Bio-green synthesis of Ni-doped tin oxide nanoparticles and its influence on gas sensing properties

Abstract

Considering the potential applications of transition metal doped nanostructured materials and the advantages of novel, cost-effective and environmentally friendly biosynthesis methods, Ni-doped SnO₂ nanomaterials have been synthesized using remnant water (ideally kitchen waste) collected from soaked Bengal gram bean (Cicer arietinum L.) extract. The structural and optical properties of the Ni-doped SnO₂ nanostructures were studied using various techniques such as UV/visible spectroscopy, FT-IR spectroscopy, X-ray powder diffraction (XRD), field-emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). The SEM and TEM images and the XRD results of the biosynthesized Ni–SnO₂ nanoparticles reveal a uniform size distribution with an average size of 6 nm and confirmed the formation of a rutile structure with the space group (P42/mnm) and the nanocrystalline nature of the products with a spherical morphology. Subsequently, Ni-doped biosynthesized SnO₂ nanoparticles were coated onto a glass substrate using the doctor blade method to form thin films. The NO₂ sensing properties of the materials have been studied in comparison with other gases. The reported gas sensing results are promising, which suggest that the Ni-dopant is a promising noble metal additive to fabricate low cost SnO₂ based sensors.

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Introduction

As an n-type semiconductor, SnO₂ has been extensively studied for various applications, including gas sensors,^1–5 transparent conducting electrodes,^6,7 catalyst support,^8–10 Li-ion battery anode materials^11,12 etc. Among these applications of SnO₂, gas sensor applications have been widely explored. A gas sensor is generally characterized by its speed (response–recovery time), selectivity and sensitivity,¹³ among which, the sensitivity is important, because higher sensitivity usually means potential sensing ability for detection of gas at a low concentration level. Among various toxic gases, nitrogen dioxide (NO₂) gas is well known as an irritant gas that can cause respiratory infections, photochemical smog, acid rain¹⁴ and it is harmful to human health and life. Thus, it is imperative to develop an applicable sensor for detecting NO₂ gas. With this motivation, extensive efforts have been focused on improving sensor sensitivity towards NO₂ gas. Studies have proved that the properties and performances of SnO₂-based devices are dramatically influenced by structural features. Therefore, most attention has been paid to the synthesis of nanostructured SnO₂ materials with the intension of tuning their properties for selectivity and sensitivity. 1D SnO₂ nanorods,¹⁵ SnO₂ nanosheets,¹⁶ SnO₂ nanoflowers,¹⁷ SnO₂ hollow spheres,¹⁸ 3D porous flower-like SnO₂,¹⁹ microporous Sn–SnO₂/carbon heterostructures,²⁰ SnO₂–carbon composite nanotubes,²¹ hollow porous SnO₂ microcubes²² etc. are a few different forms of SnO₂ nanostructures used in gas sensors.

Apart from this, one of the most important methods for modifying the materials is the introduction of dopants into the host system. Numerous experimental investigations have been performed to improve the responses of the sensors via introducing various dopants of either metal oxides^23,24 or noble metals (Pd, Au, Pt, and Ag)^25–27 considering the formation of p–n junctions or the catalytic activity of noble metals. Although noble metal additives have been proven to be very effective for improving sensor performance, their high cost restricts their potential scope for practical applications. Thus, as an economic substitute, SnO₂ doped with transition metal ions such as Co, Mn, Fe, Ni, and Cr has been explored to serve as a sensitizer to decrease the synthetic cost of sensor materials while improving their sensitivity. These dopants cause smaller SnO₂ nanostructure sizes and promote stabilization of their surface. Among the dopants, Ni is of special interest because of its capability for grain growth inhibition within the SnO₂ matrix.²⁸ Thus, with a need for a simple, economic and room temperature operated SnO₂ gas sensor, here in this work, we present a novel sensor material of Ni-doped SnO₂ nanoparticles biosynthesized by a completely green approach using remnant water (ideally kitchen waste) collected from soaked Bengal gram beans (Cicer arietinum L.). The natural bio-molecule present in the extract which is responsible for biosynthesis of the SnO₂ nanoparticles was found to be pectin. Pectin is a complex polysaccharide that is present in most primary cell walls. It is a natural part of the human diet, but does not contribute significantly to nutrition.²⁹ Pectin is commonly used in the food industry as a gelling and stabilizing agent, as pectin molecules can bind some organic and inorganic materials via molecular interactions, thus finding a use in targeted drug delivery.³⁰

