Analyzing the role of Ni dopant to change the structural, optical and photocatalytic properties of SnO2 nanoparticles

Lipan Pradhan; Shantinarayan Rout; Bipin Kumar Parida; S R Mohapatra; R Sakthivel; Debadhyan Behera

doi:10.1088/2043-6262/acebd7

1. Introduction

Metal oxide semiconductors like TiO₂, ZnO, and SnO₂ are continually drawing attention of researchers as these are promising candidates for a number of technological applications due to their light absorption properties, charge-transfer characteristics and electronic structures. Among these, bulk crystalline SnO₂ possesses tetragonal rutile structure (cassiterite phase), oxygen deficient n-type electrical conductivity and a direct band gap (E_g) equal to 3.6 eV (344 nm). The unit cell consists of two six-fold coordinated tin and four three-fold coordinated oxygen atoms. It is one among many metal oxides extensively investigated as transparent conducting material due to excellent optical and electrical properties. Tin oxide nanoparticles (NPs) have shown high active surface area, quantum confinement, larger band gap and high chemical (thermal) stability. Due to these properties, SnO₂ finds itself in many applications which include gas sensors, photocatalysts, medicines, field emission displays, photo-sensors, antistatic coatings, electrodes in lithium-ion batteries and solar cells, etc [1–5]. Dimension of these NPs is a significant aspect from the application point of view. Among various synthesis techniques, hydrothermal process offers a viable and attractive option to researchers as it adopts simple operation, low cost, easily processable and environment friendly strategy.

Like varying the experimental parameters, doping is considered as an important strategy to tune the optical and electrical properties of NPs to suit their use for any particular application. A large amount of research may be found to be devoted to modifying its optical absorption properties for use as photocatalyst. This is achieved by substituting elements or by controlling nanoparticle size and morphology or by causing internal strain within materials. Doping metal cations in SnO₂ was established to be a successful strategy to enhance the response towards visible light region and obstruct charge carrier recombination. A wide range of metal ions like Mn [6], Co [7], Ni [8], Cu [9], Zn [10], Sb [11], Bi [12] and Ce [13], etc are investigated as dopants in SnO₂ for their suitability towards degradation of hazardous dye molecules present in waste water released from food, paper, textile and leather tanning industries, etc This waste water treatment process has advantages over other processes like biodegradation, physical adsorption and reverse osmosis. These methods involve incomplete degradation of dye molecules and may lead to producing secondary pollutants in waste water.

This study reports a simple template-free hydrothermal method to synthesise undoped and 5 wt% Ni doped SnO₂ nanoparticles (SnO₂:Ni NPs). Further, a low valence cation e.g. Ni in SnO₂ creates a defect state having energy 0.26 eV below the conduction band and it acts as an electron trap by virtue of which it may hinder the recombination process of photogenerated electron–hole pairs. This is a condition essential for realisation of enhanced photocatalytic activity in nanoparticles. At the same time, although radius of Ni²⁺ ion is the same as that of Sn⁴⁺ ion, the formation of defects is expected in the host lattice SnO₂ at 5 wt% Ni doping concentration as it approaches the solubility limit [14]. However, there is still lack of complete understanding about the effect of defects on photocatalytic activity. So, there is continuous effort to understand photocatalytic applicability of SnO₂ over last few years [15, 16]. This study is an attempt in this direction to find an answer by analysing the properties of samples with the help of some basic, but essential techniques such as x-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), UV-visible spectroscopy, x-ray photoelectron spectroscopy (XPS) and micro-Raman spectroscopy. These techniques are widely accepted essential tools for the characterisation of nanosized materials to probe crystalline phase, size, strain and lattice defects, etc in solids. The synthesised NPs are expected to offer their suitability for many potential applications. In this study, the photocatalytic activity of SnO₂ NPs is investigated towards degradation of Rhodamine B (RhB) solution.

