Europium doping effect on 3D flower-like SnO₂ nanostructures: morphological changes, photocatalytic performance and fluorescence detection of heavy metal ion contamination in drinking water

Abstract

Pure and Eu-doped (1, 3, 5, 7 & 10 mol%) SnO₂ nanostructures have been successfully synthesized by a facile and simple hydrothermal method. The properties of as-synthesized samples have been investigated by various analytical techniques. It was found that the morphology of as-synthesized flower-like SnO₂ nanostructures made of intermingled small-size agglomerated nanorods can be precisely controlled by varying the Eu dopant concentration in a reasonable range. Moreover, the photocatalytic activity of SnO₂ studied by the degradation of rhodamine-B (RhB) dye in aqueous media shows excellent performance on 10 mol% europium doping, which may be attributed to its specific morphology and larger surface area as seen from BET measurements. Furthermore, sensors based on 10 mol% Eu-doped SnO₂ nanostructures show the highest fluorescence quenching efficiency (0.23) as compared to pure SnO₂ and other doped samples for the lowest concentration of Cd²⁺ (10 ppb) in drinking water with a Limit of Detection (LOD) as low as 7 ppb; 0.007 μg mL⁻¹. The formation mechanism of Eu-doped SnO₂ nanostructures has been discussed in detail.

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1. Introduction

In recent years, with prolonged urbanization and economic development, the problems arising from volatile organic reagents, toxic heavy metal ions, gases, and pollutants have become increasingly serious. Therefore, it is necessary to detect such toxic pollutants and chemicals without delay for the safety of people. Chemical sensors based on semiconductor oxides play a vital role in many areas nowadays, such as in medical diagnosis, environmental protection, and detection of toxic gases, pollutants and heavy metal ions.¹ Heavy metal ions have always been a significant threat to human health and the environment, among which Cd is of major concern, mainly due to its presence at relatively high concentrations in drinking water and its effects on human health.² Furthermore, according to the International Agency for Research and Cancer (IARC), Cd is found to be a human carcinogen.³ Cd contaminated drinking water leads to chronic renal failure.² It can cause both acute and chronic intoxication.⁴ It is highly toxic to the kidneys. Upon long exposure at low concentrations, it could become deposited in the kidneys, leading to kidney disease, fragile bones and lung damage.⁵ Therefore, developing selective and sensitive methods to detect and quantify metal ions at low concentrations for environmental monitoring is of considerable importance. Moreover, for environmental purification and organic pollutant degradation, semiconductor photocatalysts receive continuous research attention for its advanced oxidation process and moderate reaction conditions. Among various metal oxide semiconductors, SnO₂,⁶ CeO₂,⁷ and Fe₂O₃ (ref. 1) are probably the most common materials used for heavy metal ion detection and as leading photocatalysts because of their photochemical stability, low cost, non-toxicity and simple preparation.

Among them tin oxide (SnO₂), an n-type semiconductor with a wide direct band-gap (3.6 eV), has attracted extensive attraction due to its wide potential applications in photocatalysis,⁸ photodetectors,⁹ gas sensors,¹⁰ rechargeable batteries¹¹ and heavy metal ion detection.¹² Usually, the morphologies of nanosized metal oxide materials have an effect on their applications.¹³ Therefore, it is important to construct well controlled morphologies which possess high surface to volume ratio and have good performance and high reactivity.¹⁴

Till now, various SnO₂ morphologies including nanoparticles,¹⁵ hollow spheres,¹⁶ nanosheets,¹⁷ nanoflowers,¹⁸ polyhedrons¹⁹ and 3D hierarchical nanostructures²⁰ have been synthesized and extensively investigated. Among such classes of materials, three dimensional (3D) nanostructures are deemed to be the most effective, due to their large surface area, porous nanostructures formed by the adjacent building blocks, low density and better reactivity towards various applications such as photocatalysis, sensing etc. Wang et al. have prepared various morphologies of as synthesized SnO₂ nanomaterials for photocatalytic applications.²¹ Liu and co-workers reported the synthesis of hierarchical SnO₂ nanostructures made of ultrathin nanosheets.¹⁴ Ghosh et al. reported the synthesis of poppy flowerlike SnO₂ nanoflower array on Sn–foil by solvothermal method.²² Wu et al. reported the synthesis of novel SnO₂ nanoflowers by PEG and citric acid assisted hydrothermal method.²³ Chen et al. have prepared 3D SnO₂ nanoflowers with nanoporous petals using a controlled structure-preserving thermal-oxidation process.²⁴ Tan et al. have prepared SnO₂ hollow spheres via surfactant assisted hydrothermal method.²⁵ He et al. have demonstrated the synthesis and gas sensing properties of 3D SnO₂ nanostructures.²⁶ Chen et al. synthesized SnO₂ nanopolyhedrons assembled from ultrathin SnO₂ nanowires by SDS assisted hydrothermal process.¹⁹

Although, several researchers have reported the synthesis, properties and applications of nanorods assembled flower-like 3D SnO₂ nanostructures but mostly they had followed complicated experimental procedures as seen in literature.^27,28 A simple hydrothermal method is used in the present research to synthesize 3D flower-like SnO₂ nanostructures. As the sensing and photocatalytic properties of nanomaterials are highly dependent upon their morphology and size, so by tuning the morphology of nanorods assembled nanoflowers to smaller size dimension will significantly enhance their sensing and photocatalytic performance.