Here, in this work, the gas sensing properties of biosynthesized Ni-doped SnO₂ for NO₂ gas and the concentration-dependent behaviours of the sensors to NO₂ are investigated. As well as this, the gas sensing properties of Ni doped SnO₂ for NO₂ gas, are compared with its sensing of other gases like NH₃, LPG and H₂S.

Experimental

In a typical synthesis of Ni-doped tin oxide nanoparticles, 20 g of dry Bengal gram beans (Cicer arietinum L.) were soaked in 100 mL of DI water for 6 h at room temperature. Thereafter, the soaked seeds were removed and the extract was filtered using a glass-fiber filter (GF/F) to be free from particulate matter. 10 mL of an aqueous SnCl₄ (0.01 M) solution was added to 10 mL of the gram bean extract and diluted to 50 mL. To this solution, an appropriate amount of NiCl₂·6H₂O was added to 5 wt% doping. The solution was centrifuged and the powder so obtained was calcined at 600 °C to remove the organic contaminants. This powder was then coated onto the glass substrates to form thin films using the doctor blade method, annealed at 250 °C and was used for further characterizations and gas sensing applications. The Ni-doped SnO₂ nanoparticles so produced were characterized for their preliminary structural and morphological properties. Gas sensing properties of the biosynthesized Ni-doped SnO₂ nanoparticle thin film was studied in detail and its sensitivity for NO₂ gas was investigated in this work. The gas sensing properties of the biosynthesized Ni-doped SnO₂ nanoparticle thin film is also investigated for different gases like NH₃, LPG and H₂S. For better comparison of the NO₂ gas sensing properties of the biosynthesized Ni-doped SnO₂, both pure SnO₂ and Ni-doped SnO₂ nanoparticles were synthesized using a chemical method (sol–gel) as mentioned elsewhere³¹ and their NO₂ gas sensing properties were explored.

The biosynthesized nanocrystalline SnO₂ and the Ni-doped SnO₂ nanoparticles were investigated using X-ray diffraction (XRD). The XRD spectra of SnO₂ and Ni-doped SnO₂ nanoparticles (powder form) were recorded using a Bruker D8 Advance X-ray diffractometer. The X-rays were produced using a sealed tube and their wavelength was 0.154 nm (Cu Kα₁ radiation). The X-rays were detected using a fast counting detector based on silicon strip technology (Bruker LynxEye detector). The Ni-doped SnO₂ nanoparticles were sputter coated with platinum to avoid charging effects prior to their characterization using scanning electron microscopy (SEM, JEOL, Japan). On the other hand, the ultrasonically dispersed biosynthesized Ni-doped SnO₂ nanoparticles were drop-coated onto the lacey carbon coated Cu grid and dried prior to TEM analysis using a field emission transmission electron microscope (FE-TEM Tecnai G2 20). FTIR spectra were recorded using FTIR (CARRY 600Z series, Japan) to investigate the characteristics of the functional group of the samples. The optical studies were performed using a Labindia UV-Vis Spectrophotometer 3092.

The thin films of both chemically and biosynthesized, pure and Ni-doped SnO₂ nanoparticles were tested for NO₂ gas sensing. Electrical contacts of silver paste separated by 1 mm were deposited on Ni-doped SnO₂ thin films. The sensor was mounted upside down in a stainless steel test chamber (volume: 250 cm³). A desired concentration of the test gas in the chamber was achieved by injecting a known quantity of gas using a micro-syringe. A change in resistance of the film as a function of time (response curve) was recorded at an operating temperature of 200 °C for 100, 60 and 20 ppm concentrations of NO₂ gas, which was commercially procured. The response data was acquired by using a computer interfaced 6514 Programmable Electrometer. The recovery of the sensor was recorded by exposing the sensor to air. From the response curves, the sensitivity (S) was calculated using the relation:

S(%) = (R_g − R_a)/R_a × 100%

where R_a and R_g are resistances in air and test gas (NO₂), respectively.^32–34 Response and recovery times were defined as the times needed for 90% total resistance change on exposure to gas and air, respectively.

