A simple sonochemical approach for synthesis of selenium nanostructures and investigation of its light harvesting application.

Ultrasonics Sonochemistry xxx (2014) xxx–xxx

Contents lists available at ScienceDirect

Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultson

A simple sonochemical approach for synthesis of selenium nanostructures and investigation of its light harvesting application Mokhtar Panahi-Kalamuei a, Mehdi Mousavi-Kamazani b, Masoud Salavati-Niasari a,⇑, S. Mostafa Hosseinpour-Mashkani c a b c

Institute of Nano Science and Nano Technology, University of Kashan, P. O. Box 87317-51167, Kashan, Iran Young Researchers and Elites Club, Kashan Branch, Islamic Azad University, Kashan, Iran Young Researchers and Elites Club, Qom Branch, Islamic Azad University, Qom, Iran

a r t i c l e

i n f o

Article history: Received 3 July 2014 Received in revised form 9 September 2014 Accepted 10 September 2014 Available online xxxx Keywords: Selenium Sonochemical method Nanostructures Light harvesting

a b s t r a c t Selenium (Se) nanostructures were synthesized by a sonochemical method using SeCl4 as a new precursor for Se nanostructures. Moreover, hydrazine, potassium borohydride, and thioglycolic acid were used as reducing reagents in aqueous solution. Ultrasonic power, irradiation time, reducing agent, solvent, HCl, NaOH, and the surfactant were changed in order to investigate the effect of preparation parameters on the morphology and particle size of selenium. The obtained Se with different morphologies and sizes was characterized by XRD, SEM, TEM, EDS, and DRS. The selenium nanostructures exhibited enhanced photocatalytic activity in the degradation of methylene blue (MB) under visible light irradiation. Furthermore, to examine the solar cell application of as-synthesized selenium nanostructure, FTO/TiO2/Se/Pt-FTO and FTO/Se/CdS/Pt-FTO structures were created by deposited selenium film on top of the TiO2 layer and FTO glass prepared by Doctor’s blade method, respectively. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction In the past few decades, nanomaterials have been extensively studied for the degradation of organic pollutes as well as solar cell application [1–3]. Elemental selenium (Se) is a narrow band gap (1.7 eV) semiconductor which has been extensively used in solar cells, photocatalyst, xerography, and rectifiers [4–6]. It is also a photoconductor with a relatively low melting point (217 °C), high refractive index and high reactivity leading to a wealth of functional materials such as Ag2Se [7–9], CdSe [10], ZnSe [11,12], PbSe [13,14], and NiSe [15–19]. There are some methods for selenium nanostructures preparation which involve solution-phase [20], Green Synthesis [21,22], microwave-polyol method [23] and ultrasonic irradiation approach [24,25]. Among various methods for the preparation of nanostructures, ultrasonic method is more promising in terms of low cost, simply control the shape and particle size, low processing temperature, simplicity and potential for large-scale production. Recently, the ultrasonic process as a fast, convenient, and economical method has been widely used to generate novel nanostructure materials under ambient conditions [24–29]. The chemical effects of ultrasound arise from ⇑ Corresponding author. Tel.: +98 361 5555 333; fax: +98 361 555 29 30. E-mail address: [email protected] (M. Salavati-Niasari).

acoustic cavitation, which is the formation, growth, and implosive collapse of bubbles in a liquid. The growth of the bubble occurs through the diffusion of solute vapor into the volume of the bubble, while the collapse of the bubble occurs when the bubble size reaches its maximum value. When solutions are exposed to ultrasound irradiation, the bubbles are implosively collapsed by acoustic fields in the solution. According to hot spot theory, very high temperatures (>5000 K) are obtained upon the collapse of a bubble. Since this collapse occurs in less than a nanosecond, very high cooling rates (>1010 K/s) are also obtained [30,31]. These extreme conditions can drive a variety of chemical reactions to fabricate nano-sized materials. It is widely accepted that the properties of nanomaterials have a close relationship with their morphology, size, size distribution, crystallinity, [32,33] and control over the morphology and size of inorganic materials at micro- and nanoscale level is an important goal. Therefore, in this research we investigate the effect of preparation parameters such as ultrasonic power, irradiation time, reducing agent, solvent, HCl, NaOH, and surfactant on the morphology and particle size of selenium which was synthesized from SeCl4 as selenium precursor by a sonochemical method. To our knowledge, it is the first time that SeCl4 is used as the selenium source for the synthesis of Se nanostructures. Thioglycolic acid was used as both the solvent and the capping agent. It is noteworthy that, in suitable conditions, thioglycolic acid can

http://dx.doi.org/10.1016/j.ultsonch.2014.09.006 1350-4177/Ó 2014 Elsevier B.V. All rights reserved.

