# An Organic Acid-induced Synthesis and Characterization of Selenium

Authors: Charu Dwivedi; Chetan P. Shah; Krishankant Singh; Manmohan Kumar; Parma Nand Bajaj
Journal: Journal of Nanotechnology
 (2011)

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## Abstract

A simple wet chemical method has been developed to synthesize selenium nanoparticles (size 40–100 nm), by the reaction of sodium selenosulphate precursor with different organic acids in aqueous medium, under ambient conditions. Polyvinyl alcohol has been used to stabilize the selenium nanoparticles. The synthesized nanoparticles can be separated from its sol by using a high-speed centrifuge and can be redispersed in aqueous medium with a sonicator. UV-visible optical absorption spectroscopy, X-ray diffraction, energy dispersive X-rays, differential scanning calorimetry, atomic force microscopy, and transmission electron microscopy techniques have been employed to characterize the synthesized selenium nanoparticles.


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## Body

## 1. Introduction

The research on the synthesis and characterization of nanomaterials has been
stimulated by their technological applications. The first few technological uses of
these materials were as catalysts and pigments. Now, much of the hype surrounding
these materials revolves around the enhanced electrical, mechanical, and optical
properties. In this nanosize regime, the particles possess short-range structures
that are essentially the same as that of the bulk, yet they have optical and/or
electronic properties which are dramatically different from that of the bulk [1–3]. These represent a relatively new class of materials
and have come under intensive investigations because of their quantum size effects,
large surface-to-volume ratio, and the large difference in the properties of their
surface atoms. An extensive reported literature exists on the synthesis and
applications of nanoparticles of both metals, such as silver, gold, and platinum,
and semiconductors, such as CdSe, ZnSe, TiO2. However, research on
metalloid, like selenium, is scanty. Selenium is used in rectifiers, solar cells,
photographic exposure meters, and xerography [4]. It
is also used in the glass industry to eliminate bubbles and remove undesirable tints
produced by iron. Linear and nonlinear optical properties of selenium and its
reactivity towards Cd, Zn, make this an important field of study. The research on
selenium has attracted more and more attention not only because of its technological
applications, but also due to its novel role in life sciences. Selenium is also an
essential trace element in human body and has great importance in nourishment and
medicine [5]. It exists in a number of crystalline
structures, the principal ones being trigonal, consisting of helical chains, and the
less stable monoclinic form, consisting of Se8 rings [6]. Monoclinic Se (m-Se) comes in three forms,
α, β, and γ, which differ only in the way the rings are
packed [6, 7].
Amorphous selenium (a-Se) is composed of a mixture of disordered chains. Various
forms of nanoselenium can be produced, using different synthetic methods [5, 8–12]. Reduction
method is the most popular method for selenium nanoparticle preparation, which
includes chemical reduction [13],
γ-radiolytic reduction [14], bacterial
reduction [15], and so forth. However, some
literature on the formation of nanoselenium via oxidation method, such as reaction
of selenourea with hydroxyl radical [16],
electrochemical oxidation of selenide [17], and
reaction of sodium selenosulphate with acrylonitrile [18], also exists.Here, we have developed a new simple wet chemical method, employing protic acids,
such as acetic acid, oxalic acid, and gallic acid to synthesize polyvinyl
alcohol-stabilized selenium nanoparticles from aqueous sodium selenosulphate as
selenium precursor. The method is capable of producing spherical selenium
nanoparticles of size 35 to 70 nm, under ambient conditions. The synthesized
selenium nanoparticles were characterized by UV-visible optical absorption
spectroscopy, X-ray diffraction (XRD), energy dispersive X-rays (EDAX), differential
scanning calorimetry (DSC), atomic force microscopy (AFM), and transmission electron
microscopy (TEM) techniques.


