PbS micro-nanostructures with controlled

J Nanopart Res (2016)18:80 DOI 10.1007/s11051-016-3382-5

RESEARCH PAPER

PbS micro-nanostructures with controlled morphologies by a novel thermal decomposition approach Rama Gaur . P. Jeevanandam

Received: 25 December 2015 / Accepted: 23 February 2016 Ó Springer Science+Business Media Dordrecht 2016

Abstract A novel synthetic approach for the preparation of PbS micro-nanostructures with different morphologies has been reported. PbS micro-nanostructures with various morphologies such as stars, dendrites, hexapods, and cubes were synthesized by thermal decomposition of lead acetate and thiourea in ethylene glycol at 120 °C, in air, in the absence of any surfactant. The PbS micro-nanostructures were characterized using different analytical techniques such as X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, transmission electron microscopy, diffuse reflectance spectroscopy, and photoluminescence spectroscopy. The effect of different synthetic parameters such as [Pb2?:S2-] ratio, decomposition temperature, time, and source of sulfur on the morphologies of PbS was investigated and the mechanism for the formation of micronanostructures with different morphologies has also been proposed. Keywords Thermal decomposition Lead sulfide Dendrites Hexapods Nanocubes Optical properties Nanostructures Electronic supplementary material The online version of this article (doi:10.1007/s11051-016-3382-5) contains supplementary material, which is available to authorized users. R. Gaur P. Jeevanandam (&) Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee 247667, India e-mail: [email protected]

Introduction Lead sulfide is a semiconductor with narrow band gap (0.41 eV for bulk) and large Bohr radius (18 nm) (Ni et al. 2012). It is a promising material useful for various applications such as optoelectronics [e.g., photovoltaic solar cells and infrared (IR) emission/ detection devices], catalysis and sensing (Hansen et al. 2006; Hu et al. 2009; Malgras et al. 2015; McDonald et al. 2005). PbS exhibits diverse morphologies ranging from spheres, ribbons, cubes, hollow cubes, octapods, rods, tubes, wires, truncated octahedrons, dendrites, and star- and flower-shaped particles (Bashouti and Lifshitz 2008; Bu et al. 2011; Mandal et al. 2011; Podsiadlo et al. 2011; Saraidarov et al. 2007; Wang et al. 2008, 2011, 2013). The dependence of various physicochemical properties of semiconductor nanostructures on their shape and size is well documented and the size/shape control of nanomaterials provides an insight into future nanodevices (Zhou et al. 2006). Different polymorphs of the same material usually possess different properties, e.g., melting point, solubility, color, bioavailability, etc. (Lovette et al. 2008). Anisotropy in nanostructures yields a new category of smart materials, and anisotropic nanostructures exhibit unique properties and good efficiency when used for different applications. The properties of anisotropic nanostructures depend on their chemical composition, size, surface chemistry, structure, and defects (Hou et al. 2009). The differences in the properties of nanomaterials with

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various shapes but with the same composition are attributed to exposed crystal planes/faces and crystal shape is important in engineering of materials with desired properties. The difference in the surface energies of different crystal planes makes them suitable for various applications (Snyder and Doherty 2007). Different synthetic routes for PbS nanostructures with different morphologies such as aqueous route, reverse micelles, solvothermal, microwave, peristaltic pumping process, self-assembly, and thermal decomposition method have been reported in the literature (Huang et al. 2010; Liu et al. 2014; Ma et al. 2004; Mandal et al. 2011; McPhail and Weiss 2014; Ni et al. 2012; Querejeta-Fernandez et al. 2012; Wang et al. 2008). The reported methods for the synthesis of PbS dendrites and cubes involve complex multi-steps, high temperature (210–240 °C), long reaction time (12–48 h), and use of various surfactants [e.g., cetyltrimethylammonium bromide (CTAB), sodium dodecyl sulfate (SDS), cationic twin-tail surfactant (TTS), amino acids (L-cysteine, L-lysine, and L-methionine), aminocaproic acid, polyvinylpyrrolidone (PVP), etc.] (Bakshi et al. 2007; Ding et al. 2009; Huang et al. 2010; Liu et al. 2009; Shao et al. 2007; Xiang et al. 2008; Zhao and Qi 2006). Bakshi et al. have reported the synthesis of PbS nanocrystals and microcrystals by an aqueous method at 80 °C for 48 h using cationic TTS (Bakshi et al. 2007). Huang et al. and Zhao et al. have reported the synthesis of starshaped PbS nanocrystals by the thermal decomposition of thioacetamide and lead acetate in the presence of CTAB and SDS at 80 °C for 14 h (Huang et al. 2010; Zhao and Qi 2006). Ding et al. have reported solvothermal synthesis of PbS nanocubes and dendrites using lead acetate and thiourea in the presence of CTAB and SDS at 210 °C for 12 h (Ding et al. 2009). Liu et al., Xiang et al., and Shao et al. have reported the synthesis of PbS dendritic hierarchical nanostructures with the help of amino acids at high temperature (200–210 °C) and long reaction time (10–24 h) (Liu et al. 2009; Shao et al. 2007; Xiang et al. 2008). The present study involves the synthesis of PbS micro-nanostructures with different morphologies (multipodal dendrites, hexapod-like, distorted octahedrons, nanocubes, and nanoplates) by a facile thermal decomposition approach under mild conditions (120 °C, 5–180 min) starting from lead acetate and thiourea, without the use of any surfactant. The

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effect of varying the [Pb2?:S2-] ratio, thermal decomposition temperature, time, and source of sulfur on the morphologies of PbS has been investigated.

Experimental section Reagents Lead acetate dihydrate (Rankem, 99 %), thiourea (Rankem, 99 %), thioacetamide (Himedia, 97 %), ethylene glycol (Rankem, 99 %), methanol (Rankem, 90 %), and ethanol (Rankem, 90 %) were used in this study. All the chemicals were used as received except methanol which was distilled before use. Preparation of PbS micro-nanostructures The PbS micro-nanostructures with different morphologies were synthesized by a simple thermal decomposition method without the help of any surfactant or capping agent (Ni et al. 2012). In a typical synthesis, lead acetate (1 mmol) and thiourea (1–8 mmol) were dissolved in minimum amount of ethylene glycol (2 mL) at room temperature. The contents were added to 8 mL ethylene glycol which was preheated at 120 °C. The mixture was refluxed at 120 °C in air for 5–180 min and the slurry obtained was precipitated using methanol followed by washing with methanol. The product obtained was dried in a vacuum oven at about 70 °C overnight. Different PbS samples were prepared by varying the thermal decomposition time (5–180 min). The effect of varying the [Pb2?:S2-] ratio on the morphology of PbS was investigated and different [Pb2?:S2-] ratios (1:1, 1:2, 1:4, 1:6, and 1:8) were employed. Thiourea and thioacetamide were used as the sources of sulfur to understand the effect of sulfur source on the morphology of PbS micro-nanostructures. Table 1 enlists the synthetic details and nomenclature of various PbS samples that have been prepared in the present study. Characterization The PbS samples were characterized using various analytical techniques. Powder X-ray diffraction (XRD) measurements were done on a Bruker AXSD8 diffractometer using Cu Ka radiation (k = 1.5406 ˚ ) in the 2h range 5°–90° with a scan speed of 2° A


