Anisotropic growth and structure-dependent photoresponse activity of multi-level one-dimensional PbS nano-architectures

Abstract

In this work, hierarchical lead sulfide (PbS) nano-architectures were obtained via a simple one-pot hydrothermal method using a single-source precursor of Pb(II)–thiourea complex, without involving any insulating organic surfactants. Structurally, the eight-armed hierarchical PbS nanostructures were controlled to exhibit one-fold, two-fold, or three-fold hierarchy of anisotropic growth along 〈111〉 directions, under different reaction conditions. The product morphology and structural evolution in the hydrothermal process exhibit four stages: Stage 1, the nucleation of PbS nanoparticles from the complex precursor; Stage 2, the formation of cubic PbS growth bases; Stage 3, the formation of PbS nano-architectures from consumption of the previously formed PbS growth bases; and Stage 4, the deconstruction of the hierarchical PbS nanoarchitectures. Nanostructures with high energy crystal faces are attractive for designing high efficiency solar cell devices. Within the PbS octa-armed dendrites, all sub-units grow along the 〈111〉 directions with {100} facets exposed. The photoexcited electrons can be driven in the {100} facets with mixed Pb/S atoms through σ bonding consisting of the overlapping s(Pb 6s)–p(S 3p) orbitals, which significantly shortens the carrier transfer distance and reduces the carrier recombination. Drop-cast thin films prepared with octa-armed PbS dendrites, showing desired [100] structural orientation, exhibit greatly enhanced photocurrent compared to that of spray-printed thin films without any structural orientation. It is expected that these findings will be useful in understanding the formation and application of PbS and other fcc nanocrystals with different morphologies.

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1. Introduction

Hierarchical nano-architectures with oriented structures and desirable active crystal facets exhibit unique structure-dependent characteristics for both fundamental scientific research and technologically important applications.^1–10 A variety of nano-architectures of metals, chalcogenides, oxides, and macromolecules have been synthesized.^4–7 As one of the important chalcogenide semiconductors, rock-salt-structured lead sulfide (PbS) has a face-centered cubic (fcc) crystal structure, a narrow band gap of 0.41 eV,^1–3 and a large exciton Bohr radius of ∼18 nm.^4–9 PbS nanocrystals shaped like rods, wires, tubes, flowers, belts, hollow spheres, etc. have been reported.^9–15 These shape-controlled PbS crystals exhibit unique properties that are attractive for a wide range of potential applications such as solar absorbers, photodetectors, light-emitting diodes, and optical switches.^16–18 Despite the excellent progress in terms of nano-architectures and relevant growth mechanisms, the development of simple methods with a better understanding of the intrinsic mechanism of the anisotropic growth of complex nano-architectures and the material structure-related unique properties is still needed.^19–23

The formation of hierarchical nano-architectures generally involves two stages, i.e. (1) an initial nucleation of growth bases (namely seeds) with thermodynamically most stable spherical or cubic shapes, and (2) the subsequent kinetically controlled anisotropic crystal growth of the grown bases. The cubic-structured materials grown under thermodynamic control factors exhibit low-energy facets and convex-surface-dominated shapes such as cube, tetrahedron, etc.^13–15 In contrast, fast crystal growth under kinetically controlled conditions results in the formation of anisotropic morphologies exhibiting high-energy facets and concave surface.^23–25 To meet the requirements of either the kinetically controlled growth or thermodynamically controlled growth of hierarchical nano-architectures, different chemical synthetic routes utilizing several well-known underlying mechanisms for anisotropic crystal growth such as blocking of desired facets by suitable capping ligands, selective etching of undesired facets, element-specific anisotropic growth, and controlling atom deposition/diffusion rates at crystal facets have been reported.^26–32