## 2. Experimental Details

High-purity polyvinyl alcohol (PVA) of molecular weight 1,25,000 was obtained from
S.D.  fine chemicals Ltd., Mumbai, India. All the other chemicals used were of
GR grade, procured from local market. Selenium powder was purchased from Aldrich.
Aqueous solutions were prepared, using water obtained from Millipore-Q water
purification system (with conductivity of 0.6μScm−1, or less).Sodium selenosulphate (Na2SeSO3) solution was prepared by
refluxing a mixture of selenium (2 g) and Na2SO3
(20 g) in 100 mL water at 70°C, for about 7 hours
[19]. This sodium selenosulphate
(Na2SeSO3) solution
(~0.25 mol dm−3), containing unreacted
Na2SO3, was used as a stock for Se precursor. An aqueous
PVA stock solution, 1% by weight, was prepared by dissolving 1.0 g of PVA in
100 mL of water, while stirring at 80°C. The above stock
solutions were diluted to the required concentrations for different experiments.PVA-stabilized Se nanoparticles were synthesized by reaction of sodium selenosulphate
(concentration1×10-4 to 1.0×10-3 mol dm−3) with different organic
carboxylic acids (5×10-3 to 3×10-2 mol dm−3) in aqueous medium, in the presence
of PVA as a stabilizer, in the concentration range from 0.01 to 0.1%. The formation
of orange-red-coloured selenium nanoparticle sol was observed in less than one
minute. However, in the absence of PVA stabilizer, a dark red-coloured precipitate
of selenium was formed.UV-visible optical absorption spectra of the selenium nanoparticle sols were
recorded, using a double beam spectrophotometer, model Spectroscan 2600 from
Chemito. XRD patterns of the nanoparticles were recorded with a Phillips X-ray
diffractometer, model PW 1710, using a CuKα source (λ=0.15406 nm). DSC measurements were carried out, using a Mettler TA 3000
thermal analysis system (model DSC-30). About 5–10 mg of the
synthesized selenium nanoparticles and standard selenium powder were weighed into
aluminum crucibles separately, and DSC measurements of both were carried out in
N2 atmosphere, at a heating rate of 10°C/min,
from 50° to 250°C. Selenium nanoparticles,
separated from aqueous sols, using a high-speed centrifuge, at about
15000 rpm, washed with water, and dried at room temperature, were used for
XRD and thermal analysis measurements. AFM analysis of the synthesized selenium
nanoparticles was carried out, using a Solver P47 model from NT-MDT, Russia. TEM
characterization was carried out with a TECNAI 20, FEI electron microscope, using
the sample on a copper grid, coated with a thin amorphous carbon film.