Please cite this article in press as: M. Panahi-Kalamuei et al., A simple sonochemical approach for synthesis of selenium nanostructures and investigation of its light harvesting application, Ultrason. Sonochem. (2014), http://dx.doi.org/10.1016/j.ultsonch.2014.09.006

2

M. Panahi-Kalamuei et al. / Ultrasonics Sonochemistry xxx (2014) xxx–xxx

form a complex with Se4+ ions that turn to Se clusters; furthermore, it can reduce SeCl4 to Se, which means it can work as a reducing agent under ultrasonic irradiation. Moreover, the as-synthesized Se nanostructures were utilized as the photocatalyst for the degradation of methylene blue (MB) and as the photo-anode material for fabrication of FTO/TiO2/Se/Pt-FTO and FTO/Se/CdS/Pt-FTO solar cell structures to examine their solar cell application. 2. Experimental 2.1. Materials and physical measurements All the chemicals used in this method were of analytical grade and used as-received without any further purification. Ultrasonic irradiation was accomplished using a high-intensity ultrasonic probe (Sonicator 3000; Bandeline, MS 72, Germany, Tihorn, 20 kHz, 80 W cm2) immersed directly in the reaction solution. X-ray diffraction (XRD) patterns were recorded by a Philips-X’PertPro, X-ray diffractometer using Ni-filtered Cu Ka radiation at scan range of 10 < 2h < 80.Scanning electron microscopy (SEM) images were obtained on LEO-1455VP equipped with an energy dispersive X-ray spectroscopy. Transmission electron microscope (TEM) images were obtained on a Philips EM208S transmission electron microscope with an accelerating voltage of 100 kV. The energy dispersive spectrometry (EDS) analysis was studied by XL30, Philips microscope. The diffused reflectance UV–visible spectrum (DRS) of the sample was recorded by an Ava Spec-2048TEC spectrometer. Photocurrent density–voltage (J–V) curve was measured by using computerized digital multimeters (Ivium-n-Stat Multichannel potentiostat) and a variable load. A 300 W metal xenon lamp (Luzchem) served as assimilated sun light source, and its light intensity (or radiant power) was adjusted to simulated AM 1.5 radiation at 100 mW/cm2 with a filter for this purpose. 2.2. Synthesis of selenium nanostructures At first, 0.05 g (0.23 mmol) of SeCl4 as selenium precursor was dissolved in 30 ml of distilled water, TGA/water, and TGA under magnetic stirring. In the second step, different solutions were made by dissolving 0.14 mmol of different surfactants such as CTAB, SDS, TGA, PEG-600, TGA/PEG, TGA/SDS, and TGA/CTAB in 8 ml of distilled water and were added subsequently to the above solution; then, 2 ml of hydrazine (1 M), potassium borohydride (1 M) as reducing agent were added drop-wise. During these processes no sample was observed. Finally, the colorless mixtures were loaded into a beaker where the reaction was performed in an ultrasonic digestion system at room temperature. After 10 min, the colorless solutions turned red, which indicates the presence of Se2+. The color of the solution changed to black when the reaction time was prolonged to 20 min. After irradiation, the system was allowed to cool to room temperature by itself, and the obtained precipitate was collected by filtration, and washed with absolute ethanol and distilled water several times. Finally, the products were dried in vacuum at 70 °C for 5 h. The reaction conditions are listed in Table 1. 2.3. Photocatalysis experiments In order to test the photocatalytic activity of the Se nanostructures, the decolorization of methylene blue (MB) in suspension of Se nanostructures under high-pressure mercury lamp illumination was studied. A 250 W high-pressure mercury lamp (GYZ-250) was fixed at a distance of 30 cm above the surface solution. The

measured luminous intensity was 0.5 kW m2. Then, 15 mg/L of photocatalyst nanoparticles was added into 150 ml of 25 g l1 methylene blue (MB) aqueous solution. Subsequently, the aqueous suspension was magnetically stirred for 30 min in dark to achieve the adsorption–desorption equilibration to discount the adsorption of the substrate on the catalyst. Afterwards, under ambient conditions and stirring, the beakers were exposed to UV light for 30, 45, 60, 75, 90, 105, and 120 min, respectively. Then, 15 ml of sample was taken out after a determined period of time and centrifuged at 5250 rpm for 5 min. Finally, the absorbance spectra of the methylene blue (MB) solution were recorded by a UV–vis spectrophotometer (Shimadzu UV–vis spectrometer) and the decolorization rate was calculated according to the absorbance change. The decolorization efficiency (%) is calculated as decolorization efficiency (1).

