1 Introduction

Inorganic tin dioxide (SnO2) is the most important material widely used in electronic [1–12] and gas sensing [13–18] applications. For example, Agrahari et al. [19] investigated optoelectronic properties of SnO2 nanoparticles. Glot et al. [20] explored electrical properties of SnO2 ceramics for low voltage varistors. Gajendiran et al. [21] studied nano structured aggregated tin oxides (SnO2/SnO) and their structural and photoluminescence properties employing hydrothermal method. Han et al. [22] revealed first principles study of magnetic properties of stoichiometric and O deficient low index surfaces of rutile SnO2. Yang et al. [23] explored the preparation and characterization of p-type transparent conducting SnO thin films. Xu et al. [24] investigated NH3 gas sensing performance based on electrospun alkaline earth metals composited SnO2 nanofibers. Reddy et al. [25] revealed low temperature molten salt preparation of nano SnO2 as anode for lithium-ion batteries. Tao et al. [26] investigated a new way for synthesizing SnO2 nanosheets. Andersen et al. [27] studied tin dioxide as an effective antioxidant for proton exchange membrane fuel cells. Chen et al. [28] investigated low temperature synthesis of tin dioxide hollow nanospheres and their potential applications in dye sensitized solar cells and photo electrochemical type self powered ultraviolet photo detectors.

In general, the preparation of SnO2 nanostructure involves minimum neither two precursors nor high temperature, high cost and template assisted synthesis [19–28]. In recent years, mild and sustainable biosynthesis of nanostructures using green protocols has been studied extensively. However, the reports on biosynthesis of SnO2 nanostructures are scanty. For example, Vidhu et al. [29] studied biogenic synthesis of SnO2 nanoparticles and its evaluation of antibacterial and antioxidant activities. Srivastava et al. [30] employed biosynthesis of SnO2 nanoparticles using bacterium erwinia herbicola and their photo catalytic activity for degradation of dyes. In the present work, SnO2 nanocrystals were prepared employing rambutan (Nephelium lappaceum L.) peel extract mediated green synthesis route without surfactant and template at room temperature incubation and aging over a period of 1, 4 and 7 days. The prepared nanocrystals were characterized employing standard characterization techniques.

2 Experimental sections

2.1 Synthesis of SnO2 nanocrystals

Initially, 0.1 M aqueous solution of tin (IV) chloride pentahydrate \(\left[ {{\text{SnCl}}_{ 4} \cdot 5 {\text{H}}_{ 2} {\text{O}}} \right]\) was prepared. Then, 10 ml rambutan peel extract was added to 50 ml of 0.1 M \(\left[ {{\text{SnCl}}_{ 4} \cdot 5 {\text{H}}_{ 2} {\text{O}}} \right]\). The detailed synthesis procedure was given in our earlier reports [31–34]. The product was stirred about 80 °C for 2 h. The obtained product was then centrifuged; air dried and powdered using mortar and pestle. This powdered sample was calcinated in a muffle furnace at 450 °C for 1 h to get pure SnO2 nanocrystals.

2.2 Characterization of SnO2 nanocrystals

The phase and crystalline nature were identified employing X-ray powder diffraction patterns (Bruker AXS, Germany) using CuKα as radiation (1.5406 Å) source. High resolution (Dispersive Raman microscope, Horiba Jobin–Yvon; LabRAM HR UV/vis/NIR) Raman spectroscopy (excitation, 514 nm) was used to confirm SnO2. Photoluminescence (PL) spectrum was measured (under excitation by 325 nm) with a SPEX double monochromator equipped with L–N2 cooled Ge detector. Infrared (IR) spectra were recorded employing a Fourier transform infrared spectrophotometer using a Nicolet 520P FT-IR spectrometer. X-ray photoelectron spectra (XPS) were obtained using a MultiLab 2000 spectrometer (Thermo Electron Corporation, UK). Transmission electron microscopic images were obtained using a transmission electron microscope (CM 200; Philips, USA).