## 3. Results and Discussion

### 3.1. Optical Absorption Studies

The initial concentration of sodium selenosulphate determines the intensity of Se
nanoparticle sols. Typical picture of the selenium sols, produced from three
different initial concentrations of sodium selenosulphate, is shown in Figure1.Figure 1
Aqueous selenium nanoparticle sols obtained by the reaction of (A)2.0×10-4 (B) 5.0×10-4(C) 1.0×10-3 mol dm−3 sodium
selenosulphate with 2.0×10-2mol dm−3 acetic acid, in the presence
of 0.05% PVA.Figure2 shows the effect of sodium
selenosulphate and PVA concentrations on UV-visible absorption spectra of the
selenium nanoparticle sols synthesized by the reaction of sodium selenosulphate
with 2.0×10-2 mol dm−3 of different organic
acids. From Figure 2(A), it is clear that the
intensity of selenium nanoparticles increases with the increase in sodium
selenosulphate concentration. However, it shows a shoulder at around
240 nm, when the reaction was carried out with acetic acid and oxalic
acid. But it does not show any such kind of the behavior when the reaction was
carried out with an aromatic acid (i.e., gallic acid and many more). This may be
due to the overlapping of the absorption spectrum of aromatic system with the
shoulder of the selenium nanoparticles, at lower wavelengths
(<300 nm). Till now, none of the researchers have observed such
a shoulder for selenium nanoparticles. However, the absorbance at lower
wavelength region, increases significantly with acid concentration, only in the
case of oxalic acid (Figure 2(B)). Whereas, from
Figure 2(C), it is clear that, there is no
significant effect of PVA concentration, in the concentration range from 0.01 to
0.1%, on the UV-visible absorption spectra of selenium nanoparticles sols. PVA
was found to be a very efficient stabilizer for selenium nanoparticles, even at
low concentration of 0.01%.(A) Effect of sodium selenosulphate concentration on the UV-visible
absorption spectra of the selenium sols produced by the reaction of
sodium selenosulphate with5.0×10-3 mol dm−3 (a) acetic acid,
(b) oxalic acid, and (c) gallic acid, at a fixed concentration of PVA
(0.05%). (B) Effect of acid concentration, (a) acetic acid, (b) oxalic
acid, and (c) gallic acid, on the absorption spectra of the selenium
sols produced by reaction with 5.0×10-4 mol dm−3 sodium
selenosulphate, in the presence of 0.05% PVA. (C) Effect of PVA
concentration on absorption spectra of the selenium sol produced by the
reaction of 5.0×10-4 mol dm−3 sodium
selenosulphate with 5.0 × 10−3
mol dm−3 (a) acetic acid, (b) oxalic acid, and
(c) gallic acid.Further, the particle size was correlated with the nature of the UV-visible
spectra, and if the particle size is about 100 nm or more, it shows a
clear regular maxima in the visible region [20].
Comparison of the present spectra with those reported by Lin and
Wang [20] shows that the selenium
nanoparticles have an average size of about 80 nm, which is further
supported by AFM images.


### 3.2. X-Ray Diffraction Study and EDAX of the Synthesized Selenium
Nanoparticles

Crystal structure of the selenium nanoparticles was determined by XRD technique.
The typical XRD patterns of the synthesized sample, after heat treatment at
130°C, for 6 hrs, and that of the commercially
available selenium, are displayed in Figures3(a)
and 3(b), respectively. The XRD pattern of the
synthesized selenium nanoparticles, without heat treatment, is much more noisy,
with broader peaks (plot not shown), indicating amorphous nature of the
particles. It is believed that amorphous selenium is red in colour, which is
further confirmed by selenium sol presented in Figure 1. All the diffraction pattern peaks in Figure 3(a) correspond to trigonal phase, with lattice
constants a=4.362 Å and c=4.958 Å, which match very well with the reported values
(JCPDS file no. 06-362).(a) XRD pattern of the synthesized selenium nanoparticles annealed at
130°C, for 6 hrs, and (b) commercial selenium
sample. Selenium nanoparticles were synthesized by the reaction of1.0×10-3 mol dm−3 sodium
selenosulphate with 2.0×10-2 mol dm−3 acetic acid, in the presence
of 0.01% PVA.
(a)(b)Further, chemical composition of the synthesized selenium nanoparticles was also
confirmed by EDAX. The EDAX spectrum of the nanoparticles, shown in Figure4, also indicates that the nanoparticles are of
selenium only. The peak corresponding to copper and carbon arises due to
carbon-coated copper grid, on which thin film of the sample was deposited for
TEM analysis.Figure 4
EDAX pattern of the selenium nanoparticles, synthesized by the reaction of1.0×10-3 mol dm−3 sodium
selenosulphate with 2.0×10-2 mol dm−3 acetic acid, in the
presence of 0.01% PVA.


