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Original article
13 (
1
); 96-108
doi:
10.1016/j.arabjc.2017.02.004

Surfactant assisted hydrothermal synthesis of SnO nanoparticles with enhanced photocatalytic activity

, ,
 Selçuk University, Department of Chemical Engineering, Konya, Turkey

⁎Corresponding authors. Fax: +90 332 2410651. bircanhaspulat@selcuk.edu.tr (Bircan Haspulat), handankamis@selcuk.edu.tr (Handan Kamış)

Received: , Accepted: ,
© The Author(s) 2017
Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

Peer review under responsibility of King Saud University.

Abstract

SnO nanoparticles have been successfully synthesized in the presence of Triton-X 100 (TX-100) surfactant via hydrothermal method for the first time, and the photocatalytic activity under UV and visible light irradiation for the degradation of Methylene Blue (MB) and Rhodamine B (RdB) organic textile dyes was investigated. The structural, morphological and chemical characterizations were investigated by using X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), selected area electron diffraction (SAED), energy dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), Fourier Transform Infrared Spectroscopy (FTIR), UV–vis. diffuse reflectance spectroscopy (UV–vis DRS) and photoluminescence (PL) analysis. The results reveal that the addition of surfactant, TX-100, in the precursor solutions leads to reduction in crystallite size with significant changes in morphological structure of SnO nanoparticles. The synthesized SnO nanoparticles show excellent photocatalytic activity under UV or visible light irradiation. MB and RdB dyes degraded completely under UV irradiation after 90 and 150 min, respectively. Also, MB and RdB dyes degraded only 150 min later under visible light illumination with a little amount of photocatalyst (0.8 g/L). Hence, this work explores the facile route to synthesizing efficient SnO nanoparticles for degrading organic compound under both UV and visible light irradiations.

Keywords

SnO nanoparticles
Triton-X 100
Photocatalysis
Methylene blue
Rhodamine B
1

1 Introduction

Nanostructured metal oxides and semiconductor materials have attracted much attention owing to their good mechanical, magnetically, optical, electrical and physical properties (Binas et al., 2012; Lin et al., 2013; Xiao, 2012). These metal oxides, because of these properties, have been studied in a wide application area ranging from lasers, sensors, photochemistry, lithium ion batteries, photocatalysis, etc. (Gülce et al., 2013; Kim et al., 2015; Wu et al., 2015; Yates, 2009). Among these nanostructured metal oxides, stannous oxide (SnO) is an important p-type semiconductor material, with a wide band gap of Eg = 2.7 – 3.2 eV (Sun et al., 2013), has obtained a remarkable interest due to its perfect physical, chemical and morphological properties. Also, SnO can be synthesized easily by hydrothermal (Sun et al., 2013), solvothermal (Liang et al., 2013), chemical vapor deposition (Kumar et al., 2010), and pyrolysis (Hu et al., 2014) methods. SnO and SnO based functional materials have been used widely in sensors (Kuang et al., 2008), lithium ion batteries (Ning et al., 2009), solar reactor (Abanades and Chambon, 2010), photocatalysis (Cui et al., 2015). In particular, in photocatalytic organic dye degradation process, SnO nanoparticles have been used as promising materials in only a few studies (Cui et al., 2015; Xia et al., 2014; Yan et al., 2013). Also, the size and morphology of the SnO nanoparticles have an important effect on their properties mentioned above (Hu et al., 2014). Because of this reason, a lot of researchers have focused on synthesizing distinct SnO morphologies such as flowers (Liang et al., 2013), sheet (Wang et al., 2012), clinopinacoid (Iqbal et al., 2012), whiskers (Jia et al., 2004), disks (Dai et al., 2002) by chemical or physical methods.

Organic dyes are several environmental pollutants released into the environment from textile industries (Elango et al., 2015; Elango and Roopan, 2016; Skariyachan et al., 2016). Photocatalysis is one of the most effective methods suitable for the degradation of organic dyes into harmless final products. In photocatalytic reactions, very reactive species such as hydroxyl radicals and superoxide radicals are generated (Rashad et al., 2014; Robertson et al., 2012; Spasiano et al., 2015; Zhang et al., 2016b). SnO nanoparticle is an important photocatalyst candidate due to its non-toxicity, low cost and chemical stability. In spite of the fact that SnO has a lot of unique properties, only a few studies focused on the photocatalytic decolorization of dyes by using SnO nanoparticles as photocatalyst (Cui et al., 2015; Xia et al., 2014).

