 800 °C, mixed phases of S ­ nO2-t and tin oxide orthorhombic system (SnOo) were formed. Various solution pH and annealing temperatures influenced the direct band gap value. ­SnO2-t phase synthesized from the solution pH 8 and annealed at 400 °C showed a direct band gap of ~4.50 eV. The tin oxide samples annealed at various temperatures showed a slight shift in the Electronic supplementary material  The online version of this article (doi:10.1007/s10854-017-7959-2) contains supplementary material, which is available to authorized users. * Geetha Arunachalam [email protected] 1



Crystal Growth and Thin Film Laboratory, Department of Physics and Nanotechnology, SRM University, 603203 Kattankulathur, Tamil Nadu, India



Research Institute of Electronics, Shizuoka University, Hamamatsu 432‑8011, Japan

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fluorescence peak observed at ~327 nm. Raman studies of the samples obtained from Experiment 1 and Experiment 2 showed a slight shift in the vibrational frequency. I–V studies carried out to investigate the electrical properties of the ­SnO2 thin film formed by simple drop casting method revealed better ohmic contact and its suitability for gas sensing applications.

1 Introduction Metal nanoparticles prepared by different synthesizing process attracted the attention because they show modifications in the shape, size distribution, surface morphology and surface area of the nanoparticles; thus, they possess the potential to tune the physical and chemical properties of the semiconducting materials [1, 2]. Quantum confinement effect of nanomaterials paved ways to discover novel materials suitable for various applications [3, 4]. Semiconducting materials with modified optical and electronic properties possess wide range of applications and the materials with wide band gap are of specific interest in device fabrications. Thus tin oxide, an n-type semiconductor with wide direct band gap of 3.6 eV, possesses potential applications in many fields such as solid state gas sensors [5], dye sensitized solar cells [6, 7], photocatalysis [8], lithium ion storage batteries [9, 10], gas sensors [11] and phosphoproteomic research [12]. Further the Au and Ag dopants enhance the electrical and optical properties of tin oxide [13]. Tin oxide nanostructures such as nanowires [14], nanobelts [15], nanoribbions [16], nanoflowers [17], and nanorods play an important role in tuning the structural, electrical and optical properties [18]. Several methods such as mechano-chemical reaction method [19], hydrothermal method [18, 20], microwave method [21] and solid-state reaction method [22] and sol–gel method are

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adopted to prepare tin oxide nanomaterials. Among them sol–gel method is cost effective and produces better homogeniety in the preparation of nanoparticles [17, 23]. Hence, in the present work, sol–gel method is employed to synthesize tin oxide nanoparticles. Among the various phases of tin oxides, SnO is the most metastable phase, which is mostly formed at high annealing temperatures (450 and 750 °C), whereas S ­ nO2 is the stable phase [24]. ­SnO2 itself crystallizes in two phases, ­SnO2 tetragonal ­(SnO2-t) and ­SnO2 orthorhombic ­(SnO2-o). Meyer et al. [25] demonstrated that SnO phase exists due to reduction of ­SnO2 and hence the two phases of ­SnO2 and SnO coexist in the prepared samples. Nose et al. [26] also discussed about SnO oxidation to S ­ nO2. Gu et al. [27] reported that the mixed phases of ­SnO2-t and S ­ nO2-o mesocrystals show effective ethanol sensing behavior than that of the single ­SnO2-t phase. The calcination temperature also plays an important role in obtaining orthorhombic phase of ­SnO2 [28]. Diallo et al. [29] reported that annealing the single phase cassiterite ­SnO2 nanoparticles (synthesized via Aspalathus linearis) at different temperatures (400–900 °C) improved the crystallinity of the samples and controlled the size of the particles [30]. It is evidently shown that the solution pH used in the synthesis process can effectively tune the nanostructures. Further the size, morphology and phase of the tin oxide nanoparticles can be effectively modified by the process of annealing [31–33]. Hence in the present work, we report on the synthesize of tin oxide nanoparticles by sol–gel method at solution pH 6, 8, 10 and 12. As the nanoparticles synthesized at pH 8 and annealed at 400 °C showed spherical structures they are further annealed at various temperatures and their effect on the structural, morphological, optical and electrical properties are reported.