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Table 1 Reaction conditions for the synthesis of various PbS micro-nanostructures, their morphologies, and nomenclature of the samples Sample ID

Reactants

Reaction conditions

Morphology

S1

Pb(Ac)2 ? thiourea (1:1)

120 °C, 5 min

Hexapods (microstructures)

S2


120 °C, 5 min

Star-shaped nanostructures

S3


120 °C, 5 min

Dendrites

S4


120 °C, 5 min

Dendrites with elongated arms

S5


120 °C, 5 min

Dendrites with elongated arms and small nanoparticles

S2-50C


50 °C, 60 min

Irregular nanostructures

S2-80C


80 °C, 60 min

Under-developed hexapods (microstructures)

S2-120C


120 °C, 60 min

Dendrites

S2-150C


150 °C, 60 min

Dendrites with flattened arms

S2-180C


180 °C, 60 min

Broken hexapods

S2-220C


220 °C, 60 min

Broken arms of hexapods

S1-5


120 °C, 5 min

Hexapods

S1-30


120 °C, 30 min

Hexapods with ridges in arms

S1-60


120 °C, 60 min

Hexapods with deep ridges in arms

S1-90 S1-120

Pb(Ac)2 ? thiourea (1:1) Pb(Ac)2 ? thiourea (1:1)

120 °C, 90 min 120 °C, 120 min

Broken arms Truncated nanocubes

S1-180


120 °C, 180 min

Truncated microcubes

S2-5


120 °C, 5 min

Star-shaped dendrites

S2-30


120 °C, 30 min

Dendrites

S2-60


120 °C, 60 min


S2-90


120 °C, 90 min


S2-120


120 °C, 120 min

Micro-hexapods and small nanoparticles

S2-180


120 °C, 180 min

Nanocubes

S1-5-Ta

Pb(Ac)2 ? thioacetamide (1:1)

120 °C, 5 min

Irregular nanoplatelets

S2-5-Ta

Pb(Ac)2 ? thioacetamide (1:2)

120 °C, 5 min

Irregular nanoplatelets

min-1. IR spectra were recorded in the range 4000–400 cm-1 using a Perkin Elmer Fourier transform IR (FT-IR) spectrometer using KBr pellets. Field emission scanning electron microscopy (FE-SEM) images were recorded using a FEI Quanta 200F electron microscope operating at an accelerating voltage of 20 kV and equipped with an energydispersive X-ray analysis (EDXA) facility. Transmission electron microscopy (TEM) images were recorded using a Technai G2 transmission electron microscope operating at an accelerating voltage of 200 kV. Samples for the TEM measurements were prepared by placing a few drops of sample suspension in ethanol on carbon-coated copper grids which were dried in air. Optical properties of the PbS samples were studied using diffuse reflectance spectroscopy (DRS) and

photoluminescence (PL) spectroscopy. The DRS spectra were recorded at room temperature on a Varian Cary 5000 UV–vis–NIR spectrometer equipped with an integrating sphere using BaSO4 as the reference. The slit width and step size were 5.0 and 1.0 nm, respectively. PL spectra were recorded on an FLS-980 UV–vis–NIR fluorescence spectrometer (Edinburgh Photonics) at room temperature. About 2 mg of each PbS powder sample was ultrasonically dispersed in 10 mL water to prepare aqueous suspensions, and the PL emission spectra were recorded for the suspensions (kexc = 900 nm). The excitation slit width, emission slit width, and step size were 0.5, 0.5, and 5.0 nm, respectively. Surface charge of the aqueous PbS suspensions (1 mg in 10 mL Millipore water) was analyzed using a Zetasizer (Malvern Instruments).

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Results and discussion The results on structural and phase analysis, morphological and optical studies, and mechanism of the formation of various PbS micro-nanostructures have been discussed below. Structure and phase analysis Figure 1 shows the XRD patterns of PbS samples prepared using different [Pb2?:S2-] ratios (samples S1–S5). The XRD patterns of all the samples match with cubic PbS (JCPDS no. 78-1901). The peaks at 2h values of 25.98°, 30.10°, 43.10°, 51.02°, 53.47°, 62.59°, 68.95°, 71.01°, and 79.02° are attributed to (111), (200), (220), (311), (222), (400), (331), (420), and (422) reflections, respectively. The crystallite size of PbS was calculated using Debye–Scherrer equation and it increases from 14.8 to 25.9 nm with an increase in the [Pb2?:S2-] ratio from 1:1 (sample S1) to 1:8 (sample S5). The peak intensity ratio I(111)/I(200) varies from 0.95 to 0.81, and the reported I(111)/I(200) ratio, from the JCPDS database for bulk PbS, is 0.84. Peng et al. and Zhang et al. have reported I(111)/I(200) values for PbS with different morphologies (Peng et al. 2008; Zhang et al. 2005). High I(111)/I(200) ratio ([0.84) indicates restricted growth of (200) plane and low I(111)/I(200) ratio (\0.84) indicates preferred growth along the (200) plane (Peng et al. 2008). Figure 2a shows the XRD patterns of PbS micronanostructures prepared at different thermal


decomposition times (5–180 min, [Pb2?:S2-] = 1:1). The intensities of the XRD peaks increase with thermal decomposition time, indicating better crystallinity of the PbS samples prepared at longer thermal decomposition times. The XRD patterns also indicate higher crystallinity for PbS nanocubes compared to that of dendrites (see the insets). The calculated crystallite size using (200) reflection increases from 12.9 to 49.0 nm with an increase in the thermal decomposition time. The I(111)/I(200) ratio decreases from 0.84 to 0.75 (Table 2) when the thermal decomposition time is increased from 5 to 180 min and such decrease suggests anisotropic growth along the (200) plane (Devi et al. 2013; Peng et al. 2008). An increase in I(111)/I(200) indicates preferred growth along h100i direction resulting in extensive branching in dendrites (Zhang et al. 2005). The observed XRD results are in agreement with the reported results (Devi et al. 2013; Ding et al. 2009; Zhang et al. 2005; Zhou et al. 2006). Figure 2b shows the XRD patterns of PbS micronanostructures prepared at different thermal decomposition times when the [Pb2?:S2-] ratio is 1:2. The XRD patterns match with cubic PbS and the crystallite size varies from 18.9 to 50.8 nm. The I(111)/I(200) ratio decreases from 0.87 to 0.68 when the thermal decomposition time is increased from 5 to 180 min. The role of sulfur source on the morphology of PbS was also investigated, and two different sources (thiourea and thioacetamide) were used during the synthesis. Figure S1 shows XRD patterns of the PbS samples synthesized using thioacetamide (S1-5-Ta and S2-5Ta) with [Pb2?:S2-] ratios of 1:1 and 1:2. When thioacetamide was used as the source of sulfur, the I(111)/I(200) ratio values were 0.72 and 0.74 corresponding to [Pb2?:S2-] ratios of 1:1 and 1:2, respectively. FT-IR spectroscopy studies

Fig. 1 XRD patterns of PbS micro-nanostructures prepared using different [Pb2?:S2-] ratios (thermal decomposition time = 5 min and temperature = 120 °C). For more details on the samples, see Table 1

