Luminescence and photocatalytic studies of Sm³⁺ ion doped SnO₂ nanoparticles

Abstract

Sm³⁺ ion doped tetragonal SnO₂ nanoparticles have been synthesized by the polyol method and its concentration and annealing effects on photoluminescence properties are studied. The XRD results show the changes of lattice parameters with the incorporation of the Sm³⁺ ion into the SnO₂ host lattices. Fourier transform infrared (FTIR) data indicate the formation of a Sn–O bond and capping of the nanoparticles by ethylene glycol. The broad emission peaks observed around 350 to 550 nm are attributed to the emission from defects or traps present in the host SnO₂. The typical emission peaks of the Sm³⁺ ion are observed and there is efficient energy transfer between the host, SnO₂ and the dopant, Sm³⁺ ion. Further, the prepared samples are experimented towards the photocatalytic behavior on selected triphenylmethane dyes, Ethyl Violet (EV) and Brilliant Green (BG). The degradation of dyes was found to be enhancing in the presence of Sm³⁺ ions as it suppress the e⁻_CB and h⁺_VB recombination.

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Introduction

SnO₂ is an n-type semiconductor with band gap of E_g = 3.6 eV at 300 K and is used in various fields like gas-sensors, solar cells, catalysis, lithium batteries, optoelectronic devices and magnetic property applications, etc.^1–6 The shape and size of the nanoparticles can be controlled by using a capping agent. Lanthanide doped semiconductor nanomaterials have a great deal of interest due to their unique optical properties and potential application in fields like fluorescent lamps, optical communication and flat panel displays.^7–9 Amongst the lanthanides, samarium is a very active member having interesting properties. Due to its fluorescence property, it has applications in high-density optical storage, under sea communication and color displays and its emitting ⁴G_5/2 level exhibits relatively high quantum efficiency.¹⁰ Because of its multiple energy-level structure and high fluorescence efficiency this lanthanide usually acts as a great emitting center as well as a red-emission center.^7,11

There are various synthetic methods for lanthanide doped and undoped SnO₂ nanoparticles such as hydrothermal,¹² precipitation,¹³ sol–gel,¹⁴ vapour transport method,¹⁵ microwave method¹⁶ and microemulsion.¹⁷

Nowadays, the textile industry of dyes and pigments is expanding worldwide, mainly in developing countries. Dyes and pigments are used in industry for coloring of cloth, cotton, paper, leather, wool, silk and nylon. Dyes are normally in a group of complex organic materials based fundamentally on the chromospheres structure. Wastewater containing the dyes is usually toxic, resistant to biodegradation, persistent in the environment, and difficult to be treated by general methods. As a consequence, there is a continuous increase in environmental contamination and global concerns of discharge of dyestuff effluents into the aquatic system.¹⁸ The dyeing wastewater discharge not only affects the environment but also reduces light penetration through the water surface needed for photosynthesis activity of aquatic organisms. In order to minimise such accidents in the environment, semiconductor photocatalysis has become more and more attractive and important.¹⁹ In the photocatalytic process, the semiconducting material absorbs UV light energy more than or equal to the energy gap, consequently generating the holes and electrons, which further give rise to oxidation and reduction of organic dyes.²⁰ In the photocatalysis process, recombination of electron and hole plays a very important role. In order to perform photocatalysis successfully we have to reduce the recombination of electron and hole, to enhance the photocatalysis performance. Some techniques like rare-earth (or metal ion) doping of semiconductors or deposition (or coupling) of metal oxides to semiconductors can reduce (delay) the recombination of charges (electron and hole).^21–23 But not a lot of work has been reported using lanthanide doped SnO₂ nanoparticles. In this work, we have studied the photoluminescence properties of Sm³⁺ ion doped SnO₂ nanoparticles and their application towards the degradation (photocatalysis) of the dyes, Ethyl Violet (EV) and Brilliant Green (BG). The detailed mechanism of degradation of dyes by pure and doped SnO₂ nanoparticles has also been elaborated.

Experimental section

Materials

Tin metal (99.9%), samarium(III) nitrate hydrate, Sm(NO₃)₃·xH₂O (99.9%) from Sigma-Aldrich, ethylene glycol, dyes (EV and BG) from Merck, ammonia, methanol and hydrochloric acid were used as received without further purification. Double distilled water was used throughout the experiment.