2 Experimental Tin oxide nanoparticles were synthesized by sol–gel method [34] using cost effective tin (II) chloride ­(SnCl2·2H2O) and ammonia ­(NH3) solution (EMerck A.R. Grade). In this process, 1.67 g of tin chloride was dissolved in ethanol (25 ml) and the solution was stirred for 2 h at 60 °C and then dried in oven at 60 °C for 5 h. On stirring the tin chloride solution, ­NH3 solution was added dropwise to adjust the solution pH to 6, 8, 10, and 12 in independent experiments. Then the product tin oxide was centrifuged, dried in hot air oven at 60 °C and then it was annealed at 400 °C for 2 h (Experiment 1). As the tin oxide nanoparticles synthesized at solution pH 8 employing the above method formed spherical shaped nanoparticles, the synthesized nanoparticles at pH 8 were annealed at 200, 400, 600 and 800 °C for 2 h (Experiment 2). Structural analysis (X-ray diffractometer (X’Pert PAN Analytical)) with ­CuKα radiation (λ = 1.5405  Å),

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surface morphology (FEI Quanta FEG 200 scanning electron microscopy), UV-DRS measurement (Shimadzu 2450), Raman analysis (NRS-7100 (JASCO)), fluorescence spectral studies (FP-8600 (JASCO)) and electrical studies (Keithley 2636B) of the synthesized tin oxide nanoparticles were carried out. The prepared sample from solution pH 8 and annealed at 400 °C was used to deposit thin film on the cleaned glass substrates (1 cm × 1 cm) using simple drop casting technique and then it was dried. Contacts were made using silver paste, and the electrical properties of the film were studied.

3 Results and discussion 3.1 Structural analysis X-ray diffraction pattern (XRD) of the as-synthesized (AS) tin oxide nanoparticles from the precursor solution of different pH values are presented in the Fig. S1 (given in supplementary file) whereas XRD pattern recorded for the samples annealed at 400 °C are presented in Fig. 1a. Figure S1 evidently shows that XRD peaks of the as-synthesized nanoparticles are broad whereas annealing the prepared samples at 400 °C increased the intensity of the XRD peaks (Fig. 1a). Further it is also evident from the Fig. 1a that the XRD peaks of the tin oxide nanoparticles synthesized from the solution pH 8 and annealed at 400 °C become relatively intense and sharp. Since the nanoparticles synthesized at solution pH 8 and annealed at 400 °C acquired spherical structures shown in Fig. 2a, b. The tin oxide nanoparticles prepared from solution pH 8 was annealed at various temperatures ­(Ta) of 200, 400, 600 and 800 °C to investigate the modification in their properties (Experiment 2). XRD pattern presented in the Fig. 1b shows that the synthesized nanoparticles annealed at 600 °C (B3) and 800 °C (B4) consist of mixed phases of S ­ nO2-t (JCPDS Card No. 41-1445) and SnO-o (JCPDS Card No. 77-2296). The diffraction peak observed at 2θ = 39.67° [Fig. 1b (B3, B4)] is due to SnO-o phase [which is marked with the symbol (*)]. Thus the results show that only S ­ nO2-t phase is formed upto the annealing temperature 400 °C (Fig. 1b; B2 = A2) and increase in the annealing temperature to 600 and 800 °C yielded the mixed phases of ­SnO2-t and SnO-o. The intensity of XRD peak of (023)* plane corresponding to SnO-o phase is relatively less. The various growth parameters used in the present work are not effectively influenced to improve the formation of SnO-o phase. Marikannan et al. [35] reported on the formation of mixed phases of ­SnO2-o/ SnO-o thin films prepared by sol–gel spin coating method using different solvents and disclosed that both oxidation and preparation methods were the major factors influencing the formation of mixed phases. Wang et al. [36] reported

J Mater Sci: Mater Electron

Fig.  1  a XRD pattern of synthesized tin oxide nanoparticles from various solution pH and annealed at 400  °C; pH 6 (A1), 8 (A2), 10 (A3) and 12 (A4). b XRD pattern of the as-synthesized tin oxide nan-

oparticles from solution pH 8 (AS2); and annealed at 200  °C (B1), 400 °C (B2 = A2), 600 °C (B3) and 800 °C (B4)