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Figure S2 shows the typical FT-IR spectrum of PbS samples prepared by thermal decomposition at 120 °C and 5 min. The IR spectrum exhibits bands due to adsorbed moisture and organics on the surface of PbS samples. The IR bands at 3420, 2920, and 2850 cm-1 are attributed to O–H stretching, and asymmetric and symmetric C–H stretching, respectively (Silverstein et al. 2015). The IR bands at 1630 and 1120 cm-1 are ascribed to bending vibration of water molecules and deformation of O–H, respectively (Stuart 2004). The IR


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Fig. 2 XRD patterns of PbS micro-nanostructures prepared using [Pb2?:S2-] ratio of a 1:1 and b 1:2 at different thermal decomposition times. The corresponding FE-SEM images are shown in the insets

Table 2 Crystallite size and I(111)/I(200) ratio values for the PbS micro-nanostructures synthesized at different thermal decomposition times [Pb2?:S2-] = 1:1

[Pb2?:S2-] = 1:2

Time (min)

Crystallite size (nm)

I(111)/I(200)

5

12.9

0.84

5

18.9

0.87

30

17.7

0.83

30

22.0

0.85

60

32.6

0.81

60

41.8

0.81

90

43.5

0.78

90

44.2

0.75

120

44.7

0.77

120

47.7

0.72

180

49.0

0.75

180

50.8

0.68

bands at 1065 and 1405 cm-1 are ascribed to C–O stretching and C–H bending, respectively. The IR band at 605 cm-1 is attributed to Pb–S stretching (Borhade and Uphade 2012). From the IR results, it can be concluded that the PbS samples have adsorbed organics and moisture on their surface. Morphological studies The morphological studies of PbS micro-nanostructures were carried out using FE-SEM and TEM, and the morphological evolution from dendrites to nanocubes has been studied in detail. The effect of varying the thermal decomposition time, temperature, [Pb2?:S2-] ratio, and the source of sulfur on the morphology of PbS has been discussed below.

Time (min)

Crystallite size (nm)

I(111)/I(200)

Effect of [Pb2?:S2-] ratio on the morphology of PbS A series of PbS samples were synthesized by changing the [Pb2?:S2-] ratio under similar experimental conditions (thermal decomposition time = 5 min and decomposition temperature = 120 °C). Figure 3 shows the FE-SEM images of PbS micronanostructures synthesized using different [Pb2?: S2-] molar ratios. Sample S1, prepared using [Pb2?:S2-] = 1:1, exhibits particles with symmetrical hexapod-like morphology with smooth edges (Fig. 3a). The length of the hexapods is about 1 lm; each hexapod consists of six arms. The length of each arm is *350–400 nm and the diameter varies from 100 to 200 nm. On increasing the [Pb2?:S2-] ratio to 1:2 (sample S2), star-shaped dendrites are observed

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Fig. 3 FE-SEM images of PbS micro-nanostructures prepared using different [Pb2?:S2-] molar ratios (thermal decomposition time = 5 min and temperature = 120 °C): a 1:1 (S1), b 1:2 (S2), c 1:4 (S3), d 1:6 (S4), and e 1:8 (S5). Scale bar 200 nm

(Fig. 3b). The star-shaped dendrites consist of six symmetrical arms mutually perpendicular to each other, and the length of each arm is about 500 nm. A careful insight at the individual dendrite reveals its threedimensional nature with rows of secondary branches on each arm. Each row consists of branches which are parallel to each other and perpendicular to the trunk. The main trunk has the length of about 1.0–1.5 lm and a diameter of 100 nm. The secondary branches are about 80–100 nm in length and 30–50 nm in diameter.

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The PbS micro-nanostructures prepared using [Pb2?:S2-] = 1:4 (sample S3) exhibit well-developed dendrites. The length of secondary branches increases to about 100–120 nm. Further increase in the [Pb2?:S2-] ratio leads to an increase in the length of the subbranches to 150 nm ([Pb2?:S2-] = 1:6, sample S4) and 200 nm ([Pb2?:S2-] = 1:8, sample S5). The increase in length of the sub-branches is attributed to the presence of excess of sulfur present during the synthesis and the excess sulfur acts as the capping agent (Apte et al. 2011).


Effect of thermal decomposition temperature on the morphology of PbS The thermal decomposition temperature was found to profoundly influence the morphology of PbS micronanostructures. Figure 4 shows the FE-SEM images of PbS micro-nanostructures synthesized at different temperatures. Since the PbS samples, synthesized using [Pb2?:S2-] = 1:2, exhibit well-defined starshaped dendrites, the effect of temperature was studied using [Pb2?:S2-] = 1:2 and the PbS samples were prepared at different thermal decomposition temperatures (50–220 °C). The PbS particles, synthesized at 50 °C, exhibit irregular morphology (Fig. 4a). Increasing the temperature to 80 °C results in the formation of a mixture of undeveloped hexapods and small irregular particles (Fig. 4b). After raising the decomposition temperature beyond 80 °C, multiarmed dendrites are formed. Figure 4c, d shows the formation of dendrites at 120 and 150 °C. Further increase in the thermal decomposition temperature results in the destruction of the dendrites; the PbS samples prepared at 180 and 220 °C are composed of multipods without the lateral small ‘sticks’ (Fig. 4e, f). The above synthetic experiments suggest that high concentration of sulfur and increasing the thermal decomposition temperature promote the formation of dendrites but temperatures beyond 150 °C are not beneficial for the formation of dendrites. The optimum thermal decomposition temperature and [Pb2?:S2-] ratios for the formation of dendrites are 120 °C, and 1:1 and 1:2, respectively. Effect of thermal decomposition time on the morphology of PbS The precursor ratios [Pb2?:S2-] of 1:1 and 1:2 and the decomposition temperature of 120 °C were chosen for investigation on the effect of thermal decomposition time on the evolution of PbS micro-nanostructures. Different PbS samples were prepared at 120 °C using thermal decomposition times of 5, 30, 60, 90, 120, and 180 min. The yield of the product was markedly increased (from 15 to 158 mg) by increasing the decomposition time from 5 to 180 min. Figure 5 ([Pb2?:S2-] = 1:1) and Fig. 6 ([Pb2?:S2-] = 1:2) show the morphological evolution of PbS micronanostructures with thermal decomposition times. In the PbS samples prepared using [Pb2?:S2-] ratio of