A variety of complex PbS superstructures have been successfully prepared by synthetic routes including self-assembly,^32–35 organic molecule-assisted hydro/solvothermal synthesis,^36,37 aqueous solution synthetic route, etc.^38–40 Among the methods mentioned above, the materials synthesis using a single-source precursor under hydro/solvothermal conditions allows the growth of complex hierarchical nano-architectures by increasing the homogeneity of reactants at the molecular level.⁸ Furthermore, single-source precursors typically have large molecule weight, volume, and size, in which the coordinating, bridging, or chelating organic ligands can prevent fast precipitation reaction between Pb²⁺ and S²⁻ ions, and thus help to form materials with large surface area, multi-level branches or metastable structures. Moreover, different additives such as surfactants, polymers, organic coordinating ligands, or capping agents can be added to the hydro/solvothermal reaction system, which provides a simple way of forming variable shape-controlled particles by tuning the kinetics of both the complex decomposition and the anisotropic crystal growth.^36–39 So far, interesting complex PbS nano-architectures have been successfully synthesized using multiple-ligand single-source precursors such as organic acid–Pb–thiourea complexes, Pb(OH)SCN, Pb(SCOPh)₂, etc.^41–45 Thiourea is commonly employed as a sulfide source and can coordinate to metal cations such as Cd²⁺ and Pb²⁺ through the sulfur atom together with anions such as Cl⁻ and NO₃⁻ for the charge balance.²¹

In the present study, we developed a simple hydrothermal synthesis of eight-armed PbS nano-architectures using a single-source precursor of inexpensive Pb(II)–thiourea complex in the absence of any organic capping ligands, and systematically studied the formation mechanism of the hierarchical nano-architectures. Drop-casted thin films prepared from the octa-armed dendritic PbS nano-architectures with three-fold hierarchy of anisotropic growth along 〈111〉 directions exhibit a structural [100] orientation. These oriented thin films exhibit greatly enhanced photocurrent as compared to the performance of spray-painted thin films with randomly packed nano-architecture powders. This is likely due to the fact that the σ bonding consisting of the overlap of s(Pb 6s)–p(S 3p) orbitals on the {100} facets with mixed Pb/S atoms provides faster carrier transport over reduced distance. The present findings can be extended to understanding the anisotropic growth of a variety of other fcc nano-architectures with novel properties.

2. Experimental section

2.1 Preparation of single-source precursor of Pb(II)–thiourea complex

In a typical synthesis⁷ of the single-source precursor, 13.2 g of lead nitrate (Pb(NO₃)₂, 40 mmol) and 6.1 g of thiourea (SC(NH₂)₂, 80 mmol) were dissolved in a small amount of distilled water and 80 mL ethanol, respectively, and then mixed together. The transparent solution mixture was heated to 60 °C and maintained at this temperature for 58 h. A gray-colored crystalline precipitation product was collected and washed with absolute ethanol for several times, before being dried in vacuum at 60 °C for 8 h. The dried gray-colored precipitate consisted of the Pb(II)–thiourea complex (chemical formula: Pb[SC(NH₂)₂]₂(NO₃)₂), used as the single-source precursor in following reactions.

2.2 Synthesis of hierarchical PbS nano-architectures

In a typical synthesis of hierarchical PbS nano-architectures of octa-armed dendrites, 0.6 g Pb(II)–thiourea complex was dissolved in 25 mL distilled water, and then the solution was transferred into a 30 mL Teflon-lined stainless steel autoclave. The sealed autoclave was heated to 130 °C for 18 h (Fig. 1a and b) and then cooled to room temperature naturally. The precipitate was collected via centrifugation, washed with distilled water and ethanol several times, and then dried at 60 °C for 8 h for further characterization. Additionally, time-dependent growth experiments at 10 min, 0.5, 1, 3, 6, 8, 10, 18, 36, and 48 h using 0.6 g Pb(II)–thiourea complex were conducted to study the growth process of the octa-armed dendrites. Two other types of hierarchical nano-architectures in the form of octa-armed stars (Fig. 1c and d) and octa-armed cubes (Fig. 1e and f) were synthesized at 160 and 180 °C, respectively. The other procedures were the same as described for the synthesis of octa-armed dendrites.

Fig. 1 SEM images of the PbS hierarchical nano-architectures in the form of (a and b) octa-armed dendrites, (c and d) octa-armed stars, and (e and f) octa-armed cubes prepared at hydrothermal reaction temperatures of (a and b) 130, (c and d) 160, and (e and f) 180 °C using 0.6 g Pb(II)–thiourea complex precursor.