ð%Þ ¼

C0 C 100 C0

ð1Þ

where C0 is the initial concentration of dye and C is the concentration of dye after photo irradiation. 2.4. Fabrication of FTO/Se/CdS/Pt-FTO cell A selenium thin film for the solar cell was prepared by the Doctor’s blade technique. First, a suspension was provided by dispersing 0.05 g of synthesized Se powder into a solution containing 5 ml of ethanol, 0.05 g of ethyl cellulose, and 0.05 ml of Triton X100. Then, this suspension was coated on washed Fluorinetin-oxide (FTO)-coated glass substrate (1 cm 1 cm). Subsequently, the glass substrate was kept at 350 °C for 30 min to eliminate the organic compounds. Afterwards, a CdS layer was deposited on a Se thin film with chemical bath deposition (CBD). The CBD process was performed with a solution consisting of 0.01 M of Cd(NO3)2 and 0.08 M of thiourea. The bath was heated to 80 °C and then the sample was immersed. A counter-electrode was made from deposition of a Pt solution on FTO glass. Later, this electrode was placed over CdS/Se electrode. The redox electrolyte consisting of 0.05 M of LiI, 0.05 M of I2 and 0.5 M of 4-tertbutylpyridine at acetonitrile as a solvent was introduced into the cell through one of the two small holes drilled in the counter electrode. Finally, these two holes were sealed by a small square of sealing sheet and characterized by I–V test. 2.5. Fabrication of FTO/TiO2/Se/Pt-FTO cell Electrophoresis deposition (EPD) was utilized to the prepare TiO2 films. During EPD, the cleaned FTO glass remained at a positive potential (anode) while a pure steel mesh was used as the counter (cathode) electrode. The linear distance between the two electrodes was about 3 cm. Power was supplied by a Megatek Pro-grammable DC Power Supply (MP-3005D). The applied voltage was 10 V. The deposition cycle was repeated 4 times, each time 5 s, and the temperature of the electrolyte solution was kept constant at 25 °C. The coated substrates were air dried. The apparent area of the film was 1 1 cm2. The resulted layer was annealed under an air flow at 500 °C for 30 min. Electrolyte solution consisted of 120 mg/l of I2, 48 ml/l of acetone, and 20 ml/l of water. For deposition of Se powder on the FTO glass substrate, a paste of Se was initially prepared. The slurry was produced by mixing and grinding 1.0 g of the nanometer sized Se with ethanol and water in several steps. Afterwards, the ground slurry was sonicated with ultra-sonic horn (Sonicator 3000; Bandeline, MS 72, Germany) and then mixed with terpineol and ethyl cellulose as binders. After removing the ethanol and water with a rotary-evaporator, the final paste was prepared. The prepared Se paste was coated on TiO2 film by a Doctor blade technique. After that the electrode was gradually


3

M. Panahi-Kalamuei et al. / Ultrasonics Sonochemistry xxx (2014) xxx–xxx Table 1 Reaction conditions for Se nanostructures. Sample No.

Power (W)

Time (min)

Reducing agent

Surfactant

Solvent

HCl (ml)

NaOH (ml)

Product

Photodecolorization

A1 A2 A3 A4 A5 A6 A7 A8 A9 A10 A11 A12 A13 A14 A15 A16 A17 A18 A19 A20 A21

60 40 80 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60

30 30 30 15 45 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30

N2H4 N2H4 N2H4 N2H4 N2H4 KBH4 N2H4 KBH4 N2H4 N2H4 N2H4 N2H4 N2H4 N2H4 N2H4 TGA TGA TGA N2H4 N2H4 N2H4

CTAB CTAB CTAB CTAB CTAB CTAB CTAB CTAB SDS TGA PEG – TGA/PEG TGA/CTAB TGA/SDS TGA TGA TGA CTAB CTAB –

Water Water Water Water Water Water Water Water Water Water Water Water Water Water Water TGA15/Wter15 TGA20/Water10 TGA Water Water Water

– – – – – – – – – – – – – – – – – – 2 – –

– – – – – – – – – – – – – – – – – – – 2 2

Se Se Se Se Se Se – Se Se Se Se Se Se Se Se Se Se Se Se Se Se

97.91 – – – 97.97 97.26 – – – – – 91.43 93.32 96.78 94.4 – – 97.34 – – –

heated under an air flow at 450 °C for 30 min. Counter-electrode was made from deposition of a Pt solution on FTO glass. Afterwards, this electrode was placed over TiO2/Se electrode. Sealing was accomplished by pressing the two electrodes together on a double hot-plate at a temperature of about 110 °C. The redox electrolyte consisting of 0.05 M of LiI, 0.05 M of I2 and 0.5 M of 4-tertbutylpyridine in acetonitrile as a solvent was introduced into the cell through one of the two small holes drilled in the counter electrode. Finally, these two holes were sealed by a small square of sealing sheet and characterized by I–V test.

3. Results and discussion Fig. 1a–c provides a comparison of typical XRD patterns of Se nanostructures derived from different reaction conditions for A1, A6, and A18 samples, respectively. The diffraction peaks, observed in Fig. 1a–c, can be indexed to pure hexagonal phase of selenium (a = b = 4.3662 Å, c = 4.9536 Å) with space group of P3121 and JCPDS No. 73–0465. The sharp diffraction peaks manifestation shows that the obtained Se nanostructures have high crystallinity. Using Scherrer equation and XRD data, the crystallite diameters

Fig. 1. XRD patterns of the as-synthesized Se (a) sample A1 (b) sample A6 (c) sample A18.


4


(Dc) of Se nanostructures were calculated (2) as 19, 11, and 23 nm for A1, A6, and A18 samples, respectively [34]:

