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Materials Science in Semiconductor Processing

Materials Science in Semiconductor Processing 27 (2014) 267–272

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Properties of mechanochemically synthesized nanocrystallineBi2S3 particles

Erika Dutková a,n, María J. Sayagués b, Anna Zorkovská a, Concepcion Real b,Peter Baláž a, Alexander Šatka c, Jaroslav Kováč c

a Institute of Geotechnics, Slovak Academy of Sciences, Watsonova 45, 040 00 Košice, Slovakiab Institute of Material Science of Seville (CSIC-US), 410 92 Sevilla, Spainc Institute of Electronics and Photonics, Slovak University of Technology, Ilkovičova 3, 812 19 Bratislava, Slovakia

a r t i c l e i n f o

Keywords:Bismuth sulphideMechanochemical synthesisSemiconductor

x.doi.org/10.1016/j.mssp.2014.05.05701/& 2014 Elsevier Ltd. All rights reserved.

esponding author.ail address: dutkova@saske.sk (E. Dutková).

a b s t r a c t

Nanocrystalline Bi2S3 particles have been synthesized from Bi and S powders by high-energy milling in a planetary mill. Structural and microstructural characterization of theprepared particles, including phase identification, specific surface area measurement andparticle size analysis has been carried out. The optical properties were measured byspectroscopic methods and the structural stability up to 500 1C was studied by thermalanalysis. The production of orthorhombic Bi2S3 with crystallite size of about 26 nm wasconfirmed by X-ray diffraction. The nanocrystals tend to agglomerate due to their largespecific surface area. Accordingly, the average hydrodynamic diameter of the mechan-ochemically synthesized particles is 198 nm. EDS analysis shows that the synthesizedmaterial is pure Bi2S3. The band gap of the Bi2S3 nanoparticles is 4.5 eV which is widerthan that in bulk materials. The nanoparticles exhibit good luminescent properties with apeak centered at 490 and 390 nm. Differential scanning calorimetry curves exhibit a broadexothermic peak between 200 and 300 1C, suggesting recovery processes. This interpreta-tion is supported by X-ray diffraction measurements that indicate a 10-fold increase of thecrystallite size to about 230 nm. The controlled mechanochemical synthesis of Bi2S3nanoparticles at ambient temperature and atmospheric pressure remains a great chal-lenge.

& 2014 Elsevier Ltd. All rights reserved.

1. Introduction

The synthesis of one-dimensional (1D) nanostructuredmetal chalcogenides has been the focus of attentionbecause of their important physical and chemical proper-ties, as well as their applications in semiconductors,pigments, luminescence devices, solar cells, IR detectors,and thermoelectric devices.

Bismuth sulphide, Bi2S3 is known to be attractivematerial for photoelectrochemical applications as it has areasonably narrow band gap (Eg¼1.3 eV) and a reasonableincident photon to electron conversion efficiency. Bi2S3 isalso a promising semiconductor material for applicationsin photovoltaic cells and thermoelectric cooling technolo-gies because of its environmental compatibility.

Conventionally, Bi2S3 is prepared by methods such asdirect reaction of bulk bismuth with sulphur vapour in aquartz vessel [1] and thermal decomposition of variousprecursors [2]. In these methods, high temperature isrequired and the final products always contain someimpurities [3]. In the recent years, Bi2S3 nanowires,

E. Dutková et al. / Materials Science in Semiconductor Processing 27 (2014) 267–272268

nanotubes, nanoflowers and nanorods have been synthe-sized by several groups. Methods such as solvothermal,sonochemical, and hydrothermal processes, microwaveirradiation, photochemical synthesis, and electro-deposition have been reported for fabricating Bi2S3 nanos-tructures [4–9].

High-energy milling has been used to synthesize var-ious nanocrystalline chalcogenides [10–15]. We havealready published the paper dealing with the synthesisand kinetics of mechanochemical synthesis of Bi2S3 nano-particles by high-energy milling [16]. However, this paperis focused on the study of structural, surface, optical andthermal properties of mechanochemically synthesizedBi2S3 particles from the point of view of the possibleapplication of the such as-synthesized Bi2S3. The con-trolled mechanochemical synthesis of nanocrystallineBi2S3 at ambient temperature in the absence of a solventis still a great challenge.