2.3 Materials characterization

The crystal structure of the samples was studied by X-ray powder diffraction (XRD, Rigaku-Smart Lab Advance). Cu-Kα radiation (λ = 1.5408 Å) was used as the X-ray source, operating at 40 kV and 100 mA. All the samples were characterized in the 2θ range of 20–80°, with a scan-speed 0.8 s per step and step increment of 0.02° s⁻¹. The morphology and structure of the products were investigated using a field emission scanning electron microscope (FESEM, HITACHI S-4800) and transmission electron microscope (TEM, JEOL JEM-2100). The optical properties of the products were measured using a UV-Vis-NIR spectrometer (CARY 5000 spectrophotometer).

2.4 Photoelectrode performance

In order to investigate the photoelectrical properties of orientation growth, the PbS thin films were deposited by both drop-casting and spray-painting methods. For this purpose, the as-synthesized PbS nanoparticles were dissolved in hexane to form a colloidal ink with a concentration of 5–10 mg mL⁻¹. In a typical preparation, the PbS nanocrystal ink was applied directly on to an ITO glass substrate (Kaivo, sheet resistance of 7–15 Ω sq⁻¹) by drop-casting, forming oriented nanocrystal thin films after the solvents evaporated. For comparison, the reference PbS thin film was deposited on ITO glass via spray-painting using the same PbS colloidal ink. All the films were annealed under N₂ atmosphere at 300 °C for 30 min. Photoresponse devices with ITO/PbS/ITO structure, were fabricated to validate the optoelectronic properties of the as-synthesized hierarchical PbS nano-architectures. The detailed assembly of the photoresponse device is shown in the Fig. S1.† The cell performance of the fabricated photoelectrodes was characterized by measuring current–voltage (I–V) curves (Keithley 2400 Semiconductor Characterization System) over an active area is about 0.25 cm² with the voltage range of −1000 to 1000 mV, under AM1.5 simulated solar light (100 mW cm⁻², Oriel 300 W Xe lamp, and Newport AM1.5 filter).

3. Results and discussion

3.1 Morphologies and structures of PbS hierarchical nano-architectures

The morphology and microstructure of the three typical PbS products synthesized at 130, 160, and 180 °C using 0.6 g Pb(II)–thiourea complex precursor, were investigated using scanning electron microscopy (SEM). Fig. 1 shows representative SEM images of the three hierarchical PbS superstructures. As shown in Fig. 1a and b, the PbS products synthesized at 130 °C, exhibit three-fold hierarchy of anisotropic growth. The first level anisotropic growth is the two layers of four dendritic arms, about 6 μm long and 90° apart, extending radially from a central base. The second level anisotropic growth is the trident dendritic sprouts parallelly oriented along each of the above dendritic arms. The length of the sprouts gradually decreases from the end of dendritic arm to its tip. The third level anisotropic growth is the trident nanorods parallelly growing on each of the secondary dendritic sprouts. Overall, the octa-armed dendrites consist of oriented one-dimensional nanostructures at multi nano-/micron-levels. At higher hydrothermal reaction temperature of 160 °C, the reaction products (Fig. 1c and d) are octa-armed star-like nano-architectures and display two-fold hierarchy of anisotropic growth. The first level anisotropic growth is the eight spear-shaped arms, about 4.5 μm long, extending radially from a concave central base. The second level anisotropic growth is the spade-shaped units orderly stacking along each of the spear-shaped arms. These octa-armed star-like nano-architectures do not show a third level anisotropic growth of nanorods that are observed on the octa-armed dendrites (Fig. 1a and b). At much higher hydrothermal reaction temperature of 180 °C, the reaction products (Fig. 1e and f) only exhibit one level anisotropic growth which is the eight cubic arms extending radially from a concave central base. The eight cubic arms are about 1.5 μm in length and exhibit etched surface and corners surrounding a concave central base. In contrast to the dendritic arms (Fig. 1a and b) and spear-shaped arms (Fig. 1c and d) formed at lower temperatures, the octa-armed cubes do not have high-level hierarchical patterns or multi-level microstructures on the eight cubic arms. Overall, these three products indicate that the hydrothermal reaction temperature is critical to the hierarchical growth along the kinetically-controlled growth direction.