Synthesis

Pure SnO₂ and Sm_xSn_1−xO₂ (x = 0.02, 0.05, 0.07 and 0.10) nanoparticles were synthesized by the ethylene glycol route. In a typical synthesis procedure, stoichiometric amounts of the precursor of Sm³⁺ ions and tin metal were dissolved in a minimum quantity of conc. HCl to get a clear solution. The solution was evaporated five times by alternating addition of distilled water so that the excess HCl gets evaporated out. The resulting transparent solution was allowed to mix with 50 ml of ethylene glycol and heated in a round bottom flask at 40 °C for 30 minutes. Ammonia solution (stoichiometric amount) was added to the above transparent solution and was allowed to reflux at 120 °C for 2 h. The resulting white precipitate was collected by centrifugation at 10 000 rpm after washing with methanol. The precipitate was annealed at different temperatures for 2 h respectively.

Characterization

A Philips X-Ray diffractometer (PW 1071) with Cuk_α (1.5405 Å) radiation having a Ni filter was used for X-ray diffraction (XRD) study. All patterns were recorded over the angular range 10 ≤ 2θ/deg ≤ 90 with a step size of Δ2θ = 0.02. The powder samples were ground and dispersed in methanol on a glass slide and allowed to dry. The average crystallite size (t) was calculated using the Debye–Scherrer relationship, t = 0.9λ/β cos θ, where λ is the wavelength of the X-ray and β is the full width at half maximum (FWHM). The lattice parameters were calculated from the least square fitting to the diffraction peaks.

Fourier transform infrared (FTIR) spectra were recorded using an FTIR-8400S (206-72400) Shimadzu Spectrometer. Powder samples were studied by making thin pellets with KBr. A JEOL 2000 FX transmission electron microscope was used for recording TEM images. For the TEM measurements, the powder samples were ground and dispersed in methanol. A drop of the dispersed particles was put over the carbon coated copper grid and evaporated to dryness at room temperature. All the UV-visible measurements were done in a Shimadzu 2450 UV-Visible Spectrophotometer. The BET surface area measurement was performed at 77 K on a Quantachrome Instrument (V 5.02). All the luminescence spectra were recorded using a Perkin Elmer LS-55 spectrophotometer. Powder samples were dispersed in methanol and spread over the quartz slide and dried at room temperature.

Photocatalysis experiment

In the photocatalysis process (degradation of dyes), 0.233 g of the catalyst (pure SnO₂ or Sm³⁺ ion doped SnO₂ nanoparticles) was dispersed in 10 ml of deionized water and sonicated for 1 hour. Different concentrations of the dispersed nanomaterials were mixed with 1 ml of the dye solution (EV or BG) having a concentration of 2.5 × 10⁻⁴ M, as summarized in Table S1 (see ESI†). The solution mixture of the catalyst and the dye were kept in the dark prior to the excitation in order to achieve maximum adsorption of the dye on the surface of the catalyst. During the illumination time no volatility of the solution was observed.

The solution mixture was irradiated with 260 nm of UV light inside a closed chamber and kept for 10 minutes. After irradiating, the catalyst was removed by centrifugation and the absorbance of the filtrate was measured. The changes in the concentration of dyes were monitored from their characteristic absorption (λ_max) at 593 (EV) and 625 nm (BG) using a UV-Visible spectrophotometer. The absorbance at their respective λ_max of the dyes represents the aromatic part of the dyes and its decrease of absorbance indicates the degradation of dye.

The UV illumination is obtained by using a self-deozonating 150 W Xe lamp along with a stigmatic concave diffraction grating monochromator to get the desired wavelength. The slit width was kept constant at 2.5 for all the illumination processes.

Results and discussion

XRD patterns of the pure powder and 5 at% Sm³⁺ ion doped SnO₂ nanoparticles (as-prepared, 500 and 900 °C) are shown in Fig. 1(a and b). All the XRD patterns of the annealed samples could be indexed on the basis of a tetragonal system reported for SnO₂ (JCPDS Card No. 41-1445) with lattice parameters a = 4.73 Å and c = 3.18 Å. The broad peaks of the as-prepared sample indicate the low crystallinity of the sample and the crystallinity of the sample increases with the increase in annealing temperature. The lattice parameters for undoped pure SnO₂ prepared at 500 °C are a = 4.70 Å and c = 3.19 Å. In the case of Sm³⁺ doped SnO₂ nanoparticles annealed at 500 °C, the lattice parameters are a = 4.71 Å and c = 3.18 Å and those for the 900 °C annealed sample are a = 4.69 Å and c = 3.19 Å indicating that Sm³⁺ ions had incorporated into the SnO₂ lattice. As ionic radius of the Sm (R_Sm³⁺ = 0.1079 nm) is greater than the ionic radius of the Sn (R_Sn⁴⁺ = 0.076 nm),^24,25 it is expected that the lattice undergoes significant change with Sm³⁺ incorporation. In previous studies,^26,27 only small amount of lanthanides could be incorporated into the SnO₂ lattice. The crystalline size calculated using the Debye–Scherer equation from the plane (110) are found to be 20 and 34 nm for pure SnO₂ annealed at 500 and 900 °C and 20 and 35 nm for doped SnO₂ nanoparticles annealed at 500 and 900 °C.