that, tin oxide nanoparticles prepared using carbon nanofibers (CNFs) by hydrothermal method for different reaction time have produced mixed phases of ­SnO2-t and ­SnO2-o and ­SnO2/CNFs nanocomposite prepared was used for hydrogen sensor application. Gu et al. [27] reported that both ­SnO2-t and ­SnO2-o phases are resulted when annealing temperature is raised to 600 °C. Hu et al. [28] also reported that the simple chemical method yields the S ­ nO2-t and S ­ nO2-o phases of nanorods which are used for sensing Isopropanol gas. But the present work shows that the as-synthesized sample from solution pH 8 and annealed at 600 and 800 °C formed mixed phases of ­SnO2-t and SnO-o. Interplanar distance d and the values of unit cell parameters a and c of S ­ nO2-t phase were calculated from the XRD peaks of (200) and (002) planes using the relation 1/ d2 = (4sin2θ)/λ2 = (h2 + k2)/a2 + l2/c2 [34] and the average crystallite size (D) was calculated from Debye–Scherrer formula, D = 0.9λ/(βcos θ) [37], where λ is the wavelength of X-rays used (1.5405 Å), β is full width half maximum (FWHM) in radian and θ is the Bragg’s angle and (h k l) are the Miller indices. The cell parameters calculated in the present work are given in Table 1which reveals that cell parameter values obtained from Experiment 1 and Experiment 2 are nearly same. Further the present values compare well with the corresponding standard values of a = 4.738 Å and c = 3.187 Å (JCPDS Card No. 41-1445). Thus the calculated values show that the a, c and d are not influenced effectively by the various precursor solution pH and annealing temperature. The calculated average crystallite size from the XRD peaks of (110), (101) and (211) planes of ­SnO2-t phase (Experiment 1 and Experiment 2) are given in the Table 1. Experiment 2 shows when annealing temperature is

increased, the XRD peak intensity becomes sharper and the crystallite size increases systematically (Table 1). Sathyaseelan et al. [38] and Muthukumar et al. [39] have also observed the same trend due to heat treatment in the synthesized T ­ iO2 nanoparticles. The increase in crystallite size is due to the diffusion of atoms from the grain boundary to the grain [1]. Thus, the present results reveal that the nanoparticles synthesized from the solution pH 8 and annealed at 400 °C show well developed single phase tin oxide (­ SnO2-t) (Fig. 1b; B2). However the tin oxide nanoparticles obtained from pH 8 and annealed at 600 and 800 °C show the signature of SnO-o phase along with the S ­ nO2-t phase. Thus the annealing temperatures 600 and 800 °C seem to be critical which formed the SnO-o phase, a first report from this work. 3.2 Morphological analysis Morphological variation due to the influence of solution pH and annealing temperatures obtained from the Experiments 1 and 2 is displayed in the Fig. 2a, b respectively. Morphology of the tin oxide synthesized from sol–gel method strongly depends on the amount of ­H+ or ­OH− ions in the solution [40]. Tin oxide synthesized from the solution pH 6 and ­Ta 400 °C, contains slurry like morphology with no definite shape (A1). When the solution pH is 8 and T ­ a is 400 °C, the image B2 (= A2) shows agglomerated spherical shaped structures. This is enlightened by the projected image shown adjacent to image B2. On increasing the solution pH to 10 (A3) and 12 (A4), the size of the agglomerated structure is decreased and unevenly distributed for T ­ a 400 °C. The reason for modification of the surface morphology can be explained using the ­OH− and H ­ + ions concentration in the solution

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Fig.  2  a FESEM images of tin oxide nanoparticles prepared from various solution pH and annealed at 400 °C; pH 6 (A1), 8 (A2), 10 (A3) and 12 (A4). b FESEM images of as-synthesized tin oxide nanoparticles from pH 8 and annealed at different temperatures 200 °C (B1), 400 °C (B2 = A2), 600 °C (B3) and 800 °C (B4). c EDS analysis of tin oxide nanoparticles synthesized from pH 8 and annealed at 400 °C

Table 1  Comparison of cell parameters, average crystallite size and band gap energy of the tin oxide nanoparticles obtained from the Experiment 1 and Experiment 2 Tin oxide nanoparticles

Sample code Cell parameters(Å)  a = b  c Average crystallite size D (nm) Band gap ­Eg (eV)

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Experiment 1 (­ Ta = 400 °C)

Experiment 2 (pH 8)