### 3.3. Differential Scanning Calorimetry (DSC) Study

DSC thermograms of the synthesized selenium nanoparticles and standard commercial
selenium sample were recorded from 50° to
250°C. DSC thermogram of the synthesized selenium
nanoparticles showed an exothermic transition at 90°C, along
with endothermic melting peak at 217°C (Figure5(a)). Enthalpy of the transition was found to be
45.2 J/g. The repeat DSC thermogram of the same selenium sample,
recorded after bringing it to ambient temperature, did not show any exothermic
peak at the mentioned temperature (plot not shown). This clearly indicates that
the selenium particles loose their nanocrystalline nature in the first thermal
run itself, and the transition could be assigned to increase in the
crystallinity of the selenium nanoparticles. This observation is in
corroboration with the XRD results obtained with the synthesized sample and that
annealed at 130°C. DSC thermogram of the standard selenium
powder sample also shows melting peak at 217°C, without any
such exothermic peak, which is shown in Figure 5(b).DSC thermogram of (a) the selenium nanoparticles synthesized by the reaction
of1.0×10-3mol dm−3 sodium selenosulphate with 2.0×10-2  mol dm−3 acetic acid, in the
presence of 0.01% PVA, and (b) standard commercial selenium sample.
(a)(b)


### 3.4. AFM and TEM Studies

AFM and TEM are very important techniques, which are used to get the information
about particle size, shape, surface topography, and so forth. Therefore,
morphology and structure of the synthesized selenium nanoparticles were also
determined by these techniques. Typical 2D and 3D AFM images of the synthesized
selenium nanoparticles are shown in Figure6.
The 2D image of the synthesized selenium nanoparticles shows smaller individual
particles of about 10 nm size, along with larger agglomerates of sizes
upto ~150 nm while the 3D image indicates the presence of
individual spherical particles, with maximum height of 30 nm in the
z-direction.AFM images of the selenium nanoparticles formed by the reaction of1.0×10-3 mol dm−3 sodium
selenosulphate with 2.0×10-2 mol dm−3 acetic acid, in the
presence of 0.01 % PVA, (a) 2D image and (b) 3D image.
(a)(b)Transmission electron microscope image of the synthesized selenium nanoparticle
is shown in Figure7. Spherical shape of
individual nanoparticles, with size in the range of 35–70 nm, is
evident from the TEM image. AFM image is taken by suspending much higher
concentration of the selenium nanoparticles on the silicon wafer in the presence
of the PVA stabilizer. It shows individual particles in the size range of
10–150 nm, along with some larger aggregates while the TEM image
is taken at much higher magnification, with a dilute sample, after washing out
the PVA stabilizer. Thus, it shows a specific portion of the sample deposited on
the graphite-coated copper grid, having well-separated individual particles of
size in the range of 35–70 nm. These differences in the two
sample preparation methods are responsible for the observed difference in the
particle sizes. The conclusions drawn from AFM and TEM studies are in
corroboration with each other. Thus, the present method is capable of producing
spherical selenium nanoparticles.Figure 7
TEM images of the selenium nanoparticles, formed by the reaction of1.0×10-3 mol dm−3 sodium
selenosulphate with 2.0×10-2 mol dm−3 acetic acid, in the
presence of 0.01% PVA.