Fig. 1 XRD patterns of (a) pure SnO₂ and (b) 5 at% Sm³⁺ doped SnO₂ nanoparticles at different annealing temperatures (as-prepared, 500 and 900 °C).

Fig. 2 shows the FTIR spectra of Sm³⁺ (5 at%) doped SnO₂ nanoparticles at different annealing temperatures (as prepared, 500 and 900 °C). Distinctly, a broadband in the range 400–750 cm⁻¹ (580 cm⁻¹) can be observed for all the samples under study. This peak is due to the vibration of the Sn–O bond.²⁸ With the increase in the annealing temperature, the intensity of the Sn–O band increases indicating the crystallisation of the SnO₂ particles which is also confirmed by the XRD study. There are also OH⁻ peaks at 1635 cm⁻¹ (bending vibration) and a broad peak at 3453 cm⁻¹ (stretching vibration) which could be observed in the samples, vibration corresponding to C–H are observed in the region of 2700–2900 cm⁻¹ (ref. 29) indicating the prepared samples were well capped by the ethylene glycol used during the synthesis of the nanoparticles, which is absent at the higher annealing temperature, which indicates the evaporation of the capping agent.

Fig. 2 FTIR spectra of SnO₂:Sm³⁺ (5 at%) at different annealing temperatures (as-prepared, 500 and 900 °C).

Fig. 3 shows the microscopy structure of the as-prepared sample of Sm³⁺ (5 at%) doped SnO₂. The particles are spherical in shape. The sizes are 20 nm in range which is well in agreement with the calculation from XRD. The SAED shows the low crystallinity of the sample as also indicated from the XRD data. The as prepared samples have a high content of water molecules. Highly crystalline SnO₂ nanomaterials are observed on annealing at higher temperatures because of the evaporation of water and organic moiety.

Fig. 3 (a) TEM images and (b) SAED of annealed samples of SnO₂:Sm³⁺ (5 at%).

Fig. 4a shows the emission spectrum of the pure SnO₂ nanoparticles for different annealing temperatures excited at 300 nm (absorption peak of SnO₂).³⁰ A broad peak ranging from 350–550 nm is observed, which is related to the emission from defects (oxygen vacancy) or traps present in the host SnO₂.^24,31 The peak observed around 420 nm is related with the emission from defects or traps present in the host responsible for the luminescence. The intensity of the broad peak increases with the increase in annealing temperature, indicating the loss of the luminescence quencher hydroxyl ion (OH⁻) present in the sample.

Fig. 4 Emission spectra of (a) pure SnO₂ and (b) SnO₂:Sm³⁺ (5 at%) nanoparticles.

In the excitation spectrum of Sm³⁺ ion doped SnO₂ nanoparticles (Fig. S1, see ESI†), monitored at an emission wavelength of ⁴G_5/2 → ⁶H_7/2 (600 nm) of the Sm³⁺ ion, the whole spectrum can be divided into two regions. The peaks in the region 220–300 nm relate to the host absorption.³² The peak at 260 nm is related to the O⁻² to Sm³⁺ charge transfer transition. The other regions from 300–450 nm is related to the f–f transition of Sm³⁺ ions. The peaks at 365, 406 and 425 nm can be assigned to the ⁶H_5/2 → ⁴D_15/2, ⁶H_5/2 → ⁴K_11/2 and ⁶H_5/2 → ⁴M_19/2 transitions of the Sm³⁺ ion.³²