2. Materials and methodology

Tin oxide nanocrystalline powders were synthesised by hydrothermal method [17]. All chemicals taken were of analytical grade and used without further purification. 2.707 gm (0.15 M) SnCl₂ $\cdot$ 2H₂O and 7.2072 gm (1.5 M) urea were dissolved in 80 ml distilled water and stirred for 0.5 h. The suspension solution was then heated in an autoclave at 150 °C for 24 h. The precipitate obtained was washed with distilled water and harvested by processing with several centrifugation and re-dispersion cycles [18]. The product was sintered at 700 °C for 7 h in air to produce SnO₂. To prepare Ni doped SnO₂ sample, requisite amount of nickel acetate was added to the precursor solution to obtain 5 wt% with respect to Tin in the final solution and followed the same procedure as in case of SnO₂.

The phase of the samples was determined by XRD (Rikagu, Ultima IV) using CuKα radiation (λ = 1.54056 Å) and scanning 2θ at the rate of 2° min⁻¹ in the range 5°–80°. The surface morphology was investigated using FESEM (Zeiss, Gemini 300). Chemical composition of the samples was analysed qualitatively using energy dispersive x-ray analysis (EDAX). The nanoparticles are further probed for their internal crystalline structure by TEM (JEOL JEM 2100 PLUS) operating at 200 kV. Optical absorbance spectra were recorded in the wavelength range 200–800 nm using a UV–vis spectrophotometer (Cary Win 300 UV–vis). For this purpose, 10 μM solutions were prepared by dispersing each sample in ethanol. Raman measurement was carried out in the range of 350–1000 cm⁻¹ using a micro-Raman spectrometer (Horiba) with 532 nm laser excitation.

The surface chemical-state compositions of the nanostructures were analysed by XPS in a Thermo-VG Scientific ESCALab 250 microprobe with a monochromatic Al Kα x-ray source (1486.6 eV), operated with a typical energy resolution of 0.4−0.5 full width at half-maximum.

The photocatalytic activity was accounted by degrading the Rhodamine B solution. For this purpose, 0.25 g of SnO₂ NPs was mixed with 20 ppm of Rhodamine B and the resulting solution was stirred under UV illumination of peak wavelength 365 nm. The sample was measured for optical absorbance at intervals of 10 min till complete degradation of Rhodamine B solution occurred.

3. Results and discussions

3.1. X-ray diffraction

Figure 1 shows the XRD patterns of the SnO₂ and Ni doped SnO₂ NPs. Both the NPs are crystalline and showed many sharp Bragg diffraction peaks. The dominant peaks for the SnO₂ are observed at 2θ = 26.43° and 2θ = 33.88° which are assigned respectively to the (110) and (101) planes of the rutile-type cassiterite phase of SnO₂ (JCPDS card No. 71–0652). Also, the two diffraction patterns showed other prominent peaks of lower intensity, which are identified according to standard patterns [19]. Further, phase purity of the synthesised samples was investigated by Rietveld refinement of SnO₂ and 5 wt% Ni doped SnO₂ NPs. The refinement was done using FULLPROF software [20]. Rietveld refined XRD patterns indicate formation of single phase having tetragonal crystal structure with space group 'P4₂/m n m'. The fitting parameters obtained after structural refinement are depicted in table 1. The goodness of fit (χ²) for SnO₂ and 5 wt% Ni doped SnO₂ was found to be 3.41 and 4.40 respectively. This indicates a good agreement between the observed and refined patterns for the samples. Moreover, as compared to the parent sample (SnO₂), a decrease in lattice parameters in 5 wt% Ni doped SnO₂ is observed with the shifting of dominant peaks i.e. 2θ = 26.43° and 33.88° to higher values. For brevity, the peak at 2θ = 26.43° is shown as inset in figure 2. This feature could be due to the lower ionic radii of Ni²⁺ ion substituting the higher ionic radii of Sn⁴⁺ ion.

Table 1. Crystal structure parameters of SnO₂ and 5 wt% Ni doped SnO₂ nanoparticles.