SnO2-t

SnO2-t

SnO2-t/ SnO-o

pH 6

pH 8

pH 10

pH 12

Ta = 200 °C

Ta = 400 °C

Ta = 600 °C

Ta = 800 °C

A1

A2

A3

A4

B1

B2 = A2

B3

B4

4.700 3.180 11.9 3.63

4.700 3.160 10.0 4.50

4.700 3.160 6.2 4.48

4.700 3.160 6.8 3.55

– – 5.1 3.79

4.700 3.160 10.0 4.50

4.700 3.180 15.4 3.69

4.700 3.170 19.0 3.61

J Mater Sci: Mater Electron

as following: when the solution pH is 6 (­ Ta = 400 °C), it becomes relatively more acidic and hence there will be more concentration of O ­ H− ions. Reaction of N ­ H3 with the precursor solution containing tin chloride leads to the formation Sn(OH)2 compound during the reaction process and later dissociates into ­Sn2+ and ­OH− ions. When the concentration of ­Sn2+ and O ­ H− ions are more than the critical value, S ­ n2+ plays a major role in the formation of S ­ nO2 nuclei. However when pH is increased, changes in the ­OH− ions concentration occur and decrease the density of H ­ + ions than that at lower pH. Thus on increasing the solution pH from acidic to basic the concentration of ­H+ ions decreases and increase in the ­OH− ions influences to form the desired structures [40]. In the basic medium (> pH 7) the ­OH− ions interact with positively charged S ­ n2+ and form S ­ nO2. In the present work at higher pH (10, 12) hydrolysis and condensation process will be uncontrollable [40] and hence the samples A3 and A4 give irregular shape; subsequently in the Experiment 2 high annealing temperature (Ta = 600 and 800 °C) formed more agglomerated structures. Thus the FESEM images clearly show that the S ­ nO2-t nanoparticles prepared from pH 8 and on annealing at 400 °C yielded agglomerated spherical shaped nanoparticles (A2 = B2). EDS of sample (A2 = B2) confirms the presence of elements Sn and O (Fig. 2c). 3.3 Optical properties The size and shape of the nanoparticles tune the optical properties of materials that depend on the preparation parameters [41]. Figure 3a shows the UV-DRS measured in the range of 200–740 nm for ­S nO 2-t phase prepared from different solution pH and annealed at 400 °C (Experiment 1). The absorption peak observed around 278 nm

Fig.  3  a U-V DRS of tin oxide nanoparticles synthesized from various solution pH and annealed at 400  °C; pH 6 (A1), 8 (A2), 10 (A3) and 12 (A4). b U-V DRS of as-synthesized tin oxide nanopar-

(A2 = B2) for the prepared S ­ nO2-t phase from solution pH 8 and annealed at 400 °C is due to photoexcitation of the electrons from valence band to conduction band [42]. It is obvious from the spectrum that the absorption peak of the ­SnO2-t prepared from the solution pH 8 and annealed at 400 °C is shifted to 278 nm from that of the bulk S ­ nO2 value 337 nm. Thus, the blue shift indicates the quantum confinement effect due to decrease in particle size [43]. Further, the absorption edge of the prepared samples obtained from the solution pH 10 (A3) and 12 (A4) and annealed at 400 °C is shifted towards the longer wavelength to 305 and 307 nm respectively, thus indicating the red shift relative to that of the tin oxide nanoparticles prepared at solution pH 8 and annealed at 400 °C (A2). According to Beer’s law [44], transmittance and absorbance are inversely related to each other. Materials with less absorption in particular wavelength show transparent behavior in that wavelength. In the Experiment 2, the samples prepared from pH 8 and annealed at 600 °C (B3) and 800 °C (B4) show increased transmittance at ~387 nm which may be due to the increase in the crystallite size and formation of mixed ­SnO2-t and SnO-o phases (Fig. 3b). Tin oxide nanoparticles synthesized from solution pH 8 and annealed at 400 °C (B2) show the absorption peak at 278 nm, which when compared with that of the bulk shows a blue shift; thus indicating the quantum confinement effect. The direct band gap ­(Eg) is calculated using the formula, αhν = B(hν − Eg)n, where n takes the value of 1/2 for direct transition, α is an absorption coefficient, B is a constant called band tailing parameter and hν is the incident photon energy [45]. The direct optical band gap was calculated by using Tauc plot which is plotted between hν and (αhν)2 and extrapolating the straight line to energy

ticles from pH 8 and annealed at different temperatures 200 °C (B1), 400 °C (B2 = A2), 600 °C (B3) and 800 °C (B4)

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Fig.  4  a Optical band gap of tin oxide nanoparticles synthesized from various solution pH and annealed at 400 °C; pH 6 (A1), 8 (A2), 10 (A3) and 12 (A4). b Optical band gap of as-synthesized tin oxide nanoparticles from pH 8 and annealed at different temperatures 200 °C (B1), 400 °C (B2 = A2), 600 °C (B3) and 800 °C (B4)

axis at (αhν)2 = 0. Figure 4a shows the value of ­Eg of tin oxide nanoparticles synthesized from the Experiment 1 whereas Fig. 4b shows E ­ g of tin oxide nanoparticles synthesized from the Experiment 2. The shift in the absorption peaks observed is due to the change in the crystallite size. It is evident that the ­SnO2-t nanoparticles prepared from the solution pH 8 and annealed at 400 °C (B2 = A2), possess relatively high band gap (4.50 eV) when compared to that of the bulk ­SnO2 (3.62 eV), which agree well with

the reported value (4.6 eV) [46]; this may be attributed to the effect of quantum confinement.