The phase structure and crystallinity of the three PbS products were confirmed by powder X-ray diffraction (XRD). Fig. 2 shows the XRD patterns of the PbS hierarchical nano-architectures in the form of octa-armed dendrites, octa-armed stars, and octa-armed cubes, shown in Fig. 1a and b, c and d, and e and f, respectively. The peak position and relative intensity of all diffraction peaks for the three products match well with the standard powder diffraction pattern (JCPDS no. 05-0592), and can be indexed as the face centered cubic (fcc) rock salt structured PbS with lattice constant of a = 5.938 Å.⁶ No peaks of other impurities can be detected, revealing the high purity of the as-synthesized products. It is noted that the diffraction peaks for the octa-armed dendrites are wider than those of the octa-armed stars (Fig. 2b) and octa-armed cubes (Fig. 2c), indicating the smallest grain size in the octa-armed dendritic nano-architectures. This is in agreement with the SEM results on the size increment of structure units from nanorods to spear-shaped arms and cube-like arms. The sub-unit structure of a typical primary dendritic arm of the octa-armed dendrites (Fig. 1a) were investigated using SEM, TEM, and HRTEM. As seen in Fig. 3a and b, the building units of both secondary and tertiary level structures align systematically along three equivalent directions. The diameter of the nanorods at the tertiary hierarchical anisotropic growth is about 90 nm. HRTEM image (Fig. 3c) of a selected area clearly shows continuous lattice fringes with lattice spacing of d = 0.29 nm, which corresponds to the interspacing distance of {200} planes of cubic PbS. The typical selected area electron diffraction (SAED) pattern (Fig. 3d) of the dendritic arm displays strong ordered electron spots, confirming the single crystalline nature of the PbS dendrites, in agreement with the HRTEM results.^35–37 Therefore, the PbS octa-armed dendrite is a single crystal with eight dendritic arms symmetrically extending along eight equivalent 〈111〉 crystallographic directions of the cubic PbS. A schematic illustration of the structure of a typical dendritic arm observed from different angles is shown in Fig. 3e. A 3D structural model (Fig. 3f) was drawn for better understanding of the hierarchical structure and orientation of primary, secondary, and tertiary nano-architectures at multiple levels. The primary dendritic arm consists of three secondary nano-level sprouts stacking along 〈111〉 directions, exhibiting micro-level trident-like structure. Each secondary sprout is composed of three tertiary nano-level nanorods well-aligned along 〈111〉 directions. The angle between the primary, secondary, and tertiary branches is close to 70°, corresponding to the theoretical angle between 〈111〉 directions.^{20,21,27–31}

Fig. 2 XRD patterns of the as-prepared PbS hierarchical nano-architectures in the form of (a) octa-armed dendrites, (b) octa-armed stars, and (c) octa-armed cubes, shown in Fig. 1a and b, c and d, and e and f, respectively.

Fig. 3 (a) SEM image, (b) TEM image, (c) HRTEM image, and (d) SAED pattern of a typical individual dendritic arm showing the morphology, lattice fringe, crystallinity, and multi-level microstructure of the octa-armed dendrites. (e) Schematic illustration of the structure of a typical dendritic arm observed from different directions. (f) Schematic 3D structural model of a typical dendritic arm, showing the hierarchical structure and orientation of primary, secondary, and tertiary nano-architectures at multi-level.

3.2 Formation of hierarchical nano-architectures

Rock salt PbS has a fcc structure. Many reports have mentioned that PbS crystals generally nucleate as tetra-decahedron seeds, exposing six {100} and eight {111} facets.^6,7 The difference of relative growth rates, corresponding to different surface energies, of the six side {100} facets (six side faces of a cube) and the eight {111} facets (eight corners of a cube) of the growth bases determines the anisotropic growth and structure of the PbS nanoarchitectures.^17–19 By adjusting the Pb/S ratio, octahedra bounded by the most stable {111} planes can be formed at the Pb/S ratio of 1.73, and perfect cubes bounded by the less stable {100} planes will result if the Pb/S ratio is reduced to 0.58.²⁰ According to Yang group's report, the cubic shape is the thermodynamically favored morphology due to the minimized surface energy.¹⁰ By further adjusting the reaction conditions, more complicated structures, such as eight arm dendrite or butterfly-like PbS microstructures³⁰ can be formed via 〈111〉 growth from a cubic crystal. In the present study, we considered the influence of the reaction temperature, reaction duration, OH⁻ ions, and precursor concentration on the morphology and structure formation of the products.