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1:1, transition from hexapods to truncated cubes is observed (Fig. 5). At 5 min, particles with symmetrical hexapod-like morphology are observed. The length of each arm is about 350–400 nm (Fig. 5a). With an increase in the decomposition time to 30 min, the hexapod starts developing ridges in the arms and the length of arm is increased to about 450 nm (Fig. 5b). Figure 5c shows the FE-SEM image of PbS prepared at 60 min. The ridges have now become prominent and the length of arms increases to about 800 nm. Further increase in the thermal decomposition time (e.g., 90 min) leads to a distortion of multipodal morphology (Fig. 5d) and finally truncated nanocubes are formed at 120 min (Fig. 5e). The increase in thermal decomposition time from 120 to 180 min leads to an increase in the edge length of truncated cubes from about 100 to 280 nm (Fig. 5f). The morphological evolution of PbS micro-nanostructures prepared using [Pb2?:S2-] ratio of 1:2 with different thermal decomposition times exhibits a transition from multipodal dendrites to cubes (Fig. 6). The PbS sample synthesized at 5 min exhibits particles with star-shaped dendrite-like morphology (Fig. 6a). The particles comprise six symmetric arms; each arm acts as the principal axis (length *1 lm and diameter *200–300 nm) and bears small secondary sub-branches mutually perpendicular to the main axis in all the four directions. When the decomposition time is increased to 30 min, particles with dendritelike morphology are observed (Fig. 6b). With an increase in the decomposition time to 30 min, the particles with star-shaped dendrite morphology grow into dendrites with elongated sub-branches. The dendrite consists of a main trunk with small lateral secondary branches growing perpendicular to the major axis. Each of the six major arms has four rows of secondary branches and each row is composed of branches parallel to each other and perpendicular to the trunk. The main trunk has the length of about 1.0–3.5 lm and the diameter of 100 nm. The branches are about 50–200 nm in length and 50–80 nm in diameter. When the decomposition time is increased to 60 min, the sub-branches get flattened and reduction in the dimension of the micro-nanostructures is observed (Fig. 6c). Further increase in the decomposition time to 90 min (Fig. 6d) leads to the formation of a product in which the sub-branches start to fuse with each other. The secondary branches break up as small nanocubes from the main multipods. The PbS

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Fig. 4 FE-SEM images of PbS micro-nanostructures (S2, [Pb2?:S2-] = 1:2 and reaction time = 60 min) synthesized at different thermal decomposition temperatures: a 50 °C, b 80 °C, c 120 °C, d 150 °C, e 180 °C, and f 220 °C. Scale bar 200 nm

sample synthesized at 120 min shows distorted multipods to form octahedral particles (Fig. 6e). At 180 min, truncated PbS nanocubes are observed with an edge length of about 90–100 nm (Fig. 6f). Effect of using different sulfur sources on the morphology of PbS The effect of using different sulfur sources on the morphology of PbS particles was also investigated.

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Thiourea and thioacetamide were chosen as the sulfur sources for the synthesis. Figure S3a, b shows the FESEM images of PbS samples synthesized using thioacetamide ([Pb2?:S2-] = 1:1 and 1:2), and Fig. 3a, b shows the corresponding FE-SEM images of PbS samples synthesized using thiourea. The FESEM results show that no PbS dendrites with multiarmed structures are formed when thiourea is replaced with thioacetamide. When thioacetamide is used, irregular PbS particles are observed (Fig. S3).


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Fig. 5 FE-SEM images of PbS micro-nanostructures prepared using [Pb2?:S2-] ratio of 1:1 (S1) at different thermal decomposition times: a 5 min, b 30 min, c 60 min, d 90 min, e 120 min, and f 180 min. Scale bar 200 nm

Hexapods and dendrites are observed when thiourea was used with different [Pb2?:S2-] ratios (1:1 and 1:2; Fig. 3a, b). No significant change in the morphology of PbS was observed when the amount of thioacetamide and thermal decomposition time were varied during the synthesis of PbS samples. When only thiourea was used as the source of sulfur, complete evolution of the morphology of PbS micro-nanostructures from dendrites to nanocubes was observed. From the above results, one can conclude that thiourea is

indispensable for the formation of PbS dendrites with multi-armed structures. The observed results on the effect of using different sources of sulfur on the morphology of PbS are in agreement with the reported results. PbS dendrites are formed when thiourea is used and PbS particles with irregular morphology are formed when other sulfur sources are used (e.g., sodium sulfide and thioacetamide) (Ni et al. 2012; Peng et al. 2008; Phuruangrat et al. 2012; Wang et al. 2003). Phuruangrat et al. have

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Fig. 6 FE-SEM images of PbS micro-nanostructures prepared using [Pb2?:S2-] ratio of 1:2 (S2) at different thermal decomposition times: a 5 min, b 30 min, c 60 min, d 90 min, e 120 min, and f 180 min. Scale bar 200 nm

reported the synthesis of cubic- and star-shaped dendritic PbS structures by solvothermal method using thiourea and thiosemicarbazide as sulfur sources, respectively (Phuruangrat et al. 2012). The use of different lead salts (lead chloride, lead acetate, and lead nitrate) and different sulfur sources (thiourea, thiosemicarbazide, and thioacetamide) for the synthesis of different PbS nanostructures by microwave method has been reported by Phuruangrat et al (Phuruangrat et al. 2008). Hexapods and fern-like structures were formed

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when thiosemicarbazide and thiourea were used, while the use of thioacetamide resulted in the formation of nano-sized particles. Wang et al. and Peng et al. have reported the formation of PbS dendrites when thiourea was used during the solvothermal synthesis and irregular particles were obtained when thioacetamide and sodium sulfide were used (Peng et al. 2008; Wang et al. 2003). Ni et al. have reported that no dendrites are formed when sodium sulfide and thioacetamide are used as the source of sulfur (Ni et al. 2012).


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Fig. 7 a, b TEM images, c high-resolution TEM image, and d selected area electron diffraction pattern of PbS dendrites (sample S2-5, [Pb2?:S2-] = 1:2, thermal decomposition time = 5 min, and temperature = 120 °C)

TEM studies TEM analysis was carried out for the PbS micronanostructures with dendrite morphology (S2-5, Table 1) and nanocubes (S2-180). Figure 7a shows the TEM image of a PbS dendrite. The dendrite consists of a principal multipodal structure, with secondary branches grown perpendicular to the major axis. The dense dark region in the TEM image clearly shows the principal multipodal structure of the dendrite. The length and diameter of each major trunk are about 800 and 150 nm, respectively. The less dense region in the TEM image shows the secondary growth perpendicular to the major axis. The length of the secondary branch varies from about 100 to 250 nm and the diameter of the secondary branches ranges from about 40 to 80 nm. Figure 7b shows the TEM image of tip of one of the arms of the dendrite, and it

clearly shows the dense major axis and less dense secondary branches. Figure 7c shows the high-resolution image of the PbS dendrite. The d spacing value, ˚ which calculated from the lattice fringes, was 3.42 A matches with that of (111) plane of cubic PbS. The selected area electron diffraction pattern of the PbS dendrite (Fig. 7d) shows bright spots indicative of its single-crystalline nature. The bright spots are indexed to (111), (200), (220), (311), and (400) planes of cubic PbS. Figure 8 shows the TEM images and selected area electron diffraction pattern of PbS nanocubes prepared by the thermal decomposition method (sample S2180). The nanocubes appear dark and dense due to large thickness of the sample (Fig. 8a, b). The edge length of the nanocubes ranges between 200 and 300 nm. Figure 8c shows the high-resolution image of the PbS nanocube. The d spacing value, calculated

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Fig. 8 a, b TEM images, c high-resolution TEM image, and d selected electron diffraction pattern of PbS nanocubes (sample S2-180, [Pb2?:S2-] = 1:2, thermal decomposition time = 180 min, and temperature = 120 °C) Table 3 Weight and atomic percent of lead and sulfur present in PbS micro-nanostructures prepared using different starting [Pb2?:S2-] ratios S1 ([Pb2?:S2-] = 1:1)

S2 ([Pb2?:S2-] = 1:2)

S3 ([Pb2?:S2-] = 1:4)

S4 ([Pb2?:S2-] = 1:6)

S5 ([Pb2?:S2-] = 1:8)

Wt%

At.%

Wt%

At.%

Wt%

At.%

Wt%

At.%

Wt%

At.%

Pb

92.1

64.3

90.5

62.6

90.5

59.7

89.5

56.9

88.8

55.0

S

7.9

35.7

7.5

37.4

9.5

40.3

10.5

43.1

11.2

44.9

Pb:S ratio

1.80

Sample

1.67

˚ which matches from the lattice fringes, was 2.96 A with that of (200) plane of cubic PbS. The selected area electron diffraction pattern of PbS nanocubes (Fig. 8d) shows an array of highly oriented bright spots indicative of their single-crystalline nature.