The emission spectrum of Sm³⁺ doped SnO₂ nanoparticles (Fig. 4b) show the typical emission of the Sm³⁺ ion. The three sharp peaks observed at 562, 600 and 644 nm can be assigned to the ⁶G_5/2 → ⁶H_5/2, ⁶G_5/2 → ⁶H_7/2 (magnetic dipole) and ⁶H_5/2 → ⁴H_9/2 (electric dipole) transition of the Sm³⁺ ion.^32,33 The predominance of 600 nm i.e. magnetic dipole transition indicates that in our prepared samples, the Sm³⁺ ions occupy Sn⁴⁺ sites in the SnO₂ lattice (having D_2h or C_2h symmetry) except magnetic dipole allowed transitions, all other transitions are forbidden.²⁴

Fig. 5 shows the emission spectra of the 5 at% Sm³⁺ doped SnO₂ nanoparticles at different annealing temperatures as-prepared, 500 and 900 °C (Fig. 5a) and at different concentrations of Sm³⁺ (2, 5, 7 and 10 at%) at 900 °C (Fig. 5b) excited at 300 nm. The intensity of the peaks increases with the increase of annealing temperature from as-prepared to 900 °C, which is related with the loss of ⁻OH/dangling bonds present on the surface of the nanoparticles which acts as luminescence quencher through multiphonon relaxation.^34–36 The emission peaks at 562, 600 and 644 nm, correspond to the ⁶G_5/2 → ⁶H_5/2, ⁶G_5/2 → ⁶H_7/2 and ⁶H_5/2 → ⁴H_9/2 transitions of Sm³⁺ ions. The magnetic dipole mechanism plays an important role in ⁶G_5/2 → ⁶H_7/2 transition. The transition ⁶G_5/2 → ⁶H_7/2 also contains some part of magnetic dipole component caused by the interaction between the Sm³⁺ ion and the magnetic field component. The ⁶G_5/2 → ⁶H_7/2 and ⁶H_5/2 → ⁴H_9/2 transitions are mainly induced by electric dipoles. The ⁶G_5/2 → ⁶H_7/2 electric-dipole transition is very sensitive to the local environment around Sm³⁺. The f–f electric dipole transitions are forbidden in nature and can be raised owing to the mixture of the 4f^N transitional states with opposite-parity configurations.³⁷ The intensity of the emissions peaks increases with the increase in the concentration of the Sm³⁺ ion from 2 to 10 at% (Fig. 5b) which is due to the increase in the number of active absorbing centers (Sm³⁺ ions). As the Sm³⁺ ion itself doesn't have an absorption peak around 300 nm, the observance of the emission spectra corresponding to the various transitions of Sm³⁺ ions when excited at 300 nm (which is the band edge absorption of SnO₂) indicates that there is a good energy transfer from the host, SnO₂ to the dopant, Sm³⁺ ion. Energy transfer occurs when the emission of the donor is equal to the absorption of the acceptor in resonance condition, as 420 nm emission of SnO₂ is due to the defect levels and is almost overlapping the excitation peak of the Sm³⁺ ion at 406 nm, so energy transfer is possible from SnO₂ to Sm³⁺ ions. The process of energy transfer is shown in Scheme S1 (see ESI†). The incoming radiation is absorbed by SnO₂ nanoparticles and transferred to the Sm³⁺ ion and then the excited Sm³⁺ ion emits the characteristic emission peak of the Sm³⁺ ion at 652, 600 and 644 nm.

Fig. 5 Emission spectra of SnO₂:Sm³⁺ (5 at%) nanoparticles at (a) different annealing temperatures and (b) at different doping concentrations (2, 5, 7, 10 at%).

Fig. 6 shows the photocatalytic behavior of as-prepared pure (Fig. 6a and b) and Sm³⁺ (5 at%) doped (Fig. 6c and d) SnO₂ nanoparticles on the degradation reaction of the selected dyes. The maximum absorption (λ_max) of the ethyl violet dye is found to be 593 nm and that of BG is 625 nm. The intensity of the dyes decreases with the addition of SnO₂ nanoparticles when irradiated indicating the decolourisation of dyes in the presence of the pure and doped SnO₂ (catalyst), the decolourisation of dyes increase with the increase of concentration of catalyst. The phenomenon of photocatalysis can be explained as follows. By applying UV-visible light irradiation with a photon of energy equal to or higher than the band gap energy (3.6 eV), the SnO₂ catalyst generates electron–hole pairs with free electrons in the empty conduction band. The electron–hole pairs thus produced, can go across the band gap to the conduction band, and holes (h⁺) stay in the valence band as formulated in eqn (i). In the presence of aqueous solution, the h⁺ is scavenged by surface hydroxyl groups or H₂O molecules to produce highly reactive and non hydroxyl radicals (OH˙), as shown in eqn (ii) and (iii). Then, the photo-generated electrons in the conduction band could react with O₂ acceptors to produce a superoxide radical anion (O₂⁻˙) of oxygen in eqn (iv). The OH˙ are considered to be the dominant oxidizing agent contributing to the destruction of organic dyes.³⁸ Degradation mechanisms of the dye molecules under the contact with light irradiation and photocatalyst are summarized in eqn (v)–(vii).¹⁸