3 Results and discussion

The technique for the formation mechanism of metal oxide nanostructures was already discussed in our previous studies [31–34]. The same technique was employed successfully to synthesize SnO2 nanocrystals in the present study. The structure, phase and morphological variations of SnO2 nanocrystals incubated at room temperature over a period of 1, 4 and 7 days aging was described in the present study. The XRD pattern of 450 °C calcinated reaction product from the mixture of 0.1 M \({\text{SnCl}}_{ 4} \cdot 5 {\text{H}}_{ 2} {\text{O}}\) and 10 ml extract incubated with 1, 4 and 7 days of final product are shown in Fig. 1a–c. The obtained product unambiguously revealed the cassiterite type tetragonal rutile phase of SnO2 nanocrystals. The diffraction peaks (110), (101), (200), (211), (220), (002), (310), (112), (301), (202), (321), (222) and (321) were clearly detected in XRD pattern of the incubated samples which closely matched with expected cassiterite type tin dioxide nanocrystals (JCPDF card file 01-072-1147 (A)) with lattice constant a = 4.73700, b = 4.73700, c = 3.18500 Å and α = 90, β = 90, γ = 90. Rutile has tetragonal \({\text{D}}^{ 1 4}_{{ 4 {\text{h}}}}\) symmetry. All the prominent peaks in the pattern of polycrystalline SnO2 were indexed on the basis of JCPDF card file 01-072-1147 (A). No other SnO2 phases was observed and hence revealed high purity of the product [JCPDF card 01-072-1147 (A)]. The relative intensities of the peaks increased over a period of incubation period. The average crystallite size of the prepared tin oxide nanocrystals was calculated using the X-ray diffraction pattern employing the Debye Scherer formula D = kλ/(βcosθ), where λ is the wavelength of the X-ray radiation, k is a constant taken as 0.898, θ is the diffraction angle and β is the full width at half maximum [31]. The average size of the SnO2 nanocrystallites deduced from Sherrer’s formula for the strongest peak (101) was about 48.7 nm which was nearly consistent with the results observed from the TEM images. Over a period of incubation, diffraction peaks full width half maxima (FWHM) decreased with increased aging period and produced increased average crystallite size. It can be clearly revealed that the peaks were broadened at 1 day incubated product when compared to higher period of incubation.

Fig. 1
figure 1

XRD pattern: reaction product from the mixture of 0.1 M \({\text{SnCl}}_{ 4} \cdot 5 {\text{H}}_{ 2} {\text{O}}\) and 10 ml extract (a) 1, (b) 4, (c) 7 days

Full size image

In addition, the structural and electronic properties of 1, 4 and 7 days incubated product were examined employing Raman spectroscopy. Figure 2a–c shows the Raman spectra of SnO2 nanocrystals incubated at 1, 4 and 7 days respectively. In general, the expected normal lattice vibrations at Brillouin zone gamma point are Γ = 1A 1g  + 1A 2g  + 1A 2u  + 1B 1g  + 1B 2g  + 2B 1u  + 1E g  + 3E u according to P42/mnm space group. The characteristic Raman peaks observed at 249, 476, 631 and 773 cm−1 were respectively revealed infrared (IR) active (Eu), Raman active (Eg), (A1g) and (B2g) vibration modes of pure tetragonal rutile SnO2 structure. Stable rutile phase formation was clearly evidenced in 7 days of incubation treatment. The overall observation from XRD and Raman results confirmed cassiterite type tetragonal rutile structure SnO2 characteristics.

Fig. 2
figure 2

Raman spectra: reaction product from the mixture of 0.1 M \({\text{SnCl}}_{ 4} \cdot 5 {\text{H}}_{ 2} {\text{O}}\) and 10 ml extract (a) 1, (b) 4, (c) 7 days

Full size image

To explore the defect states of SnO2 nanocrystals in detail, photoluminescence measurements were carried out at room temperature. Room temperature PL spectra of 1, 4 and 7 days of reaction product mixture are shown in Fig. 3a–c at an excitation wavelength of 325 nm. The obtained nanostructured SnO2 PL spectra clearly exhibited a strong PL emission in the visible region from 400 to 600 nm ascribed to surface defects related to oxygen vacancies in the near surface region. The obtained nanostructures revealed high degree of crystallinity and broad PL emission peaked in the green emission range 520–540 nm in the visible emission. Due to SnO2 wide bandgap energy, the obtained emission band cannot be assigned to band-edge or to exciton recombination emission. The observed clear individual emission band at 543 nm may be attributed to radiative states of the visible PL emission. It is commonly attributed to oxygen vacancies of tetragonal rutile SnO2. A similar result was observed in XPS studies.

Fig. 3
figure 3

PL spectra: reaction product from the mixture of 0.1 M \({\text{SnCl}}_{ 4} \cdot 5 {\text{H}}_{ 2} {\text{O}}\) and 10 ml extract (a) 1, (b) 4, (c) 7 days

Full size image

The FTIR spectra of 1, 4 and 7 days incubated product mixture are shown in Fig. 4a–c. The spectra were recorded in the range of 500–4,000 cm−1. The absorption band observed at 405, 430 and 507 cm−1 were associated with Sn–O stretching modes of vibration. The appearance of this band clearly revealed the presence of SnO2 in the crystalline phase. No other band was observed. The obtained IR bands confirmed the formation of SnO2 nanocrystals which can be correlated in line with the XRD results.