Results and discussion

Crystallography

The representative powder X-ray diffraction patterns of pure and Ni-doped SnO₂ (5 wt%) samples are shown in Fig. 1. The diffraction peaks in the spectra were indexed to the rutile phase of the SnO₂ and the lattice parameters were calculated to be a = 4.78 Å and c = 3.21 Å, which is consistent with the literature values (a = 4.775 Å, c = 3.199 Å, JCPDS no. 77-0447), belonging to the space group of P42/mnm. No peaks due to the metallic tin, NiO, Ni or any other Sn based oxide or hydroxide were observed, indicating the high purity of the final product. There have been reports of the occurrence of traces of NiO in the Ni doped SnO₂ for an excessive dopant concentration of 50 wt% Ni.³⁵ In the present study, we have used only 5 wt% Ni as the dopant. Thus, the Ni doped SnO₂ can be expected to be phase pure. The average crystallite sizes of the pure and Ni-doped SnO₂ nanoparticles were determined from XRD peaks by using Scherrer’s formula D = Kλ/β cos θ and noted to be ∼11 nm and 6 nm, respectively, where K is the Scherrer’s constant (K = 0.9), λ is the wavelength of the incident X-rays and β is the full width at half maximum of the XRD peaks expressed in radians. It is known that for a tetragonal structure, the lattice parameters can be calculated by where h, k, l are all integers, (hkl) is the lattice plane index and a and c are lattice constants.

Fig. 1 Representative XRD spectra of pure and Ni-doped biosynthesized SnO₂ nanoparticles calcined at 600 °C.

There is no significant change in the lattice parameters and cell volume, possibly due to the nearly equal size of the dopant Ni ion (0.69 Å) with that of the host ion Sn (0.71 Å). However, upon doping with Ni a pronounced broadening of diffraction peaks is observed. This can be attributed to the decrease of particle size on doping of SnO₂ with Ni through the formation of a Ni–Sn solid solution and/or to the induced microstrain.³⁶

Scanning electron microscopy

The surface morphologies of the biosynthesized SnO₂ and Ni-doped SnO₂ nanoparticles were examined by SEM. Fig. 2 shows the micrograph of pure and Ni-doped SnO₂ nanoparticles calcined at 600 °C demonstrating spherical shaped nanoparticles with a narrow size distribution. However, to further confirm the actual size of the SnO₂ nanoparticles, TEM analysis was also employed.

Fig. 2 Representative FE-SEM image of (a) biosynthesized pure SnO₂ nanoparticles and (b) biosynthesized Ni-doped SnO₂ nanoparticles calcined at 600 °C.

Transmission electron microscopy

TEM studies indicated that both the pristine and Ni-doped SnO₂ (calcined at 600 °C) formed spherical nanoparticles and that the particle sizes decreased due to Ni doping. Fig. 3a shows the representative TEM image of pristine SnO₂ nanoparticles, while Fig. 3b shows the representative TEM image of Ni-doped SnO₂ nanoparticles, calcined at 600 °C. The particles were observed to be monodispersed and the size was noted to be ∼11 nm for pure and ∼6 nm for Ni doped nanoparticles, showing a narrow size distribution. Interestingly, no aggregation of SnO₂ nanoparticles was observed and the connected crystallites of ∼6 nm formed a random network. Selected area electron diffraction (SAED) patterns (insets in Fig. 3a and b) were obtained to investigate the crystallinity. It is observed that the SAED pattern obtained from the nanoparticles showed a ring like pattern, which confirms the polycrystalline nature of the bio-synthesized nanoparticles. It clearly indicates the growth of nanoparticles along the 110, 200, 211 planes (inset in Fig. 3b), which is in agreement with the XRD data. The particle size distribution histograms of pure and Ni-doped biosynthesized SnO₂ nanoparticles calcined at 600 °C are shown in Fig. 3c and d, respectively, demonstrating the average size of the particles.