Samples	SnO₂	5 wt% Ni doped SnO₂
Space group	P4₂/m n m	P4₂/m n m
Lattice parameters
a (Å) = b (Å)	4.74071(5)	4.73607(13)
c (Å)	3.18839(9)	3.18540(7)
Cell volume (Å³)	71.65693(9)	71.44966(8)
R-factors (%)
R_p	11.5	13.4
R_wp	14.6	16.7
χ²	3.41	4.40
<Sn-O>_avg	2.062 Å	2.058 Å

**Figure 2.** Rietveld refinement of x-ray diffraction patterns of SnO₂ and 5 wt% Ni doped SnO2 NPs.
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The crystallite size and strain in both samples were determined using Williamson-Hall plot [21]. Crystallite size was found to be 39 nm in SnO₂ and 35 nm in Ni doped SnO₂ samples. Similarly, lattice strain was determined to be 3.56 × 10⁻³ in SnO₂ and 3.99 × 10⁻³ in Ni doped SnO₂ samples. So it may be noticed that crystallite size decreases with 5 wt% Ni doping. Though both Ni²⁺ and Sn⁴⁺ have comparable ionic radii, the slight decrease in crystallite size may be attributed to formation of Ni-O-Sn. It has been reported that when Ni²⁺ ions replace Sn⁴⁺ ions, crystal defects are formed in the crystal lattice which leads to generation of oxygen vacancies (V_O) [22].

3.2. Electron microscopy

Figure 3 shows the FESEM images of SnO₂ and Ni doped SnO₂ NPs recorded at similar operating conditions. In SnO₂, spherical NPs are formed which are embedded in hollow tubular microstructures. But, in Ni doped SnO₂, the shape of NPs is elongated and irregular. It is also supported in TEM observation (figure 4). The morphology and internal crystallinity of the constituent nanoparticles are also probed by transmission electron microscope. The dark field images and corresponding selected area electron diffraction (SAED) patterns are represented in figure 4. Both samples indicate polycrystalline nature in their SAED patterns. Concentric rings corresponding to lattice planes (110), (101), (200) and (211) as depicted in XRD analysis are drawn in SAED patterns of both samples. More number of bright diffraction spots in SAED pattern of Ni doped SnO₂ shows improved crystallinity compared to undoped NPs.

**Figure 4.** Transmission electron microscopic views of (a) SnO₂ and (b) 5 wt% Ni doped SnO₂ NPs. Corresponding SAED patterns on the right hand side.
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Besides microscopic characterisation, for finding the purity or contents of materials in the prepared sample, EDAX spectra were recorded as depicted in figure 3. The two most intense peaks corresponding to SnLα and OKα in the NPs confirm the SnO₂ phase. NiKα line in the doped sample depicts the presence of Ni. The atomic % and weight % calculated by the EDAX analytic software are shown as inset in the EDAX spectrum figures. Although, EDAX poses a qualitative approach and does not reflect the true values about the concentration of various elements present in the samples, still it may be inferred that SnO₂ is Sn rich [Sn/O = 0.666], whereas almost stoichiometry is maintained in Ni doped SnO₂ sample [(Sn+Ni)/O = 0.497]. It may also be said that Ni is incorporated in the SnO₂ host at Sn sites and has significant influence on the structural properties.

At normal condition, four significant point defects exist in SnO₂, i.e. oxygen vacancies (V_O), tin vacancies (V_Sn), oxygen interstitials (O_i) and tin interstitials (Sn_i). Among these, V_O and Sn_i are major defects as they are donor defects and contribute to the n-type intrinsic conductivity of SnO₂. Further, V_O has lowest formation energy, thereby becoming dominant donor defect in SnO₂ [23]. This is also verified experimentally [24, 25]. However, presence of Sn_i states cannot be ruled out. If formation of oxygen vacancies is considered to be promoted in Ni doped samples [22], then higher oxygen content in Ni doped sample is attributed to chemisorptions of free oxygen molecules at the surface sites of nanoparticles as discussed later.