Therefore a challenging work is to control the morphology of 3D SnO₂ nanostructures by tuning the shape and size of nanorods or nanosheets filling interspaces to smaller dimensions for their enhanced significant performance. Doping method has been extensively used in order to modify the electronic structure of nanoparticles, inducing narrow energy band gap to achieve improved optical, morphological, photocatalytic²⁹ and sensing properties.³⁰ Recent experimental studies have shown that the introduction of rare earth elements like La, Nd, Sm, Yb, Ce etc. in wide band-gap semiconductors such as ZnO, SnO₂, TiO₂ etc. results in their improved sensing³¹ and photocatalytic performance.³² Rare earth doped compounds have been studied because of their fast mobility of oxygen ions and catalytic properties which are one of the important factors in chemical sensing.³¹ Various authors had reported the effect of doping of Yb,³¹ La,³³ Y³⁴ and Ce³⁵ on structural, optical, morphological, photocatalytic and sensing properties of SnO₂ nanostructures. To the best of our knowledge, no attention has yet been focused on heavy-metal ion sensing and photocatalytic performance of Eu-doped SnO₂ nanostructures. On the contrary, researchers tend to explore the synthesis, morphology, optical, electrical, gas-sensing and photoluminescence properties of Eu doped SnO₂.^36–39 So, it is necessary to investigate the photocatalytic and heavy-metal ion sensing properties of Eu-doped SnO₂ nanostructures. Moreover, the photocatalytic activity of as-synthesized Eu-doped SnO₂ nanostructures has been examined using rhodamine-B dye as a test contaminant in aqueous media. Furthermore, for sensing application, Eu-doped SnO₂ nanostructures have been effectively used for the detection of toxic heavy metals like Cd²⁺ ion in water with the lowest concentration up to 10 ppb (parts per billion) level.

2. Experimental section

A simple hydrothermal method along with mild operating conditions (reaction temp. < 200 °C) has been used for the synthesis of pure and doped SnO₂ nanostructures. The precise control over its synthesis conditions (temperature, duration, pH etc), low energy consumption and less air pollution reflect the simplicity of this method.

2.1 Synthesis of europium doped SnO₂ samples

All the chemicals and organic-solvents are analytical grade reagents and used without any further purification. To synthesize europium-doped SnO₂ samples, firstly 4 mmol SnCl₄·5H₂O and 8 mmol C₆H₅Na₃O₇·2H₂O were dissolved into 30 mL of ethanol–water (1 : 1) solution. The resulting solution was vigorously stirred for 1 h with the help of magnetic stirrer. After vigorous stirring for 1 h, the solution was loaded into Teflon-lined stainless steel autoclave and heated to 180 °C for 24 h.

After heat treatment, the autoclave was naturally cooled down to room temperature. After cooling, the resultant product was washed several times with de-ionized water and ethanol. Finally, the obtained precipitates were annealed at 600 °C for 2 h. The same procedure was followed for the preparation of europium-doped SnO₂ samples, with the addition of europium(III) nitrate pentahydrate (Eu (NO₃)₃·5H₂O) to tin(IV) chloride solution before the addition of tri-sodium citrate (C₆H₅Na₃O₇·2H₂O) and the concentration of Eu was varied from (1–10) mol%. Finally, europium-doped SnO₂ samples were collected.

2.2 Characterization

The crystallographic, morphological, compositional and optical properties of as-synthesized samples were investigated by various analytical techniques. The crystallographic parameters of the prepared samples were examined by X-ray diffractometer (Rigaku Miniflex-600 powder X-ray diffractometer, Cu-K_α1, λ = 0.154 nm) in the 2θ range varying from 20–80°, with a scanning rate and step size of 4000° min⁻¹ and 0.02° at (40 kV, 15 mA), respectively. The morphological features of the samples were studied by FEI Quanta 200 F SEM Field Emission Scanning Electron Microscope (FESEM). The chemical and elemental composition of the samples was investigated by Energy Dispersive Spectroscopy (EDS), attached with FESEM, respectively. The functional groups of the samples were determined by Fourier Transform Infrared Spectroscopy (FTIR, PerkinElmer spectrometer) in the range of 4000–400 cm⁻¹. The surface analysis of the samples was carried out by BET (Autosorb-I Quatachrome Corp.). The optical properties were investigated by UV-vis absorption spectroscopy (Hitachi U-2900) and (PL) photoluminescence spectroscopy (Fluoromax-4 spectrofluorometer) at room temperature.

2.3 Photocatalytic experiment

The photocatalytic degradation of rhodamine-B (RhB) dye was performed using pure and Eu-doped SnO₂ samples as the nano-photocatalysts under UV irradiation exposure at room temperature. An array of 4 UV tubes of 18 W each was served as the light source to provide UV-vis illumination. To carry out the experiment, firstly 40 mg of nano-photocatalyst was added into 100 mL of rhodamine-B (RhB) solution (3.2349 mg L⁻¹). Prior to UV irradiation exposure, the suspension was stirred in dark for 60 min to reach an adsorption–desorption equilibrium among rhodamine-B (RhB) dye, nano-photocatalyst and water. At certain time intervals of irradiation, 3–5 mL of samples were collected and analyzed with UV-vis absorption spectrophotometer (Hitachi U-2900) to record the remnant concentration of rhodamine-B (RhB) by monitoring the corresponding absorbance of the characteristic peak at 554 nm.

2.4 Detection of Cd²⁺ ion contamination in water by sensing the fluorescence property of Eu-doped SnO₂ nanostructures

The fluorescence detection of aqueous Cd²⁺ in water was studied using europium-doped SnO₂ NP solution at room temperature. Various concentrations of Cd(NO₃)₂ (10 ppb, 20 ppb, 30 ppb, 40 ppb, 50 ppb, 80 ppb, 100 ppb) at Parts Per Billion (PPB) were prepared from stock solution by quantitative dilution. To demonstrate the effect of Cd²⁺ ion on Eu-doped SnO₂ NPs, 500 μL portions of several concentrations of Cd²⁺ ion (0–100 ppb) were added one at a time to 3 mL of Eu-doped SnO₂ NPs solution with continuous stirring and the resulting mixture was allowed to stand for 15 min at room temperature and the photoluminescence intensity of the resulting solution was recorded on spectrofluorometer at 340 V.