Within the Pb(II)–thiourea complex precursor (Pb[SC(NH₂)₂]₂(NO₃)₂), the Pb ion is in a tetrahedral coordination with SC(NH₂)₂ coordinated to Pb ion through the S²⁻ and NO₃⁻ coordinated to Pb⁴⁺ for charge balance. Different from the usual formation of PbS through fast precipitation reaction of S²⁻ and Pb²⁺ ions, the thermal decomposition of Pb[SC(NH₂)₂]₂(NO₃)₂ complexe²⁹ results in the formation of PbS through the reaction as follows:

3Pb[SC(NH₂)₂]₂(NO₃)₂ → 3PbS + 5NH₃ + 3HN=C=S + 3HN=C–NH + 4NO₂ + 5O₂

The release rates of S²⁻ and Pb²⁺ reactants from Pb[SC(NH₂)₂]₂(NO₃)₂ are dependent on the heating rate and the reaction temperature, which allows the flexible tuning on the growth of PbS through thermodynamically controlled nucleation at lower temperatures and kinetically controlled anisotropic crystal growth at relatively higher temperature. In order to understand the formation mechanism of the octa-armed dendritic nano-architectures with three-fold hierarchy in the 〈111〉 growth directions, the discussion of the formation process is divided into four stages, with the aid of XRD and SEM studies on the morphology and structure evolution of intermediate products.

Fig. 4 and 5 show the SEM images and XRD patterns of the Pb(II)–thiourea complex and the intermediate products formed at 10 min – 48 h during the growth of the nano-architectures at 130 °C. The heat-treated Pb(II)–thiourea complex, Pb[SC(NH₂)₂]₂(NO₃)₂, is particle-shaped with sizes of up to 100 μm, as seen in Fig. 4a. Fig. 5a shows its XRD pattern. Stage 1, the nucleation of PbS nanoparticles from the complex precursor, as shown in Fig. 4b and c. The fcc structure of these PbS nanoparticles was confirmed by XRD, as shown in Fig. 5b and c. During the heating-up process of the reaction system, the Pb[SC(NH₂)₂]₂(NO₃)₂ precursor slowly decomposed from its surface, forming relatively polydispersed round nanoparticles with an average diameter of 30–100 nm. The smooth round morphology indicates that the growth of the PbS nanoparticles at this stage is thermodynamically controlled. Stage 2, the formation of cubic PbS growth bases, as shown in Fig. 4d. Part of the initially formed smooth round nanoparticles grow to be nanocubes with size of 250–300 nm with slightly concave surface. These nanocubes can grow larger till all Pb[SC(NH₂)₂]₂(NO₃)₂ precursor completely decomposes and converts to PbS. The XRD pattern (Fig. 5d) shows relatively clear peaks of fcc PbS diffraction pattern. The formation of cubic morphology suggests the kinetically controlled growth along 〈111〉 directions. Stage 3, the formation of PbS nano-architectures through consumption of the previously formed PbS growth bases, as shown in Fig. 4e–i. The XRD diffraction patterns of all these intermediate products can be indexed to pure fcc PbS, as shown in Fig. 5e–i. After 3 h reaction (Fig. 4e), the product contains PbS microcubes with size of ∼1.1 μm and some PbS nanoparticles. Each microcube exhibits concave surfaces with etched centers. After 6 h reaction (Fig. 4f), the previously observed PbS microcubes grow anisotropically along the eight cube corners. After 8 h reaction (Fig. 4g), hierarchical nano-architectures with a deep concave center and eight symmetric dendritic arms are observed. After 10 h reaction (Fig. 4h), the hierarchical nano-architectures clearly exhibit three-fold anisotropic growth. After 18 h reaction (Fig. 4i), the PbS nanoparticles are essentially disappeared and the product contains well-defined nano-architecture with octa-symmetric-dendritic arms. The product morphology and structural evolution indicate that the re-dissolution of the formed PbS nanoparticles provides the reactants for the continuous growth of the octa-armed dendrites. Stage 4, the deconstruction of the hierarchical PbS nano-architectures (Fig. 4j–l). As the reaction prolonging, the octa-armed dendrites continuously grow bigger at the expense of the dissolution of the central growth base. After 36 h reaction (Fig. 4j), the length of dendritic arms extend up to 5–6 μm and the size of the central growth base obviously decreases due to the dissolution. After 48 h reaction (Fig. 4k and l), the PbS nano-architectures are destroyed because overgrown dendritic arms break apart from the deeply etched growth base.