3.3. Optical absorption

The absorbance spectra and optical band gap for SnO₂ are shown in figure 5(a). Better transparency is found to be possessed by Ni doped SnO₂ NPs compared to SnO₂ NPs. This can be attributed to the higher oxygen content in the doped sample as found from its EDAX spectrum. As SnO₂ has direct band gap (E_g), it is determined by extrapolating the linear part of the (αhν)² versus hν plot (known as Tauc plot) to energy hν axis [26]. Accordingly, it is found to be 3.97 eV and 4.11 eV for SnO₂ and Ni doped SnO₂ respectively [figure 5(b)], which are higher than the reported value of bulk SnO₂, i.e. 3.6 eV. Band gap of a material strongly depends on the size, strain and carrier concentration. There are many reports that indicate band gap narrowing in SnO₂ upon Ni doping [27, 28]. Band gap shrinkage is explained on the basis of many body effects like electron–electron interactions and electron-impurity interactions. In this study, we observed slight increase in band gap of SnO₂ with 5 wt% Ni doping. Usually, widening of band gap energy is ascribed to Burstein–Moss effect [29]. Non-stoichiometric SnO₂ has n-type electrical conductivity. Ni substitution at Sn sites may lessen the carrier concentration. However, there may be a net increase in carrier concentration owing to other defects like oxygen vacancies in Ni doped SnO₂ NPs, thereby populating the bottom states of conduction band. Thus, transition of electrons to bottom states of conduction band is blocked. This may be the reason for observed higher optical band gap in case of doped sample.

3.4. Micro-Raman spectroscopy

SnO₂ NPs were also investigated by micro-Raman spectroscopy [figure 6]. The dominant peak is observed at 634 cm⁻¹. It further confirms the purity of the rutile SnO₂ samples. Rutile SnO₂ belongs to the space group ${D}_{4h}^{14}$ comprising two SnO₂ units. There are two metal atoms and four oxygen atoms in the unit cell. According to Diéguez et al [30], the different phonon modes of the SnO₂ at the Brillouin zone are given by the following expression.

$\begin{eqnarray*}&&{\rm{\Gamma }}={A}_{1g}+{A}_{2g}+{B}_{1g}+{B}_{2g}+{E}_{g}+2{A}_{2u}+2{B}_{1u}+4{E}_{u}.\end{eqnarray*}$

**Figure 6.** Micro-Raman spectra of SnO₂ and 5 wt% Ni doped SnO₂ NPs at room temperature (λ_exc = 532 nm).
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Among these, Raman active modes are the non-degenerated A_1g, B_1g and B_2g and the doubly degenerated E_g mode. E_u and A_2g modes are IR active modes. It may be noticed that the E_u mode has become Raman active in Ni doped SnO₂. The peak located at 634 cm⁻¹, as seen in figure 6, is attributed to the SnO₂ characteristic Raman mode A_1g. This mode is related to the expansion and contraction of Sn-O bond. Raman peaks at 474 cm⁻¹, 690 cm⁻¹ and 774 cm⁻¹ correspond to the E_g, A_2u(LO) and B_2g vibration modes respectively [30–32], where the subscript LO in A_2u(LO) stands for longitudinal optical phonon mode. The E_g mode is observed due to vibration of two oxygen atoms parallel to c-axis, but in opposite direction. Therefore, this mode may be more sensitive to oxygen vacancies than other modes. The absence of E_g mode in Ni doped SnO₂ may indicate very high oxygen vacancy concentration in it.

3.5. X-ray photoelectron spectroscopy

XPS analysis of the pure SnO₂ and Ni doped SnO₂ samples was carried out to study the surface chemical states of the elements. The data is represented in figure 7. The profile peak fitting of the XPS spectra is done after subtracting Shirley background. Part of the profile fitting is done by the Gaussian function and part by the Voigt function using OriginPro-2021 software. The C 1 s spectrum at 284.8 eV was used as a reference for calibration. The analysis confirms the presence of C, O, Sn and C, O, Sn, Ni in the pure SnO₂ nanoparticles and Ni-doped SnO₂ nanoparticles respectively. The XPS C1s spectra feature the following bands: non-oxygenated ring C–C (∼284.88 eV), C–O (∼286.35 eV), C=O (∼288.50 eV) in both pure SnO₂ and Ni-doped SnO₂ [33]. The difference in area percentage of C 1 s between both the samples may be caused due to the doping effect.