SnO₂ + hν → SnO₂ (e⁻_CB + h⁺_VB) (i)

SnO₂ (h⁺_VB) + H₂O → SnO₂ + H⁺ + OH˙ (ii)

SnO₂ (h⁺_VB) + OH⁻ → SnO₂ + OH˙ (iii)

SnO₂ (e⁻_CB) + O₂ → SnO₂ + O₂⁻˙ (iv)

Dye + OH˙ → degradation products (v)

Dye + h⁺_VB → oxidation products (vi)

Dye + e⁻_CB → reduction products (vii)

Fig. 6 Photocatalysis of dyes (EV and BG) in presence of (a, b) pure SnO₂ and (c, d) SnO₂:Sm³⁺ (5 at%) nanoparticles.

From the above discussion we observed that e⁻_CB and h⁺_VB are responsible for photocatalysis of dyes. The e⁻_CB and h⁺_VB have a chance for recombination which reduces the efficiency of the photocatalyst or undergo redox reaction (eqn (iii) and (iv)) which increase the efficiency of photocatalyst by reactions with any organics dyes which adsorbed on the surface of SnO₂ through weak van der Waals forces in dye contaminated waters,³⁹ and consequently decomposed to harmless products. The introduction of lanthanide/metal ions reduced the width of the band gap energy levels between the conduction band and valence band allows the catalyst to be active under visible light.⁴⁰ When lanthanide is introduced as a doping agent for SnO₂, these energy levels can be incorporated into the band gap of SnO₂. These new energy levels can either accept electrons from the valence band or donate electrons to the conduction band. Owing to the lower energy separation between the new energy levels and the valence band or conduction band and thereby suppressing the e⁻_CB and h⁺_VB, recombination and efficiency of photocatalysis increase. Moreover oxygen vacancy found in photoluminescence studies also increases the degradation of dyes which acted as an electron trap and delay the recombination of electron–hole pairs.⁴¹ Therefore, the degradation efficiency of dyes increase in the case of Sm³⁺ ion doped than undoped SnO₂ nanoparticles (Fig. 7a and b).

Fig. 7 Absorbance spectra of dyes (a) EV and (b) BG in presence of Sm³⁺ doped and undoped SnO₂ nanomaterials.

BET surface area measurements were performed in order to get a better understanding of the surface active site of the catalyst for the photodegradation of the dyes under study. The specific surface areas are found to be 137.41 and 167.56 m² g⁻¹ for pure SnO₂ and Sm³⁺ doped SnO₂ nanomaterials respectively. The increase in the surface area of the Sm³⁺ doped materials further confirms the enhanced photocatalytic behavior over the undoped samples. Considering the recent SnO₂ nanomaterials in the literature^42–45 the observed surface area of undoped and doped SnO₂ nanomaterials in this work is extraordinarily high. For example, porous SnO₂ nanowire,⁴³ mesoporous SnO₂ hollows⁴⁵ with surface areas up to 101 m² g^–1 were reported and applied as photocatalysts. It is believed that the high specific surface areas and reduction of the width of the band gap energy levels between the conduction band and valence band of the Sm³⁺ doped SnO₂ nanomaterials are beneficial for the diffusion of reactant molecules to the active sites, resulting in an enhanced loading of dye molecules leading to the photodegradation of the dye.

Conclusions

A spherical shape is exhibited for pure and Sm³⁺ ion doped SnO₂ nanoparticles, prepared using ethylene glycol as a capping agent and its photoluminescence properties have been studied at different annealing temperatures (as-prepared, 500 and 900 °C). The crystallinity of the samples was found to be increasing with the increase in annealing temperature, which is due to the evaporation of water and organic moiety from the surface of the nanoparticles. There are efficient energy transfers from host SnO₂ to the Sm³⁺ ion. The photocatalytic behavior of the prepared samples was investigated on selected triphenylmethane dyes, viz. Ethyl Violet and Brilliant Green. The degradation of dyes was found to be enhanced in the presence of Sm³⁺ ions.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c3nj00759f

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