3. Results and discussion

3.1 The structural characteristics

The crystallographic features of as-prepared samples are investigated by X-ray diffractometer (XRD) as shown in Fig. 1(a). All the diffraction peaks are well indexed to tetragonal rutile structure of SnO₂ (JCPDS no. 41-1445, a = b = 0.47382 nm and c = 0.31871 nm, space group P4₂/mnm (136)). Also, no diffraction peak due to any other impurity has been detected in the spectra, which indicates the purity of as-synthesized europium doped SnO₂ samples. The crystallographic parameters calculated from XRD patterns and Debye's Scherrer formula are listed in Table 1. A decrease in the value of average crystallite size (D) and lattice parameters a, b and c has been observed with the increase in the doping concentration, indicating the successful incorporation of europium ions into SnO₂ host lattice which leads to lattice distortion and suppressing the growth of larger SnO₂ nanocrystallites.⁴⁰ To investigate the effect europium doping on the crystallinity of as-prepared 3D flower-like SnO₂ nanostructures, the 2θ for (110) and (101) diffraction peaks has been monitored. Fig. 1(b) shows the comparison in the shift of 2θ for (110) and (101) diffraction peaks which indicates a slightly right shift in Eu-doped SnO₂ samples, as compared with pure SnO₂ and the maximum the concentration is, the greater is the shift observed in the spectra. The right shift observed in Eu-doped SnO₂ samples indicates the presence of compressive strain (as listed in Table 1), which is calculated using the following equation:⁴¹where θ is the Bragg angle of the peak, λ is the X-ray wavelength and β is the Full Width Half Maximum (FWHM) in radians. As seen in Fig. 1(d), an increase in the amount of compressive strain has been examined from −0.031 to −0.046 with the decrease in the value of average crystallite size for Eu-doped SnO₂ samples. Furthermore, only a right shift towards higher angle has been observed in the case of Eu-doped SnO₂ samples along with no peak broadening (as seen in Fig. 1(b)), which indicates the presence of homogenous strain in Eu-doped SnO₂ samples, as in the case of homogenously strained crystallite, the crystallites in the sample were compressed by the same amount, which causes only a shift in diffraction peak but no peak broadening. Moreover, a decrease in the values of lattice parameters has been observed with the increase in doping concentration as seen in (Table 1 and Fig. 1(c)). According to vegard's law, the subsitutional incorporation of the dopant ions into host lattice, leads to the shrinkage of lattice, while the dopant incorporation in the interstitial sites between Sn and O atoms, however, will expand the lattice.^42,43 So, the results predicted from Table 1, indicates the subsitutional doping of europium ions into SnO₂ host lattice.

Fig. 1 XRD patterns of pure and Eu-doped SnO₂ with different doping amount (a) 2θ in the range of 20–80°; (b) comparison the shift of (110) and (101) peaks; (c) plots showing the variation of lattice parameters with % Eu content and (d) plot showing the variation of crystallite size and compressive strain with Eu (mol%) content.

Table 1 Crystallographic parameters of pure and Eu-doped SnO₂ samples calculated from XRD patterns

Specimen	Lattice constant a = b (Å)	Lattice constant c (Å)	Crystallite size D (nm)	Strain (η)
Pure SnO2	4.7636	3.1949	33.58	−0.031
1 mol% Eu–SnO2	4.7566	3.1938	33.44	−0.033
3 mol% Eu–SnO2	4.7333	3.1862	32.09	−0.036
5 mol% Eu–SnO2	4.7318	3.1826	29.17	−0.039
7 mol% Eu–SnO2	4.7243	3.1822	25.67	−0.042
10 mol% Eu–SnO2	4.7216	3.1819	20.02	−0.046
Std JCPDS 41-1445	4.7382	3.1871	—

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3.2 The morphological and compositional analysis

To study the elemental composition as-prepared of Eu-doped SnO₂ samples, the EDS spectra of the samples have been recorded, as shown in Fig. 2(a) and (b). The peaks of tin (Sn), oxygen (O) and europium (Eu) are clearly observed in the spectra which confirm the presence of europium (Eu) in SnO₂ host lattice. Pure SnO₂ comprises 38.31% oxygen (O), 61.69% tin (Sn) and Eu-doped SnO₂ samples comprises 55.92% oxygen (O), 34.81 tin (Sn) and 9.27% europium (Eu) by weight. The morphological features of as-prepared samples were investigated by Field Emission Scanning Electron Microscopy (FESEM), as shown in Fig. 3(a). As confirmed by FESEM images, pure SnO₂ comprises flower-like morphology, grown in very high density. Further, it is observed that the flower-like structures are made of small-size agglomerated irregular-shape nanorods, which intermingle with each other having their one side connected with the side surface of other nanorods. The diameter of the nanorods is about 21–25 nm. The morphology of 1% Eu-doped SnO₂ is shown in Fig. 3(b). Compared with the undoped sample, the products are composed of SnO₂ nanoheets, in place of nanorods, with the average diameter of about ∼21 ± 3 nm. 3% Eu-doped SnO₂ nanostructures, as shown in Fig. 3(c) consist of hollow triangle-shape SnO₂ nanosheets, with an average diameter of ∼21 ± 4 nm. Mix morphology of small spheres and SnO₂ nanorods has been observed in the case of 5% Eu-doped SnO₂ SnO₂ (Fig. 3(d)). By, increasing the Eu dopant concentration from 5 to 7%, the morphology comprises only SnO₂ nanorods, in place of nanorods alongwith spheres (as observed in 3% Eu doped SnO₂), with an average diameter of ∼19 ± 2 nm as seen in Fig. 3(e). Maximum dopant concentration of Eu (10 mol%) in SnO₂ host lattice results in SnO₂ nanorods of longer length (Fig. 3(f)) as compared to nanorods observed in the case of (Eu : SnO₂ 7%) with an average diameter reduced to ∼15 ± 3 nm.