Because physical and chemical properties of nanomaterials do not depend only on the composition but also on the particle size, a good synthesis protocol has to provide good control over particle size. The surfactant serves as micro reactors to confine the crystal growth and it plays an important role in the controlled preparation of nanoparticles (Pal and Chauhan, 2009). TX- 100 is a biodegradable nonionic surfactant (Iyyappan et al., 2016). Nonionic surfactants are used in many industries because of their surface activity. So, they can be easily mixing with other surfactant and other additives (Zdziennicka, 2009). To the best of our knowledge, there has been no report for the synthesis of SnO nanoparticles in the presence of the TX-100 surfactant in the previous literature. In this report, SnO nanoparticles were synthesized in aqueous solution in the presence of TX-100 by using hydrothermal synthesis method, for the first time. Furthermore, as described above, SnO nanostructured materials have distinctive properties and are now widely used in lithium rechargeable batteries (Das et al., 2016; Hu et al., 2014; Kim et al., 2011; Zhang et al., 2016a), gas sensor (Chu et al., 2015b; Suman et al., 2015) and water splitting (Liang et al., 2014). However, SnO nanostructured powders have been in a few studies used as photocatalysts for degradation of organic pollutants until now (Cui et al., 2015; Kuznetsova et al., 2014; Liu et al., 2012). The photocatalytic activity of the synthesized SnO nanoparticles for photocatalytic degradation of MB and RdB dyes is investigated under UV and visible light irradiation. The prepared SnO sample was characterized by means of XRD, XPS, SEM, TEM, SAED, EDX, FTIR, PL and UV–vis DRS. Also, the effect of dye type, light source, irradiation time and photocatalyst amount on photocatalytic activity of the synthesized SnO nanoparticles was investigated.

2

2 Experimental

2.1

2.1 Materials

All chemical reagents were of analytical grade and used without further purification. Tin(II) chloride dehydrate (SnCl2·2H2O) was purchased from Alfa Aesar. Triton™ X-100 was used as a surfactant and was supplied by Sigma Aldrich. Methylene blue, rhodamine B, sodium hydroxide and ethanol were purchased from Merck. Millipore Milli-Q deionized water was used throughout the experimental work.

2.2

2.2 Synthesis of SnO nanoparticles

SnO nanoparticles were synthesized by hydrothermal method in starting solutions containing varying TX-100 ratios. Firstly, 20 mL of Stannous Chloride aqueous solution containing 0.25 M SnCl2·2H2O was prepared under magnetic stirring. Subsequently, 20 mL of varying concentrations (0 M, 0.125 M, 0.25 M and 0.5 M) of Triton X-100 aqueous solution was dropped into SnCl2 solution and a homogenous solution was obtained. Afterward, 20 mL of (0.75 M) NaOH solution was added dropwise to the above solution. This homogeneous solution transferred to a Teflon-lined stainless steel autoclave and kept in an oven at a constant 130 °C temperature for 15 h. After that, the autoclave was cooled down to room temperature naturally. The formed precipitation was collected by centrifugation and washed for several times with de-ionized water and ethanol. Lastly, the precipitation dried in vacuum oven at 60 °C for 15 h. The samples that were synthesized in different concentration ratios of Sn2+:TX-100 (1:0; 1:0.5; 1:1; 1:2) were symbolized as S1, S2, S3 and S4, respectively.

2.3

2.3 Characterization

X-ray Diffraction (XRD) analysis was carried out by the Shimadzu XRD-6000 X-ray diffractometer using Cu Kα radiation (λ = 0.15418 nm) in the scanning range 20–70° at rate of 2°/min. The chemical composition of sample was investigated using by X-ray photoelectron spectroscopy (XPS) analysis (PHI VersaProbe). The morphological analysis of the synthesized products was carried out using the ZEISS Evo LS 10 scanning electron microscope and Jeol 2100F 200 kV RTEM transmission electron microscope. The UV–vis diffuse reflectance and the FTIR spectra of the synthesized SnO nanoparticles were obtained by using the HITACHI U 3900 UV–visible diffuse reflectance spectrophotometer and the Perkin Elmer 1725 instrument, respectively. The optical absorption spectra of all the samples were obtained using the Ocean Optics HR4000 UV–visible spectrophotometer during the photocatalytic activity test. The PL spectra of the as-prepared SnO nanoparticles were recorded using a HITACHI F-2700 fluorescence spectrophotometer.