Dc ¼ Kk=bcosh

ð2Þ

where b is the breadth of the observed diffraction line at its half intensity maximum, K is the so-called shape factor, which usually takes a value of about 0.9, and k is the wavelength of X-ray source used in XRD. Energy Dispersive X-ray Analysis (EDX) was used to identify the elemental composition of selenium nanostructures (Fig. 2). The EDS spectrum of Se nanosheets (A1 sample) indicates the presence of Se element. Furthermore, neither N nor C signals are detected in the EDS spectrum, which means the product is pure and free of any surfactant or impurity. Therefore, both EDS and XRD analyses show that Se nanostructures are successfully synthesized through the sonochemical method. UV–vis absorption spectrum of Se nanostructures (A1 sample) is shown in Fig. 3. The fundamental absorption edge in most semiconductors follows the exponential law. Using the absorption data, the band gap can be estimated by Tauc’s relationship (3):

a ¼ a0ðht Eg Þn=ht

ð3Þ

where a is the absorption coefficient, ht is the photon energy, a0 and h are the constants, Eg is the optical band gap of the material, and n depends on the type of the electronic transition and can take any value between ½ and 3 [35]. The Eg value is calculated as 3.5 eV (354 nm) for the Se (A1 sample), which shows a large blue shift compared with Se bulk (1.7 eV) [36]. Changing the reaction parameters such as ultrasonic power, irradiation time, reducing agent, solvent, HCl, NaOH, and surfactant in ultrasonic method is an effective way to control morphology, particle size, and properties of products [37–40]. Therefore, in this manuscript, the effects of aforementioned parameters on the morphology and particle size of the Se nanostructure were investigated. To examine the effect

Fig. 2. EDS pattern of as-synthesized Se (a) sample A1 (b) sample A6.

Fig. 3. Diffuse reflectance spectrum of Se nanostructure (sample A1).

of ultrasonic power, three experiments were performed at 60, 40, and 80 W (A1–A3 samples, Fig. 4a–c, respectively). Ultrasonic irradiation creates bubbles which produce high temperature and energy after decomposition, where sufficient amounts of energy for formation of Se nanosheets are produced. According to the SEM image of A1 sample (Fig. 4a), the product consists of nanosheets with average diameter of 200 nm. Careful observation of the images suggests that, by changing the ultrasonic power from 60 W (Fig. 4a) to 40 and 80 W (Fig. 4b and c, respectively), the morphology and homogeneity of samples have changed, and the irregular aggregated microparticles are formed. Hence, the optimal power to accomplish a suitable and homogeneous size and morphology was obtained at 60 W in these experiments (A1 sample and Fig. 4a). Below this power, i.e. 60 W, there is not enough energy to separate particles. On the other hand, any amount of power more than 60 W will cause the agglomeration of the nanoparticles, which is due to large quantity of energy of the particles. Higher ultrasonic intensity can cause the collapse of more cavitation bubbles and a stronger shock wave, which can prevent the crystal growth. Therefore, by changing the ultrasonic intensity, we can control the dimensions of the nanosheets. Considering the different conditions and the produced morphologies, the optimum power for ultrasonic irradiation was set at 60 W, and other experiments were carried out with this power. We performed two experiments in 15 and 45 min reaction time to investigate the effect of reaction time on morphology of the samples at constant power of 60 W (A4 and A5 samples, Fig. 5a and b, respectively). According to Fig. 5a and b, decreasing the reaction time from 30 to 15 min as well as increasing it from 30 to 45 min increases and decreases the diameters of the nanosheets, respectively. There is not precipitate in the ultrasonic times below 15 min because they do not have sufficient energy for nucleation. When the reaction was performed in 15 min, the appropriate amount of energy was supplied in reaction medium, which in turn improved the nucleation process (A4 sample, Fig. 5a). Nanosheets produced at 30 min (A1 sample, Fig. 4a) are uniformly good, though their size is larger than the size of nanosheets produced at 45 min (A5 sample, Fig. 5b). Therefore, to achieve an appropriate and uniform size and morphology, the optimal time is necessary. The recommended time for this test is 30–40. To observe the effect of reducing agent on the morphology and particle size of selenium nanostructure, potassium borohydride (KBH4) was used instead of hydrazine (A6 sample, Fig. 6a). It seems changing the reducing agent produces nanoparticles with average particle size of 20 nm instead of nanosheets. Potassium borohydride causes an increase in nucleation processes, which can produce a change in the grow mechanism of particles since it is a stronger reducing agent than hydrazine. TEM images were taken to characterize the size and morphology of Se nanoparticles. The TEM images



5

Fig. 4. SEM images of Se nanostructure (a) sample A1 (b) sample A2 (c) sample A3.

Fig. 5. SEM images of Se nanostructure (a) sample A4 (b) sample A5.

of Se (A6 sample, Fig. 6b and c) show the product is mainly composed of a large amount of sphere-like nanoparticles with average particle size of 15 to 20 nm. To investigate the effect of ultrasonic irradiation on the morphology and particle size of the products, two blank tests were carried out under mechanical stirring conditions in presence of hydrazine and potassium borohydride (A7 and A8 samples, respectively). By using hydrazine as the reducing agent under mechanical stirring and in the absence of ultrasonic irradiation (A7 sample), no precipitate was collected (the reaction did not take place), and the solution remained colorless. When potassium borohydride was used under mechanical stirring in the absence of ultrasonic irradiation (A8 sample), the reaction occurred successfully and the colorless solution turned into red after 5 min and black

precipitates after 10 min (Fig. 6d). According to Fig. 6d, agglomeration of microstructures is observable in many areas, and the product mainly consists of microstructures. However, when the same reaction was performed in the presence of ultrasonic irradiation, homogeneous nanoparticles were produced (A6 sample, Fig. 6a). To reduce a materials particle size, large particles or lumps must be fractured into smaller ones. To initiate fractures, external forces are applied to the particles. Advantage of using ultrasound radiation is that it yields smaller particles and result shows an increase in the size of particles prepared without ultrasonic irradiation. In addition to, according to what was mentioned before, it is found that the stirring has no effect on the SeCl4 or hydrazine degradation, but the ultrasonic irradiation increases the rate of degradation. As a result,