Fig. 3 Representative TEM images of (a) biosynthesized pure SnO₂ nanoparticles (b) biosynthesized Ni-doped SnO₂ nanoparticles calcined at 600 °C. The insets show corresponding SAED patterns. Particle size distribution histogram plots of (c) biosynthesized pure SnO₂ nanoparticles and (d) biosynthesized Ni-doped SnO₂ nanoparticles.

Fourier transform infrared spectroscopy (FTIR)

Room temperature FTIR spectra of drop casted gram bean extract, the as prepared Ni-doped SnO₂ and Ni-doped SnO₂ calcined at 600 °C are shown in Fig. 4. The FTIR spectra of the extract resembles the IR spectra for pectin present in the extract, and thus an obvious absorption peak at about 514 cm⁻¹ can be found for the pectin–SnO₂ composite sample; this is a typical IR absorption peak originating from the stretching mode of the Sn–OH bond. The remaining peaks in the pectin–SnO₂ composite are induced by pectin, which is confirmed by comparing the IR spectrum of the composite with that of the pectin.³⁷ The peak at 1030 cm⁻¹ is assigned to the C=O or C=C double bond of pectin. The absorption peaks at 1386 and 1579 cm⁻¹ are related to stretching bands of the COO⁻ groups of pectin. It is found that the intensities of the peak around 2925 cm⁻¹ (induced by the carboxyl and CH₂ groups of pectin) can be attributed to the pectin–SnO₂ composite and are obviously weaker than that for pectin. This may originate from the participation of COO⁻ and CH₂ groups in a hydrogen bond system, which stabilizes the pectin conformation in a solid state. The above results indicate that the final product is a true composite of pectin and SnO₂. The pectin peaks were not removed even after washing the sample repeatedly, suggesting that the interactions between pectin and SnO₂ are strong. The peak at around 614–930 cm⁻¹, which refers to Sn–O stretching modes of Sn–O–Sn, appeared even after calcination at 600 °C.

Fig. 4 FTIR spectra of (a) gram bean extract (b) as prepared Ni-doped SnO₂ nanoparticles and (c) Ni-doped SnO₂ nanoparticles calcined at 600 °C.

From the synthesis mechanism summarized in Fig. 5, we can conclude that, when gram beans are soaked in de-ionised water, autolysis of the cell wall occurs, which results in the release of pectin molecules in the extract.^38,39 These molecules have a tendency to bind metal ions.⁴⁰ Thus, when tin chloride is added to the extract, the tin ions get bound to the pectin molecules as shown in Fig. 5. As the pH of the solution is increased, this tin-hydroxide–pectin gel shrinks³⁰ and inhibits the further growth of the nanoparticles. Subsequently, when it is calcined, we get very small sized nanoparticles with a narrow size distribution.

Fig. 5 Schematic representation of the biosynthesis mechanism using gram bean extract.

Absorbance and band gap of Ni-doped SnO₂

Absorption spectroscopy is a powerful, non-destructive technique to explore the optical properties of semiconducting nanoparticles. The absorption spectra of pure and Ni doped SnO₂ nanoparticles are shown in Fig. 6. The absorbance is expected to depend on several factors, such as band gap, oxygen deficiency, surface roughness and impurity centres. The absorbance spectra show an ultraviolet cut-off at around 250–290 nm, which can be attributed to the photo-excitation of electrons from the valence band to the conduction band.

Fig. 6 Band-gap and UV-Vis spectra of pure SnO₂ nanoparticles and Ni-doped SnO₂ nanoparticles calcined at 600 °C.