2.4

2.4 Measurement of photocatalytic activities

The photocatalytic degradation of MB and RB dyes in the presence of the SnO nanoparticles was performed in quartz tube reactor under the illumination of UV or visible light. Photocatalytic decolorization of the dyes under UV or visible light was carried out in a Luzchem model 4V photoreactor (Luzchem Research Inc., Canada) equipped with 8 UV C lamps with an emission at 254 nm or halogen reflector lamps (50 W, model JDR E27 Pelsan, China) with an emission at 350 to 800 nm. The dye solution (MB or RdB) was mixed with desired amount of catalysts. Before the irradiation of the dye solution, the suspension was stirred for 60 min in dark conditions to obtain adsorption-desorption equilibrium in the presence of SnO nanoparticles. After that, the suspensions were irradiated with stirring. The photocatalytic degradation of organic dyes was investigated at room temperature in the presence/absence of different catalysts under irradiation/dark for given times.

MB and RB dyes concentrations were analyzed by using UV–vis spectroscopy method. Absorbance intensity changes of characteristic bands of the dyes were measured. The degradation efficiency of dye is calculated by the following equation:

(1)
Degradation ( % ) = C 0 - C C 0 × 100 where C0 is the initial concentration of dye before irradiation and C is the concentration of dye after a certain irradiation period.

In order to investigate the photocatalytic reusability of the SnO nanoparticles, the SnO nanoparticles were used for several photocatalytic runs under UV and visible light irradiations for the both dyes. For this, 0.8 mg/mL of SnO nanoparticles was added into (1 × 10−5 M) MB or RB aqueous solutions. Photocatalytic activity test procedures as mentioned above were applied. After measurements, the catalyst was separated from solution by centrifugation and following decantation. The separated catalyst was washed with de-ionized water for many times and dried at room temperature. After that, the photocatalyst was added into a new dye solution to reuse for photocatalysis experiments. Same experimental procedure was repeated for five times.

3

3 Results and discussion

3.1

3.1 Structural and morphological characterization

Fig. 1 indicates XRD patterns of SnO nanoparticles synthesized in different concentration ratios of Sn2+/TX-100. The XRD pattern of SnO nanoparticles shows the presence of (1 0 1), (1 1 0), (0 0 2), (2 0 0), (1 1 2) and (2 1 1), respectively, indexed with the tetragonal SnO structure with the lattice constant of a = b = 3.8020, c = 4.8360 and α = β = γ = 90° (JCPDS 00-006–0395). (1 0 1), (1 1 0) and (2 0 0) crystal planes were not observed in the S1 crystal structure. As the amount of TX-100 in synthesis medium increases, peak intensity of (1 0 1), (1 1 0) and (2 0 0) crystal planes increases and the opposite peak intensity of (0 0 2) crystal plane decreases. Also, the concentration ratios of Sn2+/TX-100 increased to 1:2, some diffraction peaks of the triclinic Sn3O4 were observed (Chen et al., 2017). (−1 2 0), (1 1 1), and (−1 2 1) crystal plans of triclinic Sn3O4 were observed at 26, 27 and 32°. Compared with all SnO samples, crystal growth is highly affected by the amount of TX-100 in synthesis medium.

The XRD patterns of SnO nanoparticles synthesized different concentration ratios Sn2+/TX-100 (a) S1 (b) S2 (c) S3 (d) S4.
Figure 1 The XRD patterns of SnO nanoparticles synthesized different concentration ratios Sn2+/TX-100 (a) S1 (b) S2 (c) S3 (d) S4.

The crystal size of the SnO nanoparticles has been calculated according to Debye-Scherer (D = Kλ/β cos θ) formula, considering the most intense peak (0 0 2). Table 1 shows the estimated crystallite size of SnO nanoparticles synthesized in different concentration ratios of Sn2+/TX-100 from (0 0 2) peak using the Debye–Scherrer equation.