6


Fig. 6. SEM image of Se nanostructure (a) sample A6 (b and c) TEM images sample A6 (d) sample A8.

the benefit of using ultrasonic irradiation is both the occurrence of the reaction and production of nanoparticles. We used four surfactants in our experiment to investigate their influence on the morphology of samples at the constant power of 60 W in 30 min. Fig. 7a–c show the SEM images of Se nanostructures in presence of SDS, TGA, and PEG-600 as surfactant, respectively. The morphology of all samples is particle-like. It should be noted that PEG-600 has a strong steric hindrance effect, which prevents the particle growth and results in creation of small nanoparticles instead of large nanosheets (Figs. 1a and 7b). According to SEM images in Fig. 7a–c, it is seen that the dimension of synthesized Se using SDS is smaller than that of other surfactants. Considering the reaction conditions in the beginning of the synthesis process, Se4+ ions were formed; later, they changed into Se2+ ions (dark red solution) as well as Se nanoparticles. It can be concluded that Se2+ ions are

collected around Se particles. Therefore, anionic surfactant (SDS) is best to prevent agglomeration of Se nanoparticles. To investigate the effect of surfactant on the morphology and particle size of Se nanostructures, a blank test was performed (A12 sample, Fig. 7d). It is clear that aggregated microstructures are formed in the absence of the surfactant. In addition, the effects of mixtures of different surfactants on morphology of the products were examined and compared them with one another. The SEM image of A13 sample in presence of TGA and PEG mixture as the surfactant (Fig. 8a) shows the product mainly consists of nanoparticles as well as microsphere structures. When TGA/PEG mixture was replaced with TGA/CTAB and TGA/SDS (A14 and A15 samples, respectively), the product consisted of nanorods, nanoparticles as well as nanosheets, as shown in Fig. 8b and c, respectively. The morphology and size distribution of A15 sample was further studied by TEM (Fig. 8d). According to



Fig. 7. SEM images of Se nanostructure (a) sample A9 (b) sample A10 (c) sample A11 (d) sample A12.

7

Fig. 8. SEM images of Se nanostructure (a) sample A13 (b) sample A14 (c) sample A15 (d) TEM image sample A15.


8


Fig. 8d, the product consists of nanorods with average diameter of 30–70 nm and nanoparticles, which is in agreement with what was observed from the SEM image of A15 sample. Suslick and his colleagues have reported that organic liquids support acoustic cavitation [41]. As a result, decreasing solvent vapor pressure causes an increase in the intensity of cavitation collapse, the peak temperature reached during such collapse, and consequently, the rates of sonochemical reactions. In this work, water and TGA with different vapor pressures and viscosities were used to prepare selenium nanostructures in presence of ultrasonic irradiation for 30 min. Fig. 9a–c shows the SEM images of A16–A18 samples obtained by TGA/water (15/15), TGA/water (20/10), and TGA, respectively. According to Fig. 9a–c, the products are mainly composed of nanorods covered with nanoparticles. Comparing Figs. 9a and b, it is evident that increasing the proportion of TGA to water from 15/15 in A16 sample to 20/10 in A17 sample causes a decrease in length and diameter of rods due to the fact that the vapor pressure of TGA (0.4 Torr at 25 °C) is lower than that of water (24 Torr at 25 °C). As a result, the vapor pressure of TGA/water (20/10) solution is lower than that of TGA/water (15/15) solution. In A18 sample, TGA acts as a surfactant, solvent, and reducing agent. Thioglycolic acid as a soft template can control the morphology of the product and play a significant role in formation of nanorods (Fig. 9c). Furthermore, in suitable conditions, it can form a complex with Se4+, which turns into Se clusters. Moreover, thioglycolic acid has two functional groups, namely, –COOH and –SH. Excess concentrations of TGA leads to its absorption on the surface of Se4+ ions and creation of hydrogen as well as S–S bonds among TGA molecules. The Se nanoparticles were cross-linked via hydrogen and S–S bonds and, finally, turned to Se nanorods. HSCH2CO2H (TGA) can be readily oxidized to the corresponding disulfide [SCH2CO2H]2 under ultrasonic waves which can reduce SeCl4 to the Se. ÞÞÞ

2HSCH2 CO2 H ! ½SCH2 CO2 H2 þ 2H

ð4Þ

SeCl4 þ H2 O ! H2 SeO3 þ 4HCl

ð5Þ

4H þ H2 SeO3 ! Se þ 3H2 O

ð6Þ

4HSCH2 CO2 H þ H2 SeO3 ! ½SCH2 CO2 H4 þ Se þ 3H2 O

ð7Þ

A TEM image was taken to characterize the size and morphology of Se nanorods (A18 sample, Fig. 9d). Fig. 9d reveals that the Se nanorods prepared by the ultrasonic method consist of nanorods with average diameter size of 70–80 nm. The formation mechanism of Se nanostructures by sonochemical is probably related to the radical species generated from water molecules by absorption of the ultrasound energy. It has been known that during an aqueous sonochemical process, increasing the temperatures and pressures inside the collapsing bubbles makes water vaporize and further pyrolyze into H and OH radicals [37]: ÞÞÞ