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1.48

1.32

1.22

Elemental analysis results The elemental composition of Pb and S in the samples was determined by EDXA. Table 3 shows the weight and atomic percent of lead and sulfur present in the


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Table 4 Weight and atomic percent of lead and sulfur present in PbS micro-nanostructures prepared at different thermal decomposition times ([Pb2?:S2-] ratio = 1:1) Sample

5 min

30 min

60 min

90 min

120 min

180 min

Wt%

At.%

Wt%

At.%

Wt%

At.%

Wt%

At.%

Wt%

At.%

Wt%

At.%

Pb

92.1

64.3

90.4

52.6

89.9

50.4

86.2

49.3

86.3

49.7

82.6

43.9

S

7.9

35.7

9.6

47.4

10.7

49.6

13.8

50.8

13.7

50.3

17.4

56.1

Pb:S ratio

1.80

1.11

1.02

0.97

0.99

0.78

Table 5 Weight and atomic percent of lead and sulfur present in PbS micro-nanostructures prepared at different thermal decomposition times ([Pb2?:S2-] ratio = 1:2) Sample

5 min

30 min

60 min

90 min

120 min

180 min

Wt%

At.%

Wt%

At.%

Wt%

At.%

Wt%

At.%

Wt%

At.%

Wt%

At.%

Pb

90.5

62.6

88.6

54.7

86.3

49.7

84.1

46.9

82.6

45.9

83.9

44.7

S

7.5

37.4

11.4

45.3

13.7

50.3

15.9

53.1

17.4

55.1

16.1

55.3

Pb:S ratio

1.67

1.21

0.99

PbS samples synthesized using different [Pb2?:S2-] ratios (samples S1–S5) by averaging the values at different spots in the FE-SEM images. The Pb:S atomic ratio in the final product varies from 1.80 to 1.22. The Pb:S atomic ratio in the PbS samples decreases with an increase in the starting [Pb2?:S2-] ratio. The decrease in the Pb:S atomic ratio is ascribed to increased sulfur content in the PbS samples from S1 to S5. Tables 4 and 5 summarize the weight and atomic percent of lead and sulfur in the PbS micronanostructures synthesized at different thermal decomposition times, when the starting [Pb2?:S2-] ratios were 1:1 and 1:2, respectively. In the PbS samples synthesized using [Pb2?:S2-] = 1:1, with an increase in the thermal decomposition time from 5 to 180 min, the Pb:S atomic ratio, in the PbS samples, decreases from 1.80 to 0.78 (Table 4). Similarly, in the case of PbS samples synthesized using [Pb2?:S2-] = 1:2, with an increase in the thermal decomposition time from 5 to 180 min, the Pb:S atomic ratio decreases from 1.67 to 0.81 (Table 5). The reduction in Pb:S atomic ratio is explained as follows. At shorter thermal decomposition time, only a small portion of the reactants (lead acetate and thiourea) get converted into PbS and the excess sulfur acts as the capping agent leading to the formation of dendritic microstructures. As the thermal decomposition time is increased, yield of the product increases indicating increased conversion of the reactants to the

0.88

0.83

0.81

product. The amount of free sulfur available for capping thus decreases with increased thermal decomposition time. Optical studies Optical characterization of the PbS micro-nanostructures was carried out using DRS and PL spectroscopy. Figure 9 shows the DRS spectra of PbS samples prepared using different [Pb2?:S2-] ratios and thermal decomposition times. The absorption at about 1400 nm corresponds to band gap absorption of PbS. The band gap values of PbS samples were estimated using Tauc plots (Fig. S4). In general, the band gap of PbS samples varies from 0.76 eV (S4) to 0.92 eV (S2-60). The blue shift in the band gap with respect to the bulk band gap (0.41 eV) is attributed to decrease in the crystallite size of PbS in the micro-nanostructures. Quantum confinement effect is not appreciable due to the larger crystallite size ([Bohr radius) of the PbS samples. For PbS samples synthesized using different [Pb2?:S2-] ratios, the band gap varies from 0.76 eV (S4) to 0.81 eV (S1) (Fig. 9a). A slight shift in the band gap towards low energy was observed for the PbS sample prepared using [Pb2?:S2-] = 1:8. The shift is attributed to decrease in the crystallite size of PbS nanoparticles prepared using increased sulfur content. Excess of sulfur acts as the capping agent leading to the formation of smaller PbS nanocrystallites. Figure 9b, c

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Fig. 9 DRS spectra of PbS micro-nanostructures prepared using a different [Pb2?:S2-] molar ratios and b, c different thermal decomposition times ([Pb2?:S2-] = 1:1 and 1:2, respectively). The decomposition temperature for all the samples was 120 °C

shows the DRS spectra for the PbS samples synthesized at different thermal decomposition times, with the starting [Pb2?:S2-] ratios of 1:1 and 1:2, respectively. The band gap of PbS varies from 0.88 eV (S230) to 0.92 eV (S2-60). No significant change in the band gap of PbS was observed when thioacetamide was used as the sulfur source instead of thiourea and the band gap values for PbS samples S2-5-Ta and S1-5Ta were 0.89 and 0.90 eV, respectively. Figure 10 shows the PL spectra of PbS samples synthesized using different [Pb2?:S2-] ratios. The PL spectrum of PbS exhibits peaks at about 965, 1015, and 1085 nm. The emission peak at 965 nm is attributed to excitonic emission and the peaks at 1015 and 1085 nm are ascribed to non-radiative recombination of

123

electrons trapped in sulfur defects present in the PbS samples (Yadav and Jeevanandam 2015; Zhao et al. 2007). The intensity of emission peaks decreases with an increase in the [Pb2?:S2-] ratio. The reduction in the PL peak intensity depends on crystallinity of the PbS samples (Zhou et al. 2006). The decrease in the intensity is attributed to decrease in the crystallinity of the PbS samples. With increasing sulfur content in the reacting mixture, dendrites are formed with high concentration of defects. These defects trap the excited electrons causing non-radiative emission and the intensity of excitonic emission is decreased (Buckner et al. 2004). Figures 11 and 12 show the PL spectra of PbS samples synthesized using different thermal decomposition times with [Pb2?:S2-] ratios of 1:1