Table 1 Estimated crystallite size of SnO nanoparticles synthesized different concentration ratios Sn2+/TX-100 from (0 0 2) peak using the Debye-Scherrer equation.
Sn2+:TX-100 Crystal size (nm)
1:0 71.0
1:0.5 69.1
1:1 43.6
1:2 43.7

TX-100 was used as a surfactant to achieve the decrease in the size of SnO crystallites. It is well known that, a convenient way to prepare small sized particles is to add a surfactant, such as TX-100, to the synthesis medium (Iyyappan et al., 2016; Raja et al., 2015). Particularly, in hydrothermal synthesis, the average crystallite size has been reduced from 131.9 nm to 43.3 nm with TX-100 usage. In general, there are two steps in the formation of nanoparticles: first, a large crystal nucleus is formed and second, the growth of the formed nucleus. When a surfactant is used in the synthesis procedure, further growth of the crystals was hindered, probably because of the adsorption of the surface-active agent on the surface of the crystal nucleus. Additionally, the adsorption of the surface-active agent may also stabilize the nanoparticles (Vaiano et al., 2015).

Fig. 2 shows the SEM photographs of the samples at low and high magnification. It can clearly be seen from SEM images that morphologies of samples are remarkably distinct from each other. SnO synthesized without TX-100 has compact morphology (Fig. 2a). Also, the spherical structure is shown in morphology of S1. When TX-100 was added to synthesis medium, morphology has completely changed. When the concentration ratio of Sn2+:TX-100 is 1:0.5, the morphology of S2 has been formed by irregular microplates (Fig. 2b). While increasing the concentration ratios of Sn2+:TX-100 to 1:1 and 1:2, morphology has been formed by both irregular sheets and spherical structures (Fig. 2c and d). Fig. 2a, b and d shows that there was not a good control over the particle size. On the other hand, Fig. 2c (concentration ratio of Sn2+:TX-100) shows a greater uniformity in the particle size distribution.

SEM images of (a) S1, (b) S2, (c) S3, (d) S4 at different magnifications.
Figure 2 SEM images of (a) S1, (b) S2, (c) S3, (d) S4 at different magnifications.

Fig. 3 shows TEM images at different magnifications, SAED pattern and EDX spectrum of S3 sample. The morphology of SnO nanoparticles was composed of different size and shape. Pentagonal, rod, spherical and plate shapes were observed in low (Fig. 3a) and high (Fig. 3b) magnification TEM images. Fig. 3c shows the electron diffraction pattern of SnO. The SAED rings were indexed tetragonal SnO and corresponding (1 0 1), (1 1 0), (0 0 2), and (1 1 2) planes of SnO, respectively. These diffraction rings are attributed to the polycrystalline nature of the nanoparticles (Das et al., 2010). These results are in agreement with the XRD data. Fig. 3d presents EDS spectrum of SnO nanoparticle and summary of analysis results is shown in inset Fig. 3d. Atomic ratios of Sn and O in the synthesized nanoparticle agree well with the chemical composition of SnO nanoparticle.

TEM images at (a) low magnification, (b) high magnification, (c) the corresponding SAED pattern and (d) EDS spectrum of synthesized SnO nanoparticles.
Figure 3 TEM images at (a) low magnification, (b) high magnification, (c) the corresponding SAED pattern and (d) EDS spectrum of synthesized SnO nanoparticles.

The chemical state of SnO was clarified by using XPS spectra. Fig. 4a shows the survey spectra of SnO nanoparticles. The binding energies in the XPS spectra are calibrated using that of C 1s (285.1 eV). According to Fig. 4a, XPS spectra have been deeply adopted in the elemental quantification. The elemental quantification shows that [O]/[Sn] atomic ratio is equal to 1.05 (Zhang et al., 2011). The XPS Sn 3d spectrum for SnO nanoparticles is shown in Fig. 4b. The binding energy peak of Sn 3d5/2 occurs at 486.1 and 494 eV for SnO nanoparticles (Chu et al., 2015a; Liang et al., 2010). The XPS O 1s spectrum is shown in Fig. 4c, and the O 1s binding energy peak was observed at 529.7 eV to be associated with O in SnO nanoparticle (Liang et al., 2010). This XPS results are also confirmed with XRD results.

(a) Survey scan, (b) Sn 3d, (c) O 1s core level XPS spectrum.
Figure 4 (a) Survey scan, (b) Sn 3d, (c) O 1s core level XPS spectrum.

