Chemical Engineering Journal 215–216 (2013) 151–156

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Chemical Engineering Journal

journal homepage: www.elsevier .com/locate /cej

Structure and optical properties of Bi2S3 and Bi2O3 nanostructures synthesizedvia thermal evaporation and thermal oxidation routes

Hyunsu Kim a, Changhyun Jin a, Sunghoon Park a, Wan In Lee b, In-Joo Chin c, Chongmu Lee a,⇑a Department of Materials Science and Engineering, Inha University, 253 Yonghyun-dong, Nam-gu, Incheon 402-751, Republic of Koreab Department of Chemistry, Inha University, 253 Yonghyun-dong, Nam-gu, Incheon 402-751, Republic of Koreac Department of Polymer Science and Engineering, Inha University, 253 Yonghyun-dong, Nam-gu, Incheon 402-751, Republic of Korea

h i g h l i g h t s

" Bi2S3 nanorods were synthesized by thermal evaporation." Bi2O3 nanostructures were synthesized by thermal oxidation of the Bi2S3 nanorods." The structural evolution of the two nanomaterials was examined." The luminescence properties of the two nanomaterials were examined." The origins of the emissions are also discussed.

a r t i c l e i n f o

Article history:Received 18 August 2012Received in revised form 4 October 2012Accepted 16 October 2012Available online 10 November 2012

Keywords:Bi2S3 nanorodsBi2O3 nanostructuresThermal evaporationThermal oxidationPhotoluminescence

1385-8947/$ - see front matter Crown Copyright � 2http://dx.doi.org/10.1016/j.cej.2012.10.102

⇑ Corresponding author. Tel.: +82 32 860 7536; faxE-mail address: cmlee@inha.ac.kr (C. Lee).

a b s t r a c t

Bi2S3 nanorods with diameters of a few tens of nanometers and lengths of a few to a few tens of microm-eters were synthesized by thermal evaporation. Subsequently, Bi2O3 nanostructures were synthesized bythermal oxidation of the Bi2S3 nanorods. The structural evolution of the two nanomaterials and theirluminescence properties were examined by scanning electron microscopy, transmission electron micros-copy, X-ray diffraction, energy-dispersive X-ray spectrometry, and photoluminescence (PL) spectroscopy.The PL measurements revealed the as-synthesized Bi2S3 nanorods to have an emission band centered at�595 nm. The Bi2O3 nanorods synthesized by the thermal oxidation of Bi2S3 nanorods for 1 h showed anemission band centered at �440 nm, which is in the bluish violet region. In contrast, the Bi2O3 nanostruc-tures synthesized by thermal oxidation of Bi2S3 nanorods for 2 or 3 h showed an emission band centeredat �505 nm, in the bluish green region, and a shoulder at �380 nm. The origins of the emissions are alsodiscussed.

Crown Copyright � 2012 Published by Elsevier B.V. All rights reserved.

1. Introduction

Bismuth sulfide (Bi2S3) is a semiconducting material witha direct bandgap in the range 1.3–1.7 eV with many potentialapplications including photovoltaics [1], thermoelectrics [2], X-ray computed tomography [3], and electrochemical hydrogen stor-age[4,5]. Over the past decade, many studies on one-dimensional(1D) Bi2S3 nanostructures have been reported. A range of 1DBi2S3 nanostructures such as nanorods [6–10], nanowires[11–13], nanoribbons [14,15], nanocables [16,17], and nanotubes[18,19] have been synthesized using various methods: Ye et al.synthesized Bi2S3 nanotubes using an evaporation technique[20]; Liu et al. synthesized Bi2S3 nanowires using a simple inor-ganic-surfactant-assisted solvothermal process [21]; Flower-like

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: +82 32 862 5546.

shapes, snow flake-like structures, and nanowires have been syn-thesized via a biomolecule-assisted route [4,22]; Zhang et al.adopted a thioglycolic acid-assisted hydrothermal technique forthe synthesis of flower-like Bi2S3 nanostructures [23]; 1D Bi2S3

nanostructures were also synthesized using an anodic aluminamembrane template [18].