Fig. 4 SEM images of (a) Pb(II)–thiourea complex precursor and the intermediate products formed at hydrothermal reaction duration of (b) 10 min, (c) 30 min, (d) 1 h, (e) 3 h, (f) 6 h, (g) 8 h, (h) 10 h. (i) 18 h, (j) 36 h, and (k and l) 48 h during the hydrothermal growth of the octa-armed dendrites at 130 °C using 0.6 g Pb(II)–thiourea complex precursor.

Fig. 5 XRD patterns of (a) Pb(II)–thiourea complex precursor and the intermediate products formed at hydrothermal reaction duration of (b) 10 min, (c) 30 min, (d) 1 h, (e) 3 h, (f) 6 h, (g) 8 h, (h) 10 h, (i) 18 h, (j) 36 h, and (k) 48 h during the hydrothermal growth of the octa-armed dendrites at 130 °C using 0.6 g Pb(II)–thiourea complex precursor. The standard powder diffraction pattern of rock-salt PbS is provided at the bottom.

The above dissected growth process of the octa-armed dendritic nano-architectures indicates that the slow decomposition of the Pb[SC(NH₂)₂]₂(NO₃)₂ precursor at relative low temperature results in the nuclei's slow formation and growth rate dominated by the low surface energy.^27–29 With the progressive decomposition of the Pb[SC(NH₂)₂]₂(NO₃)₂, the synthesis solution becomes more basic because of the accumulation of the by-product of NH₃. The OH⁻ ions protect the cubic center by selectively passivating the {100} planes at the early stages of the anisotropic crystal growth, which is consistent with previous reports.^{13–16,19,20} The strong chemical interaction between OH⁻ ions and the {100} planes of the PbS results in the chemisorption of OH⁻ ions on the {100} planes and thus strongly blocks the addition of PbS nuclei to the {100} planes of growth bases, which may greatly strengthen the Berg effect^15–20 and facilitate overgrowth at corners of the cubic center and form the concave microcubes. During the growth of the nano-architectures, a large number of small particles disappear because of the Ostwald ripening process, which results in high yield of the hierarchical symmetric octa-armed dendritic nano-architectures. Duan et al. found that micro-cubes, from the center of the {100} planes and then the corner {111} planes, were etched by OH⁻ ions depending on the amount of the ammonium hydroxide.³⁰ The etching effect of the OH⁻ ions on the {100} planes is also found with depletion of the Pb[SC(NH₂)₂]₂(NO₃)₂ precursor during the later anisotropic crystal growth at Stages 3 and 4. The dissolution of {100} facets provides ions for the continued growth of the dendritic arms. The gradual dissolution of the {100} facets results in structural breakage along different 〈111〉 directions and the final structural collapse.

A speculated progress for the shape and structure evolution of hierarchical PbS nano-architectures with tunable nano- and micro-structures is provided in Fig. 6. All three types of products (octa-armed dendrites, octa-armed stars, and octa-armed cubes) possess similar formation and growth processes, including initial nucleation and formation of the nano-cube growth bases, anisotropic crystal growth of the cubic growth bases, and selective etching and continuous anisotropic crystal growth. With the hydrothermal reaction temperature increasing from 130, through 160, to 180 °C, the decomposition rate of the Pb[SC(NH₂)₂]₂(NO₃)₂ precursor rapidly increases, which results in a larger size of the growth bases and delayed anisotropic growth in the 〈111〉 directions and dissolution on {100} facets. Compared to the octa-armed dendrites, the octa-armed stars and octa-armed cubes thus exhibit conical arms and cubic arms with large sizes and no secondary and tertiary nano-/micro-level structures are observed. Besides the influence of the reaction temperature, reaction duration, and OH⁻ ions, the precursor concentration is also important in controlling the product structure of the dendritic nano-architectures. With a lower precursor concentration (0.15 g), star-like architectures with eight symmetrically distributed rods were obtained (Fig. 7a and b). In contrast, by adding extra precursor (1 g), one-, four-, and six-armed dendritic nano-architectures were formed (Fig. 7c–f) because of the overgrowth of products. At further higher precursor concentration or extended reaction duration, only deconstructed PbS pieces were obtained.