Fig. 2 (a) EDS spectra of pure SnO₂; (b) 10 mol% Eu-doped SnO₂.

Fig. 3 FESEM images of (a) pure SnO₂; (b) 1 mol%; (c) 3 mol%; (d) 5 mol%; (e) 7 mol% and (f) 10 mol% Eu-doped SnO₂ samples.

Interestingly, from FESEM images, it has been observed that the morphology of SnO₂ nanorods changes first into nanosheets, then from nanosheets to nanorods alongwith spheres and finally results into nanorods of longer length, after the doping of Eu element into pure SnO₂. The basic reason behind this is the subsitutional incorporation of Eu ions into SnO₂ host lattice, which modifies the local structure of host lattice, thereby leading to the variation in the surface energy of some crystal planes, which further changes their growth rate.⁴⁴ Therefore, europium serves as a structure-modifying agent in the growth of nanorods. It can be observed from Fig. 3(a)–(f) that the diameter and thickness of various morphologies of nanostructures is reduced gradually with the increase in the amount of Eu doping, which is the result of suppression of grain boundary migration and increasing the energy barrier for grain growth.⁴⁵

3.3 Growth mechanism

The formation mechanism of pure and europium-doped SnO₂ nanostructures is explained in Fig. 4. Firstly, the nucleation of tiny SnO₂ nanocrystals takes place from the hydrolysis of tin precursor. After nucleation process, the tiny crystals of SnO₂ aggregated and grew into SnO₂ nanorods by oriented attachment process.⁴⁶ After that, the new particles formed will further grow into another nanorod by spontaneously landing on as-formed nanorods, in order to control their total surface energy,^47,48 forming flower-like SnO₂ nanostructures as illustrated in Fig. 4(a). Here, sodium citrate, which is added along with tin precursor plays an important role to control the morphology of as-synthesized samples. When added along with tin precursor, forms a complex with tin ions because of its strong coordination ability and will lead to decrease in the concentration of free tin ions in the solution. The as-formed new complex slows down the rate of reaction, thereby forming 3D flower-like nanostructures.¹⁴ Furthermore, it has been examined that the SnO₂ morphology could be successfully controlled by varying the Eu dopant concentration in a reasonable range, which indicates the subsitutional doping of europium ions in SnO₂ host lattice.

Fig. 4 (a) Schematic formation mechanism of pure SnO₂ nanostructures; (b) (1, 3, 5, 7 & 10 mol%) Eu-doped SnO₂ nanostructures.

Firstly, compound nuclei will be formed by introducing europium ions with different molar concentrations (1.3.5.7 & 10%) into the precursor solution, due to subsitutional incorporation of europium ions into SnO₂ host lattice which takes place only with the addition of europium ions into precursor solution with the same molar ratio (1 : 1). This will lead to the generation of double-ionized oxygen vacancies, which further changes the surface energy and density of charges of each crystal face⁴⁹ and thereby leading to different growth rates of various crystal faces. Then, these primary SnO₂ nanocrystals oriented grow with the driving force of decreasing surface energy into different structures through aggregation mechanism⁵⁰ and Ostwald ripening process,⁵¹ under the influence of europium (Eu) as illustrated in Fig. 4(b).

3.4 Functional group analysis

Fig. 5 shows the FTIR spectra of as-synthesized samples. The bands in 680–721 cm⁻¹ region corresponds to the O–Sn–O bending vibration, while the bands observed in 471–618 cm⁻¹ region are due to the antisymmetric vibration of Sn–O–Sn mode of tin oxide. The broad absorption peak at ∼3426 cm⁻¹ attributed to the O–H stretching vibration.⁵² The peak at 1110 cm⁻¹ is due to Sn–OH vibration.⁵³ Peaks in the region 1448–1480 cm⁻¹ are attributed to the CH₂ bending vibrations. 1700–1729 cm⁻¹ peaks corresponds to C=O stretch.

Fig. 5 FTIR spectra of (a) pure; (b) 1 mol%; (c) 3 mol%; (d) 5 mol%; (e) 7 mol% and (f) 10 mol% Eu-doped SnO₂ samples.

Moreover, a small shift in Sn–O–Sn peak has been observed in the spectra, which can be explained by Hooke's law⁵⁴ which states that the vibration frequency is indirectly proportional to the reduced mass i.e. the vibrational frequency shifts to the lower value with an increase in the reduced mass. A slight shift of Sn–O–Sn peak to lower value has been observed with a decrease in doping concentration from 10 mol% to 0 mol%.

3.5 Optical properties

To investigate the optical properties of europium doped SnO₂ samples, the UV-vis absorption spectra has been recorded as shown in Fig. 6(a). The absorbance of light by nanoparticles is greatly influenced by the presence of impurity centers.⁵⁵ All the samples exhibit absorbance in 300–340 nm region. Moreover, a blue-shift in the absorption edges of the samples has been observed with an increase in europium dopant concentration from 0 to 10 mol%. The band-gap of pure and Eu-doped SnO₂ samples has been calculated using Tauc relation, as given below:

αhν = A(hν – E_g)ⁿ (2)

where α is the absorption coefficient, A is constant, E_g is the band-gap of semiconductor and n = 1/2 for direct band-gap semiconductor.⁵⁶ Fig. 6(b) shows the Tauc plot of (αhν)² versus the photon energy (eV) of europium-doped SnO₂ samples. The measured values of band-gap are found to be 3.60, 3.65, 3.80, 3.87, 3.90 and 4.07 eV for pure, 1%, 3%, 5%, 7% and 10% Eu-doped SnO₂ samples. It has been found that with an increase in europium-doping concentration from 0 to 10 mol%, the band-gap increases. 10% europium-doped SnO₂ samples possess larger band-gap value, which may be attributed to the variation in lattice defects.