The high-resolution O 1 s spectra for pure SnO₂ and Ni-doped SnO₂ nanoparticles are deconvoluted into two major bands as shown in figures 7(b) and (f) respectively. These peaks correspond to binding energies of 530.41 eV (Metal-Oxygen), 531.45 eV (Oxygen defect) in pure SnO₂ and 530.26 eV (Metal-Oxygen), 531.10 eV (Oxygen defect) in Ni-doped SnO₂ [34]. The higher binding energy component peak increases in Ni-doped SnO₂ sample. This component corresponds to oxygen defect sites with low oxygen coordination. Thus it can be inferred that the oxygen vacancy concentration increases in the SnO₂ nanoparticles when Ni is present due to the charge imbalance between Sn⁴⁺ and Ni²⁺. This leads to chemisorptions of a large number of free oxygen molecules on the nanoparticle surface [35]. This may be the reason for which the EDAX analysis shows higher oxygen content in Ni doped SnO₂ sample. These chemisorbed oxygen molecules may take part in the photocatalytic activity which is explained later.

The presence of Ni was confirmed by the high resolution XPS Ni 2p spectrum [figure 7(g)] of Ni-doped SnO₂ nanoparticle sample. Two typical peaks at ∼855.80 eV and 874.00 eV can be attributed to Ni 2p_3/2 and Ni 2p_1/2 respectively, and two other peaks at 861.41 eV and 880.35 eV as satellite peaks [36]. The fitted peaks at 855.85 eV and 873.10 eV correspond to Ni²⁺, whereas the peaks at 856.31 eV and 874.01 eV are ascribed to Ni³⁺ [37]. In figure 7(c), it is clearly seen that component for Ni 2p is absent in pure SnO₂ for obvious reasons.

The Sn 3d spectrum [figure 7(d)] exhibits spin–orbit doublet peaks at ∼486.59 eV (Sn⁴⁺ 3d_5/2) and ∼495.02 eV (Sn⁴⁺ 3d_3/2) for pure SnO₂. The same [figure 7(h)] is also for Ni-doped SnO₂ at ∼486.40 eV (Sn⁴⁺ 3d_5/2) and ∼494.82 eV (Sn⁴⁺ 3d_3/2). spin–orbit splitting (Δ = 8.42 eV) of doublet lines for Sn 3d spectrum confirms the presence of Sn⁴⁺ state [38]. For Ni doped SnO₂, these peaks are slightly shifted to lower binding energies which may be due to the presence of Ni ions in SnO₂.

3.6. Photocatalytic activity

The absorbance spectra of Rhodamine B solution were recorded from 400 nm to 650 nm as shown in figure 8. The surface plasmon resonance (SPR) band for Rhodamine B dye was recorded in 553 nm. The falling intensity shows the degradation of the solution with time due to presence of NPs. It may be noticed that the SPR band at 550 nm takes 100 min to disappear in presence of Ni doped SnO₂ NPs whereas the band can be still observed after the same time in case of undoped SnO₂ nanoparticles [figures 8(a)–(b)]. It took 130 min to disappear completely in case of undoped SnO₂ nanoparticles (not shown in figure). Thus, it indicates that the synthesised SnO₂ NPs are good catalysts whereas Ni doping makes it better. It may also be seen that the band position is more red-shifted to a greater extent in case of Ni doped SnO₂ nanoparticles.