Fig. 6 Illustration of the structural evolution of the hydrothermally grown PbS hierarchical nano-architectures in the form of octa-armed dendrites, octa-armed stars, and octa-armed cubes with changes in the hydrothermal reaction temperature (130–180 °C) and duration (1–36 h).

Fig. 7 SEM images of the PbS products hydrothermally synthesized at 130 °C for 36 h using (a and b) 0.15 g, and (c–f) 1 g Pb(II)–thiourea complex precursor.

3.3 Structure-dependent photoresponse property

The materials' properties largely depend on the surface atomic configuration and the degree of exposure of active crystal facets. A variety of studies have shown that some materials with specific exposed facets exhibit excellent photophysical and photochemical properties because certain crystal facets are more preferable than the others for photoexcitation, separation, and transport of the carriers (electrons or holes).^49–51 For example, rutile {011} and anatase {001} faces provide sites that are conducive to enrichment of holes, while the rutile {110} and anatase {101} faces offer sites that are conducive to enrichment of electrons.^41–52

PbS has fcc crystal structure in which the {111} facets only contain either Pb or S atoms while the {100} facets contain mixed Pb/S atoms. The electronic structure of PbS has been investigated by plane wave DFT calculations by Walsh and coworkers.^46,47,53,54 The analysis of electron density maps reveals that PbS adopts the centrosymmetric rock-salt structure, showing no stereochemical activity of the lone pair. The lone pair in PbS has essentially s character, exhibiting an approximately spherical charge distribution, due to the small overlap between the Pb 6s and the S 3p states (σ bonding through the s(Pb 6s)–p(S 3p) overlap). Generally, the density of states (DOS) curve shows that the top of the valence band consists of hybridized Pb 6s–S 3p orbitals,⁵³ whereas the bottom of the conduction band is primarily composed of Pb 6p orbitals. The band structure indicates that charge transfer upon photoexcitation occurs from the hybridized Pb 6s–S 3p orbitals to the Pb 6p orbitals. The hybridization orients the S atom charge density towards the Pb atom along the axial directions, leading to an increased overlap between Pb atom and S atom wave functions. According to the partial electronic density of states,^46,47 the 〈001〉 directions have maximum electron cloud overlapping area as compared to the 〈111〉 directions. The hybridization between the Pb 6s and S 3p states increases the bonding strength between the Pb and S atoms in the 〈001〉 directions, enhancing the covalency of the bond and increasing the electron density conducive to carrier transport in the 〈001〉 directions.

Within the PbS octa-armed dendrites, all sub-units grow along the 〈111〉 directions with {100} facets exposed. The photoexcited electrons can be driven to {100} facets with mixed Pb/S atoms through the σ bonding consisting of the overlapping s(Pb 6s)–p(S 3p) orbitals, which greatly shortens the carrier transfer distance and reduces the carrier recombination.⁴⁷ We thus investigated the photoresponse properties of the PbS octa-armed dendrites, prepared as thin films, in the dark and under illumination using a solar simulator (AM1.5G illumination, 100 mW cm⁻²). The illumination excites electrons in the valance band to the conduction band and then increases the holes in the PbS octa-armed dendrites, which enhance the conductivity of the film. The thin films were prepared by drop-casting PbS powders on ITO glass and spray-printing PbS powders on ITO glass for comparison. The drop-casting method allows slow evaporation of the solvents from the nano-architecture suspension with the particles naturally settling on the substrate. As seen in Fig. 8a, the particles within the drop-casted thin film forms a monolayer thin film consisting of dispersed vertically-standing dendritic nano-architectures on the substrate. This allows increased exposure of the {100} facets. In contrast, the spray-printed thin film contains a larger volume of randomly packed powders. Fig. 8b shows the XRD patterns of the drop-casted and spray-printed thin films. The spray-printed thin film sample shows a XRD diffraction pattern similar to the standard XRD pattern (JCPDS card no. 05-0592), with the {111}/{200} peak intensity ratio close to 1.3. The drop-casted thin film shows the same peak positions, but a significantly large {111}/{200} peak intensity ratio of 8.4, indicating the obvious [100] orientation of the thin film. The UV-Vis-near IR spectra (Fig. 8c) of both the drop-casted and spray-printed thin films show progressively enhanced light absorption with increasing wavelength from 300 to 1500 nm. Both films exhibit similar absorption for light near the UV region, while the light absorption at IR region for the spray-printed thin film is about 10% higher than that of the drop-casted thin film due to its larger volume. Although the drop-casted thin film has lower optical absorption, it still exhibits superior photoresponse property. Fig. 9 shows the I–V curves of the film tested in the darkness and under an illumination intensity of 100 mW cm⁻² (Xenon Lamp), AM1.5G. The photocurrent of the drop-casted thin film under light irradiation (Fig. 9a, 2.0954 mA at 1000 mV) is nearly twice as higher as that measured in darkness (Fig. 9b, 0.80162 mA at 1000 mV), enhanced by 161.4%. The photocurrent of the spray-printed thin film under light irradiation (Fig. 9c, 2.2643 mA at 1000 mV) is enhanced 75.6% that measured in darkness (Fig. 9d, 1.2893 mA at 1000 mV).