In order to investigate the presence of defects in as-synthesized europium-doped SnO₂ samples, the photoluminescence emission spectra have been recorded, keeping the excitation wavelength at 330 nm, as shown in Fig. 7. The emission spectra consist of two regions, one weak band in ultraviolet region (350 nm) and other strong band in visible region (666 nm). The first peak at 350 nm is assigned to Near Band Edge (NBE) emission, which is due to the radiative recombination of photo-generated charge carriers. Apart from this peak, another visible emission peak at 666 nm has been observed, which is attributed to oxygen defects related shallow and deep trap centers. Mostly, oxygen vacancies are known to be the most common defects present in metal oxide semiconductors, existing in three different charge states;⁵⁷ V⁰_O, V_O⁺ and V_O²⁺. The peak at 666 nm is attributed to the visible emission from deep trap centers, whose origin can be assigned to the radiative recombination of electrons in deep level states with the photo-excited holes in the valence band.

Fig. 6 (a) UV-vis absorption spectra; (b) Tauc plot of band gap energy for europium-doped SnO₂ samples.

Fig. 7 PL emission spectra (inset; enlarged spectra) of pure and europium-doped SnO₂ samples.

It has been observed that, with an increase in Eu dopant concentration from 1 to 10 mol%, the intensity of strong band at 666 nm gradually increases, while that of weak band at 350 nm tends to merge with each other.

During subsitutional incorporation of Sn⁴⁺ ions by Eu³⁺ ions, −1 charge of the substituted Sn⁴⁺ ion has to be compensated somewhere from the lattice in the form of oxygen vacancy. The oxygen vacancies thus created will act as radiative luminescence centers giving rise to defect levels, thereby enhancing the photoluminescence intensity of europium-doped SnO₂ doped samples.

Increase in photoluminescence intensity will dominate the photocatalytic activity. But, in our study, 10 mol% europium-doped SnO₂ sample exhibits highest photoluminescence intensity as well as highest photocatalytic activity (as seen in Fig. 10(a)). The reason behind the highest photocatalytic activity of 10% europium-doped SnO₂ sample is its larger surface area (as seen from BET; Fig. 8(b)) and more UV absorbance (as seen in Fig. 6(a)). Larger surface area and absorption of light by 10 mol% europium-doped SnO₂ leads to the presence of more photo-generated charge carriers on the surface and the carriers which are not trapped by the defect states may migrate towards the surface of nanoparticles through lattice, thereby enhancing the probability of charge transfer to the surface and dominating the radiative recombination between the carriers leading to enhancement in photocatalytic activity.

3.6 Nitrogen adsorption–desorption isotherms

The simple explanation for the enhanced photocatalytic activity of the europium-doped SnO₂ samples is their increased surface area. To study the surface analysis of pure SnO₂ by europium doping, nitrogen adsorption–desorption isotherm curves has been recorded for pure and 10 mol% europium-doped SnO₂. As shown in Fig. 8(a) and (b), the isotherm exhibits a type V H₂ hysteresis loop. The surface area of pure SnO₂ from BET analysis is found to be 30.50 m² g⁻¹, whereas it is increased to 58.01 m² g⁻¹ in the case of 10 mol% Eu-doped SnO₂. The pore size distribution curves of pure and 10 mol% Eu-doped SnO₂ are shown inset in Fig. 8(a) and (b), which reveal the pore diameter for pure and doped SnO₂ as 2.0 and 2.2 nm, respectively. The larger surface area of 10 mol% Eu-doped SnO₂ leads to its enhanced photocatalytic activity.

Fig. 8 (a) Nitrogen adsorption and desorption isotherms of pure SnO₂; (b) 10% europium-doped SnO₂; (c) pore size distribution curve of pure and (d) 10% Eu-doped SnO₂.

3.7 Photocatalytic degradation of RhB dye by europium-doped SnO₂ samples

In order to examine the photocatalytic performance of europium-doped SnO₂ samples for the degradation of organic pollutants, photocatalytic degradation experiments of RhB dye has been carried out under UV exposure. The time-dependent absorption spectra of dye solution has been recorded after every 10 min time interval of UV exposure to measure the photocatalytic performance of as-synthesized Eu-doped SnO₂ samples. Fig. 9(a)–(f) shows the variation in the time-dependent absorption spectra of RhB dye in the presence of Eu-doped SnO₂ samples. As seen in Fig. 9, the intensity of absorption maxima (λ = 553 nm) of RhB dye has been greatly influenced by all the prepared pure and doped samples and decreases rapidly with the extension of exposure time. Fig. 10(a) shows the degradation of RhB dye as a function of time by pure and europium-doped SnO₂ samples under UV exposure of 70 min. It can be seen that after 70 min of UV irradiation exposure, 10 mol% europium doped SnO₂ exhibits the highest percentage of degradation (∼90%) as compared to other samples as shown in Fig. 10(b). The main reason behind the remarkable photocatalytic activity of 10% Eu-doped SnO₂ nanostructures may be attributed to its specific morphology and smaller thickness of nanorods, as this leads to the adsorption of large no of reactive species and oxygen molecules by providing more active sites and allowing effective transport for the reactant molecules to get to the active sites. Moreover, the larger surface area of the 10% Eu-doped SnO₂ (as seen from BET measurements; Fig. 8(b)) can help to diffuse the reactant molecules to the active sites, which further results into the enhancement of loading dye molecules and excellent photocatalytic degradation.⁶ Furthermore, larger surface to volume ratio of 10% Eu-doped SnO₂ sample leads to more surface oxygen vacancies, thereby increasing its surface activity and hence photocatalytic performance.