2. Experimental

Mechanochemical synthesis of Bi2S3 was performed ina planetary ball mill (Pulverisette 6, Fritsch, Germany)starting from powders of bismuth (99.5%, Aldrich,Germany) and sulphur (99%, Ites, Slovakia) in an argonatmosphere, according to the reaction

2Biþ3S-Bi2S3 ð1Þ

The reaction is thermodynamically possible, as theenthalpy change is negative, � ΔH2981¼ 176:6 kJ=mol[17].

The milling was performed in a 250-mL tungstencarbide milling chamber with 50 tungsten carbide balls,10 mm in diameter. The rotational speed of the planetcarrier was 500 rev/min. The powder charge was 4.06 g Biand 0.94 g S, corresponding to the stoichiometry of Eq. (1).The milling time was 60 min and the mill was operated atroom temperature.

X-ray diffraction (XRD) measurements were carried outusing a D8 Advance diffractometer (Bruker, Germany)equipped with a θ/θ goniometer, Cu Kα radiation (40 kV,40 mA), secondary graphite monochromator and scintilla-tion detector. For the data treatment and analysis thecommercial Bruker processing tools have been used. Con-cretely, for the phase identification the Diffrac plus Eva andfor the Rietveld analysis and microstructure characteriza-tion the Diffrac plus Topas software have been utilized. Thecrystalline size was estimated by “double-Voight” method,using the integral breadth, since this characteristic is themost comparable to values observed by TEM.

The specific surface area was determined by the lowtemperature nitrogen adsorption method in a Gemini2360 sorption apparatus (Micromeritics, USA).

Particle size analysis was carried out employing aNanophox particle sizer (Sympatec, Germany) using thephoton cross correlation spectroscopy method. The lightsource is a build-in He–Ne laser with a maximum outputof 10 mW at the wavelength of λ¼0.6328 μm. The samplewas dispersed in water, ultrasonically de-agglomeratedafter sedimentation and the spectrum from the fine

particles present in the liquid was taken. The measuredresults were processed with the Windox 5 software.

The morphology, microstructure and composition ofthe sample were analyzed using LEO 1550 field emissionscanning electron microscope (Zeiss, Germany) and TEC-NAI G2 F30 STEM FEG, field emission scanning-transmission electron microscope, operating at 300 kV,coupled with an energy dispersive spectrometer EDS INCAdetector spectrometer. The sample was sufficiently con-ductive, thus it is not coated with any conductive materialin order to avoid charging artefacts.

A small quantity of the powder sample was suspendedin acetone and droplets of the suspension were depositedon carbon-coated copper grids for the SEM and TEM(HRTEM) analysis.

Optical studies were carried out using UV–vis spectro-photometer Helios Gamma (Thermo Electron Corporation,Great Britain) in quartz cell by dispersing of synthesizedparticles in absolute ethanol by ultrasonic stirring.

The photoluminiscence (PL) spectra at room tempera-ture were acquired at right angle on a photon count-ing spectrofluorometer PC1 (ISS, USA) in the range 370–500 nmwith an excitation wavelength of 325 nm. A 300 Wxenon lamp was used as the excitation source. The emis-sion is collected in a 25 cm monochromator with resolu-tion of 0.1 nm equipped with a photomultiplier. Formeasuring the PL intensity, the nanopowders were dis-persed in absolute ethanol.

Calorimetric measurements were carried out in aSETARAM differential scanning calorimeter DSC-111 witha sensitivity of 1 mV/s. Samples of 6 to 8 mg were used andsubsequently heated under a nitrogen atmosphere fromambient temperature to 500 1C, at a rate of 10 1C/min. Anempty crucible was used as a reference sample.

3. Results and discussion

The progress of the mechanochemical synthesis ofBi2S3 is illustrated by XRD patterns of the mixture of BiþSprecursors (a) and sample taken after 60 min of milling (b)(Fig. 1). In the starting material (pattern a) only peaksbelonging to Bi metal (JCPDS 77-7112) and S (JCPDS 78-1888) are seen. The mechanosynthesized Bi2S3 (bismuthi-nite) has orthorhombic structure (space group 62, Pnma),with refined lattice parameters a¼11.3008 Å, b¼3.9855 Å,c¼11.1439 Å. The estimated average crystallite size isD¼26 nm. The process is rather straightforward, withBi2S3 (JCPDS-74-9437) being the only solid product (pat-tern b). The kinetics of the mechanochemical synthesiswas studied in paper [16].