UV–visible spectroscopy is one of the important tools for characterizing the optical properties of nanoparticles. Optical properties of the synthesized SnO nanoparticles were investigated by using UV–vis DRS. The diffuse reflectance spectra of the products in 200–900 nm range are shown in Fig. 5a. These spectra indicate that the S3 sample shows the highest absorption capacity in UV range while the absorption band intensities of the samples in visible range are decreased with Sn2+:TX-100 ratio. The optical band gap values, Eg, of the synthesized SnO nanoparticles have been calculated from the plot between hν and (αhν)2 as shown in Fig. 5b. The optical band gap values of the synthesized SnO nanoparticles were calculated to be 2.66, 2.86, 2.72 and 2.99 eV for S1, S2, S3 and S4, respectively. The optical band gap values of the synthesized SnO nanoparticles are in good agreement with the Eg values (2.5–3.5 eV) of SnO nanoparticles which was previously reported (Hsu et al., 2014; Liang et al., 2010; Sun et al., 2013).

(a) UV–vis diffuse reflectance spectra and (b) plot of (αhv)2 versus (hν) of the synthesized SnO nanoparticle samples.
Figure 5 (a) UV–vis diffuse reflectance spectra and (b) plot of (αhv)2 versus (hν) of the synthesized SnO nanoparticle samples.

For individual semiconductor, photocatalytic activity was retarded by the recombination of most of the photogenerated charge carriers at surface or bulk trapping sites with the emission of fluorescence. Photoluminescence is one of the unique optical properties of semiconductor nanoparticles correlated with particular emission wavelengths depending upon the band structures, physical dimensions as well as the chemical environments of semiconductor nanoparticles. The room temperature PL spectra of the synthesized SnO nanoparticles with an excitation wavelength of 290 nm are shown in Fig. 6. A strong emission band centered at 310 nm was observed for S1 sample that is possibly attributed to the band edge emission of SnO (Liu et al., 2012). However, the emission band significantly decreases in intensity for S2, S3 and S4 samples. This indicates that the synthesized SnO nanoparticles in the presence of TX-100 significantly quenched the PL of SnO because of the excellent electron conductivity. In other words, the synthesis in the presence of TX-100 promotes the separation rates of photogenerated electrons and holes, which will favor the photocatalytic activity as described below (Wang et al., 2011).

PL spectra of SnO samples.
Figure 6 PL spectra of SnO samples.

FTIR spectroscopic analyses were carried out to investigate the structural characteristics of S3 sample. Fig. 7 represents the FTIR spectrum of SnO nanoparticle. The absorption peak at 545 cm−1 is associated with stretching vibrations of Sn—O (Zhang et al., 2014). Also, the broad absorption band at around 3303 cm−1 points the presence OH functional group which results from the washing process (Gajendiran and Rajendran, 2015).

FTIR spectrum of S3 sample.
Figure 7 FTIR spectrum of S3 sample.

3.2

3.2 Photocatalytic activity of the synthesized SnO nanoparticles

The photocatalytic activity of the synthesized SnO nanoparticles was investigated by the decolorization of two different organic dyes, MB and RdB under UV or visible light irradiation as followed by spectrophotometric monitoring at room temperature. These dyes are common pollutants in textile industries waste water. Fig. 8 shows the effect of Sn2+:TX-100 ratio on photocatalytic activity of the produced nanoparticles (S1, S2, S3 and S4 samples) for 60 min irradiation. It can be seen from Fig. 8, the photocatalytic activity of S3 is higher than other SnO samples (S1, S2 and S4) under both UV and visible light irradiations. Accordingly, SnO nanoparticle named S3 was chosen as the photocatalyst for degradation of MB and RdB dyes under UV and visible light irradiation.

The effect of Sn2+:TX-100 ratio on photocatalytic activity of the synthesized SnO nanoparticles under (a) UV and (b) visible light irradiation (photocatalyst amount 0.8 mg/mL; initial concentration of dyes: 1.0 × 10−5 M, irradiation time: 60 min).
Figure 8 The effect of Sn2+:TX-100 ratio on photocatalytic activity of the synthesized SnO nanoparticles under (a) UV and (b) visible light irradiation (photocatalyst amount 0.8 mg/mL; initial concentration of dyes: 1.0 × 10−5 M, irradiation time: 60 min).