On the other hand, bismuth oxide (Bi2O3) has been the focus ofscientific research owing to its unique chemical and physical prop-erties such as high-energy bandgap (2.85 and 2.58 eV for mono-clinic and tetragonal Bi2O3 phases, respectively), high refractiveindex, high dielectric permittivity, and excellent photoconductivity[18,24,25]. These versatile properties make Bi2O3 an attractive can-didate material for optical coatings, microelectronics, ceramicglasses, and gas sensors [26–30]. They are also used in the soft oxi-dation of hydrocarbons and are good electrolyte materials forapplications such as solid oxide fuel cells [31,32]. 1D nanostruc-tures of Bi2O3 have been synthesized using a range of techniques

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Fig. 1. XRD patterns of Bi2S3 and Bi2O3 nanostructures.

152 H. Kim et al. / Chemical Engineering Journal 215–216 (2013) 151–156

including hydrothermal synthesis [33], microemulsion [34], metal-organic chemical vapor deposition[35], atmospheric pressurechemical vapor deposition [36], the chemical method [37], oxida-

Fig. 2. (a) SEM image, (b) enlarged SEM image, and (c) EDX spectrum of the Bi2S3 nannanostructures (oxidation time = 1 h). (g) SEM image, (h) enlarged SEM image, and (i) E

tive metal vapor transport deposition [38,39], template-based heattreatment [40], and thermal oxidation [41]. To the best of theauthors’ knowledge, however, Bi2O3 nanostructures have not beensynthesized via thermal oxidation of Bi2S3 nanorods. Moreover,there are few reports on the structure of Bi2S3 and Bi2O3 nanostruc-tures and their luminescent properties. This paper reports the syn-thesis of Bi2S3 and Bi2O3 nanostructures using conventionalthermal evaporation and thermal oxidation techniques, respec-tively, along with their photoluminescence (PL) properties.

2. Experimental

Bi2S3 nanorods were synthesized using a thermal evaporationtechnique based on a vapor–liquid–solid mechanism. The thermalevaporation process was carried out in a conventional horizontaltube furnace. An alumina boat, 4 cm in length and 1.5 cm in diam-eter, containing Bi2S3 and S powders (350 mg, 99.99%, Sigma–Aldrich), was placed at the center of the quartz tube (zone 1,700 �C). A piece of p-type Si(100) wafer coated with a 4 nm thickAu film as a substrate was placed approximately 12 cm away fromthe alumina boat in the downstream direction (zone 2, 400 �C).After arranging the substrates, the tube was pumped down to0.75 Torr using a rotary pump. High-purity argon gas was thenintroduced into the tube at a flow rate of 60 cc/min throughoutthe synthesis process. The temperature of the evaporation source(Bi2S3 powder) in the furnace was increased to 700 �C at a rate of

orods. (d) SEM image, (e) enlarged SEM image, and (f) EDX spectrum of the Bi2O3

DX spectrum of the Bi2O3 nanostructures (oxidation time = 2 h).

H. Kim et al. / Chemical Engineering Journal 215–216 (2013) 151–156 153

30 �C/min. During synthesis, the substrate and source tempera-tures were monitored using a thermocouple. After maintainingthe temperature for 2 h, the furnace was cooled to room tempera-ture and the product was removed. Subsequently, dry oxidationwas performed. The products were transferred to a quartz tubein a horizontal oxidation furnace, which was sealed tightly andpurged with high-purity oxygen. The temperature was then in-creased to 500 �C at a heating rate of 8 �C/min and held at that tem-perature for 1, 2, or 3 h before being reduced gradually to roomtemperature. During the thermal oxidation process, the processpressure and oxygen flow rate were maintained at �0.4 Torr and�20 cc/min, respectively.

PL spectroscopy (SPEC-1403 PL spectrometer) was performed atroom temperature using a He–Cd laser (325 nm, 55 MW) as theexcitation source. Glancing angle (0.5�) X-ray diffraction (XRD)was performed using a Philips X’pert XRD diffractometer (Cu Karadiation; 0.01� step size). Scanning electron microscopy (SEM)was performed using a Hitachi S-4200 instrument. Transmissionelectron microscopy (TEM) and energy-dispersive X-ray spectrom-etry (EDXS) were carried out using a Philips CM-200 instrumentequipped with an EDX spectrometer. The film thicknesses used inthe growth rate calculations were obtained from the SEM cross-sectional views.

3. Results and discussion

XRD (Fig. 1) showed that the products obtained by thermalevaporation of a mixture of Bi2S3 and S powders comprised anorthorhombic-structured Bi2S3 phase (JCPDS card No. 17-0320).