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Fig. 10 a PL spectra of PbS micro-nanostructures prepared using different [Pb2?:S2-] ratios (thermal decomposition time = 5 min and temperature = 120 °C), and b–d the same spectra expanded in different wavelength regions

and 1:2, respectively. In general, a decrease in the intensity of PL emission peak is observed with an increase in the thermal decomposition time. The reduction in the intensity is attributed to an increase in the PbS crystallite size with an increase in the thermal decomposition time (Souici et al. 2009). PbS crystals with size greater than 10 nm do not show any significant difference in the emission properties, and no shift of excitonic emission peaks is observed. Souici et al. have reported the synthesis of PbS nanoparticles by a radiolytic method and have observed that with an increase in the radiation dose, the size of crystallite increases and intensity of the emission increases (Souici et al. 2009). The transition from dendrites to nanocubes leads to increased crystallinity, as observed from the XRD results. PbS samples with high crystallinity (nanocubes) have low concentration of defects compared to dendrites with poor crystallinity

and thus more defects (Zhou et al. 2006). As a result, the emission intensities of dendrites are more than those of PbS nanocubes. Figure S5 shows the PL spectra of PbS samples synthesized using different sources of sulfur (thiourea and thioacetamide). The PL spectrum of PbS sample synthesized using thioacetamide exhibits similar emission peaks as that of PbS synthesized using thiourea, but with a reduction in the peak intensity for the sample synthesized using thioacetamide due to an increase in crystallite size. Mechanism of formation of PbS micronanostructures The shape evolution of PbS with different morphologies could be controlled by varying the synthetic parameters such as thermal decomposition temperature, time, [Pb2?:S2-] ratio, and source of sulfur. Systematic

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Fig. 11 a PL spectra of PbS micro-nanostructures prepared using [Pb2?:S2-] = 1:1 at different thermal decomposition times (temperature = 120 °C), and b–d the same spectra expanded in different wavelength regions

studies on varying the synthetic parameters reveal that the morphology of PbS depends mainly on the degree of supersaturation of sulfide ions released from thiourea under elevated temperatures. The injection of a solution containing precursors (lead acetate and thiourea dissolved in 2 mL ethylene glycol) into ethylene glycol preheated at 120 °C causes the release of Pb?2 and S2ions which leads to the formation of PbS. Nucleation is favored due to less solubility of the product (PbS) in ethylene glycol (Phuruangrat et al. 2008). The nuclei formed get integrated with the existing nuclei, and fusion of nuclei is favored over the generation of new nuclei since this requires less energy. The growth process is kinetically and thermodynamically controlled and various synthetic parameters influence the growth process leading to the formation of PbS with various morphologies.

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At the same temperature, increasing the [Pb2?:S2-] ratio in the beginning of synthesis leads to the release of more sulfide ions from thiourea and there is an increase in the degree of supersaturation. When the [Pb2?:S2-] ratio is increased from 1:1 to 1:8, preferential growth along h100i direction is favored leading to an increase in the length of secondary branching in the dendrites. The growth and branching is dictated by the number of nuclei surrounding each particle, i.e., local supersaturation; higher degree of supersaturation favors the formation of three-dimensional dendrites. With an increase in [Pb2?:S2-] ratio from 1:1 to 1:2, the hexapods start developing lateral secondary branching over the six arms and form star-shaped dendrites (sample S2) and dendrites with extensive branching (samples S3–S5). At the same [Pb2?:S2-] ratio, increasing the thermal decomposition


Page 17 of 20

80

Fig. 12 a PL spectra of PbS micro-nanostructures prepared using [Pb2?:S2-] = 1:2 at different thermal decomposition times (temperature = 120 °C), and b–d the same spectra expanded in different wavelength regions

temperature has a similar effect on the degree of supersaturation. At temperatures greater than 150 °C, the release of sulfide ions increases which raises the monomer concentration (Peng et al. 2008). Thermal decomposition at low temperatures (e.g., 50 and 80 °C) results in the formation of irregular PbS particles (Fig. 4) with low yield due to lower local supersaturation. Increase in the thermal decomposition temperature to 180 or 220 °C also results in the formation of irregular particles. Nucleation and growth at these temperatures result in the formation of larger PbS crystals which causes strain and distortion of the dendritic micro-nanostructures. At a fixed [Pb2?:S2-] ratio and decomposition temperature, increase in the degree of supersaturation with time results in the evolution of PbS with different morphologies ranging from dendrites to cubes.

In the last few years, research has provided ample evidence to support the fact that high degree of supersaturation causes three-dimensional growth of preformed nuclei leading to the formation of dendrites (Peng et al. 2008; Wu et al. 2011). In the present study, the formation of dendritic and multipodal structures at 5 min (thermal decomposition time) is explained based on the preferential growth of particles along the h100i direction. The fast growth on six {100} faces leads to shrinkage of the faces, as evidenced by the formation of a cusp at each pod and the formation of star-shaped dendrites (Fig. 6a). The open space among the six pods consists of rough surface with high surface energy. As the reaction proceeds, the monomer concentration drops, which in turn affects the preferential growth along the h100i direction and causes subsequent filling of the free space among the six pods.

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Decreasing the supersaturation favors the formation of concave structure and leads to thermodynamically favored cubic particles (samples S1-180 and S2-180) (Wu et al. 2011). The increase in intensity of (200) planes in the XRD patterns for PbS samples synthesized at longer time (Fig. 2) supports the preferential growth along the h100i direction to form nanocubes. Peng et al. have reported that the I(111)/I(200) ratio decreases with decrease in the degree of supersaturation (Peng et al. 2008). Similar observation has been made in the present study; the I(111)/I(200) ratio decreases with increased reaction time. At shorter reaction time, the concentration of sulfide ions is high and the sulfide ions act as a capping agent and facilitates the formation of dendritic nanostructures. At longer reaction time, the conversion of reactants to PbS leads to less availability of sulfide ions for capping and hence no dendrites are formed. The negative surface charge due to sulfide ions capping on the PbS particles decreases with increased reaction time (Table 6) which supports the above mechanism, and the EDXA results are in agreement with the proposed mechanism. The Pb:S atomic ratio decreases, when the starting [Pb2?:S2-] ratio is higher and the thermal decomposition time is longer (Tables 3, 4, 5). This confirms increased conversion of the reactants (lead acetate and thiourea) to PbS and the presence of sulfur in high amount in the final product. The formation of dendrites and irregular nanoplates when thiourea and thioacetamide are used can be explained as follows. Thioacetamide easily decomposes to release sulfide ions at low temperature (e.g., *25 °C), while thiourea being stable is decomposed at relatively high temperature (*120 °C) (Peng et al. 2008; Wang et al. 2003). On changing the source of sulfur from thioacetamide to thiourea, the formation of PbS dendrites is observed. Thiourea undergoes Table 6 Zeta potential of PbS micro-nanostructures prepared at different reaction times using starting [Pb2?:S2-] ratio of 1:2 (temperature = 120 °C) Thermal decomposition time (min)

Zeta potential (mV)