Fig. 8 (a) SEM images, (b) XRD patterns, and (c) UV-Vis-near IR spectra of the octa-armed dendrites in the form of drop-casted and spray-printed thin films.

Fig. 9 Photoresponse properties of the octa-armed dendrites in the form of (a and b) drop-casted and (c and d) spray-printed thin films in the (b and d) dark and (a and c) under AM1.5G illumination.

The significantly enhanced photocurrent for the drop-casted thin film as compared to the spray-printed thin film is attributed to three factors: (1) the hybridization of s(Pb 6s)–p(S 3p) orbitals increases the electron density conducive to carrier transport in the 〈001〉 directions, which greatly shortens the carrier transfer distance and reduces the carrier recombination; (2) the octa-armed dendrites consist of self-assembled multi-level 1D nano-/micro-nanostructures, similar to the well-known 1D ZnO and TiO₂ ^39,40 nanostructures that have been suggested as better scaffolds for 3rd generation nanostructured solar cells, with better optoelectronic properties such as lower carrier recombination and more efficient electron transport; (3) the present synthetic strategy for controlling the anisotropic growth does not use organic capping agents and structure-directing agents, and thus the surface of the nanoarchitectures is essentially clean and free of insulating barriers. Thereby the photo excited carriers can efficiently transfer between bottom and top conductive ITO glass through two opposite 1D dendritic arms, which is the shortest pathway as compared to the randomly packed powders within the spray-printed thin film. This contributes to improving the photocurrent.

4. Conclusions

In summary, we reported a facial synthetic strategy for preparing hierarchical PbS superstructures by hydrothermal decomposition of a single-source precursor in H₂O. In our hydrothermal reaction system, the crystal orientation of products is mainly dependent on the precursor concentration; the product hierarchy is determined by the hydrothermal reaction temperature; and the product morphology can be achieved by adjusting the reaction duration. Based on the analysis of the product morphology and structural evolution, we proposed a formation mechanism of the complicated PbS architectures and described the growth process in four stages. We also tried to relate the crystal structure with the electron configuration and the photoresponse performance. To take advantage of the hybridization between the Pb 6s and S 3p orbitals, which increases the bonding strength between Pb and S ions in the 〈001〉 directions and thus fasters the carrier separation and transport, we drop-casted the eight-armed PbS dendritic product to form a thin film with [100] structural orientation. The drop-casted thin film exhibits photocurrent enhancement of 161.4%, which is much higher than the 75.6% observed from the spray-painted thin film. This work will make it easier to understand and control the structure of PbS towards photoelectronic applications.

Conflict of interest

The authors declare no competing financial interest.

Acknowledgements

This research was supported by the National Natural Science Foundation of China (No. 51425202, No. 21506089), Natural Science Foundation of Jiangsu Province (BK20140929, BK2012426), Natural Science Foundation of Colleges and Universities in Jiangsu Province (14KJB530007), Specialized Research Fund for the Doctoral Program of Higher Education (SRFDP), the Project Funded by the Top-notch Academic Programs Project of Jiangsu Higher Education Institutions (TAPP), the Research and Innovation Program for College Postgraduates of Jiangsu Province (No. KYLX15_0791), the China Scholarship Council (No. 201508320302), and ​SBK2016030032.

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Footnotes

† Electronic supplementary information (ESI) available: Detailed assembly of the photoresponse device. See DOI: 10.1039/c6ra09712j

‡ These authors contributed equally to this work.

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