The photocatalytic activity of S1 sample (synthesized in the absence of TX-100) is much lower than that of other SnO samples which are synthesized in the presence of TX-100. Generally, the separation rate of photogenerated electron–hole pairs is one of the important factors that mainly affects photocatalytic activity. According to the UV–vis. DRS and PL spectra analyses, S1 sample has the emission band with the highest intensity among the three samples, exhibiting the highest recombination rate and lowest transfer efficiency of photoinduced carries, thus reducing the photocatalytic performance of S1 sample. On the other hand, S2, S3 and S4 samples, having a high separation rate of photogenerated electrons and holes, favor their photocatalytic activity. Among these three samples, S3 sample has the highest photocatalytic activity due to the fact that it has the smallest particle size (Fig. 2c) the consequent increase in the specific surface area, lower fluorescence emission and the higher UV absorbance.

In order to optimize the photocatalyst amount, the effect of SnO amount on the degradation of dyes was investigated under UV and visible light irradiation. The photocatalytic experiments were carried out a fixed irradiation time (60 min). The results are displayed in Fig. 9.

The effect of the photocatalyst amount on the degradation of dyes under (a) UV and (b) visible light irradiation.
Figure 9 The effect of the photocatalyst amount on the degradation of dyes under (a) UV and (b) visible light irradiation.

The degradation efficiency of the dyes increased up to 0.8 mg/mL photocatalyst amount, and then the degradation efficiency was nearly stable for MB and RdB dyes. The increase in the degradation efficiency results from the increase in total surface area, and number of active sites on photocatalyst surface. But, when the photocatalyst was overdosed (up to 0.8 mg/mL photocatalyst amount), because of the loss in the surface area caused by agglomeration at high catalyst amount, the number active sites on SnO nanoparticle may become nearly stable (Eskizeybek et al., 2012). Thus, 0.8 mg/mL of SnO photocatalyst was chosen as the optimal amount of photocatalyst for photocatalytic degradation experiments.

The change in optical absorption spectra of MB and RdB dyes by SnO nanoparticles under UV or halogen light irradiation for different time intervals is given in Fig. 10. The disappearance of the characteristic band of MB dye shows that MB has been degraded by the synthesized SnO nanoparticles under UV and visible light irradiation in 60 and 150 min, respectively (Fig. 10a and b). Also, the degradation efficiency of MB dye is around 94% in the presence the S3 nanoparticles after only 30 min exposure time under UV light irradiation. Similarly, under halogen light irradiation, the decolorization efficiency of the MB dye is 30% after same irradiation time. Additionally, when the RdB dye is exposed to UV and visible light irradiation, the characteristic absorption band of the dye disappears using the S3 nanoparticles as a photocatalyst after 90 and 150 min, respectively (Fig. 10c and d). It can be seen clearly from Fig. 10c and d, the absorption intensity of RdB dye’s characteristic band decreases after 30 min under UV and visible light irradiation. The degradation efficiency of RdB dye is around 57% under UV light irradiation after 30 min (Fig. 10c). When the RdB is subject to visible light irradiation, the degradation efficiency is 38% after 30 min using the S3 nanoparticles as a photocatalyst (Fig. 10d). Since no new peaks were observed in the absorbance spectra of both dyes, the loss of absorbance may be due to the degradation of the dyes.

UV–vis absorption spectrum of MB and RdB dyes used the synthesized SnO nanoparticles as a photocatalyst for different time intervals under (a) UV light irradiation of MB, (b) halogen light irradiation of MB, (c) UV light irradiation of RdB, (d) halogen light irradiation of RdB (photocatalyst amount 0.8 mg/mL; initial concentration of dyes: 1.0 × 10–5 M).
Figure 10 UV–vis absorption spectrum of MB and RdB dyes used the synthesized SnO nanoparticles as a photocatalyst for different time intervals under (a) UV light irradiation of MB, (b) halogen light irradiation of MB, (c) UV light irradiation of RdB, (d) halogen light irradiation of RdB (photocatalyst amount 0.8 mg/mL; initial concentration of dyes: 1.0 × 10–5 M).