Fig. 9 Time dependent absorption spectra of rhodamine-B dye degraded in the presence of (a) pure SnO₂; (b) 1% Eu-doped SnO₂; (c) 3% Eu-doped SnO₂; (d) 5% Eu-doped SnO₂; (e) 7% Eu-doped SnO₂ and (f) 10% Eu-doped SnO₂ samples.

Fig. 10 (a) RhB degradation as function of time by europium-doped SnO₂ samples; (b) plot showing the variation of degradation rate (%) by all the samples w.r.t time.

Fig. 11 shows the proposed schematic mechanism of RhB degradation by europium-doped SnO₂ nanophotocatalysts under UV light exposure. The overall degradation process is as follows: at first, SnO₂ absorbs photon energy greater than or equal to its band gap, which leads to the excitation of electrons from valence band to conduction band, generating electron–hole pairs. The excited electrons from conduction band of SnO₂ may in turn migrate to surface deposited europium (Eu) nanoparticles because of higher energy level of conduction band of SnO₂ as compared to Fermi level (E_fe) of Eu–SnO₂ and hence get trapped by the defect states introduced by europium doping. Therefore, europium nanoparticles will act as temporary trap centers for photogenerated electrons, preventing their recombination with holes. Moreover, UV irradiation exposure leads to the excitation of rhodamine-B (RhB) dye molecules, which are adsorbed onto the surface of SnO₂ nanostructures. Upon excitation, the RhB dye molecules will transfer the electrons from LUMO into the conduction band of SnO₂. The photogenerated electrons produced from above mentioned processes will than react with oxygen molecules (O₂) molecules adsorbed on the surface of photocatalyst, forming superoxide anion radicals (O₂˙⁻). The as formed O₂˙⁻ anion radicals will than react with (e_CB⁻) electrons and proton (H⁺), forming hydrogen peroxide (H₂O₂). The hydrogen peroxide (H₂O₂), by taking an electron will dissociate into highly reactive hydroxyl radical (OH). Futhermore, the photogenerated holes will react with water molecules (H₂O) forming hydroxyl radicals (OH˙). The hydroxyl radicals (OH˙) formed are known to be the powerful oxidizing agents, which will react with dye molecules leading to de-mineralization of products into harmless substances i.e. CO₂ and H₂O. The reactions of whole process are summarized as follow:⁵⁸

SnO₂ + hν → SnO₂ (e_CB⁻ + h_VB⁺) (3)

e⁻ + O² → O₂⁻ (4)

Eu³⁺ + 3e⁻ → Eu (5)

h⁺ + H₂O → H⁺ + OH˙ (6)

O₂˙ + e_CB⁻ + 2H⁺ → H₂O₂ (7)

H₂O₂ + e_CB⁻ → OH + OH˙ (8)

OH˙ + RhB˙⁺ → CO₂ + H₂O (9)

Fig. 11 Proposed schematic mechanism of RhB degradation over the surface of europium-doped SnO₂ nanophotocatalyst under UV illumination.

3.8 The quenching effect of Cd²⁺ ion on PL of europium-doped SnO₂ samples

In the present study, the optical response of pure and europium-doped SnO₂ samples has been tested with regard to the possible fluorescence variations against the addition of different concentrations of Cd²⁺ ion (0, 10, 20, 30, 40, 50, 80 and 100 ppb; 0–100 ppb; 0–0.10 μg mL⁻¹) in the aqueous solution at pH = 6. The prepared aqueous solution of europium-doped SnO₂ samples exhibited a significant response to Cd²⁺ ion. Fig. 12(a)–(f) show the PL emission spectra at excitation wavelength λ_ex = 330 nm of the reference aqueous suspension of europium-doped SnO₂ samples in the presence of Cd²⁺ at different concentrations. As illustrated in Fig. 12, Cd²⁺ efficiently quenched the fluorescence signal ‘F’, which decreases gradually with an increase in ion concentration without having any appreciable change in the spectral profile of all the samples. The quenching of the fluorescence intensity can be quantified by a Stern–Volmer plot which describes the relationship between fluorescence intensity and quencher concentration. Fig. 12(g)–(l) shows the Stern–Volmer plots of the quenching experiment (calibration curve of F₀/F versus Cd²⁺ ion concentration in the range 0–0.10 μg mL⁻¹; 0–100 ppb) by europium-doped SnO₂ samples. For a static quenching process, plotting relative fluorescence intensity (F₀/F) against the concentration (Q) should produce a linear line.⁵⁹ The graphs shows the onset of good linearity from 0–0.10 μg mL⁻¹ Cd²⁺ concentration range describing static quenching with regression coefficient R² = 0.997, 0.997, 0.996, 0.996, 0.995, 0.996 for 0%,1%, 3%, 5%, 7% & 10% europium-doped SnO₂ samples. Here F₀ is the fluorescence intensity of the samples at initial time t = 0 and F is the fluorescence intensity at time t. The Limit of Detection (LOD) has been calculated based on IUPAC definition (3σ of the reagent blank signal divided by the slope of the calibration curve, while σ represents standard deviation⁶⁰) of all the samples, listed in Table 2. The LOD value of Cd²⁺ for 10% Eu-doped SnO₂ sample (0.007 μg mL⁻¹; 7 ppb) has found to be well below the lowest possible concentration of our experiment (10 ppb, 0.010 μg mL⁻¹), as compared to other samples. Moreover, the quenching efficiency of all the samples has been calculated using the following equation:⁶¹