Fig. 4
figure 4

IR spectra: reaction product from the mixture of 0.1 M \({\text{SnCl}}_{ 4} \cdot 5 {\text{H}}_{ 2} {\text{O}}\) and 10 ml extract (a) 1, (b) 4, (c) 7 days

Full size image

The composition and chemical valence state of the elements existing in SnO2 nanocrystals was systematically characterized employing XPS as shown in Fig. 5a–l. Figure 5a–c shows the survey XPS spectra of 1, 4 and 7 days of reaction products. From survey XPS spectra (Fig. 5a–c), only Sn and O related core levels were detected which again confirmed the purity of SnO2. The high resolution spin–orbit components (3d 3/2 and 3d 5/2) of the Sn3d peak were both observed at 496 and 487 eV as shown in Fig. 5d–f corresponding to Sn4+ in a tetragonal rutile structure  [35, 36]. And the peaks corresponding to Sn2+ were not detected. Figure 5g–i shows the O1s core level spectra. The calculated composition of SnO2 nanocrystals was given in Table 1. As the incubation and aging treatment was increased.

Fig. 5
figure 5

XPS pattern: (a–c) Survey, (d–f) Sn3d, (g–i) O1s, (j–l) C1s scan of 1, 4 and 7 days of reaction product from the mixture of 0.1 M \({\text{SnCl}}_{ 4} \cdot 5 {\text{H}}_{ 2} {\text{O}}\) and 10 ml extract

Full size image
Table 1 XPS compositional characterization of the obtained product
Full size table

TEM images of the prepared nanocrystalline SnO2 nanocrystals for different incubation and aging treatment are shown in Fig. 6a–i. The TEM images of 1 day incubated SnO2 product clearly revealed the size ranges from 100 to 250 nm. However, the average size of 4 day incubated SnO2 product were 175–350 nm. Moreover, the growth and shape of the nanostructures particularly depend on the incubation and aging treatments. 7 day incubated product was clearly revealed that the size of the obtained product was in the range of 200–400 nm. The selected area electron diffraction (SAED) patterns clearly revealed the ring as well as lattice fringe formation when incubation period was increased. Three main identified planes such as (1 1 0), (1 0 1), (2 1 1) observed in SAED pattern (Fig. 6c) were in good agreement with the results obtained from XRD pattern. The size of the particle increased when incubation and aging treatment was increased. Little agglomerated particles formation was observed at 1 day when compared to 4 and 7 days. Well isolated irregular shaped crystallites were observed at 1 day aging treatment, while more aggregated crystallites was observed at 4 and 7 days of aging treatment. The results were consistent with the crystallite size as calculated from XRD pattern. The obtained TEM micrographs clearly revealed that the grain size and agglomeration was increased at higher incubation and aging treatments.

Fig. 6
figure 6

TEM image: a–c 1, d–f 4, g–i 7 days of reaction product from the mixture of 0.1 M \({\text{SnCl}}_{ 4} \cdot 5 {\text{H}}_{ 2} {\text{O}}\) and 10 ml extract

Full size image

The obtained product results in the present study have been compared with Vidhu et al. and Srivastava et al. study. Vidhu et al. [29] studied biogenic synthesis of SnO2 nanoparticles and its evaluation of antibacterial and antioxidant activities. They emphasized an unexploited, cost effective and environmentally benign method of synthesizing bioactive tin oxide nanoparticles of size from 2.1 to 4.1 nm using Saraca indica flower. They further revealed an increase in particle size was observed with increasing annealing temperature. They could attained fine tuning of band gap as evidenced in photoluminescence emission. They further explored that oxygen vacancies play an important role on PL emission. In a separate study, Srivastava et al. [30] employed biosynthesis of SnO2 nanoparticles using bacterium erwinia herbicola and their photo catalytic activity for degradation of dyes. They were synthesized tetragonal SnO2 nanoparticles (15–40 nm) employing a green biological synthesis technique using Gram-negative bacteria Erwinia herbicola followed by an annealing treatment over 425 K. They demonstrated that the bacterial protein and biomolecules served as the template for reduction, stabilization and controlling size and aggregation of SnO2 nanoparticles. In the present work, SnO2 nanocrystals were prepared employing rambutan peel extract mediated green synthesis route without surfactant and template at room temperature incubation and aging over a period of 1, 4 and 7 days. The utilized rambutan peels has the capability of ligating SnO2 nanocrystals [31–34]. 1, 4 and 7 days of incubation and aging treatment increased the size, crystallinity and pure phase of the obtained cassiterite type tetragonal rutile SnO2 nanocrystals which could be useful and applied for potential industrial applications.

4 Conclusions

In summary, tetragonal rutile phase SnO2 nanocrystal over a period of incubation was explored employing a simple and cost effective sustainable green synthesis method. The structure, morphology and compositional property of SnO2 nanocrystal were characterized by XRD, Raman, PL, IR, XPS and TEM studies. The incubation and aging treatment affects the particle size of the synthesized samples. The obtained green synthesized pure rutile phase SnO2 nanocrystals were showed good structural, compositional and textural properties beneficial in the development of potential industrial applications.