## 3.1. Optical Absorption Studies

The initial concentration of sodium selenosulphate determines the intensity of Se
nanoparticle sols. Typical picture of the selenium sols, produced from three
different initial concentrations of sodium selenosulphate, is shown in Figure1.Figure 1
Aqueous selenium nanoparticle sols obtained by the reaction of (A)2.0×10-4 (B) 5.0×10-4(C) 1.0×10-3 mol dm−3 sodium
selenosulphate with 2.0×10-2mol dm−3 acetic acid, in the presence
of 0.05% PVA.Figure2 shows the effect of sodium
selenosulphate and PVA concentrations on UV-visible absorption spectra of the
selenium nanoparticle sols synthesized by the reaction of sodium selenosulphate
with 2.0×10-2 mol dm−3 of different organic
acids. From Figure 2(A), it is clear that the
intensity of selenium nanoparticles increases with the increase in sodium
selenosulphate concentration. However, it shows a shoulder at around
240 nm, when the reaction was carried out with acetic acid and oxalic
acid. But it does not show any such kind of the behavior when the reaction was
carried out with an aromatic acid (i.e., gallic acid and many more). This may be
due to the overlapping of the absorption spectrum of aromatic system with the
shoulder of the selenium nanoparticles, at lower wavelengths
(<300 nm). Till now, none of the researchers have observed such
a shoulder for selenium nanoparticles. However, the absorbance at lower
wavelength region, increases significantly with acid concentration, only in the
case of oxalic acid (Figure 2(B)). Whereas, from
Figure 2(C), it is clear that, there is no
significant effect of PVA concentration, in the concentration range from 0.01 to
0.1%, on the UV-visible absorption spectra of selenium nanoparticles sols. PVA
was found to be a very efficient stabilizer for selenium nanoparticles, even at
low concentration of 0.01%.(A) Effect of sodium selenosulphate concentration on the UV-visible
absorption spectra of the selenium sols produced by the reaction of
sodium selenosulphate with5.0×10-3 mol dm−3 (a) acetic acid,
(b) oxalic acid, and (c) gallic acid, at a fixed concentration of PVA
(0.05%). (B) Effect of acid concentration, (a) acetic acid, (b) oxalic
acid, and (c) gallic acid, on the absorption spectra of the selenium
sols produced by reaction with 5.0×10-4 mol dm−3 sodium
selenosulphate, in the presence of 0.05% PVA. (C) Effect of PVA
concentration on absorption spectra of the selenium sol produced by the
reaction of 5.0×10-4 mol dm−3 sodium
selenosulphate with 5.0 × 10−3
mol dm−3 (a) acetic acid, (b) oxalic acid, and
(c) gallic acid.Further, the particle size was correlated with the nature of the UV-visible
spectra, and if the particle size is about 100 nm or more, it shows a
clear regular maxima in the visible region [20].
Comparison of the present spectra with those reported by Lin and
Wang [20] shows that the selenium
nanoparticles have an average size of about 80 nm, which is further
supported by AFM images.


## 3.2. X-Ray Diffraction Study and EDAX of the Synthesized Selenium
Nanoparticles

Crystal structure of the selenium nanoparticles was determined by XRD technique.
The typical XRD patterns of the synthesized sample, after heat treatment at
130°C, for 6 hrs, and that of the commercially
available selenium, are displayed in Figures3(a)
and 3(b), respectively. The XRD pattern of the
synthesized selenium nanoparticles, without heat treatment, is much more noisy,
with broader peaks (plot not shown), indicating amorphous nature of the
particles. It is believed that amorphous selenium is red in colour, which is
further confirmed by selenium sol presented in Figure 1. All the diffraction pattern peaks in Figure 3(a) correspond to trigonal phase, with lattice
constants a=4.362 Å and c=4.958 Å, which match very well with the reported values
(JCPDS file no. 06-362).(a) XRD pattern of the synthesized selenium nanoparticles annealed at
130°C, for 6 hrs, and (b) commercial selenium
sample. Selenium nanoparticles were synthesized by the reaction of1.0×10-3 mol dm−3 sodium
selenosulphate with 2.0×10-2 mol dm−3 acetic acid, in the presence
of 0.01% PVA.
(a)(b)Further, chemical composition of the synthesized selenium nanoparticles was also
confirmed by EDAX. The EDAX spectrum of the nanoparticles, shown in Figure4, also indicates that the nanoparticles are of
selenium only. The peak corresponding to copper and carbon arises due to
carbon-coated copper grid, on which thin film of the sample was deposited for
TEM analysis.Figure 4
EDAX pattern of the selenium nanoparticles, synthesized by the reaction of1.0×10-3 mol dm−3 sodium
selenosulphate with 2.0×10-2 mol dm−3 acetic acid, in the
presence of 0.01% PVA.