QE = 1 – I/I₀ (10)

where, I₀ is the initial fluorescence intensity in the absence of analyte and I is the fluorescence intensity in the presence of corresponding analyte. Fig. 13(a)–(h) shows the bar graphs corresponding to fluorescence quenching efficiency obtained for pure and europium-doped SnO₂ samples with Cd²⁺ concentration range (0–0.010 μg mL⁻¹). Among all the samples, only 10% europium-doped SnO₂ caused maximum fluorescence quenching at lowest level of Cd²⁺ concentration (10 ppb; 0.010 μg mL⁻¹) with an efficiency of about 0.23 as compared to other samples (as seen in Fig. 13(h)), which shows that 10% europium-doped SnO₂ sample exhibits highest sensitivity towards the lowest ppb level of Cd²⁺ concentration (0.010 μg mL⁻¹; 10 ppb), with LOD of 0.007 μg mL⁻¹; 7 ppb, which might be attributed to smaller size and thickness of Eu-doped SnO₂ nanorods.

Fig. 12 PL emission spectra of the aqueous suspension of (a) pure SnO₂; (b) 1 mol%; (c) 3 mol%; (d) 5 mol%; (e) 7 mol% and (f) 10 mol% Eu-doped SnO₂ samples in the presence of Cd²⁺ at different concentrations; (g) calibration fitting curve of the linearity of Stern–Volmer plot for Cd²⁺ at different concentrations in the aqueous suspension of pure SnO₂; (h) 1 mol%; (i) 3 mol%; (j) 5 mol%; (k) 7 mol% and (l) 10 mol% Eu-doped SnO₂ samples.

Table 2 LOD and quenching efficiencies of europium-doped SnO₂ samples

Samples	Limit of detection (μg mL^−1) (LOD)	Quenching efficiency at (10 ppb)
0% Eu : SnO2	0.009; 9 ppb	0.19
1% Eu : SnO2	0.010; 10 ppb	0.17
3% Eu : SnO2	0.010; 10 ppb	0.12
5% Eu : SnO2	0.010; 9 ppb	0.10
7% Eu : SnO2	0.008; 8 ppb	0.20
10% Eu : SnO2	0.007; 7 ppb	0.23

---

It has been found that permissible limits of different heavy metal ions in drinking water has set by different agencies i.e. (WHO), (ISI), (USEPA), (CPCB) and (ICMR). But, a bias has been observed in the permissible limits of drinking water set by these different agencies. Permissible limits for drinking water parameters such as cadmium (Cd), mercury (Hg), iron (Fe), lead (Pb), arsenic (As), chlorides, fluorides, nitrite etc. are compared for different agencies in Table 3. The permissible limit of heavy metal ions and other parameters set by different agencies does not show uniformity. For cadmium (Cd), the permissible limit set by different agencies are (mg L⁻¹; μg mL⁻¹): USEPA 0.005, WHO 0.005, ISI 0.01, ICMR 0.01 and CPCB no relaxation.⁶² According to ISI and ICMR, the permissible limit of Cd in drinking water is 0.01 μg mL⁻¹; 10 ppb. Moreover, the level of different heavy-metal ions like Cd, Ni, Hg, Mn, Pb, Cu, Fe and Co in drinking water in different places has been evaluated by various researchers (Table 4) by collecting the drinking water samples and the results were analyzed using Atomic Absorption Spectrometer (AAS). The results showed that concentration of cadmium (Cd) ranges from 0.0 to 0.06 mg L⁻¹; μg mL⁻¹, for cobalt (Co) ranges from 0.027 to 0.08 mg L⁻¹; μg mL⁻¹, for copper (Cu) ranges from 0.0 to 0.08 mg L⁻¹; μg mL⁻¹, for iron (Fe) ranges from 0.0 to 23.26 mg L⁻¹; μg mL⁻¹, for lead (Pb) ranges from 0.0 to 0.19 mg L⁻¹; μg mL⁻¹, for manganese (Mn) ranges from 0.035 to 1.79 mg L⁻¹; μg mL⁻¹, for mercury (Hg) ranges from 1.380 to 3.15 mg L⁻¹; μg mL⁻¹ and for nickel (Ni) ranges from 0.00 to 0.09 mg L⁻¹; μg mL⁻¹, among which the value for cadmium (Cd) exceeded the recommended level set by USEPA and WHO (0.005 mg L⁻¹; μg mL⁻¹).⁶³ This implies that drinking water of these sampling regions is polluted and if used without any treatment, can cause polluted and if used without any treatment, can cause serious health effects to people around these areas. The detection limit of the sensor 10% Eu-doped SnO₂ (0.007 μg mL⁻¹; 7 ppb) is lower than the ISI and ICMR limit of Cd in drinking water (10 ppb; 0.01 μg mL⁻¹) which indicates that it can be used as an efficient sensors for the detection of Cd (present below 10 ppb; 0.01 μg mL⁻¹) in drinking water in such polluted areas.