H2 O ! H þ OH

ð8Þ

H þ H ! H2

ð9Þ

OH þ Other species ! Oxidized products

ð10Þ

According to the work of Nakui et al. [42], hydrazine degradation takes place under ultrasonic irradiation:

N2 H4 þ H2 O ! N2 Hþ5 þ OH

ð11Þ

N2 Hþ5 þ H2 O ! N2 H2þ 6 þ OH

ð12Þ

N2 H4 þ 4OH ! N2 þ 4H2 O

ð13Þ

Fig. 9. SEM images of Se nanostructure (a) sample A16 (b) sample A17 (c) sample A18 (d) TEM image sample A18.



9

Fig. 10. SEM images of Se nanostructure (a) sample A19 (b) sample A20 (c) sample A21.

SeCl4 þ 3H2 O ! H2 SeO3 þ 4HCl

ð14Þ

4H þ H2 SeO3 ! Se þ 3H2 O

ð15Þ

According to the above reaction procedures, either acid or base play an important role in synthesis of Se nanostructures. Therefore, the effects of HCl and NaOH on the morphology and particle size of products were examined (Fig. 10a–c). Fig. 10a, the SEM image of A19 sample in presence of HCl, shows the product is composed of agglomerated nanoparticles, nanosheets, and nanorods. According to Fig. 10b, when NaOH is used instead of HCl (A20 sample), selenium nanoparticles with the mean size of 15–25 nm and irregular shapes are produced (Fig. 10b). In comparison with A1 sample, in A20 sample where sodium hydroxide is used, the reducing effect of hydrazine is higher, leading to an increase in nucleation processes and therefore creation of small nanoparticles. When we continued the reaction procedure in presence of NaOH without

Fig. 11. Photodecolorization of methylene blue (MB) under UV illumination by blank and Se nanostructures.


10


Fig. 12. I–V characterizations of the (a) FTO/Se/CdS/Pt-FTO sample A1 (b) FTO/TiO2/Se/Pt-FTO sample A1.

CTAB (A21 sample), the product was composed of nanoparticles with average particle size of 10–15 nm, as shown in Fig. 10c. As a result, by adjusting the pH value, the surfactant could be removed. To evaluate the photocatalytic decomposition of methylene blue (MB), as-synthesized Se nanostructures were used as photocatalyst. Moreover, the photocatalytic decomposition was performed under UV-light illumination, and the degradation rate for the decomposition of methylene blue (MB) was estimated by observing the changes in absorbance (absorption intensity vs. irradiation time) obtained by UV–vis spectra. Fig. 11 exhibits the plot of the remaining dye concentration (A/A0) versus time intervals for the photocatalytic reaction of Se samples with different morphologies including nanosheets (A1 and A5 samples), nanoparticles (A6 sample), microparticles (A12 sample), nanoparticles/microspheres (A13 sample), nanoparticles/nanorods (A14 and A15 samples), and nanorods (A18 sample). The dye degradation rate on the surface of Se nanostructures is calculated by the following Eq. (16):

Degradation rate ð%Þ ¼

A0 A 100 A0

ð16Þ

where A0 and A are initial concentration and changed absorbencies of dye after ultraviolet irradiation, respectively. It can be seen that in the absence of as-synthesized Se nanostructures, an almost negligible amount of methylene blue (MB) radiation is observable (plot A) while in the presence of Se, more than 90% of MB was decolorized after 120 min. Evidently, changes in morphology have led to changes in decolorization of MB, the most of which is for the nanosheets and the least for microstructures. Compared with microstructures, nanostructures have a more effective surface, and consequently, produce a higher yield. Also, nanoparticles and nanorods have a lower yield than nanosheets. The decolorization results could be due to the more suitable surface of the nanosheets, in comparison with other aforementioned morphologies, for adsorption and decolorization. J–V characterization of a typical solar cell fabricated using in situ approach is shown in Fig. 12a and b. The measurement of the current density voltage (J–V) curve for Se (A1 sample) was carried out under the illumination of AM1.5 (100 mW/cm2). Device characteristics are as follows: Voc = 0.63 V, Jsc = 0.368 mA/cm2, FF = 59%, and N = 0.13 for FTO/Se/CdS/Pt-FTO structure and Voc = 0.56 V, Jsc = 0.975 mA/cm2, FF = 71%, and N = 0.39 for FTO/TiO2/Se/Pt-FTO structure. The Voc and Jsc of this device are in order of those obtained using non-vacuum-based techniques. In comparison to other similar works illustrated in Table 2, our method is simpler, faster, and more controllable. We have used SeCl4 as a new selenium source to prepare the Se nanostructures. Furthermore, SeCl4 is a good selenium source, which

Table 2 Different methods compared for the preparation of Se nanostructures. Method

Precursors

Morphology

Ref.

Sonochemical Sonochemical

H2SeO3, N2H4H2O Amorphous selenium SeO2 and KBH4

Nanotubes and Nanowires Nanowires

[24] [25]

Nanostructures (rod, particle, etc.)