In order to obtain the accurate kinetic data of the Rhodamine B degradation, rate constant (k) was calculated by first order kinetic reaction equation given by $\mathrm{ln}\left(C/{C}_{0}\right)=-k\,t;$ where C₀ and C are respectively the initial and final concentration of Rhodamine B solution after time t . Depending on the nature of the plots in figures 8(c)–(d), the data set is divided into two time ranges to calculate rate constant, i.e. 0–60 min and 60–100 min The values are found to be 0.009 min⁻¹ up to 60 min and 0.021 min⁻¹ after 60 min in case of undoped SnO₂. These values increase to 0.014 min⁻¹ and 0.045 min⁻¹ in case of doped sample. Stability or repeated usability of the catalyst bears importance similar to enhancement of photocatalytic activity of the nanoparticles for practical applications. To know the stability of Ni doped SnO₂, photocatalytic experiment, i.e. degradation of Rhodamine B under UV visible light, was performed for three cycles under the same conditions. The graph was plotted between irradiation time and RhB degradation as shown in figure 9(a). It depicts good stability of the Ni doped SnO₂ nanoparticle which means that it can be potential photocatalytic material for industrial waste water treatment. The slight decrease in activity observed after each cycle may be due to incomplete collection of the photocatalytic material [12].

**Figure 9.** (a) The recycling performance of 5 wt% Ni doped SnO₂ by RhB degradation under UV irradiation, (b) Schematic representation of the photocatalytic activity of Ni doped SnO₂ nanoparticle to degrade RhB dye molecules.
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In general, photocatalytic mechanism can be explained based on the generation of electron–hole pairs in NPs under illumination which interact with the dye molecules adsorbed on the NPs surface. This interaction leads to the generation of reactive oxygen species which cause breaking of the dye molecules [39]. The superior photocatalytic activity of Ni doped SnO₂ NPs has been attributed to the charge separation process facilitated by Ni²⁺ ions by inducing a defect state (0.26 eV) below the conduction band. Kumar et al observed suppression of electron hole recombination process in Ni doped ZnO enhancing its degradation efficiency [40]. Le et al [12] have proposed a mechanism in which both photogenerated electrons ( ${e}_{CB}^{-}$ ) and holes ( ${h}^{+}$ ) get involved in a series of reactions with adsorbed oxygen and water molecules to form ${}^{\bullet }{\rm{O}}_{2}^{-}$ and ${}^{\bullet }{\rm{O}}{\rm{H}}$ radicals. The reactions are given as follows.

$\begin{eqnarray*}&&{e}_{CB}^{-}+{{\rm{O}}}_{2}\to {}^{\bullet }{\rm{O}}_{2}^{-},\end{eqnarray*}$

$\begin{eqnarray*}&&{e}_{CB}^{-}+{{\rm{O}}}_{2}+{{\rm{H}}}_{2}{\rm{O}}\to {{\rm{H}}}_{2}{{\rm{O}}}_{2},\end{eqnarray*}$

$\begin{eqnarray*}&&{h}^{+}+{{\rm{H}}}_{2}{\rm{O}}\to {{\rm{H}}}^{+}+{}^{\bullet }{\rm{O}}{\rm{H}},\end{eqnarray*}$

$\begin{eqnarray*}&&{}^{\bullet }{\rm{O}}_{2}+{{\rm{H}}}_{2}{{\rm{O}}}_{2}\to {{\rm{O}}}_{2}+{{\rm{OH}}}^{-}+{}^{\bullet }{\rm{O}}{\rm{H}},\end{eqnarray*}$

$\begin{eqnarray*}&&{e}_{CB}^{-}+{{\rm{H}}}_{2}{{\rm{O}}}_{2}\to {{\rm{HO}}}^{-}+{}^{\bullet }{\rm{O}}{\rm{H}}.\end{eqnarray*}$

The ${}^{\bullet }{\rm{O}}_{2}^{-}$ and ${}^{\bullet }{\rm{O}}{\rm{H}}$ radicals react with the RhB molecule (cationic dye molecule) and break it (degradation) to form CO₂ and H₂O.