Table 3 Permissible limits for different parameters of drinking water set by different agencies i.e. (WHO), (ISI), (USEPA), (CPCB) and (ICMR)⁶²

Parameters	USEPA	WHO	ISI	ICMR	CPCB
pH (mg L^−1; μg mL^−1)	6.5–8.5	6.5–8.5	6.5–8.5	6.5–9.2	6.5–8.5
Iron (mg L^−1; μg mL^−1)	—	0.1	0.3	1.0	1.0
Sulphate (mg L^−1; μg mL^−1)	—	—	150	400	400
Calcium (mg L^−1; μg mL^−1)	—	75	75	200	200
Cadmium (mg L^−1; μg mL^−1)	0.005	0.005	0.01	0.01	No relaxation
Magnesium (mg L^−1; μg mL^−1)	—	50	30	—	100
Copper (mg L^−1; μg mL^−1)	1.3	1.0	0.05	1.5	1.5
Arsenic (mg L^−1; μg mL^−1)	0.05	0.05	0.05	0.05	No relaxation
Lead (mg L^−1; μg mL^−1)	—	0.05	0.10	0.05	No relaxation
Zinc (mg L^−1; μg mL^−1)	—	5.0	5.0	0.10	15.0
Mercury (mg L^−1; μg mL^−1)	0.002	0.001	0.001	0.001	No relaxation
Chromium (mg L^−1; μg mL^−1)	0.1	—	—	—	No relaxation

---

Table 4 Comparison of measured heavy metal ions in different places⁶³

Location	Measured concentration (mg L^−1; μg mL^−1)								Sources
	Cd	Ni	Hg	Mn	Pb	Cu	Fe	Co
Gombe	—	—	—	0.12	—	0.02	0.62	—	64
India	0.002	0.003	—	—	0.003	—	—	—	65
Ethiopia	0.017	—	—	—	—	BDL	—	0.018	66
Pakistan	0.067	0.00	—	—	0.351	0.433	1.676	—	67
Dutse	0.003	0.043	2.283	0.273	0.015	0.015	1.824	0.041	63

---

3.9 Sensing of 10% Eu-doped SnO₂ towards other heavy-metal ions

The fluorescence property of 10% Eu-doped SnO₂ in the presence of different heavy-metal ions like Cd²⁺, Ca²⁺, Ba²⁺, Cu²⁺, Zn²⁺, Sr²⁺, Na⁺, K⁺, Hg²⁺, Co²⁺ and Pb²⁺ has been tested as seen in Fig. 14(a). The fluorescence intensity has been quenched in the presence of Cd²⁺ ions whereas for other heavy-metal ions such as Ca²⁺, Ba²⁺, Cu²⁺, Zn²⁺, Sr²⁺, Na⁺, K⁺, Hg²⁺, Co²⁺ and Pb²⁺, no change in the intensity has been observed. However, for some of the metal ions like Ca²⁺, Ba²⁺, Cu²⁺, Zn²⁺, Sr²⁺, Na⁺, K⁺, Hg²⁺, Co²⁺ and Pb²⁺ it shows minimal effect i.e. 5% of fluorescence quenching. Therefore, 10% Eu-doped SnO₂ can be used as a fluorescent sensor to detect Cd²⁺ ions in drinking water.

Fig. 13 Bar graphs representing fluorescence quenching efficiency obtained for Eu-doped SnO₂ solution with Cd²⁺ at (a) 0.01 μg mL⁻¹; (b) 0.02 μg mL⁻¹; (c) 0.03 μg mL⁻¹; (d) 0.04 μg mL⁻¹; (e) 0.05 μg mL⁻¹; (f) 0.08 μg mL⁻¹; (g) 0.10 μg mL⁻¹ and (h) comparison of quenching efficiencies of Eu-doped SnO₂ samples at different Cd²⁺ ion concentrations.

Fig. 14 (a) Selectivity bar diagram of the fluorescence quenching of 10% Eu-doped SnO₂ (3 mL) towards Cd²⁺ and other heavy metal ions with a concentration of (0.10 μg mL⁻¹); (b) the fluorescence intensity of 10% europium-doped SnO₂ (3 mL) in the presence of 0.10 μg mL⁻¹ of various metal ions with an added of 0.10 μg mL⁻¹ Cd²⁺.

3.10 Selectivity of 10% Eu-doped SnO₂ sensor

For the evaluation of the selectivity of probe towards Cd²⁺ ion, the fluorescence response of 10% Eu-doped SnO₂ has been measured in the presence of other interfering metal ions individually as seen in Fig. 14(b). Some of the metal ions like Ca²⁺, Ba²⁺, Cu²⁺, Sr²⁺, Co²⁺ and Pb²⁺ does not cause any fluorescence quenching of probe even at 0.10 μg mL⁻¹ and have no significant effect on the fluorescence spectrum, whereas Zn²⁺, Na⁺, K⁺ and Hg²⁺ ions have slight turn on effect. Hence, Cd²⁺ ions can easily be deducted by the probe in the presence of above metal ions with several-fold of concentrations.

4. Conclusions

Hydrothermal method is a facile and simple method for the synthesis of europium doped SnO₂ nanostructures, providing a good yield of samples. It has been found that the dopant concentration plays an important in controlling the morphology, photocatalytic and sensing ability of the prepared products. 10% Eu-doped SnO₂ has been observed as an efficient photocatalyst with the degradation ability of (∼89%), which is mainly due to its large surface area. Europium-doped SnO₂ nanostructures have also been effectively used for the detection of Cd²⁺ ion in drinking water at ppb level by recording their fluorescence quenching. 10% Eu-doped SnO₂ nanostructures shows highest quenching efficiency as compared to pure and other doped samples at lowest concentration of Cd²⁺ (10 ppb) with the Limit of Detection (LOD) as low as 7 ppb; 0.07 μg mL⁻¹. This work demonstrates that this optimum concentration doping serves as an excellent photocatalyst for the degradation of organic pollutants and for the detection of heavy metal ions in drinking water, present even at lowest possible concentration.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work is financially supported by DST (Department of Science and Technology), New Delhi. Authors are grateful to SAIF, Panjab University; Chandigarh, SAI Lab Thapar University; Patiala, IIT Madras, IIT Kanpur for providing the necessary laboratory facilities to carry out this work.

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