5

-27.0

30

-22.8

60

-22.2

90

-21.8

120

-18.8

180

-7.8

123

controlled release of sulfide ions at high temperature compared to thioacetamide. The formation of small nuclei facilitates the assembly of PbS particles into dendritic structures. The transition of dendrites to nanocubes has been reported in the literature. Zhou et al. and Fernandez et al. have reported the shape evolution of PbS star-shaped dendrites, multipods, truncated nanocubes, and nanocubes in the presence of CTAB at longer reaction time (3 h–5 days) (QuerejetaFernandez et al. 2012; Zhou et al. 2006). Quan et al. and Zhang et al. have reported the transition from PbS nanocubes to horn-shaped dendrites (Quan et al. 2008; Zhang et al. 2005). In the present study, transition of PbS dendrites to nanocubes is observed at longer reaction time (e.g., 180 min). The synthesis of PbS dendrites by thermal decomposition method has been reported only by a few authors. McPhail et al. have reported the synthesis of PbS hexapods by thermal decomposition of lead oxide and elemental sulfur at 210 °C for 90 min under nitrogen (McPhail and Weiss 2014). Zhao et al. have synthesized star-shaped PbS nanostructures using lead acetate, thioacetamide, and acetic acid in the presence of mixed cationic/anionic surfactants (CTAB and SDS) at 80 °C for 12 h (Zhao and Qi 2006). Ni et al. have reported that the presence of CTAB is necessary for the synthesis of multi-armed PbS dendrites when lead acetate and thiourea are decomposed in ethylene glycol at 120 °C for 10 min (Ni et al. 2012). The reported thermal decomposition methods involve either high temperatures (e.g., 210 °C) or sometimes longer reaction times (e.g., 3 h) and the synthesis is carried out in the presence of surfactants (e.g., CTAB, SDS, PVP, sodium dodecylbenzenesulfonate, etc.). The present synthetic method is better than the reported thermal decomposition methods as it is carried out at lower temperature (120 °C) and less time (5 min), and the presence of inert gas or capping agents is not required. The method provides a controlled route for the synthesis of PbS micro-nanostructures at low temperatures. It is cost effective and the morphology of PbS can be easily tuned by varying the synthetic parameters.

Conclusions PbS micro-nanostructures with different morphologies (e.g., star-shaped particles, dendrites, hexapods, and nanocubes) were successfully synthesized by a


novel thermal decomposition method at relatively low temperature (120 °C) in the absence of any surfactant and inert conditions. PbS micro-nanostructures with different morphologies were obtained by varying the synthetic parameters ([Pb2?:S2-] ratio, thermal decomposition temperature, time, and source of sulfur). The morphology of PbS particles depends on the molar ratio of lead acetate to thiourea, thermal decomposition time, temperature, and source of sulfur. The transition from dendrites to nanocubes was observed on increasing the reaction time, and the presence of thiourea was found to be indispensable for the formation of dendrites; the use of thioacetamide instead of thiourea results in the formation of irregular PbS plates. Systematic studies on the synthetic parameters reveal that the formation of PbS with dendritic morphology depends on the degree of supersaturation of free sulfide ions released from thiourea at elevated temperatures. The optical properties of PbS micro-nanostructures depend on their morphology which in turn depend on the [Pb2?:S2-] ratio, thermal decomposition time, temperature, and the sulfur source. The band gap varies from 0.76 to 0.92 eV and the reduction in PL peak intensity is observed when the morphology of PbS changes from dendrites to cubes and irregular nanoplatelets. The present approach opens a new route for the synthesis of metal sulfides with controlled morphologies. The PbS micro-nanostructures with different shapes and sizes can be used in different optoelectronic and electrochemical applications. Acknowledgments The award of Junior/Senior Research Fellowship (JRF/SRF) to Ms. Rama Gaur by the Council of Scientific and Industrial Research, Government of India, is gratefully acknowledged. Thanks are also due to the Institute Instrumentation Centre, IIT Roorkee, for providing some of the facilities used in the present study.

References Apte SK, Garaje SN, Mane GP, Vinu A, Naik SD, Amalnerkar DP, Kale BB (2011) A facile template-free approach for the large-scale solid-phase synthesis of CdS nanostructures and their excellent photocatalytic performance. Small 7(7):957–964. doi:10.1002/smll.201002130 Bakshi MS, Thakur P, Sachar S, Kaur G, Banipal TS, Possmayer F, Petersen NO (2007) Aqueous phase surfactant selective shape controlled synthesis of lead sulfide nanocrystals. J Phys Chem C 111(49):18087–18098. doi:10.1021/ jp075477c

Page 19 of 20

80

Bashouti M, Lifshitz E (2008) PbS sub-micrometer structures with anisotropic shape: ribbons, wires, octapods, and hollowed cubes. Inorg Chem 47(2):678–682. doi:10.1021/ ic700706a Borhade AV, Uphade BK (2012) A comparative study on characterization and photocatalytic activities of PbS and Co doped PbS nanoparticles. Chalcogenide Lett 9(7):299–306 Bu J, Nie C, Liang J, Sun L, Xie Z, Wu Q, Lin C (2011) Synthesis of single-crystal PbS nanorods via a simple hydrothermal process using PEO–PPO–PEO triblock copolymer as a structure-directing agent. Nanotechnology 22(12):125602/1–125602/7. doi:10.1088/0957-4484/22/ 12/125602 Buckner SW, Konold RL, Jelliss PA (2004) Luminescence quenching in PbS nanoparticles. Chem Phys Lett 394(4–6):400–404. doi:10.1016/j.cplett.2004.06.138 Devi PI, Sivabharathy M, Ramachandran K (2013) Enhancement of dielectric constant in PVDF polymer using dendrite-shaped PbS nanostructures. Optik 124(19):3872–3875. doi:10.1016/j.ijleo.2012.11.038 Ding B, Shi M, Chen F, Zhou R, Deng M, Wang M, Chen H (2009) Shape-controlled syntheses of PbS submicro-/nanocrystals via hydrothermal method. J Cryst Growth 311(6):1533–1538. doi:10.1016/j.jcrysgro.2009.01.086 Hansen JA, Mukhopadhyay R, Hansen JO, Gothelf KV (2006) Femtomolar electrochemical detection of DNA targets using metal sulfide nanoparticles. J Am Chem Soc 128(12):3860–3861. doi:10.1021/ja0574116 Hou Y, Kondoh H, Ohta T (2009) PbS cubes with pyramidal pits: an example of etching growth. Cryst Growth Des 9(7):3119–3123. doi:10.1021/cg801013t Hu K, Liu P, Ye S, Zhang S (2009) Ultrasensitive electrochemical detection of DNA based on PbS nanoparticle tags and nanoporous gold electrode. Biosens Bioelectron 24(10):3113–3119. doi:10.1016/j.bios.2009.04.001 Huang T, Zhao Q, Xiao J, Qi L (2010) Controllable selfassembly of PbS nanostars into ordered structures: closepacked arrays and patterned arrays. ACS Nano 4(8):4707–4716. doi:10.1021/nn101272y Liu S, Xiong S, Bao K, Cao J, Qian Y (2009) Shape-controlled preparation of PbS with various dendritic hierarchical structures with the assistance of L-methionine. J Phys Chem C 113(30):13002–13007. doi:10.1021/jp8104437 Liu M, Leng M, Liu D, Chen F, Li C, Wang C (2014) Local supersaturation dictated branching and faceting of submicrometer PbS particles with cubic growth habit. Inorg Chem 53(21):11484–11491. doi:10.1021/ic501368y Lovette MA, Browning AR, Griffin DW, Sizemore JP, Snyder RC, Doherty MF (2008) Crystal shape engineering. Ind Eng Chem Res 47(24):9812–9833. doi:10.1021/ie800900f Ma Y, Qi L, Ma J, Cheng H (2004) Hierarchical, star-shaped PbS crystals formed by a simple solution route. Cryst Growth Des 4(2):351–354. doi:10.1021/cg034174e Malgras V, Nattestad A, Yamauchi Y, Dou SX, Kim JH (2015) The effect of surface passivation on the structure of sulphur-rich PbS colloidal quantum dots for photovoltaic application. Nanoscale 7(13):5706–5711. doi:10.1039/ c4nr07006b Mandal T, Piburn G, Stavila V, Rusakova I, Ould-Ely T, Colson AC, Whitmire KH (2011) New mixed ligand single-source