## 3.3. Differential Scanning Calorimetry (DSC) Study

DSC thermograms of the synthesized selenium nanoparticles and standard commercial
selenium sample were recorded from 50° to
250°C. DSC thermogram of the synthesized selenium
nanoparticles showed an exothermic transition at 90°C, along
with endothermic melting peak at 217°C (Figure5(a)). Enthalpy of the transition was found to be
45.2 J/g. The repeat DSC thermogram of the same selenium sample,
recorded after bringing it to ambient temperature, did not show any exothermic
peak at the mentioned temperature (plot not shown). This clearly indicates that
the selenium particles loose their nanocrystalline nature in the first thermal
run itself, and the transition could be assigned to increase in the
crystallinity of the selenium nanoparticles. This observation is in
corroboration with the XRD results obtained with the synthesized sample and that
annealed at 130°C. DSC thermogram of the standard selenium
powder sample also shows melting peak at 217°C, without any
such exothermic peak, which is shown in Figure 5(b).DSC thermogram of (a) the selenium nanoparticles synthesized by the reaction
of1.0×10-3mol dm−3 sodium selenosulphate with 2.0×10-2  mol dm−3 acetic acid, in the
presence of 0.01% PVA, and (b) standard commercial selenium sample.
(a)(b)


## 3.4. AFM and TEM Studies

AFM and TEM are very important techniques, which are used to get the information
about particle size, shape, surface topography, and so forth. Therefore,
morphology and structure of the synthesized selenium nanoparticles were also
determined by these techniques. Typical 2D and 3D AFM images of the synthesized
selenium nanoparticles are shown in Figure6.
The 2D image of the synthesized selenium nanoparticles shows smaller individual
particles of about 10 nm size, along with larger agglomerates of sizes
upto ~150 nm while the 3D image indicates the presence of
individual spherical particles, with maximum height of 30 nm in the
z-direction.AFM images of the selenium nanoparticles formed by the reaction of1.0×10-3 mol dm−3 sodium
selenosulphate with 2.0×10-2 mol dm−3 acetic acid, in the
presence of 0.01 % PVA, (a) 2D image and (b) 3D image.
(a)(b)Transmission electron microscope image of the synthesized selenium nanoparticle
is shown in Figure7. Spherical shape of
individual nanoparticles, with size in the range of 35–70 nm, is
evident from the TEM image. AFM image is taken by suspending much higher
concentration of the selenium nanoparticles on the silicon wafer in the presence
of the PVA stabilizer. It shows individual particles in the size range of
10–150 nm, along with some larger aggregates while the TEM image
is taken at much higher magnification, with a dilute sample, after washing out
the PVA stabilizer. Thus, it shows a specific portion of the sample deposited on
the graphite-coated copper grid, having well-separated individual particles of
size in the range of 35–70 nm. These differences in the two
sample preparation methods are responsible for the observed difference in the
particle sizes. The conclusions drawn from AFM and TEM studies are in
corroboration with each other. Thus, the present method is capable of producing
spherical selenium nanoparticles.Figure 7
TEM images of the selenium nanoparticles, formed by the reaction of1.0×10-3 mol dm−3 sodium
selenosulphate with 2.0×10-2 mol dm−3 acetic acid, in the
presence of 0.01% PVA.


## 4. Conclusions

Acid-induced synthesis of selenium nanoparticles has been found to be a simple and
convenient method, which can be carried out under ambient conditions. PVA was used
as a stabilizer for the selenium nanoparticles. The size of the selenium particles
was found to increase with sodium selenosulphate concentration. The effects of
higher concentration of PVA and organic acids were not that pronounced. Nanonature
of the synthesized selenium particles and increase in their crystallinity on heating
were confirmed by both XRD and DSC experiments. Spherical selenium nanoparticles of
size about 35–70 nm, as determined by AFM and TEM techniques, could
be produced. The selenium nanoparticles may serve as template, to generate other
important nanomaterials, and find applications in fabrication of nanoscale
optoelectronic devices.



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