The variation of degradation versus irradiation time of MB and RdB is shown in Fig. 11. In order to understand the effect of irradiation on photocatalytic activity, comparative experiments were done. Firstly, the degradation of MB and RdB dye solutions was investigated without catalyst in the dark medium (hydrolysis). The concentration of MB and RdB remained unchanged after 150 min under dark condition without photocatalyst (not shown in Fig. 11). Similarly, the degradation of dyes was analyzed under UV and visible light irradiation without catalyst. In this experiment set, the degradation efficiencies are around 2% and 1% for MB and RdB, respectively. Also, in order to understand the effect of adsorption mechanism of decolorization of dyes, the experiment was done under dark condition with S3 nanoparticles as catalyst. The decolorization efficiencies are only 7% and 6% for MB and RdB dyes, respectively. After the dyes reached adsorption-desorption equilibrium, the photocatalytic degradation of MB and RdB dyes was investigated under UV and visible light by using S3 nanoparticles as photocatalyst (Fig. 11). It can be seen clearly from Fig. 11a and b, when the MB dye exposed to UV and visible light irradiation, MB dye degraded using after 60 and 150 min SnO as a photocatalyst under UV and visible light irradiation, respectively. Similarly, RdB dye is degraded completely by using SnO photocatalyst under UV and visible light irradiation after 90 and 150 min, respectively.

Extend of degradation (a) MB under UV light, (b) MB under halogen light, (c) RdB under UV light, (d) RdB under halogen light irradiation with respect to time intervals over the SnO photocatalyst (catalyst amount: 0.8 mg/mL).
Figure 11 Extend of degradation (a) MB under UV light, (b) MB under halogen light, (c) RdB under UV light, (d) RdB under halogen light irradiation with respect to time intervals over the SnO photocatalyst (catalyst amount: 0.8 mg/mL).

Table 2 shows the decolorization efficiencies of MB and RdB dyes by using SnO photocatalyst under UV and visible light irradiation and dark condition after 60 min. From Table 2, it can be clearly seen that MB dyes almost completely decolorizated in 60 min under UV light irradiation. Degradation of RdB dye has been achieved to 91% in the same time under UV light irradiation. However, the degradation of MB and RdB dyes under halogen light is lower than UV light irradiation.

Table 2 The degradation efficiencies of MB and RdB dyes by using SnO photocatalyst under UV and visible light irradiation and dark condition after 60 min.
UV light Visible light Dark
% Degradation of MB 100 50 7
% Degradation of RdB 91 66 6

In the literature, the number of studies using powder SnO as photocatalyst is quite low. The photocatalytic activity of SnO—TiO2 composite was investigated under visible light by degradation of MB dye. The results were compared with the photocatalytic activity of pure SnO and pure TiO2. The MB dye decolorizated only about 10% under visible light by using SnO as photocatalyst after 100 min. (Yan et al., 2013). In another work, Decolorization of RdB dye was investigated by using SnO/Sn3O4 as photocatalyst under UV light irradiation (Xia et al., 2014). After 210 min, RdB dye nearly decolorizated. It is clear that our study has much better photocatalytic activity results than any other study in the literature. The degradation of MB and RdB dyes following pseudo-first order kinetics was calculated by relation −ln(Ct/C0) = kappt, where kapp is the apparent rate constant, C0 is the initial concentration and Ct is the concentration of dyes at the given time. Fig. 12 shows the relationship between t and −ln(Ct/C0) for MB and RdB dyes under UV and halogen light irradiation.

Comparison of the apparent rate constant of (a) MB and (b) RdB dyes under UV and visible light irradiation used SnO as a photocatalyst (catalyst amount: 0.8 mg/mL).
Figure 12 Comparison of the apparent rate constant of (a) MB and (b) RdB dyes under UV and visible light irradiation used SnO as a photocatalyst (catalyst amount: 0.8 mg/mL).

The photocatalytic activity of SnO nanoparticles under UV and visible light irradiation is evaluated by comparing the kaap value listed in Table 3. The kaap value of MB dye under UV light irradiation is 6.5 times higher than visible light irradiation. Similar results were obtained between the kaap value of RdB under UV and visible light irradiation. When RdB dye exposed to UV light irradiation the kaap value is increased up to 2.17 times, compared to the kaap value of RdB dye under visible light irradiation.

Table 3 Apparent rate constant of dyes degradation and linear regression coefficients from plot of −ln(Ct/C0) = kappt (initial dye concentrations: 1.0 × 10−5 M, catalyst amount: 0.8 mg/mL).
UV light Halogen light
kapp (min−1) R2 kapp (min−1) R2
MB 1.1261 0.944 0.1700 0.997
RdB 0.5934 0.974 0.2728 0.992

Reusability is one of the most important factors in catalysis research. To confirm the recyclability of S3 sample, the photocatalytic degradation reaction was repeated up to five cycles (Fig. 13). For each recycling run, SnO photocatalyst is collected by centrifugation, and then washed with deionized water several times. As shown in Fig. 13, no apparent deactivation of the photocatalyst was observed after five consecutive runs, which implies the high stability of the synthesized SnO nanoparticles.