The specific surface area of the mechanochemicallysynthesized Bi2S3 nanoparticles is 1.7 m2/g. This value isabout 30 times higher than the corresponding value of thestarting BiþS mixture (SA¼0.06 m2/g). Particles in thenanometer size range have a strong tendency to agglom-erate due to their relatively large specific surface area.

The particle size distribution of the nanocrystallineBi2S3 particles was characterized by photon cross correla-tion spectroscopy as shown in Fig. 2. The size of theparticles is rather uniform with an average hydrodynamic

0

250

500

750

1000

1250

1500

1750

2000

10 20 30 40 50 60 70

10 20 30 40 50 60 700

250

500

750

1000

1250

1500

1750

2000

Bi2S3

Milled for 60 min

Precursor mixture

Bi S

2 theta (degrees)

Inte

nsity

(a.u

.)

Fig. 1. XRD patterns documenting the mechanosynthesis of Bi2S3 nano-particles. (a) Mixture of Bi and S precursors before milling, (b) synthe-sized sample after 60 min of milling.

1000100

0

1

2

3

Dis

trib

utio

n de

nsity

Particle size (nm)

Fig. 2. Particle size distribution of the mechanochemically synthesizedBi2S3 (milling time 60 min).

Fig. 3. SEM micrograph of the mechanochemically synthesized Bi2S3(milling time 60 min).

E. Dutková et al. / Materials Science in Semiconductor Processing 27 (2014) 267–272 269

diameter of 198 nm. The sized dispersion has relativelynarrow, monomodal distribution profile.

The surface morphology of the mechanochemicallysynthesized Bi2S3 was investigated by SEM (Fig. 3). Bi2S3aggregated particles with irregular morphology wereobtained.

The microstructure and composition of the Bi2S3 sam-ple (after milling 60 min) was further studied using ED,TEM, HRTEM and EDS techniques and the obtained resultsare depicted in Fig. 4. The TEM image (Fig. 4a) showsagglomerated crystals and the corresponding ED pattern(inset) is formed by rings, indicating the presence of smalldiffraction domains; all the rings could be indexed in theorthorhombic system of the Bi2S3 sample with Pnma spacegroup (marked in the figure). The nanocrystalline char-acter of the sample was also corroborated in the HRTEMmicrographs (Fig. 4c and d), where nanocrystallinedomains oriented in different directions were clearlyobserved and some of the interplanar distances weremeasured and marked in the figure. The size of nanocrys-tallites shows an average between 5 and 10 nm. Theenergy-dispersive X-ray spectrum (EDS) (Fig. 4b) showsthat the synthesized material is pure bismuth sulphide(Bi¼40 at% and S¼60 at%) with no detectable impurities,the Cu and C peaks are coming from the grid. This result isconsistent with the XRD pattern presented above, thatcorresponds to single-phase Bi2S3. It is worth noting thatthe particle size of about 198 nm found with the photoncross correlation spectroscopy is large because such parti-cles are formed by clusters of crystalline nanodomains(5–10 nm) of bismuth sulphide.

Fig. 4. (a) TEM image and the corresponding ED pattern; (b) EDS spectrum and ((c) and (d)) HRTEM images of the mechanochemically synthesized Bi2S3(milling time 60 min).

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The absorption UV–vis spectrum of the Bi2S3 mechan-ochemically synthesized during 60 min (Fig. 5) shows aweak absorption peak at E280 nm. The band gap isdetermined from intercept of the extrapolated linear fitto the experimental data of the Taue plot. A bandgap of4.5 eV was estimated by extrapolating the linear part ofthe graph as shown in Fig. 5 (inset). It is quite similar tothat synthesized by other procedures [18,19]. The observedpeaks are blue shifted relative to the bulk Bi2S3 at 956 nm(1.3 eV) [20] and are assigned to the optical transitionsof the excitonic states of Bi2S3. The obvious blue shiftcould be attributed to the existence of very small Bi2S3nanocrystalline particles agglomerated into the largeclusters.