The Tauc relation is used to calculate the band gap: αhν = A(hν − E_g)ⁿ where α is the absorption coefficient, A is a constant and n = 1/2 for a direct band gap semiconductor. An extrapolation of the linear region of a plot of (αhν)² vs. hν gives the value of the optical band gap E_g. The measured band gap was found to be 3.6 eV for undoped SnO₂ nanoparticles and 3.3 eV for Ni-doped SnO₂. The bandgap of the pure SnO₂ nanoparticles is in agreement with the reported value. However, on doping with nickel, the band gap energy decreases even though the particle size decreases. This is in contrast to the normal phenomenon of quantum confinement. A similar phenomenon was observed by Ahmed et al.⁴¹ for samples containing 5% Ni concentration, where the SnO₂–SnO_2−x alloying effect was considered responsible for the band gap narrowing effect.

Gas sensing

Gas-sensing behaviours of the Ni-doped SnO₂ sensor were explored after NO₂ gas injection. The response–recovery properties of semiconducting oxide gas sensors depend on the operating temperature and gas concentrations. Table 1 shows a comparison of the NO₂ gas-sensing characteristics of sensing material in the present work and those reported in the literature. From the table, we observe that for a higher response at a lower concentration, sophisticated synthesis methods need to be used. Meanwhile in the present work a simple, economic and eco-friendly biosynthesis method is used. For better comparison of the gas sensing properties of the biosynthesized Ni doped SnO₂, the gas sensing properties of both pure and nickel doped biosynthesized SnO₂ and both pure and nickel doped chemically synthesized (sol–gel method) SnO₂ were studied and their respective response curves for 100 ppm NO₂ gas at 200 °C are shown in Fig. 7. The resulting gas responses along with response and recovery time are summarized in Table 2. The sensor resistance increased when exposed to NO₂ gas and consequently recovered to its original value on removal of the NO₂ gas. From Fig. 7 we can clearly see that the response was relatively higher for biosynthesized Ni-doped SnO₂ compared to chemically synthesized Ni-doped SnO₂. The sensor response for chemically synthesized SnO₂ was similar to the reported value.³¹ Furthermore, we observe that the sensor response increased with Ni doping for both the samples. This may be due to the reduction of particle size upon Ni doping,³⁶ which resulted in an increased surface area for adsorption of NO₂ gas. The inset in Fig. 7 shows a decrease in response time and increase in recovery time with an increase in gas concentration for biosynthesized Ni–SnO₂. The decrease in response time may be due to the large availability of vacant sites on the film for gas adsorption and the increase in the recovery time may be due to the heavier nature of NO₂ and the reaction products which are not leaving the interface immediately after the reaction, resulting in a decrease in desorption rate.³¹

Table 1 Comparison of gas-sensing characteristics of the sensing material in the present work with those reported in the literature

Sr. no.	Sensing material	Synthesis method	Analyte gas	Gas concentration	Operating temperature	Response	Ref. no.
1	SnO2 nanobelts	Carbothermal reduction method	NO2	100 ppb	200 °C	∼2	42
2	(rGO–CNT–SnO2)	Hydrothermal method	NO2	5 ppm	RT	2.53	43
3	SnO2 nanowire	Thermal evaporation method.	NO2	10 ppm	100 °C	117	44
4	Ag functionalized SnO2 microrods	Vapor–liquid–solid (VLS) method	NO2	10 ppm	200 °C	2700	45
5	Flower-like In2O3	Solvothermal	NO2	0.2 ppm	140 °C	41.1	46
6	Graphene modified by ZnO	CVD, ALD	NO2	10 ppm	200 °C	38	47
7	Hollow In2O3 microspheres	Hydrothermal	NO2	500 ppb	80 °C	737.8	48
8	Ni-doped SnO2 nanoparticles	Gram bean extract mediated synthesis	NO2	100 ppm	200 °C	40	This work

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Fig. 7 Gas sensitivity for chemically and biosynthesized pure and Ni-doped SnO₂ nanoparticle thin films. Inset shows variation in the response and recovery time of the Ni-doped SnO₂ sensors with varying NO₂ gas concentration.