Fig. 3. (a) Low magnification TEM image, (b) corresponding SAED pattern of the tip

The sharp and tall reflection peaks suggested that the Bi2S3 nano-rods were crystalline. ‘‘However, purely ‘‘sharp’’ peaks are notenough to verify that the products are single crystalline.’’ On theother hand, the product obtained by the thermal oxidation ofBi2S3 nanorods was a simple tetragonal-structured b-Bi2O3 phase(JCPDS card No. 27-0050) with lattice constants of a = 0.7742 nmand c = 0.5631 nm. The only low-temperature phase obtainedwas a-Bi2O3, whereas b-, c-, d-, and x-Bi2O3 were all high-temper-ature phases [42–44]. The b-Bi2O3 obtained existed as a metastablephase at room temperature, which exhibited promising ionic con-ductivity on deliberate doping [42–44].

Fig. 2a–c shows the SEM images and EDX spectrum of theas-synthesized Bi2S3 nanorods. Randomly oriented rod-like nano-structures with diameters of a few tens of nanometers and lengthsof a few micrometers were observed (Fig. 2a). The enlarged SEMimage of a typical Bi2S3 nanorod (Fig. 2b) showed a square pillar-like morphology with a globular particle at its tip. The EDX spec-trum (Fig. 2c) of the nanorods from zone 2 revealed a S to Bi atomicratio of �1.3, i.e., the nanorods were S-rich. The presence of Cu andC was attributed to the TEM grid.

Fig. 2d–f presents the SEM images and EDX spectrum of thenanostructures synthesized by the oxidation of Bi2S3 nanorodsfor 1 h. The morphology of the nanostructures (Fig. 2d) was sim-ilar to that of the as-synthesized Bi2S3 nanorods (Fig. 2a), but thecomposition of the former was quite different from that of thelatter. EDXS (Fig. 2f) showed that the nanostructures did notcontain S but consisted of Bi and O, suggesting that the nano-structures were a pure Bi2O3 phase, not a Bi2S3 phase. The O toBi atomic ratio in the nanostructures (Fig. 2f) was �1.8 (>3/2),i.e., the nanorods were O-rich, suggesting that the Bi2O3

of a Bi2O3 nanorod and (c) EDX spectrum focused at the tip of a Bi2O3 nanorod.

154 H. Kim et al. / Chemical Engineering Journal 215–216 (2013) 151–156

nanostructures contained Bi vacancies instead of O interstitials.During thermal oxidation, the Bi2S3 nanorods were oxidized toBi2O3 as follows [45]:

2Bi2S3 þ 3O2 ¼ 2Bi2O3 þ 6S; DG773K ¼ �1172:619 kJ=mol: ð1Þ

SEM images showed that the morphology of the product ob-tained by the thermal oxidation of Bi2S3 nanorods for 2 h(Fig. 2g) was different from that obtained by the oxidation ofBi2S3 nanorods for 1 h (Fig. 2d). The product consisted basicallyof two types of nanostructures with different morphologies: clus-ters or hierarchical aggregates with diameters of 0.5–1.5 lm andthin nanowires with diameters of a few tens of nanometers(Fig. 2h). EDXS (Fig. 2i) showed that the O to Bi atomic ratio inthe nanostructures from 2 h oxidation was similar to that in thenanostructures from 1h oxidation (�1.8).

Fig. 3a–c shows the low-magnification TEM image, correspond-ing selected area electron diffraction (SAED) pattern, and EDXspectrum focused at the tip of a Bi2O3 nanorod. As is well known,

Fig. 4. (a) Low-magnification TEM image of a typical Bi2S3 nanorod in the product frompattern of the Bi2S3 nanorods. (d) Low-magnification TEM image of a typical thin nancorresponding SAED pattern of a typical thin nanowire in the Bi2O3 nanostructures (oxidain the Bi2O3 nanostructure (oxidation time = 1 h). (h) HRTEM image and (i) correspon(oxidation time = 2 h).

Au was used as a catalyst for vapor–liquid–solid (VLS) growth ofBi2S3 nanorods. The SAED pattern exhibits the spotty pattern offace-centered cubic-structured Au lattice clearly. The EDX spec-trum also clearly indicates the existence of Au at the tip of thenanorod. The Cu and C in the spectrum were ascribed to the TEMgrid.