Room temperature PL spectrum of the mechanochemi-cally synthesized nanocrystallites is shown in Fig. 6. Withthe excited wavelength at 325 nm, the corresponding

emission peaks at E490 nm (2.51 eV) and E390 nm(3.18 eV)) could be observed for the Bi2S3 nanoparticles.It can be ascribed to a high level transition in Bi2S3semiconductor nanocrystallites. This kind of band edgeluminescence can arise from the recombination of excitonsand/or trapped electron–hole pairs [21], whereas PL back-ground could be assigned to the surface-assisted radiativerecombinations in Bi2S3 nanocrystallites of varying size.

In order to obtain information about the thermalstability of the mechanochemically synthesized Bi2S3nanoparticles, DSC analysis combined with XRD wascarried out with the aim to detect the products formedeventually after thermal treatment.

Fig. 7 shows the DSC diagram obtained for the Bi2S3sample after 60 min of milling and Fig. 8 shows the XRDpatterns of the same sample before (Fig. 8A) and after(Fig. 8B) heating to 500 1C during the DSC measurement.

200 300 400 500 600 700 800 900 10000.00

0.05

0.10

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0.25

0.30

0.35

0.40

0.45

Wavelength (nm)

Abs

orba

nce

(a.u

.)

Fig. 5. UV–vis spectrum of the mechanochemically synthesized Bi2S3.Inset: The dependence of (αhv)2 versus hv.

400 450 500 550 6000

50000

100000

150000

200000390 nm

Emission wavelength (nm)

PL In

tens

ity (a

.u.)

λEXC=325 nm

490 nm

Fig. 6. PL spectrum of the mechanochemically synthesized Bi2S3 (millingtime 60 min).

0 100 200 300 400 500-1.0

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249°C

Hea

t Flo

w (m

V)

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100°C

Fig. 7. DSC diagram of Bi2S3 after 60 min of milling.

20 30 40 50 60 70 800

1000

2000

3000

4000

Inte

nsity

(cou

nts)

2 theta (degrees)

0

1000

2000

Fig. 8. XRD patterns of Bi2S3 after 60 min of milling before DSC (A) andafter DSC (B).

E. Dutková et al. / Materials Science in Semiconductor Processing 27 (2014) 267–272 271

The calorimeter experiment shows an endothermic peakat about 100 1C due to the loss of adsorbed water and abroad exothermic peak between 200 and 300 1C (249 1C)that can be related to the processes of recovery and re-crystallization of the defects generated during milling.Successive runs (not shown in the figure) reveal neither

any exothermic peak nor any weight loss. The XRD patterntaken before the DSC experiment (Fig. 8A) shows only thebroadened peaks of the product compound. The XRDpattern recorded after the DSC run (Fig. 8B) shows thewell-developed sharp peaks corresponding to the Bi2S3compound with enhanced crystallinity and also traces of Bimetal (at 2Θ¼27.1651, 37.9341 and 39.6191.) The crystallitesize increased about 10-fold to about 230 nm after DSC,supporting the interpretation of the DSC experiments.

4. Conclusion

The preparation of the nanocrystalline Bi2S3 particleswas successfully attempted using high-energy ball millingof elemental Bi–S powder mixtures during 60 min.A single-phase product formed in a one-step combinationreaction, providing powders of 26-nm crystallites thatform submicron agglomerates. Heating to 500 1C resultedin a 10-fold increase of the grain size. The nanocrystallineBi2S3 particles possess blue–green properties under ultra-violet light excitation. The PL properties suggest that Bi2S3is a reasonably functional material, which opens up aprospect of its application in the future semiconductortechnology. Although mechanochemical synthesis is apromising option for the preparation of Bi2S3 nanocrystal-line particles, further investigations are needed to achievecontrol of the particle size and agglomeration. In thefuture, it may be of interest to consolidate these powdersinto solid pellets and study their electrical conductivityand thermoelectric properties.

Acknowledgements

The support through the Slovak Grant Agency VEGA(project 2/0027/14 and 1/0921/13) and the Slovak Researchand Developing Agency APVV (project VV-0189-10) is grate-fully acknowledged. The authors also acknowledge the sup-port of the European Regional Development Fund- projectsNANOCEXMAT 1 (ITMS 26220120019) and NANOCEXMAT 2(ITMS 26220120035).

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