Table 2 Summarised responses, response time and recovery time

Sr. no.	Sensing material	Response time (s)			Recovery time (s)			Response
	NO2 conc.	20 ppm	60 ppm	100 ppm	20 ppm	60 ppm	100 ppm
1	Chem-SnO2	26	20	14	145	225	310	20
2	Chem-Ni–SnO2	20	15	10	145	172	200	23
3	Bio-SnO2	20	15	10	90	120	145	28
4	Bio-Ni–SnO2	10	10	8	66	84	115	40

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It is well known that the NO₂ sensing mechanism of SnO₂ depends on the surface oxygen adsorbed on the SnO₂ nanoparticle surface. The sensing mechanism involves adsorption of oxygen species on the surface of tin oxide nanoparticles, which causes the removal of electrons, and thus, causes an increase in the potential barrier at the grain boundaries. The gas molecules interact with the oxygen species and produce a notable change in the electronic property of the material. Thus, the density of oxygen species on the surface defines the rate of reaction and the catalytic properties. NO₂ is an oxidizing gas with an electron affinity much higher than oxygen (0.48 eV); NO₂ can interact with SnO₂ by trapping electrons directly through the surface oxygen ions thereby forming new surface electron acceptor levels.^49,50

To explore further the picture of the NO₂ sensing mechanism, we plotted the response at different NO₂ gas concentrations as shown in Fig. 8. The inset shows a power–law relationship of response (S) and NO₂ gas concentration.

Fig. 8 Gas response curve for Ni-doped SnO₂ nanoparticle thin films for different concentrations of NO₂ gas. Inset shows log(response(S)) vs. gas concentration plot.

The power exponent value (m) was fitted as m = 0.48566. These exponent values depend on the final adsorbed species at the surface; when the adsorbed species are NO₂⁻ or O⁻, the exponent values for both are 1. In the case of 2O⁻, the exponent value is 0.5.⁵¹ Thus, our results indicate that the electrons in SnO₂ will transfer adsorbed species to the new surface due to NO₂ adsorption, increasing the surface potential barrier of SnO₂ and thus causing its resistance to increase.

Selectivity was studied as another sensor parameter. The responses of Ni-doped SnO₂ sensors towards a variety of flammable, toxic and corrosive gases including H₂S, LPG, NH₃ and NO₂ of 100 ppm at the optimal operating temperature of NO₂ (200 °C) were explored to evaluate their selectivity. Fig. 9 shows that the gas sensor of ‘Ni-doped SnO₂’ exhibited excellent selectivity to NO₂ gas when compared to other gases. The results suggest that the Ni-doped SnO₂ thin film sensor can be fabricated to monitor highly toxic NO₂ in polluted air.

Fig. 9 Gas selectivity for the Ni-doped SnO₂ sensor for different gases (H₂S, LPG, NH₃ and NO₂) of 100 ppm concentration.

Conclusions

Ni-doped SnO₂ nanoparticles were successfully biosynthesized using remnant water (ideally kitchen waste) collected from soaked Bengal gram beans (Cicer arietinum L.) making it a greener approach. This procedure resulted in the formation of well dispersed nanoparticles with a narrow size distribution and with a size of ∼6 nm. This was confirmed from XRD and TEM analyses. The SAED pattern confirmed the polycrystalline nature of SnO₂ nanoparticles. FE-SEM could confirm the narrow size distribution of the Ni-doped SnO₂ nanoparticles. Gas sensors based on the Ni-doped SnO₂ showed a gas response of 40 at 100 ppm of NO₂ gas, which is a promising result for the biosynthesized Ni-doped SnO₂ nanoparticle thin films for gas sensing applications and can be attributed to the small size and uniform distribution of Ni-doped SnO₂ nanoparticles within the thin film. The synthesis method described in this work is facile and versatile, providing opportunities to control the morphology of various other semiconducting metal oxides, with particular promise for application in gas sensors.

Acknowledgements

The authors are thankful to the UGC-DAE Consortium for Scientific Research, Indore (Project Ref. No: CSR-I/CRS-48/48) and UGC, New Delhi (F. No. 41-370/2012 (SR)) for the financial support. We are also thankful to the Department of Nanotechnology, Dr B. A. M. University for providing the laboratory facility.

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