[43]

Precipitation

dissolves well in distilled water and provides a highly reactive in aqueous solution. In our experiment, when SeCl4 is added in deionized water, a completely clear acidic solution is obtained that contains H2SeO3; however, H2SeO3 can be quickly converted to Se by hydrazine, potassium borohydride, and thioglycolic acid under ultrasonic irradiation. To our knowledge, it is the first time that thioglycolic acid is used as solvent, surfactant, and reducing agent in synthesis of Se nanostructures. As a result, under ultrasonic waves, thioglycolic acid can reduce SeCl4 to Se, where no other reductant is needed. Furthermore, controlling thioglycolic acid concentrations results in different morphologies of the product such as nanoparticles and nanorods. Moreover, for the first time, we made FTO/TiO2/Se/Pt-FTO and FTO/Se/CdS/Pt-FTO solar cell structures in order to examine solar selenium nanostructures.

4. Conclusions In summary, selenium nanostructures were successfully synthesized by sonochemical method using SeCl4 as a Se precursor, and hydrazine, potassium borohydride, and thioglycolic acid as reducing reagents. Besides, the effects of ultrasonic power, irradiation time, reducing agent, solvent, HCl, NaOH, and surfactant on the morphology and particle size of the selenium nanostructures were investigated. Water, thioglycolic acid, and a mixture of thioglycolic acid and water were employed as solvents, and the volume ratio of thioglycolic acid to water was found to play a key role in controlling the morphology of the resultant selenium samples. Furthermore, TGA can be easily oxidized to the corresponding disulfide [SCH2CO2H]2 under ultrasonic waves, which can reduce SeCl4 to Se. Moreover, photodegradation of methylene blue (MB) and solar cells tests were performed in order to investigate the light harvesting application of selenium nanostructures. Selenium nanostructures were characterized by XRD, TEM, EDS, DRS, and SEM techniques.



Acknowledgment Authors are grateful to council of University of Kashan for providing financial support to undertake this work by Grant No. (159271/199). References [1] M. Mousavi-Kamazani, M. Salavati-Niasari, Compos. Part B Eng. 56 (2014) 490–496. [2] S.M. Hosseinpour-Mashkani, M. Salavati-Niasari, F. Mohandes, K. Venkateswara-Rao, Mater. Sci. Semicond. Process. 16 (2013) 390–402. [3] M. Zahedifar, Z. Chamanzadeh, S.M. Hosseinpoor-Mashkani, Luminescence 135 (2013) 66–73. [4] S. Nath, S.K. Ghosh, S. Panigahi, T. Thundat, T. Pal, Langmuir 20 (2004) 7880– 7883. [5] L.B. Yang, Y.H. Shen, A.J. Xie, J.J. Liang, B.C. Zhang, Mater. Res. Bull. 43 (2008) 572–582. [6] J.A. Johnson, M.L. Saboungi, P. Thiyagarajan, R. Csencsits, D. Meisel, J. Phys. Chem. B 103 (1999) 59–63. [7] M. Jafari, A. Sobhani, M. Salavati-Niasari, J. Ind. Eng. Chem. 20 (2014) 3775– 3779. [8] K. Li, X. Liu, H. Wang, H. Yan, Mater. Lett. 60 (2006) 3038–3040. [9] S. Xu, L. Zhang, X. Zhang, C. He, Y. Lv, Sensors Actuat. B Chem. 155 (2011) 311– 316. [10] A. Sobhani, M. Salavati-Niasari, Mater. Res. Bull. 40 (2014) 7–14. [11] Y. Ding, S.Z. Shen, H. Sun, K. Sun, F. Liu, Sensors Actuat. B Chem. 203 (2014) 35– 43. [12] W. Zhou, S. Xue, J. Han, P. Xie, Mater. Lett. 134 (2014) 256–258. [13] M. Salavati-Niasari, B. Shoshtari-Yeganeh, F. Mohandes, Mater. Res. Bull. 48 (2013) 1745–1752. [14] M. Salavati-Niasari, B. Shoshtari-Yeganeh, M. Bazarganipour, Superlattice Microstruct. 58 (2013) 20–30. [15] M. Salavati-Niasari, A. Sobhani, Opt. Mater. 5 (2013) 904–909. [16] A. Sobhani, M. Salavati-Niasari, Superlattice Microstruct. 65 (2014) 79–90. [17] Y.C. Zhu, T. Mei, Y. Wang, Y.T. Qian, J. Mater. Chem. 21 (2011) 11457–11463. [18] A. Sobhani, M. Salavati-Niasari, F. Davar, Polyhedron 31 (2012) 210–216. [19] A. Sobhani, F. Davar, M. Salavati-Niasari, Appl. Surf. Sci. 257 (2011) 7982– 7987.