Fig.  5  a Raman spectra of tin oxide nanoparticles synthesized from various solution pH and annealed at 400  °C; pH 6 (A1), 8 (A2), 10 (A3) and 12 (A4). b Raman spectra of as-synthesized tin oxide nan-

oparticles from solution pH 8 and annealed at 200  °C (B1), 400  °C (B2 = A2), 600 °C (B3) and 800 °C (B4)

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3.4 Raman spectral analysis Raman spectral analysis is useful to get information on the crystal structure, defects and disorders of the nanoparticles [47]. Raman analysis of the tin oxide nanoparticles prepared from Experiments 1 and 2 were carried out in the range of 300–900 cm− 1 and are presented in Fig. 5a,

J Mater Sci: Mater Electron Table 2  Raman frequency of tin oxide nanoparticles with corresponding frequency assignments

Samples

SnO2 nanoparticles

Experiment 1 (­ Ta = 400 °C) pH 6 pH 8 pH 10 pH 12 Ta=200 °C Experiment 2 (pH 8) Ta=400 °C SnO2/SnO phase ­(Ta=600 °C) SnO2/SnO phase ­(Ta=800 °C)

b. The vibrational frequencies obtained from Raman spectral analyses for tin oxide nanoparticles are presented in Table 2 along with frequency assignments. The samples prepared from different pH (6–12) and annealed at temperatures (Ta = 200–800 °C) show Raman peaks at ~625 cm−1 ­(A1g), ~757 ­(B2g), ~681 ­(A2u (LO)) (LO-Longitudinal optical phonon modes) and ~323 ­cm− 1 ­(Eu (LO)) mode. In this spectrum, ­A1g corresponds to symmetric Sn–O stretching mode and ­B2g represents asymmetric Sn-O stretching vibration [48]. When annealing the sample at 200 °C (B1), the observed peak at ~557 c­ m− 1 is due to A ­ 2u transverse optical (TO) phonon mode and also the peak appeared at ~323 ­cm− 1 is due to ­(Eu) TO mode [49] which is not present when annealing temperature is increased. Rumyantseva et al. [50] also reported that the peak observed at the range of ~300–350 cm− 1 is due to low heat treatment (300 °C) and similarly the present work shows the Raman peak at ~323 cm− 1 is disappeared at increasing temperature (400–800 °C). The intensity of the peak at ~681 cm− 1, is increasing with increase in the annealing temperature (Ta = 400 °C − 800 °C). Reports show that SnO vibrational peak is observed at around ~100–300 cm− 1 [51], which is not present in this work due to less influence of SnO-o phase since only a signature of the SnO-o is observed from XRD pattern. The Raman spectrum of the sample prepared from the solution pH 8 and annealed at 400 °C shows high intensity peak at ~621 c­ m− 1 ­(A1g mode) and ~757 c­ m− 1 ­(B2g mode), which confirm the formation of ­SnO2 tetragonal rutile structure [52]. 3.5 Fluorescence properties Fluorescence (FL) is one of the useful tools used to investigate the optical properties of nanoparticles possessing direct or indirect bandgap and to understand the defects of oxygen vacancies in the oxide materials that act as the radiative centers in the luminescence processes [53]. Electrons excited from valence band to conduction band leave holes when the material is exposed to UV radiation [54]. The tin oxide

A1g ­(cm− 1) B2g ­(cm− 1) A2u (LO) ­(cm− 1)

Eu ν2 (TO) ­(cm− 1)