As explained in earlier reports [41], Ni is localised at the surface sites of nanoparticles and gets oxidised by surrounding SnO₂ nanoparticles. In this study, XPS analysis reveals the fact that Ni promotes formation of oxygen vacancies in lattice and then chemisorption of oxygen/water molecules at nanoparticle surface as confirmed by EDX analysis. Further, Ni substituted at Sn sites is expected to lower the electron density in SnO₂ nanoparticle. Hence, the improved photocatalytic activity may be preferably ascribed to the increase in oxygen vacancy concentration and hole concentration. Under UV irradiation, the hole concentration may increase further and some of the photogenerated electrons may be trapped by defects due to Ni²⁺ and Vo preventing recombination with holes. The holes may be taking part in the degradation process of RhB molecule as proposed by Le et al [12]. As shown schematically in figure 9(b), electrons are excited from valence band (VB) to conduction band (CB), when the nanoparticle is irradiated with UV light. Some electrons are lost due to recombination with holes. Transition of electrons from conduction band to Ni²⁺ and V_O states may be taking place, followed by migration to adsorbed oxygen molecules and subsequently, $\bullet {{\rm{O}}}_{2}^{-}$ radicals are formed which then react with RhB dye molecules.

4. Conclusions

The structural, optical and photocatalytic properties of SnO₂ and SnO₂:Ni NPs were probed in this work. The NPs were successfully synthesised by following a simple, template-free procedure using hydrothermal method. XRD analysis confirmed the crystalline nature of these NPs having rutile-type cassiterite phase of SnO₂. Further, Rietveld refined XRD patterns supported single phase tetragonal crystal structure having space group P4₂/m n m. The decrease in lattice parameters and crystallite size was observed in Ni doped SnO₂ with an opposite trend in lattice strain values. This is attributed to the substitution of Sn⁴⁺ ions by Ni²⁺ ions in the host lattice. FESEM and TEM results showed regular spherical shape of nanoparticles (arranged in hollow tubular form) and irregular elongated shapes in case of SnO₂ and Ni doped SnO₂ NPs respectively. The optical absorption properties were investigated with the help of UV–vis spectrophotometer. The direct optical band gap (E_g) was determined to be 3.97 eV for SnO₂ and 4.11 eV for SnO₂:Ni NPs from their Tauc plots. The higher band gap for doped NPs is explained on the basis of Burstein–Moss effect. Raman spectroscopy also confirmed the formation of rutile-type cassiterite phase of SnO₂. Peaks are observed at 474 cm⁻¹, 634 cm⁻¹, 690 cm⁻¹ and 774 cm⁻¹ corresponding to the E_g, A_1g, A_2u(LO) and B_2g vibration modes respectively. The peak at 634 cm⁻¹ is the characteristic A_1g Raman mode of SnO₂ related to expansion and contraction of Sn-O bond. XPS data analysis showed significant increase of oxygen vacancy concentration in the SnO₂:Ni NPs due to the charge imbalance between Sn⁴⁺ and Ni²⁺. The increase of oxygen vacancy concentration led to chemisorptions of more free oxygen molecules on the nanoparticle surface. The photocatalytic efficiency of the SnO₂:Ni NPs was found to be considerably higher than that of pure SnO₂ NPs. The observed high photocatalytic activity in SnO₂:Ni NPs under UV irradiation is attributed to generation of electron–hole pairs in NPs and the charge separation process facilitated by Ni²⁺ ions and V_O defects. We observed a dominant red shift in band position in case of Ni doped SnO₂ NPs. Due to the fact that Ni doped SnO₂ NPs depict faster response to degrade RhB dye molecules compared to SnO₂ NPs, it may be inferred that Ni doped SnO₂ NPs are better catalysts than undoped SnO₂ NPs for the purpose of waste water treatment in industries.

Acknowledgments

The authors gratefully acknowledge the help from Department of Geology towards XRD analysis and the financial help from Centre of Excellence for Env. Pub. Health, Ravenshaw University, Cuttack, India under OHEPEE (26913/HED/HE-PTC-WB-02-17) by Government of Odisha.

Analyzing the role of Ni dopant to change the structural, optical and photocatalytic properties of SnO₂ nanoparticles

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Abstract

1. Introduction

2. Materials and methodology