Effect of number of runs on the degradation of dyes in the presence SnO nanoparticles (a) UV light irradiation (b) visible light irradiation of MB and RdB after 60 min (catalyst concentration: 0.8 mg/mL; initial concentration of dyes: 1 × 10−5 M).
Figure 13 Effect of number of runs on the degradation of dyes in the presence SnO nanoparticles (a) UV light irradiation (b) visible light irradiation of MB and RdB after 60 min (catalyst concentration: 0.8 mg/mL; initial concentration of dyes: 1 × 10−5 M).

3.3

3.3 Possible mechanism

SnO is a typical p-type semiconductor metal oxide. When the SnO is excited with UV or visible light irradiation, SnO absorbs the photons at its interface. If the energy of photons is equal to or greater than SnO band gap (2.72 eV), electron (e−)-hole (h+) pairs are generated at SnO nanoparticles’ surface (Eq. (2)).

(2)
SnO + h ν → SnO ( e - + h + )

The photogenerated electron-hole pairs disperse to the surface of SnO nanoparticles. After that, these electron-hole pairs react with electron acceptors and donors. Thus, conduction band electron generates •O2− in Eq. (3) by reducing O2 on the surface of SnO nanoparticles and the hole product OH• by oxidizing water in Eq. (4).

(3)
SnO ( e - ) + O 2 → SnO + • O 2 -
(4)
SnO ( h + ) + H 2 O → SnO + OH • + H +

Hydrogen peroxide may be generated due to the presence of superoxide radicals (Eq. (5)).

(5)
2 • O 2 - + 2 H + → H 2 O 2 + O 2

The generated hydroxyl radicals are very active and oxidize MB and RdB dyes to mineral end product (Eq. (6)).

(6)
OH • + Dyes → H 2 O + Radicals of Dyes → mineral end products

Schematic diagram of photocatalytic mechanism is shown in Fig. 14.

Schematic diagram of photocatalytic mechanism.
Figure 14 Schematic diagram of photocatalytic mechanism.

4

4 Conclusions

In this study, SnO nanoparticles were synthesized by a novel surfactant-assisted method. By changing the concentration of the TX-100 surfactant, SnO crystals having different sizes and morphologies could be achieved. To the best of our knowledge, there has been no report for the synthesis of SnO nanoparticle in the presence of TX-100 in the previous literature. The characterization of synthesized SnO nanoparticles was completed by using XRD, SEM, TEM, XPS, UV–vis, PL and FTIR methods. The results of XRD, TEM and XPS analyses are consistent with each other. SEM, XRD, UV–vis. DRS and PL observations demonstrate that the surfactant TX-100 plays an important role in controlling the formation of precursor powders. Also from XRD analysis, it is possible to observe that the presence of TX-100 can effectively prompt the crystallization and inhibits the grain growth allowing to obtain smaller SnO crystallites. The photocatalytic performances of SnO nanoparticles were analyzed for the photocatalytic degradation of two textile dyes under both UV and visible light irradiations. The addition of TX-100 to synthesis solution led to the decrease in the size of SnO particles, and to the formation of the architecture consisting of platy units. The morphological structure and photocatalytic activities of the synthesized SnO nanoparticles changed with increasing amount of TX-100. The morphological change of SnO particles could be attributed to the suppression of the growth of SnO crystals by the micelle formation. The SnO nanoparticles showed a good photocatalytic activity on photocatalytic degradation of MB and RdB dyes under both UV and visible light irradiations. Sn2+:TX 100 ratio, light source, and amount of photocatalyst affected the photocatalytic degradation of the dyes. The synthesized SnO nanoparticles in the presence of TX-100, having a high separation rate of photogenerated electrons and holes, favor their photocatalytic activity. S3 sample has the highest photocatalytic activity due to the fact that it has the smallest particle size. The synthesized SnO photocatalyst still has great photocatalytic activity after five cycles and was able to remove also MB and RdB dyes in the presence of both UV and visible light.

Acknowledgments

The authors are grateful for financial support by The Scientific and Technical Research Council of Turkey (Project no: 113Z656) and The Administrative Units of The Research Projects of Selçuk University (Project no: 17201006). This work produced from M. Sarıbel’s M. Sc Thesis.

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