11

[20] S.Y. Zhang, J. Zhang, H.Y. Wang, H.Y. Chen, Mater. Lett. 58 (2004) 2590–2594. [21] H. Chen, J.B. Yoo, Y. Liu, G. Zhao, Electron. Mater. Lett. 7 (2011) 333–336. [22] C.H. Ramamurthy, K.S. Sampath, P. Arunkumar, M.S. Kumar, V. Sujatha, K. Premkumar, C. Thirunavukkarasu, Bioprocess. Biosyst. Eng. 36 (2012) 1131– 1139. [23] Y.J. Zhu, X.L. Hu, Mater. Lett. 58 (2004) 1234–1236. [24] X.M. Li, Y. Li, S.Q. Li, W.W. Zhou, H.B. Chu, W. Chen, I.L. Li, Z.K. Tang, Cryst. Growth Des. 5 (2005) 911–916. [25] B. Gates, B. Mayers, A. Grossman, Y. Xia, Adv. Mater. 14 (2002) 1749–1752M. [26] S.K. Suslik, Ultrasound: Its Chemical Physical and Biological Effects, VCH, Weinheim, Germany, 1988. [27] S. Koda, T. Kimura, T. Kondo, H. Mitome, Ultrason. Sonochem. 10 (2003) 149– 156. [28] M. Salavati-Niasari, G. Hosseinzadeh, F. Davar, J. Alloys Compd. 509 (2011) 134–140. [29] M. Salavati-Niasari, J. Javidi, F. Davar, A. Amini-Fazl, J. Alloys Compd. 503 (2010) 500–506. [30] K.S. Suslick, S.B. Choe, A.A. Cichowlas, M.W. Grinstaff, Nature 353 (1991) 414– 416. [31] K.S. Suslick, G.J. Price, J. Annu, Rev. Mater. Sci. 29 (1999) 295–326. [32] A.R. Tao, S. Habas, P.D. Yang, Small 4 (2008) 310–325. [33] T.K. Sau, A.L. Rogach, F. Jäckel, T.A. Klar, J. Feldmann, Adv. Mater. 22 (2010) 1805–1825. [34] M. Mousavi-Kamazani, M. Salavati-Niasari, H. Emadi, Mater. Res. Bull. 47 (2012) 3983–3990. [35] N. Mir, M. Salavati-Niasari, Mater. Res. Bull. 48 (2013) 1660–1667. [36] U. Lee, J. Choi, N. Myung, I.H. Kim, C.R. Nair-Chenthamarakshan, N.R.D. Tacconi, K. Rajeshwar, Bull. Korean Chem. Soc. 29 (2008) 689–692. [37] M. Esmaeili-Zare, M. Salavati-Niasari, A. Sobhani, Ultrason. Sonochem. 19 (2012) 1079–1086. [38] F. Tavakoli, M. Salavati-Niasari, F. Mohandes, Ultrason. Sonochem. 21 (2014) 234–241. [39] G. Kianpour, M. Salavati-Niasari, H. Emadi, Ultrason. Sonochem. 20 (2013) 418–424. [40] M. Mousavi-Kamazani, M. Salavati-Niasari, H. Emadi, Micro. Nano Lett. 7 (2012) 896–900. [41] K.S. Suslick, J.J. Gawienowski, P.F. Schubert, H.H. Wang, Ultrasonics 22 (1984) 33–36. [42] H. Nakui, K. Okitsu, Y. Maeda, R. Nishimura, Ultrason. Sonochem. 14 (2007) 627–632. [43] Y. Wu, Y. Ni, Chem. Eng. J. 187 (2012) 328–333.


Effect of pH on the sonochemical synthesis of BiPO4 nanostructures and its electrochemical properties for pseudocapacitors.

Eu-doped ZnO nanoparticles: Sonochemical synthesis, characterization, and sonocatalytic application.

Sonochemical synthesis and characterization of new one-dimensional manganese(II) coordination polymer nanostructures.

Synthesis of dendritic silver nanostructures supported by graphene nanosheets and its application for highly sensitive detection of diazepam.

A novel sonochemical synthesis of antlerite nanorods.

Water-dispersible nanospheres of hydrogen-bonded supramolecular polymers and their application for mimicking light-harvesting systems.

Sonochemical synthesis of a new nano-sized cerium(III) coordination polymer and its conversion to nanoceria.

Aqueous based reflux method for green synthesis of nanostructures: Application in CZTS synthesis.

Doped ZnO 1D nanostructures: synthesis, properties, and photodetector application.

Ambient Facile Synthesis of Gram-Scale Copper Selenide Nanostructures from Commercial Copper and Selenium Powder.

Ultrafast sonochemical synthesis of protein-inorganic nanoflowers.

Sonochemical synthesis of LiNi0.5Mn1.5O4 and its electrochemical performance as a cathode material for 5 V Li-ion batteries.

Synthesis and characterization of new light emitter symmetrical phenoxazinium salt and its potential application as sensor for assessment of Hg2+.

Fabrication of parabolic Si nanostructures by nanosphere lithography and its application for solar cells.

Synthesis of silver nanoparticle and its application.

Biogenesis of light harvesting proteins.

A simple one-step PCR walking method and its application of bacterial rRNA for sequencing identification.

Aluminum Nanoarrays for Plasmon-Enhanced Light Harvesting.

Natural strategies for photosynthetic light harvesting.

Panchromatic absorbers for solar light-harvesting.

b protein complex. An assessment of its manganese content.

A novel approach to low-temperature synthesis of cubic HfO2 nanostructures and their cytotoxicity.

Photodynamic therapy for glioblastoma: A preliminary approach for practical application of light propagation models.

Complex Photonic Structures for Light Harvesting.