123

80

Page 20 of 20

precursors for PbS nanoparticles and their solvothermal decomposition to anisotropic nano- and microstructures. Chem Mater 23(18):4158–4169. doi:10.1021/cm201064c McDonald SA, Konstantatos G, Zhang SG, Cyr PW, Klem EJD, Levina L, Sargent EH (2005) Solution-processed PbS quantum dot infrared photodetectors and photovoltaics. Nat Mater 4(2):138–142. doi:10.1038/nmat1299 McPhail MR, Weiss EA (2014) Role of organosulfur compounds in the growth and final surface chemistry of PbS quantum dots. Chem Mater 26(11):3377–3384. doi:10. 1021/cm4040819 Ni Y, Wang X, Hong J (2012) Fast reflux synthesis of multiarmed PbS dendrites, influencing factors and optical properties. RSC Adv 2(2):546–551. doi:10.1039/c1ra00769f Peng Z, Jiang Y, Song Y, Wang C, Zhang H (2008) Morphology control of nanoscale PbS particles in a polyol process. Chem Mater 20(9):3153–3162. doi:10.1021/cm703707v Phuruangrat A, Thongtem T, Thongtem S (2008) Characterization of PbS with different morphologies produced using a cyclic microwave radiation. Appl Surf Sci 254(23):7553–7558. doi:10.1016/j.apsusc.2008.01.059 Phuruangrat A, Thongtem T, Kuntalue B, Thongtem S (2012) Characterization of cubic and star-shaped dendritic PbS structures synthesized by a solvothermal method. Mater Lett 81:55–58. doi:10.1016/j.matlet.2012.04.129 Podsiadlo P, Lee B, Prakapenka VB, Krylova GV, Schaller RD, Demortiere A, Shevchenko EV (2011) High-pressure structural stability and elasticity of supercrystals selfassembled from nanocrystals. Nano Lett 11(2):579–588. doi:10.1021/nl103587u Quan Z, Li C, Zhang X, Yang J, Yang P, Zhang C, Lin J (2008) Polyol-mediated synthesis of PbS crystals: shape evolution and growth mechanism. Cryst Growth Des 8(7):2384–2392. doi:10.1021/cg701236v Querejeta-Fernandez A, Hernandez-Garrido JC, Yang H, Zhou Y, Varela A, Parras M, Kotov NA (2012) Unknown aspects of self-assembly of PbS microscale superstructures. ACS Nano 6(5):3800–3812. doi:10.1021/nn300890s Saraidarov T, Reisfeld R, Sashchiuk A, Lifshitz E (2007) Synthesis and characterization of PbS nanorods and nanowires. Physica E 37(1–2):173–177. doi:10.1016/j.physe.2006.07. 015 Shao S, Zhang G, Zhou H, Sun P, Yuan Z, Li B, Chen T (2007) Morphological evolution of PbS crystals under the control of L-lysine at different pH values: the ionization effect of the amino acid. Solid State Sci 9(8):725–731. doi:10.1016/ j.solidstatesciences.2007.06.002 Silverstein RM, Webster FX, Kiemle DJ, Bryce DL (2015) In: Brennan D (ed) Spectrometric identification of organic compounds, 8th edn. Wiley, New York Snyder RC, Doherty MF (2007) Faceted crystal shape evolution during dissolution or growth. AIChE J 53(5):1337–1348. doi:10.1002/aic.11132

123

J Nanopart Res (2016)18:80 Souici AH, Keghouche N, Delaire JA, Remita H, Etcheberry A, Mostafavi M (2009) Structural and optical properties of PbS nanoparticles synthesized by the radiolytic method. J Phys Chem C 113(19):8050–8057. doi:10.1021/jp811133b Stuart B (2004) Infrared spectroscopy: fundamentals and applications. Wiley, Chichester Wang D, Yu DB, Shao MW, Liu XM, Yu WC, Qian YT (2003) Dendritic growth of PbS crystals with different morphologies. J Cryst Growth 257(3–4):384–389. doi:10. 1016/s0022-0248(03)01470-2 Wang N, Cao X, Guo L, Yang S, Wu Z (2008) Facile synthesis of PbS truncated octahedron crystals with high symmetry and their large-scale assembly into regular patterns by a simple solution route. ACS Nano 2(2):184–190. doi:10. 1021/nn7000855 Wang Y, Dai Q, Yang X, Zou B, Li D, Liu B, Zou G (2011) A facile approach to PbS nanoflowers and their shape-tunable single crystal hollow nanostructures: morphology evolution. CrystEngComm 13(1):199–203. doi:10.1039/c004459h Wang Y, Yang X, Xiao G, Zhou B, Liu B, Zou G, Zou B (2013) Shape-controlled synthesis of PbS nanostructures from -20 to 240 °C: the competitive process between growth kinetics and thermodynamics. CrystEngComm 15(27):5496–5505. doi:10.1039/c3ce40337h Wu W, He Y, Wu Y, Wu T (2011) Self-template synthesis of PbS nanodendrites and its photocatalytic performance. J Alloy Compd 509(38):9356–9362. doi:10.1016/j. jallcom.2011.07.036 Xiang J, Cao H, Wu Q, Zhang S, Zhang X (2008) L-Cysteineassisted self-assembly of complex PbS structures. Cryst Growth Des 8(11):3935–3940. doi:10.1021/cg7007842 Yadav SK, Jeevanandam P (2015) Synthesis of PbS–Al2O3 nanocomposites by sol–gel process and studies on their optical properties. Opt Mater 46:209–215. doi:10.1016/j. optmat.2015.04.020 Zhang W, Yang Q, Xu L, Yu W, Qian Y (2005) Growth of PbS crystals from nanocubes to eight-horn-shaped dendrites through a complex synthetic route. Mater Lett 59(27):3383–3388. doi:10.1016/j.matlet.2004.09.065 Zhao N, Qi L (2006) Low-temperature synthesis of star-shaped PbS nanocrystals in aqueous solutions of mixed cationic/ anionic surfactants. Adv Mater 18(3):359–362. doi:10. 1002/adma.200501756 Zhao XS, Xu SY, Liang LY, Li T, Cauchi S (2007) Luminescent stability of water-soluble PbS nanoparticles. J Mater Sci 42(12):4265–4269. doi:10.1007/s10853-006-0679-2 Zhou G, Lu M, Xiu Z, Wang S, Zhang H, Zhou Y, Wang S (2006) Controlled synthesis of high-quality PbS starshaped dendrites, multipods, truncated nanocubes, and nanocubes and their shape evolution process. J Phys Chem B 110(13):6543–6548. doi:10.1021/jp0549881