A low-magnification TEM image of a typical as-synthesizedBi2S3 nanostructure clearly showed that the nanostructure had astraight rod-like morphology with a globular particle at the tip ofeach rod (Fig. 4a). Fig. 4b and c shows an HRTEM image and thecorresponding SAED pattern of a typical Bi2S3 nanorod, respec-tively. The existence of crystal fringes over the entire area of thenanorod in the HRTEM indicated that the Bi2S3 nanorod was a sin-gle crystal. The resolved spacing between the two parallel neigh-boring fringes (Fig. 4b) was 0.35 nm, corresponding to the {130}lattice plane group of orthorhombic Bi2S3. The SAED pattern con-firmed that the Bi2S3 nanorod consisted of an orthorhombic-struc-tured Bi2S3 phase with lattice constants of a = 1.114 nm, b = 1.130

zone 2 in the evaporation furnace. (b) HRTEM image and (c) corresponding SAEDowire in the Bi2O3 nanostructure (oxidation time = 1 h). (e) HRTEM image and (f)tion time = 1 h). (g) Low-magnification TEM image of a typical hierarchical aggregateding SAED pattern of a typical hierarchical aggregate in the Bi2O3 nanostructures

Fig. 5. Comparison of the PL spectra of the Bi2S3 nanorods grown by thermalevaporation of Bi2S3 and S powders and Bi2O3 nanostructures prepared by thermaloxidation of Bi2S3 nanorods.

Fig. 6. Schematic diagram of the evolution of Bi2O3 nanostructures from Bi2S3

nanorods during the thermal oxidation process.

H. Kim et al. / Chemical Engineering Journal 215–216 (2013) 151–156 155

nm, and c = 0.3981 nm (JCPDS card No. 17-0320, Pbnm).Low-magnification TEM revealed the thin, curved, wire-like mor-phology of the two types of nanostructures, with different mor-phologies in the Bi2O3 nanostructures synthesized by 1h thermaloxidation of Bi2S3 nanorods (Fig. 3d). Fig. 4e and f presents theHRTEM image and corresponding SAED pattern, respectively, ofthe nanowire shown in Fig. 4d. The clear spotty pattern indicatedthat the nanowire was a single crystal. The resolved spacingbetween the two parallel neighboring fringes in the Bi2O3 nano-wire was the {201} lattice plane group of the tetragonal-structuredb-Bi2O3.

On the other hand, Fig. 4g–i presents the low-magnificationTEM image, HRTEM image, and corresponding SAED pattern,respectively, focusing on the cluster-like Bi2O3 nanostructure.The clear crystal fringes with different orientations in the SAEDpattern (Fig. 4f) indicated that the nanorod was nanocrystalline.The resolved spacings between the two parallel neighboringfringes in the Bi2O3 nanostructures (Fig. 3h)were 0.23, 0.31, and0.27 nm, corresponding to the {301}, {201}, and {220} latticeplane groups of the tetragonal-structured b-Bi2O3, respectively.The existence of a halo-like diffraction pattern besides the spottypattern confirmed that the nanostructure was nanocrystalline. Afurther systematic examination might be necessary; however, webelieve that the pillar-like bodies of the Bi2S3 nanorods evolvedinto thin Bi2O3 nanowires, whereas the globular particles at thetip of the individual Bi2S3 nanorods evolved into Bi2O3 hierarchicalaggregates during thermal oxidation. The higher growth rate of theBi2O3 hierarchical aggregates than that of the thin Bi2O3 nanowiresmight be due to the higher Au concentration in the hierarchicalaggregates.

Fig. 5 shows the room temperature PL spectra of the Bi2S3 andBi2O3 nanostructures synthesized in this study. The Bi2S3 nanorodshad a broad emission band centered at approximately at 595 nm inthe yellow region and a shoulder at �425 nm in the violet region. A325 nm He–Cd laser was used as the excitation source for PL. Themain peak was similar to the PL emission band centered at�585 nm obtained for the Bi2S3 nanostructures synthesized by asolvothermal method [21]. The visible emission from the Bi2S3

nanostructures was attributed to point defects such as Bi vacanciesand S interstitials in the Bi2S3 lattice [46].