625 621 625 622 – 621 627 627

– – – 323 – – –

– 757 762 – – 757 769 765

– 681 – – – 681 – 684

samples were excited at 265 nm and the broad dominant emission peaks are observed at 327 nm (~3.79 eV) and at 332 nm (~3.74 eV) in the near band-edge emission due to band-to-band recombination process. The FL spectra of tin oxide nanoparticles prepared from the Experiment 1 and Experiment 2 are shown in Fig. 6a, b respectively. Generally, in the preparation of nanoparticles the oxygen vacancies are the most common defects which act as radiative centers in luminescence action [55]. Mishra et al. [56] reported that the ­SnO2 emission peaks occurred in the range of 400–500 nm are due to the doubly ionized oxygen vacancies and the present work shows a weak emission peak at ~409 nm (~3.0 eV) for all the samples. In the present work there are no appreciable shifts in the FL peak position for the samples prepared in the Experiment 2 whereas the peak intensity increases with increase in the annealing temperature. 3.6 I‑V characteristics SnO2-t phase prepared from solution pH 8 and annealed at 400  °C (A2) was used to deposit ­S nO 2 film on the glass substrate by simple drop casting method in order to investigate the electrical property using I–V characteristic study. The prepared sample (A2) was made as a paste and thin film was deposited using drop casting technique on the glass substrate. Then the film was dried in air at 60 °C for 1 h. The contact was made over the prepared film using silver paste and again dried in hot air oven under 60 °C for 1 h. I–V characteristics measured for the sample exhibit good ohmic behavior (Fig. 7). Resistance (R) of the sample was calculated using the standard formula R = V/I where, V is the voltage and I is the current. When temperature is constant, the resistance measured at 1V is found to be 1.6 MΩ. The ­SnO2-t phase prepared from pH 8 and annealed at 400 °C shows the low resistance. Johari et al. [57] reported that the ­SnO2 nanowires prepared using vapor–liquid–solid mechanism gives the resistance value of 5 MΩ at 200 °C which reveals good ohmic behavior.

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Fig.  6  a Fluorescence spectrum of tin oxide nanoparticles prepared from various solution pH and annealed at 400 °C; pH 6 (A1), 8 (A2), 10 (A3) and 12 (A4). b Fluorescence spectrum of as-synthesized tin

J Mater Sci: Mater Electron

oxide nanoparticles from pH 8 and annealed at different temperatures 200 °C (B1), 400 °C (B2 = A2), 600 °C (B3) and 800 °C (B4)

4 Conclusions

Fig. 7  I–V characteristics of tin oxide nanoparticles synthesized from solution pH 8 and annealed at 400 °C (A2)

Huang et al. [58] reported that doping Mn with tin oxide reduces the resistance from 1.61 × 109 Ω to 1.65 × 108 Ω. From the present results, the I–V curve shows better ohmic behavior and the resistance of the film increases with increase in the annealing temperature. Thus the results show that resistance obtained for pH 8 and annealed at 400 °C in the present work is slight less than the reported values [57, 58]. Further increase in the annealing temperature increases the current due to increase in the oxygen vacancies. This fact can also be seen from the increase in FL intensity when the annealing temperature is increased [59, 60].

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Sol–gel method based tin oxide nanoparticles were synthesized by varying pH of the precursor solution and the prepared samples were annealed at different temperatures. Overall, the prepared sample A2 annealed at 400 °C shows slightly agglomerated spherical shaped morphology and confirms the formation of single ­SnO2-t phase. The formation of mixed phases ­SnO2-t and SnO-o is due to annealing the samples at 600 and 800 °C. The absorption edge observed at 278 nm in the sample (A2) is shifted from the bulk value 337 nm due to quantum confinement effect. The Raman peaks appeared at ~621 and ~757 cm−1 confirm the formation of ­SnO2-t phase and the presence of weak FL emission peak at 409 nm is due to the oxygen vacancies in the prepared tin oxide nanoparticles. ­SnO2-t phase nanoparticles prepared from solution pH 8 and annealed at 400 °C shows an average crystallite size of ~10 nm and hence increase in the band gap value to ~4.5 eV. I–V study shows ohmic contact nature and the resistance obtained is 1.6 MΩ for the sample A2 which may be useful for sensing application. Acknowledgements  One of the authors (MP) sincerely thanks SRM University, Chennai, for the award of SRM research fellowship to carry out the work. The authors gratefully acknowledge Prof. D. John Thiruvadigal, Dean of Science and humanities, Dr. Preferencial Kala, Head, department of Physics and Nanotechnology and SRM University for extending the facilities created under (DST-FIST SR/ FST/PSI-155/2010). The authors also thank Centre for Nanoscience and Nanotechnology and to Nanotechnology Research Centre, SRM University, for extending the facilities.

J Mater Sci: Mater Electron

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