The Bi2O3 nanostructures synthesized by the thermal oxidationof Bi2S3 nanorods for 1 h showed an emission band centered at�440 nm in the bluish violet region (Fig. 5). This emission mightbe attributed to the band-to-band transition because 440 nmmatches the energy band gap of b-Bi2O3 (2.81 eV) well. On theother hand, the Bi2O3 nanostructures synthesized by the thermaloxidation of Bi2S3 nanorods for 2 or 3 h showed an emission bandcentered at �505 nm, which was in the bluish green region, and ashoulder at �380 nm (violet region) (Fig. 5). The bluish green emis-sion was in good agreement with that reported by Parke et al. forBi2O3 nanostructures synthesized using a wet method [47]. Theyreported that the blue or green emission arose from energy trans-fer among B3+ ions, which were dominant in the b-Bi2O3 phase. Onthe other hand, the Bi2O3 nanostructures were prepared by ther-mal oxidation, i.e., a dry method. Therefore, the bluish green emis-sion from the Bi2O3 nanostructures synthesized by thermaloxidation of the Bi2S3 nanorods for 2 or 3 h can be attributed tothe recombination from the conduction band to the energy levelsof deep-traps like Bi vacancy or surface states [38]. The PL emissionwavelengths of the samples annealed for 1 h are different fromthose of annealing for 2 h despite that the Bi2S3 nanorods havebeen fully thermal-oxidized to Bi2O3, and that the Bi:O ratio ofthese samples annealed for different times (1 and 2 h) is also sim-ilar. However, XRD patterns (Fig. 1) and SEM images (Fig. 2)showed that there were significant differences in structure andmorphology between the two nanostructures of the samples for

different times (1 and 2 h). The PL properties of a material mightdepend on its microstructure and morphology as well as its com-position because the densities of crystal defects and surface statesin the material depend on the morphology.

A comparison of the Bi2O3 nanostructures synthesized by thethermal oxidation of Bi2S3 nanorods for 2 h with those synthesizedby the oxidation of Bi2S3 nanorods for 3 h in PL emission intensityshowed that the latter had higher emission intensity. This differencein emission intensity might be due to the difference in morphologyof the nanostructures. The former (2 h oxidation) had a rod-likemorphology, whereas the latter (3 h oxidation) had a combinationof hierarchical aggregate and thinner wire-like morphologies(Fig. 6). The larger surface area of the latter allowed more electronsand holes to return to the ground state via optically radiative recom-bination routes, which would increase the PL intensity [48].

4. Conclusions

Bi2S3 1D nanostructures were synthesized by thermal evapora-tion. Bi2O3 nanostructures were then synthesized by thermal oxi-dation of the Bi2S3 1D nanostructures. The as-synthesized Bi2S3

1D nanostructures had a randomly oriented rod-like morphologywith a globular particle at its tip; they were a few tens of nanome-ters in diameter and up to a few tens of micrometers in length. Themorphology of the product obtained by the thermal oxidation ofBi2S3 nanorods for 2 h was significantly different from that of theas-synthesized Bi2S3 nanorods or the nanostructures synthesized

156 H. Kim et al. / Chemical Engineering Journal 215–216 (2013) 151–156

by oxidizing the Bi2S3 nanorods for 1 h. The product consisted basi-cally of two types of nanostructures with different morphologies:clusters or hierarchical aggregates with diameters of 0.5–1.5 lmand thin nanowires with diameters of a few to a few tens of nano-meters. XRD and TEM showed that the Bi2S3 and Bi2O3 nanostruc-tures were monocrystalline and nanocrystalline, respectively. PLspectroscopy revealed the as-synthesized Bi2S3 nanorods had anemission band centered at �595 nm, which might be due to pointdefects such as Bi vacancies and S interstitials in the Bi2S3 lattice.The Bi2O3nanorods synthesized by thermal oxidation of the Bi2S3

nanorods for 1 h showed an emission band centered at �440 nmin the bluish violet region, which is attributed to the band-to-bandtransition. In contrast, the Bi2O3nanostructures synthesized by thethermal oxidation of Bi2S3 nanorods for 2 or 3 h showed an emis-sion band centered at �505 nm in the bluish green region. The blu-ish green emissions from the Bi2O3 nanostructures might be due tothe Bi vacancies and surface states.

Acknowledgment

This study was supported financially by the Korean ResearchFoundation (KRF) through the 2010 Core Research Program.

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